Visual cortical evoked potentials under conditions of sequential blanking

Two separate experiments were conducted to study the visual evoked potential (VEP) correlates of sequential blanking, a phenomenon in which up to one-half of a discrete train of stimulus inputs are not perceived for certain orders of input. A PDP-7 digital computer was used to present stimuli (from 2 to 5 letter Xs) on a CRT display in both experiments. In Experiment 1 there were four basic conditions designed and counterbalanced to indicate the nature of the VEP when stimuli were blanked and when Ss reported all stimuli. All of the stimuli were of equal intensity. The main finding was that although the eight Ss did not perceive and report blanked stimuli, they did respond to them physiologically as indicated by the VEP. The implications of these findings were discussed in relation to recent studies of visual masking and metacontrast in which VEP was recorded. In Experiment 2 the normally blanked and the normally blanking stimuli were alternately increased in intensity to determine the effect on sequential blanking and the VEP. Six Ss were tested under three basic counterbalanced conditions. It was found that sequential blanking could be reliably overcome by increasing the intensity of the normally blanked stimuli. In addition, when the normally blanking stimuli were of greater intensity than the blanked stimuli, not only did perceptual suppression occur, but the evidence indicated that there was no VEP to the first of the two blanked stimuli.