Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Psychology

First Advisor

David H. Brained

Second Advisor

Geoffrey K. Aguirre

Abstract

The first stage of human color vision is the encoding of light by the long (L), medium (M), and short (S) wavelength sensitive cone photoreceptors. These signals are subsequently recombined into three post-receptoral pathways, two chromatic opponent and one luminance. The two chromatic pathways take the difference of cone activity (L-M and S-(L+M)) and the third luminance pathway sums the cone activity (L+M). Beyond the level of these post-receptoral mechanisms, the computations performed and their location in the brain are not well characterized. Across two studies, we examined the sensitivity of the underlying chromatic mechanisms using both functional magnetic resonance imaging and psychophysics. In both types of experiments, we employ the use of ‘silent substitution’ which allows for the selective modulation of activity of cone photoreceptors, and their combinations. In the first experiment, we collected BOLD fMRI responses in human V1 to spatially uniform, temporal chromatic modulations that systematically vary in chromatic direction and contrast. From this, we found that within the LM cone contrast plane, V1 is most sensitive to L-M contrast modulations and least sensitive to L+M contrast modulations. Within V1, we observe little to no change in chromatic sensitivity as a function of eccentricity. In the subsequent experiment, we measured the temporal impulse response functions associated with tracking chromatic Gabor patches and measured how the lag functions change as a function of chromatic direction and contrast, confined to the LS cone contrast plane. We measured detection thresholds for stimuli matched in their spatial, temporal, and chromatic properties. We found that for both tracking and detection, within the LS cone contrast plane, the underlying mechanisms are most sensitive to L-isolating contrast modulations and least sensitive to S-isolating contrast modulations. Across all experiments, we developed color-contrast separable models that allow us to model the relative sensitivities of the chromatic mechanisms in a way that is independent from the effect of contrast. We use these models to further quantify the sensitivities of the underlying chromatic mechanisms. This work advances our understanding of human color perception and places important constraints on building more generalized models of color processing.

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