| Summary: | Ten experiments are reported which are concerned with visual evoked response (VER) correlates of the processing of visual stimuli. In Experiments 1 and 2 VERs were elicited by letters requiring verbal and non-verbal processing respectively. The results suggest that hemispheric differences in VERs to lateralised stimuli reflecting the anatomical arrangements of the visual system are modified by the way the stimuli are processed. These conclusions were supported by a third experiment in which non-verbal processing of nonverbal stimuli was required. A further three experiments investigated VERs elicited by midline presentation of the letter and pattern stimuli. It was concluded that the right hemisphere is pre-eminent for the initial processing of visually presented stimuli and that when subjects had foreknowledge of the stimuli the P2-N2 component reflected whether or not the stimuli were subjected to verbal processing, and whether such processing was asymmetrically distributed across the hemispheres. Experiment 7 attempted to find a VER index of the limits of trans-callosal connections between the striate cortices, with inconclusive results. Two further experiments, utilising the stimuli employed in Experiments 1-6, were designed to investigate whether the P300 component of the VER reflected hemispheric asymmetries of processing. No such effects were found in Experiment 8 which provided new evidence pertaining to the relationship between P300 behavioural measures of information-processing. The results of Experiment 9 suggested that P300 could reflect asymmetrical processing, a conclusion supported by the results of the final experiment in which P300S were elicited by simple lateralised stimuli. The general conclusion drawn from these experiments is that the VERs reflected both structural and dynamic aspects of information-processing and indicated that important determinants of the flow of stimulus information in the brain are the nature of the stimulus, the task-set of the subject and the interaction of these two factors.
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