Deficient binocular combination reveals mechanisms of anisometropic amblyopia: Signal attenuation and interocular inhibition

Abstract

Amblyopia is a developmental disorder that results in deficits of monocular and binocular vision. It's presently unclear whether these deficits result from attenuation of signals in the amblyopic eye, inhibition by signals in the fellow eye, or both. In this study, we characterize the mechanisms underlying anisometropic amblyopia using a binocular phase and contrast combination paradigm and a contrast-gain control model. Subjects dichoptically viewed two slightly different images and reported the perceived contrast and phase of the resulting cyclopean percept. We found that the properties of binocular combination were abnormal in many aspects, which is explained by a combination of (1) attenuated monocular signal in the amblyopic eye, (2) stronger interocular contrast-gain control from the fellow eye to the signal in amblyopic eye (direct interocular inhibition), and (3) stronger interocular contrast-gain control from the fellow eye to the contrast gain control signal from the amblyopic eye (indirect interocular inhibition). We conclude that anisometropic amblyopia led to both monocular and interocular deficits. A complete understanding of the mechanisms underlying amblyopia requires studies of both monocular deficits and binocular interactions.

Deficient Binocular Combination of Second-Order Stimuli in Amblyopia

“…37 Thus for normal adults, since the carrier contrasts are the same in the two eyes, they will have equal weight in the binocular summation of second-order signals. 36,37 For amblyopes, however, owing to attenuation/ suppression in binocular viewing, 9 the carrier contrasts in the amblyopic eye will be less effective 17 and therefore would be expected to be given less weight in the binocular sum.…”

“…18,45 For the first-order condition, it is well documented that binocular combination of first-order gratings is contrast-gain controlled. 9,11,17,19,35,[46][47][48] Recently, we have shown that second-order binocular combination could also be explained by the contrast-gain control theory. 37 Thus for normal adults, since the carrier contrasts are the same in the two eyes, they will have equal weight in the binocular summation of second-order signals.…”

“…3-8 Recently, imbalances in binocular function have been put forward as a possible primary factor in the disorder. [9][10][11] This has led to new binocular training therapies. 12-16 Such a sensory imbalance has been demonstrated in several paradigms, for example, local phase combination, 9,17 global motion coherence, 18 and global orientation coherence.…”

“…19 Even at low spatial frequencies where the two eyes have equivalent monocular visual thresholds, the amblyopic eye still makes less of a contribution to the binocularly fused percept than the nonamblyopic eye. 9,11,[17][18][19] Moreover, the sensory imbalance tends to be more severe for tasks involving extrastriate pathways. 19 The magnitude of the imbalance in the three paradigms mentioned above is greater for the global motion coherence task than for the global orientation coherence task, and both of these have greater effects than seen in the local phase combination task.…”

“…[9][10][11] This has led to new binocular training therapies. [12][13][14][15][16] Such a sensory imbalance has been demonstrated in several paradigms, for example, local phase combination, 9,17 global motion coherence, 18 and global orientation coherence. 19 Even at low spatial frequencies where the two eyes have equivalent monocular visual thresholds, the amblyopic eye still makes less of a contribution to the binocularly fused percept than the nonamblyopic eye.…”