Asymmetric suppression outside the classical receptive field of the visualcortex

Areas beyond the classical receptive field (CRF) can modulate responses of the majority of cells in the primary visual cortex of the cat (Walker et al ., 1999). Although general characteristics of this phenomenon have been rep orted previously, little is known about the detailed spatial organization o f the surrounds. Previous work suggests that the surrounds may be uniform r egions that encircle the CRF or may be limited to the "ends" of the CRF. We have examined the spatial organization of surrounds of single-cell recepti ve fields in the primary visual cortex of anesthetized, paralyzed cats. The CRF was stimulated with an optimal drifting grating, whereas the surround was probed with a second small grating patch placed at discrete locations a round the CRF. For most cells that exhibit suppression, the surrounds are s patially asymmetric, such that the suppression originates from a localized region. We find a variety of suppressive zone locations, but there is a sli ght bias for suppression to occur at the end zones of the CRF. The spatial pattern of suppression is independent of the parameters of the suppressive stimulus used, although the effect is clearest with iso-oriented surround s timuli. A subset of cells exhibit axially symmetric or uniform surround fie lds. These results demonstrate that the surrounds are more specific than pr eviously realized, and this specialization has implications for the process ing of visual information in the primary visual cortex. One possibility is that these localized surrounds may provide a substrate for figure-ground se gmentation of visual scenes.