The brain represents visual information in maps — adjacent neurons respond to adjacent parts of the visual field. This topographic principle is well established for space, motion, and color. Whether the brain represents abstract quantities the same way — whether there exists a spatial map for number itself — has been debated. Functional MRI at standard resolution could not resolve the question because the putative numerosity maps are small and overlap with other cortical representations.
Published in Communications Biology, researchers used ultra-high-field fMRI at 7 Tesla to map numerosity responses across both visual and auditory modalities in the human brain. They found topographically organized numerotopic maps — regions where adjacent neurons prefer adjacent numbers, following a logarithmic Gaussian tuning curve. Visual numerosity maps scattered across association cortices; auditory numerosity maps appeared in anatomically distinct locations in superior temporal and premotor cortices. Both modalities used the same logarithmic coding scheme despite being spatially separate.
The structural insight is about the relationship between the coding scheme and its instantiation. The logarithmic tuning — where the difference between 1 and 2 is represented as larger than the difference between 8 and 9 — is the same in both modalities. But the maps are anatomically distinct: visual numbers in one place, auditory numbers in another. The brain does not have a single number center that counts everything. It has multiple independent maps that happen to use the same mathematical encoding. The abstraction lives in the coding scheme, not in a shared neural substrate. The brain converges on the same representation without converging on the same location.