PURPOSE: Human subjects are more sensitive to luminance decrements than luminance increments, as is evident by a lower threshold and shorter latency for dark stimuli. This asymmetry can be explained by the results of neurophysiological recordings in dLGN and V1 of cat and monkey. Specifically, V1 population responses demonstrate that black stimuli elicit a higher level of activation than white stimuli, while the response latency of OFF cells in dLGN is shorter than that of ON cells. The removal of a black or white spot often generates the perception of a negative afterimage. Here we ask whether increment and decrement stimuli also elicit asymmetrical negative afterimages? If so, can these asymmetries be attributed to the asymmetries for ON- and OFF-channels?
METHODS: To answer these questions, we performed psychophysical experiments in human observers and single-cell recordings in cat dLGN.
RESULTS: Psychophysically, we found that bright afterimages elicited by luminance decrements (darker flash spots) are stronger and last longer than dark afterimages elicited by luminance increments of equal size; yet the perceptual latency of the latter is shorter than that of bright afterimages. Neurophysiologically, we found that ON cells in dLGN (and retina by analyzing S-potentials) responded to the removal of a dark spot with higher ?ring rates and longer latencies than OFF cells to the removal of a bright spot.
CONCLUSIONS: We suggest that subcortical differences in ON- and OFF-channels can explain asymmetries between bright and dark negative afterimages which is consistent with a subcortical origin of negative afterimages.