Uncovering covert attention in complex environments with pupillometry

Spatial visual attention prioritizes specific locations while disregarding others. The location of spatial attention can be deployed without overt movements (covertly). Spatial dynamics of covert attention is exceptionally difficult to measure due to its hidden nature. One way to implicitly index the location of covert attention is via pupillary light responses (PLR), as the strength of PLR is modulated by where attention is allocated. However, this method has so-far necessitated simplistic stimuli. Here we report on a novel pupillometric method that allows to track covert attention even with highly complex stimuli. Participants (n = 36) watched movie clips while they either passively viewed the movie, or they top-down shifted covert attention to targets in the left, right, or both sides of the visual field. Using a recent toolbox (Open-DPSM), we evaluated whether luminance changes in regions presumably receiving more attention contribute more strongly to the pupillary responses – and thereby reveal covert attention. Three independent and established effects of covert attention on pupil responses were found: (1) a bottom-up effect suggesting more attention drawn to more dynamic regions in the movie, (2) a top-down effect suggesting more attention towards the instructed direction, and (3) an overall tendency to attend the left side (i.e., pseudoneglect). These findings show that pupil responses can physiologically index covert attention with our approach, even in highly dynamic and complex environments. We see considerable potential in measuring covert attention with our method in many real-life scenarios that were impossible to study before.