Receptive fields of single optic nerve fibers in a mammal with an all-cone retina. I: contrast-sensitive units.

Abstract

The purpose of this study was to determine the organization of the receptive fields of single optic nerve fibers coming from the all-cone retina of the ground squirrel (Citellus mexicanus). The anesthetized animal was held in a stereotaxic instrument and faced a tangent screen upon which stimuli were presented by 2 slide projectors. W microelectrodes, plated with Au and Pt, were used to record extracellularly from single fibers. Many of the units (218 of 410; 53%) had concentrically organized receptive fields with either excitatory ("on") centers of inhibitory ("off") surrounds, or the reverse. The centers and surrounds were mutually antagonistic. Light adapting the field center raised the sensitivity of the surround, which by itself was often unresponsive. The centers and the surrounds had identical spectral sensitivities; both received inputs from cones containing the same green-sensitive visual plgment. Moving black or white spots evoked equally strong responses for all directions of motion across the center-surround border. These cells are probably mediators of simultaneous contrast information.