Anomalous trichromats have reduced sensitivity to the L-M dimension of color space due to the reduced separation between the spectral sensitivities of their L and M cones. Despite this, previous work suggests that these observers may perceive the world to be much more colorful than their cone sensitivities would predict, potentially because of long-term adaptation that amplifies the weakened chromatic signals provided by the cones. Most of the evidence for this gain adjustment rests on subjective measures of color appearance or color salience. In the present study, we tested for neural correlates of color compensation by using fMRI to compare the cortical responses to chromatic stimuli in normal and anomalous observers. Thresholds were collected for a total of 7 anomalous trichromats (3 deuteranomals and 4 protanomals), and 6 color normal controls. Initial results showed that chromatic thresholds for the L-M axis did not predict BOLD responses, indicating neural compensation in early visual areas. In an additional experiment, we used an attentionally demanding task to ensure that top-down influences were limited. We also collected retinotopic mapping in order to independently define early visual areas (V1, V2, V3, and hV4). In this case, the group-averaged BOLD responses to L-M stimuli were not significantly greater than responses predicted by threshold, but individual participants did show evidence of compensation. The same was true when responses were normalized to responses to S-axis stimuli. Our results thus provide evidence for compensatory amplification, but suggest that the degree of compensation varies across individuals
