Functional dissociation of language and theory of mind in the developing superior temporal lobe

Language and theory of mind (ToM; the ability to infer others' mental states) are both crucial for human communication, and yet their developmental origins are unclear. Are their neural substrates distinct within the superior temporal lobe (STL) but with opposing lateralization, as in adults? Or do they emerge from common neural substrates during development, perhaps in homologous regions originally involved in more basic social processing? Here we investigate the development of this functional dissociation, and the dissociation of their underlying connectivity fingerprints in a large cohort of children (ages 3-9 years, n = 54 sessions, n = 42 subjects) and adults (n = 28). We demonstrate that children show distinct patterns of neural specificity for language and ToM in STL, just like adults. Children show no evidence of developmental 'disentangling' cross-sectionally or longitudinally. Finally, children's connectivity fingerprints predicting future language or ToM activation are almost identical to concurrent fingerprints and are largely non-overlapping across domains. While linguistic and ToM processing undergo continued neural specialization to reach the mature adult-like state, they are remarkably distinct early in human development. Our results challenge the idea that language develops from neural processors common for social communication and instead support distinct neural origins of these mental domains.