Saturation in human cones

Abstract

Increment threshold intensities were measured for a foveally presented 20msec test flash which fell on a flashed 4.1° white background field. The test field was varied in color (red or green) and size (1° or 0.57°). In one condition (green 0.57° test) the onset asynchrony of the background and test fields was also varied. The increment threshold measurements directly demonstrate saturation of theπ 4 and π 5 color mechanisms and, based on the cone signal equation given by Alpern, Rushton and Torii (1970c), provide estimates of the semi-saturation constant σ. For a given color mechanism, a single value of σ was found to be adequate for all test flash sizes and asynchrony intervals tested; however, these values of σ are about 100 times smaller than those found by Alpern et al. using the contrast-flash technique. The Alpern et al. cone signal equation is essentially a simple gain control model. It is shown that when light intensities are correctly expressed in terms of quantum catch, the gain control model can account explicitly for both the observed cone mechanism saturation in the absence of pigment bleaching and the absence of saturation in the presence of bleaching by steady backgrounds.