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Siddhart Srivatsav, John Maule, Anna Franklin, Michael Webster; Ensemble percepts of color contrast. Journal of Vision 2017;17(10):1173. doi: https://doi.org/10.1167/17.10.1173.
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Ensemble coding has been demonstrated for many attributes including color, but the metrics on which this averaging is based remain uncertain. We examined ensemble percepts for colors that varied in saturation along opponent axes, to explore how averaging occurs across saturation within and between hues. Ensembles were composed of 4 chromatic contrasts in equal log steps plus gray. They were displayed as 16 randomly positioned circles, demarcated by black borders from the gray background. Some sets included complementary colors (opposite sides of the same axis) and either did or did not include the achromatic stimulus. The hue axis varied randomly across trials to avoid learning specific colors. Targets included the four displayed contrasts and five additional levels at intermediate or higher contrasts. On each trial observers saw the ensemble for 0.5 sec and after a 1 sec delay responded whether a single displayed target was a member of the set. For sets with complementary colors (e.g. red and green), false alarms for gray targets were lower even when this gray represented the mean chromaticity. Reported membership was instead bimodal for each hue. In unimodal sets (e.g. red or green) false alarms fell precipitously at the gray boundary. These categorical effects suggest that averaging occurs primarily within rather than between complementary hues. In biased ensembles (e.g. red contrasts higher than green) the perceived membership paralleled the asymmetry (e.g. fewer false alarms for high contrast greens). This suggests that hue and saturation are not encoded as independent attributes, and instead is consistent with averaging within each hue. Our results suggest that ensemble percepts of color are not based on a simple metric like Euclidean distance, and that the overall mean of a color distribution with different hues may be available only implicitly, from the summary percepts of the different hues. Funding EY-10834
Meeting abstract presented at VSS 2017
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