Temporal dynamics of human color processing measured using a continuous tracking task.

We characterized the temporal dynamics of color processing using a continuous tracking paradigm by estimating subjects' temporal lag in tracking chromatic Gabor targets. To estimate the lag, we computed the cross-correlation between the velocities of the Gabor target's random walk and the velocities of the subject's tracking. Lag was taken as the time of the peak of the resulting cross-correlogram. We measured how the lag changes as a function of chromatic direction and contrast for stimuli in the LS cone contrast plane. In the same set of subjects, we also measured detection thresholds for stimuli with matched spatial, temporal, and chromatic properties. We created a model of tracking and detection performance to test whether a common representation of chromatic contrast accounts for both measures. The model summarizes the effect of chromatic contrast over different chromatic directions through elliptical isoperformance contours, the shapes of which are contrast independent. The fitted elliptical isoperformance contours have essentially the same orientation in the detection and tracking tasks. For the tracking task, however, there is a striking reduction in relative sensitivity to signals originating in the S cones.