Why do people’s pupils dilate when they’re surprised?

“Pupil dilation signals a spike in arousal, and our findings support the idea that these rapid fluctuations in arousal are doing something useful in the brain,” said study author Matt Nassar, an associate professor of neuroscience and of cognitive and psychological sciences at Brown. “They’re enabling us to deal with a world that often changes from one context to another context.” Corrie Pikul, News from Brown