Abstract
The adaptive-gain theory states that high Locus Coeruleus (LC) activity is associated with exploration (behavior characterized by distractibility and frequent task switching), whereas intermediate LC activity is associated with exploitation (focus on a single task). Because studies have reported a correlation between pupil size and LC activity, pupil size is often used as a convenient proxy for LC activity when testing the theory’s predictions. Here we take a data-driven approach to explore the relationships between LC activity, pupil size, emotional intensity (of movie scenes), and frequency of eye movements in an unconstrained setting. We analyzed data of participants watching the movie Forrest Gump during combined fMRI recording and eye tracking. We analyzed correlations in different frequency bands, in order to isolate whether correlations exist primarily in rapid (seconds) or slow (minutes) fluctuations. We found a positive correlation between LC activity and pupil size, confirming previous reports. But this correlation was limited to high frequencies; notably, the very slow decrease in pupil size over time, which is often seen in experiments and presumably reflects increased drowsiness, was not accompanied by a decrease in LC activity. We further found that the emotional intensity of the movie scenes affects pupil size, but not LC activity. Finally, we found that pupil size, but not LC activity, correlates with frequency of eye movements; specifically, large pupils were associated with fewer eye movements, suggesting exploitation-like, rather than exploration-like, behavior. Taken together, our results suggest that, in an unconstrained setting, there is a correlation between pupil size and LC activity, but that neither pupil size nor LC activity predicts eye-movement behavior in a way that is consistent with the adaptive-gain theory.
Acknowledgement: Netherlands Organization for Scientific Research