Increased signal diversity/complexity of spontaneous EEG, but not evoked EEG responses, in ketamine-induced psychedelic state in humans
Published in PLOS ONE, 2020
How and to what extent electrical brain activity reflects pharmacologically altered states and contents of consciousness is not well understood. This study investigated whether measures of evoked and spontaneous EEG signal diversity are altered by sub-anaesthetic ketamine compared to normal wakefulness, and how these measures relate to subjective experience. High-density 62-channel EEG was recorded in 10 healthy volunteers before and during sub-anaesthetic ketamine in a within-subject design. Evoked complexity was assessed via the perturbational complexity index (PCI) from TMS-evoked responses; spontaneous diversity was assessed by Lempel-Ziv complexity (LZc), amplitude coalition entropy (ACE), and synchrony coalition entropy (SCE).
No significant difference was found in TMS-evoked complexity (PCI) between the ketamine condition and normal wakefulness, while all spontaneous EEG measures showed significantly increased values under ketamine — and these increases correlated with subjective ratings of altered states. The results suggest that PCI and spontaneous complexity may reflect complementary aspects of altered consciousness: PCI may index the brain’s capacity for information integration (and thus the general capacity to sustain consciousness), while spontaneous complexity indexes the richness of conscious content. Sub-anaesthetic ketamine thus appears to amplify the complexity of conscious content without fundamentally impairing the brain’s capacity to sustain consciousness.
Contributions: Co-responsible for theory, design, and implementation; assisted in analysis, acquisition, interpretation, and manuscript revision.
Recommended citation: Farnes, N., Juel, B. E., Nilsen, A. S., Romundstad, L. G., & Storm, J. F. (2020). Increased signal diversity/complexity of spontaneous EEG, but not evoked EEG responses, in ketamine-induced psychedelic state in humans. PLOS ONE, 15(11), e0242056.
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