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1Laboratory of Neuromodulation and Behavior, Department of Psychiatry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6100; and 2Nencki Institute of Experimental Biology, 02093 Warsaw, Poland
Submitted 11 July 2003; accepted in final form 10 March 2004
We previously reported that noradrenergic neurons in the monkey locus coeruleus (LC) are activated selectively by target stimuli in a target detection task. Here, we varied the discrimination difficulty in this task and recorded impulse activity of LC neurons to analyze LC responses on error trials and in relation to behavioral response times (RTs). In easy and difficult discrimination conditions, LC neurons responded preferentially to target stimuli with phasic activation. These responses consistently preceded behavioral responses regardless of task difficulty. Latencies for LC and behavioral responses increased similarly for difficult compared with easy discrimination trials. LC response latencies were also shorter for fast RT trials compared with slow RT trials regardless of difficulty, indicating a close temporal relationship between LC and behavioral responses. This relationship was confirmed with response-locked histograms of LC activity, which yielded more temporally synchronized LC responses than stimulus-locked histograms. Population histograms of LC activity revealed that nontarget stimuli resulting in false alarm responses produced phasic LC activation (although smaller than for target-hit trials), and nontarget stimuli resulting in correct rejection responses yielded a small inhibition in LC activity. Population analyses also revealed that LC responses included an early, small excitatory component that was not previously detected. This early response was nondiscriminative because it was similar for target and nontarget stimulus trials. These results indicate that LC neurons exhibit early small magnitude responses that are closely linked to sensory stimuli. In addition, these cells show a later, larger magnitude response that is temporally linked to behavioral responses. These and other results lead us to hypothesize that LC responses are driven by decision processes and help facilitate subsequent behavioral responses.
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