Saturation in human cones.

Increment threshold intensities were measured for a foveally presented 20msec test flash which fell on a gashed 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 the n, and ILL color mechanisms and, based on the cone signal equation given by Alpern, Rushton and Torii (197Oc), provide estimates of the semi-saturation constant 6. For a given color mechanism a singIe value of d was found to be adequate for all test flash sizes and asynchrony intervals tested; however, these values of o 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 expticitly 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. Key Words-cone saturation; cone signals. It is well known that the human cone visual system cannot be saturated by a steady background light (Stiles, 1939, 1949, 1959). For a background light of any intensity (no matter how strong), it is always possible to see a s~ciently strong test Bash superimposed on the steady background. Quite the opposite is found in the rod system, in which a steady, moderately strong background (about 3 log scotopic td) raises the increment threshold so high that no test flash of any intensity can be detected (Aguilar and Stiles, 1954). In a series of papers, Alpern, Rushton and Torii (1970a,b,c) investigated the saturation of both rods and cones. Rather than using a conventions increment threshold paradigm, Alpern et al. employed the contrast-flash technique {Alpern, 1965; Alpern and Rushton, 1965, 1967) in which the intensity of an after-flash surrounding (but not including) the test field is adjusted so as to bring the test gash to threshold. They demonstrated saturation by adding a steady background light to the surround area only and then measuring the intensity of the after-flash required to bring a fixed test to threshold as a function of steady background intensity. Alpern et al. (1970~) also suggested a modification to the standard increment threshold procedure which involved the presentation of a brief test on a &shed background. Using this technique with a white, foveally presented test falling on a flashed white background, they showed that a back~ound of about 610g td saturates the photopic system. King-Smith and Webb (1974) used the same procedure with colored test and background fields to demonstrate saturation in each of the red, green, and blue color mechanisms. However, they provided only relative intensity measurements. The present experiments use the same experimental paradigm. Increment threshold measurements were made for a foveally presented 20msec test flash (denoted i.) which fell on a hashed, 4.1” white background field (denoted 6). In addition, conventional increment threshold measurements were made with the test held ii falling on a steady background % presented for a period sufficiently long to assure that the level of bleached pigment was at equilibrium. The present increment threshold measurements made with flashed back~ounds provide a direct demonstration of n4 and rrs color mechanism saturation. The effect of test field size is also explored, as is the effect of altering the test and background fields’ onset asynchrony. Conventional increment threshold measurements make possible a comparison between measurements made with flashed and steady backgrounds. By me~uring abso’tute (rather than relative) test light and background light intensities, it is possible to relate the present increment threshold measurements (1) to previously published experimental results and (2) to the amount of pigment bleached by the background lights. Absolute intensity measurements allow precise quantitative development of a gain control model in which the gain depends on the quantum catch of the incident background light. Absolute intensities are necessary because the fraction of cone pigment bleached affects the quantum catch. It is shown that this model can account explicitly for the saturation observed with flashed backgrounds and the absence of saturation found with steady backgrounds.