Brown University: pupil dilation in 63 people marked a brain context reset on surprise
In an experiment with colored squares, unexpected colors caused pupil dilation and stronger EEG waves, which the authors link to updating expectations.
Researchers at Brown University in Providence, Rhode Island, report that pupils tend to dilate when people meet information that contradicts their expectations, according to brown.edu. The work was done at the Carney Institute for Brain Science and published in Nature Human Behaviour. The experiment included 63 participants, which gave 57 EEG datasets and 60 pupil measurement datasets. brown.edu
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Experiment with colored squares
Participants repeatedly predicted what they were about to see. They were then shown a new set of colored squares, reported what they saw and made a new prediction. During the task the researchers recorded pupil diameter and EEG signals.
According to the publication's retelling, there were two conditions. In the change-point condition, colors of one family suddenly switched to another family. In the oddball condition, a color from a different family appeared briefly and then the original family returned. Participants did not notice changes in their pupils or brain activity.
Colors that did not match predictions elicited pupil dilation and amplified certain brain waves. brown.edu
brown.edubrown.eduOpen source ↗ These measures were related to reduced bias and to adjustment of learning. When images matched what participants expected, they were biased toward their expectations. Yet participants also learned from unexpected results and adjusted their next predictions accordingly.
Norepinephrine and the context refresh
Surprising events activate the locus coeruleus, the main source of norepinephrine, the messenger that drives the flight-or-flight response. This activity correlates with pupil diameter and with specific EEG waves. Study author Matt Nassar noted that, despite many studies showing norepinephrine spikes after surprising events, the function of these signals and their effect on behavior had been unclear.
The authors describe the norepinephrine spike as almost a refresh mechanism: a sign that the brain is adjusting to new information that changes expectations. In Nassar's words, the brain holds a mental context and replaces it when it recognizes a new situation. Pupil and EEG changes are external markers that the new context is being loaded. This limits the influence of the previous context on perception and gives a clean slate, which allows faster learning.
Per the retelling, a refresh does not always improve learning. In situations where change is likely, more is learned from new information. In stable situations the brain becomes more cautious and does not overreact to chance events.
