LOCUS-COERULEUS NEURONS IN MONKEY ARE SELECTIVELY ACTIVATED BY ATTENDED CUES IN A VIGILANCE TASK

Impulse activity was recorded extracellularly from noradrenergic neuro ns in the nucleus locus coeruleus (LC; 47 single-cell and 126 multicel l recordings) of four cynomolgus monkeys performing an oddball visual discrimination task. For juice reward, the subjects were required to r elease a lever rapidly in response to an infrequent(10-20% of trials) target cue (CS+) that was randomly intermixed with nontarget (CS-) sti muli presented on a video display. All LC neurons examined were phasic ally and selectively activated by target cues in this task. Other task events elicited no consistent response from these neurons (juice rewa rd, lever release, fix-spot stimuli, nontarget stimuli). In one animal , nontarget cues phasically inhibited LC neurons. Phasic LC excitatory responses to target cues in this task occurred at a relatively short latency (mean = 90.7 msec), approximately 200 msec prior to the behavi oral response (lever release). In addition, LC response magnitudes var ied with behavioral performance, being substantially attenuated during epochs of poor performance (high false alarm rate). There was a posit ive correlation (r = 0.30, p < 0.0001) between the latency of LC respo nses and the latency of behavioral responses to same target cues, cons istent with the possibility that LC responses may have a role in selec tive attention by facilitating responses to the CS+ stimulus. Analyses of behavioral response latencies to pairs of stimuli indicated that L C responses may facilitate behavioral responses to subsequent sensory cues, consistent with a role of this system in sustained attention/vig ilance. Moreover, responses became reduced in magnitude over time duri ng prolonged task performance (> 90 min), in parallel with a behaviora l performance decrement. These results show that LC neurons are activa ted selectively by attended stimuli that demand a rapid response in th is task, and that such LC responses may contribute to conditioned beha vioral responses.