Directional Modulation of Visual Responses of Pretectal Neurons by Accessory Optic Neurons in Pigeons Most Directional Cells in the Pigeon Nlm Prefer Horizontal Motion, but Others Prefer Vertical Motion

ÐThe nucleus lentiformis mesencephali and the nucleus of the basal optic root in birds, homologous to the nucleus of the optic tract and the terminal nuclei of the accessory optic tract in mammals, are involved in optokinetic nystagmus. The present study provides the ®rst electrophysiological evidence that reversible blockade of the pigeon nucleus of the basal optic root by lidocaine can change visual responsiveness of pretectal neurons in a direction-dependent manner. Thirty pretectal cells examined were classi®ed as unidirectional (80%), bidirectional (10%) and omnidirectional (10%) cells according to their directional selectivity. Among the unidirectional cells, seven cells changed ®ring rates in all directions of motion, 11 changed visual responses only in the preferred directions and six others did not change their responsiveness during lidocaine. Most of the bidirectional cells changed ®ring rates in the temporonasal direction, and two-thirds of the omnidirectional cells showed these changes in all directions. Thirteen lidocaine administration sites were marked within the nucleus of the basal optic root and 19 recording sites were marked within the nucleus lentiformis mesencephali. This histological veri®cation indicates that the effects of lidocaine blockade in the accessory optic nucleus on the directional selectivity and visual responsiveness of pretectal cells appear to be related, to some extent, to the location of drug injections in the nucleus of the basal optic root. This study has found that visual neurons in the nucleus of the basal optic root, which predominantly prefer vertical and backward motion, could modulate the directional selectivity and visual responsiveness of neurons in the nucleus lentiformis mesencephali, which mainly prefer horizontal motion. It is conceivable that both nuclei work together in coordination and in competition during optokinetic nystagmus. It has been shown that the nucleus lentiformis mesence-phali (nLM) in the pretectum and the nucleus of the basal optic root (nBOR) of the accessory optic system in birds are primary visual nuclei involved in generating opto-kinetic nystagmus, which stabilizes an object image on the retina by slow tracking and rapid resetting movements of the eyes. Their mammalian homologues are thought to be the nucleus of the optic tract and the terminal nuclei of the accessory optic tract, respectively. Visual neurons in these optokinetic nuclei are selective for direction and velocity of motion, and respond to motion of large-®eld patterns. In pigeons, receptive ®elds of optokinetic neurons are usually composed of an excitatory receptive ®eld and an inhibitory receptive ®eld, both of which …