Centrifugal control of the avian retina. III. Effects of electrical stimulation of the isthmo-optic tract on the receptive field properties of retinal ganglion cells

Abstract

Single unit recording techniques were used in decerebrate, immobilized domestic chicks to investigate the effects of electrical stimulation of the peripheral end of the severed isthmo-optic tract on the receptive field properties of ganglion cells in the contralateral retina. Tract stimulation always worked to enhance ganglion cell visual responses and was never observed to depress them. However, activation of the tract did not itself generate ganglion cell firing and centrifugal effects were only apparent when the retinal cells were receiving visual stimulation. Visual stimuli such as large, centred spots which were normally ineffective in driving ganglion cells because they enroached on the inhibitory surround, often produced activity when combined with tract stimuli. Experiments with spots and annuli suggest that this centrifugal enhancement probably results from suppression of the inhibitory surround mechanisms (disinhibition) since in such cases tract stimuli were only effective when paired with both spot and annulus, and not with either target alone. Occasionally, the centrifugal input seemed to increase ganglion cell excitability through a direct effect on central excitatory mechanisms (facilitation), since in these cases tract stimulation produced considerably increased firing to small, centred spots whilst failing to exert any appreciable effect on responses to either the annulus alone, or the annulus in combination with the spot. The possible functional significance of these findings is discussed.