Spatial organization and magnitude of orientation contrast interactions in primate V1.

We have explored the spatial organization of orientation contrast effects in primate V1. Our stimuli were either concentric patches of drifting grating of varying orientation and diameter or grating patches displaced in x-y coordinates around a central patch overlying the classical receptive field (CRF). All cells in the sample exhibited response suppression to iso-oriented stimuli exceeding the CRF. Changing the outer stimulus orientation revealed five response patterns: 1) orientation alignment suppression (17% of cells)-a suppressive component tuned to the same orientation as the cell's optimal, 2) orientation contrast facilitation (63%)-responses to orientation contrast stimuli exceeded those to the center stimulus alone, 3) nonorientation specific suppression (3%), 4) mixed general suppression and alignment suppression (14%), and 5) orientation contrast suppression (14%)-cross-oriented stimuli evoked stronger suppression than iso-oriented stimuli. Thus most cells (94%) showed larger responses to orientation contrast stimuli than to iso-oriented stimuli, and over one-half showed orientation contrast facilitation. There appeared to be a spatially structured organization of the zones driving the different response patterns with respect to the CRF. Nonorientation-specific suppression and orientation contrast suppression were predominantly evoked by orientation contrast borders located within the CRF, and orientation contrast facilitation was mainly driven by surround stimuli lying outside the CRF. This led to different response patterns according to border location. Zones driving orientation contrast facilitation were not necessarily contiguous to, nor uniformly distributed around, the CRF. Our data support two processes underlying orientation contrast enhancement effects: a simple variation in the strength of surround suppression drawing on the fact that surround suppression is tuned to the same orientation as the CRF and a second process driven by orientation contrast that enhanced cells' responses to CRF stimulation by either dis-inhibition or orientation contrast facilitation. We suggest these processes may serve to enhance response levels to salient image features such as junctions and corners and may contribute to orientation pop-out.