Relationship Between Interocular Suppression in Visual Cortical Neurons and the Degree of Strabismic Amblyopia

Background. Human psychophysical studies report that amblyopia is linked to robust binocular suppression during early infancy. Previously we have shown that V1 physiology of strabismic monkeys is dominated by interocular suppression (i.e., binocular responses are weaker than monocular responses) that begins with the onset of strabismus. Objective. This study investigated whether the prevalence of interocular suppression in V1 and V2 neurons is related to behaviorally determined degree of strabismic amblyopia in individual monkeys. Methods. Infant monkeys were reared with surgical strabismus (esotropia) between 3 weeks and 18 months of age. Eye alignment was monitored during early development (8-10 months). Standard operant methods were used to measure the severity of amblyopia (Amblyopia Index) in the affected and the fellow eye of each monkey. Binocular and monocular receptive field properties of V1 and V2 neurons were analyzed with standard microelectrode recording and analysis methods. Results. The strabismic monkeys (n = 9) were found to exhibit a wide range of amblyopia (i.e., Amblyopia Index ranged between 0.3 and 0.9). Binocular interactions in V1 and V2 were dominated by interocular suppression and the prevalence of suppressive units in individual monkeys varied with their amblyopia index values. The proportion of binocular suppressive units in V1 and V2 of each monkey was highly correlated with its amblyopia index. Conclusions. These results suggest that the greater the degree of amblyopia is, the higher the prevalence of binocularly suppressive neurons is in V1 and V2.