Motion perception at scotopic light levels.

Although the spatial and temporal properties of rod-mediated vision have been extensively characterized, little is known about scotopic motion perception. To provide such information, we determined thresholds for the detection and identification of the direction of motion of sinusoidal grating patches moving at speeds from 1 to 32 deg/s, under scotopic light levels, in four different types of observers: three normals, a rod monochromat (who lacks all cone vision), an S-cone monochromat (who lacks M- and L-cone vision), and four deuteranopes (who lack M-cone vision). The deuteranopes, whose motion perception does not differ from that of normals, allowed us to measure rod and L-cone thresholds under silent substitution conditions and to compare directly the perceived velocity for moving stimuli detected by either rod or cone vision at the same light level. We find, for rod as for cone vision, that the direction of motion can be reliably identified very near to detection threshold. In contrast, the perceived velocity of rod-mediated stimuli is reduced by approximately 20% relative to cone-mediated stimuli at temporal frequencies below 4 Hz and at all intensity levels investigated (0.92 to -1.12 log cd m(-2)). Most likely, the difference in velocity perception is distal in origin because rod and cone signals converge in the retina and further processing of their combined signals in the visual cortex is presumably identical. To account for the difference, we propose a model of velocity, in which the greater temporal averaging of rod signals in the retina leads to an attenuation of the motion signal in the detectors tuned to high velocities.