Project summary Vision is a highly dynamic system that continuously adapts to changes in the stimulus properties of the environment or the physiological properties of the observer. How these adaptation processes adjust to the sensitivity limits of the observer is important for understanding normal visual function and the consequences of visual deficits or disease. The proposed research examines basic mechanisms of adaptation and plasticity by examining how color vision adjusts to losses in color sensitivity as a result of inherited color deficiencies. Color- deficient observers with anomalous trichromacy have weaker sensitivity to reddish-greenish colors because of alterations in the genes coding the medium and longwave photopigments. However, there is emerging evidence that the perceptual experience of color in anomalous trichromats is stronger than their sensitivity losses predict, potentially because the neural mechanisms encoding color amplify and thus compensate for the reduced color signals available from their receptors. Color deficiencies provide an ideal model for exploring these compensatory mechanisms, by comparing observers who differ because of well-defined and highly stable properties of the initial receptors, for which they have had a lifetime to adapt. The proposed studies are designed to assess the extent and mechanisms of these compensatory adjustments and how they are manifest in and impact different aspects of color perception. The first aim will compare color perception and contrast coding in normal and color-deficient observers across a battery of tasks designed to isolate different levels and properties of color vision. The second aim will complement these behavioral assessments with direct measures of cortical neural activity in normal and color-anomalous observers using functional magnetic resonance imaging, electroencephalography, and adaptive optics to probe neural responses to color. The results of these studies will serve to better characterize the neural and perceptual consequences of color deficiencies, and more generally will characterize basic processes underlying human vision and how it adapts in response to visual losses or their corrections.