Effects of the orientation of spatially restricted surround stimuli on RF center surround interaction in macaque v2 neurons

We have previously shown that stimulating the RF surround of a V2 neuron with iso-oriented, high-contrast (80%) annulus gratings strongly suppresses its center responses initiated by high-contrast stimuli (Zhang et al, 2005). Exception to this suppressive interaction was that center responses were facilitated if center stimulus contrast was below 10-15%. To examine possible involvement of V2 neurons in global form vision, we determined whether manipulating the size, location, and orientation of high-contrast surround gratings could facilitate center responses initiated by equally high-contrast stimuli. We recorded from 80 V2 units of normally reared adult monkeys. The size of surround sinusoidal gratings was matched to that for the size of center gratings. Two identical surround stimuli were presented on the opposite sides of the center stimuli (\u27serial arrangement\u27) and the distance between the center and surround stimuli was kept at 3/4th of the center size. The orientation of surround stimuli relative to that for center stimuli was set at 0, 45, 90, 135, and 180 degrees and drifted in both directions. In the majority of V2 neurons, we found facilitation of center responses by surround gratings at one or more surround orientations. This surround facilitation was most prevalent for the iso-orientation or collinear condition (0°) if iso-oriented surround gratings were drifted in the direction opposite to that for center gratings. Orthogonal surround stimuli, regardless of drift direction, were as equally effective in facilitating center responses. Units preferring oblique surround orientations were less frequently encountered. Interestingly, at these oblique orientations, center facilitation was also greater when surround gratings were drifted in opposite direction from that for center gratings. Surround inhibition was most frequently encountered with iso-orientation surround stimuli drifted in the same direction as center stimuli. A small percentage of units showed only facilitation or suppression at all surround orientaitons. Together, surround stimuli that were different in orientation and/or drift direction were more effective in facilitating center responses. These results suggest that V2 neurons could be capable of detecting large “shape primitives” such as cornered or angled contours or collinearly arranged contours that extend way beyond RF centers of V2 neurons