We now discuss the roles of the NMT systems in the global regulation of brain activity and behavior. To understand these roles, it is necessary to synthesize a large body of data. This is best done by the construction of frameworks or theories that link consistent observations and generate testable predictions. One theory suggests that during arousal, each NMT system provides control over synchronization frequency for different areas of the brain. According to this view, the entire brain shows similar resting frequencies; high-frequency activity appears locally in those parts of the brain that are controlling behavior. Support for this theory is lacking. Studies that examine correlations between neurons during sensory stimulation have found only brief periods of synchrony. The high-density arrays of EEG electrodes that are currently available or the newer magnetoencephalographic technology can be used to test this hypothesis.

Another hypothesis that has received substantial empirical support is that each NMT system mediates different response components of behavior. This hypothesis arose from research conducted with an attentional task known as the five-choice, serial RT task. This task allows independent assessment of error rate, frequency of premature responses, number of response omissions, and response latency. Using this task, the effects of damage to NMT systems are tested in a highly systematic manner. It was found that cholinergic lesions impair the ability to discriminate visual targets (increased error rate). In contrast, NA lesions impair accuracy of responding, especially in the presence of distracters. Lesions of DA systems impair overall speed and probability of responding. Finally, 5HT lesions produce a reduction in premature responding. It is likely that some of the deficits are due to changes in alerting as well as alterations in attentive behavior.

The primary deficits in one response measure were accompanied by minor changes in the other measures. Perhaps this is due to incomplete independence of the measures. For example, impulsive responding to incorrect stimuli should increase error rates and decrease response latency. In any case, the results are consistent with the central tenet of this review that patterning of activity within NMTs is responsible for distinct aspects of aroused behavior.

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