Isoluminant stimuli are perceived to be moving slower than comparable luminance stimuli.Here we investigate whether smooth pursuit eye movements show an analogous slowing.

Eye movements were measured with a Purkinje image eye tracker while viewing 1 cpd sine wave gratings or small Gaussian spots of light that moved at four different speeds between 1 and 8 deg/s. Pursuit was determined in response to targets defined by luminance contrast (10%) and compared to pursuit for targets at or near the point of isoluminance. We obtained similar pursuit eye movements for both types of targets. Steady state eye speed was reduced by about 10% for isoluminant targets only at high temporal frequencies. Eye movement acceleration was not affected. The biggest difference was obtained with respect to latency which was 30 to 80 ms longer at isoluminance. Compared to the dramatic slowing of 30% or more that we observed psychophysically with these stimuli, there was hardly any impairment for steady state pursuit eye movements. Furthermore, the largest effects for pursuit were obtained at high temporal frequencies, where no perceptual slowing of isoluminant stimuli was found. At low temporal frequencies, eye movement steady state speed was not affected, even though the same stimuli were sometimes perceived at 30% of their veridical speed when subjects were fixating. When psychophysical judgments are made during pursuit, speed is perceived veridical for all stimuli. We conclude that the mechanisms that cause the perceptual slowing of isoluminant gratings are not involved in the generation of pursuit eye movements.