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Although mind-wandering during silent reading is well documented, to date no research has investigated whether similar processes occur during reading aloud. In the present study, participants read a passage either silently or aloud while periodically being probed about mind-wandering. Although their comprehension accuracies were similar for both reading conditions, participants reported more mind-wandering while they were reading aloud. These episodes of mindless reading were associated with nearly normal prosody, but were nevertheless distinguished by subtle fluctuations in volume that were predictive of both overall comprehension accuracy and individual sentence comprehension. Together, these findings reveal that previously hidden within the common activity of reading aloud lies: (1) a demonstration of the remarkable automaticity of speech, (2) a situation that is surprisingly conducive to mind-wandering, (3) subtle vocal signatures of mind-wandering and comprehension accuracy, and (4) the promise of developing useful interventions to improve reading.

Mind-wandering is both pervasive and detrimental to task performance. As such, identifying covert physiological measures that are associated with this off-task state could inform theories of mind-wandering and lead to interventions that improve task focus. Although previous work suggests that pupil dilation (PD) may vary between on- and off-task states, no studies have examined whether PD systematically varies within a subject as they report becoming disengaged from a task-a key step in developing useful mind-wandering prediction algorithms. In the present study, PD was measured while participants advanced through a passage one word at a time. Spontaneous mind-wandering was assessed during reading using standard thought probe methodology. Results revealed higher PD prior to off-task than prior to on-task reading. This newly discovered relationship between momentary fluctuations of attention and PD offers promise for future innovations that use these systematic changes in PD to predict and better control mind-wandering.

Although consciousness can be brought to bear on both perceptual and internally generated information, little is known about how these different cognitive modes are coordinated. Here we show that between-participant variance in thoughts unrelated to the task being performed (known as task unrelated thought, TUT) is associated with longer response times (RT) when target presentation occurs during periods when baseline Pupil Diameter (PD) is increased. As behavioral interference due to high baseline PD can reflect increased tonic activity in the norepinephrine system (NE), these results might implicate high tonic NE activity in the facilitation of TUTs. Based on these findings, it is hypothesised that high tonic mode NE leads to a generalised de-amplification of task relevant information that prioritses internally generated thought and insulates it from the potentially disruptive events taking place in the external environment.

Accumulating evidence suggests that the brain can efficiently process both external and internal information. The processing of internal information is a distinct "offline" cognitive mode that requires not only spontaneously generated mental activity; it has also been hypothesized to require a decoupling of attention from perception in order to separate competing streams of internal and external information. This process of decoupling is potentially adaptive because it could prevent unimportant external events from disrupting an internal train of thought. Here, we use measurements of pupil diameter (PD) to provide concrete evidence for the role of decoupling during spontaneous cognitive activity. First, during periods conducive to offline thought but not during periods of task focus, PD exhibited spontaneous activity decoupled from task events. Second, periods requiring external task focus were characterized by large task evoked changes in PD; in contrast, encoding failures were preceded by episodes of high spontaneous baseline PD activity. Finally, high spontaneous PD activity also occurred prior to only the slowest 20% of correct responses, suggesting high baseline PD indexes a distinct mode of cognitive functioning. Together, these data are consistent with the decoupling hypothesis, which suggests that the capacity for spontaneous cognitive activity depends upon minimizing disruptions from the external world.