Decoupling of the brain's default mode network during deep sleep

Abstract

The recent discovery of a circuit of brain regions that is highly active in the absence of overt behavior has led to a quest for revealing the possible function of this so-called default-mode network (DMN). A very recent study, finding similarities in awake humans and anesthetized primates, has suggested that DMN activity might not simply reflect ongoing conscious mentation but rather a more general form of network dynamics typical of complex systems. Here, by performing functional MRI in humans, it is shown that a natural, sleep-induced reduction of consciousness is reflected in altered correlation between DMN network components, most notably a reduced involvement of frontal cortex. This suggests that DMN may play an important role in the sustenance of conscious awareness.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

The default mode network during wake and sleep. Composite maps showing correlations with PCC during (A) wake and (B) deep sleep, and their significant difference as determined from statistical t test (C). A significant reduction of involvement of frontal regions is seen during deep sleep, whereas the posterior cingulate–inferior parietal correlations are preserved. The Z maps in A and B are both thresholded at Z = ±5.0; the t map in C is thresholded at t = ±3.5. Both positive (yellow–red) and negative (blue) correlations are shown. Z values, t values, and Talairach coordinates of all significant clusters are reported in Tables 1 and 2.

Fig. 2.

Connectivity of the main components of the DMN during wake and deep sleep. The connectivity within (disks) and between components (lines) was determined from temporal correlation analysis of average time courses within each ROI. The ROIs were defined as the voxels within each anatomic region that are significantly connected to the PCC seed during wake, using a low threshold (P = 0.0001, uncorrected). The size of the disks represents within-region connectivity, whereas thickness of lines represents between-region connectivity. During deep sleep, the posterior areas (bilateral IPC and PCC) strengthen their connectivity, whereas the connections between frontal and posterior regions are lost. See also Tables 3 and 4. MF = medial prefrontal/anterior cingulate cortex; IPl = left inferior parietal/angular gyrus; IPr = right inferior parietal/angular gyrus; PC = posterior cingulate/precuneus.

Fig. 3.

Temporal evolution of connectivity within DMN at the single-subject level. (A) Composite single-slice maps (Z = 25) of correlation with seed in PCC. Each image represents the average of 2 10-min correlation maps (for full data see Fig. S3). Periods of deep sleep (indicated by colored background) coincide with a reduced involvement of frontal regions; this is confirmed by the region-based analysis shown in B, which shows reduced correlations with ACC and MPFC in the presence of a robust correlation within PCC. Corresponding levels of band-limited EEG activity are given in C, with the following definitions: lower delta: <2 Hz; upper delta: 2–4 Hz; theta: 4–8 Hz; alpha: 8–12 Hz; and beta: 12–20 Hz).