Scientists have explained why pupils dilate when we are surprised

The study was published in the journal *Nature Human Behaviour*.

Sixty-three people took part in the experiment. They were shown coloured squares on a screen and asked to predict which colour would appear next. The participants then reported what they had seen and made a new prediction.

During the task, the researchers measured the diameter of the participants’ pupils and recorded their brain’s electrical activity using electroencephalography (EEG). For the final analysis, they were able to obtain 60 sets of data on pupillary responses and 57 EEG recordings.

When the colour did not match the prediction, the participants’ pupils dilated. At the same time, a specific component of brain activity—P3a—intensified; this had previously been linked to the brain’s attentional response to new and unexpected events.

Details

The experiment showed that people perceived expected colours with a slight bias: what they saw seemed to them to be closer to what they had expected to see.

Following an unexpected result, the influence of previous expectations diminished. Participants assessed the new information more accurately and adjusted their subsequent predictions to reflect the changed situation.

The researchers believe that a brief surge in physiological arousal helps the brain realise that the previous model of what is happening no longer works and needs to be updated. The dilation of the pupils is one of the external signs of this reorganisation.

The authors compare the reaction to unexpected events to a kind of update mechanism. The brain temporarily weakens the influence of the old context so that new information is not assessed solely through the lens of previous perceptions.

This may simultaneously reduce perceptual errors and help a person learn more quickly in changed circumstances. Such a mechanism becomes particularly useful in situations where the environment suddenly begins to behave differently from before.

According to the researchers, changes in pupil size indicate the moment when the brain is likely ‘loading’ the new context and adjusting its predictions.

Unexpected events are linked to activity in the locus coeruleus — a small region of the brainstem that serves as one of the main sources of noradrenaline. This neurotransmitter is involved in regulating wakefulness, attention and responses to significant changes.

However, in the new experiment, the researchers did not measure noradrenaline release or locus coeruleus activity directly. Pupil size and EEG readings were used as indirect physiological markers of the arousal system.

The authors specifically note that pupil dilation is non-specific and can occur for various reasons. Therefore, the conclusions are based not on a single indicator, but on a combination of data on the pupils, brain activity and the participants’ behaviour.

Why this is important

The results help to explain why the body needs a rapid physiological response to surprise. This response may not merely accompany an emotional state, but may reflect the brain’s transition to a more flexible mode of information processing.

However, the study was conducted in a laboratory setting: participants were working with colours on a screen. It is not yet possible to state with certainty that the same mechanism fully explains reactions to complex life events, danger or emotionally charged news.

Furthermore, the study identified a link between physiological signals and behavioural changes, but did not prove that pupil dilation itself causes the brain to reorganise.

Source

Study: Tiantian Li, Harrison Marble et al. Fluctuations in arousal reflect latent state transitions that facilitate behavioural optimisation.

Journal: *Nature Human Behaviour*, 2026.