Published in Annual Meeting, Neuroscience, Nov 2017.
The activity of neurons in visual cortex is modulated by what lies beyond the classical receptive field. Surrounding contrast usually suppresses responses, and the strength of this suppression is often selective for visual features. In V1, suppression is maximal when the orientation of a drifting grating presented in the surround matches the orientation presented to the center. More broadly, V1 suppression is strongest when center and surround images match, and weaker when they differ in image statistics. Less is known about surround suppression in V2, although its influence on responses is similar to that in V1 when studied with drifting gratings. We have recently shown that V2 neurons, but not V1 neurons, respond more strongly to "naturalistic" textures containing higher-order statistical dependencies than to spectrally-matched "noise" images containing only second-order dependencies. We wondered whether these stimuli might reveal differences in contextual modulation between the areas. We measured the responses of single V1 and V2 units in both anesthetized and awake, fixating macaque monkeys to naturalistic and noise images presented within an aperture that varied in diameter. In V1, suppression was similar for both types of stimuli, with a slight tendency for naturalistic images to evoke stronger suppression. This is consistent with previous work in V1, as naturalistic images contain correlations over space that impose stronger dependencies between center and surround. In contrast, surround suppression in V2 neurons was much weaker for naturalistic textures than for spectrally-matched noise images. We wondered whether these results depended more on stimulus statistics in the center of the receptive field or in the surround. Preliminary measurements with mixture stimuli suggest that suppression is actually weakest in V2 when the center is stimulated with naturalistic texture and surrounded by spectrally-matched noise. These results suggest that the statistical structure of visual context plays a more sophisticated role in modulating the selectivity of neuronal responses in V2 than it does in V1.