Enhanced visual contrast suppression during peak psilocybin effects: Psychophysical results from a pilot randomized controlled trial
This placebo-controlled study (n=6) investigates how psilocybin (25mg) affects visual surround suppression compared to placebo (100mg niacin). The study finds increased surround suppression effects under psilocybin, with stronger suppression correlating with more intense subjective visual effects.
Authors
- Swanson, L. R.
- Jungers, S.
- Varghese, R.
Published
Abstract
In visual perception, an effect known as surround suppression occurs wherein the apparent contrast of a center stimulus is reduced when it is presented within a higher-contrast surrounding stimulus. Many key aspects of visual perception involve surround suppression, yet the neuromodulatory processes involved remain unclear. Psilocybin is a serotonergic psychedelic compound known for its robust effects on visual perception, particularly texture, color, object, and motion perception. We asked whether surround suppression is altered under peak effects of psilocybin. Using a contrast-matching task with different center-surround stimulus configurations, we measured surround suppression after 25 mg of psilocybin compared with placebo (100 mg niacin). Data on harms were collected, and no serious adverse events were reported. After taking psilocybin, participants (n = 6) reported stronger surround suppression of perceived contrast compared to placebo. Furthermore, we found that the intensity of subjective psychedelic visuals induced by psilocybin correlated positively with the magnitude of surround suppression. We note the potential relevance of our findings for the field of psychiatry, given that studies have demonstrated weakened visual surround suppression in both major depressive disorder and schizophrenia. Our findings are thus relevant to understanding the visual effects of psilocybin, and the potential mechanisms of visual disruption in mental health disorders.
Research Summary of 'Enhanced visual contrast suppression during peak psilocybin effects: Psychophysical results from a pilot randomized controlled trial'
Introduction
Surround suppression is a well-established visual phenomenon in which the perceived contrast of a central stimulus is reduced by a higher-contrast surrounding stimulus. Although psychophysical studies have characterised surround suppression extensively, its neurochemical underpinnings are not well understood. Prior pharmacological probes have implicated GABAergic, cholinergic, adenosinergic, dopaminergic and noradrenergic systems, but the role of serotonergic signalling—particularly via 5-HT2A receptors implicated in psychedelic effects—has not been tested in humans. Psilocybin, a serotonergic psychedelic metabolised to psilocin and acting as a 5-HT2A agonist, produces dose-dependent alterations in visual phenomenology, yet psychophysical assessments of centre–surround interactions under psilocybin in people are lacking. This pilot study set out to measure whether peak effects of a high oral dose of psilocybin (25 mg) alter visual contrast surround suppression in healthy adults. Using a contrast-matching (two-alternative forced choice) task with three surround configurations (no surround, orthogonal surround, parallel surround), the investigators compared point-of-subjective-equality (PSE) under psilocybin versus an active placebo (100 mg niacin) in a within-subjects crossover design. The study also assessed subjective psychedelic effects with the 5D-ASC questionnaire and explored correlations between subjective visuals and changes in surround suppression. This work was performed as a small feasibility pilot prior to a larger trial and included safety monitoring for adverse events.
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Study Details
- Study Typeindividual
- Journal
- Compound
- Topics
- APA Citation
Swanson, L. R., Jungers, S., Varghese, R., Cullen, K. R., Evans, M. D., Nielson, J. L., & Schallmo, M. (2024). Enhanced visual contrast suppression during peak psilocybin effects: Psychophysical results from a pilot randomized controlled trial. Journal of Vision, 24(12), 5. https://doi.org/10.1167/jov.24.12.5
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