Scientists Reveal How Surprise Triggers Pupil Dilation and Brain 'Refresh'

Researchers at Brown University studied 63 participants to understand why pupils dilate when surprised, finding evidence that dilated pupils signal a spike in arousal that helps the brain shift into a 'refresh' mode to respond to unexpected events. The response is mediated by the locus coeruleus, a brainstem structure that releases norepinephrine as part of the fight-or-flight response, with the brain using this arousal shift to adapt when context changes.