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

Karami A., Castaldi E., Eger E., Piazza M.

Commun Biol. 2025 Jul 9;8(1):1029. doi: 10.1038/s42003-025-08395-z. PMID: 40634477; PMCID: PMC12241354. https://pmc.ncbi.nlm.nih.gov/articles/PMC12241354/

Visual numerosity, traditionally linked to the parietal cortex, is now thought to be represented across a broader cortical network, including early visual and associative areas in both streams. However, how numerosity is encoded relative to other visual features remains unclear. We conducted a whole-brain functional magnetic resonance imaging (fMRI) study with thirty-one adults performing a numerosity estimation task on visual sets varying in number, item size, total item area, field area, and density, ensuring tight stimulus control. Using model-based representational similarity analyses, we found numerosity represented independently of other visual properties in early visual areas and amplified in retinotopic and non-retinotopic associative regions across both streams. Dimensionality reduction of BOLD patterns revealed distinct geometries: a one-dimensional representation of numerical rank in early visual and ventral retinotopic areas, and a curved structure encoding rank and distance-to-endpoints in associative dorsal and ventral regions. These results demonstrate distinct neural coding schemes for numerosity across cortical regions.