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| Gary Aston-Jones |
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Department of Psychiatry, University of Pennsylvania, USA
e-mail: gaj@mail.med.upenn.edu |
Presentation: |
| 2002-10-06, 12:10-12:50 |
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| Attention and behavioral flexibility: Function of the noradrenergic locus coeruleus system in monkey. |
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| It has been proposed that the locus coeruleus (LC) regulates nonspecific arousal and thereby may participate in a wide range of functions. Our work indicates that, while the LC may indeed play an important role in arousal, it has more specific effects on behavior and may regulate cortical mechanisms involved in selective attention and task performance. In one study, we recorded impulse activity of LC neurons in monkeys performing a visual discrimination task. Phasic and tonic firing characteristics of LC neurons varied in close relation to task performance. Phasically, LC neurons were selectively activated by target cues and not by other task events including the animal's behavioral responses. The target-elicited LC responses were limited to periods of good performance, when tonic firing rates were at an intermediate level (~ 1-2 spikes/sec). Higher levels of tonic activity were associated with few or no phasic LC responses, and poor task performance. Direct manipulations of LC activity via local microinfusions yielded behavioral results consistent with the above recordings. A computational model was constructed to explore mechanisms that underlie these patterns of LC activity and their relationship to task performance. This model revealed that electrotonic coupling among LC neurons can provide a mechanism for regulating the pattern of LC activity between two modes of functioning, which may in turn regulate task performance. In one mode (high electrotonic coupling, resulting in intermediate levels of tonic LC activity and robust phasic responses to task-defined target stimuli), LC responses facilitate the processing of target stimuli while responses to distractors are reduced. In the other mode (low coupling, resulting in high tonic LC activity and reduced phasic responding), task performance worsens and non-specific responding to distractors (e.g., false alarms) is increased. This leads us to hypothesize that the LC may play a role in modulating attentional state, by favoring the processing of task-defined stimuli ("selective attention") in one mode, versus a broader sampling of stimuli in the environment ("scanning attention") in the other. Inputs to the LC from the orbital and cingulate cortices were identified in monkey using tract-tracing. Such inputs are likely sources for the selective LC responses to target stimuli, and for inputs that regulate electrotonic coupling among LC cells. Overall, these results indicate that, during waking, the LC may participate in a system that alters attentional mode to facilitate selective vs scanning attention. This in turn suggests a more specific role for the LC in regulating cognitive function than earlier hypotheses limiting its role to simple arousal. |
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