Cortical representation of the constituent structure of sentences

TLDR

Sentences possess a hierarchical structure with nested constituents rather than being simple word strings. The study used fMRI to test whether neural representations of constituents grow with constituent size, as indexed by word count. Participants viewed 12-word or pseudoword streams while fMRI recorded activity, and researchers identified regions whose activation increased proportionally with constituent size. A left‑hemispheric network split into two subsets: inferior frontal and posterior temporal regions showed constituent‑size effects regardless of lexical content, indicating abstract syntactic processing, whereas temporal pole, anterior STS, and TPJ showed size effects only with lexical content, suggesting semantic constituent encoding; delayed activation in some inferior frontal and superior temporal areas further indicated that larger nested structures take longer to compute.

Abstract

Linguistic analyses suggest that sentences are not mere strings of words but possess a hierarchical structure with constituents nested inside each other. We used functional magnetic resonance imaging (fMRI) to search for the cerebral mechanisms of this theoretical construct. We hypothesized that the neural assembly that encodes a constituent grows with its size, which can be approximately indexed by the number of words it encompasses. We therefore searched for brain regions where activation increased parametrically with the size of linguistic constituents, in response to a visual stream always comprising 12 written words or pseudowords. The results isolated a network of left-hemispheric regions that could be dissociated into two major subsets. Inferior frontal and posterior temporal regions showed constituent size effects regardless of whether actual content words were present or were replaced by pseudowords (jabberwocky stimuli). This observation suggests that these areas operate autonomously of other language areas and can extract abstract syntactic frames based on function words and morphological information alone. On the other hand, regions in the temporal pole, anterior superior temporal sulcus and temporo-parietal junction showed constituent size effect only in the presence of lexico-semantic information, suggesting that they may encode semantic constituents. In several inferior frontal and superior temporal regions, activation was delayed in response to the largest constituent structures, suggesting that nested linguistic structures take increasingly longer time to be computed and that these delays can be measured with fMRI.