Color properties of the contrast flash effect: monoptic vs dichoptic comparisons.

COLOR PROPERTIES OF THE CONTRAST FLASH EFFECT: MONOPTIC VS DICHOPTIC COMPARISONS JOHN I. YELLOTT, JR. and BRIAU A. WANDELL University of California. Irvine, CA 92664, U.S.A. (Receiced 2 September 1975; in The visibility of a brief flash of light can be greatly reduced if it is followed within a few msec by a second flash falling on an adjacent portion of the retina. This type of backward masking is widely known as “metacontrast” (Alpem, 1952, 1953; Kahne- man, 1968; Weisste-in, 1972). In an influential series of psychophysical papers, Alpem (1965), Alpem and Rushton (1965, 1967), and Alpem, Rushton and Torii (1970 ab,c,d) have used a metacontrast version of Stiles’ (1959) increment threshold technique to study spatial interactions between and within Stiles’ x mechanisms. Basically, their approach has been to plot test flash threshold luminance against the luminance of a surrounding masking flash (the meta- contrast analog of Stiles’ threshold vs radiance curve) and analyze the effects of stimulus parameters in terms of Stiles’ displacement rules (Enoch, 1972). They call the mask a “contrast flash”, and refer to the test flash threshold elevation produced by it as the “con- trast gash effect”. Using this approach, Alpem and Rushton (1965) showed that when the test flash is being detected by a given color mechanism xi (e.g. rt5 for a red flash), contrast flashes of different colors produce identical threshold elevations if their in- tensities are adjusted to be equal according to the action spectrum for xi. (This had already been shown for the rod mechanism rro by Alpem, 1965). Thus, to the extent that these psychophysically delined action spectra (i.e. Stiles’ t.v. i. curves) are the same as those of the cones themselves (cf. Enoch, 1972; King-Smith and Webb, 1974), one can say that to be equally effective at raising threshold for a flash detected by, e.g. red cones, contrast flashes of different colors must be adjusted to produce an equal response in red cones-regardless of their effect on other recep- tor systems. These results have quite reasonably been interpreted as showing that the contrast flash effect is “receptor-specific; the rods of the test flash were inhibited only by rods in the surround, no matter how strongly cones were also excited; red cones of the flash were inhibited only by red cones in the sur- round, etc.” (Rushton, 1972). This interaction presum- ably occurs in the retina, before the signals from dif- ferent receptor systems are combined by opponent process cells. On this basis, contrast flash data have been used to infer the quantitative relationship between receptor signals and light intensity (A1pe.m et a[., 1970 a,b,c,d) and these deductions (in particular, that receptor signals are linear with light intensity over a large range) have come to be widely accepted (e.g. Brindley, 1970; Rodieck, 1973). In all of these contrast flash experiments the test and contrast flashes have both been presented to the revised firm 1 March 1976) same eye (monoptic presentation). However it is well established (Weisstein, 1972, p. 236) that metacontrast masking also occurs when the test flash is delivered to one eye and the masking Bash to the other (dichop- tic presentation). We report here the results of exper- iments in which we have replicated on a small scale the essential features of the color mechanism work of Alpem and Rushton (1965) using dichoptic as well as monoptic presentation. We find, as they did, that when the test flash is red, a red contrast flash raises its threshold much more than a green contrast flash of the same luminance. However we also find that a given contrast flash is equally effective in raising test flash threshold whether presented monoptically or dichoptically. In this respect, contrast flashes act quite differently than steady backgrounds, which in our experiment, and so far as we know in all previous ones, have no effect at all dichoptically (Whittle and Challands, 1969). Our contrast flashes also differ from steady backgrounds in having what appears to be a much narrower action spectrum: the difference in effectiveness between a red light and a green one, as measured by the threshold elevation of a red test flash, is much greater when these lights are used as contrast flashes than when they are used as steady backgrounds. Thus both monoptically and dichopti- tally our results are incompatible with the assump- tion that masking takes place within a single rt mechanism. These results indicate that the contrast flash effect cannot always be reliably assumed to reflect a recep- tor-specific inhibitory signal acting within the retina, even when the color properties of the effect seem superficially to be consistent with such a model. Thus, one must be cautious about inferring the form of retinal signals from contrast flash data. EXPERIMEXT 1 The purpose here was to compare the color properties of moaoptic and dichoptic contrast flashes. The logic of our procedure was the same as that of Alpern and Rushton (1965), except for the inclusion of dichoptic presentations. As a preiiminary step, we measured the increment thresh- old for a 10msec red test flash suoerimposed. either monopticafly or dichoptically, on a steady background field. This was done for both red and areen backgrounds at luminances of 0, 0.1, 1.0 and lccd/m’. Ge then measured threshold for the same red test flash, superim- posed on a zero background and followed after 50msec by a 40 msec red or green contrast flash. The spectral com- positions of these red and green contrast flashes were the same as those of the corresponding steady backgrounds. Contrast flash luminance was 0.1, 1.0 or 10 cd/m’.