Increased signal diversity/complexity of spontaneous EEG, but not evoked EEG responses, in ketamine-induced psychedelic state in humans.
Level 4 - case-series / case-control
Small open-label, non-randomized within-subject experimental study in healthy volunteers
PubMed 33226992 · doi:10.1371/journal.pone.0242056
What was done
In an open-label within-subject study, 10 healthy volunteers underwent high-density 62-channel EEG before and during administration of sub-anaesthetic doses of ketamine. Evoked brain signal diversity was measured using transcranial magnetic stimulation (TMS) and quantified via the perturbational complexity index (PCI). Spontaneous EEG signal diversity was assessed during eyes-open and eyes-closed conditions using Lempel Ziv complexity (LZc), amplitude coalition entropy (ACE), and synchrony coalition entropy (SCE), alongside subjective ratings of altered consciousness.
What was found
The abstract reports no numerical values or effect sizes. There was no statistically significant difference in TMS-evoked complexity (PCI) between sub-anaesthetic ketamine and baseline wakefulness. All measures of spontaneous EEG signal diversity (LZc, ACE, and SCE) increased significantly under ketamine and correlated with subjective assessments of altered consciousness. Spontaneous signal diversity was also significantly higher with eyes open versus eyes closed in both wakefulness and ketamine conditions.
Why it matters
The findings suggest spontaneous EEG complexity reflects conscious content richness while TMS-evoked complexity reflects the baseline capacity to sustain consciousness.
Limits
The study had a very small sample size (n = 10) of healthy volunteers, an open-label design lacking a placebo comparator, and the abstract omits exact numerical values and effect sizes.