The Assessment of Evoked Potenfiol Contrast Thresholds Using Real-Time Retrieval

Transcription

1 June 1964 Vol. 25/6 nvestgatve Ophthalmology & Vsual Scence Journal of Dosc and Clncal Research rtcles The ssessment of Evoked Potenfol Contrast Thresholds Usng Real-Tme Retreval Wllam H. Seple,* Mark J. Kupersmrh,+ Jeremah. Nelson,* and Ronald E. Carr* Electrophysologc contrast senstvty functons (CSF) have been estmated usng lock-n amplfer sgnal retreval of the vsually evoked response (VER). These CSFs were compared wth CSFs obtaned psychophyscally usng the same stmulus condtons. The two measures of contrast senstvty behave smlarly n response to varatons of temporal and spatal frequency. The major advantage of usng real-tme retreval s speed. Threshold for a sngle spatotemporal condton can be estmated n as lttle as 2 sec, makng the applcaton of electrophysologc contrast senstvty testng feasble for clncal populatons. nvest Ophthalmol Vs Sc 25: , 1984 The need for contrast senstvty functons (CSF) n any complete assessment of vsual functonng has been long apprecated. Estmates of vsual contrast threshold performance throughout the spatotemporal doman add nformaton that s lackng n Snellen acuty performance at hgh spatal frequency and hgh contrast, whch s usefully related to age, 1 refractve error, 2 and varous ophthalmologcal and neurologcal dseases. 3 We present a method for obtanng CSFs electrophysologcally, drectly from the vsual system's evoked response. Only a few researchers have attempted to measure electrophysologcally determned contrast thresholds, 4 manly because conventonal computer averaged evoked potentals requre lengthy recordng perods to retreve responses near threshold. Wth lengthy averagng, potentals can be contamnated by fatgue and adaptaton. Consequently, t has not been feasble to obtan CSFs estmated from evoked potentals for clncal populatons. n approach that utlzes real-tme retreval of vsual responses to contnuously changng, or "swept," stmul (eg, contrast levels ncreasng wth tme) mnmzes some of these dffcultes. Such meth- From the Departments of Ophthalmology* and Neurology,t New York Unversty Medcal Center, New York, New York. Supported n part by NE grant EY-2179 and by an unrestrcted grant to the Department of Ophthalmology from Research to Prevent Blndness, nc. Submtted for publcaton: February 1, Reprnt requests: Wllam H. Seple, PhD, Department of Ophthalmology BEL5W9, New York Unversty Medcal Center, 55 Frst venue, New York, NY 16. ods have been poneered n the study of evoked potental responses to spatal frequency 5 and color 6 and extended by us to swept contrast. We recently have reported fndng an oblque effect n contrast and acuty threshold usng these methods. 7 n ths study, we show complete CSFs determned for a number of temporal frequences and compare them wth psychophyscal results obtaned under the same stmulus condtons. Subjects Materals and Methods Three men and one woman between the ages of 25 and 39 served as subjects. ll had normal vson correctable to 6/6 and no hstory of neurologcal or ophthalmologcal dsease. They were experenced psychophyscal observers and acquanted wth the purpose of the experment. ddtonal contrast senstvty thresholds for sx clncal subjects (three men and three women) are ncluded n Table 1. These were nave observers wth normal ophthalmologcal examnatons; all gave ther nformed consent to partcpate n the study. Stmul Sne-wave gratng sgnals from a pattern generator were dsplayed on a Conrac Model QQ 14N/ vdeo montor at a mean lumnance of 5 cd/m 2 (Photo Research Spectra Spot Meter PR-15). Background wall lumnance was approxmately 2 cd/m 2 ; natural pupls were typcally 4 mm n these young subjects. Contrast (L max - L mn /L max + L mn ), spatal frequency, 627

2 628 NVESTGTVE OPHTHLMOLOGY 6 VSUL SCENCE / June 1984 Vol. 25 Table 1. VER and psychophyscally denned contrast senstvty thresholds Observers Experenced normals (four subjects) SD: Nave normals (sx subjects) SD: T-test / cycle/ deg VER PT* cycles/ dt VER PT* cycles/ deg VER PT* Mean VER-nferred contrast thresholds n percent for sx nave normal observers are shown n comparson wth the four experenced observers whose results are presented n detal n the subsequent fgures. Standard devatons are shown under each mean. ll VER results were obtaned at 7 reversals/ sec. No dfference between experenced and nave observers was sgnfcant. * PT = psychophyscal thresholds. or temporal rate of alternaton could be manpulated ndvdually on a gven tral by applyng an external control voltage to the pattern generator. Counterphase flcker (square wave temporal modulaton) was set at ether 3, 7, or 43 reversals/sec, producng cortcal potentals at 3, 7, or 43 Hz, respectvely. Responses Scalp potentals were recorded from a gold cup electrode placed 2.5 cm above the non and referenced to an ear, usng the contralateral ear as ground. The amplfed EEG (Grass Model P51 U C preamplfer, gan 1,) was shared by a par of lock-n amplfers (EG & G/PR Ortec/Brookdeal 955-SC). For techncal dscusson of the methods employed see Nelson et al. 7 Expermental Paradgm Contrast thresholds were estmated by sweepng the contrast of a constant spatal frequency gratng (.5, 1, 3, 4, 8, or 12 cpd) at one temporal rate (3, 7, or 43 reversals/sec). Contrast ncreased logarthmcally from.1% to 2% over a 2-sec tral perod. logarthmc sweep was chosen to provde added resoluton near threshold 5 and to facltate extrapolaton by lnearzng the observed stmulus-response relatonshp. 4 Subjects were nstructed only to fxate on a central pont; no subjectve responses were requred. Threshold Estmaton Thresholds were estmated by extrapolatng the response slope to the pont at whch the response frst appeared. Runs were begun durng perods of stablty n the background EEG. t the begnnng of a vsually drven response (threshold), the output begns to clmb regardless of ts current restng voltage level. zero response pont s thus defned whch determnes the base lne for extrapolaton. Ths voltage may not be the electrcal zero. Laboratory tests have provded a demonstraton of the n varance of thresholds nferred by these methods when startng baselnes purposely are selected over a wde range of startng voltage levels. The experments and the techncal ratonale for our procedure are descrbed elsewhere. 7 s others have noted, a partcular strength of the technque s ts mmunty to response ampltude fluctuatons. 8 mpltude changes affect the slope of the lnear response, not the ntercept from whch the threshold s nferred. n recent work, we have used phase senstve detecton (PSD) of the response rather than a vector magntude calculaton. PSD offers slghtly better sgnal retreval by rejectng nterference on the grounds of napproprate phase angle wth respect to the reference, as well as because of napproprate frequency. Psychophyscs Psychophyscal thresholds for each stmulus condton were obtaned n separate trals. The observer adjusted the contrast of the montor usng a potentometer. scendng (ncreasng contrast to threshold) and descendng lmts were averaged to yeld the psychophyscal CSF. Results We have been able to defne contrast senstvty functons for 1 normal observers usng lock-n retreval of the vsual evoked response (VER, Table 1). The electrophysologcally determned CSF tracks the same process as the psychophyscally determned CSF as spatal frequency and temporal rate are vared, although wth a slght loss of senstvty. Results from four subjects studed n greater detal are now descrbed. Fgure 1 shows 12 lock-n retreved responses for one subject at all spatal frequences (3 reversals/sec). Usng the threshold estmaton method descrbed above, CSFs were derved from such records. Varablty of contrast thresholds among runs of the same spatotemporal condton were approxmately 3% for each observer (eg, from 1.% to 1.3%). Contrast senstvty functons at 7 reversals/sec, as well as a comparson of the VER and psychophyscal measures at ths temporal frequency, are shown for four subjects n the four panels of Fgure 2. The shapes of the functons are smlar for the two measures, although the VER s consstently less senstve than the

3 No. 6 CONTRST SENSTVTY SSESSMENT FROM EVOKED POTENTL / Seple er ol. 629 Fg. 1. Monocular raw data of WS at 7 reversals/sec. The threshold defnng sectons of twelve 2-sec sweeps of contrast have been supermposed on the X-axs (contrast). The turnaround pont at the toe of each curve gve the threshold. The number above each curve refers to the spatal frequency n cycles per degree for that run. Full scale senstvty, 5 juv l 2 SWEPT CONTRST (PERCENT) psychophyscal CSF (mean of.4 log unts less at 7 reversal/sec). The dfference would be.15 log unt less (e,.25) f the VER thresholds were compared wth only ascendng psychophyscal thresholds. n spte of ths senstvty loss, the VER determned CSF does appear to measure the same process. For example, subject MK showed a sharp, low, spatal-frequency fall-off n senstvty at 7 Hz for both the VER and psychophyscal measures, whereas, subject PW dd not show ths fall-off n ether measure. One mportant dfference between the two measures was apparent. n all subjects, the VER derved CSF showed a broader and less well-defned peak than the psychophyscal results. Monocular CSFs for two subjects are plotted n Fgure 3 wth temporal rate shown parametrcally. ll subjects showed a loss of contrast senstvty as rate of alternaton was ncreased to 43 reversals/sec. gan the VER and psychophyscally determned CSFs showed smlar behavor. For example, subject MK exhbted the largest fall-off of senstvty at the hgh temporal frequency psychophyscally and ths loss also was reflected by the VER derved senstvty. Subject WS showed the least fall off n senstvty psychophyscally and also showed less reducton n VER contrast senstvty at the hgh temporal frequency. The other two 1 subjects showed ntermedate losses n senstvty at the hgher reversal rate, agan wth the psychophyscs and VER results runnng n parallel. Dscusson We have been able to nfer contrast senstvty thresholds over a range of spatal and temporal frequences from the VER, obtanng a contrast senstvty functon n a few mnutes of recordng. Our nferred VER thresholds correlate well wth those obtaned under the same condtons usng standard psychophyscal methods n our laboratory, and show responses to varatons n temporal and spatal frequency qualtatvely smlar to those prevously reported n the lterature. 9 The current method represents an automaton of earler technques that used averaged VERs. 4 n those studes, contrast thresholds were nferred for a gven spatal frequency by extrapolatng evoked potental responses recorded separately to a successon of manually set contrasts; whereas the extrapolaton method estmates thresholds, t s generally agreed that suprathreshold VER ampltudes have lttle correlaton wth contrast senstvty. Thus, the curves of Tyler 1 for suprathreshold VER ampltude as a functon of spatal frequency show several sharp peaks rather than the smooth curve of a CSF. Estmatng CSFs by measurng the contrast requred

4 6 NVESTGTVE OPHTHLMOLOGY 6 VSUL SCENCE / June 1984 Vol. 25 ~ 1 3 / 8 1 r X \ \ \ \ \ 8 8\ ~ JN MK 8 PW ws 8 X the use of extrapolaton, sgnal to nose lmtatons may stll be a factor. Lock-n sgnal retreval technques are not mmune to all problems nherent wth the averaged VER. lthough vewng tme s lmted to 2 sec n our experments, habtuaton effects may alter the nferred threshold. 12 We employed two procedures to reduce ths result. Frst, we always swept contrast from below threshold to above. Ths mnmzes preexposure to suprathreshold contrasts. Second, when spatotemporal condtons were repeated, orentaton was alternated between horzontal and vertcal orentatons. The thresholds to these two orentaton dd not dffer 7 and were combned. rousal and attenton level also may nfluence VER ampltude, a problem that s nherent n any testng method. Durng long averagng perods attenton vares and the recorded evoked potental ampltude reflects the average attenton level. lthough attenton also may vary durng the 2-sec recordng perod we used, departure from the antcpated lnearty of the slope s mmedately evdent and entre runs can be dscarded SPTL FREQUENCY ( CYCLES/DEGREE ) Fg. 2. Contrast senstvty functons of four subjects at 7 reversals/ sec. Broken lne = average of the two monocular psychophyscal thresholds; open crcle = electrophysologcally defned thresholds for the rght eye; closed crcle = electrophysologcally defned thresholds for the left eye. 8 6> o MK PSYCHOPHYSCS r o O WS PSYCHOPHYSCS to elct a crteron response depends on suprathreshold stmul. t s not a threshold technque, and the CSF may be estmated ncorrectly f the stmulus s greatly suprathreshold.'' There are changes n the polarty and ampltude of averaged transent VER components to dfferent patterns and spatal frequences. Such changes may be assumed ncorrectly to be due to the spatal frequency manpulatons alone. mpltude changes also occur wth changes n temporal frequency. For example, the largest evoked potental occurs at 16 Hz usng low spatal frequency patterns, whereas hgh spatal frequences yeld larger ampltude evoked potental at reversal rates around 7 Hz.'' Lock-n retreval wth swept stmulaton estmaton of threshold s, however, ndependent of absolute ampltude and consequently ndependent of the factors that saturate, shape, and dstort the suprathreshold stmulus-response functon. On the average, the senstvty obtaned wth ths technque s less than we obtaned usng psychophyscal methods. t may be that the mechansms generatng a scalp potental are ancllary to the neuronal processes determnng the psychophyscal response. lso, despte 1 CO UJ cc vn ( kst SEh CC 1 CON1 1UUU C 1 r O VEP ^ VEP SPTL FREQUENCY (CYCLES/DEGREE) Fg. 3. Monocular psychophyscal and electrophysologcal contrast senstvty functons of two subjects at 3 reversal rates. Closed crcles = 3 reversals per sec; trangles = 7 reversals per sec; open crcles = 43 reversals per sec. #

5 No. 6 CONTRST SENSTVTY SSESSMENT FROM EVOKED POTENTL / Seple er ol. 631 n summary, a major dsadvantage of swept stmulaton, lock-n retreval method appears to be the level of techncal expertse needed to understand and apply the technque. There s also some varablty n the data obtaned, but ths s sometmes no greater than the nter-subject varablty found n psychophyscs usng traned subjects (Table 1). For clncal applcatons, the advantages of speed and objectvty are compellng. Key words: evoked potental, modulaton transfer functon, contrast senstvty, gratngs, vsual assessment References 1. rundale K: n nvestgaton nto the varaton of human contrast senstvty wth age and ocular pathology. Br J Ophthalmol 62:213, Enoch JM, Ohzu H, and to M: Contrast (modulaton) senstvty functons measured n patents wth hgh refractve error wth emphass on aphaka:. Theoretcal consderatons. Doc Ophthalmol 47:139, rden GB: The mportance of measurng contrast senstvty n cases of vsual dsturbance. Br J Ophthalmol 62:198, Campbell FW and Maffe L: Electrophysologcal evdence for the exstence of orentaton and sze detectors n the human vsual system. J Physol 27:635, Tyler CW, pkaran P, Lev DM, and Nakayama K: Rapd assessment of vsual functon: an electronc sweep technque for the pattern vsual evoked potental. nvest Ophthalmol Vs Sc 18:73, Regan D: Colour codng of pattern responses n man nvestgated by evoked potental feedback and drect plot technques. Vson Res 15:175, Nelson J, Kupersmth MJ, Seple W, and Carr RE: Spatotemporal condtons whch elct or abolsh the oblque effect n man. Vson Res, n press. 8. Tyler CW, Nakayama K, pkaran P, and Lev DM: VEP assessment of vsual functon. Vson Res 21:67, Robson JG: Spatal and temporal contrast-senstvty functons of the vsual system. J Opt Soc m 56:1141, Tyler CW, pkaran P, and Nakayama K: Multple spatalfrequency tunng of electrcal responses from human vsual cortex. Exp Bran Res 33:535, Regan D: ssessment of vsual acuty by evoked potental recordng: mbguty caused by temporal dependence of spatal frequency selectvty. Vson Res 18:439, Chlders DG, Doyle TC, Brnck G, and Perry NW Jr: Ensemble characterstcs of the human vsual evoked response: perodc and random stmulaton. EEE Trans Bomed Eng BME-19:48, 1972.