Abstract
BackgroundIt has been previously shown that loss of consciousness is associated with a breakdown of dominating fronto-parietal feedback connectivity as assessed by electroencephalogram (EEG) recordings. Structure and strength of network connectivity may change over time. Aim of the current study is to investigate cortico-cortical connectivity at different time intervals during consciousness and unconsciousness. For this purpose, EEG symbolic transfer entropy (STEn) was calculated to indicate cortico-cortical information transfer at different transfer times.MethodsThe study was performed in 15 male volunteers. 29-channel EEG was recorded during consciousness and propofol-induced unconsciousness. EEG data were analyzed by STEn, which quantifies intensity and directionality of the mutual information flow between two EEG channels. STEn was computed over fronto-parietal channel pair combinations (10 s length, 0.5–45 Hz total bandwidth) to analyze changes of intercortical directional connectivity. Feedback (fronto → parietal) and feedforward (parieto → frontal) connectivity was calculated for transfer times from 25 ms to 250 ms in 5 ms steps. Transfer times leading to maximum directed interaction were identified to detect changes of cortical information transfer (directional connectivity) induced by unconsciousness (p<0.05).ResultsThe current analyses show that fronto-parietal connectivity is a non-static phenomenon. Maximum detected interaction occurs at decreased transfer times during propofol-induced unconsciousness (feedback interaction: 60 ms to 40 ms, p = 0.002; feedforward interaction: 65 ms to 45 ms, p = 0.001). Strength of maximum feedback interaction decreases during unconsciousness (p = 0.026), while no effect of propofol was observed on feedforward interaction. During both consciousness and unconsciousness, intensity of fronto-parietal interaction fluctuates with increasing transfer times.ConclusionNon-stationarity of directional connectivity may play a functional role for cortical network communication as it shows characteristic changes during propofol-induced unconsciousness.
Highlights
Altered states of consciousness are associated with changes in directional and functional connectivity within a frontoparietal network
An onset of transfer entropy beginning at 25 ms transfer time may reflect effective connecting times between brain regions
No significant changes of feedback vs. feedforward transfer time are observed at consciousness and at unconsciousness
Summary
Altered states of consciousness are associated with changes in directional (effective) and functional connectivity within a frontoparietal network. This is observed as a common phenomenon during anesthesia, vegetative states and sleep [1,2,3,4,5,6,7,8,9]. Non-static characteristics of network activity and connectivity can be seen as an intrinsic mechanism of the brain to optimize effectiveness of information transfer. Aim of the current study is to investigate cortico-cortical connectivity at different time intervals during consciousness and unconsciousness For this purpose, EEG symbolic transfer entropy (STEn) was calculated to indicate cortico-cortical information transfer at different transfer times
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