Cardiac-locked auditory stimulation modulates pupil and neural dynamics

The human brain is sensitive to temporal regularities across bodily and environmental signals. Here, we investigated the pupil and neural correlates of regularity encoding established across cardiac and auditory stimuli. Auditory sequences were presented in synchrony with the heartbeat (synchronous), at a fixed pace, or without temporal regularity while recording pupillometry, electroencephalography, and electrocardiography in healthy participants. Sounds evoked typical pupil dilation in all conditions. However, only in the synchronous condition, pupil dilation progressively decreased over the course of the sequence, possibly reflecting adaptation to the repeated cardio-auditory alignment. A concurrent increase in global EEG activity suggested enhanced cortical processing in response to the synchronous sequence. Pupil constriction was driven by participants with higher heart rate, indicating that pupil adaptation mostly occurs in response to fast auditory sequences. Cardio-audio regularity encoding manifests as a pupil adaptation and an amplification of global EEG activity, likely reflecting improved temporal prediction precision.