Analysis of Normal Flicker Sensitivity and Its Variability in the Visuogram Test

Transcription

1 Investigative Ophthalmlgy & Visual Science. Vl. 32, N. 9, August 1991 Cpyright Assciatin fr Research in Visin and Ophthalmlgy Analysis f Nrmal Flicker Sensitivity and Its Variability in the Visugram Test Christpher W. Tyler Flicker sensitivity was measured in grups f yunger and lder adult bservers t assess its mean values and the test-retcst variability bth between sessins and in a single sessin. Test-retest sensitivities differed by less than 15% but tended t decrease slightly in a sessin and increase slightly between sessins. Optical blur had little effect n the measured sensitivities, implying that they were nt mediated by the edge infrmatin in the display. There were n significant differences between eyes ther than thse attributable t testing rder. Within eyes, inherent variatins in human flicker sensitivity accunted fr abut half f the variance, and within-sessin variability accunted fr mst f the remainder. Invest Ophthalml Vis Sci 32: , 1991 Sensitivity t sinusidal tempral mdulatin (flicker sensitivity) f a unifrm stimulus field is a useful technique with applicatin t phthalmlgic diagnsis. When measured by varying the amplitude f the light mdulatin at a series f fixed frequencies, flicker threshlds prvide measurements f cntrast sensitivity fr the stimulus used and define the tempral respnse characteristic f the visual pathway. These tw prperties allw flicker sensitivity t be used in varius diseases t prvide earlier detectin than previusly available tests, 12 t measure a reversible cmpnent f the visual susceptibility t disease, 3 t prvide differential diagnsis amng cnditins that are similar accrding t ther tests (eg, retinitis pigmentsa), 4 t characterize the nature f a sensry deficit, 5-6 and t prvide infrmatin as t which retinal mechanisms are affected by disease. 5 A requirement fr full clinical applicatin is the establishment f nrmal data against which individual test results can be evaluated. Few such nrmal data are available fr tempral mdulatin sensitivity, althugh the variatin with age has nw been investigated fully. 7 Several issues relevant t the detailed specificatin f nrmal flicker sensitivity and its variability by means f the tempral visugram test are nw addressed. The tempral visugram is a representatin f the lss (r enhancement) f flicker sensitiv- Frm the Smith-Kcttlewell Eye Research Institute, San Francisc, Califrnia. Supprted by Natinal Institutes f Health grants EY 6555 and RR5981. Submitted fr publicatin: June 27, 1990; accepted April 12, Reprint requests: Christpher W. Tyler, PhD, Smith-Kettlewell Eye Research Institute, 2232 Webster Street, San Francisc, CA ity in relatin t the nrmal mean sensitivity under the same cnditins. Nrmal mean flicker sensitivity fr sinusidal mdulatin f the stimulus luminance ver time is determined as the reciprcal f detectin threshld ver a large number f nrmal bservers. The tempral visugram (r lss functin) then is defined as the lg f the rati f sensitivity fr a particular bserver t the nrmal mean sensitivity (in tenths f a lg unit r decilgs [dl]). Tempral sensitivity functins were measured at many frequencies and cmpared in each eye f yunger and lder bservers. The rbustness f the functin then was evaluated fr degradatin f the image by ptical blur. The interbserver variability was cmpared at ten frequencies t determine whether there was any cnsistent variatin with frequency. The test-retest reliability was evaluated bth within a test sessin and between tw sessins separated by several weeks. Reliability was specified bth in terms f the abslute change in sensitivity with tempral frequency and the standard deviatin (SD) f the changes. Stimulus Design Materials and Methds A flicker threshld system was set up t test flicker frequencies in half-ctave steps frm 2.5 Hz upward, in additin t the critical fusin frequency (CFF). The flicker was presented in a smth, sustained fashin by means f a 0.5-sec csine envelpe fr the flicker signal, a sinusidal mdulatin arund the cntinuusly present mean level. The stimulus cnsisted f an array f light-emitting dides f 660-nm peak wavelength diffused t appear as a unifrm red disc f 5 subtense and set in an 2552

2 N. 9 FLICKER SENSITIVITY TEST NORMS / Tyler 2550 equiluminant white surrund f 400 cd/m 2. This stimulus cnfiguratin has been develped fr assessment f phtpic visin acrss a range f retinal and ptic nerve disrders. It uses lng-wavelength light t islate cne respnses and eliminate any rd cntaminatin; it als has ther advantages fr the study f retinal functin. Spatially, the stimulus was selected t be large enugh t cver the macula rather than being restricted t the fvea in extent. This larger size tended t minimize fveal edge effects, cnsequently resisting the effects f bth ptical blur and inaccurate eye fixatins. As a result, this stimulus was ptimized fr the assessment f tempral retinal functin against spatial and ptical factrs. It thus has wide applicability t a range f eye diseases affecting bth macular functin and brader retinal sensitivity and minimized cntaminatin frm defcus, ptical distrtins, and early media pacities. One imprtant issue in the study f clinical visual sensitivity lsses is cntrl f the age changes in transmissin lsses in the ptic media: the crnea, lens, and vitreus bdies. The lens grws bth harder and mre ptically dense with age, ptentially cntaminating evaluatin f retinal functin. Hwever, there is a large cvariatin in the transmissin lss with the wavelength f the light used fr the test. Lsses becme almst negligible fr lnger wavelengths, such as the peak wavelength f 660 nm chsen fr the test stimulus. The transmissin lss at 660 nm is nt measurable in yung adults, 8-9 and the increase in transmissin lss frm ages 5-70 yr is less than 0.1 lg unit (1 dl) arund this wavelength. 9 Mrever, the same resistance t ptical scatter makes lng-wavelength light the regin f the spectrum least susceptible t lsses frm early lens cataracts. This test prbe thus may be cnsidered the mst sensitive wavelength fr measurement f nnptical changes in visual functin with age. Use f lng-wavelength light has ther advantages. It is the nly regin f the spectrum that can islate the respnses f a single cne type withut adaptatin. It als maintains this islatin at all intensity levels, aviding cntaminatin frm rds and ther cne classes. Finally, it is available inexpensively in the frm f light-emitting dides, which can deliver high intensities with excellent cntrl ver the tempral prperties and linearity. Psychphysical Staircase Prcedure The test prcedure cnsisted f a yes-n task fr flicker detectin with 20% blank trials as a check n the false-alarm respnse rate. The verall paradigm cnsisted f a cyclic interleaved staircase prcedure, in which each the staircases fr all stimulus frequencies were interleaved in cyclic rtatin. Fr the fixed frequency staircases, stimulus amplitude (mdulatin depth) was decreased by ne step (ie, 0.1 lg units) fr a yes respnse, but it was increased by three steps fr a n respnse. (Thus an accurate n respnse shuld represent threshld and shuld be fllwed by three yes respnses as the staircase jumped up and tracked dwn t threshld again.) The staircase was terminated when a n respnse was btained n the same r adjacent mdulatin levels as the preceding n respnse. The threshld was cnsidered t be the average flicker amplitude at these tw criterin n respnses. The CFF first was measured alne by means f a similar descending staircase prcedure. Fr the CFF measurements nly, the staircase had a variable step size. The sequence began at 5 Hz and increased in 5-Hz steps until the first n respnse was btained. The subsequent staircase perated by 1 -Hz frequency steps n the same three-up/ne-dwn paradigm used fr the mdulatin amplitude, until tw subsequent n respnses were btained at the same r adjacent frequency steps. The resulting CFF value was used t set the subsequent range f frequencies used fr the secnd phase f the test the measurement f mdulatin sensitivity at fixed frequencies. Having selected the applicable frequency range, the algrithm then cycled thrugh the chsen test frequencies in sequence, btaining ne respnse fr each frequency befre changing t the next. Each respnse then was used t determine the value presented at that frequency the next time arund, fllwing the same algrithm as fr CFF except that each step was 1 dl (25%) f mdulatin amplitude. The staircase fr each frequency prceeded independently (althugh temprally intermixed) until tw subsequent n respnses were btained n the same r adjacent flicker levels. Each eye was tested separately, with the untested eye being ccluded by an paque patch. A typical test run fr ne eye wuld take abut 10 min fr ten frequencies; run lengths varied frm apprximately r mre trials, accrding t when the criteria were met. The ptimal values fr the terminatin criteria were ascertained by cmputer simulatin f hundreds f runs and als were fund t give psychphysical threshlds with clse t the simulated variability even in untrained bservers. Test-Retest Prcedure The requirement fr test-retest reliability was that each eye be tested twice in ne sessin and twice again in a further sessin separated by a substantial lapse f time. Bth eyes f 33 nrmal bservers were tested in

3 2554 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991 Vl. 32 tw sessins separated by at least 1 mnth but n mre than 4 mnths. The testing rder always was t start with the right eye and then test the left eye, fllwed by a secnd test fr each eye in sequence. The same prcedure was fllwed in the secnd test sessin. Observers The bservers were drawn frm a ppulatin f hspital vlunteers and respndents t lcal advertisements. The average age f the bservers was 38 ± 11.9 yr. Infrmed written cnsent was btained frm each bserver after the nature f the prcedures was explained fully accrding t the prtcl apprved fr this prject by the Jint Cuncil n Human Research. The absence f cular pathlgy was assessed by btaining a clinical histry and direct phthalmic evaluatin. It was cnsidered ptimal t use lenient inclusin criteria fr nrmal subjects because this prvides the mst useful sample fr clinical evaluatin f the effective nrmal ppulatin. Data Analysis A significant aspect f the data analysis was the use f Winsrizatin t derive rbust estimates f the errr terms n the sensitivity means. 10 Winsrizatin is a prcess in which data falling mre than a set multiple f the SD frm the mean is crpped frm the sample and the SD recmputed n the reduced sample. This new SD then is used as the basis f further iteratins f the same prcedure until the values stabilize, and n mre data are crpped. The value f the Winsrized estimate f the ppulatin SD is mre rbust against statistical fluctuatins than the full sample SD fr small data samples. 10 It has the further advantage that it tends t extract the SD f a nrmal ppulatin frm an verall distributin cntaminated by a small prprtin f abnrmal values. This type f mixed distributin is typical f clinical ppulatins, where a small number f patients are mixed in with a large grup frm a hmgeneus nrmal distributin. It is als typical f human psychphysical respnse patterns, where mst data are btained under cnditins f high attentiveness, but ccasinal lapses may result in a few wild values in the distributin. A similar situatin is encuntered in aging ppulatins, where mst individuals may be nrmal, althugh a few may have undetected age-specific diseases which distrt their results relative t the nrmal grup. Suppse such a distributin cnsists f a large ppulatin with a small SD and a small prprtin with a large SD, perhaps with a different mean s that the cmbined distributin is asymmetric. The Winsrizatin prcedure prgressively crps the (asymmetric) tails f the distributin, because its criterin SD is dminated by the cntributin f the larger ppulatin. The crpping will cntinue until it reaches the tails f the large ppulatin, where it will stp as it wuld if that had been the sle distributin. Winsrizatin therefre is effective at extracting the nrmal distributin and excluding the small abnrmal ne. Winsrizatin ften is viewed with the suspicin that it may reduce the measured value iteratively t prvide an unrealistically lw estimate f the ppulatin SD. This is nt the case if the Winsrizatin is dne prperly. Cnsider a situatin where there are 20 elements in a sample and a high crpping criterin such as 2 SD. On a statistical basis, nly ne f the elements shuld fall utside these bunds n a typical trial. The effect f crpping will reduce the estimated SD by abut 10%, but the likelihd is that n further crpping will ccur when the criterin is reapplied. The slight reductin in the SD estimate may be cmpensated by adjusting the P criterin fr statistical tests applied t this ppulatin. The cnclusin frm these arguments is that Winsrizatin is a pwerful technique fr increasing the reliability and validity f SD estimates in ppulatins cntaining a small prprtin f deviant respnses. Its widespread use may be advcated fr develpment f clinical nrms against which disease states may be tested. Results Nrmal Sensitivity in the Tempral Visugram Tempral visugram sensitivities were measured fr 59 bservers with n clinical symptms f cular pathlgy. The data were screened fr reliability f the bservers in tw ways. The first was derived frm the false-psitive respnse rate. The test prcedure included 20% blank trials as an indicatin f the bservers' attentin t the task and respnse criterin. This resulted in the presentatin f an average f 8 ± 3 blank trials during a typical test run. The bservers were allwed n mre than tw false-psitive respnses fr an acceptable perfrmance; hwever n ne was excluded frm further analysis based n this criterin. The secnd screening prcedure was t Winsrize the data t eliminate utliers. 710 First, the means and SDs were cmputed fr each data set in each frequency cnditin. All bservers with data falling beynd 2 SD frm the mean fr that frequency cnditin and age grup were remved frm the verall sample (even if ther data fr the same bserver fell within the range). A new mean and SD then were cmputed fr the Winsrized data fr each age grup

4 N. 9 FLICKER SENSITIVITY TEST NORMS / Tyler 2555 and cnditin, and the prcedure iterated until it stabilized with n mre remvals. In practice, usually ne and n mre than tw iteratins sufficed fr the prcedure t stabilize in each grup. A ttal f eight (13.6%) bservers were excluded by this criterin. The bservers then were divided int tw age grups, thse yr f age (n = 28; mean age, 34 ± 11.0 yr) and thse yr f age (n = 23; mean age, 68 ± 7.7 yr). The excluded bservers cnsisted f five frm the yunger grup (mean age, 45 ± 9.2 yr) and three frm the lder grup (mean age, 69 ± 4.7 yr). These exclusin data prvide n evidence that the exclusins were biased with respect t age either between r within the age grups. The tempral visugram sensitivities btained are shwn in Figure 1, separately fr right and left eyes. Fr the yunger grup, the sensitivities f the tw eyes Right Eye Left Eye cc c -1 O) M -2 <D D P yrs yrs L-R MEANS Tempral Frequency (Hz) Fig. 2. Mean difference between right and left eye sensitivities in visugram frmat (but nte rdinate scale difference frm Fig. 1). Dashed line nrmalized sensitivity level fr right eyes. Open symbls mean sensitivity lss in left eye relative t right eye fr yunger bservers. Filled symbls mean sensitivity lss in left eye relative t right eye fr lder bservers. :\j B yrs N = 23 Cmbined l ld \ \ Tempral Frequency (Hz) Fig. 1. (A) Mean flicker mdulatin sensitivities fr right (filled symbls) and left (pen symbls) eyes f bservers between 15 and 55 years ld. (B) Mean flicker mdulatin sensitivities fr bservers aged years ld. Dashed line mean f data frm yunger bservers frm (A). D were similar, rising t a peak at 10 Hz and declining prgressively mre steeply t higher frequencies. The frm f this functin is as expected frm the riginal studies f flicker sensitivity. 1 ' 12 A cmparisn f the sensitivities fr these grups befre and after the Winsrizatin prcess shwed that all the pst-winsrizatin values were within 0.2 dl (abut 1%) f their pre- Winsrizatin cunterparts. The Winsrizatin prcess did nt, therefre, intrduce significant bias in the sensitivity results. The lder grup shwed a slightly reduced sensitivity relative t the yunger nes, averaging abut a 1 -dl (~25%) reductin acrss tempral frequency but with a slight increase t higher frequencies. This is the pattern f behavir expected frm a previus study f aging effects with the same stimulus. 7 Hwever, ur results shw that there was a difference between the tw eyes in bth age grups. These results are displayed in a visugram frmat with enlarged vertical scaling in Figure 2. Taking the right eye as a baseline, the visugram shws the relative gain r lss in sensitivity exhibited by the left eye. At all frequencies in bth age grups, the left eyes were less sensitive than the right by an average f abut 0.4 dl (~ 10%). The effect was small but statistically significant (student t-test, P < 0.01). The pattern f sensitivity difference was different in the tw age grups, hwever. The yunger bservers shwed nly a small eye difference up t 20 Hz, then appeared t switch t a substantially larger difference reaching abut 0.7 dl (~ 18%). The lder bservers, hwever,

5 2556 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991 Vl. 02 shwed a larger difference up t 20 Hz, which seemed t be reduced thereafter. The number f bservers was nt sufficient t allw a secure interpretatin f these different patterns, but the cnsistent empiric result f a lwer sensitivity in the left eye when tested secnd shuld be taken int accunt in setting the limits f nrmal sensitivity levels fr the visugram test. Effects f Optical Blur The flicker image was blurred by measuring the flicker sensitivity functin befre and after intrductin f a +10 D spheric lens befre the right eyes f six bservers (mean age, 54 ± 19.2 yr). This pwer was chsen s that the stimulus appeared t be a raised csine blur circle with full cntrast at the center but maximally gradual change acrss the edge. At the viewing distance f 28 cm, the +10 D pwer wuld have been reduced by 100/28 r 3.57 D, ensuring that the residual blur was still D. The means and their standard errrs (SEM) fr the blurred-nrmal sensitivity differences were pltted in the frm f a visugram as a functin f frequency in Figure 3. The data fell very clse t the zer line f n effect but shwed an average drp f abut 1 dl that appears t be unifrm acrss frequency. A small sensitivity lss is t be expected frm this degree f blur because it wuld reduce the average cntrast acrss the riginal stimulus area by 50%, r 3 dl (n the assumptin that the stimulus was blurred t a single raised csine prfile with full cntrast at the center). Sensitivity therefre wuld be expected t be reduced by 0-3 dl depending n the integratin area f the mechanisms determining flicker sensitivity Tempral Frequency (Hz) Fig. 3. Effect f blurring stimulus with a +10 D lens. Pints shw average flicker lss relative t unblurrcd stimulus fr six bservers. Errr bar indicates ±1 SDfthe lss rati averaged ver all frequencies. Interbscrver Variability Fr cmparisn f a patient's results with the nrmal ppulatin statistics, it is imprtant t knw the variability f test sensitivities acrss the nrmal ppulatin. A study 7 f this variability in a large ppulatin f self-tested bservers shwed it t have an average SD f abut 1.8 dl thrughut yung adulthd. Hwever, this study had three shrtcmings fr ur purpses. It did nt invlve cntrlled labratry testing cnditins, nly three frequencies were tested, and the SD was nt cmputed fr lder individuals wh cnstitute much f the clinical ppulatin. My study readdressed all three f these deficiencies. The SD fr the sensitivities f each eye fr each f the age grups is shwn in Figure 4. The values range frm dl but d nt deviate significantly frm a unifrm distributin fr any cnditin. The average SD acrss frequency was 1.75 dl fr the yunger bservers and 1.70 dl fr the lder grup (values similar t thse btained in the previus study 7 ). Thus there is n indicatin that the lder bservers had greater variability than the yunger nes, either n an individual basis r acrss the ppulatin (either type f variability wuld increase the ppulatin SD measured). These values are nly 11.6% and 14.8% lwer than the crrespnding values befre Winsrizatin. Thus the exclusin prcedure did nt prduce a large r age-related reductin in the SD values, althugh sme reductin is inherent in the prcedure. Nrmal Test-Retest Reliability Between Sessins The reliability measure apprpriate t cmparisns f patients with nrmal subjects is test-retest reliability between sessins. Each persn is tested in a different sessin, s they shuld be cmpared with the variability in a single bserver when tested in different sessins. Virtually all variability in attentinal and bichemical states then is equivalent between the tw situatins. The between-sessin reliability was btained by cmparisn f the results in the first test n each eye between the tw testing sessins as described. The mean sensitivity changes in the first pair f tests frm the first t the secnd testing sessin are shwn in Figure 5A. The first bservatin frm these data was that the changes between sessins were small, with nne exceeding 0.6 dl (15%). At best, these effects are barely statistically significant n an individual frequency basis; the criterin change fr significance (P < 0.01) was 0.74 dl fr therighteyes and 0.63 dl fr the left eyes. Althugh the changes were small, hwever, they were almst all in the psitive directin, s that measured sensitivity was cnsistently better in the secnd sessin than the first. There was als

6 N. 9 FLICKER SENSITIVITY TEST NORMS / Tyler 2557 Right Eye Left Eye - 5? CT 9 _J «w 2- S cc.-tr.t: Q eg ^ ' A In yrs ld (N=28) B yrs ld (N = 23) - 1 I Stand Flicker Frequency (Hz) Fig. 4. Standard deviatins fr the sensitivities fr each eye as a functin f tempral frequency. (A) Yunger bservers. (B) lder bservers. Filled bars right eyes, pen bars left eyes. Nte that abscissa has nninterval frequency spacing. The data shw n cnsistent variatin with frequency. a tendency fr the difference t increase tward the higher frequency, as indicated by a significant crrelatin (P < 0.01) between sensitivity increase and test frequency f r = 0.55 fr the tw eyes cmbined. Between-sessin variability was unifrm acrss frequency (Fig. 5B) averaging 1.82 dl fr the right eye and 1.54 dl fr the left, fr a cmbined average f 1.68 dl. Nte that these values cmbine the cntributins f variability fr the tw sessins. Because distributin variances are additive, the estimated variance fr a single sessin shuld be half f the cmbined variance fr the tw sessins. Variability is pltted in terms f its SD (square rt f the variance); therefre the estimated SD fr a single sessin shuld be smaller by the square rt f 2 (0.707) than the values pltted. The average SD f the single-sessin variability thus wuld be estimated t be 1.19 dl. The average slpe f the variability acrss tempral frequency was nt stat Right Eye Left Eye (N = 33) CT c 03.c O c c i_ I.llt.l.l "--0-5 O Frequency (Hz) Fig. 5. (A) Mean sensitivity change between first and secnd sessins fr each eye f nrmal bservers (averaged fr full age range frm years at each frequency). Filled bars right eyes, pen bars left eyes. Abscissa again has uneven frequency spacing. Mst values increase slightly between sessins, especially at higher frequencies. (B) Standard deviatin f tcst-rctest variability between first and secnd sessins fr each eye f the same bservers.

7 2558 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991 Vl. 32 tistically significant and accunted fr nly ±12.5% f the ttal variance. Nrmal Test-Retest Reliability Within a Sessin Within-sessin reliability is the measurement apprpriate fr cmparisn f different test cnditins run n a single bserver in a test sessin. It includes nt nly the inherent testing variability but als fatigue and practice effects accumulated during the test prcedure. The results fr the 33 test-retest bservers are shwn separately fr the tw eyes in Figures 6A and 6B. The mean sensitivity change frm the first t the secnd test fr each eye (Fig. 6A) was always negative except at ne frequency (2.5 Hz). It seems, therefre, that nrmal bservers tend t d mre prly with repeated testing in a sessin, althugh the reductin in sensitivity averaged less than 1 dl. The pattern f reductin was, as wuld be expected, similar between the tw eyes but appeared t increase with tempral frequency. The relatinship with frequency was significant at P < 0.01, as indicated by the crrelatin value f fr the tw eyes cmbined. The variability f the test-retest data within the sessin, hwever, was relatively unifrm acrss frequency (Fig. 6B), with an average SD acrss eyes f 1.62 dl. Applying the same crrectin fr tw distributins that was used fr the between-sessin variability gave an estimated SD fr a single sessin f 1.14 dl. The average slpe was nt statistically significant and accunted fr a full range f nly ±13% arund the mean level. Effects f Optical Blur Discussin The data in Figure 3 shw that the tempral visugram test was highly insensitive t refractive errr because a degree f diptric errr beynd anything that nrmally wuld be encuntered has nly a marginal effect n sensitivity and is frequency independent. This result emphasizes the rbustness f tempral sensitivity in the frm we tested, with respect t bth the spatial parameters f the stimulus and the refractive status f the patients under diagnsis. The blur data in Figure 3 als are interesting frm the pint f view f the mechanisms underlying tempral sensitivity. It has been suggested 13 that tempral sensitivity has a large cntributin frm lcal edge effects, especially at lw tempral frequencies. If this were a differential effect (that culd perate at lw tempral frequencies but nt at high tempral frequencies where the spatitempral phase f the perceived flicker becmes indeterminate), the shape f the flicker sensitivity functin wuld be affected by the degree f blur. Our data shw n such effects and therefre suggest that edge effects d nt prvide a detectable cntributin t flicker sensitivity in this stimulus cnfiguratin, at least dwn t the lwest tested frequency f 2.5 Hz. This may be because the edges f the stimulus are viewed in the near periphery (at 2.5 eccentricity if the bservers are fixating accurately) where edge sensitivity is less than in the fvea. Reducing the edges by prfund blurring therefre Right Eye Left Eye (N =33) ) B " Q) ^ i Q -D 10- g ) Frequency (Hz) 03 n 111 l -L J Fig. 6. (A) Mean sensitivity change, and (B) SD f test-retest variability within the first sessin (between first and secnd runs fr each eye), in the same frmat as Figure 5. Nte that almst all mean values decrease during the sessin, especially at higher frequencies.

8 N. 9 FLICKER SENSITIVITY TEST NORMS / Tyler 2559 has little effect n the fveal cntributin t the measured sensitivities. The implicatin f a failure t use edge infrmatin is supprted by the fact that the tempral sensitivities were much lwer than thse fr spatially mdulated targets at the ptimum spatial frequency. 14 It thus appears that the reprted flicker sensitivities are mediated by detectrs in the area stimulated rather than edge-selective mechanisms emphasizing the regins lcal t the edge f the flickering area. Right-Left Eye Difference The data in Figure 2 shw that, as tested, the left eye had a lwer sensitivity than the right by an average f abut 0.4 dl. Mrever, the pattern f lss with respect t tempral frequency in yunger adults was similar t the eye effect; lw frequencies were the least affected and high frequencies, the mst. Hwever, the ppsite pattern was seen in the lder bservers. The mean age differences acrss tempral frequency were significant {P < 0.01) by the chi-square test. A pssible explanatin fr this empiric left-eye inferirity is that the clinical prcedure was always t test the right eyefirst,fllwed by the left eye. The left eye sensitivities thus might have suffered frm fatigue r ther time-linked factrs during the testing sessin. This pssibility was addressed by the results f the within-sessin test-retest data; these shwed a tendency tward reduced sensitivity fr the secnd test during a sessin relative t the first (Fig. 6). It therefre is apprpriate t cmpensate fr the expected test-retest changes in the eye effect. Hwever, the test-retest data were separated by an intervening test f the ther eye in each case; therefre they represented a separatin f twice as much time and effrt as the intereye cmparisn. Accrdingly, the cmpensatin used was ne half the values shwn in Figure 6A, averaged acrss eyes. This represents the best estimate f the effect f test rder alne n the left-right eye cmparisn. The resulting difference in sensitivity between the tw eyes is shwn fr yunger and lder bservers in the frm f a visugram fr the left eye relative t the right eye sensitivities in Figure 7. This plt indicates that the left-right differences became insignificantly different frm zer after crrectin fr rder effects. The dashed lines shw the average 5% cnfidence intervals f ± 1.95 SEM based n the SEM f dl fr the yunger grup and dl fr the lder grup averaged acrss frequency. These cnfidence intervals encmpass the crrected differences at all frequencies; all the differences therefre are within measurement errr. The left-right difference seen in Figure 2 was attributable entirely t the fact that the right eye was tested first. as -1 si - 2 « MPENSATED MEANS a yrs yrs 5 1 V5 Tempral Frequency (Hz) Fig. 7. Left-right eye differences crrected fr within-sessin changes fr yunger and lder bservers. Differences arc all less than abut 0.5 dl. Dashed lines shw average cnfidence intervals f ± 1.95 SEM fr the tw grups arund the means, averaged acrss tempral frequency and age grup. Nte that neither grup f bservers shws significant intcrcular differences exceeding these cnfidence intervals at any tempral frequency. Partitining the Ppulatin Variance The verall ppulatin SD, averaged acrss the tw age grups, was 1.73 dl, giving a ttal variance f 2.98 dl. A very similar value f the ppulatin SD was btained in the previus study f ver 800 bservers. 9 This variance may be partitined between the interbserver, between-sessin, and within-sessin cntributins by the additive variance mdel. 15 This mdel relies n the assumptin that the factrs influencing variability with and between sessins are independent and nt related t individual differences. A sample f crrelatins in the full data set were evaluated fr interdependence f test-retest variability with individual differences, but n cnsistent crrelatin trends were fund. It therefre appears that the independent variability assumptin is a reasnable assumptin fr these data. The mean between-sessin variance averaged ver each bserver was 1.42 dl, based n the estimated single-sessin SD f 1.19 dl. Cmparing this variance with the verall ppulatin variance f 2.98 dl shws that the between-sessin variance accunted fr 47.7% f the measured ppulatin variance. Because the variance between sessins includes essentially all the influences n an individual that culd enter int the grup measurement, it fllws that nly 53.3% f the variance was caused by stable variatins between individuals. The implicatin is that inherent flicker

9 2560 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991 Vl. 32 sensitivity is relatively similar acrss individuals and wuld have a SD f 0.53X2.98= 1.26 dl if it culd be measured withut any test variability. It shuld be nted that this analysis applies t the Winsrized data with extreme respnses remved and therefre determines the "typical" sectin f the bserver ppulatin and test variability distributins lying within ± 2 SD f the mean (95% f the ppulatin and its respnses). The remaining 5% cnstitute extreme bservers r respnses that may nt have the same relatinship between the ppulatin and test variabilities. The SD f 1.26 dl fr the typical members (95%) f the ppulatin implies a full range f underlying sensitivity variatin f 5 dl, r abut a 300% difference frm the lwest t the highest. The same apprach may be taken t partitining the ppulatin variance int within-sessin variance, althugh there may be ther influences perating between tests in a single sessin that d nt perate n the first test in each sessin acrss bservers. Examples are the fatigue and practice effects mentined previusly. Any attempt t partitin the ppulatin variance n the basis f within-sessin variance is therefre an upper bund (there culd always be less within-sessin variance perating in the test ppulatin than was measured in sequential tests in the sessin). The within-sessin variance had the lw value f 1.30 dl, accunting fr 43.6% f the ppulatin variance. This means that the ttal variance can be partitined int this 43.6% frm within-sessin variance, 4.1% frm between-sessin variance, and 53.3% frm inherent individual variatin. It thus appears that the majr cmpnents f the flicker variability are the stable individual differences between bservers and the immediate within-sessin test effects, with nly a small prtin attributable t variability between sessins. Key wrds:flicker,tempral sensitivity, nrmal, variability, blur Acknwledgment The authr thanks R. H. Stamper, Chairman f the Department f Ophthalmlgy, Pacific Presbyterian Medical Center, San Francisc, Califrnia, fr making their facilities available. References 1. Tyler CW: Specific deficits f flicker sensitivity in glaucma and cular hypertensin. Invest Ophthalml Vis Sci 20:204, Stamper RL and Tyler CW: Effect f glaucma n central visual functin. In Recent Advances in Glaucma, Tich U and David R, editrs. Amsterdam, Elsevier, 1984, Buncic JR, Tytla ME, and Trpe GE: Flicker sensitivity in cular hypertensin befre and during hyptensivc treatment. ARVO Abstracts. Invest Ophthalml Vis Sci 27(Suppl):158, Tyler CW, Ernst WJK, and Lyness AL: Phtpicflickersensitivity lsses in simplex and multiplex retinitis pigmentsa. Invest Ophthalml Vis Sci 25:1035, Tyler CW and Ernst WJK: Psychphysical evaluatin f the tempral respnse f the retina in cular disease. Eye Science 2:109, Ernst WJK, Tyler CW, Clver GC, Nble BA, and Faulkner DJ: Flicker studies f retinitis pigmentsa. In Research in Retinitis Pigmentsa, Vl 62, Zrenner E, Krastel H, and Gebel HH, editrs. Oxfrd, Pergamn, 1987, : 7. Tyler CW: Tw prcesses cntrl variatins inflickersensitivity ver the life span. J Opt Sc Am [A] 6:481, Van Nrren DV and Vs JJ: Spectral transmissin f the human cular media. Visin Res 14:1237, Werner JS: Develpment f sctpic spectral sensitivity and the absrptin spectrum f the human cular media. J Opt Sc Am [A] 72:247, Tukcy JW and McLaughlin DH: Less vulnerable cnfidence and significance prcedures fr lcatin based n a single sample: Trimming/Winsrizatin. Sankhya A 25:331, de Lange H: Research in the dynamic nature f the human fvea-crtex systems with intermittent and mdulated light: I. Attenuatin characteristics with white and clred light. J Opt Sc Am [A] 48:777, Kelly DH: Visual respnses t time-dependent stimuli: I. Amplitude sensitivity measurements. J Opt Sc Am [A] 51:422, Tulunay-Kcescy U: Variables determiningflickersensitivity in smallfields.j Opt Sc Am [A] 60:390, Rbsn JG: Spatial and tempral cntrast-sensitivity functins f the visual system. J Opt Sc Am [A] 56:1141, Winer BJ: Statistical Principles in Experimental Design. New Yrk, McGraw-Hill, 1962,