Neural mechanisms of color appearance in the human visual system

The appearance of a uniform surface depends on the color of the surrounding light. Such shifts in appearance are thought to reflect an important step in visual processing that contributes to the stable appearance of objects and surfaces across different viewing conditions. Though the mechanisms underlying effects of surround color have been well characterized psychophysically, less is understood about how these mechanisms are implemented in the nervous system. This dissertation seeks to increase our understanding of how activity in early visual cortical areas relates to color appearance.;Three experiments measured fMRI activity for test surfaces that are physically identical but appear different when placed on different backgrounds. In Experiments 1 and 2, subjects performed a demanding behavioral task that directed their attention to the appearance of the stimuli. We found the strongest fMRI activity for the edge between the test surface and background, and this edge signal extended across several millimeters of cortex. The effect of different backgrounds on activity for the test surface was not apparent in Experiment 1, and we found that degrading the edge in Experiment 2 did not significantly change our results. We propose that differences in task difficulty across the different background conditions contributed to the pattern of results observed in Experiments 1 and 2.;In Experiment 3, subjects viewed the different background conditions while performing a demanding letter recognition task at fixation. We observed an extended edge response similar to the one observed in Experiments 1 and 2. In addition, we found that placing the test surface on different backgrounds influenced activity in area V1. We propose that this effect is due to neurons that encode color appearance, neurons that encode scene segmentation mechanisms that depend on appearance, or some combination of the two.