vmPFC damage reduces mind-wandering, but not other classes of off-task thought.

Ventromedial prefrontal cortex (vmPFC) is consistently engaged during mind-wandering, but its role in mind-wandering is still unclear. The present study tests the hypothesis that vmPFC is necessary for the (endogenous) generation of stimulus-independent and task-unrelated thought (mind-wandering), but not for externally triggered forms of off-task thought. To this aim, we studied off-task thought in vmPFC patients and brain-damaged and healthy controls, sampling the occurrence of different off-task experiences during a vigilance task, including mind-wandering, external distractions, and task-related thoughts. Moreover, we experimentally manipulated the presence of task-irrelevant cue-words capable to elicit mind-wandering (Standard condition vs. Cued condition). vmPFC patients showed reduced endogenously generated mind-wandering compared to the control groups (Standard condition), but also a weakened tendency to experience mind-wandering in response to cue-words (Cued condition). By contrast, vmPFC patients normally reported on (hence could become aware of) other types of off-task mental content, such as external distractions and task-related thoughts. These findings indicate that vmPFC integrity is necessary for the generation of mind-wandering, be this driven endogenously or by (minimal) cues, possibly by initiating the mental construction of personal (past and future) events that typically fuel mind-wandering. Without such internally generated content, attention is less likely to shift inward.