Psychedelics as a treatment for disorders of consciousness

Disorders of consciousness (DoC) — including the vegetative state (VS) and the minimally conscious state (MCS) — are severe outcomes of acquired brain injury in which wakefulness may be present but conscious awareness is absent or only minimally detectable. A variety of interventions have been tried, from pharmacological agents (e.g. zolpidem, amantadine) to invasive neuromodulation (deep brain stimulation, vagal nerve stimulation) and non-invasive stimulation (transcranial electrical or magnetic stimulation), but none have produced consistently reliable improvements in conscious awareness or functional recovery. Parallel to this, a resurgence of clinical research into classic psychedelics has produced findings relevant to basic and clinical questions about consciousness. Scott and colleagues propose exploring psilocybin as a candidate treatment to increase conscious awareness in patients with DoC. Their rationale rests on two recent and convergent empirical observations: measures of brain complexity (as indexed by metrics such as the perturbational-complexity index, PCI, and Lempel–Ziv complexity, LZC) appear to track levels of conscious awareness, and psychedelic drugs robustly increase these complexity measures in healthy humans. The authors frame a research agenda to test whether psilocybin-induced increases in brain complexity could translate into increased conscious content or detectable behavioural signs of awareness in DoC patients, while acknowledging substantial ethical and practical challenges.

Scott and colleagues survey conceptual, empirical and practical grounds for their proposal, beginning with a concise account of how ‘‘brain complexity’’ is operationalised and measured. Complexity is treated primarily in its ‘‘differentiation’’ aspect, measurable with entropy-like metrics; PCI quantifies the complexity of deterministic EEG responses to transcranial magnetic stimulation, and LZC quantifies compressibility or signal diversity in spontaneous EEG. Empirical work reviewed shows that these measures are high in wakefulness, reduced in unconscious states (non-REM sleep, anaesthesia) and lower in VS than in MCS patients, while patients with preserved awareness but impaired motor output (locked-in syndrome) retain high complexity values. They then summarise evidence that classic psychedelics raise measures of brain complexity above normal waking levels. Human studies with psilocybin, LSD and ketamine have reported elevated LZC and related metrics across EEG, magnetoencephalography and fMRI, and the magnitude of complexity increases has been correlated with the subjective intensity of the psychedelic experience. Turning to mechanisms, the authors highlight the role of 5-HT2A receptor agonism as the principal pharmacological action of psilocybin/psilocin. These receptors are densely expressed on high-level cortical regions (including default-mode network nodes) and on layer 5 pyramidal neurons that are important integrative elements of cortical circuitry; presynaptic 5-HT2A receptors at thalamo-cortical synapses may influence thalamo-frontal connectivity. Behavioural and animal evidence associating 5-HT2A agonism with enhanced cognitive flexibility and neural plasticity is presented as plausibly relevant to promoting richer conscious content. An important distinction in the authors' argument is between conscious content and arousal. They note that LZC and related entropy measures appear to index conscious content rather than general arousal: stimulant drugs that primarily increase arousal (e.g. amphetamines) have not been shown to raise complexity measures, whereas psychedelics do. From this perspective, increasing conscious content (richness of experience) may be more relevant to elevating awareness in DoC than simply increasing arousal. The paper also addresses conceptual limits of a unidimensional ‘‘level’’ model of consciousness. Recent calls for a multidimensional taxonomy of conscious capacities are acknowledged, and the authors recognise that measures such as PCI and LZC are unidimensional indices that may not capture the full heterogeneity of conscious states. They present their proposal as a test of whether increasing a complexity index translates into meaningful changes across other dimensions of consciousness. Ethical considerations receive extended treatment. The authors acknowledge the dual concerns of protecting patients who cannot consent while also pursuing research that could benefit this vulnerable group. They argue that ethically proportionate research is possible and note that modern clinical work with psychedelics has established safety frameworks. Psilocybin is characterised in the paper as having low toxicity and addiction potential; common acute effects emerge 30–60 minutes after oral dosing (10–25 mg), peak at 2–3 hours and largely abate by 6 hours, while intravenous administration produces a faster, shorter effect. The authors highlight the risk of transient distress or a ‘‘bad trip’’ but argue that such effects are rare under supported clinical conditions and could be mitigated by careful environmental control and the presence of familiar carers. Practical and scientific limitations are emphasised. The investigators caution that extensive neuronal loss in some DoC patients may preclude clinically meaningful benefit, and they express scepticism about the translational value of many animal acquired-injury models for this question. To bridge experimental uncertainty, they propose a staged research pathway: initial experiments in sedated healthy human volunteers (measuring LZC and/or PCI before and after psilocybin), sleep studies testing effects in non-REM sleep, and comparative studies against stimulants to disentangle effects on arousal versus content. For early-phase patient studies the authors recommend an incremental, adaptive design prioritising safety and signal detection. Key practical recommendations include dose-escalation (with reference to clinical dose ranges of 25–40 mg and a demonstrated physiological safety of 30 mg/70 kg in volunteers), exclusion of participants with a history of psychotic disorder or abnormal resting ECG, and careful management of serotonergic antidepressant use because of receptor downregulation and attenuated psychedelic responses. Neurophysiological monitoring is recommended as a minimum (continuous scalp EEG for offline LZC analysis), with PCI (EEG + TMS) included if practicable. Cardiorespiratory monitoring and behavioural observation for psychological distress are advised. Clinical outcome measures for later trials should be standard behavioural indices such as the Coma Recovery Scale–Revised (CRS‑R) or Wessex Head Injury Matrix, and the authors stress the importance of assessing covert awareness (e.g. command-following paradigms) if behavioural responsiveness is absent. Finally, the authors outline translational possibilities for later-stage research: combining psychedelic administration with rehabilitative care, exploring ‘‘micro‑dosing’’ schedules that prioritise drug‑assisted rehabilitation over acute psychedelic experiences, and using pre‑dose EEG features to prognosticate or predict treatment response. Throughout, they reiterate the exploratory nature of the proposal and the need for caution, rigorous monitoring and ethical oversight.

Scott and colleagues conclude that DoC encompass both a major scientific opportunity and substantial ethical and pragmatic challenges. They emphasise that recent evidence linking psychedelics to increased brain complexity, together with accumulating data supporting the safety of psilocybin in controlled settings, justify an open‑minded but cautious programme of research to test whether psychedelics can elevate conscious awareness in patients with DoC. The authors call for carefully designed, ethically proportionate studies to explore this possibility rather than for immediate clinical application.