The Relationship Between Fusion, Suppression, and Diplopia in Normal and Amblyopic Vision.
OBJECTIVE
Single vision occurs through a combination of fusion and suppression. When neither mechanism takes place, we experience diplopia. Under normal viewing conditions, the perceptual state depends on the spatial scale and interocular disparity. The purpose of this study was to examine the three perceptual states in human participants with normal and amblyopic vision.


METHODS
Participants viewed two dichoptically separated horizontal blurred edges with an opposite tilt (2.35°) and indicated their binocular percept: "one flat edge," "one tilted edge," or "two edges." The edges varied with scale (fine 4 min arc and coarse 32 min arc), disparity, and interocular contrast. We investigated how the binocular interactions vary in amblyopic (visual acuity [VA] > 0.2 logMAR, n = 4) and normal vision (VA ≤ 0 logMAR, n = 4) under interocular variations in stimulus contrast and luminance.


RESULTS
In amblyopia, despite the established sensory dominance of the fellow eye, fusion prevails at the coarse scale and small disparities (75%). We also show that increasing the relative contrast to the amblyopic eye enhances the probability of fusion at the fine scale (from 18% to 38%), and leads to a reversal of the sensory dominance at coarse scale. In normal vision we found that interocular luminance imbalances disturbed binocular combination only at the fine scale in a way similar to that seen in amblyopia.


CONCLUSIONS
Our results build upon the growing evidence that the amblyopic visual system is binocular and further show that the suppressive mechanisms rendering the amblyopic system functionally monocular are scale dependent.
McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montréal, Québec, Canada 2Traumatic Brain Injury Program, The Research Institute of the McGill University Health Center, Montréal, Québec, Canada.
Daniel P
DP
Spiegel
McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montréal, Québec, Canada.
Alex S
AS
Baldwin
McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montréal, Québec, Canada.
Robert F
RF
Hess
D000293:Adolescent; D000328:Adult; D000550:Amblyopia / Q000503:physiopathology*; D002648:Child; D004172:Diplopia / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D012684:Sensory Thresholds / Q000502:physiology*; D015357:Vision Disparity / Q000502:physiology*; D015348:Vision, Binocular / Q000502:physiology*; D014792:Visual Acuity; D055815:Young Adult