Real-time tracking of pupil-phase fluctuations reveals state-dependent modulation of temporal attentional capacity

Pupil diameter is widely used as an index of arousal and brain state, yet it remains unclear whether slow pupil-linked state fluctuations systematically modulate the effective capacity of temporal attention. Here, we tested this question using an auditory attentional blink paradigm, in which participants were required to detect the first (T1) and second (T2) targets. In Experiment 1, trial-by-trial analyses revealed that successful T2 detection, given correct T1 detection (T2|T1), was associated with smaller baseline pupil size. Furthermore, analyses focusing on slow pupil fluctuations (< 0.2 Hz) revealed that the T2|T1 detection accuracy increased during the pupil dilation phase occurring 0-2 seconds after pupil constriction. In Experiment 2, we used real-time pupillometry to trigger stimulus presentation during predefined phases of ongoing slow pupil dynamics. This closed-loop manipulation produced reliable phase-dependent differences in T2|T1 detection accuracy. Critically, the effect of pupil phase remained significant in a linear mixed-effects model that included baseline pupil size as a covariate, indicating that it cannot be explained by baseline pupil size alone. Together, these findings demonstrate that temporal attentional capacity is shaped not only by arousal level but also by the phase of slow pupil-linked brain-state fluctuations. Our results suggest that the attentional blink reflects a dynamically regulated, state-dependent limitation rather than a fixed processing bottleneck.