Abstract:
A system and method for performing a vision examination includes displaying a series of visual stimuli in a line by line pattern display for observation by a patient and detecting the patient&#39;s visual evoked potential in response to the visual stimuli. Electrical signals representative of the visual evoked potentials for each stimulus of each series of visual stimuli displayed is converted to digitized data, recorded and measured. The measured evoked potential data is then evaluated and compared to certain predetermined values in order to detect whether or not the measured data is reliable. The presentation of visual stimulus is synchronized with the rate of sampling the responsive visual evoked potential signals by determining if a predetermined specific line of the line by line display has been displayed and by generating interrupt request signals to a computer in order to initiate the sampling of the visual evoked potential signals and to reflash the contents of a video RAM for the next frame of image displays.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of medical examination for diagnosis and more particularly, to a system and method for audio and visual testing by detecting and measuring evoked potentials as a result of perceived sensory stimuli. In particular, the invention is directed to a system and method for vision examination using evoked potential in which the presentation of periodic sensory stimulus and the sampling rate for recording evoked potential responsive signals are synchronized. 
     BACKGROUND 
     It is common in the field of medical examinations to assist in diagnosis to conduct various types of tests, whether conducted within a hospital environment, laboratory or physician&#39;s office. Such tests can range from simple patient observation to the use of complex examination and diagnostic equipment in which electrical stimuli are applied to a patient and the resulting electrical response signals from the patient are recorded, measured and analyzed. An electrocardiogram is an example of such an examination and diagnostic test in which electrical response signals from stimuli are recorded and analyzed. Electrocardiogram signals are indicative of a patient&#39;s heart condition and may be used to detect a heart attack or other cardiac condition. Another familiar medical diagnostic test is an electroencephalogram which uses stimuli to generate electrical signals from the brain of a patient which can be measured in the form of electrical potentials (referred to as evoked potentials) and which indicate the patient&#39;s brain activity. Data recorded from an electroencephalogram test are useful for determining such things as seizures or to assist the physician in the diagnosis of brain damage. Other examination procedures also use evoked potentials for diagnosing a variety of other diseases, including diseases of the central nervous system, auditory system and the visual system. Evoked potentials are typically determined by measuring electrical responses to sensory stimuli. When stimulation is applied to a particular sense of a human being, a corresponding brain potential is evoked at an information processing part of the brain that functions to manage the particular sense. Such evoked brain potentials are usually detected and measured by detecting electrical signals using electrodes positioned on the skin of the human head in the area of the information processing center of the brain corresponding to the particular sense involved. 
     Visual evoked potentials (VEP) are the evoked potentials in response to visual stimulation and are particularly useful to assist in diagnosing ophthalmic diseases in infants and young children, because such individuals are not always able to indicate responsiveness to visual stimuli or to verbalize the occurrence of vision failure. The retina contains more than 130 million light-receptor cells. These cells convert light into nerve impulses that are processed for certain features, which are transmitted by the optic nerve to the brain, where they are interpreted. Muscles attached to the eye control its movement. Birth defects, trauma from accidents, disease and age-related deterioration of the components of the eye can all contribute to eye disorders. Information processing in the brain is electrochemical in nature. Evoked potentials are the electrical responses of the brain elicited by sensory stimulation. The electrical responses of the brain produced by visual stimulation are visual evoked potentials. Changes in these visual evoked potentials can be used to pinpoint anomalies along the visual pathways. These visual pathways are interconnected linkages of cells, beginning with photoreceptor cells in the retina, passing through horizontal cells, bi-polar cells and amacrine cells to ganglion cells, which wind together to form optic nerve fibers leading to cells in the brain&#39;s thalamus which then leads to cells in the visual cortex. The retina does not register images and transmit them, unaltered, to the brain. Instead, it selects and abstracts biologically useful features of information in the patterns, which strike it, and transmits a selectively filtered message to the brain by means of interactions within and among neural networks. Further processing of the information then takes place in the brain by means of similar but more complex interactions in neural networks there. Anomalies in this electrical transmission are variations from the expected pattern in the reaction of cells along the visual pathways. They are believed to provide useful insight into many diseases and conditions affecting the brain, central nervous system, the eye and the ear. Therefore, one way to detect possible visual impairment in infants and small children is to record and measure visual evoked potentials in response to visual stimulation. Visual evoked potential analyzers can be used in screening for diseases and conditions of the brain, central nervous system, the eye and the ear. They detect abnormalities in the functioning of a patient&#39;s brain by analyzing the electrical responses of the brain, which occur when certain rapidly changing patterns of light displayed on a video screen, are viewed. These electrical responses are called potentials. Sensors attached non-invasively to the scalp permit measurements of visual evoked potentials and are widely used in basic research in vision and as an aid in the diagnoses of neurological and ophthalmic disorders. However, since these sensors record visual evoked potentials from large areas of the brain, relating changes in these recorded waves to specific neural processes has previously proven difficult or impossible. 
     Visual evoked potential systems have heretofore been used to test infant response to visual stimuli in order to determine the possible presence of amblyopia. Failure to detect amblyopia as early in life as possible could lead to incurable vision problems in adulthood. However, if detected early, amblyopia can be effectively treated. Accordingly, it has been found desirable to conduct visual evoked potential tests on infants and other humans. One such system, known as the VENUS System, was heretofore commercialized by Neuroscientific Corp. and is described in an article entitled “An Electrophysiological Technique for Assessment of the Development of Spatial Vision,” Optometry and Vision Science, Vol. 74, No. 9, Sep. 9, 1997. 
     Ear infections, or otitis media, are a major reason for doctor visits among preschoolers in the U.S., accounting for more than 24 million trips a year to the family physician. The problem is a serious one with the treatment of children under two years of age for such infections having tripled between 1975 and 1990. Children who suffer from repeated ear infections before age six often experience temporary hearing loss, speech and language delays, coordination difficulties and, in some cases, permanent hearing loss. Likewise many infants, about 4 in 1000 births, have hearing impairment problems that cause delays in speech, language and cognitive development. In many instances, hearing loss is not detected until the child is two to three years old and not speaking properly. The present invention can also be adapted to screen for malfunctions in hearing among infants and children thereby providing early detection for both eye and ear problems. 
     A concern relating to testing or examinations using VEP is that the sampling rate of the VEP responses might not be synchronized with the periodic presentation of the visual stimulus. Failure to achieve such synchronization will make it difficult to read and interpret the resulting data and may lead to unreliable results as well as a possible wrong diagnosis. VEP responses that are not synchronized with the sampling rate require subjective, and therefore, unreliable analysis. Methods heretofore used to achieve such synchronization have employed external triggering signals to initiate data sampling, and therefore have lacked the capability of flexibility in the sampling rate and have resulted in large systems that were not compact and appealing for use by technicians and other medical personnel. 
     OBJECTS OF THE INVENTION 
     It is accordingly an object of the present invention to provide a system and method of synchronizing the VEP sampling rate with the presentation of the visual stimulus that overcomes the disadvantages of the prior techniques. 
     A more specific object of the present invention is to provide a system and method of synchronizing the VEP sampling rate with the presentation of visual stimulus by determining if a particular line of a line by line display of the stimulus pattern has been displayed and by generating interrupt request signals to the CPU of the computer that controls both the display of the stimuli and the sampling of VEP data in order to initiate and conduct sampling of-VEP data and to reflash the contents of a video RAM for the next frame of image displays. 
     Yet a further object of the present invention is to provide a system and method of synchronizing the VEP sampling rate with the presentation of the visual stimulus which utilizes commercially available components for generating interrupt signals and line counting. 
     Other objects, features and advantages of the invention will be apparent to those skilled in the art after appreciating the invention from the description hereinbelow. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore directed to a system and method for performing vision examinations which includes displaying a series of visual stimuli in a line by line pattern display for observation by a patient and detecting the patient&#39;s visual evoked potentials in response to the visual stimuli. Electrical signals representative of the visual evoked potentials for each stimulus of each series of visual stimuli displayed is converted to digitized data, recorded and measured. The presentation of the visual stimulus is synchronized with the rate of sampling the responsive visual evoked potential signals by determining if a particular line of the line by line display has been displayed and by generating interrupt request signals on a computer in order to initiate and conduct the sampling of the visual evoked potential signals and to reflash the contents of a video RAM for the next frame of image displays. 
     The foregoing and other features, objects and advantages of the present invention are more fully described with reference to the following drawings annexed hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the overall architecture of the present invention; 
     FIG. 2 is an illustration of one type of visual stimulus used in the present invention; 
     FIG. 3 is a block diagram illustrating a prior art architecture of a synchronization technique; 
     FIG. 4 is a block diagram illustrating the architecture of the synchronization system of the present invention; 
     FIG. 5 is flow chart illustrating the process of synchronizing the visual evoked potential sampling rate with the visual stimulus; and 
     FIG. 6 is a flow chart illustrating the process of the interrupt service routine used in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Generally, the system of the present invention incorporates a visual stimulator, data acquisition means with amplifiers to enhance VEP signals and a monitor to view results. Specifically, the system  10  of the present invention, as depicted in FIG. 1, includes a visual evoked potential recording and measuring device (or data acquisition component)  11  coupled to a central processing unit of a computer  14  for controlling the operations and functions of the VEP recording and measuring device  11 . VEP recording and measuring device  11  includes an amplifier  12  for enhancing the VEP signals being acquired and an analog to digital converter  13  for converting the signals into a form for digital use. The amplifier is optically isolated for patient safety, has a high common mode rejection ratio, low noise and stability for low signal and frequency application. Computer  14  is coupled to a monitor  15  for displaying the data captured by the VEP recording and measuring device  11  and for providing a means to convey information concerning the operation of a test being conducted on a patient  17  to an operator. Keyboard  16 , also connected to computer  14 , provides a means to input information to the computer relating to a subject being tested. The responses in the brain to the stimuli are picked up by sensors attached non-invasively to the patient&#39;s scalp and are amplified, digitized, recorded and analyzed by the data acquisition component. Disposable electrodes  20 ,  21  and  22  are positioned on the scalp of the patient  17  over the visual cortex, the frontal cortex, and the parietal cortex, respectively. Electrodes  20 ,  21  and  22  are connected via hard wire to the VEP recording and measuring device  11 . A visual stimulus generating device  23  is also connected to and controlled by computer  14  for generating visual stimuli to be perceived by the patient. A hood  24  may also be used and positioned between the stimulus generating device  23  and the patient  17  in order to enhance attention by the patient to the visual stimulus being displayed. The stimuli are highly variable arrays of one or two-dimensional light patterns. These stimuli can be rapidly modified or varied (i.e. at ˜60 Hz or higher frame rate) and luminance contrast and main luminance can be altered through a full range of the gray scales from white to black. By permitting different regions of a pattern to be varied independently or contrasted with a static area, the system can make specific and detailed measurements of visual evoked potentials and perform all of the conventional visual evoked potential tests. 
     The present invention incorporates software carried by computer  14  for displaying a graphical user interface (GUI) on the monitor  15  upon initiation of the examination procedures. The GUI allows an operator to enter patient data such as name, date of birth, identification number, etc. After the patient data is entered into the system, the vision examination will begin with the presentation of visual stimuli on the stimulus display screen  25  of the stimulus device  23  for observation by the patient. The generation of the visual stimuli by device  23  is initiated and controlled by computer  14 . The stimuli consists of sweeps of variable spatial patterns in the form of horizontal gratings that vary from thick to thin with the presentation of each such pattern lasting approximately one second. FIG. 2 illustrates an example of the type of stimulus display presented to the patient. The display consists of a pattern  30  of alternating and contrasting horizontally oriented light bands  18  and dark bands  19 . The system of the invention causes the stimulus generating device  23  to present a series of patterns on the visual display screen  25 . A series typically consists of six different patterns to be displayed. The display of each pattern lasts approximately one second. Each pattern differs from other patterns by the thickness of each band. In presenting the series, the first displayed pattern will have the thickest bands and each successive pattern displayed will have narrower bands. A number of sets of displays (each set consisting of five sweeps of a series) will be presented to the patient for observation by each eye of the patient. A set consists of five sweeps of a series, where a sweep is the continuous consecutive display of the six patterns of a series. By entering commands on an input device, such as keyboard  16  connected to computer  14 , an operator may vary the series and sets of displays and after each series or set is complete, the operator can initiate presentation of a next series or set of displays. A set of displays will be presented to each of the eyes of the patient. Tracking information about each visual stimulus display will be presented on monitor screen  15  so that the operator will be able to track display activity. Upon completion of presentation of the sets of visual stimuli, the system of the invention will present the results of the tests on monitor screen  15 . 
     A feature of the invention is the synchronization of the periodic visual stimulus and the sampling rate for recording the VEP signal responses using computerized technology. This method of synchronization includes use of interrupt request signals (IRQ) that are generated from a computer graphics card and an interrupt handling routine to sample the VEP responses and display the stimulus frame by frame at the same time. The IRQ signals generated from the graphics card are horizontal or vertical synch related signals. Thus, the IRQ signals are synchronized with the monitor frame rate. Using the IRQ signal as a timing control, the synchronization between the period of the stimulus and the VEP sampling rate is achieved. 
     Visual evoked potential signals, generated in response to visual stimulus, are digitized by the analog to digital converter  13  for computer processing. In order to analyze the VEP steady state response in a frequency domain and minimize the calculation error in the VEP Fourier components, the analog to digital conversion VEP sampling rate must be synchronized with the stimulus display. Once a periodic stimulus is presented to a subject, the VEP produced has a transient procedure at a certain starting period, for example 100 ms. After this period the VEP response reaches its steady state, that is, the response statistically becomes a repeatable periodic signal. A periodic signal can be expressed as a sum of sinusoids with various amplitudes, phases and multiple frequencies. These frequency components are referred to as Fourier components. 
     A conventional way to implement synchronization involves the use of triggering signals generated from a proprietary computer graphics card to activate analog to digital conversion of the VEP sampling. Such conventional technique is illustrated in FIG.  3 . In this technique, a proprietary computer graphics card  350  interfaces with an analog to digital (also referred to as “ADC”) conversion card  351 . ADC card  351  and graphics card  350  interface with the computer via a computer bus  352  (such as an industry standard architecture (ISA) or a peripheral component interconnect (PCI) bus). When displaying the visual stimulus on the display screen  25 , the graphics card  350  sends frame rate synchronized triggering signals to the ADC card  351 . ADC card  351  records a sample of the VEP in response to the stimuli immediately when it receives each trigger. The frequency ratio of triggering signal and visual stimulus screen frame rate is preset by the computer on the graphics card  350  by means of which the synchronized sampling rate can be adjusted by the user. For example, the software on the computer can preset the trigger rate to be twice in one frame. So, if the frame rate is 75 Hz, then the sampling rate activated by the trigger is 150 Hz. The host CPU  353  of the computer is connected to the bus via a bridge  354 . The graphics card  350  generates regular video signals to the visual stimulus display screen  25  for stimulus display. During the VEP sampling, graphics card  350  also generates frame rate synchronized triggers  360  which are transmitted to the ADC card  351  to trigger each data sampling. 
     FIG. 4 illustrates the architecture for the system of the present invention for synchronizing the VEP sampling rate with the visual stimulus. In this embodiment, no external triggering is required. Both the graphics card  450  and the ADC card  451  are commercially available products that interface with the computer CPU  453  through a standard PCI bus  452  and bridge  457 . Graphics card  450  generates regular video signals to the visual stimulus generating device  23  for stimulus display of the patterns. Card  450  incorporates a line count register used to count the horizontal lines displayed on the display screen  25 . During the VEP examining process, graphics card  450  further generates interrupt signals  454  (IRQ) via a commercially available interrupt controller  455 . Interrupt controller  455  communicates with the computer CPU  453  across a host bus  456 . The timing of the interrupt signals are determined by the display line count which can be set by the computer  14  on the graphics card  450 . When the CPU  453  receives each interrupt signal from the graphics card  450 , it suspends its current program and establishes an interrupt service/handling routine. In this routine, the CPU  453  activates the ADC card  451  to sample the VEP and reflashes the video RAM, i.e., loads the pattern data into the video card for the next frame display. 
     Graphics card  450  also has a line comparator. When the line count reaches the value set in the line comparator, card  450  generates the interrupt signal  454 . By setting a predetermined value in the line comparator, the timing of the interrupt can be controlled by the computer. It also sets up the line count in the line comparator for the next interrupt signal. 
     Thus, the synchronization of presenting the stimulus with the data sampling is completely controlled by the system software without any hardware modifications. FIG. 5 illustrates the process of the interrupt technique for synchronizing the VEP sampling rate with the visual stimulus. At step  501 , the graphics card initiates and conducts display of the stimulus on the visual stimulus monitor, line by line. Whether or not the specific or predetermined line of the visual image has been displayed is determined at step  502 . If it has been, the video RAM contents are pushed into the display device  23 / 25  in order to display the pattern and the line count is reset to zero at step  503 . If the predetermined specific line of the visual stimuli image has not been displayed, step  503  is skipped and step  504  determines whether or not the line of the visual image display has reached the preset count in the graphics card line comparator. If it has, then an interrupt request signal (IRQ) is generated to the computer CPU  453  (as indicated by dotted line  454 ). The application program  506  on the computer CPU  453  controls the interrupt initialization process. Program  506  sets the line count on the graphics card for the first interrupt occurring in a frame and initializes the video RAM content for the first pattern to display. Then the CPU generates the IRQ to the application routine  508  (for normal system operation and monitoring). When the CPU receives each interrupt signal generated from the graphics card, it suspends program  508  and goes to the interrupt service routine  507 . In this routine, the computer activates the ADC card to sample the VEP and reflashes the contents of the video RAM for the next frame of pattern to be displayed. 
     The interrupt service routine is described in connection with FIG.  6 . The first step  601  clears the interrupt requests and reads data from the ADC card to sample the VEP. Whether this is the last interrupt in the frame is determined at step  602 . If it is, the line counter is set for the first interrupt in the next frame at step  603  and if not, the line counter is set for the next interrupt in the same frame at step  604 . In either event, the video RAM contents for the next visual image to be displayed is initiated at step  605  and the interrupt service routine is terminated and service returns to the application routine. 
     The invention has been described and illustrated in connection with certain preferred embodiments which illustrate the principals of the invention. However, it should be understood that various modifications and changes may readily occur to those skilled in the art, and it is not intended to limit the invention to the construction and operation of the embodiments shown and described herein. For example, the system and method of the invention are useful for a variety of medical examinations where sensory stimuli are used to produce evoked potentials from a patient in response to the stimuli. Such examinations might relate to auditory capability, ambulatory capability, neuro response and others, as well as vision capability. Accordingly, additional modifications and equivalents may be considered as falling within the scope of the invention as defined by the claims herein below.