Pupil dilation as an indicator of future thinking

Background: The pupil typically dilates in reaction to cognitive load. In this study, we, for the first time, investigated whether future thinking (i.e., the ability to generate hypothetical scenarios in the future) would result in pupil dilation. 

Methods: We recorded pupil dilation of participants during two conditions: past and future thinking. In past thinking, we invited participants to retrieve past personal events, while in future thinking, we invited them to imagine an event that may occur in the future. 

Results: Analysis demonstrated a larger pupil size during future than past thinking. Results also demonstrated longer retrieval time of future events compared with past ones, suggesting that future thinking perhaps requires more cognitive load than for past thinking. Interestingly, retrieval times during past and future thinking were positively correlated with pupil size. 

Discussion: The finding that future thinking activates pupil dilation could be due to the fact that while both past and future thinking require retrieving information from memory, future, but not past, thinking additionally requires the ability to recombine this information into novel scenarios.