Background: In imaging functional connectivity (FC) analyses of the resting brain, alterations of FC during unconsciousness have been reported. These results are in accordance with recent electroencephalographic studies observing impaired top–down processing during anesthesia. In this study, simultaneous records of functional magnetic resonance imaging (fMRI) and electroencephalogram were performed to investigate the causality of neural mechanisms during propofol-induced loss of consciousness by correlating FC in fMRI and directional connectivity (DC) in electroencephalogram.
Methods: Resting-state 63-channel electroencephalogram and blood oxygen level–dependent 3-Tesla fMRI of 15 healthy subjects were simultaneously registered during consciousness and propofol-induced loss of consciousness. To indicate DC, electroencephalographic symbolic transfer entropy was applied as a nonlinear measure of mutual interdependencies between underlying physiological processes. The relationship between FC of resting-state networks of the brain (z values) and DC was analyzed by a partial correlation.
Results: Independent component analyses of resting-state fMRI showed decreased FC in frontoparietal default networks during unconsciousness, whereas FC in primary sensory networks increased. DC indicated a decline in frontal–parietal (area under the receiver characteristic curve, 0.92; 95% CI, 0.68–1.00) and frontooccipital (0.82; 0.53–1.00) feedback DC (P < 0.05 corrected). The changes of FC in the anterior default network correlated with the changes of DC in frontal–parietal (rpartial = +0.62; P = 0.030) and frontal–occipital (+0.63; 0.048) electroencephalographic electrodes (P < 0.05 corrected).
Conclusion: The simultaneous propofol-induced suppression of frontal feedback connectivity in the electroencephalogram and of frontoparietal FC in the fMRI indicates a fundamental role of top–down processing for consciousness.