Visual evoked cortical responses and selective dioptic masking with pattern flashes of different spatial frequencies

Human cortical visually evoked responses (VERs) to pairs of stimuli presented in rapid succession were investigated in an attempt to assess the electrophysiological nature of temporal visual processing as a function of the spatial frequency of the stimuli involved. Four stimuli, all of which were of an equal mean luminance level, consisted of a diffuse flash and square checkerboard patterns of three spatial frequencies: 0.5, 1.0, and 4.0 cycles/degree (check-sizes subtending 60, 30, and 7.5 min of arc visual angle). Stimuli were presented both singly and in all 16 possible pairwise combinations with a 40 msec interflash interval interposed between the pairs. Both the psychophysical reports and the VERs to the various stimulus configurations were analyzed in order to test whether the existence of visual information channels selectively tuned to a specific range of spatial frequencies would be revealed in terms of selective masking effects among the various stimulus combinations. Analysis of the VER data was based primarily on the magnitude of variability of the VERs, resulting from variations in the pattern stimulation from the first or second flash. The variability measure indicated the degree to which stimulus pattern processing of one flash of the pair was impaired by the nature of pattern in the preceding or following flash (forward and backward masking effects respectively)