Abstract
Mind wandering (MW) refers to the shift of attention away from a primary task towards thoughts unrelated to the task. Here we show that significant new insight into the dynamics of this process can be gained by using pupillometry. Participants performed a monotonous vigilance task which was intermixed with task-irrelevant verbal cues. At fixed times, we interrupted them and asked what they were thinking about immediately prior to the probe and what had triggered their thought. We observed pupil dilation after the presentation of verbal cues reported to have triggered MW, compared with other verbal cues with similar emotional content. Thus, MW is associated with pupil dilation. We also analysed the pupil-constriction response to the task-stimuli (vertical and horizontal bars, to be categorized as targets and non-targets for the vigilance task), and found that this was unchanged during MW. We conclude that pupil size provides an index of MW, objective and covert and that this may be exploited in further studies to understand whether and how MW affects the processing of sensory stimuli.