Global field synchrony during general anaesthesia.

BACKGROUND

Anaesthetic agents may disrupt consciousness by inhibiting long-range synchronization of brain activity. In the current study, the patterns of widespread and spatially localized synchrony during anaesthesia are investigated using a measure called global field synchrony (GFS).

METHODS

The EEG obtained during routine surgery in 29 patients was analysed with GFS over the following frequency bands: δ (1.5-3.5 Hz), θ (3.5-7.5 Hz), α1 (8-10 Hz), α2 (10.5-12 Hz), β1 (12.5-18 Hz), β2 (18.5-21 Hz), β3 (21.5-30 Hz), γ1 (30.5-40 Hz), and γ2 (60-80 Hz). In addition, localized GFS estimations over aggregate brain areas were performed. GFS was estimated over 2 s non-overlapping windows. The differences in GFS values between 'wakefulness' and 'anaesthesia' were assessed with the two-sided Wilcoxon rank-sum tests (α=0.05).

RESULTS

Anaesthetic administration caused significant GFS changes in all frequency ranges and electrode combinations studied: (i) widespread synchrony increased in the α2 and β1 ranges and decreased in all other ranges, with the exception of α1 and β2, where no specific pattern was identified; and (ii) localized synchrony decreased in all areas in the δ and γ2 ranges, while location-specific changes were observed in the remaining frequency ranges. The most consistent findings were statistically significant decreases over all areas in the γ2 range, with GFS decrease over the central-right temporal being the most consistent change.

CONCLUSIONS

Significant frequency- and location-dependent changes in GFS were induced by anaesthetic administration, with more robust changes identified in the γ range. GFS can act as an aid for further and more detailed analysis regarding the particular combinations of frequency ranges and spatial locations that are most informative for the study of anaesthetic-induced unconsciousness.