Amplitude changes in the visual evoked cortical potential with backward masking.

A series of three experiments examined backward visual masking effects and visual evoked cortical potential correlates of such masking. In Experiment I, which was concerned with the effects of sequential sets of like stimuli (grids), it was found that when backward masking occurred, it was accompanied by decreased visual evoked potential (VEP) amplitudes. Experiment II replicated the first experiment using different stimuli (letter B's) and several new subjects. Again, backward masking was accompanied by decreased VEP amplitudes. In Experiment III, which examined the effects of sequential sets of unlike stimuli (B's and grids), it was found that when earlier stimuli differed in configuration from later stimuli, there was an absence of backward masking and VEP amplitude changes also failed to appear. Thus, when backward masking did not occur, no changes in the VEP were observed. The results are discussed in terms of interactions between excitatory and inhibitory activities produced at the visual cortex by the earlier and subsequently presented stimuli. Various concepts regarding the visual system mechanisms involved in backward masking, including integration and interruption hypotheses, may help to explain the complexity of findings obtained thus far in studies using VEP correlates of backward visual masking.