Effects of psychedelics on human oscillatory brain activity.

This chapter reviews the effects of classic psychedelics on human oscillatory brain activity, as measured by resting-state electroencephalography (EEG) and magnetoencephalography (MEG). Across moderate to high doses of LSD, psilocybin, ayahuasca, and DMT, a consistent reduction in alpha power (8-13 Hz) emerges, particularly in occipital regions. Below 30 Hz, desynchronization is typical, although DMT can preserve or even increase delta/theta activity, possibly reflecting its immersive, immersive visual phenomenology. Complementing these spectral findings, measures of signal diversity (e.g., Lempel-Ziv complexity) reliably increase during psychedelic states, indicating a more variable and unpredictable pattern of neural firing. Retrospective subjective ratings of the psychedelic experience often fail to align consistently with M/EEG changes, possibly because fleeting, key experiences are obscured by data averaging or recording short segments of a long experience. In contrast, real-time evaluations of subjective intensity and plasma levels robustly covary with changes in spectral power and complexity, highlighting the potential for objective, real-time EEG biomarkers of drug activity. Limited research on functional connectivity and cortical travelling waves suggest that directed, top-down control may decrease while bottom-up signaling increases, indicating a transient reversal of typical hierarchical organization, though replications are warrented. Future work should implement more unified methodological approaches, alongside high-resolution behavioral sampling, to further our understanding of how these altered brain dynamics give rise to the distinctive qualities of the psychedelic experience. Notably, EEG has yet to be evaluated in clinical studies, and future work should aim to explore the relationship between acute EEG changes and clinical responses to psychedelic therapy.