A study found that the dilation of pupils when people face something unexpected is linked to the brain adjusting its state to adapt to new information.

Gigazine reported on Nov. 8 that a Brown University research team said the reaction could be a brain switching signal needed for learning and behavioural adjustment, rather than surprise itself.

The experiment compared two situations. One was a "change-point condition" in which colours in the same category continued and then suddenly shifted to a different category. The other was an "oddball condition" in which, while the same category continued, a different colour appeared once and then the sequence returned to the original flow.

Participants showed pupil dilation and stronger activity in specific brainwaves when they saw an unexpected colour. Noradrenaline also surged at the time. The team said the brain rapidly adjusts its internal state to take in new information.

The researchers said they saw the reaction as related to activation of the locus coeruleus. They said when an unexpected event occurs, the locus coeruleus becomes active and releases the neurotransmitter noradrenaline. Noradrenaline is involved in the "fight-or-flight response" and raises arousal by promoting activity across multiple neurons.

The researchers suggested pupil dilation is a side effect of this brain reset. Participants did not notice changes in their pupils or brain activity themselves.

The study found that this "refresh" environment did not always increase learning. In environments where change happened often, it was advantageous to learn more from new information, but in stable environments the brain responded more cautiously and was not excessively drawn to incidental events. That means responses to surprise do not always work the same way, but are adjusted depending on the stability and volatility of the surroundings.

The study is meaningful in that it broadened explanations of why noradrenaline is released. It also showed that changes in pupil size could potentially be used as an observable signal of the brain processing prediction errors and adjusting its next behavioural strategy.