Spatial grouping modulates the link between individual alpha frequency and temporal integration windows in crowding

Previous research has linked endogenous alpha oscillations ([~]7-13 Hz) to temporal integration windows in visual perception, with higher individual alpha frequency (IAF) predicting improved temporal segregation. Here, we investigated whether alpha-rhythmic temporal integration is a factor in visual crowding and whether this relationship is mediated by spatial grouping mechanisms. 47 participants performed a Vernier discrimination task, in which we manipulated both the stimulus onset asynchrony (SOA) between flankers and targets, and the spatial configuration of the flankers. Specifically, flankers were arranged to either induce crowding or "uncrowding", through the manipulation of good-Gestalt properties. Our results show that crowding has a temporal integration period of around 170 ms but this varies substantially across individuals. Importantly, resting-state IAF predicted individual variance in temporal integration windows: individuals with faster endogenous alpha rhythms could begin to segregate targets from distractors at shorter SOAs. Crucially, this effect was specific for crowding-inducing flankers and disappeared when flankers led to uncrowding. These results suggest that top-down spatial grouping can overwrite the temporal integration constraint imposed by alpha oscillations, highlighting both the relevance of alpha for understanding limits on peripheral visual processing as well as the flexible and context-dependent role of alpha in temporal integration. SIGNIFICANCE STATEMENTThe frequency of alpha-band activity varies across individuals and has previously been linked to temporal integration windows for low-level stimulus properties (e.g., flashes). However, the relevance of individual alpha frequency (IAF) for everyday perception is less clear. We tested whether IAF predicts temporal integration in spatial crowding, a phenomenon that strongly limits perceptual abilities. We found that individuals with higher IAF were better able to spatially segregate targets from flankers at shorter flanker-target offsets, indicating that they spatially integrated the flankers over a smaller temporal window. Interestingly, this effect disappeared when crowding was alleviated by flanker configurations following good-Gestalt principles. Our results suggest that bottom-up mediated spatio-temporal constraints on perception are linked to alpha oscillations but can be overridden by top-down spatial cues.