Previous research has established that orientation selectivity depends to a great extent on suppressive mechanisms in the visual cortex. In this study, we investigated the spatial organization and the time-course of these mechanisms. Stimuli were presented in circular windows of "optimal" and "large" radii. The two stimulus sizes were chosen based on an area-response function measured with drifting gratings at high contrast. The "optimal" size was defined as the smallest radius that elicited the peak response (average value of 0.45°), whereas "large" was defined as two to five times the optimal size. We found that the peak amplitude of tuned enhancement and untuned suppression varied <10% on average with stimulus radius, indicating that they are mainly concentrated in the classical receptive field. However, tuned suppression-in those cells that showed it-was significantly stronger with large stimuli, indicating that this component has a contribution from beyond the classical receptive field. These results imply that spatial context (in large stimuli) enhances orientation selectivity by increasing tuned suppression. We also characterized the time evolution of enhancement, of untuned suppression, and of tuned suppression. The time-course of tuned suppression was markedly slower in time-to-peak and longer in its persistence than untuned suppression. Therefore tuned suppression is likely to be generated by long-range recurrent connections or cortico-cortical feedback, whereas untuned suppression is mainly generated locally in V1.
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