Perception & Psychophysics 1998.60 (3).491-502

Characteristics ofanisometropic suppression: Simple reaction time measurements

MICHAEL J. PIANTA and MICHAEL KALLONIATIS University ofMelbourne, Melbourne, Victoria, Australia

The characteristics of artificially induced anisometropic suppression were investigated in ob­ servers with normal and abnormal binocular vision (anisometropic amblyopia) by using a simple re­ action time paradigm. Reaction time was measured as a function of stimulus intensity for various stimulus durations. For all conditions, the reaction time increased as stimulus intensity decreased toward threshold. Wefound that traditional techniques for modeling this trend were inadequate, so we developed a simple visuogram method for comparing these functions. Using this technique, re­ action time versus intensity functions are shown to be shape-invariant for all conditions examined. This means that, although reaction times are longer during induced anisometropic suppression or in anisometropic amblyopia, they are the same ifcontrast is normalized to equate threshold. The shape­ invariant nature of these functions is also consistent with the notion that a single mechanism medi­ ates detection under these conditions. Temporal summation was investigated at both threshold (method of limits) and suprathreshold (criterion reaction time) levels. Again, because of shape in­ variance, the suprathreshold results mirror the threshold results. The critical duration (the duration at the intersection of the complete summation and zero summation regions) is not affected by any of the conditions. However, the critical intensity (the intensity for the zero summation region) is higher for the amblyopic eyes, as compared with the normal or nonamblyopic eyes. Induced anisometropic suppression always increases the critical intensity, with a smaller increase occurring for the ambly­ opic eyes. This suggests that amblyopic eyes do not have a need for strong suppression.

Suppression ofall or part ofthe visual field ofone eye els, alternating periods ofsuppression and dominance are is often associated with abnormal binocular vision. It oc­ experienced; and, with large levels, suppression is constant curs under binocular viewing conditions and is generally (Simpson, 199I). Also, increasing the amount ofinduced assumed (see, e.g., Burian & von Noorden, 1974) to repre­ anisometropia increases the size ofthe suppression scotoma sent an adaptive mechanism that attempts to eliminate (Simpson, 1991). confusion (resulting from different images falling on cor­ It has been hypothesized that chronic suppression is re­ responding retinal positions) and diplopia (resulting from sponsible for the developmentofamblyopia (see, e.g., Duke­ the same images falling on disparate retinal positions). Elder & Wybar, 1973). It seems logical that anisometropic Suppression that occurs in abnormal binocular vision is amblyopia (a developmental anomaly that occurs when the usually called clinical suppression. However, suppression image in one eye is habitually defocused because ofan im­ can also occur in individuals with normal binocular vision. balance in the refractive status between the eyes) may be Anisometropic suppression may be induced by defocus­ a sensory consequence ofanisometropic suppression, be­ ing the image in one eye relative to the other (Humphriss, cause both conditions develop in response to differential 1959, 1960, 1963, 1982; Humphriss & Woodruff, 1962; interocular image focus and both result in the functional Schor, Landsman, & Erickson, 1987; Simpson, 1991, 1992; loss ofvisual information from one eye. Ifthis hypothesis Simpson, Smith, Harwerth, & Kalloniatis, 1990; Wolfe & were true, it would be reasonable to assume that common Owens, 1979). Under these conditions, the amount ofsup­ neural mechanisms underlie these conditions. pression depends on the level ofanisometropia: With small We have used a reaction time paradigm to compare the levels, suppression does not occur; with intermediate lev- mechanisms responsible for the detection ofdifferent du­ ration stimuli during induced anisometropic suppression and anisometropic amblyopia. Previous studies have used reaction times to differentiate the response properties of This work was supported, in part, by National Health and Medical Research Council Grant 950607 and by a Special Initiatives Grant (from sustainedand transient mechanisms in normal (Harwerth, the University ofMelbourne) to M.K. We thank three anonymous refer­ Boltz, & Smith, 1980; Harwerth & Levi, 1978a) and abnor­ ees for their helpful suggestions on a previous version ofthis manuscript mal binocular vision (Harwerth & Levi, 1978b; Levi, Har­ and associate editor R. P.O'Shea for valuable editorial comments. Cor­ werth, & Manny, 1979). These studies provide evidence respondence concerning this article should be addressed to M. Kalloni­ that, in amblyopia, the sustained neural channels are more atis, Department ofOptometry and Vision Sciences, University ofMel­ bourne, Melbourne, Victoria, 3052, Australia (e-mail: m.kalJoniatis@ severely affected (i.e., have a higher intensity requirement) optometry.unimelb.edu.au). than are the transient channels. Different duration stimuli

491 Copyright 1998 Psychonomic Society, Inc. 492 PIANTA AND KALLONIATIS

Synoptophore Targel8 5 (a) (b) L,

~ ~ AS

~

M, P,

~

W

ST

F,

L4

FS,

M P2 II 2

~ F2 LS

RightEye

Figure 1. Schematic of the Maxwellian view optical system and modified synoptophore. Inset shows the synoptophore targets. Target a is viewed by the tested eye; target b is viewed by the other eye. Readers capable of free fusion may experience alternating periods ofsup­ pression and dominance ofthe central region of the targets. The predominance of the cross center can be reduced by defocusing the right eye. (S, light source; L, lens; AS, aperture stop; FS, field stop; P, pellicle; F, fixed neutral density filters; W, circular neutral density wedge; M, plane front surface mirrors; ST, shutter; T, background targets; D, diffusers; B, blur lenses.) are used, in an attempt to tap into the sustained and tran­ present during normal binocular vision (O'Shea, 1987; sient channels: The transient mechanism should be more O'Shea & Crassini, 1981). sensitive to brief-duration stimuli, and the sustained mech­ anism should be more sensitive to long-duration stimuli. METHOD Measuring reaction times as a function ofstimulus duration also allows us to examine temporal summation at both Apparatus Temporal summation was measured with a binocular four-chan­ threshold and suprathreshold levels for the two conditions. nel optical system, shown schematically in Figure I. Two channels Reaction times have also been used to assess binocular ri­ were part ofa Maxwellian view optical system; the others were the valry suppression and to determine whether suppression is left and right channels ofa synoptophore. CHARACTERISTICS OF ANISOMETROPIC SUPPRESSION 493

Table 1 nance rose to maximum in 3 msec; at offset, the luminance fell to zero Observer Details in 1.5 msec. All flash durations reported were calculated by the Observer Age Eye Refractive Error Corrected Acuity width-at-half-height ofthe pulse. M.K. 35 L +0.25/-0.50 x 160 6/4.5+ Observers R plano/-0.25 x 80 6/4.5+ M.P. 25 L +3.50/-1.75 x 160 6/6+ Six observers participated in this experiment; each gave informed R +3.75/-1.75 x 10 6/6+ consent. Two ofthese, the authors (M.K. and M.P.), had normal (cor­ G.N. 25 L* +5.50/-1.50 x 70 6/6- rected) visual acuity and normal binocular vision; both had knowl­ R +0.75 6/4.5 edge of the purpose ofthe experiments. Four observers had hyper­ S.D. 26 L* +5.00/-4.50 x 29 6/19 metropic anisometropic amblyopia. Two ofthe amblyopic observers R +1.25 6/4.5 (G.N. and S.D.) had a general idea ofthe purpose ofthe experiments; L.y. 22 L +0.50/-0.25 x 170 6/6++ the other two (L.Y.and P.O.) were naive. R* +4.25/-1.00 x 180 617.5+ The diagnosis ofanisometropic amblyopia was based on our clin­ P.D. 29 L* +1.25/-3.50 x 15 617.5-- ical assessment, interpreted in terms ofthe following definition pro­ R +0.25/-0.50 x 170 6/3.8 posed by Ciuffreda, Levi, and Selenow (1991): *Amblyopic eye. Amblyopia canbe definedas a unilateral (or infrequently bilateral)con­ dition in which the best correctedvisual acuity is less than 6/6 in the absence of any obviousstructuralor pathological anomalies but with One channel ofthe Maxwellian view was used for the test field; one or more of the following occurringbefore the age of 6 years: am­ the other channel provided a background field during pilot experi­ blyogenic anisometropia, constantunilateral esotropiaor exotropia, am­ ments in which the synoptophore background targets were not used. blyogenic bilateralisometropia, amblyogenic unilateral orbilateral astig­ matism,imagedegradation. Anisometropia was induced by inserting positive lenses in the syn­ optophore at positions BL or BR, out of the optical path of the test Each observer was orthophoric at the time oftesting. One ofthe am­ field. The combination ofthe test field and the synoptophore targets blyopic observers (G.N.) experienced intermittent exotropia during was made possible by the replacement of the plane mirrors in the childhood and when fatigued but usually showed no eye movement synoptophore with pellicles PL and PRo Presentation ofthe test field to cover test. None ofthe otherobservers had a history ofstrabismus to the left or right eye was achieved by sliding the synoptophore or showed any abnormality of ocular alignment on examination. along rails to the right or left, respectively. Normal synoptophore None of the observers had anomalous retinal correspondence (as controls for interpupillary distance and phorias were available and tested by the Bagolini striated glasses). The presence ofreduced cor­ permitted precise alignment ofthe targets. rected monocular vision in the absence ofany otherocularabnormal­ Experiments were automated via a personal computer (IBM com­ ityjustifies the inference that anisometropia was present early in life. patible) fitted with a digital input/output board (PC-14B; Boston Both of the observer's pupils were dilated (two drops of 0.5% Technology) and a sound cara (SoundBlaster Pro, Version 2.0, Cre­ Tropicamide) 15 min prior to the experiment, and full spectacle cor­ ative). The experimental paradigm was controlled by a C++ program rections were worn. Visual angles were not corrected for magnifica­ (Borland C++, Version 3.1). The digital input/output board was used tion effects. Details for each observer are supplied in Table I. to operate the shutter (Uniblitz), monitor the observer's responses, and accurately time the stimulus and foreperiod durations (using an Procedure on-board timer). The sound card generated ready signals, feedback Prior to formal data collection, each observer was provided with signals, and white noise in headphones worn by the observer. The ob­ two or more sessions ofpractice, in order to minimize learning ef­ server's head was held steady by having the observer bite on a dental fects during the course of the experiment. Also, prior to each ses­ wax impression (Kerr) connected by a rigid bar to the synoptophore. sion, a short practice (5-10 min) was conducted to ensure proper alignment ofthe observers and to prepare them for the main session. Stimuli Observers wore earplugs and headphones playing white noise to Background field targets are shown in Figure I (inset). The line mask the sound ofthe shutter and other extraneous noises. The ready width for each target was 0.55°, and the missing central region of signals and warning signals were also supplied through the head­ target (b) was 1.00° in diameter. Under monoptic conditions, only phones. An experimental session consisted of 10 runs for up to seven target a was viewed; the other eye was occluded. Under dichoptic stimulus durations and lasted from 1.5 to 2.5 h. conditions, both targets were used: target a was viewed by the eye Increment thresholds were determined as a function of stimulus being tested, the other eye viewed target b. Both background fields duration with a modified method of limits that incorporated a sim­ had the same luminance and gave a retinal illuminance of 1,720 td ple reaction time paradigm (Harwerth et aI., 1980; Kalloniatis & (based on a dilated pupil size of8 mm for each observer). This value Harwerth, 1990, 1991). In each trial, after a ready signal (a brief ensured that observers were at least 2 log units into the region ofWeber tone), the observer depressed a hand-held response button that ini­ adaptation (Blackwell, 1946; Fuortes, Gunkel, & Rushton, 1961). tiated an exponentially distributed foreperiod with a mean wait of The test field was a 0.65° diameter disk. Foveal spatial sununation 500 msec. (This type of foreperiod has a constant hazard function, (i.e., critical area) reported for amblyopic eyes is smaller than this which means that the time elapsed since the warning signal does not value (Flynn, 1967; Grosvenor, 1957; Miller, 1955; Weber, 1988); affect the momentary tendency for the reaction stimulus to appear; thus, any changes in spatial summation should not affect thresholds thus, there are no cues for the observer as to when to expect the re­ for observers with anisometropic amblyopia. The test was presented action stimulus.) The stimuli were presented for a range ofdurations monocularly in the center ofthe continuous background target as a that varied pseudorandomly. Each stimulus offset occurred either rectangular pulse. Illuminance was measured after each session with after the duration ofthe stimulus elapsed or after the response ofthe an illuminance meter (Gossen PANLUX), and retinal illuminance observer (i.e., it was response-terminated), whichever occurred first. was calculated with the method described by Nygaard and Frumkes Therefore, the stimulus duration specified in the experiments was the (1982). The test field was attenuated by fixed neutral density filters maximum duration. (Note that, except for the longest duration stim­ and a circular neutral density wedge. ulus at intensities far above threshold, the maximum duration always Stimulus duration was calibrated with a digital storage CRO occurred.) Observers were instructed to respond as quickly as pos­ (Gould 20 MHz, Type 1425) and a photodiode (Radiometric filter, sible to the stimulus, without making early releases. If the observer Model 1158, United Detector Technologies). At onset, the lumi- released the button within the response interval (starting 100 msec 494 PIANTA AND KALLONIATIS

800 to be dominant. The suppression condition is the opposite, with the -u MK background blurred for the eye being tested, forcing it to suppress. I Nonnal E 600 If n Dichoplic Dominance Analysis -Gl f ~ For each condition, mean reaction times were plotted as a function E ofintensity. Pieron (1914, 1920) was first to suggest that these func­ i= 400 c l!jIlj<¥.,L tions follow a very simple law: 0 E(l) = r + kt:», (I) ti 200 o III 0 13msec where E(I) is the expected reaction time, 1is a measure ofintensity, CIl 200msec a:: • rois an irreducible time constant, k is a reducible time constant, and 0 f3 is an exponent. Mansfield (1973) and Kohfeld, Santee, and Wal­ 0 2 3 4 lace (1981 a, 1981b) have presented various methods for fitting this empirical formula to the data. However, more recently, Luce (1986) log I (td) has proposed that a bettermethod is to fit Equation I to the data with Figure2. Mean reaction time (in milliseconds) plotted as a func­ a least squares fitting procedure. Luce compared results ofthe three tion ofintensity for two durations, measured underthe conditions methods and found that, depending on the method used, the value of indicated (Observer M.K.). Error bars show standard errors of ro could be affected by tens ofmilliseconds and, more importantly, the mean. Reaction times are shortest for the highest test intensity f3 by factors of2 or 3. We fitted our reaction time data with the least and then increase as test intensity is decreased toward threshold squares method suggested by Luce. (indicated by the vertical dotted lines). Thresholds for the 13- and For each nominal duration, psychometric functions were derived 260-msec test flashes occur at 1.63 and 1.13 log td, respectively. from the number of responses obtained at each stimulus intensity Subsequent reaction time figures have been normalized (by trans­ during the method of limits. A cumulative Gaussian with two free­ lating the data along the x-axis to position threshold at 1.0 log td) floating parameters (mean and standard deviation) was fitted to to simplify comparison ofthe shape ofthe functions. these psychometric functions (SigmaPlot for Windows, Jandel Sci­ entific). Threshold was defined as the intensity required to produce 50% correct (i.e., the mean of the fitted Gaussian) and has been plot­ after stimulus onset and lasting 1,000 msec after stimulus offset), it ted against duration in Figure 6 and in the first column of Figure 7. was assumed that the observer detected the stimulus (a hit), and the Since analysis of threshold-duration functions does not provide a reaction time was recorded as the time from the onset ofthe stimu­ useful description ofthe temporal response ofa linear system (Wat­ lus to the observer's button release. The response was reinforced son, 1986), we used a classical two-line curve fit with two free pa­ with a briefhigh-pitched tone. A response during the foreperiod or rameters: the critical duration (te) and the critical intensity (Ie)' within 100 msec ofthe stimulus onset (an early release) was penal­ Equation 2A models the Bloch's law (or total temporal summation) ized by a long-duration deep tone (time-outof5 sec). Ifthere was no region ofthe curve and has a fixed slope of - I. Equation 28 models response before the end ofthe response interval (a miss), it was in­ the region ofzero temporal summation and has a fixed slope ofzero. dicated by a briefdeep tone. After feedback, there was a I-sec delay log 1 = log Ie+ log t, - log t for t :::; te (2A) before the ready signal ofthe next trial. After each trial that resulted in a hit, the intensity ofthe stimulus was decreased by 0.1 log units. log 1 = log Ie for t > i, (2B) For each consecutive miss after the first miss after a hit, there was a In these equations, 1is the threshold intensity and t is the duration. .5 probability that the run would terminate. Alignment of the ob­ We also fitted threshold-duration functions with the following server was checked after each run. function: As noted in the introduction, induced anisometropic suppression may not result in constant suppression of the more defocused eye. 1/t, = I + te / t. (3) For the normal observers, near to constant suppression was achieved A useful property ofthis function is that it has a smooth transition with 2.0, 3.0, and 6.0 dioptres ofanisometropia. In order to produce between the regions of complete and zero temporal summation. constant suppression in the nonamblyopic eye ofthe observers with However, the goodness-of-fit (as assessed by chi-squared) was not anisometropic amblyopia, we were forced to use 6.0 dioptres ofani­ consistently better with Equation 3 than with Equations 2A and 28. sometropia. We used the same amount ofanisometropia for the am­ For this reason and to enable comparison with previously published blyopic eye. Even with these levels, there were rare occasions on data, we have used Equations 2A and 28 to model our threshoId­ which suppression and dominance would alternate. To maximize the duration data. chance that stimuli were presented during the phase we were inter­ ested in, observers were advised to initiate a trial at the beginning of the suppression or dominance phases. Distributions for the durations RESULTS of the rivalry-like phases were measured subjectively with a timer (Fox, Todd, & Bettinger, 1975; Levelt, 1965; Walker, 1975). These Reaction Times distributions were fitted with gamma distribution (Lehky, 1988) and compared with the foreperiod distribution. In all cases (except for Figure 2 shows the reaction times as a function oftest in­ Observer M.K. with 1.0 dioptre ofanisometropia, see Results), there tensity for 13- and 200-msec test durations (measured under was less than 6% overlap (i.e., on average, the stimulus may have the conditionsindicated). Reactiontime versus intensity func­ been presented during the wrong phase in only lout of 17 trials). tions have the same basic properties for an conditions: The There were three situations examined in this experiment: monoc­ reaction time is shortest for the highest test intensity and ular, dominance, and suppression. No suppression can occur in the then increases as test intensity is decreased toward thresh­ monocular condition (by definition, suppression is a purely binocular phenomenon), so it is used as a control condition for 2 ofthe observers old (indicated by the vertical dotted lines). The 13-msec with amblyopia. The dominance condition is achieved when both data appear to the right ofthe 200-msec data, because the eyes view the background targets, with the anisometropia blurring threshold for the 13-msec test flash is 0.5 log units higher the background for the eye not being tested, forcing the tested eye than the threshold for the 200-msec test flash (compare to CHARACTERISTICS OF ANISOMETROPIC SUPPRESSION 495

800 ....,...... ----,~------, (a) MK (b) PO Normal Non·amblyopic 600 Dichoptic Dichoptic Dominance Dominance ts msec 13.5 msec 400 g - '0 =224 (0.656) '0 = 257 (0.985) en 200 E k =5900 (0.962) k =516 (0.988) ~ =1.10 (0.971) ~ =0.51 (0.996) -II) O-l---,..-,.--,--.-'"""T"""----r-...... ---i E j:: c 800 ..,--..,..------, o (c) GN (d) SO ~ Non·amblyopic Amblyopic m 600 Dichoptic Dichoplic a: Suppression Dominance BO.3ms9C 13.5 msec 400

200 '0 = 273 (0.965) '0 = 324 (0.949) k = 2180 (0.994) k =5413 (0.996) ~ =0.97 (0.996) ~ =1.49 (0.998)

o 2 3 4 0 2 3 4

log I (td) Figure 3. Mean reaction time (in milliseconds) plotted as a function of intensity for various ob­ servers and conditions. Error bars show standarderrors orthe mean. The lines represent the results of the curve fit using the method suggested by Luce (1986), and the parameters and their depen­ dencies (in parentheses) are shown. The parameters k and f3(and often '0) have dependencies close to one, indicating that the equation is over-parameterized.

Figure 6a, open circles). Figures 3a-3d and 4a-4d show one another. This indicates that the equation is too compli­ similar data, but these data have been normalized by trans­ cated and is probably over-parameterized. Note that higher lation along the x-axis to position threshold test intensity at dependencies result when the data are variable or when 1.0 log td in order to simplify comparison ofthe function the data are collected over a smaller range of intensities. shapes (the data shown in Figure 4a are the normalized data However, even with data obtained from an experienced ofFigure 2). At near threshold test intensities, typical reac­ observer over a wide range ofintensities, k and {3 are still tion times were between 500 and 800 msec. dependent on each other (Figure 3a). Thus, k and {3 alone To obtain parameters that could be used to characterize should not be used to describe the shape ofthese functions. and compare the shape ofthe mean reaction time versus As the approach proposed by Luce (1986) could not be intensity functions, we fitted the data with the equation sug­ used to uniquely describe the shape ofthe functions and gested by Luce (1986). However,the two shape parameters as previously used methods have also been shown to be (k and (3) derived with this procedure were not indepen­ inadequate, we developed a new method for comparing dent. Figure 3 illustrates this dependency problem when mean reaction time versus intensity curves. Graphs com­ applied to select data from a variety ofconditions and ob­ paring two parameters were plotted (nonamblyopic vs. am­ servers. The values of the parameters returned from the blyopic, monocular vs. dominant, dominant vs. suppressed, curve fit are indicated on each graph; the dependencies or durations inside Bloch's law vs. durations outside Bloch's are shown in parentheses. The dependence ofa parameter is law), and separate graphs were produced for each combi­ defined to be nation of the other parameters. The threshold values of both functions were normalized to I log td. In other words, dependence the reaction time versus intensity curves being compared (variance of the parameter, other parameters constant) were translated along the x-axis, so that threshold corre­ (variance of the parameter, other parameters changing) . sponded to 1 log td. This superimposed the functions for the two conditions and exposed any shape differences. (4) To further emphasize shape differences, difference plots (or visuograms) were derived. These graphs plot the The parameters k and {3 (and often ro) have dependencies difference in mean reaction time as a function ofintensity near one, suggesting that they are strongly dependent on for each ofthe graphs discussed above. Ifthe mean reac- 496 PIANTA AND KALLONIATIS

800 (a) MK (b) MK Nonnal Normal 600 Dichoplic Dichoplic Dominance Suppression

400 ~ -en E 200 0 13 msec o 13msec -Gl • 200msllC • 200msllC E i= 0 c 0 800 :;= (e) MK (d) MK U Normal Normal :8 600 Diehoptic Dichopllc II: 13 msec 200msac 400 ~ 200 0 Suppression 0 Suppression • Dominance • Dominance 0 I I 0 2 3 4 o 2 3 4 log I (td)

400 u (e) MK -I E 200 -Gl U C l! 0

:! o Graph(a), p = -17.4:1: 36.2 is -200 o Graph(b), ~ = 9.2 ., 60.0 t- A Graph(e), p =10.8:I: 55.5 Il: v Graph(d), u =-22.6 :I: 57.8 -400 2 3

log I (td) Figure 4. Mean reaction time (in milliseconds) plotted as a function of intensity (normalized rel­ ative to 1 log td) for normal Observer M.K. Error bars show standard errors ofthe mean. Panels a­ b: A comparison of durations within and outside Bloch's law, during dominance and suppression. Panels c-d: A comparison of suppression and dominance for durations within and outside Bloch's law. Panel e: A visuogram showing the difference in mean reaction time between the functions plot­ ted on each of the other graphs (open symbols minus fiDed symbols). The visuogram shows no sys­ tematic variation, indicating that the mean reaction time versus intensity curves are the same shape for each condition. tion time versus intensity functions are different shapes, show that there is no difference in the shapes ofthe func­ there will be a systematic variation in the visuogram plot. tions for the two conditions, and this is confirmed by the Ifthere is a constant increase or decrease ofthe mean re­ visuogram (Figure 4e). The similarity ofthe shapes ofthe action time for one of the functions, there will be a shift functions indicates that it is likely that detection is medi­ up or down of all points on the visuogram. To highlight ated by the same mechanism, under these experimental these types of changes, the mean and standard deviation conditions (Harwerth & Levi, 1978a, 1978b). for each plot as a whole are indicated in the figure legend. Figure 5 shows visuograms for the amblyopic Observers Figure 4 shows mean reaction time plotted as a function G.N., S.D., L.V, and P.D.,respectively. Only a selection of ofnormalized intensity for Observer M.K. Figures 4a-4d parameter combinations has been shown for each ofthese CHARACTERISTICS OF ANISOMETROPIC SUPPRESSION 497

400 -,------, (a) GN (b) so 200

o Ad.. ~~ or

j o N2S(B-L), P = -3.6 :t 26.1 o A20(B-L), p'" -18.3:t 56.7 E -200- c N2(S-O)L, P =-22.0 :t 24.6 C N2(S·O)L, P =-6.4 :t 23.1 .. (A-N)20L, P = -18.6:t 25.2 .. (A-N)20L, p. 10.0 :t 53.7 - v N(1-2)Ol, P = -7.4:t 37.4 v N(1·2)OL, p", -1.5 :t 35.8 I

(e) LV (d) PO

200

o N2S(B-L), P =-'0.9 :t 28.5 -200 o N20(B·L), P = -22.5 :t 26.7 c A2(S·O)B, P = 0.3 :t 35.9 c A2(S-O)L, P =-27.3 :t 33.0 .. (A·N)2SB, p. 7.3:t 20.8 .. (A-N)20L, P = 2.6 :t 35.9 v (A-N)2SL, P '" -6.7 :t 34.3 -400 ~--,...---,...---_r_----i 2 3 2 3 log I (td) Figure 5. Visuograms for amblyopic Observers C.N., S.D., L.v.,and P.O. The legend contains a key and the mean and standard deviation for each plot separately. The visuograms show no sys­ tematic variation, indicating that the mean reaction time versus intensity curves are the sameshape for each cendltien, [Legend key: Each legend entry contains five characters. The two parameters that are compared in the visuogram are shown in parentheses. Therefore, the key can be considered in four sections, in the foUowingorder. Eye tested: N, nonamblyopic; A, amblyopic. Viewing condi­ tion: t, monoptic; 2, dichoptic. Suppression phase: S, suppression; 0, dominance. Stimulus dura­ tion: B, brief(13.5 msec); L, long (80.3 msee), The two parameters that are compared are shown in parentheses.] observers, because the results are the same for all condi­ were derived from the reaction time data by plotting the tions. The shapes ofthe functions are very similar for every test intensity for a criterion reaction time at each duration. combination ofparameters in each observer. Thus, in both The shape-invariant nature ofthe reaction time versus in­ amblyopic and nonamblyopic eyes, it is likely that detec­ tensity functions predicts that the suprathreshold results tion is mediated by the same mechanism (Harwerth & would have the same critical duration (te) as the threshold Levi, 1978a, 1978b). results, but with critical intensity (Ie) increasing as crite­ There is some suggestion that longer duration stimuli rion reaction time decreased. Plots of suprathereshold­ result in longer reaction times, because the mean of the duration functions (not shown) confirmed this prediction. mean reaction time differences for these conditions are all Consequently, we discuss only the threshold data below. negative; however, this effect is small. For the dominant Figure 6 shows dichoptic threshold-duration functions and the suppressed condition, the mean reaction times of for 2 normal observers under various levels of induced the normalized curves are the same. Thus, the mean ofthe anisometropia (0-3.0 dioptres). The dominance condition reaction time differences for these conditions are close to (no anisometropia) is an example ofa normal threshold­ zero. Therefore, when the intensity (or Weber contrast) is duration function. The critical durations are within the normalized to equate threshold, there is no difference in normal range expected at this level ofadaptation, as are the reaction time for the dominant condition and the sup­ critical intensities (Roufs, 1972). For the suppressed con­ pressed condition. The same applies for the amblyopic dition (3.0 dioptres of anisometropia), there is a general and nonamblyopic eyes. increase in threshold at all durations (by about 0.6 log units), as compared with the control condition. In other Temporal Summation words, Ie increases while te remains the same. Threshold-duration functions were plotted with thresh­ For ObserverM.K., thresholds measured for 2.0,3.0, and olds derived by the procedure outlined in the Method sec­ 6.0 (not shown) dioptres ofanisometropia were all simi­ tion. Similar functions, but for suprathreshold stimuli, lar. The same result was found for Observer M.P.with 3.0 498 PIANTA AND KALLONIATIS

(by 0.4-0.6 log units), with a minimal increase in tc'These (a) MK 3 results compare favorably with the results for normal ob­ 0 No aniso .. +1.000 aniso servers. The results for the amblyopic eye indicate that the .. +2.00 0 aniso thresholds for the monocular condition (Observers G.N. 0 +3.00 0 aniso and S.D. only) are again similar to those obtained for the 2 dominance condition. However, with suppression, there is E c a general increase in threshold (by 0.1-0.4 log units) and '0 ~ C>--- again a minimal increase in tc'There is little difference in tc - : 0 between nonamblyopic and amblyopic eyes under any of -'0 '0 the conditions. That is, for each condition there is a gen­ .c ell eral difference in threshold for the amblyopic eye, as com­ ..G) pared with the nonamblyopic eye. For the monocular con­ .c (b) MP l- 3 dition, this difference is 0.5-0.6 log units; for the dominant 0 Noaniso e:» c +3.00 0 aniso condition, the difference is 0.3-0.6 log units; and for sup­ .2 pression, the difference is only 0.1-0.2 log units. The second column in Figure 7 illustrates the changes 2 that occur in the critical intensity. There is only a small

c u change (less than 0.2 log units) in I, for the monocular and ~ dominance conditions. However, suppression causes a 0-- 0 common pattern ofchange: Ic increases for both the non­

'I amblyopic and amblyopic eyes, with this increase being 10 100 1000 greater for the nonamblyopic eye. Therefore, the threshold elevation that accompanies suppression results in the Duration (msec) threshold-duration functions becoming similar for the nonamblyopic and amblyopic eyes. To facilitate interpreta­ Figure 6. Threshold-duration functions for two normal ob­ servers under dichoptic conditions for various levels of ani­ tion ofthese results in terms ofthe Weber contrast, Table 2 sometropia (indicated in the legend). Standard errors of the shows the converted I c values. mean are smaUer than the symbol size, except where indicated. The last column ofFigure 7 highlights the constancy of Solid lines through the data represent the curve fitting results. the critical duration. There is no difference between tc for Induced anisometropic suppression causes an equal increase in the monocular condition and t, for the dichoptic condition, threshold at all durations. The results for 6.0 dioptres of ani­ sometropia were similar to the results for 3.0 dioptres of ani­ and suppression does not cause any reliable pattern ofchange sometropia and are not shown, for clarity. in tc' Some observers show an increase in tc with suppres­ sion; others show a decrease; however, these changes are small (not more than 0.2 log units). The nonamblyopic and 6.0 (not shown) dioptres ofanisometropia. This sug­ eyes tend to have a longer tc than do the amblyopic eyes; gests that anisometropic suppression is complete with however, these differences are also small (not more than 2.0 dioptres ofanisometropia and that further increases in 0.2 log units). Therefore, temporal integration does not the level ofanisometropia will not result in a change to the appear to be notably affected by either induced aniso­ suppression. The results for 1.0 dioptre ofanisometropia metropic suppression or anisometropic amblyopia. lie between the dominant and suppressed conditions. For this condition, the observer reported frequent alternations DISCUSSION of suppression and dominance, similar to binocular ri­ valry. Standard errors ofthe mean for these points are larger This study presents reaction time and temporal sum­ than those for the other results, which is to be expected mation data for 4 observers with anisometropic ambly- because, for this condition, the test flash is sometimes pre­ sented during suppression and at other times during dom­ Table 2 inance. This will result in the mean threshold lying some­ The Critical Intensity (/<) Expressed where between the mean threshold for dominance and in Terms of Weber Contrast suppression (in a position dependent on the temporal char­ Weber Contrast (MIl) acteristics ofthe alternations). Observer Eye Monoptic Dominance Suppression Threshold-duration functions for the 4 amblyopic ob­ G.N. Nonamblyopic 0.4 0.5 2.0 servers (G.N., S.D., L.Y., and P.D.) are shown in the first Amblyopic I.5 1.0 3.2 column ofFigure 7. All observers display similar charac­ S.D. Nonamblyopic 0.5 0.5 1.9 teristics. Results for the nonamblyopic eye show that the Amblyopic 1.8 2.1 3.0 L.y. Nonamblyopic 0.7 1.8 threshold-duration function for the monocular condition Amblyopic 2.3 2.7 (Observers G.N. and S.D. only) is very similar to that for the P.D. Nonamblyopic 0.4 2.0 dominance condition (both I c and tc are similar). However, Amblyopic 1.1 2.3 with suppression, there is a general increase in threshold Note-All values are percentages. CHARACTERISTICS OF ANISOMETROPIC SUPPRESSION 499

Threshold-duratlon Critical Intensity Critical duration

3 GN 3 GN GN 100 2 2 gf- -! ~ :~ 10

3 SO 3 SO so 100

2 2 -'0::.. • i~ '0 '0 -E :~ .c j 10 -_u E I LV ~ LV LV .c 3 ,g 3 ..u ... - 100 ~ ,g 2 2 ~ ! ~ ;--- 1 10

3 PO 3 PO PO 100 2 2 t==- ~ : i 1 10 I 10 100 1000 0 2 4 6 o 2 4 6 Duration (msec) Anisometropia (0) Anisometropia (D)

o Non-amblyopic eye.dominance • Amblyopic eye.dominance v Non-amblyopic eye,suppression ... Amblyopic eye,suppression o Non-amblyoplc eye,monocular • Amblyopic eye,monocular

Figure 7. Threshold-duration functions, critical intensities (Ie)' and critical durations (te) for 4 observers with anisometropic am­ blyopia. The eye tested and the viewing condition are identified in the legend. Amblyopia and induced anisometropic suppression both cause an equal increase in threshold at all durations. There is no notable change in the critical duration for any condition. With in­ duced anisometropic suppression the critical intensity increases more for the nonamblyopic eye than it does for the amblyopic eye.

opia, on the basis ofthe criteria ofCiuffreda et a1. (1991) Reaction Times and our clinical assessment. However, we acknowledge The data obtained indicate that mean reaction time ver­ that the use ofa traditional clinical definition ofamblyopia sus intensity functions obey the principles of shape in­ (e.g., a two-line difference in acuity) would not classify 2 variance (i.e., only the curves' relative position along the ofthe observers (L.V and G.N.) as amblyopic. Given the x-axis changes, not the shape ofthe curve) for a variety of definition noted above and other data (see Results), we conditions: duration within Bloch's law versus duration consider these to be cases of mild amblyopia. We regard outside Bloch's law,dichoptic (dominance) versus monoc­ Observer P.D. to be a case ofmoderate amblyopia and Ob­ ular, dominance versus suppression, and nonamblyopic server S.D. to be a case ofmore severe amblyopia. Thus, versus amblyopic eye. The principle of shape invariance we have attempted to cover a range ofamblyopia severity. means that, for all criterion reaction times, the horizontal 500 PIANTA AND KALLONIATIS

displacement between the curves will be the same, indi­ from 0.3 to 0.6 log units between observers (i.e., there is cating the same relative effect at suprathreshold levels. no change to te, but there is an increase in Ie)' Grosvenor For any of the conditions of this study, there does not (1957) reported an elevated threshold for wide bar stimuli appear to be a difference in shape between the curves for a in the fovea of the amblyopic eye of0.3-0.6 log units at duration inside Bloch's law and those for a duration out­ different background intensities. Grosvenor also found the side Bloch's law.Therefore, there is no change to temporal increase in threshold to be constant at three durations (10, summation (Ie) at suprathreshold levels (ifusing a criterion 50, and 200 msec) for all but the brightest background in­ reaction time paradigm) for any ofthese conditions. tensity (estimated to be 3,000 td, assuming a 4-mm pupil). These results are in conflict with those ofUeno (1977, An increase in threshold (about 0.5 log units) for large area 1978), who demonstrated that the exponent ofthese func­ foveal stimuli in the amblyopic eye has also been reported tions decreased with increasing stimulus duration. He by Flynn (1967). found a monotonic increase in t, with increasing criterion Harwerth and Levi (I978b) and Loshin and Jones (1982) reaction time and different exponents for different condi­ studied contrast sensitivity as a function ofexposure du­ tions. There are several possible reasons for the difference ration for specific spatial frequencies. They found that t, between our data and the results of Ueno. First, Ueno varied as a function ofthe spatial frequency, with longer (1977) used a constant foreperiod design, which results in integration times for higher spatial frequencies. The am­ a hazard function that increases with time. This type of blyopic eyes had normal i, values and normal summation foreperiod is unsatisfactory, because there is evidence for slopes (0.7 for gratings) at low spatial frequencies. How­ response criterion changes with a changing hazard func­ ever, at high spatial frequencies, te values were longer, and tion. In other words, this design allows the observer to base the summation slopes were shallower (slopes as flat as his or her response on an estimate ofthe foreperiod rather 0.4). They also found that Ievaried as a function ofspatial than on only the reaction stimulus (Luce, 1986).Catch trials frequency: At low spatial frequencies, there was a small in­ are often used to suppress time estimations; however, Ueno crease in Ie (about 0.1 log units for 0.5 cycles per degree), (1977) did not use catch trials. Second, in both studies, and, at high spatial frequencies, there was a larger increase Ueno (1977, 1978) did not give feedback to the observers. (about 0.5 log units for 8.0 cycles per degree). These re­ Thus, the observers do not have the opportunity to establish sults suggest that, had we used a high spatial frequency a value for the tradeoff between errors and speed (Luce, stimulus, we might have reached different conclusions. 1986); consequently, their criteria may vary. Finally, the Threshold-duration functions obtained for the ambly­ fitting paradigm used by Ueno (1977, 1978) is unreliable opic eye with monocular viewing are similar to those ob­ (Luce, 1986). tained with the dominance condition. This may seem sur­ There are differences in mean reaction time between the prising, considering that visual acuity in the amblyopic amblyopic and the nonamblyopic eyes before normalization, eye is reduced further when measured with the nonam­ with the difference becoming smaller at higher intensities blyopic eye open (Pugh, 1954; Schor, Terrell, & Peterson, or contrasts. This finding is in agreement with numerous 1976; von Noorden & Leffler, 1966). However, visual acu­ studies (Ciuffreda et aI., 1991; Hamasaki & Flynn, 1981; ity and increment thresholds are very different tasks. This Mackenson, 1958; von Noorden, 1961). However, there finding implies that (for Observers G.N. and S.D.) under appear to be no differences in mean reaction time between these conditions, there is little or no inhibitory effect ex­ the amblyopic and the nonamblyopic eyes when the in­ erted on the amblyopic eye from the nonamblyopic eye. tensity is normalized to equate thresholds. It seems that During induced anisometropic suppression in the nor­ the increase in reaction time for the amblyopic eye is a re­ mal eye or in the nonamblyopic eye, there is an increase in sult ofthe contrast threshold deficit ofthat eye alone, and, threshold that is constant for all durations (i.e., no change ifcontrast is scaled in threshold units, the reaction times to te, an increase in Ie)' The magnitude ofthe threshold ele­ for the two eyes are equivalent. vation ranges from 0.4 to 0.6 log units between observers. Induced anisometropic suppression in the amblyopic eye Temporal Summation causes a threshold elevation of0.1-0.5 log units. This ele­ Threshold-duration functions obtained for the nonam­ vation is again constant for all durations. For most ob­ blyopic eye during monocular viewing are similar to those servers, threshold is elevated by 0.1-0.3 log units. One ob­ obtained for the normal and nonamblyopic eyes during server showed a threshold elevation of0.5 log units. The dominance. For these conditions, critical durations (Ie) range threshold elevation caused by induced anisometropic sup­ from 20 to 42 msec, critical intensities (Ie) range from 6.5 pression in the amblyopiceye isalways less than that induced to I 1.7 td, and Weber contrast ranges from 0.4% to 0.7%. in the nonamblyopic eye or in normal eyes. This result These parameters are within the ranges obtained by other supports that ofHolopigian, Blake, and Greenwald (1988), authors for normal observers (under monocular conditions) who found that the degree ofamblyopia and the depth of at similar background intensities (Graham & Kemp, 1938; suppression were inversely related, which implies that am­ Herrick, 1956; Keller, 1941; Roufs, 1972). blyopic eyes do not have a need for strong suppression. During dominance, there is an increase in threshold for Are induced anisometropic suppression and binocular the amblyopic eye relative to the normal or nonamblyopic rivalry suppression related? It seems sensible to suggest eye (for both monoptic and dichoptic viewing). This ele­ that these conditions may be related, because they both vation ofthreshold is constant for all durations and ranges occur in response to conflicting monocular visual input and CHARACTERISTICS OF ANISOMETROPIC SUPPRESSION 501 they both result in the functional loss of vision from one will be greatest in neurons that are most responsive to eye. This experiment provides two lines ofevidence that stimulus characteristics that are suppressed. Therefore, suggest that they may be related. First, the threshold eleva­ anomalies of the amblyopic visual system should reflect tion during anisometropic suppression is equal for all du­ anomalies ofthe normal visual system during suppression rations, suggesting nonselectivity-a feature ofbinocular (Hubel et al., 1975). This reasoning relies on the presence rivalry suppression (Blake & Fox, 1974;Fox& Check, 1966, ofsuppression mechanisms early in human visual devel­ 1968; Holopigian, 1989; Wales & Fox, 1970). Second, the opment. Birch, Shimojo, and Held (1985) and Shimojo, threshold elevation is ofsimilar magnitude to that reported Bauer, 0'Connell, and Held (1986) have shown binocular for binocular rivalry suppression (Wales & Fox, 1970). rivalry to be present at about 4 months ofage in humans Is induced anisometropic suppression related to ani­ (preferential looking); therefore, it is likely that other sup­ sometropic amblyopia? It seems reasonable to suggest that pression mechanisms are also present. these conditions are related, because the instigating cir­ In conclusion, the major finding from this study is that cumstances (dissimilar monocular images) are compara­ reaction time versus intensity functions are shape-invariant, ble in the two cases. A similar line ofargument that is used which is consistent with the notion that a single mecha­ to relate anisometropic suppression to binocular rivalry nism mediates detection under these experimental condi­ can be used here. In the amblyopic eye, there is a thresh­ tions. This also means that, although reaction times are old elevation that is nonselective for duration. This result longer during induced anisometropic suppression or in is qualitatively (nonselectivity) and quantitatively (mag­ anisometropic amblyopia, they are equal ifcontrast is nor­ nitude ofthe threshold elevation) similar to the result for malized to equate threshold. Temporal summation, as as­ induced anisometropic suppression in normal eyes or in sessed by the critical duration, does not change for any of the nonamblyopic eye. However, Blake (1989) pointed out these conditions. 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