Dreaming as mind wandering: evidence from functional neuroimaging and first-person content reports

Abstract

Isolated reports have long suggested a similarity in content and thought processes across mind wandering (MW) during waking, and dream mentation during sleep. This overlap has encouraged speculation that both "daydreaming" and dreaming may engage similar brain mechanisms. To explore this possibility, we systematically examined published first-person experiential reports of MW and dreaming and found many similarities: in both states, content is largely audiovisual and emotional, follows loose narratives tinged with fantasy, is strongly related to current concerns, draws on long-term memory, and simulates social interactions. Both states are also characterized by a relative lack of meta-awareness. To relate first-person reports to neural evidence, we compared meta-analytic data from numerous functional neuroimaging (PET, fMRI) studies of the default mode network (DMN, with high chances of MW) and rapid eye movement (REM) sleep (with high chances of dreaming). Our findings show large overlaps in activation patterns of cortical regions: similar to MW/DMN activity, dreaming and REM sleep activate regions implicated in self-referential thought and memory, including medial prefrontal cortex (PFC), medial temporal lobe structures, and posterior cingulate. Conversely, in REM sleep numerous PFC executive regions are deactivated, even beyond levels seen during waking MW. We argue that dreaming can be understood as an "intensified" version of waking MW: though the two share many similarities, dreams tend to be longer, more visual and immersive, and to more strongly recruit numerous key hubs of the DMN. Further, whereas MW recruits fewer PFC regions than goal-directed thought, dreaming appears to be characterized by an even deeper quiescence of PFC regions involved in cognitive control and metacognition, with a corresponding lack of insight and meta-awareness. We suggest, then, that dreaming amplifies the same features that distinguish MW from goal-directed waking thought.

Keywords: default mode network; dreaming; first-person report; introspection; memory consolidation; mind wandering; neurophenomenology; spontaneous thought.

Figure 1

Neural substrate of REM sleep vs. waking rest. Significant meta-analytic clusters contributing to the neural substrate of REM sleep (as a proxy for dream mentation). Axial slices are displayed in Talairach space, with 3 mm skip. Color bars indicate likelihood that peaks represent actual peaks of difference at a given voxel. Activations (REM > waking rest) are in red-yellow, deactivations (REM < waking rest) in blue-green.

Figure 2

Multiple fMRI methods defining the default mode network. Key cortical areas contributing to the default mode network, as revealed by distinct fMRI methods and study designs. Data are based on a meta-analysis by Buckner et al. (2008) that included 4 blocked design fMRI studies, 10 event-related fMRI studies, and 4 studies of functional connectivity. Colors for each panel of images denote the number of studies finding significant effects at a given voxel (see color bars and numeric labels at right). Note the similarity in the pattern of regions recruited, regardless of method (A), and the strong convergence across all methods (B). Reproduced with permission from Buckner et al. (2008); originally adapted from Shannon (2006). ER, event-related; HFC, hippocampal functional connectivity.

Figure 3

Tentative model of dreaming as intensified mind wandering. A preliminary model of dreaming as an intensified version of waking mind wandering. Intensity of audiovisual imagery, number of bizarre or implausible elements, and activity in DMN regions all appear to increase from waking, goal-directed thought, through waking spontaneous thoughts, to dream mentation. The opposite trend may hold for activity levels in prefrontal executive regions such as anterior cingulate cortex and dorsolateral prefrontal cortex, which are highly active in goal-directed waking thoughts and tasks, only somewhat active during mind wandering (Christoff et al., 2009), and mostly quiescent during dreaming/REM sleep (Table 2). Solid lines represent subjective, experiential elements; dashed lines represent brain activity levels as measured by regional cerebral blood flow using PET, or BOLD (blood-oxygen-level-dependent) signal using fMRI. DMN, default mode network; PFC, prefrontal cortex.