Opposing motion inhibits responses of direction-selective ganglion cells in the fish retina.

Abstract

Inhibitory influences in receptive fields (RFs) of the fish retinal direction-selective ganglion cells (DS GCs) were investigated. Responses of the fast retinal DS GCs were recorded extracellularly from their axon terminals in the superficial layer of tectum opticum of immobilized fish. The data were collected from two cyprinid species - Carassius gibelio, a wild form of the goldfish, and the barbel fish Labeobarbus intermedius. Visual stimuli were presented to the fish on the monitor screen within a square area of stimulation occupying approximately 11 × 11° of the visual field. DS GCs were stimulated by pairs of narrow stripes moving in opposing directions. One of them entered central (responsive) area of cell receptive field (RRF) from the preferred, and the other one from the null side. Stimuli merged at center of stimulation area, and subsequently moved away from each other. It was shown that the cell response evoked by the stripe coming from the preferred side of RF was inhibited by the stimulus coming from the opposite direction. In the majority of units recorded inhibitory effect induced by the null-side stimulus was initiated in the RF periphery. As a rule, inhibitory influences sent from the RF periphery were spread across the entire central area of RF. Modifications of the inhibitory influences were investigated throughout the whole motion of paired stimuli. Evident inhibitory effects mediated from the null direction were recorded during the approach of stimuli. When stripes crossed each other and moved apart inhibition was terminated, and cell response appeared again. Null-side inhibition observed in fish DS GCs is most likely induced by starburst-like amacrine cells described in morphological studies of different fish species. Possible mechanisms underlying direction selectivity in fish DS GCs are discussed.