Normal subjects showed a right hemisphere processing superiority in reaction times to visual stimuli under conditions of sustained visual attention. This asymmetry was predicted based on findings in studies of blood flow, clinical populations, and physiology on lateralization of the noradrenergic system. The asymmetry was eliminated when an auditory event was delivered simultaneously with the visual target suggesting a common pathway between voluntary sustained alerting and more automatic alerting effects of external stimuli. The act of sustaining alertness also interacted with covert orienting of attention as would be expected if alerting influenced the efficiency of covert shifts of attention.

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