We have tested the hypothesis that in humans the signals carried by ON- and OFF-pathway respectively are processed for the perception of motion by two distinct physiological substrates. In vertebrates, onset of a bright visual stimulus is signaled to the CNS by ON-center retinal ganglion cells; onset of a dark stimulus is transmitted by OFF-center cells. We chose apparent motion generated by successive presentation of two adjacent lines as a stimulus. Lines presented on a bright background were either darker or brighter than this background. Delayed onset of a pair of bright or dark lines elicits apparent motion at the same time fulfilling the constraint of stimulating either ON- or OFF- center ganglion cells, respectively. We determined the threshold delay needed for subjects to perceive the temporal order of the onset of the two lines for various angular separations. The threshold delay for a pair of bright lines stayed low for separations from 2′ to 7′. The threshold delay for a pair of dark lines was low only within a narrow range of separations centered around 3′. The variation of thresholds with line distance must reflect the existence of a limited processing zone for the perception of motion. The diameter of the processing zone for bright lines is about twice as large as that for dark lines. This suggests that in humans the separation of ON- and OFF-pathways extends to the early stages of motion perception. To test this hypothesis independently, thresholds were determined when a bright and a dark line were presented in succession. This was done for a separation of 3′ where thresholds for a pair of lines with equal contrast are similarly low. Temporal order was perceived correctly only when the delay was at least two to four times as high as the threshold delays found for the equal contrast stimuli.
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