Isolating syntax in natural language: MEG evidence for an early contribution of left posterior temporal cortex

Although the construction of syntactic structures is considered a fundamental component of language processing, insights concerning its neurobiological basis have remained elusive. This may be due, in part, to the inherent difficulty of isolating incremental syntax from other components of language, such as semantic composition. However, while pure manipulations of syntax are challenging to design, natural language grammars do have a sparse set of constructions presenting the possibility. Here we examined one such case, English post-nominal adjectives (mountain TALL enough for a strenuous hike), which were contrasted with semantically parallel but structurally simpler noun-adjective sequences in an MEG experiment. We observed a sharp activity increase in the left posterior temporal lobe (PTL) when syntactic composition was more straightforward, approximately 200 msec after adjective onset. The semantic fit between the noun and adjective was also varied, but this affected anterior temporal cortex, consistent with prior work. These findings offer a unique demonstration of the relevance of posterior temporal cortex for syntactic processing in natural language. We also present connectivity evidence that the syntax-related PTL responses were relayed to ipsilateral inferior frontal and anterior temporal regions. The combined results draw an initial picture of the rapid spatio-temporal dynamics of the syntactic and semantic composition network in sentence processing.