Brown University study links pupil dilation to surprise

Researchers have identified a link between pupil dilation and the brain's response to unexpected events. A new study led by Matt Nassar, an associate professor at Brown University, suggests that when a person is surprised, the pupils dilate as part of a transient arousal reaction.

Nassar explains that following a surprise, the brain appears to switch into a ‘refresh’ mode. The dilation serves as a visible indicator of a sudden increase in arousal, which helps the brain cope with rapid shifts in environmental context. This physiological response is partially regulated by the locus coeruleus, a cluster of neurons in the brainstem.

Beyond light sensitivity, pupil size can also reflect cognitive effort, physical activity, emotional arousal, or the effects of certain medications. Additionally, pupil fluctuations have been observed to coordinate with breathing patterns, typically expanding during exhalation and contracting during inhalation.