Active and predictive adjustment of pupil size for accommodating visual regularity

The visual system efficiently encodes information by summarizing visual regularities that could be quantified by summary statistics. While summary statistics are thought to be processed in several cortical and subcortical areas, we investigated whether the earliest stage of visual processing, the pupil, is involved in this process. In three experiments, participants either performed ensemble orientation estimation with or passively viewed bar arrays with different bar orientation distributions while their pupil size was recorded. We found that pupil size increased when the orientation distribution became more dispersed and was closely linked to participants estimation performance. Furthermore, pupillary responses occurred automatically during passive viewing and could even predict ensemble orientation estimation performance. Moreover, when anticipating a dispersed distribution, the pupil dilated in advance. Our findings reveal a new cognitive role of the pupil - it actively and predictively adjusts its size to facilitate the extraction of visual regularity.