Number words recruit numerosity-related cortex in 3- to 5-year-old children.

Learning to map number words onto their ordinal and quantitative meanings is critical in the acquisition of formal mathematics. Previous neuroimaging work in adults suggests that the intraparietal sulcus (IPS) represents verbal, symbolic number (i.e., number words) as well as non-verbal, non-symbolic numerosity (i.e., number of items in an array), but it is unclear when this overlap emerges in development. If regions that represent number words rely on representations of numerosity, then there should be functional overlap between number word and numerosity representations in the brain, even in children as young as 3-5 years of age. Using fMRI, we compared neural responses to number word sequences in an auditory task with neural responses to numerosity in a visual task. Across multiple whole-brain analyses, we found that the bilateral IPS is involved in perception of number word sequences, even in very young children. Moreover, we found that child-to-adult neural synchrony during the auditory task was significantly correlated with children's formal math ability. Lastly, we found significant overlap between auditory number word processing and visual numerosity processing, suggesting that even early in development, similar neural substrates are involved in both verbal, symbolic number, and non-verbal, non-symbolic numerosity.