| The majority of the brain's norepinephrine is found in the brainstem nucleus locus coeruleus (LC). The LC projects throughout the central nervous system. Therefore, the timing of neuronal activity in the LC represents an adrenergic signal that is broadcast throughout the brain. However, little is known about the relationship between the timing of LC activity and behavior, in part because of the difficulty in accessing this small, deep brain structure. To help overcome this difficulty, we developed an MRI method that allows us to localize electrode trajectories relative to brain regions of interest. This method has substantially improved our ability to find and record from LC neurons in awake, behaving monkeys. We have used these recordings to characterize the relationship between the timing of LC neuronal activity and goal-directed sensory-motor processing. Early studies implicated phasic LC activity in goal-directed sensory processing. However, more recent studies suggest that phasic LC activity may be more related to the behavioral response. To further clarify LC's role in sensory-motor processing, we recorded from single-LC neurons in monkeys performing a motor-control task known as countermanding. For a given trial, the monkey is shown a peripheral visual stimulus to use as a saccade target. On a small fraction of the trials, sometime after the target is shown but before the saccade is initiated, a "stop-signal" is given. The exact time of occurrence of this signal is the critical variable that determines whether or not it is effective and stops the monkey from making the saccade. Performance is typically modeled as a race between dynamic "go" and "stop" processes, the winner of which commits the subject to initiate or withhold the saccadic response. We found phasic LC activity to be related both to the "go" sensory event and the "go" commitment. The presence of LC activity related to the "go" sensory event does not depend on the subsequent behavior of the monkey. LC activity is markedly absent in relation to the "stop" sensory event and the "stop" commitment on stop-signal trials. Thus, LC encodes task-relevant sensory and motor events leading to overt, but not withheld, actions. |
