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Retinal ganglion cells of the pigeon accessory optic system.

After the demonstration of its involvement in visuomotor coordination, the accessory optic system has been the subject of renewed interest. An interesting feature of its functional organization is that the retinal ganglion cells generating projections to accessory optic nuclei appear to be, at least in birds, the displaced ganglion cells of Dogiel. Retinal extracellular unit recordings were performed for 21 pigeons. Neurons were identified by antidromic activation from primary visual relays, including the nucleus of the basal optic root of the accessory optic system, the optic tectum, and the thalamic dorsolateral nuclear complex. Ganglion cells (N = 68) of the accessory optic system were always found deeper in the retina, close to the boundary between the inner plexiform and the inner nuclear layers, i.e. displaced from the ganglion cell layer itself. Their latencies to antidromic activation ranged from 1.3 to 4.6 ms (mean +/- SD, 2.3 +/- 0.5 ms). The receptive fields were very large, peripheral and apparently non-organized. Responses to stationary photic stimuli were weak, and the main feature of these cells appeared to be high sensitivity to slowly moving targets, with some degree of directional selectivity. These response properties indicate that the ganglion cells of the accessory optic system in pigeons could well detect visual image displacement on the retina. Together with other anatomical, physiological and behavioral data, the present study provides information linking the accessory optic system to oculomotor control functions.