Abstract:
An apparatus for determining a subject&#39;s threshold for perceiving acceleration includes a motion platform to execute motions and to receive a response to the executed motions from a subject on the motion platform and a feedback system in communication with the motion platform to receive the subject&#39;s responses, and to determine a next motion. The next motion has at least one feature determined based on the response of the subject to the executed motions. The apparatus also includes a controller connected to the motion platform and the feedback system to cause the motion platform to execute the motions defined by a motion set

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to the vestibular system, and in particular, to the diagnosis of vestibular dysfunction. 
       BACKGROUND 
       [0002]    The vestibular system of the inner ear enables one to perceive body position and movement. In an effort to assess the integrity of the vestibular system, it is often useful to test its performance. Such tests are often carried out at a vestibular clinic. 
         [0003]    Vestibular clinics typically measure reflexive responses like balance or the vestibulo-ocular reflex to diagnose a subject&#39;s vestibular system. The vestibulo-ocular reflex is one in which the eyes rotate in an attempt to stabilize an image on the retina. Since the magnitude and direction of the eye rotation depend on the signal provided by the vestibular system, observations of eye rotation provide a basis for inferring the state of the vestibular system. 
         [0004]    Measurements of eye movement are useful for diagnosing some failures of the vestibular system. However, some patients report perceptual vestibular problems and still test normal on standard diagnostic tests that assess the vestibulo-ocular reflex. This demonstrates that current diagnostic techniques do not adequately assess all aspects of vestibular function, especially perceptual aspects. 
         [0005]    The failure of vestibulo-ocular reflex measurement might result because reflexive vestibular responses and vestibular perception use different neural pathways. The failure may also arise because some disorders involve subtleties that are not assessed by measuring the vestibulo-ocular reflex. For example, vestibulo-ocular reflex tests typically assess responses to motions with relatively large amplitudes. But it may also be important to conduct tests having motions with smaller amplitudes. 
         [0006]    Therefore, existing devices and methods fail to assess and/or characterize perceptual responses evoked by vestibular stimulation, particularly in clinical settings. 
       SUMMARY 
       [0007]    In one aspect, the invention features an apparatus for determining a subject&#39;s threshold for perceiving acceleration. The apparatus includes a motion platform to execute motions and to receive a response to the executed motions from a subject on the motion platform and a feedback system in communication with the motion platform to receive the subject&#39;s responses, and to determine a next motion. The next motion has at least one feature determined based on the response of the subject to the executed motions. The apparatus also includes a controller connected to the motion platform and the feedback system to cause the motion platform to execute the motions defined by a motion set. 
         [0008]    In another aspect, the invention features a computer-readable medium having encoded thereon software for determining a subject&#39;s threshold for perceiving acceleration. The software includes instructions for causing a data processing system to carry out the steps of (a) commencing execution of a first motion set defining a sequence of motions; (b) recording responses of the subject to the motions in the sequence of motions defined by the first motion set; (c) determining that the responses of the subject satisfy a condition; (d) terminating execution of the first motion set; (e) at least in part on the basis of the subject&#39;s responses to the motions in the sequence of motions defined by the first motion set, defining a second motion set defining a sequence of motions; (f) commencing execution of the second motion set; and (g) recording responses of the subject to the motions in the sequence of motions defined by the second motion set. 
         [0009]    In another aspect, the invention features a method for determining a subject&#39;s threshold for perceiving a motion. The method includes steps: (a) commencing execution of a first motion set defining a sequence of motions; (b) recording responses of the subject to the motions in the sequence of motions defined by the first motion set; (c) determining that the responses of the subject satisfy a condition; (d) terminating execution of the first motion set; (e) at least in part on the basis of the subject&#39;s responses to the motions in the sequence of motions defined by the first motion set, defining a second motion set defining a sequence of motions; (f) commencing execution of the second motion set; and (g) recording responses of the subject to the motions in the sequence of motions defined by the second motion set. 
         [0010]    Embodiments may include one or more of the following features. The controller can be configured to cause the motion platform to execute a sinusoidal acceleration. The motion set can define an acceleration having a maximum magnitude and the feature includes the maximum magnitude. The feature can include a direction of the motion. Each motion can belong to a type selected from the group consisting of translation, rotation, and a combination of translation and rotation. The feedback system can detect the subject&#39;s vestibulo-ocular reflex. 
         [0011]    Embodiments may also include one or more of the following features. The method can also include determining a maximum magnitude and frequency for each motion of the first motion set before commencing execution of a first motion set. Determining that the responses of the subject satisfy a condition can include determining how many recorded responses are correct and comparing the number of correct responses to a threshold. Defining the second motion set can include determining a maximum magnitude and frequency for each motion of the second motion set based on the recorded responses. The method can also include examining the responses of the subject to the motions defined by the first motion set and determining the subject&#39;s threshold before commencing execution of the second motion set. The method can also include, after step (g), defining the second motion set to be a new first motion set, and repeating the steps (c) to (d). The method can also include determining a motion threshold on the basis of the responses of the subject; selecting a new frequency; and repeating the steps (a) to (g) for determining thresholds corresponding to the new frequency. Recording responses of the subject can include observing the subject&#39;s vestibulo-ocular reflex. 
         [0012]    All publications, patent applications, patents, and other references mentioned herein are incorporated by reference herein in their entirety. 
         [0013]    Other features and advantages of the invention will be apparent from the following detailed description, from the claims, and from the drawings, in which: 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of a vestibular test system; 
           [0015]      FIG. 2  is a schematic diagram of a motion platform in the vestibular test system of  FIG. 1 ; 
           [0016]      FIG. 3  is a flow chart exemplifying a method of a vestibular test using the vestibular system of  FIG. 1 ; 
           [0017]      FIG. 4  is a motion list describing a motion set; 
           [0018]      FIG. 5  is a portion of a feedback list; 
           [0019]      FIG. 6  shows a time series of acceleration amplitude A; 
           [0020]      FIG. 7  shows a motion package; 
           [0021]      FIG. 8  shows a portion of statistical results; and 
       
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       [0022]    Referring to  FIG. 1 , a vestibular test system  10  includes a motion platform  12 , a controller  14 , and a feedback system  16 . 
         [0023]    The motion platform  12  holds a subject whose vestibular system is to be tested, and moves that subject in response to instructions from the controller  14 . Generally, each motion provided by the motion platform  12  is described by a motion profile that includes information about the direction of motion and other features related to the motion. For example, a motion can be a translational motion along any of the three perpendicular axes x, y, and z of a coordinate system centered on the motion platform  13 . Generally, the z axis is vertical to the ground on which the motion platform  12  rests, and the x and y axes define a plane parallel to the ground. In some embodiments, the motion platform  12  is arranged so that the subject is oriented with his spine parallel to the z axis. 
         [0024]    The motion platform  12  can also provide various rotational motions such as a roll, which is a rotation about the x axis; a pitch, which is a rotation about they axis; and a yaw, which is a rotation about the z axis. 
         [0025]    In some embodiments, the motion profile includes the amplitude and frequency of the velocity and acceleration of the motion. The amplitude of the acceleration and velocity vary with time, whereas the frequency remains constant. For example, a translational motion starts with a zero velocity, accelerates to a maximum velocity, and decelerates to zero again. In some embodiments, the acceleration is sinusoidal and can be expressed as 
         [0000]        a ( t )= A  sin(2πƒ t )   (1)
 
         [0000]    where a(t) is the acceleration at time t, A is the acceleration amplitude, and ƒ is the frequency. With such an acceleration, starting from zero, the translational velocity v(t) at time t is 
         [0000]        v ( t )= A /(2πƒ)[1−cos(2πƒ t )]  (2)
 
         [0000]    Similarly, a rotational motion can include a sinusoidal angular acceleration and an angular velocity, both of which are expressed in a manner similar to the translational acceleration and velocity of equations (1) and (2). 
         [0026]    Referring to  FIG. 2 , a motion platform  12  moves a subject  17  seated thereon according to motion profiles, as discussed above. A suitable motion platform  12  is a MOOG series 6DOF2000E (manufactured by Moog Inc., East Aurora, N.Y.). 
         [0027]    The motion platform  12  includes an actuator to allow the subject  17  to unambiguously communicate his perception of motion. A suitable actuator is a pair of buttons  19   a,    19   b.  However, the actuator can also be a joystick, a pair of joysticks, a pair of switches, or even foot pedals. 
         [0028]    In some embodiments, the subject  17  communicates his perception of motion to an operator by speaking or by gesture. In such cases, the operator provides a suitable input to an actuator, for example by using a keyboard to record data indicative of the subject&#39;s perception of motion. 
         [0029]    During a test, the subject  17  presses one of the buttons  19   a,    19   b  to indicate that he perceives motion. The particular button pressed indicates the subject&#39;s perception of the motion&#39;s direction. The number of times that the subject  17  presses the correct button  19  provides a basis for assessing his vestibular system. To avoid confusing the vestibular system with false signals caused by the subject&#39;s own head motion, an adjustable head brace  21  holds the subject&#39;s head in place such that it is bisected by the xz plane. 
         [0030]    Referring back to  FIG. 1 , when the subject  17  presses a button  19   a,  the feedback system  16  transmits a corresponding response signal to the controller  14 . In some embodiments, the feedback system  16  includes two buttons. The subject can press either button to indicate his perception of the motion he is subjected to. 
         [0031]    The choice of which button to press and when to press it is based on a pre-agreed rule between the subject  17  and a test operator. For example, the subject can agree to press the first button upon perceiving an upward translational motion and to press the second button upon perceiving a downward translational motion. The rule can be adjusted upon agreement between the subject and the test operator. 
         [0032]    The buttons  19   a,    19   b  are electronically connected to a processor that records, on a feedback list  27 , which buttons were pressed and what motion was occurring at the time. Periodically, the processor  25  examines the feedback list  27  and causes the controller  14  to determine the motion profile of the next motion to be applied to the subject  17 . In this way, the vestibular test system  10  automatically and adaptively changes motion profiles in response to the subject&#39;s perception of motion. 
         [0033]    The processor  25  determines the motion profile parameters, such as acceleration amplitude A, velocity amplitude A/πƒ, frequency ƒ, and direction, on the basis of the feedback list  27 . In response to instructions from the processor  25 , the controller  14  then causes the motion platform  12  to move in a manner consistent with the determined motion profile. In some embodiments, the controller  14  includes a microprocessor or a computer. 
         [0034]    Referring to  FIG. 3 , a vestibular test  18  using the vestibular test system  10  in  FIG. 1  determines a subject&#39;s motion threshold by moving the subject in a manner defined by the motion set. Each motion set includes several motions, each characterized by a motion profile. In the embodiment described herein, each motion profile characterizes a sinusoidal acceleration with a uniform frequency ƒ and acceleration amplitude A. The subject&#39;s motion threshold at a particular frequency ƒ is the acceleration amplitude A at which the subject perceives motion at that frequency. 
         [0035]    The vestibular test  18  starts with a set-up step  20  in which the subject  17  is seated and stabilized on the motion platform  12  at the initial position. 
         [0036]    The setup step  20  is followed by a training step  24 , the purpose of which is to enhance the likelihood that the subject accurately communicates his perception of motion by pressing the correct button  19 . The training step  24  includes trial tests to train the subject  17  to press the correct button in response to perceiving a particular motion. To assist the process, the training step  24  can be performed in a lighted room to enable the subject  17  to use vision to help perceive the motion. 
         [0037]    To minimize the influence of non-vestibular cues regarding motion direction, the remaining steps of the vestibular test  18  are performed without any visual cues indicating the motion. For example, the movement may be applied in the dark or with a visual display, e.g., a small light emitting diode, that moves with the subject during any motion. To reduce reliance on wind cues, the subject&#39;s skin surfaces are covered, for example, with long sleeves, or gloves, and a visor is attached to the head holder  21 . To reduce reliance on audio cues, the subject  17  is provided with earplugs to reduce the external noise by about 20 dB, and exposed to white noise (circa 60 dB). Reliance on tactile cues is minimized by evenly distributing pressure via padding. 
         [0038]    Multiple motion sets are provided to the subject  17  to obtain a motion threshold for the subject  17  at a selected frequency ƒ. In one embodiment, each motion set includes motions of the same type. For example, a motion set would include nothing but yaw motions, or nothing but pitch motions. 
         [0039]    Referring to  FIG. 4 , a motion set  26  is described by a list of motion profiles  50   a - e , which defines a sequence of motions. In  FIG. 4 , there are five motion profiles  50   a - e  corresponding to five translations along they axis. Each motion profile  50   a - e  is characterized by a direction, an acceleration amplitude A, and the selected frequency ƒ. In some embodiments, all motion profiles  50   a - e  in the motion set  26  have the same acceleration amplitude A and frequency ƒ, but each has a random direction. For example, the first motion profile  50   a  of the motion set  26  is a translation in the positive direction of the x axis, and is labeled as “+x.” The second motion profile  50   b  is a translation in the negative direction along the x axis, and is labeled as “−x”. The direction, “+x” or “−x”, for each motion profile  50   a - e  of the motion set  26  is randomly assigned. 
         [0040]    The acceleration amplitude A is determined based on the subject&#39;s responses to previous motion sets  26  applied to the subject  17 . For example, when the motion set  26  is the first motion set to be applied to the subject  17  along a particular axis or about a particular axis, the acceleration amplitude A is chosen to be about 10 to 20 times larger than the motion threshold of a normal subject for that particular type of motion. 
         [0041]    When the motion set  26  is not the first motion set to be applied to the subject  17 , the acceleration amplitude A is determined based on the previous motion sets applied. The motion profiles  50   a - e  comprising a motion set  26  are determined prior to applying, to the subject  17 , the motions specified in the motion set  26 . 
         [0042]    Typically, before beginning each motion, an alert is sent, for example, in the form of a tone or light, to the subject  17  to notify the subject that a motion is going to start. In some embodiments, after each motion ends, another alert is sent to the subject  17  to notify the subject  17  that a response is now required if the subject has not responded already. In cases where the acceleration amplitude was too low for the subject  17  to detect any motion, this response would essentially be a guess. 
         [0043]    The response of the subject  17  to each motion of the multiple motion sets, together with the motion&#39;s direction, for example, “+x” for positive direction of x axis and “−x” for the negative direction as described in  FIG. 4 , acceleration amplitude A and frequency ƒ are recorded in a feedback list  28 , as shown in  FIG. 5 . The result of the subject&#39;s perception of each motion can be marked in the form of numbers, alphabets, or symbols, for example, “0” for failure and “1” for correct perception. 
         [0044]    Referring back to  FIG. 3 , following creation of a motion set (step  28 ), the controller selects the first motion profile of that motion set (step  30 ) and causes the motion platform  12  to execute the motion defined by that motion profile (step  32 ). This motion set is referred to as the “incumbent motion set.” 
         [0045]    If the subject&#39;s perception is correct (step  34 ), the feedback list, in which the subject&#39;s responses have been accumulated, is inspected to determine the total number T c  of correct results for the incumbent motion set (step  52 ). If the total number (T c ) of correct results is less than a threshold τ c  (step  54 ), then the next motion in the incumbent motion set is selected (step  56 ) and applied to the subject (step  32 ). 
         [0046]    If T c  is equal to the threshold τ c , then the test is too easy for the subject, and no further motions are selected from the incumbent motion set. As a result, in some cases execution of the motion set is terminated prior to completing the sequence of motions defined by the motion set. Instead of completing the sequence of motions, a decreased acceleration amplitude A −  is calculated (step  58 ) to make the test more difficult. This process is repeated until a subject provides an incorrect response. 
         [0047]    In one embodiment, the decreased acceleration amplitude will be 50% of the acceleration amplitude at which the total number of correct responses reached the threshold. For other embodiments, the decreased acceleration amplitude might be some other fixed percentage (e.g., 75%) of the most recent acceleration amplitude at which the total number of correct responses reached the threshold. 
         [0048]    In yet other embodiments, the reduction in the acceleration amplitude may equal 50% of the most recent increase in the acceleration amplitude. Other embodiments may decrease the acceleration amplitudes by other percentages (e.g., 25%) of the most recent increase in the acceleration amplitude. 
         [0049]    If, on the other hand, the subject&#39;s response is wrong (step  34 ), the feedback list is inspected to determine the total number T w  of wrong results recorded thus far for the incumbent motion set (step  36 ). 
         [0050]    If T w  is less than a threshold τ w  (step  38 ), then the next motion profile in the motion set is selected (step  40 ) and its corresponding motion applied to the subject (step  32 ). If T w  is equal to the threshold τ w , the test is assumed to be too difficult for the subject, and no further motion profiles are selected from the incumbent motion set. Instead, to make the test easier, an increased acceleration amplitude A +  is calculated (step  42 ). As a result, in some cases, execution of the sequence of motions is terminated prior to completion of all motions in the motion set. 
         [0051]    In some embodiments, the increase in the acceleration amplitude might equal some percentage (e.g., 50%, 60%, 70%, etc.) of the difference between the current acceleration amplitude and the acceleration amplitude at which the subject last correctly identified the direction of motion. 
         [0052]    The test can terminate if the difference ΔA between the original acceleration amplitude A and either the decreased acceleration amplitude A −  or the increased acceleration amplitude A +  is smaller than a threshold Δc (step  44 ). When this occurs, the vestibular test is regarded as having been completed for the selected frequency and type of motion, and the motion threshold for the subject is determined on the basis of the acceleration amplitude, for example by evaluating the mean of the original amplitude (step  46 ) and either the increased or decreased amplitude. The vestibular test then proceeds to a new selected frequency and/or motion type (step  48 ). 
         [0053]    Alternatively, the test can terminate upon occurrence of a pre-defined number of local minima in the sequence of acceleration amplitudes used during the test. 
         [0054]    Referring to  FIG. 6 , the acceleration amplitude applied to the subject  17  varies depending on whether the subject  17  responds correctly or incorrectly. In general, when the subject  17  responds correctly, the amplitude decreases; and when the subject responds incorrectly, the amplitude increases. As the test progresses, the resulting time series of acceleration amplitudes naturally develops maxima and minima. For example, the particular time series shown in  FIG. 6  features two local minima, one at the third motion test, the other at the sixth motion test. 
         [0055]    As is apparent from  FIG. 6 , the subject  17  responded correctly to the tests in which the acceleration amplitude progressively decreases from A 0  to A 1  and responded incorrectly when the acceleration amplitude was further reduced to A 2 . The subject  17  further responded correctly to tests with acceleration amplitudes A 1 &gt;A 3  and failed the next test with acceleration amplitude A 4 . A local minimum in the time series is thus formed each time a subject responds incorrectly after having responded correctly two or more times. 
         [0056]    In one embodiment, the test would terminate if the acceleration amplitude yielding the incorrect response was the 2 nd  local minimum in the sequence of acceleration amplitudes tested. Other embodiments might terminate at the 3 rd  local minimum or the 4 th  local minimum. 
         [0057]    In an alternative embodiment, the increased acceleration amplitude would be an acceleration amplitude at which the subject most recently attained a particular score, i.e., a particular number of correct answers. By using this acceleration more than once, one can average out the effects of noise or other variations that may otherwise corrupt the test results. In such an embodiment, the test  18  skips the step  44  of  FIG. 3  and continues with step  28  directly. To complete the test  18 , each acceleration amplitude is used in, for example, less than two, three, or four motion sets. 
         [0058]    The method described in  FIG. 3  can be used to systematically determine other motion thresholds at different frequencies for the same or other types of motions, including translation, pitch, roll, yaw, or a combination thereof. For each type of motion, the relationship between the motion thresholds and the motion frequency ƒ defines a vestibulogram for the subject  17 . Because of the systematic manner in which the method acquires such information, and the minimal intervention required, the method is particularly suited for clinical use. Moreover, the method described herein can be used to efficiently collect data to create a graph of motion threshold as a function of frequency, referred to herein as a “vestibulogram,” for each of several motion directions. 
         [0059]    The determination or calculation of the increased acceleration amplitudes A +  and A −  can vary. In some embodiments, when the feedback list  28  indicates that the subject  17  has yet to experience an acceleration amplitude greater than the last amplitude A, A +  is set to be, for example, 60%, 50%, 40%, 30%, or 20%, greater than A. In other cases, the feedback list  28  indicates that the subject  17  has experienced an acceleration amplitude A 0  that is greater than the most recently used acceleration amplitude A but less than all other acceleration amplitudes recorded on the feedback list  28 . When this occurs, the increased acceleration amplitude A +  is set to be between A and A 0 , for example, 60%, 50%, 40%, 30%, or 20% of (A 0 −A) greater than A. 
         [0060]    Conversely, when the feedback list  28  indicates that the subject  17  has yet to experience an acceleration amplitude less than the most recent acceleration amplitude A, A −  is set to be, for example, 60%, 50%, 40%, 30%, or 20%, of the most recent acceleration amplitude A. In some cases, the feedback list  28  includes an acceleration amplitude A 0  that is less than the most recent acceleration amplitude A but greater than all other acceleration amplitudes recorded on the feedback list  28 . When this occurs, the decreased acceleration amplitude A −  is set to be between A and A 0 , for example, 60%, 50%, 40%, 30%, or 20% of (A−A 0 ) less than A. 
         [0061]    The thresholds τ w , τ c , and Δc can also vary with different embodiments. For example, there exist embodiments in which τ w  is 1, 2, or 3. There also exist embodiments in which τ c  is 2, 3, or 4. Additional embodiments include those in which Δc is 5%, 4%, or 3% of the last acceleration amplitude A. However, these are by no means the only criteria for stopping the current motion set. For example, when the subject does not correctly perceive the motion applied, T w  can instead represent the total number of sequential wrong results on the feedback list, while T c  represents the total number of correct results on the feedback list. 
         [0062]    In other embodiments, each of the motion sets  26  includes motion profiles that define motions of different types. For example, a motion set can have some motion profiles defining a pitch, while other motion profiles define a yaw. Or, a motion set can have motion profiles defining motions with different acceleration amplitudes A. Or, a motion set can have motion profiles defining motions with different frequencies ƒ. In such embodiments, the criteria for creating and applying new motion sets to the subject to complete the vestibular test vary according to different motion profiles chosen for the motions within the same motion set. 
         [0063]    In some embodiments, a coarse motion threshold A c  of an subject can be determined using the vestibular test of  FIGS. 1-3  using relatively low thresholds τ w , τ c , and Δc. For example, τ w  equals 1, τ c  equals 2, and/or Δc is 25% of the last acceleration amplitude A. A follow-up vestibular test based on the coarse motion threshold A c  can then be used to determine a final motion threshold A ƒ . The follow-up vestibular test is conducted using the same test system disclosed in connection with  FIGS. 1 and 2  but with motion sets that differ from those discussed in connection with  FIG. 4 , and methods that differ from those disclosed in connection with  FIG. 3 . 
         [0064]    Referring to  FIG. 7 , the follow-up vestibular test applies, to the subject, a motion package  54  that includes k motion sets. Each of the k motion sets includes a total of p motions, each of which is characterized by a direction of the motion, e.g., “+x” or “−x” and an amplitude A of the motion. To generate each motion set, one first generates a standard motion set  52   a  having p motions without motion direction assignment. The motion amplitudes of the motions in the standard motion set  52   a  are chosen to define a range between A c  and A c +nΔA, where n is an integer and ΔA is an amplitude change, for example, 2% of A c . Each amplitude can appear repeatedly within the standard motion set  52   a  so that p≧n+1. Each motion set, for example, motion set  52   b,  of the motion package  54  is generated by randomly selecting p motions from the sequence of the motions in the standard motion set  52   a  and randomly assigning a direction to each motion within each motion set. 
         [0065]    The subject responds to each motion of the motion package  54 . The number of right answers and wrong answers for each amplitude is then recorded in an answer table  56 , as shown in  FIG. 8 . For example, the illustrated answer table  56  indicates that when a total number of k motions with amplitudes A c  was applied, the subject correctly perceived q of them and incorrectly perceived k-q of them. The statistical results in the answer table  56  can be analyzed using a generalized linear model or averaged normal cumulative distribution to yield a more refined estimate of the final motion threshold A ƒ . Detailed information for the generalized linear model is provided by McCullagh P, Nelder J A (1983),  Generalized Linear Models.    
         [0066]    In other embodiments, the vestibular test described in  FIGS. 1-3  can be modified to be adapted for VOR analysis to use the vestibulo-ocular reflexes as an indicator of the subject&#39;s perception of motion. For example, the subject can be positioned on the motion platform  12  of  FIG. 2  and be subjected to the motion tests of  FIG. 3 . However, instead of pushing buttons  19  ( FIG. 2 ) at the end of each motion to indicate the perception of motion, the direction of the subject&#39;s reflexive eye movements can be measured and compared to the direction of the motion applied. The result of the comparison after each motion list is recorded on a feedback list similar to the feedback list  28  of  FIG. 5  and analyzed using methods identical to those described earlier. 
         [0067]    Having described the invention, and a preferred embodiment thereof, we claim, as new and secured by Letters Patent: