Li L-Miao¹ (), Y. Emre Turkmen¹, Bilge Sayim¹; ¹École Normale Supérieure, PSL University, CNRS

A central question in numerosity perception is whether the visual system has a dedicated “number sense” that extracts numerosity directly from visual input. Numerosity perception is subject to a radial-tangential anisotropy: items arranged radially are perceived as less numerous than items arranged tangentially. Numerosity is usually underestimated. Redundancy masking (the reduction of reported numbers in repeating patterns; RM) has been proposed as a mechanism underlying both underestimation and this anisotropy; however, a direct link between RM and typical estimation-range numerosity perception has not been established. We generated radial and tangential arrangements by placing discs within an angular region extending from fixation, with the opening angle determining numerosity. Five reference numerosities (9, 13, 18, 24, 30) with probes at ±1–4, and a small-numerosity condition (3, 4, 5) with ±1 probes, were tested. Discs formed local triplets oriented radially or tangentially, with slightly larger spacing to the remaining discs. Each local triplet reproduced the classical RM stimulus, and larger numerosities were constructed from multiple triplets. On each trial, a radial and a tangential arrangement were presented to the left and right of fixation (one reference, one probe). Participants judged which side was more numerous. As expected, small numerosities showed a robust radial-tangential anisotropy. Importantly, large numerosities showed the same pattern: radial arrangements were consistently perceived as less numerous than tangential arrangements. The persistence of the anisotropy when multiple triplets mapped onto a larger space suggests that the numerosity difference (relative under/over-reporting in radial and tangential arrangements, respectively) observed for small numerosities generalizes to larger numerosities. Compression of visual information emerges with as few as three items and persists across larger numerosities, suggesting that limitations in spatial vision fundamentally shape perceived numerosity. While a dedicated number-sense mechanism may contribute, RM may be the primary mechanism driving underestimation and radial-tangential anisotropy in numerosity perception.

Acknowledgements: ANR-24-FRAL-0016-01