A spate of research has examined how individuals regulate effortful processing in service of goal-directed behaviors. One key challenge in developing an account of this regulation is quantifying the momentary amount of cognitive effort exerted by an individual in service of their goals. A growing body of literature has suggested using task-evoked pupil dilations as a potential psychophysiological index of cognitive effort; however, it remains unclear whether pupil diameter indexes effort exertion or merely reflects task load, as both are tightly intertwined. Here, we attempt to disentangle these disparate accounts of pupil diameter by leveraging individual differences in executive function (as measured by Stroop interference) and a motivational manipulation (i.e., monetary incentives) while participants complete a task-switching paradigm. In line with both the effort and demand accounts, we observed larger task-evoked pupillary responses (TEPRs) for trials in which there was a task switch versus a task repetition. Additionally, we found that larger phasic pupillary responses at baseline (without reward incentives) predicted smaller switch costs. Mirroring this pattern, individual differences in reward-induced switch cost reductions were predicted by reward-induced increases in phasic pupil diameter. Finally, we observed that the interrelationship between effort and pupil diameter at baseline was modulated by individual differences in Stroop interference costs. Together, these findings provide support for an effort account of TEPRs, and suggest that pupillometry is a viable index of cognitive effort.
Keywords: Cognitive effort; Executive function; Motivation; Psychophysiology; Reward.