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Noradrenergic locus coeruleus (LC) neurons were recorded in monkeys performing a visual discrimination task, and a computational model was developed addressing the role of the LC brain system in cognitive performance. Changes in spontaneous and stimulus-induced patterns of LC activity correlated closely with fluctuations in behavioral performance. The model explains these fluctuations in terms of changes in electrotonic coupling among LC neurons and predicts improved performance during epochs of high coupling and synchronized LC firing. Cross correlations of simultaneously recorded LC neurons confirmed this prediction, indicating that electrotonic coupling in LC may play an important role in attentional modulation and the regulation of goal-directed versus exploratory behaviors.
Neuromodulators, such as norepinephrine (NE) and dopamine, have long been thought to play a role in regulating nonspecific aspects of behavior, such as motivation and arousal. However, recent evidence indicates that these systems may play a more specific role in task-related cognitive processes. Brainstem dopaminergic neurons respond selectively to stimuli that predict reward (1). Stimulus-specific activity has also been observed in LC neurons. Recent studies found that LC neurons in monkeys performing a visual discrimination task exhibited short-latency stimulusevoked (phasic) responses to target (CS+) stimuli but not to distractor (CS-) stimuli or other task events (2). The latencies of these LC responses were substantially shorter than, and temporally correlated with, the latencies of corresponding behavioral responses, indicating that LC activity may affect task responding. However, the mechanisms that govern LC activity or its effect on behavior have remained unclear.
LC neurons were recorded in four Cynomoigus monkeys performing a visual discrimination task (2). This task required the monkey to respond to infrequent visual target stimuli but not to frequent distractors (3). In many of our recordings, LC neurons changed levels of tonic discharge several times (Fig. IA), in association with alterations in task performance. We divided behavioral performance into epochs of "good" and "poor" performance, on the basis of the frequency of false alarm (FA) errors produced [as described previously (2, 4)]. Signal detection sensitivity (d') was substantially higher in epochs of good compared with those of poor performance, so the difference between these cannot be explained by a simple change in response criterion (4). Furthermore, although mean lever response times (RTs) were not systematically different between...