Consequences of fixational eye movements for chromatic sensitivity

The human eyes are always in motion. Fixational eye movements (FEM) continually shift gaze even when attending to a single point, displacing stimuli on the retina over an area as large as the foveola. Previous research with achromatic stimuli has shown that the temporal luminance modulations from FEM enhance sensitivity to spatial luminance modulations in the scene, especially for high spatial frequencies. We here examine whether the same is true for chromatic modulations. It has been proposed that FEM enhance sensitivity to color, but the classical experiments that studied this by eliminating retinal image motion focused on the consequences of adaptation during unnaturally long periods of fixation. We examine how the retinal motion resulting from FEM influences chromatic sensitivity during stimulus exposures of durations that are comparable to natural fixation. In a forced-choice procedure, emmetropic observers (N=6) were asked to report the orientation (±45°) of an isoluminant red-green grating (1 or 10 cycles/deg) superimposed on a Brownian noise colored background. Subjects maintained fixation at the center of the display while their eye movements were recorded at high resolution via Dual Purkinje Imaging. We compared performance in the presence and absence of retinal image motion. The latter was achieved by moving the stimulus on the display to counteract eye movements by means of a custom-built system for gaze-contingent display. For each subject, we selected a contrast that yielded approximately 90% correct during normal fixational motion. Discrimination was greatly impaired under retinal stabilization, resulting in approximately 60% correct on average across subjects. Unlike with luminance stimuli, for which FEM consequences are clearest at high spatial frequency, the drop in performance with image stabilization was similar for chromatic stimuli at both high and low spatial frequencies. These results indicate that temporal modulations from FEM play an important role in the perception of color.