Electroencephalographic signatures of attentional and cognitive default modes in spontaneous brain activity fluctuations at rest

Abstract

We assessed the relation between hemodynamic and electrical indices of brain function by performing simultaneous functional MRI (fMRI) and electroencephalography (EEG) in awake subjects at rest with eyes closed. Spontaneous power fluctuations of electrical rhythms were determined for multiple discrete frequency bands, and associated fMRI signal modulations were mapped on a voxel-by-voxel basis. There was little positive correlation of localized brain activity with alpha power (8-12 Hz), but strong and widespread negative correlation in lateral frontal and parietal cortices that are known to support attentional processes. Power in a 17-23 Hz range of beta activity was positively correlated with activity in retrosplenial, temporo-parietal, and dorsomedial prefrontal cortices. This set of areas has previously been characterized by high but coupled metabolism and blood flow at rest that decrease whenever subjects engage in explicit perception or action. The distributed patterns of fMRI activity that were correlated with power in different EEG bands overlapped strongly with those of functional connectivity, i.e., intrinsic covariations of regional activity at rest. This result indicates that, during resting wakefulness, and hence the absence of a task, these areas constitute separable and dynamic functional networks, and that activity in these networks is associated with distinct EEG signatures. Taken together with studies that have explicitly characterized the response properties of these distributed cortical systems, our findings may suggest that alpha oscillations signal a neural baseline with "inattention" whereas beta rhythms index spontaneous cognitive operations during conscious rest.

Fig. 1.

Brain regions where BOLD fMRI signal is positively (green) or negatively correlated (red) with spontaneous power fluctuations in EEG frequency bands at rest (A,8–12 Hz; B,17–23 Hz). The results from a fixed effects group analysis (see Methods) are overlaid onto a rendering of a template brain and visualized at a threshold of P < 0.001, uncorrected. Note that activation in all clusters was also significant (P < 0.05) after correction for multiple comparisons, and see Table 1 for results at the level of a random effects analysis.

Fig. 2.

Brain regions where BOLD fMRI signal at rest correlates with the reference regions indicated in green (stereotactic coordinates: A, –50, –52, 50; B, –36, –62, 36). The image data set is the same as in Fig. 1, and the functional connectivity results are visualized at a height threshold of T = 10.00.

Fig. 3.

Mean correlation over all subjects of BOLD fMRI signal time courses with power changes in different EEG frequency bands for all scalp electrode positions. Brain activity data were derived from a volume of interest (5 mm sphere) around a voxel in the left parietal (A, stereotactic coordinates: 50, –52, 50) and retrosplenial cortex (B, stereotactic coordinates: 2, –36, 30). Note that, at the latter site, we observed no strong negative correlation of BOLD signal with the beta-3 band (compare location of peak effects in Table 1). Correlation with the individual power time courses was projected onto a standard template of the corresponding electrode positions.