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Abstract

The ability to track the statistics of our surroundings is a key computational challenge. A prominent theory (Dayan & Yu, 2006) proposes that the brain monitors for unexpected uncertainty--events which deviate substantially from model predictions, indicating model failure. Norepinephrine (NE) is thought to play a key role in this process by serving as an interrupt signal, initiating model-resetting. However, evidence is from paradigms where participants actively monitored stimulus statistics. To determine whether NE routinely reports the statistical structure of our surroundings, even when not behaviourally relevant, we used rapid tone-pip sequences that contained perceptually salient pattern-changes associated with abrupt structural violations vs. emergence of regular structure.

Phasic pupil dilations were monitored to assess NE. We found that even though both transition directions (regular-to-random and random-to-regular) are clearly detectable behaviorally and both evoke strong MEG (Barascud et al, 2016) and EEG (Southwell et al, 2016) responses, only abrupt structural violations evoked pupil dilation. The results demonstrate that NE tracks unexpected uncertainty on rapid time scales relevant to sensory signals.