Neural architecture of human language: Hierarchical structure building is independent from working memory.

Using functional magnetic resonance imaging (fMRI), we show that the neural substrate of language does not overlap with that for verbal working memory when we carefully define verbal working memory in sentence processing. Object-Subject-Verb (OSV) sentences in Japanese were contrasted with canonical Subject-Object-Verb (SOV) sentences, which had less hierarchy in linguistic structure. This contrast revealed the posterior part of Broca's area and the left posterior middle temporal gyrus (pMTG) as the neural bases for hierarchical structure building. Furthermore, we changed verbal working memory load in OSV sentences by adding modifiers to the subject or object noun phrases; this resulted in the activation in the op9, which is situated in the frontal operculum and is adjacent to, but not situated in, Broca's area. The neuroanatomical segregation of language processing from verbal working memory suggests independence of the faculty of language from the verbal working memory system, providing evidence for the domain-specificity of language in human cognition.