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Abstract

Although the spatial and temporal properties of rod-mediated vision have been extensively characterized, very 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 directly compare 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 about 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.