Optical Imaging of Contextual Interactions in V1 of the 1 Behaving Monkey

24 Interactions in primary visual cortex (V1) between simple visual elements such as short bar 25 segments are believed to underlie our ability to easily integrate contours and segment 26 surfaces. We used intrinsic signal optical imaging in alert fixating macaques to measure the 27 strength and cortical distribution of V1 interactions amongst collinear bars. A single short 28 bar stimulus produced a broad-peaked hill of activation (the optical point spread) covering 29 multiple orientation hypercolumns in V1. Flanking the bar stimulus with a pair of identical 30 collinear bars led to a strong nonlinear suppression in the optical signal. This nonlinearity 31 was strongest over the center bar region, with a spatial distribution that cannot be explained 32 by a simple gain control. It was a function of the relative orientation and separation of the 33 bar stimuli in a manner tuned sharply for collinearity, being strongest for immediately 34 adjacent bars lying on a smooth contour. These results suggest intracortical interactions 35 playing a major role in determining V1 activation by smooth extended contours. Our 36 finding that the interaction is primarily suppressive when imaged optically, which 37 presumably reflects the combined inhibitory and excitatory inputs, suggests a complex 38 interplay between these cortical inputs leading to the collinear facilitation seen in the 39 spiking response of V1 neurons. This disjuncture between the facilitation seen in spiking 40 and the suppression in imaging also suggests that cortical representations of complex 41 stimuli involve interactions that need to be studied over extended networks and may be 42 hard to deduce from the responses of individual neurons. 43 44