Abstract
This chapter summarizes several studies that investigate the hypothesis that locus coeruleus (LC) activity can influence the acquisition of a new task contingency. Three adult Cynomolgus monkeys have been overtrained for several months on one stimulus contingency in the task. Subsequently, reversal has been instituted unexpectedly by abruptly reversing the meaning of the target and nontarget stimuli. In this task, LC neurons have been selectively and phasically activated by CS+ stimuli. The studies indicate that phasic responses to CS+ stimuli are specifically related to their meaning and not driven by their sensory characteristics. After reversal of the stimulus-response contingency, all LC neurons tested reversed their stimulus-response profile and became selectively responsive to the new target (old nontarget) stimuli and have been unresponsive to the old target stimuli (new nontarget). To analyze the temporal relationship between LC and behavioral responses during acquisition of a new stimulus contingency, the rate of reversal of LC phasic responses has been compared with the rate of behavioral reversal for the same task epochs. In particular, by this point after reversal, LC neurons exhibited very little response to old target (new nontarget) stimuli. The early changes that occur in LC tonic and stimulus-evoked activity after task reversal indicate that the LC may play an active role in acquisition of stimulus significance. The widespread efferent anatomy of the norepinephrine (NE)-LC system indicates that it may be involved in a very general aspect of learning rather than for learning one type of task only. Specifically, LC could play a role in acquisition of a new contingency by altering the relative salience of target and nontarget stimuli.