Inversion of direction-selectivity to anterior fields in neurons of nucleus of the optic tract in rabbits with ocular albinism.

Extracellular recordings were made of 46 well-isolated single units in the nucleus of the optic tract (NOT) from 27 rabbits which were paralyzed but unanesthetized. Rabbits were selected from 3 varieties with ocular albinism. These units showed short latencies (mean = 2.2 msec, S.D. = 0.4) to chiasmic stimulation suggestive of monosynaptic innervation from retinal ganglion cells. Receptive fields were large and receptive field centers were confined to the projection of the visual streak. All units showed direction selectivity in which preferred stimulus motions were always nearly horizontal in the anterior or posterior direction. The direction selectivity of the units was broad in that excitatory acceptance angles were 180 degrees. Single units recorded from NOT in albino rabbits could be classified into 4 types based on regions of direction selectivity. The most common (64%) type of unit had receptive fields with two regions of opposed direction selectivity. The preferred direction of motion was anterior in lateral and posterior visual fields and was posterior in anterior visual fields. Two smaller groups of units (13% and 11%) had receptive fields with only one preferred direction. When the receptive field was anterior, the preferred direction was posterior. When the receptive field was lateral or posterior, the preferred direction was anterior. The least commonly encountered type of unit (7%) had receptive fields with two regions of direction selectivity. Anterior visual fields gave rise to anterior direction selectivity and lateral and posterior visual fields gave rise to posterior direction selectivity. The characteristics of NOT units are similar to those described for NOT units in pigmented rabbits except for an inversion of direction selectivity to stimulation from anterior visual fields. NOT units in albino rabbits show posterior direction selectivity when stimulation arises from anterior visual fields. The difference in directional preference in the visual receptive fields of the units in the NOT of albino rabbit may be responsible for the inversion of optokinetic pursuit eye movements characteristic of albino rabbits.