Receptive fields of single optic nerve fibers in a mammal with an all-cone retina. II: directionally selective units.

Abstract

Some units (93 of 410, 23%) in the ground squirrel''s [Citellus mexicanusl optic nerve were directionally selective. Their receptive fields consisted of "on-off" centers (0.5-1.0[degree]) encompassed by antagonistic surrounds. The entire field center was sensitive to preferred movement of black or white stimuli (0.1 to 30%ec), even over very short distances (5[image])- Null movement inhibited any spontaneous activity. Very slow null movement (0.1-0.3[degree]/sec.) excited a unit. A directionally selective ganglion cell receives excitatory inputs from many sequence-discriminating subunits (bipolars?) which have the same preferred direction of motion. A subunit is excited directly by 1 set of cones and inhibited indirectly (via horizontal cells?) by a second laterally-displaced group of cones. The sequence of illumination of the 2 populations of receptors determines the directional-discriminating ability of the subunits, and, ultimately, of the ganglion cell. These units are connected to green-sensitive cones (peak at 525nm). There was no correlation between any aspect of the directionally selective units and their receptive-field position on the retina.