Properties of Arousal Systems Explain Some Aspects of Behavioral Performance

It is important to understand not only how the NMTs are associated with different kinds of behavior but also how their activity causes variations in a specific behavior. The classic description that relates arousal to cognitive performance is Yerkes-Dodson law, which states that the optimal performance on cognitive and motor tasks depends on a moderate level of arousal or vigilance. Low and high levels produce suboptimal performance. When graphed, Yerkes-Dodson law resembles an inverted U. The decline in performance at high levels of arousal suggests the recruitment of inhibitory processes that reverse the beneficial effects of optimal arousal.

The most likely neural structure that mediates vigilant behavior is the LC. Optimal vigilance means that sensory information is processed efficiently and irrelevant stimuli are ignored. Two properties of the LC/NA system seem well suited to the task. First, application ofNA to cortical cells causes an increase in the signal-to-noise ratio, as discussed previously. Second, by increasing NA at sensory sites, the LC filters out signals that differ only slightly from the background activity. Therefore, better filtering of meaningful stimuli is a natural consequence of optimal arousal levels.

LC neurons also show "baseline" activity that varies monotonically with arousal level in the absence of vigilant behavior. One consequence of this tonic activity is that increased release of NA onto thalamic neurons increases their activity in a monotonic manner. These increases are passed on to cortical neurons and presumably account for better performance. What causes the decline in performance with high arousal? There are several possibilities, including higher thresholds for b receptor-mediated inhibition, recruitment of inhibitory GABA interneurons, and stimulation of presynaptic (a2) autoreceptors. A definitive answer to this question remains to be provided.

Another quantitative aspect of performance to be explained is that in repetitive tasks, performance worsens over time. In one such task, printed letters are rapidly presented to human observers who report when a letter is repeated. Performance of this task varies with arousal level. Early in the task period, subjects are vigilant and performance is high; later, it is lower, presumably a result of the decline in arousal. Drugs that affect NA increase arousal and improve performance, decreasing the number of missed pairs. This suggests that the signal was more detectable, which is consistent with the cellular effects of NA.

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