Increased signal diversity/complexity of spontaneous EEG, but not evoked EEG responses, in ketamine-induced psychedelic state in humans.
Nadine Farnes, Bjørn E. Juel, André S. Nilsen, Luis G. Romundstad, Johan F. Storm
PLoS One November 23, 2020 DOI: 10.1371/journal.pone.0242056 (opens in new tab)
Study at a glance
AI-extracted from the abstract| Characteristics | Open-label within-subject experimental study Peer reviewed |
|---|---|
| Sample size | 10 |
| Population | Healthy volunteers |
| Intervention | Ketamine |
| Dose | sub-anaesthetic doses |
| Measures | perturbational complexity index (PCI), Lempel Ziv complexity (LZc), amplitude coalition entropy (ACE), synchrony coalition entropy (SCE) |
| Topics | Ketamine Esketamine |
| Keywords | Psychedelic state Brain complexity Consciousness alterations EEG Measurement |
| Citations | 81 |
| Key findings | Sub-anaesthetic ketamine left TMS-evoked brain complexity (PCI) unchanged but significantly increased spontaneous EEG signal diversity (LZc, ACE, SCE), and these increases correlated with subjective altered states of consciousness. The authors propose that PCI reflects the brain's capacity to sustain consciousness while spontaneous complexity reflects the complexity of conscious content, so ketamine may heighten content complexity without affecting the level of consciousness. |
Abstract
How and to what extent electrical brain activity reflects pharmacologically altered states and contents of consciousness, is not well understood. Therefore, we investigated whether measures of evoked and spontaneous electroencephalographic (EEG) signal diversity are altered by sub-anaesthetic levels of ketamine compared to normal wakefulness, and how these measures relate to subjective experience. High-density 62-channel EEG was used to record spontaneous brain activity and responses evoked by transcranial magnetic stimulation (TMS) in 10 healthy volunteers before and during administration of sub-anaesthetic doses of ketamine in an open-label within-subject design. Evoked signal diversity was assessed using the perturbational complexity index (PCI), calculated from EEG responses to TMS perturbations. Signal diversity of spontaneous EEG, with eyes open and eyes closed, was assessed by Lempel Ziv complexity (LZc), amplitude coalition entropy (ACE), and synchrony coalition entropy (SCE). Although no significant difference was found in TMS-evoked complexity (PCI) between the sub-anaesthetic ketamine condition and normal wakefulness, all measures of spontaneous EEG signal diversity (LZc, ACE, SCE) showed significantly increased values in the sub-anaesthetic ketamine condition. This increase in signal diversity correlated with subjective assessment of altered states of consciousness. Moreover, spontaneous signal diversity was significantly higher when participants had eyes open compared to eyes closed, both during normal wakefulness and during influence of sub-anaesthetic ketamine. The results suggest that PCI and spontaneous signal diversity may reflect distinct, complementary aspects of changes in brain properties related to altered states of consciousness: the brain’s capacity for information integration, assessed by PCI, might be indicative of the brain’s ability to sustain consciousness, while spontaneous complexity, as measured by EEG signal diversity, may be indicative of the complexity of conscious content . Thus, sub-anaesthetic ketamine may increase the complexity of the conscious content and the brain activity underlying it, while the level or general capacity for consciousness remains largely unaffected.
Comparable studies
Other non-randomized and open-label trials on ketamine, most cited first.
Citations in the library
Cited by 53
- Consciousness among delta waves: a paradox? Brain March 6, 2021
- Consciousness and complexity: a consilience of evidence Neuroscience of Consciousness 2021
- Microdosing with psilocybin mushrooms: a double-blind placebo-controlled study Translational Psychiatry August 2, 2022
- TMS-EEG and resting-state EEG applied to altered states of consciousness: oscillations, complexity, and phenomenology. iScience May 19, 2023
- Are we really unconscious in "unconscious" states? Common assumptions revisited. Frontiers in Human Neuroscience 2022
and 48 more in the library
Cites 27
- Neural correlates of consciousness: progress and problems Nature Reviews Neuroscience April 20, 2016
- A Theoretically Based Index of Consciousness Independent of Sensory Processing and Behavior Science Translational Medicine August 14, 2013
- Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin Proceedings of the National Academy of Sciences January 23, 2012
- Psychometric Evaluation of the Altered States of Consciousness Rating Scale (OAV) PLoS One August 31, 2010
- The Standardized Psychometric Assessment of Altered States of Consciousness (ASCs) in Humans Pharmacopsychiatry July 1, 1998
and 22 more in the library