Complexity of brain signals decreases under anesthesia

What happens in the brain when we lose consciousness? This is a key question in the neuroscience of consciousness. In a new paper, led by Sackler Centre Ph.D. student Michael Schartner, supervised by Adam Barrett and myself, we describe a reduction in the complexity of brain responses under (propofol) general anaesthesia. This work builds directly on a recent study by Casali and colleagues which showed that the Lempel-Ziv complexity (a measure of compressibility of data) of brain EEG responses evoked by TMS pulses diminished across a wide range of unconscious (and impaired consciousness) states.

We extend this important work by showing that complexity measures reliably predict conscious level on spontaneous as well as on TMS-evoked brain signals. We also compare different complexity measures, including Lempel-Ziv complexity and complementary measures of 'coalition entropy', which measure the variability in how different EEG channels behave together. Altogether, we find that general anasethesia induced a robust decrease in the complexity of spontaneous brain electrical responses.

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