Distinct neural representational geometries of numerosity in early visual and association regions of dorsal and ventral streams

Visual numerosity, a feature of visual sets traditionally associated with the parietal cortex, has recently been suggested to be encoded in a broader range of cortical regions, including early visual and associative cortices of both streams. However, a clear picture of how numerosity information is represented alongside other non-numeric features across the cortex is still lacking. To address this, we conducted a whole-brain 3T fMRI study with thirty-one adult subjects (eighteen females) engaged in a numerosity estimation task involving visual sets varying in number, item size, total item area, total field area, and density, thus allowing us to perform a tight control of the stimulus properties. Using model-based semipartial correlation representational similarity analyses, we found that numerosity is represented independently of other visual properties already in early visual areas and amplified in both retinotopic and non-retinotopic associative areas along both the dorsal and ventral streams. Dimensionality reduction analyses of BOLD activity patterns revealed clearly distinct neural representational geometries of numerosity across brain regions: a one-dimensional representation of numerical rank in early visual and in associative retinotopic regions of the ventral stream, and a curved structure encoding rank-order together with distance to end-point in associative regions of both the dorsal and ventral stream. These findings indicate that numerosity is represented over and above other visual features broadly in the brain but with substantially different neural coding schemes across regions, and are discussed in light of two non necessarily exclusive interpretations.