Responses of primate locus coeruleus neurons to simple and complex sensory stimuli

Single and multiple unit recordings were obtained from locus coeruleus (LC) of unanesthetized, chair-restrained monkeys during presentation of a range of sensory stimuli. Tonic activity was higher during alertness or agitation than during behavioral inattentiveness and drowsiness. Low-level, simple auditory stimuli elicited no response, while more intense stimuli evoked phasic discharges in LC activity. The most pronounced responses were elicited by aversive air puffs and by multi-modal naturalistic stimuli such as interactions with the experimenter. The results suggest that sensory stimuli effective in eliciting LC discharge have specific stimulus attributes. It is proposed that the LC is tuned to specifically respond to stimuli which are conspicuous to that species: stimuli which by their physical or behavioral properties evoke a change in the focus of attention. The LC response would thereby contribute to adaptive behavioral responses to such unexpected imperative stimuli. This hypothesis is consistent with earlier suggestions that the LC contributes to behavioral functions such as vigilance and alarm and provides a rigorous framework for future experiments.