Effect of stimulus size on the dynamics of orientation selectivity in Macaque V1

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 degrees), 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.