Cone signals in the cat's retina

Abstract

The discharges of ganglion cells in cat retina were recorded under conditions intended to isolate the cone system. Stiles'' 2-color threshold technique permitted the study of the photopic system at its highest sensitivity. The absolute sensitivity of a ganglion cell, expressed in equivalent photons of .lambda.max [wavelength for maximum absorption] at the cornea per impulse discharged, was about 2500 times less when driven by cones than when driven by rods. This ratio improved to around 200 when allowance was made for the much smaller fraction absorbed by cones of photons incident on the cornea. The number of extra impulses discharged in response to a brief flash was approximately proportional to the number of photons in the flash, up to a limit. There was a region in the middle of the receptive field within which the area of a test spot and its illumination for threshold varied inversely. A flash extending over the peripheral part of the receptive field raised threshold above its minimum, presumably as a result of surround antagonism. Assessed from area-threshold curves, the balance of center-surround antagonism in the photopic receptive field did not seem to depend on background illumination. The threshold for a small (0.2.degree.) flash confined to the middle of the receptive field was independent of background illumination until the background exceeded a particular level, the dark light (Io). In different units this ranged about a mean of 7.89 log photons (560 nm equivalent)/degree per s. For backgrounds that exceeded Io, threshold followed approximately Weber''s law up to the highest illuminations that could be produced. With test flashes that filled the center of the receptive field the Weber fraction (test flash illumination/background illumination) in some units fell below 1%. Changes in the time course and latency of response accompanied the changes in sensitivity caused by alterations in background illumination. Responses of X- and Y-cells became more transient and faster. The loss of sensitivity to a test flash brought about by a steady background light depended on the size of that light. Sensitivity varied inversely with background area within a central region that matched closely the summing area for test flashes.