Visual crowding involves delayed frontoparietal response and enhanced top‐down modulation

Crowding, the disrupted recognition of a peripheral target in the presence of nearby flankers, sets a fundamental limit on peripheral vision perception. Debates persist on whether the limit occurs at early visual cortices or is induced by top-down modulation, leaving the neural mechanism for visual crowding largely unclear. To resolve the debate, it is crucial to extract the neural signals elicited by the target from that by the target-flanker clutter, with high temporal resolution. To achieve this purpose, here we employed a temporal response function (TRF) approach to dissociate target-specific response from the overall electroencephalograph (EEG) recordings when the target was presented with (crowded) or without flankers (uncrowded) while subjects were performing a discrimination task on the peripherally presented target. Our results demonstrated two components in the target-specific contrast-tracking TRF response-an early component (100-170 ms) in occipital channels and a late component (210-450 ms) in frontoparietal channels. The late frontoparietal component, which was delayed in time under the crowded condition, was correlated with target discrimination performance, suggesting its involvement in visual crowding. Granger causality analysis further revealed stronger top-down modulation on the target stimulus under the crowded condition. Taken together, our findings support that crowding is associated with a top-down process which modulates the low-level sensory processing and delays the behavioral-relevant response in the high-level region.