Strength of low-frequency phase entrainment to external stimuli is associated with fluctuations in the brain's internal states

The brain attends to environmental rhythms by aligning the phase of internal oscillations. However, the factors underlying fluctuations in the strength of this phase entrainment remain largely unknown. In the present study we examined if the strength of low-frequency EEG phase entrainment to rhythmic stimulus sequences varied with pupil size and posterior alpha-band power, thought to reflect arousal level and excitability of posterior cortical brain areas, respectively. We recorded pupil size and scalp EEG while participants carried out an intermodal selective attention task, in which they were instructed to attend to a rhythmic sequence of visual or auditory stimuli and ignore the other perceptual modality. As expected, intertrial phase coherence (ITC), a measure of entrainment strength, was larger for the task-relevant than for the task-irrelevant modality. Across the experiment, pupil size and posterior alpha power were strongly linked with each other, and ITC tracked pupil size: with increasing pupil size, entrainment was higher to the task-relevant stimulus sequence and lower to the task-irrelevant stimulus sequence. Exploratory analyses showed that a temporal relation between ITC and posterior alpha power emerged in the time periods around pupil peaks and pupil troughs. These results indicate that endogenous sources contribute distinctly to the fluctuations of EEG phase entrainment.