Pupil dilations prior to freely timed actions reflect the reported timing of conscious intention

Freely timed actions are typically preceded by a slow anticipatory buildup of cortical brain activity, which has been extensively studied. However, such free actions are also preceded by slow pupil dilations in both humans and other animals, which have barely been examined. We investigated the neurocognitive significance of antecedent pupil dilations (APDs) in a voluntary-action paradigm. Participants performed self-paced actions and reported the timing of movement, conscious intention, or other events using a clock. APDs began a second or more before movement, and control conditions suggest that they did not reflect processing related to reporting demands, motor execution, or general anticipation. Critically, APD timing covaried with the reported timing of intention awareness but did not covary with the reported timing of overt movement or an external stimulus. Furthermore, decoding algorithms could distinguish APDs with above-chance accuracy more than 500 milliseconds before button-press. Our results suggest that APDs reflect a shift in awareness prior to movement onset and potentially offer a non-invasive method of predicting spontaneous movements before they occur.