Simultaneous EEG and fMRI of the alpha rhythm

Abstract

The alpha rhythm in the EEG is 8-12 Hz activity present when a subject is awake with eyes closed. In this study, we used simultaneous EEG and fMRI to make maps of regions whose MRI signal changed reliably with modulation in posterior alpha activity. We scanned 11 subjects as they rested with eyes closed. We found that increased alpha power was correlated with decreased MRI signal in multiple regions of occipital, superior temporal, inferior frontal, and cingulate cortex, and with increased signal in the thalamus and insula. These results are consistent with animal experiments and point to the alpha rhythm as an index of cortical inactivity that may be generated in part by the thalamus. These results also may have important implications for interpretation of resting baseline in fMRI studies.

Figures

Fig. 1

EEG recorded simultaneously with fMRI. Grey bars indicate 90 ms segments where MR gradient-related artifact was removed. Note clear presence of alpha rhythm in the bipolar channels containing occipital electrodes.

Fig. 2

Regions where MR signal increased (red-yellow) and decreased (blue-cyan) with elevations in alpha power for a representative single subject. The bottom bar shows the Pearson correlation value between signal intensity and alpha power modulation.

Fig. 3

Time course of the average MRI percentage signal change for ROIs in which BOLD signal was positively (top) and negatively (bottom) correlated with alpha rhythm for the representative single subject shown in Fig. 2. At center is the alpha power time course convolved with a hemodynamic response function, which was used as the independent response model to create the tomographic map of alpha activity.