RECEPTIVE-FIELD PROPERTIES OF VISUAL NEURONS IN THE AVIAN NUCLEUS LENTIFORMIS MESENCEPHALI

The receptive field (RF) properties of visual neurons extracellularly recorded from the nucleus lentiformis mesencephali (nLM) in pigeons (C olumba livia) were quantitatively analyzed using a workstation compute r. These cells were actively spontaneous, and direction-and velocity-s elective. Using spatial gratings as visual stimuli, these cells could be divided into three groups: uni- (74%), bi- (17%), and omnidirection al (9%) cells in terms of their directionality. On the basis of their velocity selectivity, they could be named slow cells (84%), preferring low velocity (0.1-11 degrees/s), and fast cells (14%), preferring rap id motion (34-67 degrees/s), with one cell (2%) responding maximally t o an intermediate velocity of 18 degrees/s. These two properties were correlated in the way that all unidirectionals were slow cells, omnidi rectionals were fast cells, and bidirectionals were either slow or fas t cells including the intermediate cell. Using small targets as visual stimuli, it was found that the majority of cells examined had RFs tha t each consisted of an excitatory RF (ERF) and an inhibitory RF (IRF) that overlapped. The unidirectionals were mainly of this type of RF st ructure, whereas the omnidirectionals apparently had ERFs alone. The d irection preference of ERF was opposite to that of IRF for unidirectio nal cells tested, whereas they were perpendicular to each other for on e bidirectional cell. The overall responses of these cells resulted fr om interaction between excitation and inhibition induced by directiona lly different motion. Under certain conditions, visual responses of a particular cells to a small target moving through its ERF were equal i n responsive strength to those to whole-field gratings swept over the screen. It was suggested that optokinetic nystagmus produced by whole- field gratings results from population activity of large group(s) of n eurons in some optokinetic nuclei, at least one of which is nLM.