Stronger visual surround suppression under psilocybin: A psychophysical and EEG pilot study

Perception of visual contrast depends on the surrounding spatial context. Typically, the salience of a central target is reduced by a high contrast surrounding stimulus, an effect known as surround suppression. Although this phenomenon is well-studied, the role of specific neurotransmitter systems during surround suppression in human vision remains unclear. Psilocybin is a serotonin (5-HT2A) receptor agonist known to affect visual perception (e.g., psychedelic visual phenomena). We asked whether surround suppression may be altered by psilocybin. In a double-blind crossover pilot study, healthy adults completed psychophysical (n = 6) and electroencephalography (EEG; n = 5) measures of surround suppression after taking either 25 mg of psilocybin or placebo (100 mg niacin), in separate sessions. We found that psilocybin increased the strength of surround suppression, as measured in both our psychophysical contrast matching task, and in the strength of the visual N1 component from EEG. Accuracy on catch trials was not significantly impaired under psilocybin. Our results, although preliminary and limited by a small sample size, suggest that serotonergic neuromodulation plays a role in regulating the strength of surround suppression. Our findings may also be relevant for understanding differences in visual perception in psychiatric conditions, such as the weaker surround suppression reported in individuals undergoing major depressive episodes.