Clinical suppression and amblyopia. · No. 3 SUPPRESSION AND AMDLYOPIA / Holopigian er al. 445 University, were paid an hourly wage to serve in this experiment. Informed consent was

Investigative Ophthalmology & Visual Science, Vol. 29, No. 3, March 1988Copyright © Association for Research in Vision and Ophthalmology

Clinical Suppression and AmblyopiaKaren Holopigian,*^: Randolph Blake,* and Mark J. Greenwaldf

In individuals with abnormal binocular vision, such as strabismics and anisometropes, it is common forall or part of one eye's view to be suppressed so binocular confusion and diplopia are eliminated. Weexamined the relation between the depth of suppression (the amount by which the monocular contrastincrement threshold for an eye was elevated by stimulation in the contralateral eye) and the degree ofamblyopia (difference in monocular contrast thresholds for the two eyes). There was a significantnegative correlation between suppression and amblyopia, so that clinical suppressors with no ambly-opia exhibited deep suppression (ie, large threshold elevation) while observers with amblyopia exhib-ited weaker or no suppression. This negative correlation was found when the two eyes viewed ortho-gonally oriented contours as well as identically oriented contours. These results suggest that when aneye is amblyopic there is no longer a need for strong suppression of that eye by the contralateral eye.Invest Ophthalmol Vis Sci 29:444-451,1988

Individuals with abnormal binocular vision, suchas strabismics and anisometropes, often suppress partof one eye's view. This phenomenon may be eitherunilateral, such that one eye is chronically sup-pressed, or bilateral, with dominance and suppressionalternating between the two eyes. It is generally as-sumed1 that this clinical suppression is adaptive inthat it eliminates confusion (resulting from differentimages falling on corresponding retinal locations)and diplopia (arising from the left and right eyeimages falling on noncorresponding retinal loca-tions).

Another condition commonly associated with stra-bismus and anisometropia is amblyopia, a chronicreduction in monocular vision. Amblyopia may im-pair visual performance as measured by Snellen andgrating acuity,2 contrast sensitivity,3"6 vernier acuity,7

contrast matching8 and stereoacuity.9"12

Although amblyopia and suppression both develop

From the "Departments of Psychology and Neurobiology/Physi-ology, Northwestern University, Evanston, Illinois, and the fDe-partment of Ophthalmology, Northwestern University MedicalSchool, Chicago, Illinois.

J Present address: Department of Ophthalmology, N.Y.U. Med-ical Center, New York, New York.

This research was conducted as a partial requirement for thePhD degree for KH. Portions of this work were presented at the1987 ARVO meeting, May 4-8, Sarasota, Florida.

Supported by NSF grant BNS 8418731 to RB, by NSF grantNS07223-05 to the Neurobiology and Physiology Department atNorthwestern University, a Northwestern University dissertationyear grant to KH and a grant from Children's Memorial Hospital.KH is currently supported by a grant from the RP Fighting Blind-ness to the Retina Clinic at NYU Medical Center.

Submitted for publication: June 4, 1987; accepted September 24,1987.

Reprint requests: Department of Ophthalmology, N.Y.U. Medi-cal Center, 550 First Avenue, New York, NY 10016.

in response to conflicting monocular visual input tothe two eyes, the relationship between these entitiesremains unclear. There is some speculation thatlong-term chronic suppression is actually responsiblefor the development of amblyopia in one eye. Indeed,Sireteanu and Fronius13 and Sireteanu14 found thatportions of the visual field that exhibited deeper in-terocular suppression (assessed with a luminance de-tection task) were also more amblyopic (ie, hadpoorer monocular acuity); areas less strongly sup-pressed had better acuity. The view that suppressioncauses amblyopia cannot be entirely correct, how-ever, for it is known that many individuals with clini-cal suppression have equal visual acuity in the twoeyes.

To clarify the relationship between clinical sup-pression and amblyopia, we examined the correlationbetween the degree of amblyopia and the depth ofsuppression in a group of strabismic and anisome-tropic observers. In these experiments, the depth ofsuppression was defined as the amount by which thecontrast increment threshold for an eye was raised bythe simultaneous presentation of a stimulus to thecontralateral eye. Amblyopia was indexed by themagnitude of the difference between right and left eyecontrast thresholds. To our surprise, we found thatthe degree of amblyopia and the depth of suppressionwere inversely related. Clinical suppressors withequal monocular vision showed large amounts of in-terocular suppression, while those with amblyopia ex-hibited a much smaller suppression effect.

Materials and MethodsObservers

Nine individuals with clinical suppression, soli-cited from the student population at Northwestern

444

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No. 3 SUPPRESSION AND AMDLYOPIA / Holopigian er al. 445

University, were paid an hourly wage to serve in thisexperiment. Informed consent was obtained after thenature of the procedure had been explained fully. Allobservers exhibited a suppression scotoma and werestrabismic and/or anisometropic. To be classified as aclinical suppressor, the observer had to exhibit sup-pression with the Bagolini striated glass test. Thismethod is the least dissociating (and therefore closestto normal viewing conditions) of the standard clinicaltests for suppression.15 The test was administeredwith the room lights on, so items in the room wereclearly visible to the two eyes, thus minimizing disso-ciation. Although the existence of a suppression sco-toma with the Bagolini lenses was the criteria for theclassification of suppression, for purposes of compar-ison, suppression was also assessed with the Worth 4Dot test. At the viewing distance of 33 cm, each dotsubtended 1 degree, and the entire field subtended 6degrees. Both the Bagolini and the Worth 4 Dot testswere administered under normal viewing conditionsfor these observers. In addition, all observers under-went a comprehensive ophthalmologic evaluation,including refraction, ophthalmoscopy and clinicalassessment of visual acuity, ocular alignment, stere-opsis, binocularity, and binocular and monocularfixation patterns. The observers' visual characteristicsare listed in Table 1.

All psychophysical testing was conducted at North-western University. For purposes of comparison, wealso tested four observers with good visual acuity andnormal binocular vision. Three of the normal ob-servers were paid for their participation and werecompletely naive as to the purposes of the experi-ment. All clinical and normal observers wore theirbest refractive correction during psychophysicaltesting.

Apparatus and Procedure

The observer stereoscopically viewed two identicalCRT screens situated side by side. The CRT screenswere at a viewing distance of 96 cm and had an aver-age luminance of 30.6 c/m2. For all testing sessions,the face of each CRT was masked to a circular region1.2 degrees in diameter. These small fields were usedso that suppression measures would be obtained froma retinal area completely suppressed and so thatwholesale binocular rivalry (ie, not piecemeal) wouldbe experienced by normal observers tested undercomparable conditions.

At the start of the first testing session, each observerwas taught to adjust the stereoscope mirrors to opti-cally align the dichoptically viewed CRTs, usingnonius lines to gauge alignment. During this processfusion was disrupted using a cover/uncover maneu-ver to eliminate any fusional maintenance of ocular

alignment. For this alignment procedure, which pre-ceded each testing session, precision and care werestressed.

Contrast thresholds: For all observers, monocularcontrast thresholds were assessed for both horizontaland vertical sinusoidal gratings of 3.3 c/deg. In thedepth of suppression experiments, described in thenext section, it was important to have a large range ofcontrasts available above threshold in order to deter-mine the contrast increment threshold during sup-pression. Therefore, a spatial frequency close to thepeak of the contrast sensitivity function was desir-able. Since a 3.3 c/deg grating is close to the peak andprovides four complete light and dark cycles acrossthe 1.2 degree field, this spatial frequency was usedfor both the contrast threshold and the depth of sup-pression measurements.

Contrast thresholds were measured using a two-al-ternative temporal forced-choice staircase procedure.The staircase estimated the 71% correct detectionlevel using a rule which incremented the contrast fol-lowing each incorrect response and decremented thecontrast following each two correct responses. Con-trast was initially changed in 3 dB steps, but subse-quently was changed in 1 dB steps following twostaircase reversals. Each staircase was terminatedafter 12 reversals, and the last five reversals wereaveraged to yield an estimate of the contrast thresh-old. Two independent staircases were randomly in-terleaved in one experimental run. At least fourthresholds were averaged for each data point.

The observer adapted to the prevailing light levelfor 1 min before the start of each staircase run. Theobserver triggered each trial, which consisted of two 1second intervals, denoted by tones. During one ran-domly selected interval, a sinusoidal grating patternwas ramped on for 1 second (rise time and fall timeboth equal to 250 msec); during the other interval nograting was presented. The observer's task was to sig-nal to the computer which interval contained thegrating; feedback was provided.

Contrast thresholds were assessed separately for thetwo eyes, with presentation order randomized. Thesemonocular threshold measurements were performedunder two conditions. In one condition the non-tested eye viewed a uncontoured raster, and in theother condition the non-tested eye was occluded witha black eyepiece. For all control observers and sevenof the nine clinical suppressors, these two testing pro-cedures yielded equivalent thresholds. In two clinicalobservers, thresholds were considerably lower whenthe contralateral eye was occluded (rather than view-ing the uncontoured raster) and for these two individ-uals all subsequent thresholds were measured in thismanner.


o

Table 1. Observer characteristics

A. Strabismic suppressors

Age/genderTreatment history:

Refraction: ODOS

Snellen acuity: ODwith correction

OSFixation: OD

OSOcular alignment:*

Near:

Far:

Binocular status:Worth 4 Dot:

Bagolini lens:

Titmus stereo:

BG

26/MOcclusion at 10 yr

-1.50-1.50

20/30+

20/20Near centralNear central

RET s 2 PD4-6 PD with ACT

RET s 2 PD4-6 PD with ACT

Scotoma OD

Central scotoma OD

100"

CD

20/MSurgery at 3 yr (OD)

+4.75+ 1.50

20/30 - 2

20/15CentralCentral

RET = 8 PD, LHT = 2 PD16 PD with ACT

RET = 8 PDLHT = 2 PD

Scotoma OD

Central scotoma OD

3000" or poorert

AS

24/FSurgery at 5 yr (OS),

at 7 yrs (OD)-2.75-3 .50

20 /20-

20/25CentralCentral

AET = 10 PDR H T = 5 P DAET = 10 PDR H T = 3 P D

Rapid alternatingsuppressionbetween OD andOS

Rapid alternatingsuppressionbetween OD andOS

140"

MM

20/FSurgery at 2 yr

+3.75 + 2.00 X 100+4.25 + 0.75 X 60

20/20

20/20CentralCentral

AET = 20 PD, RHT = 3-5 PDET = 35 PD with ACT

AET = 20 PD, RHT = 3-5 PDET = 40 PD with ACT

Alternating suppression

Central scotoma OD withperipheral ARC

Poorer than 3000"

WV

28/MOcclusion at 8

-2.75-2.75

20/20+

20/16CentralCentral

RET = 10 PD

RET= 6 P D

Scotoma OD

Centralscotoma OD

3000"

PD

28/MSurgery (OS) at 1

yr-2 .50

-2 .75 + 0.50 X 852 0 / 2 0 -

20/20Not testedNot tested

AET = 20-25 PDRHT = 5 PDAET= 15 PDRHT = 10 PD

Alternatingsuppression

Alternatingsuppression

Poorer than 3000"

B. Anisometropic suppressors

LP

24/FOcclusion

-11.00-4 .00

15/100

20/20CentralCentral

Orthot

OrthoJ

ScotomaOD

ScotomaOD

400"

KJ

24/FOcclusion at

9yr-0.25-3.25

20/30-

20/20+CentralCentral

Ortho

Ortho

ScotomaOD

50"

MT

20/FOcclusion from 8-

12 yr-1.25

+4.00+ 1.25X9020/20

20/80Not testedNot tested

Ortho

LET = 2 PD

Scotoma OS

Scotoma OS

50"

zm

>

<m

o

>O

8

<*•̂~.i^i

>

nm

r>m

qn

* Except as indicated cover test deviation and alternate cover measurements were identical.t Gave one positive response, one negative response to the Titmus Fly (3000 sec) in separate testing

sessions.X By history intermittently esotropic. Frequently manifested in childhood, currently only with fatigue

or ethanol ingestion.

£ Response to Worth 4 Dot was difficult to interpret.Abbreviations: OD = right eye, OS = left eye, PD = prism diopters, ET = esodeviation, RET = right

esotropia, LET = left esotropia, ACT = alternating esotropia, RHT = right hypertropia, LHT = lefthypertropia, ACT = alternate cover test, Ortho = orthophoric.

CDCo

O


No. 3 SUPPRESSION AND AMDLYOPIA / Holopigian er al. 447

Depth of suppression: Contrast increment thresh-olds were determined using a two-alternative spatialforced-choice procedure. For the initial baselinemeasures, the observer viewed a horizontal, 3.3 c/degsinusoidal grating of 10% contrast with one eye, whilethe other eye received no stimulation. The observercontrolled the trial presentation and was told to trig-ger trials only when the horizontal grating was clearlyand completely visible. Each trial involved the abruptaddition of a contrast increment to either the top orthe bottom half of the grating, with the incrementedhalf determined randomly. The contrast incrementoccurred almost immediately (25 msec) following theobserver's trigger press and lasted 200 msec. Whenthe contrast increment was removed, the grating re-turned to its original contrast. The contrast incre-ment was positioned spatially so that its boundarycorresponded to a zero crossing and produced no lu-minance edge or change in overall luminance.

Following each trial, the observer signaled whichhalf of the grating received the increment; feedbackwas again provided. Over trials, the contrast incre-ment was varied in 2 and then 1 dB steps, again fol-lowing a staircase rule which converged on the 71%correct level. Two independent staircases were ran-domly interleaved during a single run. The monocu-lar contrast increment threshold was used as the base-line for measuring suppression. For the normal ob-servers, the right eye was tested for this baselinemeasure, and for each clinical suppressor the eyewhich was usually suppressed when viewing with theBagolini lenses was tested.

For the suppression measures, gratings were pre-sented simultaneously to both eyes. When suppres-sion was measured for orthogonal gratings, (ORTHOcondition) the tested eye still viewed a horizontalgrating, but the contralateral eye now viewed a 3.3c/deg vertical grating of 10% contrast. For this condi-tion, observers were told to trigger trials when theysaw only the vertical grating, with no trace of thehorizontal grating. The contrast increment, however,was still added to the top or bottom of the horizontalgrating, so the staircase now assessed the amount ofcontrast increment needed for the suppressed eye todetect the increment. All other aspects of the staircaseprocedure remained the same.

Orthogonal gratings are used to measure binocularrivalry suppression. For clinical suppressors, how-ever, the visual stimuli for suppression under normalviewing conditions are presumed to be identical ornearly identical contours seen by the two eyes.16 Wetherefore also measured the depth of suppression bypresenting both the eyes with horizontal gratings(IDENT condition), and added the contrast incre-ment to the suppressed horizontal grating. The grat-

CQ•D\j

coaiCO0)wQ.

a3

CO

*- 0a.a>a

- 5

MB MW QH KH

ORTHO

Fig. 1. Histogram showing average depth of suppression in dB forfour normal observers. Depth of suppression is the ratio of thecontrast increment threshold for dichoptic viewing conditions rela-tive to the monocular baseline. A suppression effect of 6 dB meansthat, on average, twice as much contrast increment must be addedto the suppressed stimulus for it to be detected binocularly as whenit was viewed monocularly. The conditions shown are: (1) ortho-gonally oriented gratings with the tested eye suppressed (ORTHO);(2) identically oriented gratings (IDENT); and (3) orthogonallyoriented gratings with the tested eye dominant (DOM). The stand-ing contrast of the gratings was 10% and the spatial frequency was3.3 c/deg. Each value is the mean of four to six threshold estimates.Standard error bars are shown.

ings were identified by high contrast markers (blackspots 3.6 min of arc) located 21 min above the centerof the circular field in the left eye and 21 min below inthe right eye. Observers were instructed to triggertrials only when the appropriate marker (ie, the topone) was visible and the other marker was suppressed.

All observers, both control and clinical, were testedwith both orthogonally and identically oriented grat-ings. When the gratings were orthogonal, all ob-servers experienced suppression. When viewing iden-tically oriented gratings, however, the control ob-servers did not experience suppression—bothgratings were seen continuously, as noted by the con-tinuous presence of both markers. Since neither grat-ing was ever suppressed, this provides one test of fu-sion in normal observers. The clinical observers, onthe other hand, reliably suppressed one eye's view.For the amblyopic suppressors, the marker viewed bythe poorer eye was suppressed, while visibility of themarkers fluctuated between the eyes for the alternat-ing suppressors.

ResultsOur index of the depth of suppression was the ratio

of the monocular baseline increment threshold rela-tive to the increment threshold obtained under binoc-ular conditions. It is important to note that the depthof suppression is not a measure of amblyopia, sinceamblyopia is reflected as overall higher monocularincrement thresholds.

Figure 1 shows the results obtained for the fournormal observers. When the gratings were identically


448 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / March 1988 Vol. 29

C 20o

CO

a.0)Q

;

1r—

LP CD BG AS MM MT KJ WV PD LP CD BG AS MM MT KJ WV PD

ORTHO IDENT

Fig. 2. Histogram showing average depth of suppression in dB forclinical suppressors. Depth of suppression is the ratio of the con-trast increment threshold for dichoptic viewing conditions relativeto the monocular baseline. The conditions shown are: (1) ortho-gonally oriented gratings with the tested eye suppressed (ORTHO);and (2) identically oriented gratings with the tested eye suppressed(IDENT). The standing contrast of the gratings was 10% (20% forobserver LP) and the spatial frequency was 3.3 c/deg. Each value isthe mean of four to six threshold estimates. Standard error bars areshown.

oriented (IDENT), there was no suppression effect,and thresholds were equivalent to those for the mon-ocular baseline condition. It is interesting that thiscondition did not produce the dichoptic masking ob-served by Legge.17 He found that the threshold for asinusoidal grating was greatly elevated when a supra-threshold grating of the same orientation and spatialfrequency was simultaneously presented to the con-tralateral eye. In the Legge experiment, however,both the test and the mask gratings were brieflyflashed for 200 msec, while in the present experimentthe grating viewed by the contralateral eye waspresent continuously and only the contrast incrementto the ipsilateral eye was flashed.

For the normal observers, the magnitude of thesuppression effect for orthogonal gratings (ORTHO)ranged from 4.2 dB to 7.1 dB, with an average of 5.6dB. Increment thresholds for the horizontal gratingwere also measured under conditions of dominance(DOM condition), where trials were triggered onlywhen the horizontal grating was clearly visible andthe vertical was suppressed.* (Note that incrementthresholds were assessed in the same eye for all theseconditions; only the instructions concerning when totrigger trials changed.) These dominance thresholdswere equivalent to the monocular baseline thresh-olds. A repeated measure analysis of variance indi-cated that there were significant differences among

* For the clinical suppressors, this condition could not be tested,because most had no periods of complete dominance by the habit-ually suppressed eye.

these three conditions for normal observers, F (2,6)= 7.2, P < 0.05. A Neuman-Keuls post-hoc test re-vealed that the magnitude of suppression for theORTHO condition was significantly different thanthe magnitude for the IDENT and DOM conditionswhile the latter two did not differ.

Figure 2 shows the results for the clinical suppres-sors. Two findings are noteworthy. First, the depth ofsuppression varied greatly among these observers,ranging from 0 to greater than 20 dB. Second, for anyone clinical suppressor, the depth of suppression fororthogonal gratings closely corresponded to the depthof suppression for identical gratings. (The same pat-tern of results was obtained when the observersviewed two vertically oriented gratings.) The Pearsoncorrelation coefficient for the two measures of sup-pression was 0.94, and a repeated measure analysis ofvariance indicated no significant differences betweenthe depth of suppression for orthogonally vs. identi-cally oriented gratings, F (1,8) = 0.002, P > 0.05.

Why are there such large differences in the magni-tude of the suppression effect across clinical ob-servers? One possibility is that the depth of suppres-sion is related to the degree of amblyopia. To exam-ine this possibility we compared depth of suppressionfor both orthogonally and identically oriented grat-ings to the degree of amblyopia for these same stim-uli. Amblyopia was indexed as the ratio of the con-trast thresholds for the poorer eye to that of the bettereye for a 3.3 c/deg grating of the appropriate orienta-tion.

For both orthogonally and identically orientedgratings, the correlation between depth of suppres-sion and degree of amblyopia in the clinical suppres-sors was negative. Observers with severe amblyopiashowed almost no suppression, while observers withsmall amounts of amblyopia demonstrated largeamounts of suppression. The correlation (Pearson r)for these two indices was —0.674 for orthogonal ori-ented gratings and —0.670 for identically orientedgratings. Both of these correlations are significantlydifferent from zero (df =7,P< 0.05).

The ratio of Snellen acuities between amblyopicand nonamblyopic eyes was also compared to thedepth of suppression. This index of amblyopiashowed similar negative correlation with depth ofsuppression. The Pearson r values were -0.698 fororthogonal gratings and -0.645 for identically ori-ented gratings. Only the first correlation is signifi-cantly different from zero, indicating that Snellenacuity reduction is not as good a predictor of depth ofsuppression as is contrast threshold elevation. Thesenegative correlations are shown graphically as scat-terplots in Figure 3. The top scatterplot shows therelationship between the depth of suppression and


No. 3 SUPPRESSION AND AMBLYOPIA / Holopigion er ol. 449

the grating sensitivity difference for orthogonal grat-ings and the second scatterplot shows the relationshipbetween the depth of suppression and the interocularacuity ratios, also for orthogonal gratings.

We also examined the relationship between depthof suppression and stereopsis. There was no signifi-cant correlation between stereopsis and the depth ofsuppression for either orthogonal gratings (r = 0.454)or identical gratings (0.528).

Our results are summarized in Table 2, which listsdepth of suppression ratios, contrast threshold ratios,Snellen acuity ratios and stereoacuities for the clinicalsuppressors.

Table 2. Summary data

CD

0O

c

Q53=T3

Q)

gto

O

o2a>

7

6

5

4

3

2

1

Depth of suppression (dB)

Fig. 3. Top, scatterplot demonstrating the relationship betweenthe depth of suppression (in dB) and the contrast sensitivity differ-ence (in dB) for orthogonal gratings of 3.3 c/deg. The data shownare for the nine clinical suppressors tested in this study. Bottom,scatterplot demonstrating the relationship between the depth ofsuppression (in dB) and the interocular acuity ratio (where a ratioof 1.0 indicates no difference between the eyes). The data shown arefor the nine clinical suppressors tested in this study.

Observer

LPCDBGASMMMTKJWVPD

LPCDBGASMMMTKJWVPD

Orthogonally oriented gratings

Depth ofsupp

-0.403.909.75

12.3020.505.873.30

11.5317.06

Contrastratio

20.407.808.804.701.303.003.162.251.13

Acuityratio

6.702.00

i

.50

.25

.001.00.50.25.00

Identically oriented gratings

1.603.308.00

16.4022.602.304.10

11.1014.80

20.409.546.735.822.60

10.964.521.573.35

6.702.00

.50

.25

.00t.00.50.25.00

Stereoacuity

4003000

100140

30005050

30003000

4003000

100140

30005050

30003000

Discussion

Across the clinical suppressors tested, there was asignificant negative correlation between depth ofsuppression and the degree of amblyopia for sinusoi-dal gratings with both orthogonally and identicallyoriented patterns, and a significant negative correla-tion between depth of suppression and Snellen acuityfor orthogonally oriented gratings. In other words,much stronger suppression effects were present inclinical observers with equal visual acuity than inthose with amblyopia. Before the implications ofthese results can be considered, certain proceduralquestions need to be addressed.

Is it possible that the eyes of the clinical observersbecame misaligned in the course of a trial, so that thesuppressed eye was directed at some point off theCRT screen? We reject this hypothesis for severalreasons. Great care was taken to ensure that eyes wereproperly aligned at the start of a testing session. Ob-servers were asked to recheck alignment periodicallyduring testing by alternately occluding each eye'sview, and to realign the CRT screens, if necessary.Furthermore, the magnitude of the suppression effectfor a given observer was remarkably consistent,across the conditions tested here and several othersets of experiments,18 making it difficult to attributethe results to occasional eye misalignment. Finally,when these observers were asked to report on thevisibility fluctuations of the patterns, all observers re-ported some periods in which the grating presented to


450 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Morch 1988 Vol. 29

the usually suppressed eye became visible, indicatingthat both eyes were viewing the CRT screens simulta-neously.

Another possibility is that the initial baseline in-crement thresholds were artificially elevated for someof the clinical suppressors, due to stimulation of thedominant eye even when it was occluded. Suppose,for example, that the dominant eye received somestray light from the CRTs while the monocularthreshold was being measured in the contralateraleye. This stray light might have provided sufficientstimulation to produce suppression of the tested eye.This would have elevated the monocular baseline,and since the depth of suppression was computed as aratio relative to this baseline, a reduction in the over-all suppression effect would have resulted. To excludethis possibility, monocular increment thresholdswere remeasured under three conditions for one ob-server who had a small suppression effect (LP). Thenontested eye either viewed a uniformly luminousraster, viewed a dark occluder, or was carefullypatched to exclude all light from the eye. There wasno significant difference in the monocular thresholdsobtained in these three conditions, F (1,2) = 2.37, P> 0.05, confirming that stray light entering the domi-nant eye did not affect the monocular threshold.

Is it possible that some clinical suppressors weresimply more careful than others to trigger trials onlyduring periods of complete suppression, thereby pro-ducing apparent differences in the magnitude of thesuppression effect? If some trials were triggered whenthe contralateral eye was not completely dominant,the magnitude of the suppression effect would be un-derestimated. This seems unlikely for several reasons.For one, the clinical suppressors with deeper ambly-opia showed the least suppression, yet deeply ambly-opic eyes are suppressed for long periods of time dur-ing binocular viewing. Hence, for amblyopic sup-pressors, the likelihood of a trial's being inadvertentlytriggered during a nonsuppression state is actuallylower than it is for other observers. Careless triggeringof trials would thereby produce a greater suppressioneffect for amblyopic suppressors, not the smaller oneactually observed. Also, the magnitude of suppres-sion was very uniform among the four normal ob-servers and comparable to the magnitude of suppres-sion found in other experiments on binocular ri-valry.19"21 As three of the four normals wereinexperienced psychophysical observers, they had nomore experience with the task than did the clinicalsuppressors.

A final concern is whether the suppressed eye couldhave become dominant before the contrast incre-ment was extinguished, thus allowing detection. Thecontrast increment occurred 25 msecs after a trial was

triggered and remained present for only 200 msecs. Inother experiments on these same observers we mea-sured the durations of dominance and suppressionunder identical stimulus conditions. For the clinicalsuppressor with the most rapid alternations in view-ing state, the average duration of any one viewingstate was greater than 500 msec. For other observers,viewing states averaged 1000 msec or longer. Thelength of these suppression durations makes it im-probable that the suppression state changed duringthe course of the trial. We conclude that the differ-ences among observers were not due to any method-ological difficulties and now turn to the task of inter-preting these findings.

Do anisometropes differ from strabismics with re-spect to suppression? Our anisometropic observers,on average, showed weaker suppression effects thanthe strabismic observers. Conceivably this was be-cause the stimulus for suppression in anisometropia—blur—is less effective than is the misalignment thatoccurs in strabismus. More likely, it was due to thefact that the anisometropes in this study were moreamblyopic than the strabismics. A larger group ofanisometropes, with a wider range of amblyopia,needs to be tested to determine whether suppressionin anisometropia varies over the same range as sup-pression in strabismus.

Previous investigators have reported that the depthof suppression measured with a luminance incrementtask and the degree of amblyopia represented interms of acuity difference between the eyes vary simi-larly across the visual field in clinical suppressors,such that areas of deep suppression are also deeplyamblyopic.1314 Can these findings be reconciled withour results? We believe so. The tasks employed bySireteanu and associates were different from ours:they measured the depth of suppression using a lumi-nance increment, not a contrast increment, and con-trasted it to grating acuity, not contrast sensitivity.They compared different points in the visual field ofindividual observers, not corresponding (foveal)points in different observers, as we did. The conflictmay thus be more apparent than real. In addition,while Sireteanu does not present data for all of theobservers she tested, interobserver comparisons,where possible, suggest that the depth of suppressionshe found in the foveal region was much greater inobservers with alternating strabismus and equal vi-sual acuity14 than in strabismic and anisometropicobservers with unilateral suppression and ambly-opia.13 This is entirely consistent with our observa-tions.

What, then, can one conclude concerning the rela-tionship between amblyopia and suppression? Theimpaired monocular functioning of the amblyope's


No. 3 SUPPRESSION AND AMDLYOPIA / Holopigion er ol. 451

fovea evidently reduces the requirement for suppres-sion to eliminate diplopia for binocular viewing con-ditions. We believe that suppression is a beneficialresponse which is used by the visual system as eco-nomically as possible. Our results should not be in-terpreted to mean that amblyopia is a similarly pur-poseful response. Suppression alone is quite sufficientto eliminate diplopia in the nonamblyopic individualwithout requiring a sacrifice of monocular acuity.Neither our data nor those of other investigatorsdemonstrate a direct causal correlation between am-blyopia and suppression. We suspect their mecha-nisms are quite distinct.

Are binocular rivalry suppression and clinical sup-pression in fact equivalent? Since both forms of sup-pression apparently serve to protect the visual systemfrom non-fusable binocular input, some authors havepostulated that they are products of the same mecha-nism.22 In our study, however, binocular rivalry sup-pression and clinical suppression showed a differentpattern of results. In normal observers, identicallyoriented dichoptically viewed gratings produce no ev-idence for transient suppression while orthogonallyoriented gratings evoked alternating suppression andelevated increment thresholds during suppression.This elevation in threshold was relatively uniformamong the normal observers (on the order of 0.30log-units) and never approached the larger magni-tudes (1.0 log-unit and greater) observed in someclinical suppressors. For the clinical suppressors, themagnitude of suppression varied greatly among ob-servers, yet was remarkably consistent for any indi-vidual observer, regardless of the orientations of thedichoptic stimuli. In this respect, the suppression thatoccurs when clinical suppressors view orthogonallyoriented patterns does not resemble binocular rivalrysuppression in normal observers but is nearly identi-cal to the type of suppression the clinical observersexperience when viewing identical contours.

A difference between binocular rivalry suppressionin normal observers and clinical suppression was alsonoted by Smith et al.23 They found that normal ob-servers exhibited a wavelength-specific abnormalityin the spectral increment sensitivity function duringbinocular rivalry suppression that was not presentduring normal viewing or binocular rivalry domi-nance states. Clinical suppressors did not show thiswavelength-selective loss, regardless of whether thetest gratings were orthogonally or identically ori-ented. These differences between binocular rivalrysuppression and clinical suppression seem funda-mental, and pose a real challenge to theorists whopostulate that their mechanisms are identical.

Key words: suppression, amblyopia, strabismus, anisome-tropia, binocular rivalry

AcknowledgmentWe thank William Seiple for comments on an ear-

lier version of this paper.

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Clinical suppression and amblyopia. · No. 3 SUPPRESSION AND AMDLYOPIA / Holopigian er al. 445 University, were paid an hourly wage to serve in this experiment. Informed consent was

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