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Abstract Early mathematical development is thought to depend on visuospatial processing, yet neural evidence for this relationship in young children has been limited. We examined the neural mechanisms supporting numerical and visuospatial processing in 4- to 8-year-old children and adults using functional magnetic resonance imaging (fMRI), with three tasks: numerical matching, geometric shape matching, and number line estimation. We found that specialization for numerical and geometric processing in parietal cortex exists by 4–8 years of age, and that children exhibited greater conjunctive activation between numerical and geometric tasks throughout the parietal cortex compared to adults. During the number line task, children’s neural activity significantly overlapped with activity from both number and geometric shape matching tasks, whereas adults’ activity only overlapped with the number task. These findings provide the first neural evidence that number line estimation relies on both numerical and geometric processing in children, whereas it depends primarily on number-specific processing in adults. 

In industrialized groups, adults implicitly map numbers, time, and size onto space according to cultural practices like reading and counting (e.g., from left to right). Here, we tested the mental mappings of the Tsimane’, an indigenous population with few such cultural practices. Tsimane’ adults spatially arranged number, size, and time stimuli according to their relative magnitudes but showed no directional bias for any domain on any spatial axis; different mappings went in different directions, even in the same participant. These findings challenge claims that people have an innate left-to-right mapping of numbers and that these mappings arise from a domain-general magnitude system. Rather, the direction-specific mappings found in industrialized cultures may originate from direction-agnostic mappings that reflect the correlational structure of the natural world.