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.