CHARACTERISTICS OF SURROUND INHIBITION IN CAT AREA-17

The effects of stimuli falling outside the 'classical receptive field' and their influence on the orientation selectivity of cells in the ca t primary visual cortex are still matters of debate. Here we examine t he variety of effects of such peripheral stimuli on responses to stimu li limited to the receptive field. We first determined the extent of t he classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a ma ximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cel l just began to respond. These two estimates always agreed closely. We then presented an optimum grating of medium-to-high contrast filling the classical receptive field while stimulating the surround with a dr ifting grating that had the same parameters as the central stimulus bu t was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of ori entations, while none showed explicit facilitation under these conditi ons. For II cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the sur round stimulus. In the majority of cells, suppression was weakest for a surround grating oriented orthogonal to the cell's optimum. Nine of these cells (28%) exhibited maximum inhibition at the optimum orientat ion for the receptive field itself, but for 12 cells (38%) there was a pparent 'release' from inhibition for surround gratings at or near the cell's optimum orientation and direction, leaving inhibition either m aximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the ex istence of a subliminal facilitatory mechanism, tightly tuned at or ne ar the cell's optimum orientation, extending outside the classical rec eptive field. For just two cells of 13 tested the preferred orientatio n for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell's optimum. The contra st gain for central stimulation at the optimal orientation was measure d with and without a surround pattern. For nine of 25 cells tested, su rround stimulation at the cell's optimum orientation facilitated the r esponse to a central grating of low contrast (less than or equal to 0. 1) but inhibited that to a higher-contrast central stimulus: the contr ast-response gain is reduced but the threshold contrast is actually de creased by surround stimulation. Hence the receptive field is effectiv ely larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell's optimu m, while suppression within the receptive field has been shown to be l argely non-selective for orientation. Inhibition by orientations flank ing the optimum could serve to sharpen orientation selectivity in the presence of contextual stimuli and to enhance orientational contrast; and it may play a part in orientation contrast illusions.