To identify the brain areas involved in syntagm formation, Christophe Pallier and I studied how brain activity varies with the complexity of syntactic structures. We created a set of stimuli with a fixed number of words, but varying phrase sizes. By measuring brain activity on functional MRI as participants read these sets of words, we identified a brain network whose activity increases with the size of the syntagms. This network comprises four regions located along the left superior temporal sulcus, two regions in the left inferior frontal gyrus (Broca's area) and the left putamen. A fraction of this network continues to activate when the content words are replaced by equivalent pseudowords, forming an experimental version of Lewis Carroll's famous poem Jabberwocky . Again, these were the pSTS and IFGtri areas, suggesting that these areas implement a syntactic computation that continues to function even in the absence of lexical-semantic information.
These results have been replicated and extended in numerous recent experiments, in oral and written language, and even in sign language. They lead us to distinguish two networks involved in the Combinatorics of language words. Syntactic combinations of two words (e.g. a pronoun and a conjugated verb, " he plays ") are sufficient to activate the core syntactic regions (pSTS+IFGtri). Semantic combinations, such as adjective + noun (" red boat ") or noun + noun (" plaid jacket "), activate the temporal pole or angular gyrus instead.
