In the neurobiology of syntax, a methodological challenge is to vary syntax while holding semantics constant. Changes in syntactic structure usually correlate with changes in meaning. We approached this challenge from a new angle. We deployed word lists-typically, the unstructured control in studies of syntax-as both test and control stimuli. Three-noun lists ("lamps, dolls, guitars") were embedded in sentences ("The eccentric man hoarded lamps, dolls, guitars...") and in longer lists ("forks, pen, toilet, rodeo, lamps, dolls, guitars..."). This allowed us to minimize contributions from lexical semantics and local phrasal combinatorics: The same words occurred in both conditions, and in neither case did the list items locally compose into phrases (e.g., "lamps" and "dolls" do not form a phrase). Crucially, the list partakes in a syntactic tree in one case but not the other. Lists-in-sentences increased source-localized MEG activity at ;250-300 ms from each of the list item onsets in the left inferior frontal cortex, at ;300-350 ms in the left anterior temporal lobe and, most reliably, at ;330-400 ms in left posterior temporal cortex. In contrast, the main effects of semantic association strength, which we also varied, localized in the left temporoparietal cortex, with high associations increasing activity at ;400 ms. This dissociation offers a novel characterization of the structure versus word meaning contrast in the brain: The frontotemporal network that is familiar from studies of sentence processing can be driven by the sheer presence of global sentence structure, while associative semantics has a more posterior neural signature.
- Word lists
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