Neural codes for visual numerosity independent of other quantities are present both in the dorsal and in the ventral stream of the human brain

Representing the number of items in sets (numerosity) is a core and evolutionary ancient ability. In a recent fMRI study Castaldi et al. (eLife 2019) showed that visual numerosity is represented independently from other visual features starting from early visual areas and progressively amplified along the dorsal stream hierarchy up to parietal areas. However, because in this study we recorded the brain activity from a restricted brain volume, and performed the analyses only in a series of retinotopically organized regions of interest along the dorsal stream, we still miss an exhaustive picture of the full network of regions encoding pure numerosity across the whole brain. In this research advance, we extend the findings of Castaldi et al. in two significant ways. First, we recorded the whole brain which allowed us to characterize the neural responses to numerosity beyond the dorsal stream using a searchlight analysis in addition to an ROIs approach. Second, in addition to the classical model-based analytical approach, such as RSA, we compared the neural representational geometries across the different regions using multi-dimensional scaling, an hypothesis-free approach capable of unveiling latent neural dimensions that might otherwise go unnoticed. Our results confirm that numerosity is represented over and above other visual features in early visual areas and progressively enhanced in associative areas of the dorsal stream but also, notably, of the ventral stream. We also found that numerosity representations in association areas differ substantially from those of early visual areas, and that in associative regions of both streams numerosity is represented on a similar manifold akin to a curved number line, a structure suggestive of their similar involvement in numerosity-based decision making. Taken together, these results call for an important revision of the existing neurocognitive models of number cognition. Impact statement Visual numerosity is represented in associative areas of dorsal and ventral streams on a neural manifold that also reflects decision variables, suggesting a common role in numerosity decision-making. ### Competing Interest Statement The authors have declared no competing interest.