Late cortical dynamics mediate the symmetry-induced numerosity illusion

Numerosity perception enables us to quickly estimate the number of objects in a visual scene, a fundamental skill supporting efficient interaction with the environment. In this study, we tracked how the human brain transforms physical inputs into numerical percepts. Using EEG we measured the neural dynamics to assess whether numerosity is perceived directly or inferred from correlated non-numerical features, and how these signals are integrated with contextual cues that bias perception. We leveraged on a visual illusion in which dot arrays appear less numerous when arranged symmetrically rather than randomly. Participants viewed dot arrays of varying physical or perceived numerosity, illusorily altered by the dots spatial arrangement. Both univariate and multivariate analyses revealed that physical numerosity was decodable from occipitoparietal electrodes as early as [~]50 ms after stimulus onset, independently of low-level features such as dot size or convex hull. Spatial arrangement was represented later, from [~]150 ms, and perceived numerosity, biased by symmetry-induced grouping, emerged at a similar latency. These results indicate that early neural signals encode physical numerosity directly, whereas the symmetry-induced underestimation arises from later grouping processes that integrate spatial information to shape perceived numerosity.