The perceived speed of drifting chromatic gratings is mechanism-dependent

The perceived speed of chromatic motion was investigated for gratings that stimulate each chromatic mechanism [L-M and S-(L+M)] in isolation and for gratings that stimulate both chromatic systems. The observers' task consisted of adjusting the speed of a drifting achromatic grating to match the perceived speed of an isoluminant chromatic grating, drifting at 8 deg/s (temporal frequency of 4 Hz). Every observer reported a substantial decrease in perceived speed for chromatic gratings modulated along the S-(L+M) (blue-yellow) cardinal axis compared to other directions in color space. One observer even reported motion standstill for gratings modulated along this axis. Further testing demonstrates that the perceived speed of an isoluminant chromatic grating depends solely on the extent to which it stimulates the L-M (red-green) mechanism. Thus, under the conditions that were tested, the S-(L+M) postreceptoral mechanism does not appear to contribute significantly to determining the perceived speed of chromatic motion.