The effects of the preferential 5-HT2A agonist psilocybin on prepulse inhibition of startle in healthy human volunteers depend on interstimulus interval

Prepulse inhibition (PPI) of the acoustic startle response (ASR) is an operational measure of sensorimotor gating: a weak non-startling prepulse reduces the magnitude of a subsequent startle reflex, and this gating process is thought to limit sensory overload and support selective attention. Reductions in PPI have been observed in schizophrenia and related disorders and are widely used in translational psychopharmacology to probe neurobiological mechanisms relevant to psychosis. Animal studies have shown that serotonergic hallucinogens acting at 5-HT2A receptors (for example DOI) reliably disrupt PPI in rats, whereas a prior small human study reported an increase in PPI after psilocybin, raising questions about cross-species comparability and the relevance of hallucinogen challenge models to schizophrenia. Vollenweider and colleagues set out to clarify these discrepant findings by testing dose-dependent effects of the mixed 5-HT2A/1A agonist psilocybin on PPI in healthy volunteers. The study tested PPI across a range of interstimulus intervals (ISIs) from short (30, 60 ms) to long (120–2000 ms) to capture both pre-attentive and attention-sensitive gating, and concomitantly assessed subjective altered states (5D-ASC) and sustained attention (Frankfurt Attention Inventory, FAIR). The authors hypothesised, based on animal literature, that psilocybin would reduce PPI in humans in a dose-dependent manner.

This double-blind, placebo-controlled, within-subjects study enrolled 20 volunteers screened for startle responsiveness; 16 subjects (nine female, seven male; mean age 26.4 years, range 21–32) who showed robust startle were included. Exclusion criteria included personal or first-degree family history of major psychiatric disorders and extreme scores on certain personality subscales; minimal prior recreational drug use was present in some participants. Women were tested in the first 5 days of the follicular phase to reduce menstrual-cycle effects on PPI. Ethical approval and regulatory authorisation for psilocybin were obtained. Each subject attended four experimental sessions in randomized, counterbalanced order separated by four weeks and received placebo and three doses of psilocybin reported as 115, 215, and 315 mg/kg in identical capsules. Sessions took place in a calm laboratory; subjects abstained from alcohol the day before and from food or caffeine for 2 h before testing. Psilocybin/placebo was administered at 0900 h. Startle testing occurred 90 and 165 min after dosing to sample peak and post-peak drug effects. The FAIR attention task was administered at 0, 105, 180, and 360 min, and the 5D-ASC ratings at about 125 and 200 min. The eye-blink component of the ASR was recorded by EMG (orbicularis oculi) with standard electrode placement and filtering. Sessions comprised 58 trials with three trial types: pulse-alone (PA; 115 dB, 40 ms), prepulse–pulse (PP) trials where a 86 dB, 20 ms prepulse preceded the pulse by one of five ISIs (30, 60, 120, 240, 2000 ms), and non-stimulus trials. Startle measures analysed were startle reactivity (PA magnitude), %habituation, and %PPI (percentage reduction of PA magnitude by PP trials). Trials with artefacts were excluded; subjects with >50% error trials would have been excluded but none of the 16 final participants met that criterion. Subjective effects were measured with the 5D-ASC, a 94-item visual-analogue scale covering Oceanic Boundlessness, Anxious Ego Dissolution, Visionary Restructuralization, Auditory Alterations, and Reduction of Vigilance. Sustained attention was measured by the FAIR, yielding indices of efficiency (E), performance capacity (P), performance quality (Q) and continuity (C). Statistical analyses used repeated-measures ANOVAs (three-way and two-way as appropriate) with treatment (placebo and three doses), session (peak/post-peak), and block or PP condition as within-subject factors; Tukey post hoc tests followed significant effects. Pearson correlations explored relationships between PPI and psychological/attentional measures. Significance was set at p < 0.05.

Subjective effects: Psilocybin produced dose-dependent altered states beginning 20–40 min after dosing, peaking around 60–90 min and resolving by 6 h. On the 5D-ASC, most dimensions increased dose-dependently at peak and to a lesser extent at post-peak assessment. During the peak phase, Oceanic Boundlessness (OB), Anxious Ego Dissolution (AED), Visionary Restructuralization (VR) and Reduction of Vigilance (RV) all showed significant increases, with reported effect sizes (Cohen’s d) for OB ranging ~0.76–1.76 across doses and similar sizable effects for other dimensions; Auditory Alterations increased but remained less prominent and no subject reported hearing voices. Vegetative side-effects (nausea, transient nervousness, vertigo, somnolence) were occasional; no persistent psychotropic effects were reported in follow-up. Sustained attention: Psilocybin impaired FAIR performance in a dose-dependent manner. The FAIR performance capacity score (P) and continuity score (C) were significantly reduced across low, medium and high doses during both peak and post-peak phases (peak effect sizes d ≈ 0.86–1.27), while the quality score (Q) declined significantly only after the high dose during peak (d ≈ 0.95). The efficiency score (E), reflecting comprehension of the task, was not significantly affected. Startle amplitude and habituation: Psilocybin did not significantly alter startle reactivity (PA magnitude) or habituation. There was a main effect of block consistent with habituation across the session, but no drug × block interaction and %habituation was unchanged across doses. PPI (%PPI): The core finding was an ISI-dependent, dose-related effect of psilocybin on %PPI assessed at the peak drug effect. During the first (peak) assessment, psilocybin reduced %PPI at the short 30 ms ISI (significant reductions for low, medium and high doses; effect sizes d ≈ 0.71–0.83), had no effect at the 60 ms ISI, and increased %PPI at longer ISIs. Specific significant increases were: 120 ms for the medium dose (d ≈ 0.63); 240 ms for all doses (d ≈ 0.40–0.90); and 2000 ms for medium and high doses (d ≈ 0.88 and 0.92, respectively). During the post-peak assessment the drug × PP-condition interaction was weaker, with only trends indicating the highest dose still reduced %PPI at 30 ms and increased PPI at 240 and 2000 ms. Correlations: Pooling peak-phase data across doses (n = 48 observations), %PPI at the 30 ms ISI correlated positively with attentional performance on the FAIR: P score (R = 0.50, p < 0.0003) and continuity C score (R = 0.49, p < 0.0004). No significant correlations were found between %PPI and the 5D-ASC dimensions or between %PPI at long ISIs and attentional or subjective measures (correlation coefficients elsewhere ranged from −0.06 to 0.18).

Vollenweider and colleagues interpret their results as showing that psilocybin exerts dual, ISI-dependent effects on human PPI: a disruption at a short, presumably pre-attentive 30 ms ISI and facilitation at longer, attention-sensitive ISIs (120–2000 ms), without altering baseline startle magnitude or habituation. The short-ISI PPI reduction correlated with impaired sustained attention, supporting a link between early sensorimotor gating deficits and higher-order attentional disturbance, a pattern the authors relate to observations in schizophrenia. The authors position these findings in relation to prior literature: the increase in PPI at longer ISIs replicates an earlier small human study with psilocybin, while the short-ISI disruption aligns with animal work showing 5-HT2A agonists (for example DOI) disrupt PPI. They highlight a cross-species complexity because DOI in rats tends to reduce PPI across ISIs, whereas psilocybin in humans shows opposite effects depending on ISI. Regarding mechanisms, the investigators suggest that psilocybin’s mixed pharmacology (strong 5-HT2A agonism and moderate 5-HT1A activity as reported) might explain the bidirectional, ISI-dependent effects: 5-HT2A stimulation in subcortical modulatory regions could underlie short-ISI disruptions, whereas concurrent 5-HT1A-mediated cortical effects might mask or reverse PPI deficits at longer ISIs. They also note that downstream interactions with glutamate and dopamine systems or species-specific receptor transduction differences could contribute. The authors acknowledge that the present study cannot resolve mechanistic questions and recommend pretreatment or receptor-blocking studies to test receptor-specific hypotheses. Limitations and further research suggested by the authors include the absence of direct mechanistic manipulation (for example 5-HT1A or 5-HT2A antagonists), the possibility that the highest dose tested may not have produced a fully psychotic-like state, and species differences that complicate extrapolation from rodent DOI studies. They propose that future work should employ graded ISI startle paradigms when comparing pharmacological effects across species and investigate higher doses and receptor-specific pretreatments to clarify sites and mechanisms of action. Finally, the study is presented as the first to model concurrently PPI deficits and sustained attention impairment with a hallucinogen in healthy humans, thereby lending support to the role of serotonergic systems in sensorimotor gating and its relation to cognitive dysfunction in schizophrenia.