Functional role of efferents to the avian retina. II. Effects of reversible cooling of the isthmo-optic nucleus.

Efferents to the retina in the bird arise in the isthmo-optic nucleus of the caudal midbrain, and terminate on amacrine cells in the retina. The functional role of these efferents was studied by determining the receptive field properties of 107 optic tract fibers in the lightly anesthetized adult pigeon, and quantitating their responses to specific moving stimuli. While the recording from these fibers continued, the isthmo-optic nucleus was cooled by a thermoelectric cooling probe, and the response properties of the cells redetermined. Recording was maintained in half of the units long enough to observe recovery from cooling, and in several units the entire procedure was repeated. In 77 of the 107 units, responsiveness to all stimuli was decreased by removing efferent influences, whereas specific receptive field properties such as motion sensitivity or directionaltiy were not altered. All of the major receptive field types were affected in a similar fashion, irrespective of their position in the visual field. Responses to stimuli that did not involve the antagonistic surround were similarly affected by removal of the efferents, as were units were both weak and strong antagonistic surrounds. Efferents exert their influence on retinal ganglion cells by way of the amacrine cells on which they terminate. Data available on amacrines in Necturus indicates that they are inhibitory to ganglion cells. If amacrines have a similar role in the pigeon, then it may be stated that decreased activity in the centrifugal fibers leads to enhanced inhibition throughout the receptive fields of ganglion cells, and increased activity in the efferents produces disinhibition.