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Alan Robinson, Donald MacLeod; The strength of the McCollough effect does not increase linearly with saturation: implications for the cortical color code. Journal of Vision 2011;11(11):344. doi: https://doi.org/10.1167/11.11.344.
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© ARVO (1962-2015); The Authors (2016-present)
There is no generally agreed upon formula for expressing saturation and chromatic contrast. Here we investigate saturation coding in early visual cortex by way of the McCollough orientation-contingent color-aftereffect. We have previously shown the McCollough aftereffect strength is linear with induction time, suggesting that the effect is proportional to stimulus effectiveness (Vul, Krizay, & MacLeod, 2008). Is the same true for the saturation of the inducer? Subjects adapted to a fully saturated red and green pair of equal-luminance inducing gratings, produced by a wide gamut LCD. Aftereffect strength was assessed using a nulling technique, and comparing pre- and post-adapt nulls. Halving the inducing saturation of the gratings (by halving the difference in cone excitations) reduced the aftereffect strength only slightly. This suggests a compressive coding of saturation, where equal successive increments in cone contrast produce diminishing returns in the cortical code for saturation. Our result predicts that anomalous trichromats should see a larger McCollough effect. In anomalous trichromats, the apparent difference between red and green is greatly reduced owing to the proximity in peak spectral sensitivity (λmax) between the long-wave and middle-wave cones. During adaptation this only slightly reduces the induced aftereffect, because of the saturation non-linearity we found. In order to null the resulting aftereffect, however, the color-deficiency of the retina must be overcome by adding significantly more chromaticity to the null than a normal viewer would require. Initial evidence supports this prediction; the one anomalous subject tested so far had a McCollough effect several times larger than normal.
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