Inhibition of alpha oscillations through serotonin-2A receptor activation underlies the visual effects of ayahuasca in humans

Ayahuasca is a traditional Amazonian plant brew containing the psychedelic indole N,N-dimethyltryptamine (DMT) together with β-carboline monoamine-oxidase inhibitors (harmine, harmaline, tetrahydroharmine). Previous research has shown that ayahuasca produces robust subjective effects and broad-band reductions in spontaneous brain oscillatory power, particularly in the alpha band over posterior visual cortex; similar alpha reductions have been noted with other classical psychedelics such as psilocybin. However, while classical psychedelics are generally thought to act via the serotonin-2A (5-HT2A) receptor, the specific contribution of 5-HT2A activation to ayahuasca’s neurophysiological and psychological effects had not been experimentally tested in humans. Valle and colleagues designed a human pharmacological study to test the role of the 5-HT2A receptor in ayahuasca’s effects. The investigators hypothesised that despite ayahuasca’s multi-component chemistry, its characteristic psychedelic phenomenology and the decreases in cortical current density—especially in the alpha band—depend largely on 5-HT2A receptor activation. To test this, they combined oral ayahuasca with pretreatment using the selective 5-HT2A antagonist ketanserin in a double-blind, randomized crossover experiment and measured EEG oscillations, intracerebral current density estimates, and multiple subjective effect scales.

The study used a double-blind, randomised, balanced crossover design with four experimental sessions per participant, each one week apart. Twelve healthy volunteers (5 female, 7 male; mean age 35) with prior psychedelic experience (≥10 lifetime uses) completed all sessions. Exclusion criteria included current or past psychiatric disorders, substance dependence, significant illness and pregnancy. Participants abstained from psychoactive drugs for two weeks prior to the study and urine drug screens were performed on each experimental day. On each session day participants first received either placebo or 40 mg ketanserin; one hour later they received either encapsulated freeze-dried ayahuasca (individualised to 0.75 mg DMT/kg body weight) or placebo, yielding four treatment conditions: placebo+placebo, placebo+ayahuasca, ketanserin+placebo and ketanserin+ayahuasca. The freeze-dried ayahuasca batch was chemically characterised: per gram it contained 6.51 mg DMT, 13.14 mg harmine, 1.35 mg harmaline and 11.55 mg tetrahydroharmine; at the chosen DMT dose participants ingested approximately 1.51 mg/kg harmine, 0.16 mg/kg harmaline and 1.33 mg/kg THH. Participants remained in the laboratory for 8 hours; key measurements were taken at baseline and 90 minutes after the second treatment when ayahuasca effects were expected to peak. Electrophysiological recordings comprised 3-minute, eyes-closed EEG at 19 standard scalp locations. Signals were band-pass filtered (0.1–45 Hz), digitised at 250 Hz, and preprocessed to remove ocular and movement/muscle artifacts using blind source separation followed by automated epoch rejection; data were segmented into 5-second epochs. Power spectral density was computed using periodograms and averaged; absolute power was extracted for delta (0.5–3.5 Hz), theta (3.5–7.5 Hz), alpha (7.5–13 Hz) and beta (13–35 Hz) bands. Frequency variability was indexed by the centroid deviation (centre-of-gravity frequency) across 0.5–35 Hz. Three-dimensional intracerebral current density was estimated using standardized LORETA (sLORETA) constrained to 6,239 cortical grey matter voxels; analyses used the EEG cross-spectral matrix and squared current density values per voxel and frequency band. Subjective effects were assessed with multiple validated instruments: the Hallucinogen Rating Scale (HRS), the Addiction Research Center Inventory (ARCI), the Altered States of Consciousness questionnaire (APZ) and a battery of visual analogue scales (VAS) rating peak effects retrospectively. Statistical analyses followed International Pharmaco-EEG Group guidelines. Paired t-tests compared drug-induced EEG changes at the 90-minute time point versus baseline; sLORETA voxel-wise contrasts used baseline-corrected, log-transformed values with non-parametric permutation testing to control for multiple comparisons. Subjective measures were analysed with repeated-measures ANOVA across the four treatments, with post-hoc paired t-tests and Bonferroni correction where appropriate. Significance was set at p<0.05.

Twelve participants completed the four-session protocol. Ayahuasca (0.75 mg DMT/kg) produced widespread decreases in absolute EEG power across delta, theta and alpha bands relative to placebo; no significant changes were seen in the beta band. The alpha rhythm showed marked desynchronisation and increased frequency variability (centroid deviation). Ketanserin alone tended to produce effects opposite to ayahuasca: increases in frontal delta power and broader increases in theta, with minimal effects in alpha and beta. Pretreatment with ketanserin before ayahuasca not only offset the ayahuasca-induced delta and theta decreases but produced larger, widespread increases in these bands; importantly, ketanserin completely blocked the alpha-band decreases and normalised centroid deviation. sLORETA source analyses located the strongest ayahuasca-induced current density decreases in the alpha band to posterior regions: occipital, parietal and temporal cortices, with the maximal decrease in primary visual cortex (Brodmann area 18). Smaller alpha decreases also appeared in frontal regions. Theta decreases were localised mainly to lateral and medial frontal areas (peak in superior frontal gyrus, BA 10), while delta decreases were seen in temporal regions (peak in inferior temporal gyrus, BA 20). Ketanserin alone induced frontal theta increases (middle frontal gyrus, BA 6) but did not alter alpha or delta current density; when given prior to ayahuasca it abolished alpha decreases, reversed theta decreases into increases (peaking at precentral gyrus, BA 44), and converted delta decreases into increases in occipital and parietal areas (peak cuneus, BA 30). On subjective measures, ayahuasca significantly increased scores across most HRS subscales (Somaesthesia, Affect, Perception, Cognition, Intensity) except Volition. Ketanserin alone produced no significant subjective effects versus placebo. Ketanserin pretreatment substantially reduced several ayahuasca-induced HRS domains: Affect decreased by 62%, Perception by 56% and Intensity by 36%; however, Volition scores were paradoxically higher after the ketanserin+ayahuasca combination (≈76% increase relative to ayahuasca alone), reflecting greater incapacitation on that subscale. ARCI results showed ayahuasca increased MBG (euphoria), LSD (somato-dysphoric) and A (amphetamine-sensitive) subscales. Ketanserin pretreatment blocked the MBG increase by 80% and reduced the A subscale by 53%, bringing these scores to levels not significantly different from placebo. By contrast, ketanserin did not block the LSD subscale; scores remained elevated after the combination. Notably, ketanserin+ayahuasca produced marked increases in the PCAG (sedation) scale and reduced BG (perceived intellectual efficiency) below placebo. APZ scores increased with ayahuasca on Oceanic Boundlessness (OSE), Dread of Ego-Dissolution (AIA) and Visionary Restructuralization (VUS). Pretreatment with ketanserin reduced these increases (65% reduction for OSE, 55% for AIA, 44% for VUS), although VUS remained significantly above placebo, indicating residual visual phenomena. On VAS items, ayahuasca raised scores for most items except 'bad effects', 'fear' and 'loss of contact with external reality'. Ketanserin reduced several ayahuasca-related VAS scores: 'any effect' by 38%, 'good effects' by 62%, 'liking' by 61%, 'time' by 13%, 'feeling high' by 55%, 'changes in external reality' by 59% and 'visions' by 38%. Some items (e.g. 'any effect', 'time', 'feeling high', 'visions') remained significantly above placebo after the combination, indicating partial blockade. Correlation analyses linked neurophysiology and subjective experience. Greater decreases in alpha current density correlated with stronger VAS visual effects (r=-0.593, p=0.042) and trended with APZ-VUS (r=-0.512, p=0.059). Decreases in theta correlated with changes in VAS 'contact with external reality' (r=-0.591, p=0.043). Delta decreases correlated with changes in contact with external reality (r=-0.724, p=0.008). These correlations were attenuated and lost statistical significance when ketanserin preceded ayahuasca. The authors note two participant outliers who reported higher VAS-Visions after ketanserin+ayahuasca than after ayahuasca alone; removing them changed group means but did not fully account for the partial blockade.

Valle and colleagues interpret their findings as evidence that 5-HT2A receptor activation plays a central role in the visual and some affective effects of ayahuasca. Ayahuasca at the studied dose induced broad reductions in EEG power from delta to alpha, with the strongest alpha-band current density decreases localised to posterior visual cortex (BA 18). Those posterior alpha decreases correlated with subjective visual phenomena reported with eyes closed, consistent with the view that suppression of inhibitory alpha in visual networks increases cortical excitability and can underlie visionary experiences. Pretreatment with the 5-HT2A antagonist ketanserin blocked the alpha decreases, attenuated visual and affective subjective ratings, and weakened the correlations between alpha suppression and visual effects, supporting the hypothesis that 5-HT2A agonism mediates these neurophysiological and perceptual changes. The authors situate their results relative to prior work showing similar alpha reductions after psilocybin and other psychedelics, and to neuroimaging evidence linking reduced alpha power to increased regional metabolism and BOLD signal. They report concordant source-localisation findings in parietotemporo-occipital regions and, to a lesser extent, medial prefrontal/anterior cingulate areas; these regions are implicated in visual processing, affect and cognition and may relate to putative therapeutic mechanisms of ayahuasca. The blockade by ketanserin was partial rather than complete, which the investigators discuss in light of differences between DMT and other psychedelics. They note possible contributions from non-5-HT2A mechanisms of DMT such as 5-HT1A agonism, TAAR agonism, substrate activity at serotonin and vesicular monoamine transporters, and sigma-1 receptor modulation. These alternative mechanisms could explain residual subjective effects and the sedative profile observed with the ketanserin+ayahuasca combination (for example, increased ARCI-PCAG scores). Two participants showed atypical increases in visual scores after the combination, and removing them reduced but did not eliminate residual visual reports, further emphasising partial blockade. Key limitations acknowledged by the authors include the selective recruitment of experienced psychedelic users, and incomplete detail in the extraction about precise histories of other drug use; this constrains generalisability. The investigators recommend future studies to assess the contributions of other molecular targets—particularly sigma-1 receptor agonism—to the perceptual, affective and cognitive effects of DMT and ayahuasca. In sum, the data indicate that 5-HT2A activation is a prominent contributor to ayahuasca-induced alpha suppression in posterior cortex and to its visual phenomenology, though other mechanisms likely also participate.