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 cat primary visual cortex are still matters of debate. Here we examine the variety of effects of such peripheral stimuli on responses to stimuli limited to the receptive field. We first determined the extent of the classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a maximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cell 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 drifting grating that had the same parameters as the central stimulus but was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of orientations, while none showed explicit facilitation under these conditions. For 11 cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the surround 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 orientation for the receptive field itself, but for 12 cells (38%) there was apparent 'release' from inhibition for surround gratings at or near the cell's optimum orientation and direction, leaving inhibition either maximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the existence of a subliminal facilitatory mechanism, tightly tuned at or near the cell's optimum orientation, extending outside the classical receptive field. For just two cells of 13 tested the preferred orientation for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell's optimum. The contrast gain for central stimulation at the optimal orientation was measured with and without a surround pattern. For nine of 25 cells tested, surround stimulation at the cell's optimum orientation facilitated the response to a central grating of low contrast (< or =0.1) but inhibited that to a higher-contrast central stimulus: the contrast-response gain is reduced but the threshold contrast is actually decreased by surround stimulation. Hence the receptive field is effectively larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell's optimum, while suppression within the receptive field has been shown to be largely non-selective for orientation. Inhibition by orientations flanking 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.