The pupillary light response reflects exogenous attention and inhibition of return

Here we show that the pupillary light response reflects exogenous (involuntary) shifts of attention and inhibition of return. Participants fixated in the center of a display that was divided into a bright and a dark half. An exogenous cue attracted attention to the bright or dark side of the display. Initially, the pupil constricted when the bright, as compared to the dark side of the display was cued, reflecting a shift of attention towards the exogenous cue. Crucially, this pattern reversed about one second after cue presentation. This later-occurring, relative dilation (when the bright side was cued) reflected disengagement from the previously attended location, analogous to the behavioral phenomenon of inhibition of return. Indeed, we observed a strong correlation between 'pupillary inhibition' and behavioral inhibition of return. We conclude that the pupillary light response is a complex eye movement that reflects how we selectively parse and interpret visual input.