Patent Publication Number: US-11033453-B1

Title: Neurocognitive training system for improving visual motor responses

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 16/940,260 entitled “System And Method For Vision Testing And/Or Training”, filed on Jul. 27, 2020; which is a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 16/672,485 entitled “System And Method For Vision Testing And/Or Training”, filed on Nov. 3, 2019, now U.S. Pat. No. 10,722,114; which is a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 16/277,659 entitled “System And Method For Vision Testing”, filed on Feb. 15, 2019, now U.S. Pat. No. 10,463,250; which is a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 16/022,061 entitled “Eye Movement Measurement Device”, filed on Jun. 28, 2018, now U.S. Pat. No. 10,264,964; and further claims the benefit of U.S. Provisional Patent Application No. 62/526,140, entitled “Eye Movement Measurement Device”, filed on Jun. 28, 2017, the disclosure of each of which is hereby incorporated by reference as if set forth in their entirety herein. 
     This patent application also is a continuation-in-part of U.S. Nonprovisional patent application Ser. No. 16/120,237 entitled “Neurocognitive Training System For Improving Visual Motor Responses”, filed on Sep. 1, 2018, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a neurocognitive training system for improving visual motor responses. More particularly, the invention relates to a neurocognitive training system for improving visual motor responses of individuals having impairments and/or athletes seeking performance enhancement. 
     2. Background 
     The widely used conventional method for the measurement of eye movements is the scleral search coil (SSC) method. This method involves inserting contact lenses embedded with coils on the cornea and measuring the electromagnetic signals induced in the coils. 
     The SSC method provides very accurate measurements of both low-speed and high-speed eye movements in three dimensions. However, the method is not suitable for routine clinical testing because of its invasiveness and because of its relatively long preparation time. 
     Today for clinical testing, video recording and analysis of pupil movements is the method of choice. The method is non-invasive, but high-speed cameras are exponentially more expensive. Therefore, most cameras in the video-based clinical eye movement measurement systems are limited to 250 Hz. 
     An alternative method is using a non-video based infrared technology that can provide much higher speeds (approximately 2,000 Hz or more). However, these systems also have limitations. For example, there is electronic drift and measuring eye movements in complete darkness is difficult and not as accurate. 
     What is needed, therefore, is a non-invasive eye movement measurement device that is capable of accurately measuring of both low-speed and high-speed eye movements in a clinical setting. Moreover, an eye movement measurement device is needed that enables both low-speed and high-speed eye movements to be accurately measured without requiring a long preparation time. Furthermore, a need exists for an eye movement measurement device that utilizes hybrid technology so as to minimize the limitations associated with conventional devices relying upon a single type of measurement means. 
     The vestibulo-ocular reflex (VOR) involves the activation of the vestibular system of a person so as to cause eye movement. More particularly, the vestibulo-ocular reflex (VOR) operates so as to stabilize images on the retina during head movement. As a result of the vestibulo-ocular reflex (VOR), a head movement in one direction will produce eye movements in the opposite direction, thus preserving the images viewed by the person on the center of the visual field. Conventional tests are known for assessing the vestibulo-ocular reflex (VOR) of subjects, but these tests have numerous limitations and drawbacks, such as not being readily customizable for particular subjects, which can affect the accuracy and reliability of these tests. 
     What is needed, therefore, is a system for testing and/or training the vision of a subject that implements a customizable and accurate protocol for testing and/or training the vestibulo-ocular reflex (VOR) and/or visual motor performance of a subject. In addition, a method for testing the vision of a subject is needed that accurately tests the vestibulo-ocular reflex (VOR) and/or visual motor performance of a subject by utilizing a test that is customized for the particular subject being tested. 
     Further, many individuals experience concussions in the course of their lives. For example, athletes often sustain concussions while playing a particular sport, such as football or soccer. Frequently, one of the main consequences of a concussion is the impairment of vestibular function. Impairments in vestibular function are known to adversely affect dynamic visual acuity, and to cause blurred vision and poor visual focus when the concussed individual displaces his or her head. Visual motor exercises have been demonstrated to improve impairments in individuals with vestibular dysfunction, but to realize such an improvement, the concussed individual must perform the exercises in a regimented manner on a regular and frequent basis. 
     In addition, many athletes are seeking performance enhancement of their visual-motor system. Unlike concussed individuals, these individuals have full function of all motor control tasks and they are looking to improve their athletic performance by training their visual-motor system to perform faster and react more quickly during competition. 
     What is needed, therefore, is a neurocognitive training system for improving the visual motor performance of concussed individuals. Moreover, a neurocognitive training system is needed for enhancing the visual motor performance of athletes so that the athletes&#39; visual motor system performs faster and reacts more quickly during athletic competition. Furthermore, a need exists for a neurocognitive training system as part of the rehabilitation regime for an orthopedic and/or neurological injury. 
     BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Accordingly, the present invention is directed to a neurocognitive training system for improving visual motor responses that substantially obviates one or more problems resulting from the limitations and deficiencies of the related art. 
     In accordance with one or more embodiments of the present invention, there is provided a neurocognitive training system for improving visual motor responses. The neurocognitive training system includes a user input device, the user input device configured to output an input signal based upon an input response by the user; a visual display device having an output screen, the visual display device configured to display one or more visual objects on the output screen so that the one or more visual objects are visible to the user; and a data processing device, the data processing device operatively coupled to the user input device and the visual display device. The data processing device is programmed to generate and display one or more visual objects on the output screen of the visual display device in a plurality of different sequential locations on the output screen; receive an input signal from the user input device based upon an input response by the user; and determine, based upon the input signal received from the user input device, whether the user performed a correct action with respect to the one or more visual objects displayed on the output screen in the plurality of different sequential locations. 
     In a further embodiment of the present invention, the one or more visual objects generated and displayed on the output screen by the data processing device comprise a first visual object having a first color or shape and a second visual object having a second color or shape, the first color or shape being different from the second color or shape. When the user is presented with the first visual object having the first color or shape, the data processing device is programmed to determine whether the user performs the correct action by selecting the first visual object; and when the user is presented with the second visual object having the second color or shape, the data processing device is programmed to determine whether the user performs the correct action by not selecting the second visual object. 
     In yet a further embodiment, after the user selects the first visual object displayed on the output screen, the data processing device is further programmed to remove the first visual object from the output screen, and to generate and display an additional one of the first and second visual objects in another random location on the output screen. 
     In still a further embodiment, when the user does not select the first visual object displayed on the output screen within a predetermined period of time, the data processing device is further programmed to remove the first visual object from the output screen and to regenerate and redisplay the first visual object in another random location on the output screen. 
     In yet a further embodiment, when the one or more visual objects are generated and displayed on the output screen by the data processing device in the plurality of different sequential locations, the contrast between the one or more visual objects and a screen background is progressively reduced such that the one or more visual objects become increasing more difficult for the user to identify; and when the user is presented with the one or more visual objects with successively reduced contrast, the data processing device is programmed to determine whether the user performs the correct action by selecting each of the one or more visual objects. 
     In still a further embodiment, the data processing device is programmed to reduce the contrast between the one or more visual objects and the screen background by progressively making a color of the one or more visual objects closer to a background color of the screen background. 
     In yet a further embodiment, the user input device comprises a measurement assembly and the input signal comprises one or more measurement signals outputted by one or more measurement devices of the measurement assembly, the one or more measurement signals being generated based upon the user&#39;s contact with a surface of the measurement assembly. The data processing device is configured to receive the one or more measurement signals that are generated based upon the user&#39;s contact with the surface of the measurement assembly and to compute one or more numerical values using the one or more measurement signals, the data processing device being configured to control the movement of at least one manipulatable visual element by using the one or more computed numerical values; and the one or more visual objects generated and displayed on the output screen by the data processing device comprise a visual target defining a boundary, and the data processing device is programmed to determine whether the user performs the correct action by determining whether the user moves the at least one manipulatable visual element into the boundary of the visual target. 
     In still a further embodiment, the data processing device is further programmed to determine whether the at least one manipulatable visual element has remained in the boundary of the visual target for a predetermined period of time; and the data processing device has determined that the at least one manipulatable visual element has remained in the boundary of the visual target for the predetermined period of time, the data processing device is additionally programmed to generate and display the visual target in another random location on the output screen. 
     In yet a further embodiment, the user input device comprises at least one of: (i) a touchscreen user interface, (ii) a voice recognition device, (iii) one or more buttons, (iv) a keyboard, (v) a clicking device, (vi) a joystick, and (vii) a pointing device. 
     In still a further embodiment, the visual display device is a touchscreen visual display device with a touchscreen user interface, and the user input device comprises the touchscreen user interface. 
     In yet a further embodiment, the visual display device is a head-mounted visual display device, and the user input device comprises at least one of a voice recognition device and a touchpad. 
     In accordance with one or more other embodiments of the present invention, there is provided a neurocognitive training system for improving visual motor responses. The neurocognitive training system includes a user input device, the user input device configured to output an input signal based upon an input response by the user; a visual display device having an output screen, the visual display device configured to display one or more visual objects on the output screen so that the one or more visual objects are visible to the user; and a data processing device, the data processing device operatively coupled to the user input device and the visual display device. The data processing device is programmed to generate and display a plurality of visual objects on the output screen of the visual display device arranged in a predetermined pattern for a predetermined period of time; after the predetermined period of time has expired, remove the plurality of visual objects from the output screen and receive input signals from the user input device based upon input responses by the user that are indicative of the user&#39;s perceived replication of the plurality of visual objects arranged in the predetermined pattern; and determine, based upon the input signals received from the user input device, whether the user has correctly identified a location and sequence of the plurality of visual objects arranged in the predetermined pattern. 
     In a further embodiment of the present invention, the plurality of visual objects generated and displayed on the output screen by the data processing device are arranged in a predetermined grid pattern. 
     In yet a further embodiment, the user input device comprises at least one of: (i) a touchscreen user interface, (ii) a voice recognition device, (iii) one or more buttons, (iv) a keyboard, (v) a clicking device, (vi) a joystick, and (vii) a pointing device. 
     In still a further embodiment, the visual display device is a touchscreen visual display device with a touchscreen user interface, and the user input device comprises the touchscreen user interface. 
     In accordance with yet one or more other embodiments of the present invention, there is provided a neurocognitive training system for improving visual motor responses. The neurocognitive training system includes a user input device, the user input device configured to output an input signal based upon an input response by the user; a visual display device having an output screen, the visual display device configured to display one or more visual objects on the output screen so that the one or more visual objects are visible to the user; and a data processing device, the data processing device operatively coupled to the user input device and the visual display device. The data processing device is programmed to generate and display one or more pairs of displaceable visual objects, the one or more pairs of displaceable visual objects configured to be displaced about respective center points on the output screen of the visual display device; receive an input signal from the user input device based upon an input response by the user that identifies a tracked one of the displaceable visual objects from each of the one or more pairs of displaceable visual objects; and determine, based upon the input signal received from the user input device, whether the tracked one of the displaceable visual objects from each of the one or more pairs of displaceable visual objects identified by the user corresponds to a target displaceable visual object from each of the one or more pairs of displaceable visual objects that the user was intended to track while the target displaceable visual object was displaced on the output screen of the visual display device. 
     In a further embodiment of the present invention, the one or more pairs of displaceable visual objects generated and displayed on the output screen by the data processing device are configured to orbit the respective center points on the output screen of the visual display device for a predetermined period of time prior to the user identifying the tracked one of the displaceable visual objects from each of the one or more pairs of displaceable visual objects. 
     In yet a further embodiment, the one or more pairs of displaceable visual objects generated and displayed on the output screen by the data processing device comprise a plurality of pairs of displaceable visual objects, and the plurality of pairs of displaceable visual objects generated and displayed on the output screen by the data processing device are configured to orbit the respective center points on the output screen of the visual display device for the predetermined period of time prior to the user identifying the tracked one of the displaceable visual objects from each of the plurality of pairs of displaceable visual objects. 
     In still a further embodiment, the user input device comprises at least one of: (i) a touchscreen user interface, (ii) a voice recognition device, (iii) one or more buttons, (iv) a keyboard, (v) a clicking device, (vi) a joystick, and (vii) a pointing device. 
     In yet a further embodiment, the visual display device is a touchscreen visual display device with a touchscreen user interface, and the user input device comprises the touchscreen user interface. 
     It is to be understood that the foregoing summary and the following detailed description of the present invention are merely exemplary and explanatory in nature. As such, the foregoing summary and the following detailed description of the invention should not be construed to limit the scope of the appended claims in any sense. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a front view of an eye movement measurement device, according to a first embodiment of the invention; 
         FIG. 2  is a right side view of the eye movement measurement device of  FIG. 1 ; 
         FIG. 3  is a front view of an eye movement measurement device, according to a second embodiment of the invention; 
         FIG. 4  is a right side view of the eye movement measurement device of  FIG. 3 ; 
         FIG. 5  is a top view of the eye movement measurement device of  FIG. 3 ; 
         FIG. 6  is a schematic diagram illustrating the functionality of the video-based sensors and the non-video-based sensors of the eye movement measurement devices of  FIGS. 1 and 3 ; 
         FIG. 7  is a block diagram of constituent components of the eye movement measurement devices of  FIGS. 1 and 3 ; 
         FIG. 8  is a schematic diagram illustrating the manner in which a video-based sensor is capable of being used for the alignment of non-video-based sensors; 
         FIG. 9  depicts an example alignment of an eye using an image captured by the video-based sensor of  FIG. 8 ; 
         FIG. 10A  is a schematic diagram depicting an eye that is centered with respect to two sets of non-video-based sensors; 
         FIG. 10B  is a schematic diagram depicting an eye that is displaced to the left by a first distance relative to two sets of non-video-based sensors; 
         FIG. 10C  is a schematic diagram depicting an eye that is displaced to the left by a second distance relative to two sets of non-video-based sensors, wherein the second distance of  FIG. 10C  is greater than the first distance of  FIG. 10B ; 
         FIG. 11A  is a first bar graph illustrating numerical weights applied to the sensor output values of the non-video-based sensors in  FIG. 10A ; 
         FIG. 11B  is a second bar graph illustrating numerical weights applied to the sensor output values of the non-video-based sensors in  FIG. 10B ; 
         FIG. 11C  is a third bar graph illustrating numerical weights applied to the sensor output values of the non-video-based sensors in  FIG. 10C ; 
         FIG. 12  is a diagrammatic perspective view of a first exemplary vision testing system that utilizes a laptop computing device, according to a further embodiment of the invention; 
         FIG. 13  is a block diagram of constituent components that may be utilized in the embodiment of the vision testing system of  FIG. 12 ; 
         FIG. 14  is an illustration of an exemplary type of optotype that may be utilized in the embodiments of the vision testing systems described herein; 
         FIG. 15  is a diagrammatic view of the directions of subject head rotation that are capable of being measured with the motion sensing device of the vision testing systems described herein; 
         FIG. 16  is a partial flowchart illustrating a procedure for vision testing of a subject carried out by the system illustrated in  FIG. 12 , according to an embodiment of the invention; 
         FIG. 17  is a continuation of the flowchart of  FIG. 16 , which illustrates additional steps of the procedure for vision testing of a subject, according to an embodiment of the invention; 
         FIG. 18  is a continuation of the flowchart of  FIG. 17 , which illustrates yet additional steps of the procedure for vision testing of a subject, according to an embodiment of the invention; 
         FIG. 19  is a subject screen image of the visual display device with an exemplary visual indicator that may be utilized during the performance of the vision test, according to an embodiment of the invention; 
         FIG. 20  is a subject test screen image of the visual display device prior to the performance a static visual acuity test or a visual processing time test, wherein the exemplary visual indicator is displayed together with the exemplary optotype, according to an embodiment of the invention; 
         FIG. 21  is a graph illustrating a head angular velocity curve generated during the performance of a head impulse test, according to an embodiment of the invention; 
         FIG. 22  is a diagrammatic perspective view of a second exemplary vision testing system with eye movement tracking, according to yet a further embodiment of the invention; 
         FIG. 23  is a block diagram of constituent components that may be utilized in the vision testing system with eye movement tracking of  FIG. 22 ; 
         FIG. 24  is a diagrammatic perspective view of a third exemplary vision testing/training system, according to still a further embodiment of the invention; 
         FIG. 25  is a block diagram of constituent components that may be utilized in the embodiment of the vision testing/training system of  FIG. 24 ; 
         FIG. 26  is a screen image of a first testing/training mode displayed on the output screen of the visual display device of the vision testing/training system of  FIG. 24 ; 
         FIG. 27  is a screen image of a second testing/training mode displayed on the output screen of the visual display device of the vision testing/training system of  FIG. 24 ; 
         FIG. 28  is an exemplary screen image of an optic flow pattern, which may be used to create an optokinetic illusion for a user during testing or training performed using the vision testing/training system of  FIG. 24 ; 
         FIG. 29  is a diagrammatic perspective view of a neurocognitive training system, according to an illustrative embodiment of the invention; 
         FIG. 30  is a block diagram of constituent components that may be utilized in the illustrative embodiment of the neurocognitive training system described herein; 
         FIG. 31  is a screen image of a first training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 32  is a screen image of a second training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 33  is a screen image of a third training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 34  is a screen image of a fourth training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 35  is a screen image of a fifth training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 36  is a screen image of a sixth training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; 
         FIG. 37  is a diagrammatic timeline graphically depicting the time periods involved in computing the total reaction score for the user, according to a further illustrative embodiment of the invention; 
         FIG. 38  is a screen image of a seventh training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention; and 
         FIG. 39  is a screen image of an eighth training mode displayed on the output screen of the visual display device of the neurocognitive training system, according to the illustrative embodiment of the invention. 
     
    
    
     Throughout the figures, the same parts are always denoted using the same reference characters so that, as a general rule, they will only be described once. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     A first illustrative embodiment of an eye movement measurement device is seen generally at  100  in  FIGS. 1, 2, and 7 . As shown in  FIGS. 1, 2, and 7 , the eye movement measurement device  100  of the first illustrative embodiment generally comprises a support frame  132  configured to be worn on the head of a user; a plurality of light sources  110 ,  112 ,  114 ,  116  mounted on the central support structure  106  of the support frame  132 , the plurality of light sources  110 ,  112 ,  114 ,  116  configured to illuminate the eyes  10 ,  20  of the user; a pair of video-based sensors  102 ,  104  mounted on the support frame  132 , the pair of video-based sensors  102 ,  104  configured to detect low speed eye movements of the eyes  10 ,  20  of the user and output a plurality of first signals based upon the detected low speed eye movements; a pair of non-video-based sensors  118 ,  120  mounted on the central support structure  106  of the support frame  132 , the pair of non-video-based sensors  118 ,  120  configured to detect high speed eye movements of the one or more eyes of the user and output a plurality of second signals based upon the detected high speed eye movements; a head position detection device  144  (see  FIG. 7 ), the head position detection device  144  configured to detect a position of the head of the user and output one or more third signals that are representative of the detected position of the head of the user; and a data processing device  136  (see  FIG. 7 ) operatively coupled to the plurality of light sources  110 ,  112 ,  114 ,  116 , the pair of video-based sensors  102 ,  104 , the pair of non-video-based sensors  118 ,  120 , and the head position detection device  144 , the data processing device  136  configured to receive the plurality of first signals that are generated based upon the detected low speed eye movements, the plurality of second signals that are generated based upon the detected high speed eye movements, and the one or more third signals that are generated based upon the detected position of the head of the user, and the data processing device  136  configured to determine one or more gaze directions of the user from the plurality of first signals output by the pair of video-based sensors  102 ,  104 , the plurality of second signals output by the pair of non-video-based sensors  118 ,  120 , and the one or more third signals output by the head position detection device  144 . 
     In the illustrative embodiment, the eye movement output values are given in terms of degrees of displacement. The center gaze is labeled as 0 degrees. Horizontal and vertical deviations of a line that projects out of the center of pupil from the center gaze, measured in degrees, represent the horizontal and vertical eye movements. Prior to data collection, a calibration is necessary to convert the measured parameters into actual eye movements. 
     In the illustrative embodiment, the calibration process involves asking the user to look back and forth among two or more pre-specified dots placed in front of the user that generate known eye angles. The values derived during the calibration process are then used to convert the measured parameters into eye movements. 
     Now, with reference again to  FIGS. 1, 2, and 7 , the plurality of light sources  110 ,  112 ,  114 ,  116  of the eye movement measurement device  100  will be described. As shown in the front and side views of  FIGS. 1 and 2 , in the illustrative embodiment, each of the plurality of light sources  110 ,  112 ,  114 ,  116  comprises one or more light emitting diodes (LEDs) configured to illuminate the one or more eyes of the user. The data processing device  136  is configured to activate the one or more light emitting diodes prior to detection of the low speed eye movements by the pair of video-based sensors  102 ,  104  and prior to detection of the high speed eye movements by the non-video-based sensors  118 ,  120 . The light emitting diodes remain on during the entirety of the eye movement testing to illuminate the eyes of the user. In the illustrative embodiment, the plurality of light emitting diodes may be in form of infrared light emitting diodes. 
     Referring to  FIGS. 1 and 2 , the pair of video-based sensors  102 ,  104  of the eye movement measurement device  100  will be explained. In the illustrative embodiment, each of the pair of video-based sensors  102 ,  104  comprises a video camera mounted proximate to a respective eyebrow  40  of the user. In one exemplary embodiment, the pair of video cameras may comprise a pair of infrared cameras. Although, in an alternative embodiment, rather than being in the form of infrared cameras, the video cameras may be in the form of black-and-white cameras or color cameras. In the illustrative embodiment, each of the video cameras is in the form of a high-speed camera (e.g., a 250 Hz camera or other suitable high-speed camera). Also, in the illustrative embodiment, the video-based sensors  102 ,  104  measure low-speed eye movements, which are eye movements less than or equal to 100 degrees per second. 
     Next, the pair of non-video-based sensors  118 ,  120  will be described with reference to  FIGS. 1 and 2 . In the illustrative embodiment, the non-video-based sensors  118 ,  120  utilize direct infrared illumination and sensing means (see  FIGS. 1 and 2 ). In the illustrative embodiment, the nasal portion of both eyes  10 ,  20  of the user is illuminated by infrared light and the infrared sensors  118 ,  120  are used to measure the amount of reflected infrared light (as shown in the diagrammatic view of  FIG. 6 ). For example, as the corneal bulge (i.e., the protruding spherical cornea) of the eyes  10 ,  20  of the user moves away from the light sources  110 ,  112 ,  114 ,  116 , less light is reflected to the infrared light detectors  118 ,  120 , which may be in the form of photodiodes or other types of sensors for detecting infrared light. In the illustrative embodiment, the illuminators  110 ,  112 ,  114 ,  116  and sensors  118 ,  120  may be attached to the nose bridge component  122  of the head-mounted support frame  132  by means of the central support structure  106 . The measurement principle of the illustrative non-video-based sensors  118 ,  120  is based on taking the differences between the averages of reflected light from the areas of the eyes  10 ,  20  near the nose  30  of the user. In the illustrative embodiment, the non-video-based sensors  118 ,  120  measure high-speed eye movements, which are eye movements greater than 100 degrees per second. In the illustrative embodiment, the sampling rate of the non-video-based sensors  118 ,  120  may be 1,000 Hz or more. In other embodiments, rather than using direct infrared illumination and sensing means for the non-video-based sensors  118 ,  120 , the non-video-based sensors may be in the form of ultrasound sensors, electooculographic sensors, and combinations thereof. 
     In the illustrative embodiment, the data from the video-based sensors  102 ,  104  and the non-video-based sensors  118 ,  120  is collected in parallel. The video-based sensors  102 ,  104  track the centers of the pupils and convert them to eye movements. The non-video-based sensors  118 ,  120  track the curvature of the cornea and convert it to eye movements. Using the time stamps from both the video-based sensors  102 ,  104  and the non-video-based sensors  118 ,  120 , the eye movement data from the sensors  102 ,  104 ,  118 ,  120  is synchronized. The measured values from the video sensors  102 ,  104  are used to correct for the drift of the non-video tracings. Conversely, the measured values from the non-video sensors  118 ,  120  are used to add samples to the tracings from the video sources. The combined tracings derived from this process can accurately represent both slow and fast eye movements. 
     As mentioned above, in the illustrative embodiment, the measurements from both the video-based and non-video-based sensors  102 ,  104 ,  118 ,  120  are eventually converted to represent eye movements in degrees. The time stamps from both sets of sensors  102 ,  104 ,  118 ,  120  are used to synchronize the recordings. The video-based sensors  102 ,  104  have a better low-frequency performance, which is used to correct for drift of the non-video-based sensors  118 ,  120 . The non-video-based sensors  118 ,  120  have a better high-frequency performance, which is used to increase the number of sampling points during fast eye movements. In addition, after the data processing, eye velocities in degrees per second are eventually calculated. At the same time, the head velocities can be measured using a head position detection device  144 , which may be in the form of an inertial measurement unit (IMU) embedded in the support frame  132  worn by the user. The performance of the vestibulo-ocular reflex (VOR) is quantified by comparing the head velocity versus the eye velocities. The ratio of slow eye velocities (VOR eye movements) to head velocities, as well as the number and velocity of fast eye movements (saccades) are used to quantify the subject/user performance. 
     As shown in  FIGS. 1 and 2 , the eye movement measurement device  100  further includes a pair of dichroic mirrors  124 ,  126  mounted to the lower portion of the support frame  132 . Each of the dichroic mirrors  124 ,  126  is secured to the lower portion of the support frame  132  by a respective pair of spaced-apart mounting brackets  128 . In the first illustrative embodiment, it can be seen that the field of view of the user is substantially unobstructed by the dichroic mirrors  124 ,  126  because the mirrors  124 ,  126  are positioned below the eyes  10 ,  20  of the user. In the schematic diagram of  FIG. 6 , it can be seen each dichroic mirror  124 ,  126  is disposed in the path  134  of the reflected eye image between the eye  10 ,  20  and the respective camera  102 ,  104 . As shown in  FIG. 6 , the image of the eye  10 ,  20  is reflected off the dichroic mirror  124 ,  126 , and captured by the lens of the camera  102 ,  104 . 
     Turning to  FIG. 7 , the head position detection device  144  of the eye movement measurement device  100  will be explained. In the illustrated embodiment, the head position detection device  144  is mounted on the support frame  132  that contains the pair of video-based sensors  102 ,  104  and the pair of non-video-based sensors  118 ,  120 . The head position detection device  144  may be in the form of an inertial measurement unit (IMU) mounted on the support frame  132  worn by the user. 
     In the illustrative embodiment, it can be seen that the head-mounted support frame  132  of the eye movement measurement device  100  is in the form of an eyeglass frame worn by the user. In one or more other embodiments, the head-mounted support frame  132  may be in the form of a pair of goggles worn by the user. As shown in  FIGS. 1 and 2 , a head strap  130  is attached to the opposed sides of the support frame  132  for attaching the eye movement measurement device  100  to the head of the user. In the illustrative embodiment, the head strap  130  is resilient so that it is capable of being stretched to accommodate the head size of the user, and then, fitted in place on the head of the user. The head strap  130  can be formed from any suitable stretchable fabric, such as neoprene, spandex, and elastane. Alternatively, the head strap  130  could be formed from a generally non-stretchable fabric, and be provided with latching means or clasp means for allowing the head strap  130  to be split into two portions (e.g., the head strap  130  could be provided with a snap-type latching device). 
     In one or more further embodiments, eye movement measurement device  100  may additionally comprise a frame slippage detection device that is configured to detect a slippage of the support frame  132  relative to the head of the user. In the one or more further embodiments, the frame slippage detection device may be in the form of one or more externally disposed video cameras that are used to detect the slippage of the support frame  132 . In the illustrative embodiment, the frame slippage detection device is in the form of a stationary camera disposed in front of the eye movement measurement device  100  for detecting the slippage of the support frame  132 . 
     Also, in one or more further embodiments, the eye movement measurement device  100  may additionally comprise a light-proof cover mounted on the support frame  132 . The light-proof cover is configured to at least partially surround a front of the support frame  132  so as to permit the eye movements of the user to be detected in darkness. As such, the light-proof cover enables the eye movements of the user to be recorded in complete darkness or nearly complete darkness. In one or more embodiments, the light-proof cover is in the form of a pull-down visor, such as that used on welding goggles. In one or more alternative embodiments, light-proof cover is in the form of a snap-on cover that snaps around the support frame  132 , and in front of the dichroic mirrors  124 ,  126 . 
     Referring again to  FIG. 1 , it can be seen that, in the illustrative embodiment, the eye movement measurement device  100  further comprises a target generation device (e.g., a target light source  108 ) mounted on the support frame  132 . The target generation device  108  is configured to generate a target and project the target onto a surface (e.g., a wall surface) in front of the user such that the target is visible by the eyes  10 ,  20  of the user. The target generation device  108  may be in the form of a laser light source or a light emitting diode. 
     In addition, in one or more embodiments, the eye movement measurement device  100  further comprises a data storage device mounted on the support frame  132 . The data storage device comprises non-volatile data storage means, and is configured to store eye movement data so as to enable the eye movement data to be subsequently downloaded to a remote computing device. 
     Now, turning to  FIG. 7 , it can be seen that the illustrated data acquisition/data processing device  136  (e.g., a computing device) of the eye movement measurement device  100  includes a microprocessor  138  for processing data, memory  140  (e.g., random access memory or RAM) for storing data during the processing thereof, and data storage device(s)  142 , such as one or more hard drives, compact disk drives, floppy disk drives, flash drives, or any combination thereof. As shown in  FIG. 7 , the light sources  110 ,  112 ,  114 ,  116 , the pair of video-based sensors  102 ,  104 , and the pair of non-video-based sensors  118 ,  120  are operatively coupled to the data acquisition/data processing device  136 . Also, as illustrated in  FIG. 7 , the target light source  108  and the head position detection device  144  are operatively coupled to the data acquisition/data processing device  136 . In some embodiments, the data acquisition/data processing device  136  can be in the form of a desktop computer or laptop computer remotely located from the support frame  132  of the eye movement measurement device  100 , while in other embodiments, the data acquisition/data processing device  136  can be embodied as a mini-computer mounted on the support frame  132 . 
     In the illustrative embodiment, the data processing device  136  is operatively coupled to the light sources  110 ,  112 ,  114 ,  116 , the pair of video-based sensors  102 ,  104 , the pair of non-video-based sensors  118 ,  120 , the target light source  108  and the head position detection device  144  by means of a wireless connection. In an alternative embodiment, the data processing device  136  may be operatively coupled to the light sources  110 ,  112 ,  114 ,  116 , the pair of video-based sensors  102 ,  104 , the pair of non-video-based sensors  118 ,  120 , the target light source  108  and the head position detection device  144  by means of a hardwired connection. 
       FIGS. 3-5  illustrate an eye movement measurement device  200  according to a second illustrative embodiment of the present invention. With reference to these figures, it can be seen that, in many respects, the second illustrative embodiment is similar to that of the first embodiment. Moreover, some parts are common to both such embodiments. For the sake of brevity, the elements that the second embodiment of the eye movement measurement device has in common with the first embodiment will only be briefly mentioned, if at all, because these components have already been explained in detail above 
     Initially, referring collectively to  FIGS. 3-5 and 7 , it can be seen that, like the first illustrative embodiment, the eye movement measurement device  200  of the second illustrative embodiment generally comprises a support frame  232  configured to be worn on the head of a user; a plurality of light sources  210 ,  212 ,  214 ,  216  mounted on the central support structure  206  of the support frame  232 , the plurality of light sources  210 ,  212 ,  214 ,  216  configured to illuminate the eyes  10 ,  20  of the user; a pair of video-based sensors  202 ,  204  mounted on the support frame  232 , the pair of video-based sensors  202 ,  204  configured to detect low speed eye movements of the eyes  10 ,  20  of the user and output a plurality of first signals based upon the detected low speed eye movements; a pair of non-video-based sensors  218 ,  220  mounted on the central support structure  206  of the support frame  232 , the pair of non-video-based sensors  218 ,  220  configured to detect high speed eye movements of the one or more eyes of the user and output a plurality of second signals based upon the detected high speed eye movements; a head position detection device  144  (see  FIG. 7 ), the head position detection device  144  configured to detect a position of the head of the user and output one or more third signals that are representative of the detected position of the head of the user; and a data processing device  136  (see  FIG. 7 ) operatively coupled to the plurality of light sources  210 ,  212 ,  214 ,  216 , the pair of video-based sensors  202 ,  204 , the pair of non-video-based sensors  218 ,  220 , and the head position detection device  144 , the data processing device  136  configured to receive the plurality of first signals that are generated based upon the detected low speed eye movements, the plurality of second signals that are generated based upon the detected high speed eye movements, and the one or more third signals that are generated based upon the detected position of the head of the user, and the data processing device  136  configured to determine one or more gaze directions of the user from the plurality of first signals output by the pair of video-based sensors  202 ,  204 , the plurality of second signals output by the pair of non-video-based sensors  218 ,  220 , and the one or more third signals output by the head position detection device  144 . 
     As explained above for the first illustrative embodiment, the eye movement output values are given in terms of degrees of displacement. The center gaze is labeled as 0 degrees. Horizontal and vertical deviations of a line that projects out of the center of pupil from the center gaze, measured in degrees, represent the horizontal and vertical eye movements. 
     Now, with reference again to  FIGS. 3-5 and 7 , the plurality of light sources  210 ,  212 ,  214 ,  216  of the eye movement measurement device  200  will be described. As shown in the front and side views of  FIGS. 3 and 4 , in the illustrative embodiment, each of the plurality of light sources  210 ,  212 ,  214 ,  216  comprises one or more light emitting diodes (LEDs) configured to illuminate the one or more eyes of the user. The data processing device  136  is configured to activate the one or more light emitting diodes prior to detection of the low speed eye movements by the pair of video-based sensors  202 ,  204  and prior to detection of the high speed eye movements by the non-video-based sensors  218 ,  220 . The light emitting diodes remain on during the entirety of the eye movement testing to illuminate the eyes of the user. As described above for the first embodiment, in the illustrative embodiment, the plurality of light emitting diodes may be in form of infrared light emitting diodes. 
     Referring to  FIGS. 3 and 4 , the pair of video-based sensors  202 ,  204  of the eye movement measurement device  200  will be explained. In the illustrative embodiment, each of the pair of video-based sensors  202 ,  204  comprises a video camera mounted proximate to a respective temple of the user (i.e., each video camera  202 ,  204  is disposed outwardly from a respective eye of the user—see  FIG. 3 ). In one exemplary embodiment, the pair of video cameras may comprise a pair of infrared cameras. Although, in an alternative embodiment, rather than being in the form of infrared cameras, the video cameras may be in the form of black-and-white cameras or color cameras. In the illustrative embodiment, each of the video cameras is in the form of a high-speed camera (e.g., a 250 Hz camera or other suitable high-speed camera). Also, in the illustrative embodiment, the video-based sensors  202 ,  204  measure low-speed eye movements, which are eye movements less than or equal to 100 degrees per second. 
     Next, the pair of non-video-based sensors  218 ,  220  will be described with reference to  FIGS. 3 and 4 . In the illustrative embodiment, the non-video-based sensors  218 ,  220  utilize direct infrared illumination and sensing means (see  FIGS. 3 and 4 ). In the illustrative embodiment, the nasal portion of both eyes  10 ,  20  of the user is illuminated by infrared light and the infrared sensors  218 ,  220  are used to measure the amount of reflected infrared light (as shown in the diagrammatic view of  FIG. 6 ). For example, as the corneal bulge (i.e., the protruding spherical cornea) of the eyes  10 ,  20  of the user moves away from the light sources  210 ,  212 ,  214 ,  216 , less light is reflected to the infrared light detectors  218 ,  220 , which may be in the form of photodiodes or other types of sensors for detecting infrared light. In the illustrative embodiment, the illuminators  210 ,  212 ,  214 ,  216  and sensors  218 ,  220  may be attached to the nose bridge component  222  of the head-mounted support frame  232  by means of the central support structure  206 . The measurement principle of the illustrative non-video-based sensors  218 ,  220  is based on taking the differences between the averages of reflected light from the areas of the eyes  10 ,  20  near the nose  30  of the user. In the illustrative embodiment, the non-video-based sensors  218 ,  220  measure high-speed eye movements, which are eye movements greater than 100 degrees per second. In other embodiments, rather than using direct infrared illumination and sensing means for the non-video-based sensors  218 ,  220 , the non-video-based sensors may be in the form of ultrasound sensors, electooculographic sensors, and combinations thereof. 
     As described above for the first illustrative embodiment, the data from the video-based sensors  202 ,  204  and the non-video-based sensors  218 ,  220  is collected in parallel. The video-based sensors  202 ,  204  track the centers of the pupils and convert them to eye movements. The non-video-based sensors  218 ,  220  track the curvature of the cornea and convert it to eye movements. Using the time stamps from both the video-based sensors  202 ,  204  and the non-video-based sensors  218 ,  220 , the eye movement data from the sensors  202 ,  204 ,  218 ,  220  is synchronized. The measured values from the video sensors  202 ,  204  are used to correct for the drift of the non-video tracings. Conversely, the measured values from the non-video sensors  218 ,  220  are used to add samples to the tracings from the video sources. The combined tracings derived from this process can accurately represent both slow and fast eye movements. 
     As mentioned above, in the illustrative embodiment, the measurements from both the video-based and non-video-based sensors  202 ,  204 ,  218 ,  220  are eventually converted to represent eye movements in degrees. The time stamps from both sets of sensors  202 ,  204 ,  218 ,  220  are used to synchronize the recordings. The video-based sensors  202 ,  204  have a better low-frequency performance, which is used to correct for drift of the non-video-based sensors  218 ,  220 . That is, the output data from the video-based sensors  202 ,  204  may be used to determine the amount by which the sampling curve produced by the non-video-based sensors  218 ,  220  is shifted or offset due to the effects of sensor drift. The non-video-based sensors  218 ,  220  have a better high-frequency performance, which is used to increase the number of sampling points during fast eye movements. That is, the additional sampling points generated by the non-video-based sensors  218 ,  220  may be used to fill-in sampling points between the sampling points generated using the video-based sensors  202 ,  204  by using an interpolation algorithm, etc. (e.g., a quadratic interpolation algorithm). For example, the equation provided below may be used to align the curves generated by the video-based sensors  202 ,  204  and the non-video-based sensors  218 ,  220  so that the drift and additional sampling point corrections may be applied:
 
 {right arrow over (N)}=a·I·{right arrow over (S)}+b   (1)
 
     where: 
     {right arrow over (N)}: interpolated values for a series of time points obtained from the video-based sensor(s); 
     I: identity matrix; and 
     {right arrow over (S)}: sensed values for the same time points obtained from the non-video-based sensors. 
     In addition to applying the drift and additional sampling point corrections, in the illustrative embodiment, a cross-correlation technique may also be applied so as to correct for an unknown time shift due to an unknown latency in the video output data generated by the video-based sensors  202 ,  204  (i.e., cameras). In addition, after the data processing, eye velocities in degrees per second are eventually calculated. At the same time, the head velocities can be measured using the head position detection device  144 , which may be in the form of an inertial measurement unit (IMU) embedded in the support frame  232  worn by the user. The performance of the vestibulo-ocular reflex (VOR) is quantified by comparing the head velocity versus the eye velocities. The ratio of slow eye velocities (VOR eye movements) to head velocities, as well as the number and velocity of fast eye movements (saccades) are used to quantify the subject/user performance. 
     As shown in  FIGS. 3-5 , the eye movement measurement device  200  further includes a pair of dichroic mirrors  224 ,  226  mounted to the central support structure  206  of the support frame  232 . In the second illustrative embodiment, it can be seen that the dichroic mirrors  224 ,  226  are disposed in front of the eyes  10 ,  20  of the user so as to generally cover the field of view of the user, rather than being positioned below the eyes  10 ,  20  of the user, as in the first illustrative embodiment. Each of the dichroic mirrors  224 ,  226  is configured so as to reflect the eye image back to the camera  202 ,  204 , yet still allow the user to see through the mirror  224 ,  226  so that the surrounding environment is visible to him or her. Particularly, in the schematic diagram of  FIG. 6 , it can be seen each dichroic mirror  224 ,  226  is disposed in the path  234  of the reflected eye image between the eye  10 ,  20  and the respective camera  202 ,  204 . As shown in  FIG. 6 , the image of the eye  10 ,  20  is reflected off the dichroic mirror  224 ,  226 , and captured by the lens of the camera  202 ,  204 . 
     Turning to  FIG. 7 , the head position detection device  144  of the eye movement measurement device  200  will be explained. Similar to that described above for the first illustrative embodiment, the head position detection device  144  is mounted on the support frame  232  that contains the pair of video-based sensors  202 ,  204  and the pair of non-video-based sensors  218 ,  220 . The head position detection device  144  may be in the form of an inertial measurement unit (IMU) mounted on the support frame  232  worn by the user. 
     In the illustrative embodiment, it can be seen that the head-mounted support frame  232  of the eye movement measurement device  200  is in the form of an eyeglass frame worn by the user. In one or more other embodiments, the head-mounted support frame  232  may be in the form of a pair of goggles worn by the user. As shown in  FIGS. 3 and 4 , a head strap  230  is attached to the opposed sides of the support frame  232  for attaching the eye movement measurement device  200  to the head of the user. In the illustrative embodiment, the head strap  230  is resilient so that it is capable of being stretched to accommodate the head size of the user, and then, fitted in place on the head of the user. The head strap  230  can be formed from any suitable stretchable fabric, such as neoprene, spandex, and elastane. Alternatively, the head strap  230  could be formed from a generally non-stretchable fabric, and be provided with latching means or clasp means for allowing the head strap  230  to be split into two portions (e.g., the head strap  230  could be provided with a snap-type latching device). 
     In one or more further embodiments, eye movement measurement device  200  may additionally comprise a frame slippage detection device that is configured to detect a slippage of the support frame  232  relative to the head of the user. In the one or more further embodiments, the frame slippage detection device may be in the form of one or more externally disposed video cameras that are used to detect the slippage of the support frame  232 . In the illustrative embodiment, the frame slippage detection device is in the form of a stationary camera disposed in front of the eye movement measurement device  200  for detecting the slippage of the support frame  232 . 
     Also, in one or more further embodiments, the eye movement measurement device  200  may additionally comprise a light-proof cover mounted on the support frame  232 . The light-proof cover is configured to at least partially surround a front of the support frame  232  so as to permit the eye movements of the user to be detected in darkness. As such, the light-proof cover enables the eye movements of the user to be recorded in complete darkness or nearly complete darkness. In one or more embodiments, the light-proof cover is in the form of a pull-down visor, such as that used on welding goggles. In one or more alternative embodiments, light-proof cover is in the form of a snap-on cover that snaps around the support frame  232 , and in front of the dichroic mirrors  224 ,  226 . 
     In addition, in one or more further embodiments, the non-video-based sensors (e.g., infrared sensors) may be located relative to the optical of axis of the video-based sensor(s) (e.g., a camera) according to a known relationship (e.g., a predetermined distance or distances between the sensors, etc.) so as to enable the video-based sensor(s) to be used for the alignment of the non-video-based sensors. For example, as shown in the schematic diagram  300  of  FIG. 8 , non-video-based sensors  302 ,  304  are arranged on opposite sides of the video-based sensor  306 , which may be in the form of a camera  306 . More specifically, the non-video-based sensors  302 ,  304  are spaced apart from the camera optical axis (as indicated by the centerline in  FIG. 8 ) by known horizontal distances. In  FIG. 8 , the actual position of the eye  310  is shown offset to the right of the optical axis of the camera  306 . As shown in  FIG. 8 , the desired position of the eye  308  is centered on the camera optical axis. Because a known fixed relationship exists between the location of the camera  306  and the non-video-based sensors  302 ,  304  in  FIG. 8 , the non-video-based sensors  302 ,  304  may be aligned in accordance with the actual position of the eye  310  by using the image information obtained from the camera  306 . For example,  FIG. 9  depicts an example alignment of an eye using an image  312  captured by the camera  306  in  FIG. 8 . In  FIG. 9 , the eye  318  with pupil  320  depicts the actual location of the eye as captured by the camera  306 , while the centered eye  314  with pupil  316  depicts the desired position of the eye in the image. The offset distance between the eye  318  and the eye  314  in  FIG. 9  may be used to adjust the non-video-based sensors  302 ,  304  of  FIG. 8  to account for the non-centered position of the eye  318 . 
     Next, referring to  FIGS. 10A-10C and 11A-11C , an exemplary manner in which the non-video-based sensors may be calibrated so as to compensate for the misalignment of an eye relative to the non-video-based sensors will be described. Initially, as shown in  FIG. 10A , the central axis  334  of the eye  322  is shown centered on, and bisecting the dimensional line  328  extending from the center location of the first set of non-video-based sensors  336  to the center location of the second set of non-video-based sensors  338 . Because the eye  322  is centered relative to both sets of non-video-based sensors  336 ,  338  in  FIG. 10A , a weight of 1.0 is applied to the centermost sensors  340 ,  342  of each non-video-based sensor set  336 ,  338 , while a weight of 0.5 is applied to the two sensors disposed on opposite sides of the centermost sensors  340 ,  342 . The weighting of the non-video-based sensors  336 ,  338  is graphically illustrated in the bar graph  352  of  FIG. 11A . As shown in  FIG. 11A , the weights of 1.0 that are applied to the centermost sensors  340 ,  342  are represented by the center bar  356 , while the weights of 0.5 that are applied to the two sensors disposed on opposite sides of the centermost sensors  340 ,  342  are represented by the bars  354 ,  358 . 
     Turning  FIG. 10B , the central axis  334  of the eye  324  is shown offset to the left of the midpoint of the dimensional line  328  by a first distance  330 . Because the eye  324  is misaligned to the left relative to both sets of non-video-based sensors  336 ,  338  in  FIG. 10B , a weight of 1.0 is applied to the sensors  344 ,  346  of each non-video-based sensor set  336 ,  338 , while a weight of 0.5 is applied to the two sensors disposed on opposite sides of the sensors  344 ,  346 . The weighting of the non-video-based sensors  336 ,  338  is graphically illustrated in the bar graph  360  of  FIG. 11B . As shown in  FIG. 11B , the weights of 1.0 that are applied to the sensors  344 ,  346  are represented by the bar  364 , while the weights of 0.5 that are applied to the two sensors disposed on opposite sides of the sensors  344 ,  346  are represented by the bars  362 ,  366 . 
     Next, as shown in  FIG. 10C , the central axis  334  of the eye  326  is shown offset to the left of the midpoint of the dimensional line  328  by a second distance  332 , which is greater than the offset distance  330  of  FIG. 10B . Because the eye  326  is misaligned to the left relative to both sets of non-video-based sensors  336 ,  338  in  FIG. 10C  by a greater distance than that of  FIG. 10B , a weight of 1.5 is applied to the leftmost sensors  348 ,  350  of each non-video-based sensor set  336 ,  338 , while a weight of 0.5 is applied to the sensors disposed on right sides of the sensors  348 ,  350 . The weighting of the non-video-based sensors  336 ,  338  is graphically illustrated in the bar graph  368  of  FIG. 11C . As shown in  FIG. 11C , the weights of 1.5 that are applied to the sensors  348 ,  350  are represented by the bar  370 , while the weights of 0.5 that are applied to the sensors disposed on right sides of the sensors  348 ,  350  are represented by the bar  372 . 
     In the embodiment of  FIGS. 10A-10C and 11A-11C , the misalignment of the eye relative to the non-video-based sensors is compensated for by selectively applying weights to particular sensors in sensor sets. Although, in other embodiments, the misalignment of the eye relative to the non-video-based sensors may be compensated for by translating and/or rotating the non-video-based sensors using actuators based upon the information obtained from the video-based sensor(s). 
     Referring again to  FIG. 3 , it can be seen that, in the illustrative embodiment, the eye movement measurement device  200  further comprises a target generation device (e.g., a target light source  208 ) mounted on the support frame  232 . The target generation device  208  is configured to generate a target and project the target onto a surface (e.g., a wall surface) in front of the user such that the target is visible by the eyes  10 ,  20  of the user. The target generation device  208  may be in the form of a laser light source or a light emitting diode. 
     In addition, in one or more embodiments, the eye movement measurement device  200  further comprises a data storage device mounted on the support frame  232 . The data storage device comprises non-volatile data storage means, and is configured to store eye movement data so as to enable the eye movement data to be subsequently downloaded to a remote computing device. 
     The data processing device  136  of the eye movement measurement device  200  is the same as that described above for the first embodiment. In the illustrative embodiment, the data processing device  136  is operatively coupled to the light sources  210 ,  212 ,  214 ,  216 , the pair of video-based sensors  202 ,  204 , the pair of non-video-based sensors  218 ,  220 , the target light source  208  and the head position detection device  244  by means of a wireless connection. In an alternative embodiment, the data processing device  136  may be operatively coupled to the light sources  210 ,  212 ,  214 ,  216 , the pair of video-based sensors  202 ,  204 , the pair of non-video-based sensors  218 ,  220 , the target light source  208  and the head position detection device  144  by means of a hardwired connection. 
     In the illustrative embodiment, the eye movement measurement devices  100 ,  200  are configured to determine the gaze direction of a subject. Once the subject has been outfitted with the eye movement measurement device  100 ,  200 , the eye movement of the subject is determined based upon the output data from the video-based sensors  102 ,  104 ,  202 ,  204  and the non-video-based sensors  118 ,  120 ,  218 ,  220  of the eye movement tracking device  100 ,  200 , while the head position of the subject is determined based upon the output data from the head position detection device  144 . The gaze direction of the subject is generally equal to the sum of the head movement and the eye movement, and is given in degrees per second. 
     The eye movement measurement devices  100 ,  200  described above may be used for a variety of different eye tests. For example, the eye movement measurement devices  100 ,  200  may be used in head impulse testing in which the subject&#39;s head is moved rapidly either in the horizontal or vertical planes. The examining clinician or physician stands behind the subject while holding the head. The subject is instructed to stare at a stationary dot placed about one meter straight ahead. During several rapid head movements either horizontally (right-left) or vertically (up-down), the eye and head velocities will be determined by the device and used to quantify the performance of the semicircular canals within the vestibular system. 
     In one or more further embodiments, a vision testing system is disclosed that is particularly configured for head impulse testing on a subject. An illustrative embodiment of a vision testing system for head impulse testing is seen generally at  400  in  FIG. 12 . In the illustrative embodiment, the vision testing system  400  generally comprises a laptop computing device  402  with a visual display device  404 . The laptop computing device  402  is one exemplary form of a data processing device and/or data processing and data acquisition device. In  FIG. 12 , the laptop computing device  402  is disposed on an adjustable height table  406  so that the height of the laptop computing device  402  is selectively adjustable by a user. In one or more embodiments, the height of the table  406  is adjusted prior to each test session such that the center of the visual display device  404  of the laptop computing device  402 , which contains the one or more visual objects  410 , is generally horizontally aligned with the eyes of the subject  418  (i.e., so the subject  418  is looking straight at the visual object  410  on the visual display device  404  during the testing). 
     The one or more visual objects  410  (e.g., optotypes), which the subject identifies during each vision test described herein, are displayed on the visual display device  404  of the laptop computing device  402  in the illustrative embodiment. Specifically, as shown in  FIG. 12 , the visual object  410  is displayed on the output screen of the laptop visual display device  404  within a circle  412 . The circle  412  circumscribes the visual object  410  and focuses the attention of the subject  418  on the central region of the output screen containing the visual object  410 . 
     While a circle  412  is used to circumscribe the visual object  410  in the illustrative embodiment of  FIG. 12 , it is to be understood that other suitable shapes could be used to circumscribe the visual object  410  as well. For example, the visual object  410  may be alternatively circumscribed by a square or rectangle, as these shapes are also capable of effectively focusing the subject&#39;s attention on the central region of the output screen of the visual display device  404  containing the visual object  410 . 
     In addition, while the visual object  410  in  FIG. 12  is in the form of a Tumbling E optotype, it is to be understood that other suitable visual objects  410  may be used in place of the Tumbling E. For example, the visual object  410  alternatively may comprise a Landolt C optotype or an optotype comprising different letters of a recognized alphabet (e.g., different letters of the English alphabet). That is, in some embodiments, the subject  418  may identify different letters of a recognized alphabet, rather than different orientations of the same letter or optotype. Also, in other embodiments, the visual object  410  alternatively may comprise any other identifiable symbol (e.g., a crescent, a star, etc.). 
     In one or more embodiments, different letters or objects may be displayed in succession during a particular test (e.g., during the head impulse test described hereinafter). For example, during a particular test, a Tumbling E optotype may be displayed first, then followed by the letter “K”, a crescent symbol, a star symbol, etc. In this exemplary fashion, the letters that are displayed to the subject  418  can be consistently varied during the performance of the testing. 
     Referring again to  FIG. 12 , it can be seen that the head  430  of the subject  418  is fitted with a motion sensing device  428  disposed thereon. In particular, in the illustrative embodiment, the motion sensing device  428  is removably coupled to the head of the subject  418  using a stretchable, elastic headband  426  (i.e., a resilient band  426 ). The motion sensing device  428  is configured to measure a velocity or speed of the head  430  of the subject  418  when the head  430  of the subject  418  is displaced by the clinician  414  during the head impulse test described herein. That is, the motion sensing device  428  determines the velocity of the subject&#39;s head  430  during head impulse test (e.g., the angular velocity of the subject&#39;s head  430  in degrees per second). 
     In the illustrative embodiment, the motion sensing device  428  may comprise a three-dimensional motion sensing device (i.e., an inertial measurement unit (IMU)) having a 3-axis accelerometer, a 3-axis rate gyroscope, and a 3-axis compass (i.e., a 3-axis magnetometer). Also, the illustrative motion sensing device  428  may comprise a wireless data connection to the laptop computing device  402 . In particular, the laptop computing device  402  may comprise a data transmission interface unit that is operatively connected to one of the output ports of the laptop computing device  402 , such as the universal serial bus (USB) port of the laptop computing device  402 . As such, the laptop computing device  402  provided with the data transmission interface unit wirelessly communicates with the motion sensing device  428  using a local wireless network. In addition, the exemplary motion sensing device  428  is both lightweight (e.g., less than 30 grams) and compact in size (e.g., less than 40 mm by 70 mm by 20 mm) so that it is generally comfortable for the subject  418  to wear on his or her head  430 . 
     Next, referring to  FIG. 15 , an explanation of the three (3) directions of head rotation that the motion sensing device  428  is capable of detecting will be described. First, the motion sensing device  428  is configured to detect the rotation of the head  430  of the subject  418  about the yaw axis  466  of rotation as indicated by the curved arrow  472  in  FIG. 15 . The curved arrow  472  about the yaw axis  466  indicates the common side-to-side movement of the subject&#39;s head  430  during the vision testing. Secondly, the motion sensing device  428  is configured to detect the rotation of the head  430  of the subject  418  about the pitch axis  462  of rotation as indicated by the curved arrow  468  in  FIG. 15 . The curved arrow  468  about the pitch axis  462  indicates the up-and-down movement of the subject&#39;s head  430  during the vision testing. Thirdly, the motion sensing device  428  is configured to detect the rotation of the head  430  of the subject  418  about the roll axis  464  of rotation as indicated by the curved arrow  470  in  FIG. 15 . The curved arrow  470  about the roll axis  464  indicates the tilt-right and tilt-left movement of the subject&#39;s head  430  during the vision testing. In addition to the ability to determine head rotation about all three (3) axes of rotation, the use of a three-dimensional motion sensing device  428  advantageously permits the determination of whether the subject  418  is rotating his or her head  430  purely about the desired axis of rotation (e.g., the yaw axis) or whether there is also off-axis rotation of the subject&#39;s head  430  during the prescribed rotation (e.g., the subject is rotating his or head  430  about a combination of the yaw axis and the roll axis, or about a combination of the yaw axis and the pitch axis). Because off-axis rotation may adversely affect the accuracy of the vision testing results, it is important to determine if off-axis rotation is present during the vision testing of the subject  418 . The utilization of a three-dimensional motion sensing device  428  enables the determination of this off-axis rotation. 
     Now, an illustrative manner in which the computing device  402  of the vision testing system  400  performs the head rotation calculations from the output of the motion sensing device  428  will be explained in detail. In particular, this calculation procedure will describe the manner in which the angular position and the angular velocity of the head of the subject  418  may be determined using the signals from the motion sensing device  428  (i.e., the head-mounted IMU) of the vision testing system  400 . As explained above, in one or more embodiments, the motion sensing device  428  may be in the form of an IMU, which includes the following three triaxial sensor devices: (i) a three-axis accelerometer sensing linear acceleration {right arrow over (a)}′, (ii) a three-axis rate gyroscope sensing angular velocity {right arrow over (ω)}′, and (iii) a three-axis magnetometer sensing the magnetic north vector {right arrow over (n)}′. The motion sensing device  428  (i.e., the IMU) senses in the local (primed) frame of reference attached to the IMU itself. Because each of the sensor devices in the IMU is triaxial, the vectors {right arrow over (a)}′, {right arrow over (ω)}′, {right arrow over (w)}n′ are each 3-component vectors. A prime symbol is used in conjunction with each of these vectors to symbolize that the measurements are taken in accordance with the local reference frame. The unprimed vectors that will be described hereinafter are in the global reference frame. 
     The objective of these calculations is to find the angular position or orientation {right arrow over (θ)}(t) and the angular velocity {right arrow over (ω)}(t) of the head of the subject  418  in the global, unprimed, inertial frame of reference. The initial orientation in the global frame of reference may be either known in advance or derived from {circumflex over (Θ)} 0 , as will be explained below with regard to the rotation transformation matrix. 
     For the purposes of the calculation procedure, a right-handed coordinate system is assumed for both global and local frames of reference. The global frame of reference is attached to the Earth. The acceleration due to gravity is assumed to be a constant vector {right arrow over (g)}. Also, for the purposes of the calculations presented herein, it is presumed the sensor devices of the inertial measurement unit (IMU) provide calibrated data. In addition, all of the one or more output signals from the IMU are treated as continuous functions of time. Although, it is to be understood the general form of the equations described herein may be readily discretized to account for IMU sensor devices that take discrete time samples from a bandwidth-limited continuous signal. 
     The angular velocity {right arrow over (ω)}(t) of the subject&#39;s head rotation is obtained by coordinate transformation using the IMU output signal(s) as follows:
 
{right arrow over (ω)}( t )={right arrow over (Θ)}( t ){right arrow over (ω)}( t )  (2)
 
where {right arrow over (Θ)}(t) is the matrix of the rotation transformation that rotates the instantaneous local frame of reference into the global frame of reference.
 
     The orientation {circumflex over (θ)}(t) of the subject&#39;s head rotation is obtained by single integration of the angular velocity using the IMU output signal(s) as follows: 
     
       
         
           
             
               
                 
                   
                     
                       θ 
                       → 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         θ 
                         ¯ 
                       
                       0 
                     
                     + 
                     
                       
                         ∫ 
                         0 
                         t 
                       
                       ⁢ 
                       
                         
                           
                             ω 
                             → 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         dt 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       θ 
                       → 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         θ 
                         → 
                       
                       0 
                     
                     + 
                     
                       
                         ∫ 
                         0 
                         t 
                       
                       ⁢ 
                       
                         
                           
                             Θ 
                             → 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         
                           
                             
                               ω 
                               → 
                             
                             ′ 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         dt 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     There are two aspects to the coordinate transformation matrix {right arrow over (Θ)}(t) calculation: (i) the initial value {right arrow over (Θ)}d 0 ≡{right arrow over (Θ)}(0) at t=0 and (ii) subsequent updates to this value. The updates may be integrated from the angular velocity, i.e., the time derivative {dot over (Θ)} of the rotation transformation matrix may be set as a function of the angular velocity, and then the coordinate transformation matrix becomes: 
                       Θ   →     ⁡     (   t   )       =       ∫   0   t     ⁢           Θ   .               ⁡     (     τ   ,         ω   →     ′     ⁡     (   τ   )         )       ⁢   d   ⁢   τ               (   5   )               
The value at any point in time may be derived from known local and global values of both the magnetic north vector and the acceleration of gravity. Those two vectors are usually non-parallel. This is the requirement for the {right arrow over (Θ)}({right arrow over (g)}′,{right arrow over (n)}′,{right arrow over (g)},{right arrow over (n)}) to be unique. The knowledge of either of those vectors in isolation gives a family of non-unique solutions {right arrow over (Θ)}({right arrow over (g)}′,{right arrow over (g)}) or {circumflex over (Θ)}({circumflex over (n)}′,{right arrow over (n)}). Both are unconstrained in one component of rotation. The {right arrow over (Θ)}({right arrow over (g)}′,{right arrow over (n)}′,{right arrow over (g)},{right arrow over (n)}) has many known implementations, with the common one being the Kabsch algorithm.
 
     In one or more embodiments, if a known starting global orientation is assumed, and the time-derivative of the rotation transformation matrix as a function of the angular velocity in the local frame is used, it is possible to obtain the matrix without the need for the accelerometer and magnetometer in the IMU. 
     Advantageously, in one or more embodiments, the motion sensing device  428  described above requires no manual calibration step or setup time, other than putting the device  428  on the head  430  of the subject  418 . That is, there is no required manual calibration and/or tare step (e.g., to calibrate the accelerometer with respect to gravity and/or to zero the gyroscope) that must be performed prior to the execution of a particular test (i.e., the head impulse test). Obviating the need for manual calibration and/or a manual tare step advantageously saves valuable time during the execution of a particular test series. In order to avoid these laborious and tedious manual steps, at the beginning of every head impulse test trial, the motion sensing device  428  (i.e., inertial measurement unit  428 ) is automatically re-tared (or re-zeroed) to the subject&#39;s head position at that time by the computing device  402 . With each of these zeroing events, the computing device  402  also checks the orientation of the motion sensing device  428  in order to make sure that it is on the subject&#39;s head  430  correctly and/or to make sure the subject  418  is still and has his or her head held in the correct orientation. The computing device  402  checks the orientation of the motion sensing device  428  by using the data from the linear accelerometer, and by determining which axis the gravity vector lies in. If the computing device  402  detects no issues with the orientation of the motion sensing device  428 , nothing happens on the screen (i.e., no message is displayed in order to save time). Conversely, if the computing device  402  determines that there is a problem with the orientation of the motion sensing device  428 , a flag pops up on the screen and the test will not begin (i.e., the head impulse test will not begin). In order to begin the test, the clinician must adjust the motion sensing device  428  or the subject&#39;s head  430  to the correct position and press “okay” on the keyboard of the computing device  402  to dismiss the flag. Upon the “okay” input, the motion sensing device  428  will tare to its position at that time and the test will begin. In these one or more embodiments, the flag only pops up once per trial. 
     Also, advantageously, in these one or more embodiments, the motion sensing device  428  does not require a flat and/or still surface for using the motion sensing device  428  (i.e., the inertial measurement unit). In addition, in these one or more embodiments, the motion sensing device  428  does not comprise an electrical cord that operatively connects it to the computing device  402 . That is, the motion sensing device  428  is wirelessly coupled to the computing device  402  without the use of any electrical wires. Because the motion sensing device  428  is not tethered (i.e., it contains no electrical cord), it can advantageously accommodate various configurations and distances from the computing device  402 . In other words, the configuration of the vision testing system  400  in a room is not limited by the physical limitations imposed upon the system  400  by the fixed length of an electrical cord connecting the motion sensing device  428  to the computing device  402  (e.g., the electrical cord of a tethered motion sensing device  428  may be too short to accommodate a desired testing configuration). 
     Referring again to  FIG. 12 , it can be seen that a clinician  414  is standing behind a subject  418  seated on a chair  420 . During the head impulse test described herein, wherein the subject&#39;s head  430  is to be displaced back-and-forth, the clinician  414  may move the subject&#39;s head  430  using her arms  432  so that the velocity and range of motion of the subject&#39;s head  430  is capable of being more accurately controlled. 
     As shown in the illustrative embodiment of  FIG. 12 , it can also be seen that the subject  418  is holding a user input device  424  in his right hand. The user input device  424  is configured to output an input signal in response to a manipulation of the user input device by a system user (i.e., the subject  418 ). In the illustrative embodiment, the user input device  424  is operatively coupled to the laptop computing device  402  by wireless data transmission means. For example, the user input device  424  may comprise a wireless mouse that wirelessly communicates with the laptop computing device  402 . In other embodiments, the user input device may alternatively comprise (i) a keyboard (e.g., the keyboard of the laptop computing device  402  in  FIG. 12 ), (ii) a wireless clicking device, or (iii) a voice recognition device that allows the subject  418  to verbally input the subject&#39;s response (e.g., to record the subject&#39;s perceived orientation of the optotype). 
     During the vision testing of the subject  418 , after the subject  418  identifies the perceived configuration of the visual object  410  that is displayed on the visual display device  404 , the subject  418  may use the user input device  424  in order to enter and transmit his or her response (i.e., the perceived configuration of the visual object  410 ) to the laptop computing device  402 . In one or more embodiments, the user input device  424  is configured to accept at least four different responses of the subject  418 , wherein each of the at least four different responses of the subject  418  correspond to different configurations of the visual object  410 . For example, turning to  FIG. 14 , it can be seen that, if the visual object  410  is in the form of a Tumbling E optotype, a first configuration of the visual object  410   a  is one in which the Tumbling E is pointing up (see the top row of  FIG. 14 ), a second configuration of the visual object  410   b  is one in which the Tumbling E is pointing down, a third configuration of the visual object  410   c  is one in which the Tumbling E is pointing to the right, and a fourth configuration of the visual object  410   d  is one in which the Tumbling E is pointing to the left. Next, referring to the middle row of  FIG. 14 , it can be seen that these same four configurations of the Tumbling E can be displayed on the visual display device  404  in a smaller overall size (e.g., Tumbling E optotypes  410   e ,  410   f ,  410   g ,  410   h ). Finally, with reference to bottom row of  FIG. 14 , it can be seen that these same four configurations of the Tumbling E can be displayed on the visual display device  404  in an even smaller overall size (e.g., Tumbling E optotypes  410   i ,  410   j ,  410   k ,  410   l ). 
     Now, turning to  FIG. 13 , it can be seen that the data acquisition/data processing device (i.e., the computing device  402 ) of the illustrative vision testing system  400  comprises a microprocessor  434  for processing data, memory  436  (e.g., random access memory or RAM) for storing data during the processing thereof, and data storage device(s)  438 , such as one or more hard drives, compact disk drives, floppy disk drives, flash drives, or any combination thereof. As shown in  FIG. 13 , the motion sensing device  428  and the visual display device  404  are operatively coupled to the computing device  402  such that data is capable of being transferred between these devices. Also, as illustrated in  FIG. 13 , a plurality of data input devices  408 ,  424 , such as the wireless mouse  424 , keyboard, and touchpad  408 , are operatively coupled to the computing device  402  so that a user is able to enter data into the computing device  402 . In some embodiments, the computing device may be in the form of a laptop computer (as shown in  FIG. 12 ), while in other embodiments, the computing device may be embodied as a desktop computer. 
     In accordance with the illustrative embodiment of the invention, a flowchart illustrating a procedure for head impulse testing of a subject carried out by the system  400  is set forth in  FIGS. 16-18 . Referring initially to  FIG. 16 , the procedure commences at  500  (e.g., after the operator or clinician enters a start command to the computing device  402  by depressing a particular key on the keyboard), and in step  502 , the subject is positioned in front of the output screen of the visual display device  404  such that a visual object  410  (e.g., an optotype) is visible to the subject  418 . Then, while the subject  418  maintains a generally fixed position of his or her head  430  (i.e., keeping his or her head  430  as still as possible), one or more configurations of the visual object  410  having a first initial size are displayed on the output screen of the visual display device  404  in step  504 . For example, referring to  FIG. 14 , one or more configurations  410   e ,  410   f ,  410   g ,  410   h  of the Tumbling E optotype may be displayed one-at-time on the output screen of the visual display device  404 . In other exemplary embodiments, other suitable optotypes may be used in place of the Tumbling E, such as a Landolt C or letters of a recognized alphabet (e.g., the English alphabet). In one or more embodiments, the visual object (i.e., optotype) having the first initial size is displayed within approximately 500 milliseconds after the operator or clinician enters the start command to the computing device  402 . The visual object (i.e., optotype) having the first initial size may be sufficiently large so that nearly all of the subjects tested will correctly identify the configuration of the first optotype that is displayed on the screen, and may be displayed for a time duration of approximately 500 milliseconds on the screen. After the visual object (i.e., optotype) disappears from the output screen of the visual display device  404 , the computing device  402  may be specially programmed to implement a wait state, wherein the computing device  402  waits to record the subject&#39;s response. 
     With reference to  FIGS. 19 and 20 , exemplary subject screen images  600 ,  612 , which may be displayed on the visual display device  404  during the first set of successive trials when the subject  418  maintains the generally fixed position of his or her head  430 , will now be explained. Initially, referring to  FIG. 19 , it can be seen that a set of instructions  602  for the subject or patient may be displayed at the top of the screen image  600 . The subject or patient instructions may specify that the manner in which the subject or patient is supposed to carry out the test (e.g., by instructing the patient to sit still and focus on the circle  604 , and by instructing the patient to indicate the orientation of the optotype after it disappears from the screen). The set of instructions  602  may be initially displayed on the output screen by the computing device  402 , and then subsequently hid from view (i.e., suppressed thereafter) so as not to unnecessarily distract the subject  418  during the testing. As additionally shown in  FIG. 19 , a solid black circle  604  is displayed in the approximate center of the screen. The solid black circle  604  comprises a plurality of crosshairs  606  disposed therein (e.g., spaced 90 degrees apart from one another, and disposed at 0 degrees, 90 degrees, 180 degrees, and 360 degrees), and extending inwardly towards the center of the circle  604 . Referring again to  FIG. 19 , it can be seen that, in the bottom right-hand corner of the screen, the number of trials  608  completed thus far, together with the maximum number of trials  610  that may be completed during the first set of successive trials is displayed. Because  FIG. 19  depicts the starting screen image  600  for the first set of successive trials (i.e., when the subject&#39;s head is stationary), the number  608  is zero (0) in  FIG. 19 . In this illustrative embodiment, the maximum number of trials  610  performed in conjunction with the first set of successive trials is fourteen (14). However, it is to be understood that, in other embodiments of the invention, the maximum number of trials  610  performed in conjunction with the first set of successive trials may be higher or lower than fourteen (14). Also, in an alternative embodiment, a small box with the optotype size and a small box with the optotype display time may be placed in the bottom right-hand corner of the screen  600 , rather than the number of trials  608  completed thus far, and the maximum number of trials  610 . In this alternative embodiment, the number of trials  608  completed thus far, and the maximum number of trials  610  may be placed in test summary data on the operator or clinician screen. Turning to  FIG. 20 , it can be seen that the screen image  612  is generally the same as the screen image  600  of  FIG. 19 , except that an optotype  410 ′ is displayed in the center of the circle  604 . While a Tumbling E optotype is utilized in the illustrative embodiment of  FIG. 20 , it is to be understood that, in an alternative embodiment, a Landolt C optotype or another type of optotype may be used in place of the Tumbling E optotype. In the exemplary screen image  612  of  FIG. 20 , it can be seen that the optotype is pointing to the left, so a correct identification of the optotype by the subject or patient would require that he or she indicates that the optotype is pointing to the left. In addition, as depicted in  FIG. 20 , the number of trials  608  is equal to one (1) in order to indicate that the subject or patient has completed one trial of the first set of successive trials (i.e., when the subject&#39;s head is stationary). 
     The circle  604  depicted in  FIGS. 19 and 20 , which is generated by the computing device  402 , is large enough to fit the largest size optotype that will be displayed to the subject or patient during the static testing. The crosshairs  606  extend inward in order to prevent the circle  604  from needing to change size. Advantageously, in order to avoid becoming a distraction to the subject during the testing, the size of the circle  604  remains constant (i.e., it is static). Because the size of the circle  604  is constant during the testing, it must be large enough to accommodate the largest optotype that will be displayed to the subject during the testing. Consequently, this results in a circle  604  that is significantly larger than the smallest sized optotypes displayed during the testing, which in turn, makes it difficult for the subject to tell where the small sized optotypes will appear on the screen. The subject&#39;s ability to find the optotype on the screen is not something that it is desired to be factored into the testing. Thus, the crosshairs  606  are provided within the circle  604  in order to help direct the gaze of the subject or patient during the display of the optotype, especially that of a small optotype. Like the circle  604 , the crosshairs  606  are static during the testing (i.e., they do not undergo changes in size or shape) so as not to distract the subject during the testing. Also, as best shown in  FIG. 19 , the crosshairs  606  do not extend all the way to the center of the circle  604  so as to prevent them from interfering with the display of the small optotypes. In addition, both the circle  604  and the crosshairs  606  remain on the output screen of the visual display device throughout the duration of the testing. 
     Now, turning again to  FIG. 16 , in step  506 , the computing device or data processing device  402  determines whether or not the subject  418  correctly identifies one or more configurations (e.g.,  410   e ,  410   f ,  410   g ,  410   h ) of the visual object  410  having the first initial size. For example, the subject  418  may be presented with one or more optotypes having the first initial size (e.g. a single optotype  410   g  pointing down). This optotype  410   g  is of the first initial size, which may be a size scale factor of 0.75. After the optotype is displayed to the subject  418 , the subject  418  may utilize a user input device  424  (e.g., a wireless mouse) in order to transmit the response of the subject  418  to the computing device  402  (e.g., the wireless mouse may have four buttons, each of which corresponds to one of the four configurations of the optotype). Alternatively, the response of the subject  418  may be entered using the keyboard of the computing device  402  by using the directional arrows and space bar on the keyboard. For example, the “up” arrow key on the keyboard is used to record an optotype pointing up, the “down” arrow key on the keyboard is used to record an optotype pointing down, the “left” arrow key on the keyboard is used to record an optotype pointing to the left, the “right” arrow key on the keyboard is used to record an optotype pointing to the right, and the spacebar is used to record an unknown orientation of the optotype. If the spacebar is depressed on the keyboard, the computing device  402  randomly chooses the response of the subject  418 . After receiving the subject&#39;s response, the computing device  402  is specially programmed to determine if the subject correctly determined the configuration of the optotype that was displayed on the output screen of the visual display device  404 . For example, if the optotype  410   g  of  FIG. 14  was displayed on the screen, the subject  418  must indicate that optotype is pointing down to get the identification correct. Any other answer would be incorrect for this configuration of the optotype. Once the subject&#39;s answer or response is evaluated as to its correctness, the computing device  402  is specially programmed to record whether or not the subject  418  identified the optotype correctly. In one exemplary embodiment, if the subject  418  identifies the configuration of the optotype  410   g  correctly, the computing device  402  is specially programmed to display one or more optotypes having a physical size that is smaller than the first optotype (e.g., the second trial may comprise one or more optotypes that have a size scale factor of 0.6875). For example, the second optotype may comprise the optotype  410   j  in  FIG. 14 , which has a smaller size than the optotype  410   g  in the middle row of  FIG. 14 . Conversely, if the subject  418  identifies the configuration of the first optotype  410   g  incorrectly, the computing device  402  is specially programmed to display one or more optotypes having a physical size that is larger than the first optotype (e.g., the second optotype may have a size scale factor of 1.0). For example, in this case, the second optotype may comprise the optotype  410   c  in the top row of  FIG. 14 , which has a larger size than the optotype  410   g  in the middle row of  FIG. 14 . In one or more embodiments, as described hereinafter, the computing device  402  is specially programed to execute a parameter estimation algorithm in order to determine the physical size of the successive optotype that is displayed on the output screen of the visual display device  404 . Also, in one or more embodiments, the computing device  402  is specially programmed to display the successive optotype on the screen approximately five-hundred (500) seconds after it receives the subject&#39;s response regarding the preceding optotype. 
     In this manner, as specified in step  508  of  FIG. 16 , the computing device  402  is specially programmed to incrementally decrease or increase the size of the visual object on the output screen of the visual display device  404  during a first set of successive trials (e.g., during a maximum number  610  of fourteen (14) trials as indicated in  FIGS. 19 and 20 ). For example, if the subject identifies the configuration of a particular-sized optotype correctly, the optotype size of the next optotype displayed on the screen is generally decreased, while the optotype size of the next optotype displayed on the screen is generally increased if the subject fails to identify the configuration of the optotype correctly. In the exemplary embodiment, after each optotype is displayed on the output screen of the visual display device  404 , the computing device  402  is specifically programmed to determine whether or not the subject  418  correctly identified the configuration of the optotype. 
     In one or more embodiments, the size of the visual object presented to the subject  418  during the first set of successive trials generally changes with each successive trial (e.g., the subject  418  does not receive five (5) optotypes of the same size in consecutive order during the testing). In contrast to the static visual acuity testing, the size of the visual object presented to the subject  418  during the head impulse test trials, as described hereinafter, remains constant during each successive trial (e.g., the subject  418  receives optotypes of the same size during the head impulse testing). 
     In step  510  of  FIG. 16 , after the subject  418  has completed a predetermined number of trials of optotype identification, the computing device  402  is specifically programmed to determine a size parameter (e.g., a size scale factor) of a first threshold version of the optotype based upon the performance of the subject  418  during the first set of successive trials during which the subject  418  maintained the generally fixed position of his or her head  430 . For example, after a particular end condition is reached, the computing device  402  determines that the first threshold version of the optotype has a size scale factor of 0.40. In one embodiment, the size scale factor of the first threshold version of the optotype is a calculated value that is determined using one or more statistical equations or formulas (e.g., using a parameter estimation algorithm as described hereinafter). 
     The computing device  402  may be specifically programmed with multiple end conditions for determining when to end the first set of successive trials. For example, the computing device  402  may automatically stop the first set of successive trials as soon as any one or combination of the following conditions occur: (i) the subject has completed a total of fourteen (14) trials of optotype identification (i.e., a maximum allowable number of trials is reached), (ii) a predetermined number of reversals has occurred, or (iii) the user has stopped the first set of successive trials (e.g., the operator or clinician initiates the cessation of the successive trials by depressing a particular key on the keyboard). 
     In one or more embodiments, the computing device  402  may be specially programmed to automatically stop the first set of successive trials when both the computed number of reversals for the test series has exceeded the predetermined number of reversals and the computed number of trials for the test series has exceeded the predetermined number of trials. Initially, the manner in which the total number of reversals is calculated by the computing device  402  will be explained. During the vision testing, a reversal is determined in accordance with the following equation:
 
diff=[ y ( i− 1)− y ( i )]*[ y ( i )− y ( i+ 1)]  (6)
 
     where: 
     diff: product of the slope before and after a stimulus point; 
     y(i): stimulus value at trial number i; 
     y(i−1): stimulus value for the trial preceding trial number i; 
     y(i+1): stimulus value for the trial following trial number i; 
     i: trial number for which the reversal is being determined. 
     After computing the difference value determined using equation (6) for a given trial number, if it is determined by the computing device  402  that the difference value is less than or equal to zero, then the reversal counter is incremented using the following equation:
 
rev=rev+1  (7)
 
That is, the reversal number counter is incremented by one. Conversely, if it is determined by the computing device  402  that the difference value is greater than zero, then the reversal counter is not incremented and the number of reversals stays the same. Also, if the computed difference value is less than or equal to zero, and thus the reversal counter is incremented, the computing device  402  may be specially programmed to next determine if the total computed number of reversals is greater than or equal to the predetermined number of reversals that has been set (e.g., a total number of eight (8) reversals). After which, the computing device  402  may be specially programmed to further determine if the total number of trials that have been completed (value of i) is greater than or equal to a predetermined number of trials (e.g., fourteen (14) trials of optotype identification). When both the total computed number of reversals is greater than or equal to the predetermined number of reversals (e.g., eight (8) reversals), and the total number of trials that have been completed is greater than or equal to the predetermined number of trials (e.g., fourteen (14) trials), then the computing device  402  may be specially programmed to automatically stop the first set of successive trials of the vision testing. It is to be understood that the abovedescribed procedure may be used to determine the stopping point of the visual processing time, and head impulse test series of successive trials as well.
 
     Then, in step  512  of  FIG. 16 , the computing device  402  is specifically programmed to generate a quantitative assessment of the subject&#39;s performance during the first set of successive trials, and finally, in step  514  of  FIG. 16 , to output the quantitative assessment of the subject&#39;s performance during the first set of successive trials on the output screen of the visual display device  404 . For example, in an exemplary embodiment, the computing device  402  may generate a report in order to display the test results from the first set of successive trials. 
     After determining the size parameter of the first threshold version of the optotype in step  510  above, and generating and displaying the quantitative results in steps  512  and  514 , the visual processing time of the subject  418  may be determined by the computing device  402  in steps  516 - 522  of  FIGS. 16 and 17 . The purpose of this test is to determine how long the optotype needs to be displayed on the screen of the visual display device  404  in order for the subject  418  to comprehend the displayed orientation of the optotype. Like the first set of successive trials described above, the visual processing time test is a static-type test, so there are no head movements made by the subject or patient during the performance of the visual processing time test. The size scale factor of the first threshold version of the optotype (e.g., in units of log MAR or relative log MAR) that was determined in conjunction with the first set of successive trials described above is used by the visual processing time test. In particular, the optotype size during the performance of the visual processing time test is determined by increasing the optotype size determined in conjunction with the first set of successive trials by two sizes (e.g., by 0.2 log MAR). This optotype size remains constant for all trials of the visual processing time test. The variable in the visual processing time test is the time duration during which the optotype is displayed on the screen. The stimulus level (i.e., selected display time of the optotype on the screen) may be determined by the computing device  402  using the same parameter estimation algorithm that selects the optotype size in the first set of successive trials described above. 
     In one or more exemplary embodiments, during the visual processing time test, the shortest amount of time that the optotype is displayed on the screen of the visual display device  404  is twenty (20) milliseconds (i.e., because of the monitor refresh rate), while the maximum amount of time that the optotype is displayed on the screen of the visual display device  404  is seventy (70) milliseconds. The upper limit of seventy (70) milliseconds was selected because, if the subject or patient has a visual processing time greater than 70 milliseconds he or she will be unable to complete the dynamic trials of the head impulse test. An individual with a visual processing time greater than 70 milliseconds would be unable to complete these dynamic trials because the optotype must be displayed on the visual display device  404  while the head of the subject or patient is being displaced. If the display time of the optotype needed to be much longer than 70 milliseconds, it would not be possible to display the optotype only during the subject&#39;s head movement, and conversely, to not display it when the subject&#39;s head is at maximum amplitude and head velocity is zero (0). 
     In one or more embodiments, the computing device  402  may be specially programmed to prompt the operator or clinician to input the step change (i.e., resolution) in the visual processing time between trials. The default test setting for the resolution may be ten (10) milliseconds, while the computing device  402  may be specially programmed to accept a resolution range between ten (10) milliseconds and fifty (50) milliseconds, inclusive. 
     After the operator or clinician enters a start command to the computing device  402  by depressing a particular key on the keyboard, the visual processing time test begins. Turning again to the flowchart of  FIG. 16 , it can be seen in step  516  that, while the subject  418  maintains a generally fixed position of his or her head  430  (i.e., keeping his or her head  430  as still as possible), one or more configurations of the visual object  410  are displayed on the output screen of the visual display device  404  for an initial display time (e.g., the optotype is displayed for ninety (90) milliseconds on the screen). For example, referring to  FIG. 14 , one or more configurations  410   a ,  410   b ,  410   c ,  410   d  of the Tumbling E optotype may be displayed one-at-time on the output screen of the visual display device  404  for an initial display time. In an alternative exemplary embodiment, one or more configurations of a Landolt C optotype may be displayed one-at-time on the output screen of the visual display device  404  for an initial display time. In other exemplary embodiments, other suitable optotypes may be used in place of the Tumbling E or Landolt C, such as letters of a recognized alphabet (e.g., the English alphabet). In one or more embodiments, the visual object  410  (i.e., optotype) is displayed in approximately 500 milliseconds after the operator or clinician enters the start command to the computing device  402 . After the visual object (i.e., optotype) disappears from the output screen of the visual display device  404 , the computing device  402  may be specially programmed to implement a wait state, wherein the computing device  402  waits for the subject or clinician to record the subject&#39;s response. 
     During the visual processing time test, the same solid black circle  604  and crosshairs  606 , which was described above in conjunction with  FIGS. 19 and 20 , is used to present the optotype to the subject  418 . That is, the patient screen during the visual processing time test is generally the same as the screen images  600 ,  612  in  FIGS. 19 and 20 . 
     Now, with reference to  FIG. 17 , in step  518 , the computing device or data processing device  402  determines whether or not the subject  418  correctly identifies the one or more configurations (e.g.,  410   a ,  410   b ,  410   c ,  410   d ) of the visual object  410  that is displayed on the visual display device  404  for the initial display time. For example, the subject  418  may be presented with a single optotype  410   b  pointing down (see  FIG. 14 ). After the optotype is displayed to the subject  418 , the subject  418  or the clinician  414  may utilize a user input device  424  (e.g., a wireless mouse) in order to transmit the response of the subject  418  to the computing device  402  (e.g., the wireless mouse may have four buttons, each of which corresponds to one of the four configurations of the optotype). Alternatively, the response of the subject  418  may be entered using the keyboard of the computing device  402  by using the directional arrows and space bar on the keyboard. For example, the “up” arrow key on the keyboard is used to record an optotype pointing up, the “down” arrow key on the keyboard is used to record an optotype pointing down, the “left” arrow key on the keyboard is used to record an optotype pointing to the left, the “right” arrow key on the keyboard is used to record an optotype pointing to the right, and the spacebar is used to record an unknown orientation of the optotype. If the space bar is depressed on the keyboard, the computing device  402  randomly chooses the response of the subject  418 . After receiving the subject&#39;s response, the computing device  402  is specially programmed to determine if the subject correctly determined the configuration of the optotype that was displayed on the output screen of the visual display device  404 . For example, if the optotype  410   b  of  FIG. 14  was displayed on the screen, the subject  418  must indicate that optotype is pointing down to get the identification correct. Any other answer would be incorrect for this configuration of the optotype. Once the subject&#39;s answer or response is evaluated as to its correctness, the computing device  402  is specially programmed to record whether or not the subject  418  identified the optotype correctly. In one exemplary embodiment, if the subject  418  identifies the configuration of the optotype  410   b  correctly, the computing device  402  is specially programmed to display one or more subsequent optotypes for a display time that is shorter than the initial display time (e.g., the second trial may display the subsequent optotype on the screen for a display time of only 50 milliseconds, rather than the 60 milliseconds that was used during the initial trial). Conversely, if the subject  418  identifies the configuration of the first optotype  410   b  incorrectly, the computing device  402  is specially programmed to display one or more subsequent optotypes for a display time that is longer than the initial display time (e.g., the second trial may display the subsequent optotype on the screen for a display time of 70 milliseconds, rather than the 60 milliseconds that was used during the initial trial). Also, in one or more embodiments, the computing device  402  is specially programmed to display the successive optotype on the screen approximately five-hundred (500) milliseconds after it receives the subject&#39;s response regarding the preceding optotype. 
     In this manner, as specified in step  520  of  FIG. 17 , the computing device  402  is specially programmed to incrementally decrease or increase the display time for the visual object on the output screen of the visual display device  404  during a set of successive trials (e.g., during a maximum number  610  of fourteen (14) trials as indicated in  FIGS. 19 and 20 ). During the set of successive trials, various configurations of the same-sized visual object may be displayed on the output screen of the visual display device  404  for identification by the subject. For example, if the subject identifies a configuration of the same-sized optotype correctly, the screen display time for next-displayed optotype is generally decreased, while the screen display time for next-displayed optotype is generally increased if the subject fails to correctly identify the configuration of the optotype correctly. In the exemplary embodiment, after each optotype is displayed on the output screen of the visual display device  404  for a particular display time, the computing device  402  is specifically programmed to determine whether or not the subject  418  correctly identified the configuration of the optotype. In step  522  of  FIG. 17 , after the subject  418  has completed a predetermined number of trials of optotype identification, the computing device  402  is specifically programmed to determine a threshold visual processing time for the subject  418  based upon the performance of the subject  418  during the set of successive trials during which the subject  418  maintained the generally fixed position of his or her head  430 . For example, after a particular end condition is reached, the computing device  402  determines that the visual processing time for the subject  418  is fifty-five (55) milliseconds. In one embodiment, the visual processing time for the subject  418  is a calculated value that is determined using one or more statistical equations or formulas (e.g., using a parameter estimation algorithm as described hereinafter). 
     As described above for the first set of successive trials, the computing device  402  may be specifically programmed with multiple end conditions for determining when to end the second set of successive trials. For example, the computing device  402  may automatically stop the second set of successive trials as soon as any one or combination of the following conditions occur: (i) the subject has completed a total of fourteen (14) trials of optotype identification (i.e., a maximum allowable number of trials is reached), (ii) a predetermined number of reversals has occurred (refer to explanation above), or (iii) the user has stopped the first set of successive trials (e.g., the operator or clinician initiates the cessation of the successive trials by depressing a particular key on the keyboard). 
     Then, in step  524  of  FIG. 17 , the computing device  402  is specifically programmed to generate a quantitative assessment of the subject&#39;s performance during the visual processing time test, and finally, in step  526  of  FIG. 17 , to output the quantitative assessment of the subject&#39;s performance during the visual processing time test on the output screen of the visual display device  404 . For example, in an exemplary embodiment, the computing device  402  may generate a report in order to display the test results from the visual processing time test. 
     Referring again to  FIG. 17 , the dynamic portion of the head impulse test will be explained in detail. Initially, in step  528  of  FIG. 17 , when the subject&#39;s head  430  is displaced at or above a predetermined minimum velocity or speed (e.g., at or above 100 degrees per second) as measured by the motion sensing device  428 , one or more configurations of the optotype  410  having a third size are displayed in succession on the output screen of the visual display device  404 . In one or more embodiments, the optotype of step  528  has a third size that is greater than the size of the threshold version of the optotype determined in step  510 . In these embodiments, the size of the threshold version of the optotype determined in step  510  may be multiplied by a scale factor (e.g., 1.5 or 1.75) to arrive at the third size (i.e., the increased size) of the optotype to be used in step  528 . For example, in one or more embodiments, an optotype size that is two lines above the optotype size determined in step  510  on a Tumbling E eye chart may be used for the head impulse test. In other words, in one or more embodiments, the optotype size during the performance of the dynamic portion of the head impulse test is determined by increasing the optotype size determined in conjunction with the first set of successive trials by two sizes (e.g., by adding 0.2 log MAR to the static visual acuity determined during the first set of successive trials). During step  528 , different configurations of the tumbling E optotype having the third size may be displayed one-at-time on the output screen of the visual display device  404 . In the illustrative embodiment, the subject&#39;s head  430  may be displaced by the clinician  414  abruptly rotating the subject&#39;s to the right, or to the left, about the yaw axis  466  of  FIG. 15  (i.e., approximately in a horizontal plane). For example, during one trial, the clinician  414  may rotate the subject&#39;s head  430  to the right about the yaw axis  466  between −20 degrees and 20 degrees (i.e., between 20 degrees to the left and 20 degrees to the right). Then, in the next trial, the clinician  414  may rotate the subject&#39;s head  430  to the left about the yaw axis  466  between 20 degrees and −20 degrees (i.e., between 20 degrees to the right and 20 degrees to the left). However, during the trials of the head impulse test, it is to be understood that the clinician  414  may randomly vary the rotation of the subject&#39;s head  430  between right and left rotations (i.e., the clinician  414  does not always alternate between right and left rotation, but rather may successively rotate in the same direction during consecutive trials). In the illustrative embodiment, the optotype  410  is displayed on the output screen of the visual display device  404  when the angular velocity of the head  430  of the subject  418  meets or exceeds a predetermined minimum angular velocity. For example, an exemplary head angular velocity curve  620 ,  626  generated during the performance of a head impulse test is illustrated in the graph  614  of  FIG. 21 . As shown in this figure, the y-axis  616  of the graph  614  is the head angular velocity in degrees per second, while the x-axis  618  of the graph  614  is the time in milliseconds (msec). In the graph  614  of  FIG. 21 , it can be seen that the head angular velocity curve  620 ,  626  has a first portion  620  where the head of the subject  418  is being rotated in a first direction (e.g., to the right) and a second portion  626  where the head of the subject  418  is being rotated in a second direction (e.g., to the left). In the first portion  620  of the head angular velocity curve  620 ,  626  for head rotation to the right, the computing device  402  initially determines when the head angular velocity reaches a first value  622 . Once the computing device  402  has determined that the head angular velocity has reached the first value  622 , the computing device  402  concludes that head  430  of the subject  418  is undergoing a head impulse displacement. The initial determination that the first threshold value  622  has been reached enables the computing device  402  to differentiate between mere noise in the head angular velocity signal measured the inertial measurement unit  428 , and the actual occurrence of a head impulse displacement. Upon determining that the first threshold value  622  has been reached, the computing device  402  next determines when the head angular velocity reaches a second value  624  in the first portion  620  of the head angular velocity curve  620 ,  626  (i.e., the second value  624  is the minimum angular velocity for optotype display). Once the computing device  402  has determined that the head angular velocity has reached the second value  624 , the computing device  402  displays the optotype  410  on the output screen of the visual display device  404 . Referring again to  FIG. 21 , it can be seen that these same two threshold values  628 ,  630  are determined by the computing device  402  for the left head rotation in the second portion  626  of the head angular velocity curve  620 ,  626 . 
     Also, during the dynamic portion of the head impulse test, the computing device  402  may be specially programmed to randomly choose the orientation of the optotype that is displayed on the screen. After the optotype disappears during the dynamic portion of the head impulse test, the subject  418  enters an optotype orientation response (i.e., the perceived configuration of the visual object  410 ) into the user input device  424 , which transmits the response to the laptop computing device  402 . 
     In alternative exemplary embodiments, rather than rotating the head  430  of the subject  418  about the yaw axis  466  in  FIG. 15 , the clinician  414  may alternatively rotate the head  430  of the subject  418  about the pitch axis  462  (i.e., approximately in a vertical plane) or about the roll axis  464  (i.e., in a roll plane). The computing device  402  is specially programmed to allow the clinician  414  to selectively choose any one of these rotational directions when performing the dynamic portion of the head impulse test. 
     Referring again to the flowchart of  FIG. 17 , the computing device or data processing device  402  determines whether or not the subject  418  correctly identifies one or more configurations (e.g.,  410   a ,  410   b ,  410   c ,  410   d ) of the visual object  410  having the third size in step  530 . For example, the subject  418  may be presented with one or more optotypes having the third size (e.g., a single optotype  410   a  pointing up). In the illustrative embodiment, during the dynamic set of trials performed during the head impulse test, each of the optotypes has the same physical size (i.e., each optotype has the same increased third size that is determined based upon the threshold size computed for the first series of trials carried out in steps  500 - 510  above). After each optotype in the trial is displayed to the subject  418 , the subject  418  may utilize the user input device  424  (e.g., a wireless mouse) in order to transmit his or her response to the computing device  402  (e.g., the wireless mouse may have four buttons, each of which corresponds to one of the four configurations of the optotype). After receiving each of the subject&#39;s responses, the computing device  402  is specially programmed to determine if the subject  418  correctly determined the configuration of the optotype that was displayed on the output screen of the visual display device  404 . For example, if the optotype  410   a  of  FIG. 14  was displayed on the screen, the subject  418  must indicate that the optotype is pointing up to get the identification correct. Any other answer would be incorrect for this configuration of the optotype. Once the subject&#39;s answer or response is evaluated as to its correctness, the computing device  402  is specially programmed to record whether or not the subject  418  identified the optotype correctly. 
     In this manner, as set forth in step  532  of  FIG. 17 , the computing device  402  is specially programmed to determine whether or not the subject  418  correctly identified the configuration of the optotype  410  during a third set of successive trials (i.e., during the dynamic set of trials of the head impulse test). During the third set of successive trials, the angular velocity or speed of the head  430  of the subject  418  is measured by the motion sensing device  428  as the clinician  414  abruptly rotates the subject&#39;s to the right, or to the left, about the yaw axis  466  of  FIG. 15  during a predetermined maximum number of trials (e.g., during fourteen (14) trials). 
     In step  534  of  FIG. 18 , after the subject  418  has completed a predetermined number of trials of optotype identification (e.g., fourteen (14) total trials), the computing device  402  is specifically programmed to determine the total number of visual object configurations (e.g., optotype configurations) that the subject  418  has identified correctly during the dynamic portion of the head impulse test (i.e., during the third set of successive trials). As an example, after the subject  418  completes one such series of trials of optotype identification, the computing device  402  may determine that the subject correctly identified nine (9) out of fourteen (14) optotype configurations correctly. 
     Next, in step  536  of  FIG. 18 , the computing device  402  is specifically programmed to generate a quantitative assessment of the head impulse test based upon the total number of visual object configurations (e.g., optotype configurations) that the subject  418  identified correctly during the dynamic portion of the head impulse test (i.e., during the third set of successive trials). Then, in step  538  of  FIG. 18 , the computing device  402  is specifically programmed to output the quantitative assessment of the head impulse test for the subject  418  on the output screen of the visual display device  404 . The head impulse testing procedure concludes at step  540  in  FIG. 18 . 
     In one or more embodiments, the head impulse test may comprise a plurality of different sets of successive trials performed at different head velocities. For example, the first set of successive trials may be performed using a predetermined minimum velocity or speed of approximately 100 degrees per second (or 100 degrees per second), the second set of successive trials may be performed using a predetermined minimum velocity or speed of approximately 150 degrees per second (or 150 degrees per second), and the third set of successive trials may be performed using a predetermined minimum velocity or speed of approximately 200 degrees per second (or 200 degrees per second). In these one or more embodiments, different sets of successive trials may be performed at incrementally higher velocities or speeds over the course of the head impulse test. 
     In one or more embodiments, a baseline visual processing time and static visual acuity value must have been previously stored in the computing device  402  in order to permit the subject  418  to perform the dynamic portion of the head impulse test. For example, if no baseline visual processing time and static visual acuity was previously determined on the day of the head impulse test, the subject  418  must first complete the static visual acuity testing and the visual processing time testing prior to performing the dynamic portion of the head impulse test. 
     As briefly mentioned above, the stimulus levels and threshold determinations during the first and second sets of successive trials of the vision testing may be determined by the computing device  402  by means of executing a parameter estimation algorithm. In particular, during the first static set of successive trials during the head impulse test, the optotype size selected for each trial, and the threshold optotype size computed in step  510 , may be computed using the parameter estimation algorithm. Similarly, during the visual processing time test performed in conjunction with the head impulse test, the optotype display time selected for each trial, and the threshold visual processing time computed in step  522 , may be computed using the parameter estimation algorithm. As such, the parameter estimation computations performed by the computing device  402  will be explained in detail hereinafter. 
     Initially, the parameter estimation computational procedure during the first static set of successive trials during the head impulse test will be explained. In an exemplary embodiment, the optotype range for the set of static successive trials (i.e., static visual acuity (SVA) range) may be −0.3 log MAR to 1.0 log MAR, and it may be divided into steps of 0.05 log MAR, thereby resulting in a total of twenty-seven (27) steps overall. The initial optotype size that is presented to the subject by the computing device  402  is 0.4 log MAR. The initial optotype size of 0.4 log MAR is presumed to be easy for most subjects to recognize so that the configuration thereof will be properly identified. If the subject incorrectly identifies the configuration of the initial optotype, the computing device  402  increases the optotype size by four (4) steps so that the next optotype size presented to the subject is 0.6 log MAR. After every incorrect answer, the computing device  402  increases the optotype size by four (4) steps until the upper range limit of 1.0 log MAR is reached. The subject may be given one or more chances to answer correctly at the upper range limit of 1.0 log MAR. If the subject fails to correctly identify the configuration of the optotype with a size of 1.0 log MAR after having been given one or more chances, the computing device  402  will end the static portion of the test. 
     Conversely, if the subject correctly identifies the configuration of the initial optotype, the computing device  402  decreases the optotype size by three (3) steps so that the next optotype size presented to the subject is 0.25 log MAR. After every correct answer, the computing device  402  decreases the optotype size by three (3) steps until the subject identifies the optotype configuration incorrectly. At this point, the computing device  402  presumes that the subject&#39;s threshold must lie somewhere between the optotype sizes identified correctly and incorrectly. The process is continued, and the computing device  402  increases the optotype size by two (2) steps (0.1 log MAR) for every incorrect answer, and decreases the optotype size by one (1) step (0.05 log MAR) for every correct answer. Each time the subject correctly identifies the optotype configuration, the computing device  402  checks the number of correct answers and the number of incorrect answers for that particular threshold. When the computing device  402  determines that the percent ratio of correct answers to the total number of answers (correct/(correct+incorrect)*100) is between 50% and 75%, then the computing device  402  determines that the present optotype size is the subject&#39;s threshold. The computing device  402  further compares the number of correct and incorrect answers between the current optotype size and the next smaller optotype size. If the computing device  402  determines that the ratio of correct answers to the total number of answers is between 50% and 66%, then the static series of trials is ended, and the mean of the two (2) adjacent points (i.e., optotype sizes) is taken as the threshold. The static series of trials is continued by the computing device  402  until one of the two (2) conditions described above is fulfilled. For example, the total number of static trials conducted by the computing device  402  may be thirty (30) overall trials. 
     Now, the manner in which the computing device  402  carries out the parameter estimation computational procedure for the threshold visual processing time determination will be explained. The computational procedure for threshold visual processing time of the subject uses a visual processing time range of 35 milliseconds to 70 milliseconds, and it is divided into steps of 5.0 milliseconds, thereby resulting in a total of eight (8) steps overall. For the threshold visual processing time of the subject, the computing device  402  does not begin with a large initial step size. Rather, if the subject identifies the configuration of the optotype incorrectly, the computing device  402  increases the visual processing time by two (2) steps (10.0 milliseconds), while if the subject identifies the configuration of the optotype correctly, the computing device  402  decreases the visual processing time by one (1) step (5.0 milliseconds). During the visual processing time computational procedure, the computing device  402  performs the same two conditional comparisons described above for the static trials (i.e., determining whether between 50% and 75%, and whether between 50% and 66%). 
     In accordance with a further illustrative embodiment of the invention, the subject may be outfitted with an eye movement tracking device  474  during the head impulse testing procedure so that the eye movement of the subject  418  may be tracked during the head impulse test (e.g., refer to  FIG. 22 ). In particular, while the steps  528 - 532  in  FIG. 17  are performed, the eye movement of the subject  418  may be continually tracked, and the eye position of the subject  418  may be correlated with subject&#39;s identification of the optotype to determine if there is a specific relationship between the position of the subject&#39;s eyes and whether or not the subject  418  correctly identifies the optotype. 
     Referring to  FIG. 22 , an exemplary embodiment of a system for testing the vision of a subject with eye movement tracking is seen generally at  401  in  FIG. 22 . The system  401  illustrated in  FIG. 22  may be used for the head impulse testing procedure described above to determine if there a specific relationship between the position of the subject&#39;s eyes and whether or not the subject  418  correctly identifies the optotype. As shown in  FIG. 22 , the system  401  generally comprises a laptop computing device  402  (i.e., a computing device that is capable of collecting, storing, and processing data), which in turn, is operatively coupled to a head motion sensing device  428  and an eye movement tracking device  474 . In  FIG. 22 , it can be seen that the eye movement tracking device  474  may be in the form of goggles or glasses worn on the head of the subject  418 . The goggles or glasses worn on the head of the subject  418  may be in the form of one of the eye movement measurement devices  100 ,  200 , which incorporate both video-based and infrared sensing means. In the illustrative embodiment, the eye movement measurement devices  100 ,  200  may be operatively connected to the laptop computing device  402  by means of either a wired connection or a wireless connection. In addition, the head motion sensing device  428  is removably coupled to the head  430  of the subject  418  using a stretchable, elastic headband  426  (i.e., a resilient band  426 ). In one or more alternative embodiments, the head motion sensing device may integrated into the goggles containing the eye movement tracking device  474  (e.g., the head motion sensing device  474  may be in the form of an inertial measurement unit (IMU) integrated in the goggles worn by the subject  418 ). 
     In one or more embodiments, the stretchable, elastic headband  426  that attaches the head motion sensing device  428  to the head  430  of the subject  418  may include a strap and a compressible piece of material disposed on an interior surface of the strap. When the head motion sensing device  428  is attached to the head  430  of the subject  418  by the headband  426 , the compressible piece of material is configured to be disposed between a portion of the strap and the head  430  of the subject  418  so as to provide a cushion effect for the subject  418  (i.e., to improve the comfort of the subject  418  while wearing the device  428 ). In an exemplary embodiment, the compressible piece of material disposed on the interior surface of the strap may comprise a piece of foam, a polymeric material, such as silicone, or a composite material comprising a gel. 
     Now, turning to  FIG. 23 , it can be seen that the computing device  402  of the vision testing system with eye movement tracking  401  may comprise a microprocessor  434  for processing data, memory  436  (e.g., random access memory or RAM) for storing data during the processing thereof, and data storage device(s)  438 , such as one or more hard drives, compact disk drives, floppy disk drives, flash drives, or any combination thereof. As shown in  FIG. 23 , the head motion sensing device  428 , the eye movement tracking devices  474 ,  476 , and the visual display device  404  are operatively coupled to the computing device  402  such that data is capable of being transferred between these devices. Also, as illustrated in  FIG. 23 , one or more data input devices, such as the wireless mouse  424 , a keyboard, and a touchpad, are operatively coupled to the computing device  402  so that a user is able to enter data into the computing device  402 . In some embodiments, the computing device  402  may be in the form of a laptop computer (as shown in  FIG. 22 ), while in other embodiments, the computing device  402  may be embodied as a desktop computer. 
     In an alternative embodiment, rather than the eye movement tracking device  474  being integrated into goggles or glasses worn on the head of the subject  418 , the eye movement tracking device may be in the form of one or more remote eye movement tracking devices  476  (e.g., in the form of eye movement tracking devices  476  disposed on the top surface of table  406 ). In this alternative embodiment, the eye movement tracking devices  476  (e.g., video-based and infrared sensing means) may capture the movement and/or position of the subject&#39;s eyes while he or she performs the head impulse testing procedure described above. As shown in  FIG. 22 , the eye movement tracking devices  476  may be in the form of video-based and infrared sensing means placed in front of the subject or patient  418 . In the illustrative embodiment of  FIG. 22 , each of the eye movement tracking devices  476  may be configured to track one of the eyes of the subject  418  (i.e., the left eye movement tracking device  476  tracks the center of the pupil for left eye of the subject  418 , while the right eye movement tracking device  476  tracks the center of the pupil for right eye of the subject  418 ). In the illustrative embodiment, the eye movement tracking devices  476  may be operatively connected to the laptop computing device  402  by means of either a wired connection or a wireless connection. Also, if markers are placed on the head  430  of the subject  418  (e.g., on the forehead of the subject  418 ), the movement of the head  430  of the subject  418  may also be determined using the devices  476 , in addition to the eye movement of the subject  418 . In one or more further embodiments, the eye movement tracking device  474  described above, which is disposed in the goggles or glasses worn by the subject  418 , may be used in conjunction with the eye movement tracking devices  476  so as to form a hybrid system for tracking the eye movement of the subject  418 . 
     Because the eye position of the subject  418  measured by the eye movement tracking devices  474 ,  476  is indicative of whether or not the subject  418  is able to see the visual object (i.e., the optotype) or not, in some further embodiments, the determination of the eye position for the subject  418  enables the head impulse testing procedure to be performed without requiring the subject  418  to identify the optotype (i.e., upon the determination by the computing device  402  that there is a specific relationship between the position of the subject&#39;s eyes and whether or not the subject  418  correctly identifies the optotype). For example, if the gaze direction of the subject&#39;s eyes, as determined using the eye movement tracking devices  474 ,  476 , is not in the direction of the visual object (i.e., the optotype), the computing device  402  may determine that the subject  418  will not be able to identify the configuration of the optotype correctly. Conversely, if the gaze direction of the subject&#39;s eyes, as determined using the eye movement tracking devices  474 ,  476 , is consistent with the location of the visual object (i.e., the optotype), the computing device  402  may determine that the subject  418  will be able to identify the configuration of the optotype correctly. However, in the other embodiments of the invention, it is preferred to have the subject  418  identify the optotype during the head impulse test while his or her eye movement is simultaneously measured. 
     In accordance with yet a further illustrative embodiment of the invention, a vision testing/training system is disclosed that is particularly configured for tracking eye movements and/or eye positions of a user  721  performing a vision test or vision training session. An illustrative embodiment of a vision testing/training system with eye movement tracking is seen generally at  700  in  FIG. 24 . In the illustrative embodiment, the vision testing/training system  700  generally comprises a visual display device  710  and a data processing device and/or data processing and data acquisition device  714  (e.g., a computing device or a small-form-factor personal computer, such as the Intel® NUC—see  FIG. 25 ). The small-form-factor personal computer  714  is one illustrative form of a data processing device and/or data processing and data acquisition device. In  FIG. 24 , the small-form-factor personal computer  714  may be mounted on the back of the visual display device  710  (e.g., mounted on the back panel of a touchscreen visual display device with output screen  712 ). In one or more embodiments, the screen images of the training modes described hereinafter are displayed on the output screen  712  of the visual display device  710  so that the user  721  is able to interact with one or more visual objects in the screen images. 
     In the illustrative embodiment of  FIG. 24 , the visual display device  710  is disposed on an adjustable height stand or cart  711  so that the height of the visual display device  710  is selectively adjustable by a user  721 . Advantageously, prior to the testing/training session of the user  721 , the height of the stand  711  may be adjusted such that the approximate center of the visual display device  710  is generally horizontally aligned with the eyes of the standing user  721  (i.e., so the user  721  is generally looking at the central portion of the visual display device  710  during the testing/training). 
     As shown in the illustrative block diagram of  FIG. 25 , the vision testing/training system  700  further includes one or more user input devices  712 ,  724 . The user input device  712 ,  724  is configured to output a first signal based upon an input response by a user  721 . In the illustrative embodiment, the user input device  712  may comprise the touchscreen user interface of the visual display device  710 . In other embodiments, the user input device may alternatively comprise at least one of: (i) a voice recognition device, (ii) a wireless remote control with one or more buttons, (iii) a keyboard (i.e., a virtual or physical keyboard), (iv) a clicking device, (v) a joystick, and (vi) a pointing device. Also, in one or more further embodiments, the primary user input device  712  may comprise the touchscreen user interface of the visual display device  710 , and the supplementary user input device  724  may comprise one of: (i) a voice recognition device, (ii) a wireless remote control with one or more buttons, (iii) a keyboard (i.e., a virtual or physical keyboard), (iv) a clicking device, (v) a joystick, and (vi) a pointing device. 
     In the illustrative embodiment, during the testing/training of the user  721 , the user  721  may use the user input device  712 ,  724  in order to enter and transmit his or her response to the data processing device  714  (e.g., the selection of a particular visual object on the output screen  712  of the visual display device  710 ). 
     In addition, as shown in  FIGS. 24 and 25 , the vision testing/training system  700  further includes an eye movement tracking device  725 . During the vision testing/training conducted by the system  700 , the eye movement tracking device  725  is configured to track eye movement and/or eye position of the user  721 , and output a second signal based upon the tracked eye movement and/or eye position of the user  721 . As shown in  FIG. 25 , the eye movement tracking device  725  is operatively coupled to the computing device  714  such that data is capable of being transferred to the computing device  714  from the eye movement tracking device  725 . The eye movement tracking device  725  of the system  700  may be in the form of the eye movement measurement devices  100 ,  200  described above, or the eye movement tracking device  725  may comprise a different type of eye movement tracking device. 
     Now, turning again to  FIG. 25 , it can be seen that the data processing device  714  (e.g., the small-form-factor personal computer  714 ) of the vision testing/training system  700  comprises a microprocessor  716  for processing data, memory  718  (e.g., random access memory or RAM) for storing data during the processing thereof, and data storage device(s)  720 , such as one or more internal solid state drives, external flash drives, or any combination thereof. As shown in  FIG. 25 , the visual display device  710  is operatively coupled to the computing device  714  such that data is capable of being transferred between these devices (e.g., the visual display device  710  may be a touchscreen visual display device with a touchscreen user interface as described above). Also, as illustrated in  FIG. 25 , one or more data input devices  712 ,  724 , such as the touchscreen user interface or a voice recognition sensor are operatively coupled to the computing device  714  so that a user is able to enter data into the computing device  714 . In one or more alternative embodiments, the computing device  714  may be in the form of a laptop computing device or a desktop computer, rather than the small-form-factor personal computer of the illustrative embodiment. Also, in one or more alternative embodiments, the visual display device  710  may be in the form of a head-mounted visual display device (e.g., a display incorporated in a pair of goggles), and the user input device  724  may be in the form of a voice recognition device or a touchpad interface. 
     Referring again to  FIG. 25 , it can be seen that the illustrative vision system  700  may further include a force measurement assembly  722  for measuring the ground reaction forces and/or moments of the user. In particular, the force measurement assembly  722  may comprise a static dual force plate that is configured to rest on the floor of the room in which the system  700  is disposed (see  FIG. 24 ). The dual force plate  722  comprises a plurality of force transducers or load cells for measuring the forces and/or moments generated on the two plate surfaces thereof by respective feet of the user. As such, the center of pressure (COP), center of gravity (COG), and/or sway angle of the user may be determined while the user undergoes training on the force measurement assembly  722 . For example, in the illustrative embodiment, the dual force plate  722  may use the force plate technology described in U.S. Pat. No. 8,544,347, the entire disclosure of which is incorporated herein by reference. 
     In addition, as illustrated in  FIG. 24 , the force measurement assembly  722  is operatively coupled to the data processing device  714  by virtue of an electrical cable  723 . In one embodiment, the electrical cable  723  is used for data transmission, as well as for providing power to the force measurement assembly  722 . Various types of data transmission cables can be used for cable  723 . For example, the cable  723  can be a Universal Serial Bus (USB) cable or an Ethernet cable. Preferably, the electrical cable  723  contains a plurality of electrical wires bundled together, with at least one wire being used for power and at least another wire being used for transmitting data. The bundling of the power and data transmission wires into a single electrical cable  723  advantageously creates a simpler and more efficient design. In addition, it enhances the safety of the training environment for the user. However, it is to be understood that the force measurement assembly  722  can be operatively coupled to the data processing device  714  using other signal transmission means, such as a wireless data transmission system. If a wireless data transmission system is employed, it is preferable to provide the force measurement assembly  722  with a separate power supply in the form of an internal power supply or a dedicated external power supply. 
     Now, the acquisition and processing of the load data carried out by the illustrative embodiment of the vision testing/training system  700  will be described. Initially, a load is applied to the force measurement assembly  722  by the user disposed thereon. The load is transmitted from the first and second plate components of the dual force plate to its force transducer beams. In the illustrative embodiment, each plate component of the dual force plate is supported on a pair of force transducer beams disposed thereunder. In the illustrative invention, each of the force transducer beams includes a plurality of strain gages wired in one or more Wheatstone bridge configurations, wherein the electrical resistance of each strain gage is altered when the associated portion of the associated beam-type force transducer undergoes deformation (i.e., a measured quantity) resulting from the load (i.e., forces and/or moments) acting on the first and second plate components. For each plurality of strain gages disposed on the force transducer beams, the change in the electrical resistance of the strain gages brings about a consequential change in the output voltage of the Wheatstone bridge (i.e., a quantity representative of the load being applied to the measurement surface). Thus, in the illustrative embodiment, the pair of force transducer beams disposed under the plate components output a total of six (6) analog output voltages (signals). In the illustrative embodiment, the six (6) analog output voltages from dual force plate are then transmitted to a preamplifier board (not shown) for preconditioning. The preamplifier board is used to increase the magnitudes of the transducer analog voltages, and preferably, to convert the analog voltage signals into digital voltage signals as well. After which, the force measurement assembly  722  transmits the force plate output signals S FPO1 -S FPO6  to a main signal amplifier/converter. Depending on whether the preamplifier board also includes an analog-to-digital (A/D) converter, the force plate output signals S FPO1 -S FPO6  could be either in the form of analog signals or digital signals. The main signal amplifier/converter further magnifies the force plate output signals S FPO1 -S FPO6 , and if the signals S FPO1 -S FPO6  are of the analog-type (for a case where the preamplifier board did not include an analog-to-digital (A/D) converter), it may also convert the analog signals to digital signals. In the illustrative embodiment, the force plate output signals S FPO1 -S FPO6  may also be transformed into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ) by the firmware of the dual force plate by multiplying the voltage signals S FPO1 -S FPO6  by a calibration matrix prior to the force plate output data being transmitted to the data processing device  714 . Alternatively, the data acquisition/data processing device  714  may receive the voltage signals S FPO1 -S FPO6 , and then transform the signals into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ) by multiplying the voltage signals S FPO1 -S FPO6  by a calibration matrix. 
     After the voltage signals S FPO1 -S FPO6  are transformed into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ), the center of pressure for each foot of the user (i.e., the x and y coordinates of the point of application of the force applied to the measurement surface by each foot) may be determined by the data acquisition/data processing device  714 . If the force transducer technology described in U.S. Pat. No. 8,544,347 is employed, it is to be understood that the center of pressure coordinates (x P     L   , y P     L   , x P     R   , x P     R   ) can be computed in the particular manner described in that patent. Also, as described below, rather than computing two sets of center of pressure coordinates (i.e., one for each foot of the user), a single set of overall center of pressure coordinates (x P , y P ) may be computed in one or more embodiments. 
     In one or more alternative embodiments, the data processing device  714  determines the vertical forces F Lz , F Rz  exerted on the surface of the first and second force plates by the feet of the user and the center of pressure for each foot of the user, while in another embodiment where a six component force plate is used, the output forces of the data processing device  714  includes all three (3) orthogonal components of the resultant forces acting on the two plate components (i.e., F Lx , F Ly , F Lz , F Rx , F Ry , F Rz ) and all three (3) orthogonal components of the moments acting on the two plate components (i.e., M Lx , M Ly , M Lz , M Rx , M Ry , M Rz ). In yet other embodiments of the invention, the output forces and moments of the data processing device  714  can be in the form of other forces and moments as well. 
     In the illustrative embodiment, where a single set of overall center of pressure coordinates (x P , y P ) are determined for the force measurement assembly  722 , the center of pressure of the force vector {right arrow over (F)} applied by the user to the measurement surface of the force plate  722  is computed as follows: 
                     x   P     =       -     M   y         F   Z               (   8   )                 y   P     =       M   x       F   Z               (   9   )               
where:
 
x P , y P : coordinates of the point of application for the force (i.e., center of pressure) on the force plate assembly  722 ;
 
F Z : z-component of the resultant force acting on the force plate assembly  722 ;
 
M x : x-component of the resultant moment acting on the force plate assembly  722 ; and
 
M y : y-component of the resultant moment acting on the force plate assembly  722 .
 
     In addition, in a further embodiment, the vision testing/training system  700  further comprises a data interface configured to operatively couple the data processing device  714  to a remote computing device (e.g., remote laptop or desktop computing device) so that data from the data processing device  714  is capable of being transmitted to the remote computing device. In one or more embodiments, the data interface may comprise a wireless data interface or a wired data interface operatively coupling the data processing device  714  to the remote computing device. 
     Now, with reference to the screen images of  FIGS. 26 and 27 , the vision testing/training modes carried out by the vision testing/training system  700  will be described in detail. In the illustrative embodiment, the data processing device  714  is programmed to execute each of the testing/training modes described hereinafter. 
     In the first testing/training mode carried out by the illustrative vision testing/training system  700 , a target appears randomly on the output screen  712  of the visual display device  710  in either green or red. The user must touch the green targets and avoid the red targets. Once the user touches the target, it will disappear, and another target will appear on the output screen  712 . If the user does not touch the target after a predetermined period of time has elapsed, the target will disappear and then the target will reappear after a specified time period. This process continues for the duration of the testing/training mode. 
     In particular, with reference to the screen image  726  depicted in  FIG. 26 , in the first illustrative testing/training mode, the data processing device  714  is programmed to generate and display a plurality of visual objects on the output screen  712  of the visual display device  710  in a plurality of different sequential locations on the output screen  712 . For example, as shown in  FIG. 26 , the targets that are generated by the data processing device  714  may appear sequentially in a plurality of different locations  728  on the output screen  712  that are arranged in a circular grid pattern. For example, in the illustrative embodiment, the user may first be presented with a first visual object having a first color or shape (e.g., a target that is green in color, which is represented by the target  730  in  FIG. 26  with the diagonal hatching pattern). In this case, because the first target is in the form of a green target, the correct action to be taken by the user is to select the green target by using the user input device (e.g., the touchscreen user interface of the visual display device  710 ). Once the user touches the target on the touchscreen user interface of the visual display device  710 , the data processing device  714  receives an input signal from the touchscreen user interface based upon an input response by the user. Then, the data processing device  714  is programmed to determine whether the user has performed the correct action by selecting the first visual object. In this case, because the first target was green, the user performed the correct action by selecting the target on the output screen  712 . In addition, the data processing device  714  is programmed to determine, based upon the output signal received from the eye movement tracking device  725 , whether the user is generally looking in a direction that corresponds to a location of the first target  730  on the output screen  712  of the visual display device  710 . Advantageously, determining the gaze direction of the user allows the user&#39;s score for a particular vision test and/or vision training session to be correlated with the user&#39;s eye direction. The gaze direction determination also enables one to determine how the eye movement and the motor skills of the user integrate. Also, after the user touches the first green target using the touchscreen user interface of the visual display device  710 , the target disappears, and a second target appears on the output screen  712 . For example, in the illustrative embodiment, the user may be presented with a second visual object having a second color or shape (e.g., a target that is red in color, which is represented by the target  732  in  FIG. 26  with the criss-cross hatching pattern). In this case, because the second target is in the form of a red target, the correct action to be taken by the user is to not select the red target on the output screen  712 . The data processing device  714  is programmed to determine whether the user has performed the correct action by not selecting the first visual object. In this case, because the second target was red, the user performed the correct action by not selecting the target on the output screen  712 . In addition, the data processing device  714  is programmed to determine, based upon the output signal received from the eye movement tracking device  725 , whether the user is generally looking in a direction that corresponds to a location of the second target  732  on the output screen  712  of the visual display device  710 . Also, after a predetermined time period has elapsed, the second target disappears from the output screen  712 , and a third target appears on the output screen  712  for evaluation by the user (e.g., the third target may be in the form of either a red or green target). In the illustrative embodiment, the first training mode continues in this manner for a predetermined duration of time while the user is presented with either a green target requiring selection by the user or a red target requiring non-selection by the user in different locations on the output screen  712 . 
     In one or more embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  are displaced to different discrete locations on the output screen  712 ; and the data processing device  714  is further programmed to determine saccadic eye movements of the user (i.e., the rapid movement of the eyes of the user between fixation points) from the output signal outputted by the eye movement tracking device  725  as the user gazes at the one or more visual objects  730 ,  732  in the different discrete locations on the output screen  712 . During saccadic testing, the user&#39;s eyes are jumping from one target to another target. 
     In one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  are displaced in a continuous manner on the output screen  712 ; and the data processing device  714  is further programmed to determine smooth pursuits eye movements of the user (i.e., the opposite of saccades) from the output signal outputted by the eye movement tracking device  725  as the user attempts to follow the one or more visual objects  730 ,  732  being displaced on the output screen  712 . In these embodiments, the eye movement tracking device  725  is used to determine how closely the eyes of the user follow a target  730 ,  732  on the output screen  712  of the visual display device  710 . 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  are displaced to different discrete locations on the output screen  712 ; and the data processing device  714  is further programmed to determine anti-saccadic eye movements of the user from the output signal outputted by the eye movement tracking device  725  as the user looks away from the one or more visual objects  730 ,  732  in the different discrete locations on the output screen  712 . For example, in these one or more other embodiments, the user may be presented with one or more visual objects  730 ,  732  on one side of the screen, and then the user is instructed to look at the other side of the screen away from the one or more visual objects  730 ,  732 . When a subject has sustained a concussion or has a disease, such as Parkinson&#39;s disease, dementia, Alzheimer&#39;s disease, etc., the cognitive skill during the anti-saccade test cannot overcome the reflex of the user. This type of test could be used for a return-to-play determination after a user has sustained a concussion, or it could be used for the diagnosis of one of these diseases. 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to: (i) determine a first vision performance score based upon an ability of the user to gaze in the direction that corresponds to the location of the one or more visual objects  730 ,  732  on the output screen  712  of the visual display device  710 , the first vision performance score being determined using the second signal outputted by the eye movement tracking device  725 ; (ii) determine a second motor skills score based upon an ability of the user to perform the correct action with respect to the one or more visual objects  730 ,  732  displayed on the output screen  712 , the second motor skills score being determined using the first signal outputted by the user input device  712 ,  724 ; and (iii) determine a combined performance score for the user as a function of the first vision performance score and the second motor skills score of the user (e.g., a combined performance score based on a summation of the first vision performance score and the second motor skills score of the user). 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  comprise a series of black and white stripes that are displaced horizontally and/or vertically on the output screen  712 ; and the data processing device  714  is further programmed to determine optokinetic eye movements of the user from the second signal outputted by the eye movement tracking device  725  as the user gazes at the one or more visual objects  730 ,  732  with the series of horizontally and/or vertically displaced black and white stripes on the output screen  712 . In these one or more embodiments, as the black and white stripes are displaced horizontally and/or vertically on the output screen  712 , the eye movement tracking device  725  is used to determine if the eyes of the user are following the one or more visual objects  730 ,  732 . The eye movement tracking device  725  may also be used to quantify eye movements with optokinetic patterns. 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to determine a visual midline shift of the user from the second signal outputted by the eye movement tracking device  725  as the user gazes at the one or more visual objects  730 ,  732  on the output screen  712 . Visual midline shift often happens after a person has sustained a concussion. With visual midline shift, a person tends to see objects shifted to the right or left. 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to determine a cognitive load (i.e., amount of working memory resources) for the user based on a pupil dilation of one or more eyes of the user determined from the second signal outputted by the eye movement tracking device  725  as the user gazes at the one or more visual objects  730 ,  732  on the output screen  712 . For example, if a subject understands what is being said to him or her, then his or her pupil will dilate in an oscillatory manner. However, if the subject does not understand what is being said to him or her, then his or her pupil will not dilate much. As such, pupil dilation can be used in the context of a concussion test to determine if a person has sustained a concussion. If virtually no dilation is taking place when someone is talking to the person, then the person has likely sustained a concussion. 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  are displaced into and out of the output screen  712 ; and the data processing device  714  is further programmed to record eye movements of the user as the user gazes at the one or more visual objects  730 ,  732  being displaced into and out of the output screen  712  so that the convergence and/or divergence of the eye of the user is able to be determined. That is, the data processing device  714  uses the recording features that are available with the eye movement tracking device  725  to measure the convergence and divergence of the eyes of the user. 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the one or more visual objects  730 ,  732  generated by the data processing device  714  and displayed on the output screen  712  of the visual display device  710  are rotated in a first direction on the output screen  712 ; and the data processing device  714  is further programmed to determine a torsional eye movement of the user from the second signal outputted by the eye movement tracking device  725  as one or more eyes of the user rotate in a second direction as the user attempts to follow the one or more visual objects  730 ,  732  being rotated on the output screen  712 , the second rotational direction of the one or more eyes of the user being opposite to the first rotational direction of the one or more visual objects  730 ,  732  on the output screen  712 . For example, the torsional movement of the eye may be assessed in order to determine if a patient has vertigo. 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the balance of the user is analyzed using the force measurement assembly  722 . In these one or more embodiments, the data processing device  714  is configured to receive the one or more measurement signals that are generated based upon the user&#39;s contact with the surface of the force measurement assembly  722  and to compute one or more numerical values using the one or more measurement signals, the data processing device  714  is further configured to determine a balance parameter for the user based on the one or more computed numerical values. In these one or more embodiments, the data processing device  714  is further programmed to: (i) determine a first vision performance score based upon an ability of the user to gaze in the direction that corresponds to the location of the one or more visual objects  730 ,  732  on the output screen  712  of the visual display device  710 , the first vision performance score being determined using the second signal outputted by the eye movement tracking device  725 ; (ii) determine a second motor skills score based upon an ability of the user to perform the correct action with respect to the one or more visual objects  730 ,  732  displayed on the output screen  712 , the second motor skills score being determined using the first signal outputted by the user input device  712 ,  724 ; (iii) determine a third balance score using the balance parameter for the user determined from the one or more numerical values computed using the one or more measurement signals of the force measurement assembly  722 ; and (iv) determine a combined performance score for the user as a function of the first vision performance score, the second motor skills score, and the third balance score of the user. In these one or more embodiments, the eye movement tracking device  725  is used with force measurement assembly  722  and a touchscreen visual display device to measure combined eye/balance/hand assessments. 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700  (see  FIGS. 24-26 ), the data processing device  714  is programmed to: (i) generate and display at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710 ; (ii) displace the at least one visual object  730 ,  732  across the output screen  712  of the visual display device  710 ; and (iii) determine, based upon the signal received from the eye movement tracking device  725 , whether the user  721  is able to accurately track the at least one visual object  730 ,  732  as the at least one visual object  730 ,  732  is displaced across the output screen  712  of the visual display device  710 . 
     In these one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , prior to determining whether the user  721  is able to accurately track the at least one visual object  730 ,  732  as the at least one visual object  730 ,  732  is displaced across the output screen  712  of the visual display device  710 , the data processing device  714  is further programmed to maintain the at least one visual object  730 ,  732  in a generally stationary position on the output screen  712 , and to determine whether the user  721  is able to maintain a center point of fixation of his or her eyes on the at least one visual object  730 ,  732  that is stationary. Also, in these one or more other embodiments, the data processing device may be programmed to generate and display the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  in a pop-up manner after the at least one visual object  730 ,  732  is maintained in the generally stationary position on the output screen  712 . In addition, in these one or more other embodiments, when determining whether the user  721  is able to maintain a center point of fixation of his or her eyes on the at least one visual object  730 ,  732  in the generally stationary position on the output screen  712 , the data processing device  714  is further programmed to determine a degree of stability of the center point of fixation of the eyes of the user  721 . That is, the data processing device  714  determines the stability of the user&#39;s central fixation when the user  721  is asked to fix his or her eyes on a target and not move. The data processing device  714  determines the ability of the eyes of the user  721  to stay on the target, and the variability of this ability is a measure of the “stability” of the eyes of the user  721  (i.e., a measure of fixational stability). 
     Further, in these one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , a center point of fixation may be required on a target, which subsequently moves after an arbitrary amount of time (400 milliseconds to 5000 milliseconds). The goal is for the eyes of the user  721  to stay on the target for a given amount of time, which if successfully performed, counts as successfully “hitting” the target. In these one or more other embodiments, the performance of the user  721  in maintaining his or her pursuit eye movement is tracked. 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the user  721  is first instructed to fixate his or her eyes at some point on the screen  712 . Once that condition is satisfied for an arbitrarily selected amount of time, a target is spawned by the data processing device  714 , and the user  721  makes a saccadic eye movement toward the target. The eyes of the user  721  are then required to fixate on this second target. The eye movement tracking device  725  measures the accuracy (gain) of each saccade in consecutively spawned target. Then, the data processing device  714  compares the center of fixation calculated from the eye movement tracking device  725 , when the user  721  makes saccadic eye movements with the eyes and moves the center of fixation cursor to the target. 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to determine whether both eyes of the user are looking at the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  so as to assess binocular vision capabilities of the user  721 . In these other embodiments, eye tracking is used to determine whether the eyes of the user  721  are looking at the same position on the screen  712 , and thus whether there is binocular vision in response to a target being displayed on the screen  712 . 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , when determining whether the user  721  is able to accurately track the at least one visual object  730 ,  732  as the at least one visual object  730 ,  732  is displaced across the output screen  712  of the visual display device  710 , the data processing device  714  is programmed to determine saccadic eye movements of the user  721  from the signal outputted by the eye movement tracking device  725  as the user  721  attempts to track the at least one visual object  730 ,  732  as the at least one visual object  730 ,  732  is displaced across the output screen  712 . 
     In yet one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to determine binocular stability of the user  721  during pursuit eye movements that involve tracking the at least one visual object  730 ,  732 , saccadic eye movements that involve tracking the at least one visual object  730 ,  732 , and fixation of the eyes of the user on the at least one visual object  730 ,  732 . 
     With reference to  FIG. 28 , an exemplary screen image  764 , which may be used during the first testing/training mode carried out by the illustrative vision testing/training system  700 , is depicted. The optic flow pattern in the screen image  764  of  FIG. 28  may be used during smooth pursuit eye movements that involve tracking the at least one visual object  730 ,  732  and/or saccadic eye movements that involve tracking the at least one visual object  730 ,  732 . The optic flow pattern in the screen image  764  of  FIG. 28  creates an optokinetic illusion for the user  721  that is able to induce illusory feelings of self-motion. During testing or training, the screen image  764  may be used as an illusory background, and the at least one visual object  730 ,  732  may be displaced across the illusory background. In other embodiments, rather than using the screen image  764  of  FIG. 28 , a star field pattern may be used as the illusory background for the user  721 . 
     In still one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the system further comprises a head position measurement device (e.g., inertial measurement unit  428  described above) configured to measure an angular position of a head of the user  721 . In these one or more other embodiments, the data processing device  714  is further programmed to determine an angular position of the head of the user  721  while the user  721  attempts to track the at least one visual object  730 ,  732  as the at least one visual object  730 ,  732  is displaced across the output screen  712  of the visual display device  710 , and assess a performance of the user  721  based upon a combination of the angular position of the head of the user  721  determined from the head position measurement device  428  and eye tracking data determined from the eye movement tracking device  725 . In these one or more embodiments, the data processing device  714  may be programmed to utilize head tracking head position in unison with eye tracking to measure the angle of head movement versus eye movement comparison in response to a target being spawned in the vision field of the patient. Also, using the output of the head position measurement device  428  and the eye movement tracking device  725 , the data processing device  714  is able to determine the gaze point of the user  721  on the output screen  712  of the visual display device  710 . Then, during a training routine, the user  721  may be required to fixate his or her eyes on a target while simultaneously using the center of pressure trace from the force balance plate to hover over the same target. During this training routine, the user  721  is require to hover his or her vision center point of fixation and center of pressure over the target for an arbitrary period (i.e., the user  721  is able to converge his or her vision center point of fixation and center of pressure on the same target). 
     In these one or more other embodiments of the first testing/training mode carried out by the illustrative vision testing/training system  700 , the head position measurement device may comprise at least one of the following: (i) one or more inertial measurement units, (ii) a video camera, (iii) an infrared sensor, (iv) an ultrasonic sensor, (v) a light source configured to project a light beam onto a surface, and (vi) a markerless motion capture device. 
     In the second vision testing/training mode carried out by the illustrative vision testing/training system  700 , the center of pressure of the user, which is determined by the data processing device  714  from the force and moment output data of the force measurement assembly  722  described above, is used to control the displacement of a cursor on the output screen  712  of the visual display device  710 . In the second vision testing/training mode, the user must move the cursor into a circular zone of a target that is randomly generated on the output screen  712 . Once the cursor has remained in the circular zone of the target for a user-specified amount of time, the target disappears, and is randomly generated in another location on the output screen  712  by the data processing device  714 . 
     In particular, with reference to the screen image  754  depicted in  FIG. 27 , in the illustrative second vision testing/training mode, the data processing device  714  is programmed to generate and display a visual target  758  defining a boundary  762  on the output screen  712  of the visual display device  710 . The data processing device is further programmed to generate and display a displaceable cursor  756  on the output screen  712 . In one illustrative embodiment, the user input device for the second testing/training mode comprises the force measurement assembly  722  described above. In the second testing/training mode, the center of pressure (COP) determined from the force and moment output data of the force measurement assembly  722  in the manner described above (e.g., see equations (8) and (9) above), is used to control the movement of the displaceable cursor  756  on the output screen  712  of the visual display device  710 . For example, after being presented with the visual target  758  on the screen  712 , the correct action to be taken by the user is to displace the cursor  756  into the interior region  760  defined by the circular boundary  762  of the visual target  758 . Based upon the user&#39;s movement on the force measurement assembly  722  (i.e., shifting his weight to the right, to the left, up, or down), the cursor  756  is displaced accordingly on the screen  712  based upon the computed COP of the user. During the second testing/training mode, the data processing device  714  is programmed to determine whether the user performs the correct action by displacing the cursor  756  into the interior region  760  defined by the boundary  762  of the visual target  758 . The data processing device  714  is further programmed to determine whether the displaceable cursor  756  has remained within the interior region  760  of the visual target  758  for a predetermined period of time. And, when the data processing device  714  has determined that the displaceable cursor  756  has remained within the interior region  760  of the visual target  758  for the predetermined period of time, the data processing device is additionally programmed to generate and display the visual target  758  in another random location on the output screen  712  of the visual display device  710 . In the illustrative embodiment, the second testing/training mode continues in this manner for a predetermined duration of time while the user is presented with visual targets  758  in different locations on the output screen  712 . For each of the visual targets  758  displayed in the second testing/training mode, the user is required to displace the cursor  756  into the interior region  760  of the visual target  758  by controlling the movement of the cursor  756  by shifting his or her weight on the force measurement assembly  722  on which he or she is standing during the second testing/training mode. 
     In one or more embodiments of the second testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is further programmed to determine, based upon the output signal received from the eye movement tracking device  725 , whether the user is generally looking in the direction that corresponds to the location of the one or more visual objects  756  on the output screen  712  of the visual display device  710  as the user moves the cursor  756  into the circular boundary  762  of the visual target  758 . In these one or more embodiments, the eye movement tracking device  725  is used with eye and balance integration on the vision testing/training system  700 . For example, as the cursor  756  is displaced based upon the center of pressure (COP) determined from the force measurement assembly  722 , the eye movement tracking device  725  may be used to determine if the user&#39;s eyes engage in a smooth pursuits movement or a saccadic movement (i.e., a jumping movement). In these one or more embodiments, the movement of both eyes of the user and the balance of the user may be analyzed with regard to the cursor  756  on the output screen  712  of the visual display device  710 . The data processing device  714  is programmed to determine if the user is looking at the cursor  756 , and to determine if the user is keeping the balance cursor  756  on the visual target  758 . 
     In one or more other embodiments of the second testing/training mode carried out by the illustrative vision testing/training system  700 , the eye movement tracking device  725  may be used to control the cursor  756  rather than the force measurement assembly  722 . In these one or more other embodiments, after being presented with the visual target  758  on the screen  712 , the correct action to be taken by the user is to displace the cursor  756  into the interior region  760  defined by the circular boundary  762  of the visual target  758 . Based upon the movement of the user&#39;s eyes measured using the eye movement tracking device  725  (i.e., moving his eyes to the right, to the left, up, or down), the cursor  756  is displaced accordingly on the screen  712  based upon the computed eye movement. In these one or more other embodiments, during the second testing/training mode, the data processing device  714  is programmed to determine whether the user performs the correct action by displacing the cursor  756  into the interior region  760  defined by the boundary  762  of the visual target  758  by using the eye movement tracking device  725  to control the position of the cursor  756 . The data processing device  714  is further programmed to determine whether the displaceable cursor  756  has remained within the interior region  760  of the visual target  758  for a predetermined period of time. And, when the data processing device  714  has determined that the displaceable cursor  756  has remained within the interior region  760  of the visual target  758  for the predetermined period of time, the data processing device is additionally programmed to generate and display the visual target  758  in another random location on the output screen  712  of the visual display device  710 , and the second testing/training mode proceeds in the manner described above. 
     In yet one or more other embodiments of the second testing/training mode carried out by the illustrative vision testing/training system  700 , when the visual display device  710  is in the form of a head-mounted visual display device (e.g., a display incorporated in a pair of goggles), the displaceable cursor  756  and the visual target  758  are displayed on the head-mounted visual display device. In these one or more embodiments, the head-mounted visual display device may be in the form of a virtual reality headset or an augmented reality headset. 
     In still one or more other embodiments of the second testing/training mode carried out by the illustrative vision testing/training system  700 , the visual display device  710  is not a touchscreen visual display device with a touchscreen user interface because the eye movement tracking device  725  is used in place of the touchscreen user interface. 
     In yet one or more other embodiments of the second testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is programmed to independently control the center of the background or target location based on the center of fixation of the user  721 . For example, a person walking through a path may be displayed on the output screen  712  of the visual display device  710 , and the user  721  may control the movement of the person on the screen  712  with his or her eyes. Also, in these one or more other embodiments, the data processing device  714  is programmed to independently control the center of the background to move to the center of fixation of the user  721  in real-time. Essentially, the data processing device  714  is programmed to move the background center to the new eye position, so the point of focus is always the same and the peripheral vision moves with the eye movement. Another way is to move the background to adjust to the center of fixation. If the user  721  looks left, the center shifts left, as if the user  721  is turning their head to the left. In addition, the data processing device  714  may be programmed to move the center of the background based on the eye position of the user  721 . For example, if a user  721  is looking at a scene and he or she looks left, the scene follows his or her gaze and moves left such that the left side of the scene becomes the center of the screen and the new focal point for the user  721  (i.e., syncing peripheral and central vision). 
     In the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , the data processing device  714  is programmed to compute a plurality of numerical values using the one or more signals that are generated based upon the user&#39;s contact with the surface of the force measurement assembly  722  (see  FIGS. 24 and 25 ). The data processing device  714  is further programmed to control a movement parameter of at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  by using a first one of the plurality of numerical values (see  FIG. 26 ), and to assess the probability that the user  721  will fall based upon a combination of the first one of the plurality of numerical values and a second one of the plurality of numerical values. 
     In the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , the first one of the plurality of numerical values computed using the one or more signals of the force measurement assembly  722  comprise x and/or y coordinates specifying the center of pressure of a force vector applied by the user  721  on the force measurement assembly  722 , and the movement parameter of the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  is controlled by a fore/aft displacement of the center of pressure of the user  721  on the force measurement assembly  722 . In other embodiments, rather than using a force measurement assembly  722 , a balance measurement assembly also may be used in the vision testing/training system  700  to generate the plurality of numerical values. 
     In one or more embodiments of the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , the movement parameter of the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  comprises a speed or acceleration of the at least one visual object  730 ,  732  in an interactive game, a virtual reality scenario, and/or an immersive graphic environment. 
     In one or more other embodiments of the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , the movement parameter of the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  comprises a displacement of the at least one visual object  730 ,  732  in an interactive game, a virtual reality scenario, and/or an immersive graphic environment. 
     In one or more embodiments of the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , the second one of the plurality of numerical values computed using the one or more signals comprise a shear force applied by the user  721  on the force measurement assembly  722 , and the data processing device  714  is configured to assess the probability that the user  721  will fall based upon a combination of the center of pressure of the user and the shear force applied by the user  721  on the force measurement assembly  722 . 
     In one or more embodiments of the third vision testing/training mode carried out by the illustrative vision testing/training system  700 , a turning direction of the at least one visual object  730 ,  732  on the output screen  712  of the visual display device  710  to the right or left is controlled by a lateral displacement of the center of pressure of the user  721  on the force measurement assembly  722 . 
     It is apparent from the above detailed description that the vision testing/training systems  400 ,  401 ,  700  and the methods performed thereby significantly advance the field of vision and vestibular assessment. For example, the vision testing/training systems  400 ,  401 ,  700  described herein implement a customizable and accurate protocol for testing/training the vestibulo-ocular reflex (VOR) and/or visual motor performance of a subject. In addition, the method described herein accurately tests the vestibulo-ocular reflex (VOR) and/or visual motor performance of a subject by utilizing a test that is customized for the particular subject being tested. 
     In accordance with still a further illustrative embodiment of the invention, a neurocognitive training system for improving visual motor responses of individuals having impairments and/or athletes seeking performance enhancement is disclosed. 
     Hereinafter, the neurocognitive training system is described herein, in an exemplary manner, with reference to computer system architecture and exemplary processes carried out by the computer system. In one or more embodiments, the functionality described herein can be implemented by computer system instructions. These computer program instructions may be loaded directly onto an internal data storage device of a computing device (e.g., an internal data storage device of a tablet computing device). Alternatively, these computer program instructions could be stored on a portable computer-readable medium (e.g., a flash drive, etc.), and then subsequently loaded onto a computing device such that the instructions can be executed thereby. In other embodiments, these computer program instructions could be embodied in the hardware of the computing device, rather than in the software thereof. It is also possible for the computer program instructions to be embodied in a combination of both the hardware and the software. 
     This description describes in general form the computer program(s) required to carry out the neurocognitive training and/or testing of a user. Any competent programmer in the field of information technology could develop a system using the description set forth herein. 
     For the sake of brevity, conventional computer system components, conventional data networking, and conventional software coding will not be described in detail herein. Also, it is to be understood that the connecting lines shown in the block diagram(s) included herein are intended to represent functional relationships and/or operational couplings between the various components. In addition to that which is explicitly depicted, it is to be understood that many alternative or additional functional relationships and/or physical connections may be incorporated in a practical application of the system. 
     1. Illustrative Neurocognitive Training System 
     An illustrative embodiment of the neurocognitive training system is seen generally at  800  in  FIGS. 29 and 30 . In the illustrative embodiment, the neurocognitive training system  800  generally comprises a visual display device  810  and a data processing device and/or data processing and data acquisition device  814  (e.g., a computing device or a small-form-factor personal computer, such as the Intel® NUC). The small-form-factor personal computer  814  is one illustrative form of a data processing device and/or data processing and data acquisition device. In  FIG. 29 , the small-form-factor personal computer  814  may be mounted on the back of the visual display device  810  (e.g., mounted on the back panel of a touchscreen visual display device with output screen  812 ). In one or more embodiments, the screen images of the training modes described hereinafter are displayed on the output screen  812  of the visual display device  810  so that the user is able to interact with one or more visual objects in the screen images. 
     In the illustrative embodiment of  FIG. 29 , the visual display device  810  is disposed on an adjustable height stand or cart  811  so that the height of the visual display device  810  is selectively adjustable by a user. Advantageously, prior to the training session of the user, the height of the stand  811  may be adjusted such that the approximate center of the visual display device  810  is generally horizontally aligned with the eyes of the standing user (i.e., so the user is generally looking at the central portion of the visual display device  810  during the training). 
     As shown in the illustrative block diagram of  FIG. 30 , the neurocognitive training system  800  further includes one or more user input devices  812 ,  824 . The user input device  812 ,  824  is configured to output a signal based upon an input response by a user. In the illustrative embodiment, the user input device  812  may comprise the touchscreen user interface of the visual display device  810 . In other embodiments, the user input device may alternatively comprise at least one of: (i) a voice recognition device, (ii) a wireless remote control with one or more buttons, (iii) a keyboard (i.e., a virtual or physical keyboard), (iv) a clicking device, (v) a joystick, and (vi) a pointing device. Also, in one or more further embodiments, the primary user input device  812  may comprise the touchscreen user interface of the visual display device  810 , and the supplementary user input device  824  may comprise one of: (i) a voice recognition device, (ii) a wireless remote control with one or more buttons, (iii) a keyboard (i.e., a virtual or physical keyboard), (iv) a clicking device, (v) a joystick, and (vi) a pointing device. 
     In the illustrative embodiment, during the neurocognitive training of the user, the user may use the user input device  812 ,  824  in order to enter and transmit his or her response to the data processing device  814  (e.g., the selection of a particular visual object on the output screen  812  of the visual display device  810 ). 
     Now, turning again to  FIG. 30 , it can be seen that the data processing device  814  (e.g., the small-form-factor personal computer  814 ) of the neurocognitive training system  800  comprises a microprocessor  816  for processing data, memory  818  (e.g., random access memory or RAM) for storing data during the processing thereof, and data storage device(s)  820 , such as one or more internal solid state drives, external flash drives, or any combination thereof. As shown in  FIG. 30 , the visual display device  810  is operatively coupled to the computing device  814  such that data is capable of being transferred between these devices (e.g., the visual display device  810  may be a touchscreen visual display device with a touchscreen user interface as described above). Also, as illustrated in  FIG. 30 , one or more data input devices  812 ,  824 , such as the touchscreen user interface or a voice recognition sensor are operatively coupled to the computing device  814  so that a user is able to enter data into the computing device  814 . In one or more alternative embodiments, the computing device  814  may be in the form of a laptop computing device or a desktop computer, rather than the small-form-factor personal computer of the illustrative embodiment. Also, in one or more alternative embodiments, the visual display device  810  may be in the form of a head-mounted visual display device (e.g., a display incorporated in a pair of goggles), and the user input device  824  may be in the form of a voice recognition device or a touchpad interface. 
     Referring again to  FIG. 30 , it can be seen that the illustrative neurocognitive training system  800  may further include a force measurement assembly  822  for measuring the ground reaction forces and/or moments of the user. In particular, the force measurement assembly  822  may comprise a static dual force plate that is configured to rest on the floor of the room in which the system  800  is disposed. The dual force plate  822  comprises a plurality of force transducers or load cells for measuring the forces and/or moments generated on the two plate surfaces thereof by respective feet of the user. As such, the center of pressure (COP), center of gravity (COG), and/or sway angle of the user may be determined while the user undergoes training on the force measurement assembly  822 . For example, in the illustrative embodiment, the dual force plate  822  may use the force plate technology described in U.S. Pat. No. 8,544,347, the entire disclosure of which is incorporated herein by reference. 
     In addition, as illustrated in  FIG. 29 , the force measurement assembly  822  is operatively coupled to the data processing device  814  by virtue of an electrical cable  823 . In one embodiment, the electrical cable  823  is used for data transmission, as well as for providing power to the force measurement assembly  822 . Various types of data transmission cables can be used for cable  823 . For example, the cable  823  can be a Universal Serial Bus (USB) cable or an Ethernet cable. Preferably, the electrical cable  823  contains a plurality of electrical wires bundled together, with at least one wire being used for power and at least another wire being used for transmitting data. The bundling of the power and data transmission wires into a single electrical cable  823  advantageously creates a simpler and more efficient design. In addition, it enhances the safety of the training environment for the user. However, it is to be understood that the force measurement assembly  822  can be operatively coupled to the data processing device  814  using other signal transmission means, such as a wireless data transmission system. If a wireless data transmission system is employed, it is preferable to provide the force measurement assembly  822  with a separate power supply in the form of an internal power supply or a dedicated external power supply. 
     Now, the acquisition and processing of the load data carried out by the illustrative embodiment of the neurocognitive training system  800  will be described. Initially, a load is applied to the force measurement assembly  822  by the user disposed thereon. The load is transmitted from the first and second plate components of the dual force plate to its force transducer beams. In the illustrative embodiment, each plate component of the dual force plate is supported on a pair of force transducer beams disposed thereunder. In the illustrative invention, each of the force transducer beams includes a plurality of strain gages wired in one or more Wheatstone bridge configurations, wherein the electrical resistance of each strain gage is altered when the associated portion of the associated beam-type force transducer undergoes deformation (i.e., a measured quantity) resulting from the load (i.e., forces and/or moments) acting on the first and second plate components. For each plurality of strain gages disposed on the force transducer beams, the change in the electrical resistance of the strain gages brings about a consequential change in the output voltage of the Wheatstone bridge (i.e., a quantity representative of the load being applied to the measurement surface). Thus, in the illustrative embodiment, the pair of force transducer beams disposed under the plate components output a total of six (6) analog output voltages (signals). In the illustrative embodiment, the six (6) analog output voltages from dual force plate are then transmitted to a preamplifier board (not shown) for preconditioning. The preamplifier board is used to increase the magnitudes of the transducer analog voltages, and preferably, to convert the analog voltage signals into digital voltage signals as well. After which, the force measurement assembly  822  transmits the force plate output signals S FPO1 -S FPO6  to a main signal amplifier/converter. Depending on whether the preamplifier board also includes an analog-to-digital (A/D) converter, the force plate output signals S FPO1 -S FPO6  could be either in the form of analog signals or digital signals. The main signal amplifier/converter further magnifies the force plate output signals S FPO1 -S FPO6 , and if the signals S FPO1 -S FPO6  are of the analog-type (for a case where the preamplifier board did not include an analog-to-digital (A/D) converter), it may also convert the analog signals to digital signals. In the illustrative embodiment, the force plate output signals S FPO1 -S FPO6  may also be transformed into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ) by the firmware of the dual force plate by multiplying the voltage signals S FPO1 -S FPO6  by a calibration matrix prior to the force plate output data being transmitted to the data processing device  814 . Alternatively, the data acquisition/data processing device  814  may receive the voltage signals S FPO1 -S FPO6 , and then transform the signals into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ) by multiplying the voltage signals S FPO1 -S FPO6  by a calibration matrix. 
     After the voltage signals S FPO1 -S FPO6  are transformed into output forces and/or moments (e.g., F Lz , M Lx , M Ly , F Rz , M Rx , M Ry ), the center of pressure for each foot of the user (i.e., the x and y coordinates of the point of application of the force applied to the measurement surface by each foot) may be determined by the data acquisition/data processing device  814 . If the force transducer technology described in U.S. Pat. No. 8,544,347 is employed, it is to be understood that the center of pressure coordinates (x P     L   , y P     L   m, x P     R   , x P     R   ) can be computed in the particular manner described in that patent. Also, as described below, rather than computing two sets of center of pressure coordinates (i.e., one for each foot of the user), a single set of overall center of pressure coordinates (x P , y P ) may be computed in one or more embodiments. 
     In one or more alternative embodiments, the data processing device  814  determines the vertical forces F Lz , F Rz  exerted on the surface of the first and second force plates by the feet of the user and the center of pressure for each foot of the user, while in another embodiment where a six component force plate is used, the output forces of the data processing device  814  includes all three (3) orthogonal components of the resultant forces acting on the two plate components (i.e., F Lx , F Ly , F Lz , F Rx , F Ry , F Rz ) and all three (3) orthogonal components of the moments acting on the two plate components (i.e., M Lx , M Ly , M Lz , M Rx , M Ry , M Rz ). In yet other embodiments of the invention, the output forces and moments of the data processing device  814  can be in the form of other forces and moments as well. 
     In the illustrative embodiment, where a single set of overall center of pressure coordinates (x P , y P ) are determined for the force measurement assembly  822 , the center of pressure of the force vector {right arrow over (F)} applied by the user to the measurement surface of the force plate  822  is computed as follows: 
                     x   P     =       -     M   y         F   Z               (   10   )                 y   P     =       M   x       F   Z               (   11   )               
where:
 
x P , y P : coordinates of the point of application for the force (i.e., center of pressure) on the force plate assembly  822 ;
 
F Z : z-component of the resultant force acting on the force plate assembly  822 ;
 
M x : x-component of the resultant moment acting on the force plate assembly  822 ; and
 
M y : y-component of the resultant moment acting on the force plate assembly  822 .
 
     In addition, in a further embodiment, the neurocognitive training system  800  further comprises a data interface configured to operatively couple the data processing device  814  to a remote computing device (e.g., remote laptop or desktop computing device) so that data from the data processing device  814  is capable of being transmitted to the remote computing device. In one or more embodiments, the data interface may comprise a wireless data interface or a wired data interface operatively coupling the data processing device  814  to the remote computing device. 
     2. Neurocognitive Training Modes of the Neurocognitive Training System 
     Now, with reference to the screen images of  FIGS. 31-36 , the neurocognitive training modes carried out by the neurocognitive training system  800  will be described in detail. In the illustrative embodiment, the data processing device  814  is programmed to execute each of the training modes described hereinafter. 
     In the first training mode carried out by the illustrative neurocognitive training system  800 , a target appears randomly on the output screen  812  of the visual display device  810  in either green or red. The user must touch the green targets and avoid the red targets. Once the user touches the target, it will disappear, and another target will appear on the output screen  812 . If the user does not touch the target after a predetermined period of time has elapsed, the target will disappear and then the target will reappear after a specified time period. This process continues for the duration of the training mode. 
     In particular, with reference to the screen image  826  depicted in  FIG. 31 , in the first illustrative training mode, the data processing device  814  is programmed to generate and display a plurality of visual objects on the output screen  812  of the visual display device  810  in a plurality of different sequential locations on the output screen  812 . For example, as shown in  FIG. 31 , the targets that are generated by the data processing device  814  may appear sequentially in a plurality of different locations  828  on the output screen  812  that are arranged in a circular grid pattern. For example, in the illustrative embodiment, the user may first be presented with a first visual object having a first color or shape (e.g., a target that is green in color, which is represented by the target  830  in  FIG. 31  with the diagonal hatching pattern). In this case, because the first target is in the form of a green target, the correct action to be taken by the user is to select the green target by using the user input device (e.g., the touchscreen user interface of the visual display device  810 ). Once the user touches the target on the touchscreen user interface of the visual display device  810 , the data processing device  814  receives an input signal from the touchscreen user interface based upon an input response by the user. Then, the data processing device  814  is programmed to determine whether the user has performed the correct action by selecting the first visual object. In this case, because the first target was green, the user performed the correct action by selecting the target on the output screen  812 . Also, after the user touches the first green target using the touchscreen user interface of the visual display device  810 , the target disappears, and a second target appears on the output screen  812 . For example, in the illustrative embodiment, the user may be presented with a second visual object having a second color or shape (e.g., a target that is red in color, which is represented by the target  832  in  FIG. 31  with the criss-cross hatching pattern). In this case, because the second target is in the form of a red target, the correct action to be taken by the user is to not select the red target on the output screen  812 . The data processing device  814  is programmed to determine whether the user has performed the correct action by not selecting the first visual object. In this case, because the second target was red, the user performed the correct action by not selecting the target on the output screen  812 . Also, after a predetermined time period has elapsed, the second target disappears from the output screen  812 , and a third target appears on the output screen  812  for evaluation by the user (e.g., the third target may be in the form of either a red or green target). In the illustrative embodiment, the first training mode continues in this manner for a predetermined duration of time while the user is presented with either a green target requiring selection by the user or a red target requiring non-selection by the user in different locations on the output screen  812 . 
     In the second training mode carried out by the illustrative neurocognitive training system  800 , the user is presented a number of targets in a grid pattern for a set time period. After the time period has elapsed, the targets disappear from the output screen  812 , and the user must repeat the pattern by touching the grid locations where the targets were presented, and replicating the sequence in which the targets were presented. This process continues for the duration of the training mode with targets being arranged in different patterns during each trial of the training mode. 
     In particular, with reference to the screen image  834  depicted in  FIG. 32 , in the second illustrative training mode, the data processing device  814  is programmed to generate and display a plurality of visual objects on the output screen  812  of the visual display device  810  arranged in a predetermined pattern for a predetermined period of time. For example, as shown in  FIG. 32 , the targets  838  that are generated by the data processing device  814  may be arranged in a predetermined grid pattern (e.g., the targets  838  may be disposed in any of the locations  836  in the circular grid pattern of  FIG. 32 ). In the illustrative embodiment of  FIG. 32 , it can be seen that a total of five (5) targets  838  are presented on the output screen  812  for identification by the user. After the predetermined period of time for display of the targets  838  has elapsed, the data processing device  814  is programmed to remove the plurality of targets  838  from the output screen  812  and receive input signals from the user input device (e.g., the touchscreen user interface of the visual display device  810 ) based upon input responses by the user that are indicative of the user&#39;s perceived replication of the location and sequence of the plurality of targets  838  arranged in the predetermined pattern. That is, the user selects the positions on the output screen  812  where he or she believes the targets  838  were located, and the user selects the positions in sequence in which he or she believes the targets  838  were presented. Then, the data processing device  814  is programmed to determine, based upon the input signals received from the user input device, whether the user has correctly identified the location and sequence of the plurality of targets  838  arranged in the predetermined grid pattern. In the illustrative embodiment, the second training mode continues in this manner for a predetermined duration of time while the user is presented with different quantities of targets that are arranged in varying grid patterns on the output screen  812  for identification by the user. 
     In the third training mode carried out by the illustrative neurocognitive training system  800 , targets appearing sequentially on the output screen  812  of the visual display device  810  get progressively closer to the color of the background throughout the duration of the training mode. In this third training mode, the primary objective is to train the contrast sensitivity of the user (e.g., the athlete). 
     In particular, with reference to the screen image  840  depicted in  FIG. 33 , in the illustrative third training mode, the data processing device  814  is programmed to generate and display a plurality of visual objects on the output screen  812  of the visual display device  810  in a plurality of different sequential locations on the output screen  812 . For example, as shown in  FIG. 33 , the targets  842  that are generated by the data processing device  814  may appear sequentially in a plurality of different locations on the output screen  812  (e.g., the target  842  depicted in the darker color in  FIG. 33  is displayed first on the screen  812 , then the targets  842  depicted in lighter colors in  FIG. 33  are displayed sequentially after the first target  842 ). In the third training mode, the contrast between the targets  842  and the screen background is progressively reduced such that the targets  842  become increasing more difficult for the user to identify. For example, in the illustrative embodiment, the user may first be presented with the first target  842  (e.g., the target  842  depicted in the darkest color in  FIG. 33 ) that has a particular color which is easily distinguishable from the screen background (e.g., a first target with a bright red color that is easily distinguishable from a black background color). After being presented with the target  842  on the screen  812 , the correct action to be taken by the user is to select the target  842  by using the user input device (e.g., the touchscreen user interface of the visual display device  810 ). Once the user touches the target  842  on the touchscreen user interface of the visual display device  810 , the data processing device  814  receives an input signal from the touchscreen user interface based upon an input response by the user. Then, the data processing device  814  is programmed to determine whether the user has performed the correct action by selecting the first target  842 . Also, after the user touches the first target  842  using the touchscreen user interface of the visual display device  810 , the target  842  disappears, and a second target  842  with a reduced contrast appears on the output screen  812  (e.g., one of the targets  842  in lighter colors in  FIG. 33 ). For example, in the illustrative embodiment, the second target  842  having the reduced contrast may be a darker shade of red that is harder for the user to distinguish from the black screen background. In the illustrative embodiment, the third training mode continues in this manner for a predetermined duration of time while the user is presented with targets  842  having successively reduced contrast compared to the screen background. That is, in the illustrative third training mode, the data processing device  814  is programmed to continually reduce the contrast between the targets  842  and the screen background by progressively making a color of the one or more visual objects closer to a background color of the screen background (e.g., starting with a target  842  that is bright red, and then gradually changing the color of the target  842  such that it becomes closer and closer to the black background color). 
     In the fourth training mode carried out by the illustrative neurocognitive training system  800 , single or multiple pairs of orbiting targets appear on the output screen  812  of the visual display device  810 . The user must track one of the targets in each pair as they are displaced in an orbital path for a predetermined time period. Once the time period is over, the orbiting ceases and the user must identify the target or targets that he or she was tracking. This process continues for the duration of the training mode with orbiting targets being arranged in different patterns during each trial of the training mode. 
     In particular, with reference to the screen image  844  depicted in  FIG. 34 , in the fourth illustrative training mode, the data processing device  814  is programmed to generate and display one or more pairs of displaceable visual objects  846 ,  848  on the output screen  812  of the visual display device  810 . The one or more pairs of displaceable visual objects  846 ,  848  are configured to continually orbit in a predetermined path on the output screen  812  of the visual display device  810 . For example, as shown in  FIG. 34 , the three (3) pairs of displaceable visual targets  846 ,  848  orbit in respective circular orbit paths  850 . In  FIG. 34 , arrows  852  are used to identify the three (3) displaceable visual targets  848  that are to be followed by the user during the trial of the training mode. The three (3) displaceable visual targets  846  orbit about the same circular paths  850  as the displaceable visual targets  848 , but they are not to be followed by the user. After the displaceable visual targets  846 ,  848  have orbited in the circular path  850  for a predetermined period of time, the data processing device  814  is programmed to stop the orbital movement of the displaceable visual targets  846 ,  848  so that the user is able to identify the displaceable visual targets  848 . The data processing device  814  is programmed to receive input signals from the user input device (e.g., the touchscreen user interface of the visual display device  810 ) based upon input responses by the user that are indicative of the user&#39;s perceived identification of the orbiting visual targets  848 . That is, the user selects the targets on the output screen  812  that he or she believes are the orbiting visual targets  848 . A selection of any one of the orbiting visual targets  846  by the user constitutes an incorrect answer because they are not the targets that the user is required to follow on the output screen  812 . Then, the data processing device  814  is programmed to determine, based upon the input signals received from the user input device, whether the user has correctly identified the orbiting visual targets  848  that have been displaced in the orbital paths  850  on the output screen  812  of the visual display device  810 . In the illustrative embodiment, the fourth training mode continues in this manner for a predetermined duration of time while the user is presented with different quantities and patterns of orbiting targets to track and identify using the user input device (e.g., the touchscreen user interface of the visual display device  810 ). 
     In the fifth training mode carried out by the illustrative neurocognitive training system  800 , the center of pressure of the user, which is determined by the data processing device  814  from the force and moment output data of the force measurement assembly  822  described above, is used to control the displacement of a cursor on the output screen  812  of the visual display device  810 . In the fifth training mode, the user must move the cursor into a circular zone of a target that is randomly generated on the output screen  812 . Once the cursor has remained in the circular zone of the target for a user-specified amount of time, the target disappears, and is randomly generated in another location on the output screen  812  by the data processing device  814 . 
     In particular, with reference to the screen image  854  depicted in  FIG. 35 , in the illustrative fifth training mode, the data processing device  814  is programmed to generate and display a visual target  858  defining a boundary  862  on the output screen  812  of the visual display device  810 . The data processing device is further programmed to generate and display a displaceable cursor  856  on the output screen  812 . In the illustrative embodiment, the user input device for the fifth training mode comprises the force measurement assembly  822  described above. In the fifth training mode, the center of pressure (COP) determined from the force and moment output data of the force measurement assembly  822  in the manner described above (e.g., see equations (10) and (11) above), is used to control the movement of the displaceable cursor  856  on the output screen  812  of the visual display device  810 . For example, after being presented with the visual target  858  on the screen  812 , the correct action to be taken by the user is to displace the cursor  856  into the interior region  860  defined by the circular boundary  862  of the visual target  858 . Based upon the user&#39;s movement on the force measurement assembly  822  (i.e., shifting his weight to the right, to the left, up, or down), the cursor  856  is displaced accordingly on the screen  812  based upon the computed COP of the user. During the fifth training mode, the data processing device  814  is programmed to determine whether the user performs the correct action by displacing the cursor  856  into the interior region  860  defined by the boundary  862  of the visual target  858 . The data processing device  814  is further programmed to determine whether the displaceable cursor  856  has remained within the interior region  860  of the visual target  858  for a predetermined period of time. And, when the data processing device  814  has determined that the displaceable cursor  856  has remained within the interior region  860  of the visual target  858  for the predetermined period of time, the data processing device is additionally programmed to generate and display the visual target  858  in another random location on the output screen  812  of the visual display device  810 . In the illustrative embodiment, the fifth training mode continues in this manner for a predetermined duration of time while the user is presented with visual targets  858  in different locations on the output screen  812 . For each of the visual targets  858  displayed in the fifth training mode, the user is required to displace the cursor  856  into the interior region  860  of the visual target  858  by controlling the movement of the cursor  856  by shifting his or her weight on the force measurement assembly  822  on which he or she is standing during the fifth training mode. 
     In the sixth training mode performed using the illustrative neurocognitive training system  800 , the center of pressure of the user is determined by the data processing device  814  from the force and moment output data of the force measurement assembly  822  described above, while the user performs a series of trials with varying conditions. In addition to training, the sixth mode of the system  800  also may be used for the testing of the user. 
     In the illustrative embodiment, the sixth training mode performed using the neurocognitive training system  800  comprises eight (8) conditions under which a user is trained or tested (i.e., eight training or testing stages). In accordance with the first training and/or testing condition, a user simply stands in a stationary, upright position on the force measurement assembly  822  in a normal stance with his or her eyes open. In particular, the user stands still with his or her feet approximately placed on the force measurement assembly  822  according to instructions on the output screen  812  of the visual display device  810 . While standing still on the force measurement assembly  822  during the trial(s) of the first condition, the hands of the user are placed on his or her hips, and the user looks straight ahead with his or her eyes open. If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the first training condition. Then, the user begins the initial trial of the first training and/or testing condition. In addition, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. For example, quantifiable errors that may be recorded by the clinician include: (i) the user lifting his or her hands off his or her hips, (ii) the user opening his or her eyes during a closed eyes trial, (iii) the user stepping off the force measurement assembly  822  or moving from the starting position, and (iv) an inability of the user to maintain action with the beat of the metronome for two or more consecutive beats. During the first condition, the data processing device  814  may be programmed to generate and display a solid color background image on the output screen  812  of the visual display device  810 . 
     In accordance with the second training and/or testing condition of the sixth mode, a user stands in a stationary, upright position on the force measurement assembly  822  in a normal stance with his or her eyes closed. In particular, the user stands still with his or her feet approximately placed on the force measurement assembly  822  according to instructions on the output screen  812  of the visual display device  810 . While standing still on the force measurement assembly  822  during the trial(s) of the second condition, the hands of the user are placed on his or her hips, and the eyes of the user are closed. If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the second training condition. Then, the user begins the initial trial of the second training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. Similar to the first condition, during the second condition, the data processing device  814  may be programmed to generate and display a solid color background image on the output screen  812  of the visual display device  810 . 
     In accordance with the third training and/or testing condition, a block of foam is placed on the top surface of the force measurement assembly  822  before a user stands on the force measurement assembly  822 . Then, the user stands in a stationary, upright position on the top surface of the foam, which is disposed on the force measurement assembly  822 , in a normal stance with his or her eyes open. In particular, similar to the first condition described above, during the third condition, the user stands still with his or her feet approximately placed on the force measurement assembly  822  according to instructions on the output screen  812  of the visual display device  810 . While standing still on the foam during the trial(s) of the third condition, the hands of the user are placed on his or her hips, and the user looks straight ahead with his or her eyes open. If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the third training condition. Then, the user begins the initial trial of the third training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. Similar to the first and second conditions, during the third condition, the data processing device  814  may be programmed to generate and display a solid color background image on the output screen  812  of the visual display device  810 . 
     In accordance with the fourth training and/or testing condition, the user stands in a stationary, upright position on the top surface of the foam, which is disposed on the force measurement assembly  822 , in a normal stance with his or her eyes closed. In particular, similar to the third condition described above, during the fourth condition, the user stands still with his or her feet approximately placed on the force measurement assembly  822  according to instructions on the output screen  812  of the visual display device  810 . While standing still on the foam during the trial(s) of the fourth condition, the hands of the user are placed on his or her hips, and the eyes of the user are closed. If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the fourth training condition. Then, the user begins the initial trial of the fourth training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. Similar to the first three conditions described above, during the fourth condition, the data processing device  814  may be programmed to generate and display a solid color background image on the output screen  812  of the visual display device  810 . 
     In accordance with the fifth training and/or testing condition of the sixth mode, a user stands in a stationary, upright position on the force measurement assembly  822  with his or her feet close together and his or her eyes closed while the user rotates his or her head back-and-forth. In particular, the user stands still with his or her feet approximately placed on the force measurement assembly  822  according to instructions on the output screen  812  of the visual display device  810 . While standing still on the force measurement assembly  822  during the trial(s) of the fifth condition, the hands of the user are placed on his or her hips, and the eyes of the user are closed while the user rotates his or her head back-and-forth in an oscillatory manner. For example, the user may rotate his or her head plus or minus 30 degrees from a neutral center position in sync with an audible metronome (e.g., at 120 beats per minute). If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the fifth training condition. Then, the user begins the initial trial of the fifth training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. 
     In accordance with the sixth training and/or testing condition of the sixth mode, a user stands in an upright position on the force measurement assembly  822  with his or her eyes open while the user rotates his or her body back-and-forth. In particular, the user stands at the approximate center of the force measurement assembly  822  with his or her feet close together, and both of his or her arms extended out in front with his or her hands clasped together and his or her thumbs straight up. While focusing on his or her thumbs, the user rotates his or her entire body back-and-forth in an oscillatory manner. For example, while focusing on his or her thumbs, the user rotates his or her entire body plus or minus 30 degrees from a neutral center position (i.e., over an overall angular range of 60 degrees) in sync with a metronome set to a predetermined number of beats per minute (e.g., 40 beats per minute) and/or a visual object displaced across the output screen  812  of the visual display device  810 . If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the sixth training condition. Then, the user begins the initial trial of the sixth training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. As shown in  FIG. 36 , during the sixth condition, the data processing device  814  may be programmed to generate and display a screen image  864  on the output screen  812  of the visual display device  810  that includes a ball  866  being displaced across the output screen  812  along the path  868 , which operates as a visual metronome. That is, in the illustrative embodiment, the user tries to rotate his or her entire body in sync with the ball  866  displaced across the output screen  812  so that he or she maintains the proper body rotation timing. In an alternative embodiment, rather than displaying the screen image  864  of  FIG. 36  on the output screen  812  during the performance of the sixth condition, the data processing device  814  may be programmed to generate and display a stationary screen image on the output screen  812  of the visual display device  810 . In this alternative embodiment, the stationary screen image may include a plurality of vertical indicator lines that assist the user in achieving the proper rotational displacement path (e.g., the stationary screen image may contain a center vertical indicator line that represents the neutral center position, a first vertical indicator line disposed on a left side of the center vertical indicator line that represents the −30 degree rotational displacement limit, and a second vertical indicator line disposed on a right side of the center vertical indicator line that represents the 30 degree rotational displacement limit). 
     In accordance with the seventh training and/or testing condition, similar to the third and fourth conditions described above, a block of foam is placed on the top surface of the force measurement assembly  822  before a user stands on the force measurement assembly  822 . Then, the user stands in a stationary, upright position on the top surface of the foam, which is disposed on the force measurement assembly  822 , in a normal stance with his or her eyes closed while the user rotates his or her head back-and-forth. In particular, while standing still on the foam during the trial(s) of the seventh condition, the hands of the user are placed on his or her hips, and the eyes of the user are closed while the user rotates his or her head back-and-forth in an oscillatory manner. For example, the user may rotate his or her head plus or minus 30 degrees from a neutral center position in sync with an audible metronome (e.g., at 120 beats per minute). If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the seventh training condition. Then, the user begins the initial trial of the seventh training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. 
     In accordance with the eighth training and/or testing condition, similar to the seventh condition described above, a block of foam is placed on the top surface of the force measurement assembly  822  before a user stands on the force measurement assembly  822 . Then, the user stands in an upright position on the foam with his or her eyes open in a narrow stance while the user rotates his or her body back-and-forth. In particular, the user stands in a narrow stance with both of his or her arms extended out in front, his or her hands clasped together, and his or her thumbs straight up. While focusing on his or her thumbs, the user rotates his or her entire body back-and-forth in an oscillatory manner. For example, while focusing on his or her thumbs, the user rotates his or her entire body plus or minus 30 degrees from a neutral center position (i.e., over an overall angular range of 60 degrees) in sync with an audible metronome set to a predetermined number of beats per minute (e.g., 40 beats per minute) and/or a visual object displaced across the output screen  812  of the visual display device  810 . If a clinician is present during the testing and/or training, the clinician may instruct the user as to the proper form during the eighth training condition. Then, the user begins the initial trial of the eighth training and/or testing condition. As described above, if a clinician is present during the training and/or testing, the clinician may use a user input device  824 , such as a wireless remote control with a plurality of buttons, to record any errors performed by the user. Similar to the sixth condition described above, during the eighth condition, the data processing device  814  may be programmed to generate and display the screen image  864  on the output screen  812  of the visual display device  810  that includes the ball  866  being displaced across the output screen  812  along the path  868  (refer to  FIG. 36 ), which operates as a visual metronome. That is, in the illustrative embodiment, the user tries to rotate his or her entire body in sync with the ball  866  displaced across the output screen  812  so that he or she maintains the proper body rotation timing. In an alternative embodiment, as described above for the sixth condition, rather than displaying the screen image  864  of  FIG. 36  on the output screen  812  during the performance of the eighth condition, the data processing device  814  may be programmed to generate and display the stationary screen image explained above. 
     In the seventh training mode carried out by the illustrative neurocognitive training system  800 , similar to the fifth training mode described above, the center of pressure of the user, which is determined by the data processing device  814  from the force and moment output data of the force measurement assembly  822 , is used to control the displacement of a cursor on the output screen  812  of the visual display device  810 . In the seventh training mode, the user must move the cursor into circular zones of a plurality of different targets that are randomly generated on the output screen  812 . Once the cursor has remained in a circular zone of one of the targets for a predetermined amount of time, the user is prompted to displace the cursor to one or more additional targets in other locations on the output screen  812  by the data processing device  814 . 
     In particular, with reference to the screen image  890  depicted in  FIG. 38 , in the illustrative seventh training mode, the data processing device  814  is programmed to generate and display a plurality of visual targets  892  defining respective boundaries on the output screen  812  of the visual display device  810 . The data processing device  814  is further programmed to generate and display a displaceable cursor  894  on the output screen  812 . In the illustrative embodiment, the user input device for the seventh training mode comprises the force measurement assembly  822  described above. In the seventh training mode, the center of pressure (COP) determined from the force and moment output data of the force measurement assembly  822  in the manner described above (e.g., see equations (10) and (11) above), is used to control the movement of the displaceable cursor  894  on the output screen  812  of the visual display device  810 . For example, in the seventh training mode, the user is initially required to hover the displaceable cursor  894  controlled by his or her center of pressure (COP) trace over the target sensitivity area of the initial target  892  (i.e., the shaded area defined by the target  892  on the left side of the screen image  890  in  FIG. 38 ). When the displaceable cursor  894  is within the sensitivity area of the target  892 , a clock generated by the data processing device  814  begins to count down toward zero. Once the clock reaches zero, the second target  892  (i.e., the target  892  on the right side of the screen image  890  in  FIG. 38 ) will spawn by the data processing device  814  generating a pulsating concentric ring  898  around the outer zone of the target  892 , and a second clock generated by the data processing device  814  begins counting down. This is the “reaction time” clock, which counts how long it takes for the user to move the displaceable cursor  894  out of the initial target sensitivity area and when the user has made it to the final target, reported as the “Time to Target.” In the illustrative embodiment, the seventh training mode continues in this manner for a predetermined duration of time while the user is presented with visual targets  892  in different locations on the output screen  812 . In the seventh training mode, a pulsating concentric ring  896  within the target sensitivity area of the target  892  may be used to designate that the target  892  is active (i.e., the ring  896  moves in and out concentrically to indicate that the target  892  on the left side of the screen image  890  in  FIG. 38  is active). For each of the visual targets  892  displayed in the seventh training mode, the user is required to displace the cursor  894  into the target sensitivity area of the visual target  892  by controlling the movement of the cursor  894  by shifting his or her weight on the force measurement assembly  822  on which he or she is standing during the seventh training mode. 
     Advantageously, in the seventh training mode, the data processing device  814  may be programmed to record the user&#39;s body reaction time and eye-body coordination to give a unique measurement of reaction time. Also, in the illustrative embodiment, the user is instructed to “hover” his or her body weight over the second target, requiring an additional measure of body control and stability. 
     In one or more embodiments of the seventh training mode, the data processing device  814  measures not only the traditional eye/hand reaction time, but also how quickly the body can accurately shift balance based on a visual stimulus. The data processing device  814  measures in various directions, such as up, down, right and left, but also at different separation or distances between targets. The data processing device  814  breaks down the process into two parts, the initial time it takes the subject to see target  892 , process the information and start the cursor to move; and the subsequent time it actually takes the cursor to move to the second target  892  and select it. The first part is the visual component and the second part is the motor component. Both of these skills can be affected post-concussion and post traumatic brain injury. 
     Also, in one or more embodiments of the seventh training mode, the data processing device  814  is programmed with several variables that may be selected by the user beyond the background, touch sensitivity, and target type. One variable is grid size, the larger the grid size chosen, the great the distance between the targets  892 . The other variable is target size, which also relates to sensitivity. Also, in the seventh training mode, the data processing device  814  selects random pacing by default, which means that the time from a user moving the displaceable cursor  894  on the initial target  892 , to when the user is prompted to move it to the next target  892 , varies randomly. This is done so that the user cannot predict when to start moving his or her hand. An auditory clue may also be given by a user selecting the appropriate box on the output screen  812  of the visual display device  810 . 
     In the eighth training mode carried out by the illustrative neurocognitive training system  800 , a target appears randomly on the output screen  812  of the visual display device  810  with a “tunnel” video background. The user must then touch the target. Once the user touches the target, it will disappear, and another target will appear on the output screen  812 . If the user does not touch the target after a predetermined period of time has elapsed, the target will disappear and then the target will reappear after a specified time period. This process continues for the duration of the training mode. 
     In particular, with reference to the screen image  900  depicted in  FIG. 39 , in the eighth illustrative training mode, the data processing device  814  is programmed to generate and display a plurality of visual objects on the output screen  812  of the visual display device  810  in a plurality of different sequential locations on the output screen  812 . For example, as shown in  FIG. 39 , a first target  902  that is generated by the data processing device  814  may appear on “tunnel” video background displayed on the output screen  812 . Advantageously, the “tunnel” video background is generated by the data processing device  814  so as to make the user feel as if he or she is immersed in the scene, moving either forward or backward depending on the movement of the concentric lines in  FIG. 39 . In the eighth illustrative training mode, the data processing device  814  may be programmed to control the point of origin  904  of the “tunnel” video background based on the center of pressure (COP) of the user determined from the force measurement assembly  822 , such that as the user moves, the center of the “tunnel” video background also moves. In the exemplary screen image  900  depicted in  FIG. 39 , the point of reference of the target  902  may either be fixed to the screen or fixed to the “tunnel” video background, which comprise concentric white and black circles in the exemplary case. When the concentric rings in the screen image  900  of  FIG. 39  are moving from the outer periphery to the center, then the user perceives the effect of moving away from the output screen  812  (i.e., moving backwards). Conversely, when the concentric rings in the screen image  900  of  FIG. 39  are moving from the center to the outer periphery, then the user perceives the effect of moving into the output screen  812  (i.e., moving forwards). Once the user touches the target  902  on the touchscreen user interface of the visual display device  810 , the data processing device  814  receives an input signal from the touchscreen user interface based upon an input response by the user. Then, the data processing device  814  is programmed to determine whether the user has performed the correct action by selecting the target  902 . After the user touches the first target  902  using the touchscreen user interface of the visual display device  810 , the target  902  disappears, and a second target appears on the output screen  812 . 
     In a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed to generate a visual target and ‘hit zone’ that are individually scalable. That is, the data processing device  814  is programmed so as to enable the radius about the visual target that registers a ‘hit’ by a user to be made smaller or larger than the actual image used for the visual target. In particular, in this further embodiment, the data processing device  814  is programmed to form each target from two separate images. The first image is the visual cue that the user selects. In this further embodiment, the first image may be a basic circle, a football, a smiley face, a flower, or any other suitable visual object. The second image generated by the data processing device  814  forms ‘sensitivity zone’, which is the area about the visual cue that the user can touch and it will register as a hit. The sensitivity zone can either be hidden or shown to the user when a test is running. Both of these images can be scaled independently of one another to adjust the behavior of the test. However, in this further embodiment, the sensitivity zone is never smaller than the visual cue, meaning that if a user ever touches the cue directly, it will register as a hit. In this further embodiment, the sensitivity zone generated by the data processing device  814  has four levels of scaling (i.e., 100%, 125%, 150%, 200%). These are factors of the size of the visual cue, so if the visual cue has a diameter of X, the smallest possible sensitivity zone is X, and the largest possible sensitivity zone is 2*X. Increasing the size of the sensitivity zone allows the user to be less accurate when touching a target, and still have it register as a hit. This is useful when focusing on the speed of a patient hitting the target and not necessarily the accuracy, or when using a visual cue that is small or difficult for the user to see. 
     In yet a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed to keep track of a number of different statistics regarding the user being trained and/or tested. For example, in this further embodiment, the data processing device  814  may be programmed to record each of the following parameters regarding the performance of the user: (i) the time to hit (i.e., how long the user takes to hit a target once it appears), (ii) accuracy (i.e., how close the user&#39;s touch is to the actual center of a presented target), and (iii) accuracy by zone (i.e., how accurate the user is in the various portions of the output screen  812  of the visual display device  810 ). In this further embodiment, the data processing device  814  is programmed so as to enable a user to view and compare all of these performance parameters across different tests and sessions. Also, in this further embodiment, there are two discrete events that are recorded by the data processing device  814  every time a target appears on the screen and the user touches it. These discrete events are: (i) time to hit and (ii) target accuracy. Supplementary data is stored with these discrete events to allow for a number of data comparisons and extrapolations. In this further embodiment, the ‘time to hit’ is the amount of time that passes from a target appearing and the user successfully pressing the target. In this embodiment, mis-hits (hitting the wrong target), are discarded and not stored. Likewise, in this embodiment, the very first target appearance in a test is not stored either. This is to account for start-up time, the patient getting acclimated to the test, and a ‘fuzzy’ reference as to when to actually begin timing. As such, timings are stored from the appearance of the second target until the end of the test. In addition to the timing, in this embodiment, a number of additional parameters are stored, such as the test type, the target position on the screen, target and sensitivity scaling, visual cue used, background type used, together with any test-specific options or parameters. In this further embodiment, the ‘target accuracy’ is linked closely to the target detection explained above. When a user successfully hits a target, that touch on the screen is at a given x/y coordinate (vector), this is compared to the centered position of the target and gives us a magnitude between the actual position of the target, and the location pressed by the user. The lower the magnitude, the closer to the ‘true center’ of the target that the user touched. As with the ‘time to hit’ explained above, all supplementary data is stored with this magnitude. Using the above events with their supplementary data, the data processing device  814  may be further programmed to generate useful data graphs and comparisons for the user. For example, in one or more embodiments, the data processing device  814  may be configured to generate data graphs, such as ‘Accuracy by Target Scaling’, ‘Time to Hit Target on the Left Side of Screen versus Right Side of Screen’, ‘Overall Accuracy with a Specific Visual Cue’, ‘Accuracy with and without Audio Cues’, etc. 
     In still a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed to ensure that none of the visual targets appear off the output screen  812  of the visual display device  810 . That is, the data processing device  814  establishes a ‘safe zone’ regardless of the screen size, resolution, or target size so that a visual target is not permitted to appear off the screen  812 . 
     In yet a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed so as to permit the user to create custom backgrounds and visual targets. In particular, the data processing device  814  is programmed so as to allow the user to add his or her own customized backgrounds and targets to an installation folder, then the data processing device  814  is programmed to add the customized backgrounds and targets to a list of available choices for the background and targets. For example, the user may upload one or more images containing customized background for use during the different training modes described above. In the illustrative embodiment, the images containing the customized backgrounds may be in .png or .jpg format, and be capable of fitting a resolution of approximately 1920×1080 pixels. If the image containing the customized background has a different native resolution, it is scaled accordingly. In addition to the customized backgrounds, the data processing device  814  also is programmed so as to allow the user to select from a plurality of standard backgrounds, such as solid color backgrounds, outdoor backgrounds depicting sports fields, such as baseball fields, football fields, and soccer fields, and indoor backgrounds depicting the interior of sports arenas, such as basketball arenas and ice hockey rinks. In the illustrative embodiment, similar to the customized backgrounds, the images containing the customized targets may be in .png or .jpg format, and be capable of fitting a resolution of approximately 1920×1080 pixels. If the image containing the customized target has a different native resolution, it is scaled accordingly. In addition to the customized targets, the data processing device  814  also is programmed so as to allow the user to select from a plurality of standard targets, such as targets having a simple geometric shape (e.g., circular targets and square targets) and targets in the shape of objects used in various sports (e.g., footballs, soccer balls, and hockey pucks). 
     In still a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed to calculate the reaction time of the user based upon a plurality of different input data parameters. In this further embodiment, the data processing device  814  is programmed to generate three stages for the reaction time of the user: (i) reaction to stimulus, (ii) time to stimulus, and (iii) accuracy. Also, in this further embodiment, the data processing device  814  is programmed to track three input modes of the user (i.e., touch down, lift, and travel time) individually so as to generate the composite score for the user. In addition, in this further embodiment, the reaction time is a training that can measure how fast a user can react to a visual and/or audial stimulus. In general, during one or more training modes described above, with reference to the timeline  870  depicted in  FIG. 37 , the user places his or her finger on a target at point  872 , some time passes, and then a second target appears at point  880  and the user has to move his or her finger from the first target to the second target as quickly as possible. In this embodiment, the training flow involves the following elements: (i) ready state, (ii) stimulus, (iii) reaction time, and (iv) flight time. During the ‘ready state’ condition  874  in  FIG. 37 , the user places and holds their finger on a designated target. This is analogous to ‘On your marks, get set’ to a sprinter. The ‘ready state’ condition  874  tells the software loaded on the data processing device  814  that the user is ready and the test actually begins at  876  in  FIG. 37  once the user has had their finger in place for 500 milliseconds (ms) (this duration is adjustable, but this is the default). In this embodiment, the stimulus appears at  880  in  FIG. 37  according to a few parameters. Once the ‘ready state’ time period  874  is concluded, the stimulus appears after an arbitrary amount of time  878  has passed (see  FIG. 37 ). This time may be constant and set beforehand, or a random amount of time between 500 milliseconds (ms) and 5 seconds (s) may be used, all of which are user selectable. In this embodiment, the ‘reaction time’  882  is the amount of time that passes from the stimulus appearing at  880 , and the user lifting their finger at  884  from the ready state to move to the next stimulus target (refer to  FIG. 37 ). In this embodiment, the ‘flight time’  886  is the amount of time that passes from the user lifting his or her finger at  884  in reaction to the next stimulus appearing, and the user actually pressing the stimulus at  888  in  FIG. 37 . In this embodiment, the data processing device  814  determines the total reaction score as a function of both the reaction time and flight time. As one example, the data processing device  814  may compute the total reaction score for the user based upon the following formula: 10−(Reaction Time+Flight Time). This formula gives a theoretical maximum (though impossible) score of 10, and the worst possible score of 0 (meaning it took more than 10 seconds for the user to react and travel to the target). As with all targets accuracy other supplementary data is tracked and stored. 
     In yet a further embodiment of the neurocognitive training system  800 , the data processing device  814  may be programmed to rank and score the training of the user based upon the output of the force measurement assembly  822  (i.e., based upon the user&#39;s center of pressure determined from the force measurement assembly  822  being in sync with a displaceable visual target on the output screen  812 ). For example, in this further embodiment, the user&#39;s center of pressure determined from the force measurement assembly  822  is compared to the displacement of a ball across the output screen  812  (e.g., the data processing device  814  determines if the user displaces his or her center of pressure in sync with the ball  866  displaced back-and-forth on the screen image  864  of  FIG. 36 ). In this further embodiment, the timing of the change of direction of the user also may be compared to the change in the direction of the stimulus (i.e., how long does it take for the user to react to the change in the displacement direction of the ball  866  on the output screen  812 ). 
     In still a further embodiment of the neurocognitive training system  800 , rather than determining the center of pressure of the user from the force measurement assembly  822 , the displacement of the center of gravity (COG) for the user may be estimated from one or more inertial measurement units attached to the head of the user. More specifically, in this further embodiment, the displacement of the center of gravity (COG) for the user may be estimated by the data processing device  814  using the position of the head determined from the one or more inertial measurement units attached to the head of the user (i.e., computed using the equation (17) below). For example, the height of the user may be entered into the data processing device  814 , and then using the entered height of the user, the center of gravity (COG) of the user may be estimated to be a predetermined distance (e.g., a specified number of inches) from the one or more inertial measurement units attached to the head of the user. Also, if the user is presumed to bend only at his or her ankles, the user may be modeled as an inverted pendulum by the data processing device  814 , thus permitting the postural sway displacement and/or the postural sway angle for the user to be determined by the data processing device  814  based upon the positional output data from the one or more inertial measurement units attached to the head of the user. For example, the average of the sway displacement of the user may be presumed to be the upright zero sway position of user, and then the deviations from the zero sway position of the user determined by the positional output data from the one or more head-mounted inertial measurement units may be used to determine the postural sway displacement and/or the postural sway angle. 
     In this further embodiment, the one or more inertial measurement units may comprise a triaxial (three-axis) accelerometer sensing linear acceleration {right arrow over (a)}′, a triaxial (three-axis) rate gyroscope sensing angular velocity {right arrow over (ω)}′, a triaxial (three-axis) magnetometer sensing the magnetic north vector and a central control unit or microprocessor operatively coupled to each of accelerometer {right arrow over (n)}′, gyroscope, and the magnetometer. In addition, the one or more inertial measurement units each may comprise a wireless data interface for electrically coupling the inertial measurement unit to the data processing device  814 . 
     Next, an illustrative manner in which the data processing device  814  of the neurocognitive training system  800  performs the inertial measurement unit (IMU) calculations will be explained in detail. In particular, this calculation procedure will describe the manner in which the orientation and position of the head of the user could be determined using the signals from the one or more inertial measurement units (IMUs) of the system  800 . As explained above, in this further embodiment, each of the one or more inertial measurement units may include the following three triaxial sensor devices: (i) a three-axis accelerometer sensing linear acceleration {right arrow over (a)}′, (ii) a three-axis rate gyroscope sensing angular velocity {right arrow over (ω)}′, and (iii) a three-axis magnetometer sensing the magnetic north vector {right arrow over (n)}′. Each inertial measurement unit senses in the local (primed) frame of reference attached to the IMU itself. Because each of the sensor devices in each IMU is triaxial, the vectors {right arrow over (a)}′, {right arrow over (ω)}′, {right arrow over (n)}′ are each 3-component vectors. A prime symbol is used in conjunction with each of these vectors to symbolize that the measurements are taken in accordance with the local reference frame. The unprimed vectors that will be described hereinafter are in the global reference frame. 
     The objective of these calculations is to find the orientation {right arrow over (θ)}(t) and position {right arrow over (R)}(t) in the global, unprimed, inertial frame of reference. Initially, the calculation procedure begins with a known initial orientation {right arrow over (θ)} 0  and position {right arrow over (R)} 0  in the global frame of reference. 
     For the purposes of the calculation procedure, a right-handed coordinate system is assumed for both global and local frames of reference. The global frame of reference is attached to the Earth. The acceleration due to gravity is assumed to be a constant vector {right arrow over (g)}. Also, for the purposes of the calculations presented herein, it is presumed the sensor devices of the one or more inertial measurement units (IMUs) provide calibrated data. In addition, all of the signals from the IMUs are treated as continuous functions of time. Although, it is to be understood the general form of the equations described herein may be readily discretized to account for IMU sensor devices that take discrete time samples from a bandwidth-limited continuous signal. 
     The orientation {right arrow over (θ)}(t) is obtained by single integration of the angular velocity as follows: 
                       θ   →     ⁡     (   t   )       =         θ   →     0     +       ∫   0   t     ⁢           ω   _     →     ⁡     (   t   )       ⁢   dt                 (   12   )                   θ   →     ⁡     (   t   )       =         θ   →     0     +       ∫   0   t     ⁢         Θ   →     ⁡     (   t   )       ⁢           ω   _     →     ′     ⁡     (   t   )       ⁢   dt                 (   13   )               
where {right arrow over (Θ)}(t) is the matrix of the rotation transformation that rotates the instantaneous local frame of reference into the global frame of reference.
 
     The position is obtained by double integration of the linear acceleration in the global reference frame. The triaxial accelerometer of each IMU senses the acceleration {right arrow over (a)}′ in the local reference frame. The acceleration {right arrow over (a)}′ has the following contributors: (i) the acceleration due to translational motion, (ii) the acceleration of gravity, and (iii) the centrifugal, Coriolis and Euler acceleration due to rotational motion. All but the first contributor has to be removed as a part of the change of reference frames. The centrifugal and Euler accelerations are zero when the acceleration measurements are taken at the origin of the local reference frame. The first integration gives the linear velocity as follows: 
                       v   →     ⁡     (   t   )       =         v   →     0     +       ∫   0   t     ⁢       {         a   →     ⁡     (   t   )       -     g   →       }     ⁢   dt                 (   14   )                   v   →     ⁡     (   t   )       =         v   →     0     +       ∫   0   t     ⁢       {           Θ   →     ⁡     (   t   )       ⁡     [           a   →     ′     ⁡     (   t   )       +     2   ⁢       ω   →     ′     ×         v   →     ′     ⁡     (   t   )           ]       -     g   →       }     ⁢   dt                 (   15   )               
where 2{right arrow over (ω)}′×{right arrow over (v)}′(t) is the Coriolis term, and where the local linear velocity is given by the following equation:
 
 {right arrow over (v)} ′( t )={right arrow over (Θ)} −1 ( t ) {right arrow over (v)} ( t )  (16)
 
The initial velocity {right arrow over (v)} 0  can be taken to be zero if the motion is being measured for short periods of time in relation to the duration of Earth&#39;s rotation. The second integration gives the position as follows:
 
                       R   →     ⁡     (   t   )       =         R   →     0     +       ∫   0   t     ⁢         v   →     ⁡     (   t   )       ⁢   d   ⁢   t                 (   17   )               
At the initial position, the IMU&#39;s local-to-global rotation&#39;s matrix has an initial value {right arrow over (Θ)}(0)≡{right arrow over (Θ)} 0 . This value can be derived by knowing the local and global values of both the magnetic north vector and the acceleration of gravity. Those two vectors are usually non-parallel. This is the requirement for the {right arrow over (Θ)} 0 ({right arrow over (g)}′,{right arrow over (n)}′,{right arrow over (g)},{right arrow over (n)}) to be unique. The knowledge of either of those vectors in isolation gives a family of non-unique solutions {right arrow over (Θ)} 0 ({right arrow over (g)}′,{right arrow over (g)}) or {right arrow over (Θ)} 0 ({right arrow over (n)}′,{right arrow over (n)}) that are unconstrained in one component of rotation. The {right arrow over (Θ)} 0 ({right arrow over (g)}′,{right arrow over (n)}′,{right arrow over (g)},{right arrow over (n)}) has many implementations, with the common one being the Kabsch algorithm. As such, using the calculation procedure described above, the data processing device  814  of the system  800  may determine the orientation {right arrow over (θ)}(t) and position {right arrow over (R)}(t) of one or more body portions of the user. For example, the orientation of the head of the user may be determined by computing the orientation {right arrow over (θ)}(t) and position {right arrow over (R)}(t) of two points on the head of the user (i.e., at the respective locations of two inertial measurement units (IMUs) disposed on the head of the user).
 
     In one or more alternative embodiments, rather than using one or more inertial measurement units (IMUs) that include an accelerometer, a gyroscope, and a magnetometer, a single accelerometer may be used to simply measure the displacement of the head of the user (e.g., by using equation (17) described above). As explained above, the acceleration output from the accelerometer may be integrated twice in order to obtain the positional displacement of the head of the user. 
     It is readily apparent that the neurocognitive training system  800  described above offers numerous advantages and benefits for various treatment and performance enhancement applications involving neurocognitive training and/or testing. For example, the neurocognitive training system  800  may be used to diagnose and/or treat the effects of a concussion or traumatic brain injury (TBI) for either preventative maintenance or as part of a return to play protocol. In particular, the abovedescribed training modes of the neurocognitive training system  800  may be used to improve performance as a preventative measure and/or objectify visual-motor performance as a tool to determine return to play readiness in concussed athletes or athletes who have sustained a TBI. As another example, the neurocognitive training system  800  may be used by an athlete seeking performance enhancement of the visual-motor system. Specifically, functional improvements resulting from training on the aforedescribed neurocognitive training system  800  will translate to enhanced vision and athletic performance. As yet another example, the neurocognitive training system  800  may be used as part of a rehabilitation regime for an orthopedic and/or neurological injury. In particular, the neurocognitive training system  800  may be used to provide a unique rehabilitation modality that requires controlled movement and functional performance. In addition, in such an application, the aforedescribed neurocognitive training system  800  may be used in conjunction with other rehabilitation tools, such as a stability apparatus. 
     While reference is made throughout this disclosure to, for example, “one embodiment” or a “further embodiment”, it is to be understood that some or all aspects of these various embodiments may be combined with one another as part of an overall embodiment of the invention. That is, any of the features or attributes of the aforedescribed embodiments may be used in combination with any of the other features and attributes of the aforedescribed embodiments as desired. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is apparent that this invention can be embodied in many different forms and that many other modifications and variations are possible without departing from the spirit and scope of this invention. 
     Moreover, while exemplary embodiments have been described herein, one of ordinary skill in the art will readily appreciate that the exemplary embodiments set forth above are merely illustrative in nature and should not be construed as to limit the claims in any manner. Rather, the scope of the invention is defined only by the appended claims and their equivalents, and not, by the preceding description.