Psilocybin alters visual contextual computations

Introduction

Psilocybin is a serotonergic classical psychedelic known to alter perception and large-scale brain dynamics, yet the underlying computational changes remain unclear. Contextual computations—how local sensory inputs are integrated with surrounding information—are ubiquitous across sensory and cognitive systems and have been mechanistically linked to phenomena such as divisive normalization and surround suppression in visual cortex. Earlier work has related contextual visual illusions, such as the Ebbinghaus illusion, to properties of primary visual cortex and to circuit-level mechanisms that could be relevant to broader cognitive effects of psychedelics. Aqil and colleagues set out to test whether psilocybin alters visual-contextual computations at both behavioural and neural levels, and to capture those changes with an explicit computational model. They used a randomised, double-blind, placebo-controlled crossover design administering placebo, 5mg and 10mg psilocybin, and combined psychophysical testing of the Ebbinghaus illusion with ultra-high-field (7T) fMRI mapping and population receptive field (pRF) modelling. The study aimed to determine whether psilocybin modifies contextual perception and contextual modulation of cortical responses, and whether a single-timecourse computational model could link these effects.