Evoked Cortical Potentials and Information Processing.

The present report details the results of five separate experiments. Experiment I examines the visual evoked potential VEP to a target stimulus when it is perceptually masked by a second stimulus, and again when it is disinhibited. In experiment II, the effects of continguity of target initial and mask later visual stimuli on backward masking and the VEP was examined. Significant VEP amplitude decreases occurred when the target was followed closely in time by either of the masks. Experiment III tested the effects of differing numbers of corner masks on perception of, and VEP to, the target stimuli. In experiment IV we asked whether corner masking stimuli would be more effective than non-corner masks with respect to effects on perception and the VEP. Both corner and non-corner masks occupied less area than the target stimuli 57. The fifth experiment examined the effects of a randomly generated noise pattern on a target. The target was a letter T and the mask overlapped and crisscrossed the T at many points along its contour. The random visual pattern proved to be a very effective mask perceptually and also led to sharp decreases in VEP amplitude. The general finding of VEP attenuation with backward masking was interpreted in terms of excitatory-inhibitory interactions at the visual cortex which occur when later presented stimuli bound, or spatially overlap, earlier presented target stimuli.