Visual responses of neurons in the nucleus of the basal optic root to stationary stimuli in pigeons.

The nucleus of the basal optic root of the accessory optic system in pigeons is involved in generating optokinetic nystagmus, which stabilizes object images on the retina by compensatory eye movements. Previous studies have indicated that basal optic neurons are selective for the direction and velocity of motion. The present study shows that these optokinetic cells also respond to stationary stimuli and thereby could be categorized into three groups. The first group of cells (69.1%) responds to stationary gratings orthogonal to the preferred direction but not to gratings parallel to the preferred direction. They do not respond to stationary random-dot patterns without any orientational cues. The second group of cells (7.4%) almost equally discharges a series of bursts in response to stationary gratings with any orientations and to random-dot patterns as well. The third group of cells (23.5%) is responsive to motion but not to stationary gratings and random-dot patterns. The receptive field of basal optic cells is composed of an excitatory field and an inhibitory field, both of which overlap or occupy different regions in the visual field. The aforementioned properties may be attributed to the excitatory receptive field, whereas the inhibitory receptive field is functional when visual stimuli are moving in the direction opposite to the preferred direction of basal optic cells. The functional significance of visual responses of optokinetic neurons to stationary patterns is discussed.