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

Abstract

Syntax is the engine that allows us to create an infinitude of linguistic expressions, and the construction of syntactic structures, such as noun phrases and verb phrases, is considered a fundamental component of language processing. Nevertheless, insights concerning the neurobiological basis of syntax have remained elusive, in part because it is difficult to isolate syntax from semantic composition. Consequently, many studies of syntax have relied on meaningless artificial stimuli, such as jabberwocky expressions or artificial grammars. However, while pure manipulations of syntax are challenging to design, natural language grammars do have a sparse set of constructions presenting this 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 ms 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.

Neural basis of basic composition: what we have learned from the red–boat studies and their extensions

“…Summary of MEG results on various stages of language comprehension during reading. The LATL combinatory activity is situated in the middle of a cascade of temporally overlapping processes, including activation of argument structure representations in the angular and supramarginal gyri (dark blue, [63]), initial stages of retrieval operations in left inferior frontal cortex if the current item is the second element of a long-distance dependency (orange, [64]) and posterior temporal activity potentially reflecting more structurally based combinatory operations ( purple, [65]). This complex processing stage at 200-250 ms is preceded by visual word form recognition in the fusiform gyrus, including form-based decomposition of morphologically complex words (turquoise, [66]), and followed by N400 type lexical access effects in middle temporal cortex, reflected in MEG by so-called M350 activity (light green, [67]).…”

“…LATL corresponding to the work on composition discussed in this paper, middle temporal cortex representing effects of lexical level manipulations [67] and the most posterior centre a recent result aiming to vary syntactic composition while keeping constant the string of concepts described by open class items [65]. In our work, the left IFG (LIFG) has shown effects of retrieval during the processing of long-distance dependencies (red blob in figure 4; [64,70]), consistent with a large haemodynamic literature, but no effects of basic composition, at least with the analysis methods we have used so far.…”

Disentangling semantic composition and semantic association in the left temporal lobe

“…For example, the left pMTG in verb-noun combination shows the strongest model fit in the left middle temporal gyrus 21 , and both noun and verb phrases (e.g., "the frightened boy" and "frightened the boy") inducing greater activity in the superior temporal sulcus compared to word lists 22 . Moreover, post-nominal adjective-noun composition correlates with activity in the left medial to posterior temporal lobe when semantics is held constant 23 .…”

“…Compared with high-associative compositional phrases, low-associative compositional phrases form a newly-encountered meaning, hence likely involve more syntactic processing for a successful parsing. There is ample evidence that the LMTL supports syntactic combination [20][21][22][23] . For example, the left pMTG in verb-noun combination shows the strongest model fit in the left middle temporal gyrus 21 , and both noun and verb phrases (e.g., "the frightened boy" and "frightened the boy") inducing greater activity in the superior temporal sulcus compared to word lists 22 .…”

“…We restricted the regression analysis to a mask including the left angular gyrus (AG), the left ventromedial prefrontal cortex (vmPFC), the left middle temporal gyrus (LMTG), the left inferior frontal gyrus (LIFG), and the left medial temporal lobe. The left AG and vmPFC have also been implicated in previous literature on conceptual combination 1,18 , and the LIFG and the LMTG have been suggested to underlie syntactic combination [19][20][21][22][23] . The medial temporal lobe, on the other hand, has been held critical for associative memory in addition to the anterior temporal lobe [24][25][26][27][28][29] .…”