LOCUS-COERULEUS NEURONAL-ACTIVITY IN AWAKE MONKEYS - RELATIONSHIP TO AUDITORY P300-LIKE POTENTIALS AND SPONTANEOUS EEG

These experiments were designed to test the hypothesis that novel audi tory stimuli lead to phasic and/or tonic increases in locus coeruleus (LC) cell firing, which may be a necessary condition for the occurrenc e of P300 potentials. Event-related potentials (ERPs) and LC unit acti vity were simultaneously recorded from three awake macaque monkeys exp osed to an auditory ''oddball'' paradigm. Oddball stimuli resulted in probability-sensitive potentials resembling the human P3a component. T wenty-five percent (3/12) of LC units showed small phasic enhancements of LC firing after infrequent but not frequent tones. A comparison be tween histograms elicited by the two types of stimuli revealed signifi cant effects of stimulus sequence. This pattern suggested a slight act ivation by rare tones, followed by a brief inhibition of firing in the subsequent trial. These data suggest that changes in LC activity duri ng oddball paradigms are subtle, heterogeneous, and influenced by the subject's level of arousal and vigilance.