Described is research using computer techniques to study the brain's perceptual systems in both normal and pathological groups, including hyperactive children (6-12 years old). Reviewed are the early studies of A. Petrie, M. Buchsbaum, and J. Silverman using the electroencephalograph to obtain AER (average evoked response) records of schizophrenics. The use of the AER to investigate how the brain reacts to changes in stimulus intensity is explained. Summarized are major findings concerning AER abnormalities in schizophrenics, manic-depressives, pure depressives, hyperactive children, and in normal behavior. It is proposed that stimulus intensity control serves as an adaptive protective role in the face of potentially excessive stimulation; that schizophrenics might be representing this adaptive tendency to an exaggerated degree; and that hyperactive children who improve with drug therapy are those who, prior to treatment, have the most abnormal AER patterns. (SBH)