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Abstract

The ability to generate and sustain an internal train of thought unrelated to external reality frees an agent from the constraints of only acting on immediate, environmentally triggered events. The current paper proposes that such thought is produced through cooperation between autobiographical information provided by the default mode network and a frontal–parietal control network which helps sustain and buffer internal trains of thought against disruption by the external world. This hypothesis explains at least two features of the literature on internally guided thought. First, access to the top-down control system is a generally accepted prerequisite of conscious experience; this explains why activation of this system and default mode activity is often observed together during periods of internally guided thought. Second, because the top-down attentional control system has a limited capacity, internally and externally driven streams can come into conflict, with the result that perceptual information must be denied attentional amplification if the internal stream is to be maintained. This explains why internal thought is routinely associated with a state of perceptual decoupling, reflected in both measured anticorrelations between the default mode network and sensory areas and the manner in which task unrelated thoughts compromise task performance. This paper offers a hypothesis that should help to constrain and guide interpretations, investigations, and analyses of the neural processes involved in internally driven cognition.