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

When people encounter new information that challenges their expectations, their pupils are likely to dilate. This physiological response is a sign that something is happening in the brain to help the person adapt to the new situation, according to a study led by Brown University researchers.

“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.”

Nassar is part of a research team at Brown’s Carney Institute for Brain Science that studied the function and purpose of spikes in physical alertness. Their findings, published in Nature Human Behaviour, show that the spikes are a signal of the brain’s transition into a new mode, instantly changing how the person perceives and learns from what is happening around them.

Tracking the brain’s reset signal

Surprising events elicit activity in the part of the brain known as the locus coeruleus, which is the primary source of norepinephrine, the chemical messenger that drives the body’s fight-or-flight stress response. This activity is correlated with a change in pupil diameter as well as with specific brain waves measured by electroencephalography (EEG). Despite many studies showing spikes in norepinephrine and other markers of arousal in response to surprising events, the function of these physical signals and how they might shape behavior has been unclear, Nassar said.

The team designed an experiment involving colored squares on a screen, during which participants repeatedly made predictions about what they were about to see. Participants were then shown a new set of squares, reported what they saw and made new predictions about what they would see next. During this time, the researchers collected physiological data, including changes in pupil diameter and EEG signals.

“We wanted to capture the phenomenon associated with two overarching ideas about how the arousal system affects behavior,” said study co-author Harrison Marble, who earned a bachelor’s degree in neuroscience from Brown in 2023 and is now a research assistant and manager in Nassar’s lab. “One of them is related to learning, and the other is related to perceptual bias.”

The experiment included 63 participants, resulting in 57 EEG datasets and 60 pupil measurement datasets.

When surprise clears expectations

The researchers found that surprising colors—those that didn’t match predictions—elicited pupil dilation and amplified certain brain waves. They also found that these measurements were related to reductions in bias and adjustments in learning: When images looked like what the participants thought they were going to see, participants were biased toward their expectations. Yet pupil and brain measurements showed that participants were also able to learn from unexpected results and adjust their subsequent predictions accordingly.

In surprising situations, the researchers found, the norepinephrine spike is almost like a refresh mechanism—it’s a sign the brain is adjusting to new information that changes expectations.

“The brain holds on to some mental context, and then when it recognizes that you’re in a new situation, you replace that context,” Nassar said. “Changes in pupil diameter, as well as specific EEG readings, are external markers showing that the brain is loading that new context. This both limits the effect that the previous context has on perception and also provides a clean slate, unencumbered by previous expectations, thereby allowing one to learn faster.”

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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