Gamma Coherences in the Default Mode Resting State as a Measure of Consciousness Level

Recent findings indicate that the default mode resting state network (DMRSN) is closely associated with conscious awareness. Thus changes in consciousness level could be reflected in turn in the DMRSN functional state. One of the most important requirements for proper network function is connectivity within the network structural constituents. One of the measures of functional connectivity is analysis of the EEG coherences. In this study we have examined changes in the mean coherences in the gamma spectral band 30–45 Hz in the DMRSN during general anaesthesia (GA) in the open thorax surgery were investigated in twenty patients. They were compared with physiologic EEG findings in twenty alert subjects and the mathematical model of brain death. Results of this study indicate that the decrease in coherences in the physiologic EEG and the patients with one lung ventilation (OLV) groups was roughly proportional to the increasing distance between the electrodes. This created some kind of a structure in the DMRSN. In the OLV group the main finding was a decrease in the coherences in the gamma band. Ten of the anaesthetized subjects showed a clear burst suppression pattern. During this condition the coherences in the pairs connecting the posterior parts of the network decreased. Moreover, the differences between the connections of the anterior to posterior parts of the DMRSN and connections between the posterior parts of the DMRSN were almost lost. However, they showed still highly significant differences in all items when compared with the mathematical model of brain death. In summary the results suggest that the functional connectivity in the DMRSN assessed by mean coherences could be a reliable method for assessing the depth of GA and may be useful for understanding disorders of consciousness in general.