Suppressive interactions between nearby stimuli in visual cortex reflect crowding

Crowding is a phenomenon in which visual object identification is impaired by the close proximity of other stimuli. The neural processes leading to object recognition and its breakdown as seen in crowding are still debated. To assess how crowding affects the processing of stimuli in the visual cortex, we recorded steady-state visual evoked potentials (SSVEPs) elicited by flickering target and flanker stimuli while manipulating the spacing of these stimuli (Experiment 1) as well as target similarity (Experiment 2). Participants performed an orientation discrimination task while accuracy and speed of behavioural responses, along with frequency-tagged SSVEPs elicited by target and flanker stimuli, were recorded. Decreasing target-flanker distance reduced both behavioural performance and target-elicited SSVEP amplitudes. Estimates of the critical spacing, a measure of the spatial extent of crowding, from both behavioural data and SSVEP amplitudes were similar. Additionally, manipulating target similarity affected both measures in the same way. These findings establish a clear connection between the suppression of stimulus processing by nearby flankers in the visual cortex and crowding, and demonstrate the usefulness of SSVEPs in studying the cortical mechanisms of visual crowding.