Evoked potential indices of an early and late stage of attentional processing were recorded from schizophrenic and normal subjects during dichotic listening tasks. Despite slow and inaccurate detections, the schizophrenic subjects were able to focus selectively to different ears but only at a fast stimulation rate, showing integrity of the early selective stage. They showed an abnormal late stage, indicating inefficiency in processing information from detected targets. Marked deficits at a slow stimulation rate and during divided attention suggest that the schizophrenic attention disorder is one of control and maintenance of a selective processing strategy rather than of general slowness or absence of selectivity.

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Two auditory brainstem evoked potential studies were carried out to investigate the role of sensory influences, independent of factors such as attention and arousal, in groups varying in extraversion. In the first experiment, the rateof stimulus presentation was either 11, 41 or 81 clicks/sec, stimulus intensity being held constant at 70dB above the threshold of a normal young adult sample. Neither the latency nor the amplitude of any ofthe brainstem components significantly differed among the introverted, ambiverted or extraverted groups for any rate of presentation. In the second experiment, repetition rate was either 41 or 81 clicks/sec and intensity established at 20. 30, 50 and 70 dB. Peak V showed a highly consistent and significant increase in latency and decrease in amplitude with decreasing intensity. Again, however, no inter-group differences were found. The results suggest that N1-P2 effects that have been found at higher levels of the brain are probably not due to parallel changes in the periphery or the brainstem. The possibility that non-sensory factors, such as attention and motivation, mi^t have accounted for these cortical differences is also discussed. DESCRIPTORS: Extraversion. Auditory brainstem, Brainstem evoked potential. Intensity effects. Rate of stimulus presentation effects.