Orientation anisotropy in the spatial matrix of facilitatory subfields of V2 neurons of macaque infants

We previously reported that the spatial matrix of facilitatory subfields (subunits) in the receptive fields of individual V2 neurons in adult monkeys exhibits an orientation anisotropy; the homogeneity of the subfields is greater if a neuron preferred the cardinal orientations compared to oblique orientations. This result suggested that the convergence of V1 input on to a single V2 neuron is more orderly for V2 neurons preferring the cardinal orientations, and that this may perceptually contribute to a higher sensitivity for vertically or horizontally oriented contours in visual scenes (oblique effects). In this study, we asked whether a similar orientation anisotropy exists in V2 of infant monkeys shortly after birth. To address this issue, we used dynamic two dimensional noise stimuli and a reverse correlation (LSRC) method to obtain the spatial matrix of subfields with facilitatory profiles in V2 neurons of 4-, 8-, and 16-week-old macaque monkeys (Macaca mulatta). Extracellular recording was made in individual V2 neurons of anesthetized and paralyzed monkeys and the results from infants of different ages were compared to those obtained in adults. We found that the oblique effect in the spatial matrix of subfields was absent in V2 at 4 and 8 weeks of age but emerged sometime between 8 and 16 weeks; the average maximal orientation differences between adjacent subfields were smaller for those neurons preferring the cardinal orientations. The results suggest that the oblique effect in the spatial matrix of facilitatory subfields may not be attributed to an innate mechanism, but appears to result from an experience-dependent change in the wiring between V1 and V2. However, the effects of optical factors including higher-order aberrations have not been ruled out