Comparisons of the spatial matrix of subfields between multiple nearby V2 neurons in amblyopic monkeys

Amblyopia is a developmental vision deficit caused by experiencing binocular imbalance during early development. Despite some interesting theories based on many perceptual and modeling studies, the neural basis of vision deficits associated with amblyopia is poorly understood except for the well established ocular dominance imbalance in V1 of monocularly form deprived animals. In this study we employed a new approach to study vision deficits in amblyopic monkeys that may give us an insight into a neural basis of position uncertainty, distortion, and/or deficit orientation discrimination in human amblyopes. We simulated anisometropic amblyopia by having infant macaque monkeys wear defocusing lens in one eye between 3 weeks and 3 months of age. When they matured we obtained their spatial contrast sensitivity functions to determine the depth of amblyopia. We recorded action potentials from multiple nearby units with a single electrode. We employed dynamic two dimensional noise stimuli and a reverse correlation (LSRC) method to reveal subfields within the receptive field of each V2 neuron. The spatial maps of these subfields were compared between multiple nearby neurons with respect to their preferred orientations, spatial frequencies, and the maximal strength of responses. We quantified the heterogeneity of the subfield maps (heterogeneity index) for each unit (within comparison) and between units (across-unit comparison). We found that 1) in normal monkeys, the heterogeneity index was very low both within a given unit and across nearby units. 2) In amblyopic monkeys, for the within-unit comparison, the heterogeneity index of the subfield maps driven by the amblyopic eye was similar to that for the fellow eye, but both were slightly but significantly higher than in normal monkeys. 3) For the across-units comparison, the heterogeneity index of the subfield maps of V2 neurons for the amblyopic eye was far greater than that for the fellow eye, while the index for the fellow eye was also significantly greater than that in normal monkeys. 4) The abnormally high heterogeneity indices in amblyopic monkeys did not result from weak or noisy responses in amblyopic monkeys. 5) V2 neurons of amblyopic monkeys exhibited robust binocular suppression when dichoptic sine wave gratings were used for stimuli. The results suggest that the fine circuitry supporting the feed forward connections from V1 to V2 and the local connections within V2 appear to be disrupted in amblyopic monkeys, and that robust binocular suppression may be involved, at least in part, with the abnormally large differences in the subfield maps of multiple nearby units in amblyopic monkeys