Study Explains Why Our Pupils Dilate When We Encounter Unexpected Information

Pupil dilation in response to unexpected information may signal that the brain is switching into a new operating mode to adapt to changing circumstances, according to researchers at Brown University. The findings were published in Nature Human Behaviour.

The scientists found that when people encounter information that contradicts their expectations, their pupils dilate and their brain activity changes. These responses help the brain discard outdated assumptions more quickly and learn from new information.

The study involved 63 participants, who viewed sequences of colored squares on a screen, predicted what they expected to see next, and were then presented with unexpected outcomes. Throughout the experiment, researchers monitored changes in pupil size and recorded brain activity using electroencephalography (EEG).

The results showed that surprising events activated a brain region known as the locus coeruleus, which is responsible for releasing norepinephrine (also called noradrenaline), a neurotransmitter involved in the body's stress response and heightened alertness.

According to the researchers, this surge of norepinephrine acts as a kind of "reset" mechanism, allowing the brain to replace its previous internal context with a new one. By reducing the influence of outdated expectations, the brain becomes better able to adapt to changing situations and incorporate new information.

The researchers note that people naturally interpret the world through the lens of their existing expectations. However, the ability to rapidly revise those expectations when confronted with surprising events is a fundamental part of learning and adaptation.

Changes in pupil size, together with specific patterns of brain activity, may therefore serve as observable indicators that the brain is updating its internal model of the world in response to new information.

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