Why do people's pupils dilate when they're surprised? Researchers explain

When individuals come across unexpected information that conflicts with their initial expectations, their pupils often enlarge. This phenomenon, as revealed by a study led by researchers at Brown University, indicates that the brain is undergoing a rapid transition into a new mode of functioning. Pupil dilation is a physiological response that signals an increase in arousal, and the study's findings suggest that these fluctuations in arousal serve a useful purpose in the brain.

The research team, comprising neuroscientists and cognitive scientists at Brown's Carney Institute for Brain Science, examined the role of spikes in physical alertness during unexpected events. Their study, published in Nature Human Behaviour, demonstrates that these spikes serve as a signal that the brain is adjusting to new information and altering how the individual perceives and learns from their surroundings.

During the experiment, participants were shown a series of colored squares on a screen and asked to predict what they would see next. After each set of squares, the researchers collected data on the participants' pupil diameter and EEG signals. They discovered that surprising colors—those that did not align with the participants' predictions—caused pupil dilation and enhanced certain brain waves. Moreover, these measurements correlated with a reduction in bias and facilitated learning.

The norepinephrine spike, released by the locus coeruleus, the brain's primary source of this neurotransmitter, is almost like a "refresh mechanism" for the brain. When confronted with unexpected information, the brain temporarily discards the previous mental context and establishes a new one, enabling faster learning and adjustment to the new situation. This mechanism allows the brain to reduce the influence of prior expectations and facilitate a more efficient processing of new information.