Glossary
Locus coeruleus
What is the locus coeruleus?
The locus coeruleus is a small brainstem nucleus that is the primary source of norepinephrine in the brain. It modulates arousal, attention, stress responses, and the balance between focused and exploratory behavior.
How it works
Tonic activity is ongoing background firing; phasic activity consists of brief bursts associated with particular events. In adaptive gain theory, the phasic mode combines a moderate background rate with bursts linked to task-relevant processing. It is associated with effective engagement in the current task.
The model contrasts this with a tonic mode: elevated background firing, weaker or absent task-linked bursts, and reduced engagement in the current task. When that task’s value declines over time, this pattern is proposed to support exploration of alternatives. These modes describe ends of a continuum, not two wholly separate states. The theory draws substantially on monkey recordings and computational models.
Applied example
Consider an animal choosing whether to continue a rewarding task or explore alternatives. Adaptive gain theory asks how different activity patterns in this system contribute to that shift. The choice also involves wider brain circuits; the locus coeruleus is not a switch that controls it alone.
Why it matters
The locus coeruleus contributes to regulation of behavioral state. Adaptive gain is a model of how it may interact with wider circuits to influence task engagement and exploration.
Evidence note: Aston-Jones and Cohen (2005) propose this account from neurophysiology and modeling.