Undirected thought: neural determinants and correlates.

While goal-directed thinking has received the lion's share of neuroscientific attention, its counterpart--the undirected thought flow that comes to mind unbidden and without effort--has remained largely on the sidelines of scientific research. Such undirected thought, however, forms a large part of our mental experience. The last decade of neuroscientific investigations marked a resurgence of interest and work into the neural basis of undirected thought. This article reviews the current status of the field and examines the research on the three most frequently discussed categories of undirected thought: spontaneous thought, stimulus-independent thought, and mind wandering. The terminology and paradigms for investigating undirected thought are still being developed, while research is gradually moving beyond strictly task- and rest-based paradigms and towards incorporating introspective first-person reports in order to better understand this phenomenon. It is impossible to say at this point that undirected thinking is preferentially linked to any one particular brain system. Although its connection to the default network has been disproportionately emphasized in the literature, other brain networks such as the executive system and the temporal lobe memory network appear to be equally involved. In addition to reviewing the literature, this article also presents novel findings regarding the functional connectivity between large-scale brain networks during mind wandering. These findings reveal the presence of positive functional connectivity between regions of the default and executive networks and negative functional connectivity between the default network and primary sensory cortices. Thus, the default and executive networks can closely cooperate in supporting undirected thought processes, and seem to do so at times when the primary sensory cortices are not busy with the processing of perceptual information from the external environment. This article is part of a Special Issue entitled The Cognitive Neuroscience of Thought.