Fast and slow neurons in the nucleus of the basal optic root in pigeons.

The nucleus of the basal optic root (nBOR) is involved in the generation of the optokinetic response. Previous studies showed that most nBOR neurons exhibit direction-selectivity in response to largefield motion. We investigated the responses of pigeon nBOR neurons to drifting sine wave gratings of varying spatial and temporal frequency (SF,TF). Two groups of neurons were revealed. The first group preferred gratings of low SF (mean, 0.07 cycles per degree (cpd)) and high TF (mean, 0.76 Hz) ('fast' stimuli). The second group preferred gratings of high SF (mean, 0.56 cpd) and lower TF (mean, 0.33 Hz) ('slow' stimuli). Previous studies have demonstrated fast and slow neurons in pretectal nucleus lentiformis mesencephali, which is also involved in the generation of the optokinetic response.