Abstract
The modulation sensitivity of visual neurons can be influenced by remote stimuli which, when presented alone, cause no change in the ongoing discharge rate of the neuron. We have investigated these non-classical surround effects in primate retina. The extracellular responses of 16 parasol (MC) and 8 midget (PC) parafoveal ganglion cells were recorded from the retina of anaesthetized and paralyzed macaques. Receptive fields were stimulated with drifting sinusoidal gratings presented on a CRT within circular or annular windows that were centered on the receptive field. The remainder of the screen was held at the mean luminance. The spatial frequency was set to be above the peak for the neuron under study. When the outer diameter of a high contrast grating patch (0.75 Michelson contrast) was increased, the responses of all ganglion cells rapidly increased to a maximum. The responses of PC cells were little influenced by extending the grating patch further (mean reduction from peak was 11%, SD 0.12). For MC cells, however, increasing the size suppressed responses (mean 42%, SD 0.14) relative to that at the peak. The magnitude of suppression asymptoted at an average diameter of 1.92 deg (SD 0.69) and thus suppression cannot have arisen from the classical receptive field or been due to scattered light: the largest inner diameter of an annulus that alone evoked a response was 0.75 deg (SD 0.38). Annuli that did not generate a response also did not cause a change in the ongoing firing rate, suggesting that the suppressive surround acts in a non-linear manner. Suppression was present at contrasts above 0.25 and temporal frequencies up to 15 Hz. It was at least as strong at spatial frequencies at the very limit of the resolution of each cell. This suggests that surround suppression is a non-linear summation of subunits over a large area; we do not know if it is the same mechanism as the contrast gain controls known to be present in MC cells (Bernadete and Kaplan, 1992).