It has been shown that performance in vigilance tasks deteriorates greatly after loss of sleep. It is hypothesized that deterioration in these and other tasks is largely due to a breakdown in the ability to attend selectively t o relevant information in the task. An experiment is described in which subjects' performance in a task involving ignoring irrelevant information was shown to deteriorate more after sleep loss than performance on the same task without irrelevant information present. It is suggested that sleep plays an important part in maintaining selective attention and the symptoms shown by sleep-deprived subjects are largely due t o a failure in selective attention.One of the most clearly observable effects of sleep deprivation is the rapid deterioration in performance on tasks of vigilance. A number of workers (Wilkinson, 1958(Wilkinson, , 1960Corcoran, 1962Corcoran, , 1964 Willlams et al., 1959) have demonstrated subjects' breakdown in performance in such tasks. The main feature of the tasks used in these studies is that they necessitate the subject attending over relatively long periods of time to detect small infrequent changes in a generally unchanging stimulus display. Clearly, then, anything which reduces the subject's ability to attend to one source of stimuli rather than to others will result in a deterioration in performance in vigilance tasks. The fact that deterioration due to sleep loss is most marked in those tasks requiring selective attention to one source of stimuli would seem to suggest that one function of sleep is the maintenance of this selective attention and that a result of sleep deprivation is a breakdown in this type of attention.From the above we can make the following prediction: that there will be greater deterioration in performance after sleep loss in the situation where there are irrelevant signals than in the situation where there are no irrelevant signals, and that the degree of this deterioration will increase with the number of irrelevant signals present. The type of task which would most effectively test this prediction would be one in which the subject had to respond to one set of signals of constant complexity, and at the same time ignore other irrelevant signals, the number of which would be varied. The task chosen was similar to that used by Rabbitt (1964) in his study on ignoring irrelevant information.

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The effects of cigarette smoking on the balance of activity of the cerebral hemispheres were studied in two experiments. Experiment 1 examined the effects of smoking on lateralisation of EEG alpha and beta power in six male smokers, and revealed a dose-related biphasic action. Low doses of nicotine as measured by residual butt analysis increased left hemisphere activity in a dose related manner, while higher doses (greater than 1.1 mg) reversed this effect. Experiment 2 examined the time course of the changes in EEG alpha, beta, theta and delta lateralisation in 11 male smokers. During the initial period of smoking there was a reduction in EEG alpha, reflecting cortical activation, followed by an increased in alpha power towards the end of the cigarette. These changes were accompanied in smokers taking moderate nicotine doses, by an initial shift towards left hemisphere activation, followed by a reduction in left hemisphere activation relative to the right. Initial changes in delta and theta power were negatively correlated with nicotine dose. Higher nicotine doses were associated with greater shifts towards right hemisphere activation, as indicated by beta and delta lateralisation, and also with greater decreases in subjective arousal. These results were interpreted as supporting the hypothesis that the stimulant action of lower doses of nicotine involves the activation of a left hemisphere "Go" system, while the sedative effect of higher doses involves increased activation of the right hemisphere "NoGo" system.

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