Pupil size dynamics predict momentary changes in self-reported arousal.

Subjective arousal, or feelings of activation and alertness, can dynamically modulate affect, behavior, and cognition. Momentary changes in subjective arousal have typically been assessed with self-report in humans, which may alter those very feelings. Although researchers have traditionally used pupil size as a physiological index of arousal, links between pupil dynamics and momentary changes in subjective arousal remain unclear. Here, we combined continuous pupillometry with repeated self-report probes of changing arousal as individuals engaged in a modified Monetary Incentive Delay task. Across Virtual Reality (N = 28; 12 females, 16 males) and Functional Magnetic Resonance Imaging (N = 27; 15 females, 12 males) studies, tonic and phasic pupil measures jointly predicted subsequent self-reported arousal beyond stated incentive value and at a temporal lag of approximately 1.5-3.5 seconds. Neural activity associated with pupil dynamics further predicted pupil size, self-reported arousal, and behavioral motivation in held-out data. Together, these convergent findings establish pupil size as a temporally-precise marker of momentary fluctuations in self-reported arousal.Significance Statement Subjective feelings of arousal can critically shape people's subsequent thoughts and behavior, yet tracking its continuous fluctuations remains challenging. Here, we investigated the association of pupil size with momentary changes in self-reported arousal. Across virtual reality and FMRI studies, we demonstrate that pupil size robustly predicts subsequent self-reported arousal with a specific lag of 1.5-3.5 seconds. We further dissociate these dynamics into separable tonic and phasic pupil components that reflect changes in arousal over different timescales. Crucially, neural activity driving these pupil dynamics also predicted self-reported arousal. These findings establish pupil size as a temporally precise index of momentary changes in self-reported arousal, linking physiological changes to affective experience.