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Clinical Suppression and Amblyopia

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Clinical Suppression and Amblyopia Investigative Ophthalmology & Visual Science, Vol. 29, No. 3, March 1988 Copyright © Association for Research in Vision and Ophthalmology Clinical Suppression and Amblyopia Karen Holopigian,*^: Randolph Blake,* and Mark J. Greenwaldf In individuals with abnormal binocular vision, such as strabismics and anisometropes, it is common for all or part of one eye's view to be suppressed so binocular confusion and diplopia are eliminated. We examined the relation between the depth of suppression (the amount by which the monocular contrast increment threshold for an eye was elevated by stimulation in the contralateral eye) and the degree of amblyopia (difference in monocular contrast thresholds for the two eyes). There was a significant negative 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 an eye 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, such in response to conflicting monocular visual input to as strabismics and anisometropes, often suppress part the two eyes, the relationship between these entities of one eye's view. This phenomenon may be either remains unclear. There is some speculation that unilateral, such that one eye is chronically sup- long-term chronic suppression is actually responsible pressed, or bilateral, with dominance and suppression for the development of amblyopia in one eye. Indeed, alternating between the two eyes. It is generally as- Sireteanu and Fronius13 and Sireteanu14 found that sumed1 that this clinical suppression is adaptive in portions of the visual field that exhibited deeper in- that it eliminates confusion (resulting from different terocular suppression (assessed with a luminance de- images falling on corresponding retinal locations) tection task) were also more amblyopic (ie, had and diplopia (arising from the left and right eye poorer monocular acuity); areas less strongly sup- images falling on noncorresponding retinal loca- pressed had better acuity. The view that suppression tions). causes amblyopia cannot be entirely correct, how- Another condition commonly associated with stra- ever, for it is known that many individuals with clini- bismus and anisometropia is amblyopia, a chronic cal suppression have equal visual acuity in the two reduction in monocular vision. Amblyopia may im- eyes. pair visual performance as measured by Snellen and To clarify the relationship between clinical sup- grating acuity,2 contrast sensitivity,3"6 vernier acuity,7 pression and amblyopia, we examined the correlation contrast matching8 and stereoacuity.9"12 between the degree of amblyopia and the depth of Although amblyopia and suppression both develop suppression in a group of strabismic and anisome- tropic observers. In these experiments, the depth of From the "Departments of Psychology and Neurobiology/Physi- suppression was defined as the amount by which the ology, Northwestern University, Evanston, Illinois, and the fDe- contrast increment threshold for an eye was raised by partment of Ophthalmology, Northwestern University Medical the simultaneous presentation of a stimulus to the School, Chicago, Illinois. J Present address: Department of Ophthalmology, N.Y.U. Med- contralateral eye. Amblyopia was indexed by the ical Center, New York, New York. magnitude of the difference between right and left eye This research was conducted as a partial requirement for the contrast thresholds. To our surprise, we found that PhD degree for KH. Portions of this work were presented at the the degree of amblyopia and the depth of suppression 1987 ARVO meeting, May 4-8, Sarasota, Florida. were inversely related. Clinical suppressors with Supported by NSF grant BNS 8418731 to RB, by NSF grant NS07223-05 to the Neurobiology and Physiology Department at equal monocular vision showed large amounts of in- Northwestern University, a Northwestern University dissertation terocular suppression, while those with amblyopia ex- year grant to KH and a grant from Children's Memorial Hospital. hibited a much smaller suppression effect. KH is currently supported by a grant from the RP Fighting Blind- ness to the Retina Clinic at NYU Medical Center. Materials and Methods Submitted for publication: June 4, 1987; accepted September 24, Observers 1987. Reprint requests: Department of Ophthalmology, N.Y.U. Medi- Nine individuals with clinical suppression, soli- cal Center, 550 First Avenue, New York, NY 10016. cited from the student population at Northwestern 444 Downloaded from iovs.arvojournals.org on 09/27/2021 No. 3 SUPPRESSION AND AMDLYOPIA / Holopigian er al. 445 University, were paid an hourly wage to serve in this alignment. For this alignment procedure, which pre- experiment. Informed consent was obtained after the ceded each testing session, precision and care were nature of the procedure had been explained fully. All stressed. observers exhibited a suppression scotoma and were Contrast thresholds: For all observers, monocular strabismic and/or anisometropic. To be classified as a contrast thresholds were assessed for both horizontal clinical suppressor, the observer had to exhibit sup- and vertical sinusoidal gratings of 3.3 c/deg. In the pression with the Bagolini striated glass test. This depth of suppression experiments, described in the method is the least dissociating (and therefore closest next section, it was important to have a large range of to normal viewing conditions) of the standard clinical contrasts available above threshold in order to deter- tests for suppression.15 The test was administered mine the contrast increment threshold during sup- with the room lights on, so items in the room were pression. Therefore, a spatial frequency close to the clearly visible to the two eyes, thus minimizing disso- peak of the contrast sensitivity function was desir- ciation. Although the existence of a suppression sco- able. Since a 3.3 c/deg grating is close to the peak and toma with the Bagolini lenses was the criteria for the provides four complete light and dark cycles across classification of suppression, for purposes of compar- the 1.2 degree field, this spatial frequency was used ison, suppression was also assessed with the Worth 4 for both the contrast threshold and the depth of sup- Dot test. At the viewing distance of 33 cm, each dot pression measurements. subtended 1 degree, and the entire field subtended 6 Contrast thresholds were measured using a two-al- degrees. Both the Bagolini and the Worth 4 Dot tests ternative temporal forced-choice staircase procedure. were administered under normal viewing conditions The staircase estimated the 71% correct detection for these observers. In addition, all observers under- level using a rule which incremented the contrast fol- went a comprehensive ophthalmologic evaluation, lowing each incorrect response and decremented the including refraction, ophthalmoscopy and clinical contrast following each two correct responses. Con- assessment of visual acuity, ocular alignment, stere- trast was initially changed in 3 dB steps, but subse- opsis, binocularity, and binocular and monocular quently was changed in 1 dB steps following two fixation patterns. The observers' visual characteristics staircase reversals. Each staircase was terminated are listed in Table 1. after 12 reversals, and the last five reversals were All psychophysical testing was conducted at North- averaged to yield an estimate of the contrast thresh- western University. For purposes of comparison, we old. Two independent staircases were randomly in- also tested four observers with good visual acuity and terleaved in one experimental run. At least four normal binocular vision. Three of the normal ob- thresholds were averaged for each data point. servers were paid for their participation and were The observer adapted to the prevailing light level completely naive as to the purposes of the experi- for 1 min before the start of each staircase run. The ment. All clinical and normal observers wore their observer triggered each trial, which consisted of two 1 best refractive correction during psychophysical second intervals, denoted by tones. During one ran- testing. domly selected interval, a sinusoidal grating pattern was ramped on for 1 second (rise time and fall time Apparatus and Procedure both equal to 250 msec); during the other interval no The observer stereoscopically viewed two identical grating was presented. The observer's task was to sig- CRT screens situated side by side. The CRT screens nal to the computer which interval contained the were at a viewing distance of 96 cm and had an aver- grating; feedback was provided. age luminance of 30.6 c/m2. For all testing sessions, Contrast thresholds were assessed separately for the the face of each CRT was masked to a circular region two eyes, with presentation order randomized. These 1.2 degrees in diameter. These small fields were used monocular threshold measurements were performed so that suppression measures would be obtained from under two conditions. In one condition the non- a retinal area completely suppressed and so that tested eye viewed a uncontoured raster, and in the wholesale
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