Receptive field structure of monkey V2 neurons for encoding orientation contrast

Many neurons in V2 are orientation selective, but the possibility that orientation selectivity is not uniform across the receptive field has received scant attention. To better understand the role of V2 neurons in the analysis of orientation and orientation contrast, we examined the orientation tuning of V2 neurons at different locations within their receptive fields. Extracellular recordings were made from single neurons in area V2 of anesthetized, paralyzed monkeys. Orientation and spatial frequency tuning functions were measured for each cell with drifting sinusoidal gratings that covered the cell's receptive field. Then, small stationary sinusoidal gratings (∼1/3-1/4 of the receptive field diameter) of various orientations were presented, for 40msec each and in rapid succession, at 19 locations in and near the receptive field in a random sequence. The sequence was re-randomized, for both orientation and position, and repeated up to 100 times. The spike train recorded during the stimulus presentation was cross-correlated with the stimulus sequence to assess the spatial map of orientation tuning. Half of the neurons tested (21 of 42) showed significant spatial non-uniformity in orientation tuning. In many of these cells, there were spatially offset subregions tuned to different orientations that usually were near-orthogonal. The relative positions of subregions varied from cell to cell. The observed non-uniform receptive fields may be useful for encoding orientation contrast and may play an important role in the analysis of image attributes of intermediate spatial complexity, such as corners, angles, curvature, texture, and texture boundaries.