The ability to generate and sustain an internal train of thought independent of external reality frees an agent from acting only on immediate environmental events. This paper proposes that such thought arises from cooperation between autobiographical information from the default mode network and a frontal-parietal control network that sustains and buffers internal trains of thought against disruption. This hypothesis explains two features: first, access to the top-down control system is a prerequisite of conscious experience, explaining why its activation and default mode activity are often observed together during internally guided thought.
People vary in how often they have thoughts unrelated to the task at hand, and this variation is linked to longer reaction times when a target appears while baseline pupil diameter is elevated. Because increased baseline pupil diameter can indicate higher tonic activity in the norepinephrine system, the findings suggest that high tonic norepinephrine activity may facilitate task-unrelated thoughts. The authors hypothesize that a high tonic mode of norepinephrine broadly reduces the amplification of task-relevant information, thereby prioritizing internally generated thoughts and shielding them from potentially disruptive external events.