Synchronization Of Neural Signals Under General Anesthesia

Synchronization is becoming an essential tool to describe neurophysiological mechanisms of communication between brain regions under general anesthesia. Different synchronization measures have their own properties to reflect the changes of EEG activities during different anesthetic states. However, the synchronization changes measured by different synchronization measures in different brain areas during propofol-induced anesthesia are not fully elucidated.Two-channel electroencephalogram data from seven volunteers who had undergone a brief standardized propofol anesthesia were adopted to calculate the synchronization indexes. Prediction probability of synchronization indexes with Bispectral Index and propofol effect-site concentration, the coefficient of variation and the correlation coefficients were computed. Different synchronization measures had their own property in evaluating the synchronization changes between two-channel electroencephalogram data. Permutation cross mutual information and determinism could predict Bispectral Index and propofol effect-site concentration better than nonlinear interdependence, mutual information based on kernel estimation and cross correlation. There was a significant decrease in unconscious state and a significant increase in recovery state for permutation cross mutual information and nonlinear interdependence, while the trends were opposite for mutual information based on kernel estimation, determinism and coherence based on wavelet transformation. Phase synchronization based on phase locking value dropped significantly in unconscious state, whereas it had no significant synchronization in recovery state.Then, the intracranial 64-channel electrocorticogram signals from three epilepsy patients who undergone a brief standard propofol general anesthesia were analyzed by using phase-amplitude coupling. The time-varying phase-amplitude modulogram and modulation index based on the Kullback-Leibler distance were adopted to analyze the phase-amplitude modulations between low-frequency oscillations(slow oscillation(SO)(<1Hz), δ oscillation(1-4Hz) and θ oscillation(4-8Hz)) and high-frequency oscillations(α oscillation(8-13Hz), β oscillation(13-30Hz) and γ1 oscillation(30-40Hz)) in frontal, parietal, occipital and temporal cortices. The propofol had various effects on phase-amplitude coupling between different frequency oscillations in different cortical areas. The “trough-max” pattern of phase-amplitude modulation occurred in parietal and occipital cortices in awake state, while the “peak-max” pattern occurred in frontal, parietal and occipital cortices in unconscious state. A new discovered phase-amplitude modulation pattern, which was called “fall-pattern” and existed in frontal, parietal and occipital cortices after loss of consciousness. Also, propofol strengthen the phase-amplitude coupling after loss of consciousness.