Visually evoked potentials and selective masking with patterned flashes of different spatial frequencies

Abstract

Human visually evoked responses (VERs) to pairs of checkerboard patterned light flashes (40 msec interstimulus interval) were investigated in an attempt to assess the electrophysiological nature of temporal visual processing as a function of the spatial frequency of patterned stimuli involved. Four stimuli, all of equal mean luminance, were employed: a diffuse flash and checkerboards with fundamental spatial frequencies of 0.5, 1.0 and 4.0 c/deg (check-sizes of 60′, 30′ and 7.5′). VERs from four adult humans were recorded monopolarly over the occipital area of the scalp (O = to ear reference) and averaged. The purpose of the experiment was 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. The results indicated that the amplitude of VERs approximately 110 msec after stimulation was particularly sensitive to the presence and size of the pattern. Data were quantified in terms of variations in amplitude of this component as a function of the nature of pattern in the first and second flash. This variability measure indicated that pattern processing of one flash of the pair was impaired by the presence of pattern in the preceding or following flash (forward and backward masking effects respectively). When the first flash contained pattern, the contribution of the second flash was always less than that of the first, forward masking being most pronounced. The 7.5 and 30′ check-sizes were found to be the most effective forward maskers, and elicited the largest amplitude negative potentials at the 110-msec latency after flash onset. Forward masking, furthermore, was specific to the spatial frequency of the pattern in the first (masking) flash, electrophysiological masking being greatest when identical as compared to dissimilar spatial frequencies were presented in the flash pair. This finding of selective masking lends support to the notion that there are multiple visual channels, each selectively sensitive to a limited range of spatial frequencies.