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Serial and Parallel Combinatorics: An MEG investigation into lexical and compositional processing during parallel presentation
Poster Session E, Sunday, September 14, 11:00 am - 12:30 pm, Field House
Nigel Flower1, Liina Pylkkänen1; 1New York University
Language comprehension is often thought of as a serial, word-by-word process, as assumed in many theories of sentence processing. However, in reading, multiple words are visible at once, allowing us to ask to what degree processing becomes parallel when the stimulus itself has no temporal structure that unfolds serially over time. It has already been demonstrated that an N400 effect for a full sentence presented at once is timed similarly to classic N400s elicited for single words (Wen et al., 2019), showing no cost of reading multiple words. Further, neural reflexes of phrasal structure when glancing at short, written sentences are elicited as early as 130 ms (Fallon & Pylkkänen, 2024), which also is hard to account for under a serial model. In this MEG study (n = 24), we used rapid parallel visual presentation (RPVP) and directly focused on the serial vs parallel question. We first identified composition related neural activity by contrasting combinatory and non-combinatory expressions at three levels of length. Then, for each effect of length that was only observed for combinatory stimuli, we asked whether processing within it showed sequential or simultaneous effects of the lexical and bigram frequencies within the expression. The stimuli were full sentences (e.g., the cats are nice), shorter phrases (the cats) and bare determiners (the), as well as lists of one, two, or three plural nouns. Composition related signals were first identified in a broadly defined language mask. Then a two-stage GLM assessed sensitivity to lexical frequency (proxy for lexical access) and bigram frequency (proxy for compositional processing) within these regions. Effects of composition were observed in the left anterior temporal lobe (LATL) at 237–349 ms (p = 0.005) and at 355–418 ms (p = 0.043). GLM analysis revealed the first cluster was sensitive to the frequency of the first bigram of both four-word sentences and two-word phrases (p = 0.016), and the second to the second bigram of four-word sentences (p = 0.009)—suggesting serial composition in the LATL. A composition effect was also observed in the middle temporal gyrus at 303-339ms (p = 0.049). GLM results on this activity showed simultaneous lexical frequency effects for the first two words, but only in grammatical sentences . In sum, our results reveal serial composition in the LATL, and later lexical and combinatory processing occurring in parallel in the MTG. This pattern departs from the predictions of fully serial models, and instead indicates that the brain appears to recruit both serial and parallel mechanisms.
Topic Areas: Syntax and Combinatorial Semantics, Reading