Patent Application: US-25756503-A

Abstract:
for electrophysiological assessment of visual function , a head mounted stereo display for displaying a stimulus is used to generate a retinal or cortical response . in particular , a method for objective electrophysiological assessment of visual function of at least one eye of a subject includes presenting a visual stimulus to at least one eye of the subject , recording at least one of a retinal response and a cortical response generated as a result of the presenting , analyzing said response and , as a result of said analysing , forming a map of the visual function of the at least one eye of the subject . the invention also relates to a system for such electrophysiological assessment .

Description:
fig1 shows a schematic of the apparatus of vep recording using virtual reality goggles ( 1 ), which present the display to the subject . the goggles are connected to a computer ( 2 ) with a linked video board that generates the multifocal stimulus . recording electrodes on the scalp ( 5 ) and a ground reference eletrode ( shown on tho earlobe ), detect the vep signal from one or more recording channels ( in this case four channels are shown ). the signals are conducted to an amplifier ( 3 ), before being processed by software for presentation on the operators display ( 4 ). results can be compared for each eye , or between the two eyes of a subject with respect to normal reference values . fig2 shows a schematic of the apparatus for multifocal erg recording using virtual reality goggles ( 1 ). the set up is the same as in fig1 except that the recording electrode is placed in contact with the eye or eyelid . a ground electrode is required ( shown on the earlobe ). only one channel recording is required for the erg . fig3 is an example of multifocal multichannel vep recording from the right and left eye of a normal subject , fig3 a shows the responses achieved using a conventional screen ( 22 inch hitachi monitor ) to present the stimulus . a cortically scaled dartboard stimulus was generated with 60 different areas of pattern stimulation using the objectivision perimeter . the trace array shown in the figure represents the responses generated from each part of the visual field tested out to 27 degrees of eccenricity temporally and 34 degrees nasally . for graphics purposes the central areas are relatively enlarged to show the raw vep signal within that are . fig3 b shows a multifocal multichannel vep recordings from the some normal subject as in fig3 a , recorded using virtual reality goggles to present the same stimulus instead of the conventional monitor . the same objectivision system was used . the responses are of similar order of magnitude in the two techniques , although there is source variation in amplitude across the field . due to the specifications of the goggles used , the display was limited to 21 degrees temporally and 27 degrees nasally . fig4 provides a comparison between subjective perimetry findings and the objective vep assessment of the visual field using virtual reality goggles . fig4 a slows the grayscale and pattern deviation printout from a subjective humphrey visual field test of the right eye of a glaucoma patient . an inferior arcuate scotoma ( blind spot ) is shown in the visual field . fig4 b shows the multifocal multichamnel vep recording from the same eye as in fig4 a , recorded using virtual reality goggles . analysis of the signals demonstrates loss of vep responses corresponding to the inferior scotoma in fig4 a , with more extensive reductions in the superior field than seen on the humphrey . the amplitude deviation plot shades areas according to probability of abnormality when compared to a reference range of normal values extrapolated from the reconventional screen objectivision system . this suggests that the technique is capable of detecting visual field loss in glaucoma , just as it is with the use of the conventional large screen . it may also demonstrate more significant glaucomatous damage than suspected on conventional humphrey field testing . five glaucoma patients have been tested with the virtual reality goggles and the scotomas were detected in all five cases . examination of multifocal vep data from normal subjects using conventional crt monitors demonstrated that the amplitude of the multi - focal vep is not age - dependant ( contrary to most electrophysiology parameters , eg the pattern erg ). in fact , some elderly people produce vep responses of higher amplitude . individual variation in the thickness of the scalp or subcutaneous tissue may cause inter - individual differences in vep amplitude due to variable impedence of bone and fat . direct measurement of the thickness or impedance of these tissues is not currently practical . however , the impedance will also affect the amplitude of the spontaneous brain activity ( eeg ) in a similar fashion to the vep . to confirm this we conducted a study using the objectivision vep perimeter of the correspondence between spontaneous eeg amplitude ( 99 % confidence interval ) and multifocal vep amplitude ( largest amplitude of a trace ). the study included 34 normal subjects . the results demonstrated a strong correlation between the eeg amplitude aid vep ( correlation coefficient r = 0 . 81 ). the scatterplot for the correlation is shown in fig5 . an alternative method to measure background eeg activity is to calculate a fourier power spectrum of the eeg . therefore , if the level of spontaneous eeg activity is calculated during the recording . it provides an indirect measure of the overall registration of brain signals for that individual for the electrode positions used . whilst it is recognised that eeg amplitude is determined by many additional factors other than conductivity , it is proposed that scaling of an individual &# 39 ; s vep responses according to their eeg levels , relative to normal population eeg values , helps to reduce inter - individual vep variability . the eeg amplitude is approximately 1000 × the amplitude of the vep , so it is reasonable to assume that the vep signals themselves will have little contribution to the raw eeg levels . in analysis of multifocal vep recordings the eeg raw data is actually examined by cross - correlation techniques to extract the vep signals . when recording from an individual , the overall level of the raw eeg ( 99 % confidence interval ) as recorded during each run of the vep recording , can be used to provide an individual &# 39 ; s scaling factor . the vep extracted is then scaled by the eeg scaling factor . the value of that technique of the invention of vep scaling was confirmed by examining the data from 50 normals . the coefficient of variation for all 60 visual field test points had a mean value of 50 . 1 %. when the results were scaled according to background eeg values the coefficient of variation for all 60 visual field test points was produced to 28 . 2 % by using eeg scaling , the sensitivity of the test was also improved . in a study of 60 glaucoma cases using the objectivision system for multifocal vep perimetry , several glaucoma cases were not flagged as abnormal using the unscaled data since the subjects had overall large signals compared with normal , even though focal relative reductions could be seen when examining the trace arrays . with the data scaled according to eeg levels however , these subjects were identified as having localised reductions their vep amplitudes and the scotomas were flagged appropriately . the eeg raw data can contain a large component of alpha rhythm signals and also spikes of electrocardiogram signals . if these are not excluded from the scaling factor applied , then some subjects will have their data inadvenently scaled down lower than is appropriate . this can introduce false positive results in the vep . one technique for rectifying this problem is to examine the raw signal by fourier analysis and any alpha - rhythm spikes and electrocardiogram signals can be identified . these can then be excluded from the spectrum before calculating a scaling coefficient . therefore scaling of the vep amplitude based on amplitude of spontaneous brain activity eliminates part of the variability between individuals caused by differences in conductivity of tissues . this technique has application in analysing multifocal vep signals recorded with conventional crt monitors , plasma screens , lcd screens , or with virtual reality goggles . the method and system of this invention will find wide use in the medical field , specifically in the field of ophthalmology . the foregoing describes only some embodiments of the invention and modifications can be made thereto without departing from the scope of the invention . 1 . daseler h a & amp ; sutter e e . vis rerearch 1997 ; 37 ( 6 ) 675 - 790 2 . klistomner al , et al invest ophthamol vis sci 1998 ; 39 ( 6 ): 937 - 950 3 . kiistomer a j , et al aust n z j ophthalmol 1998 ; 26 : 91 - 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