CONDITIONED-RESPONSES OF MONKEY LOCUS-COERULEUS NEURONS ANTICIPATE ACQUISITION OF DISCRIMINATIVE BEHAVIOR IN A VIGILANCE TASK

Impulse activity was recorded extracellularly from noradrenergic neuro ns in the nucleus locus coeruleus of three cynomolgus monkeys performi ng a visual discrimination (vigilance) task. For juice reward, the sub jects were required to release a lever rapidly in response to an impro bable target stimulus (20% of trials) that was randomly intermixed wit h non-target stimuli presented on a video display. All locus coeruleus neurons examined were phasically and selectively activated by target stimuli in this task. Other task events elicited no consistent respons e from these neurons (juice reward, lever release, fix spot stimuli, n on-target stimuli). With reversal of the task contingency, locus coeru leus neurons ceased responding to the former target stimuli, and began responding instead to the new target (old non-target) stimuli. In add ition, the latency of locus coeruleus response to target stimuli incre ased after reversal (by about 140 ms) in parallel with a similar incre ase in the latency of the behavioral response. These results indicate that the conditioned locus coeruleus responses reflect stimulus meanin g and cognitive processing, and are not driven by physical sensory att ributes. Notably, the reversal in locus coeruleus response to stimuli after task reversal occurred rapidly, hundreds of trials before revers al was expressed in behavioral responses. These findings indicate that conditioned responses of locus coeruleus neurons are plastic and easi ly altered by changes in stimulus meaning, and that the locus coeruleu s may play an active role in learning the significance of behaviorally important stimuli. (C) 1997 IBRO. Published by Elsevier Science Ltd.