Functional connectivity in the resting brain: a network analysis of the default mode hypothesis

Abstract

Functional imaging studies have shown that certain brain regions, including posterior cingulate cortex (PCC) and ventral anterior cingulate cortex (vACC), consistently show greater activity during resting states than during cognitive tasks. This finding led to the hypothesis that these regions constitute a network supporting a default mode of brain function. In this study, we investigate three questions pertaining to this hypothesis: Does such a resting-state network exist in the human brain? Is it modulated during simple sensory processing? How is it modulated during cognitive processing? To address these questions, we defined PCC and vACC regions that showed decreased activity during a cognitive (working memory) task, then examined their functional connectivity during rest. PCC was strongly coupled with vACC and several other brain regions implicated in the default mode network. Next, we examined the functional connectivity of PCC and vACC during a visual processing task and show that the resultant connectivity maps are virtually identical to those obtained during rest. Last, we defined three lateral prefrontal regions showing increased activity during the cognitive task and examined their resting-state connectivity. We report significant inverse correlations among all three lateral prefrontal regions and PCC, suggesting a mechanism for attenuation of default mode network activity during cognitive processing. This study constitutes, to our knowledge, the first resting-state connectivity analysis of the default mode and provides the most compelling evidence to date for the existence of a cohesive default mode network. Our findings also provide insight into how this network is modulated by task demands and what functions it might subserve.

Figure 1

Map of the resting-state neural connectivity for the PCC. The blue arrow indicates the approximate location of the PCC peak [−2 −51 27]. The approximate locations of the eight significant clusters are labeled A–H in descending order of the cluster's t score (A corresponds to the cluster with the highest t score). A [−51 −65 27], left IPC. B has a maximum at [−2 55 −18] in the OFC, but extends superiorly into the MPFC and the vACC (seen at z = +2). C [53 −61 27], right IPC. D [−16 49 38], MPFC just left of midline. E [−44 20 41], left DLPFC. F [−12 −35 0], posterior left PHG. G [18 54 32], MPFC just right of midline. H [−58 −18 −14], left ITC. Map is superimposed on transverse sections of the group-average structural scans. The numbers below each image refer to the z plane coordinates of Talairach and Tournoux. The left hemisphere of the brain corresponds to the left side of the image. Height and extent thresholds were set at P < 0.001. t score scale is shown on the right.

Figure 2

Map of the resting-state neural connectivity for the vACC. The blue arrow indicates the approximate location of the vACC maximum [2 38 −2]. A [2 −51 27], PCC extending into the precuneus. B [4 −14 34], rostral PCC. C [4 9 −6], nucleus accumbens. D [4 −16 −3] is in the hypothalamus with some extension into the rostral midbrain. For other details, see Fig. 1.

Figure 3

Comparison of the PCC connectivity patterns during the visual processing task (Upper) and the resting-state (Lower). The blue arrows indicate the approximate location of the PCC peak [−2 −51 27]. For other details, see Fig. 1.

Figure 4

Connectivity maps showing regions inversely correlated with the left VLPFC, the right VLPFC, and the right DLPFC. In each case, the only significant cluster was in the PCC. Maxima were at [−4 −55 25] for the left VLPFC, [0 −49 30] for the right VLPFC, and [−4 −49 26] for the right DLPFC. Height threshold was P < 0.01; extent threshold was P < 0.05. For other details, see Fig. 1.