TMS-EEG and resting-state EEG applied to Altered States of Consciousness: Oscillations, Complexity, and Phenomenology

Introduction

Altered states of consciousness (ASCs) produced by serotonergic psychedelics such as psilocybin involve profound changes in mood, perception, self-awareness and sensory-emotional processing. Earlier research has shown that spontaneous electroencephalographic (EEG) signal diversity is elevated during these drug-induced ASCs compared with typical wakefulness and dreaming; however, it is unclear whether that increased diversity reflects richer causal interactions across brain regions or instead arises from more unstructured or chaotic neuronal activity. Measuring the brain's response to direct perturbations with Transcranial Magnetic Stimulation (TMS) and analysing the ensuing EEG response provides a way to probe causal interactions and integration, quantified here by the Perturbational Complexity Index (PCI), as distinct from spontaneous measures such as Lempel-Ziv complexity (LZc). Ort and colleagues set out to test whether the dissociation previously observed between spontaneous EEG diversity and perturbational complexity during sub‑anesthetic ketamine also holds for a classical 5-HT2A agonist, psilocybin. Using a placebo-randomized, double-blind within-subject design, they compared spontaneous EEG and TMS-evoked EEG measures across key cortical sites and related neurophysiological changes to participants' reported phenomenology, aiming to distinguish chaotic ongoing activity from alterations in the underlying causal interactions that support consciousness.