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Kara Emery, Vicki Volbrecht, David Peterzell, Michael Webster; Color vs. motion: Decoding perceptual representations from individual differences. Journal of Vision 2019;19(8):8. doi: https://doi.org/10.1167/19.8.8.
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Color and motion share a common three-dimensional structure, yet it is unclear to what extent this structure leads to analogous representations in visual coding. We analyzed individual differences in the perceived direction of color (2D hue angle) and motion (2D trajectory) to compare how the visual system represents these attributes. Previously we measured individual differences in color appearance for color-normal observers using a standard “hue scaling” task in which observers judged the proportion of red/green and blue/yellow in each hue. Factor analyses of these settings revealed eight non-opponent factors with loadings that were narrowly-tuned for different hue angles. This factor pattern argues against a representation of hue in terms of two underlying chromatic axes, and instead points to a code in terms of multiple, independent processes, in which neither the cone-opponent (LvsM and S) nor perceptually-opponent (e.g. red-green or blue-yellow) directions appear privileged. To test for a corresponding code for motion space, we developed a “motion scaling” task, in which observers rated the proportion of left/right and up/down motion in moving dots. Factor analyses of these settings revealed factors whose loadings instead varied roughly sinusoidally with stimulus angle or which were narrowly tuned to one of the cardinal directions. In contrast to the factor pattern for hue, this pattern points to a metrical code for motion in terms of the cardinal (horizontal and vertical) axes. These differences suggest that while the “spaces” defining color and motion are fundamentally similar, the perceptual representations of directions within these spaces are fundamentally different.
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