Neurophysiological dynamics of phrase-structure building during sentence processing

Abstract

Although sentences unfold sequentially, one word at a time, most linguistic theories propose that their underlying syntactic structure involves a tree of nested phrases rather than a linear sequence of words. Whether and how the brain builds such structures, however, remains largely unknown. Here, we used human intracranial recordings and visual word-by-word presentation of sentences and word lists to investigate how left-hemispheric brain activity varies during the formation of phrase structures. In a broad set of language-related areas, comprising multiple superior temporal and inferior frontal sites, high-gamma power increased with each successive word in a sentence but decreased suddenly whenever words could be merged into a phrase. Regression analyses showed that each additional word or multiword phrase contributed a similar amount of additional brain activity, providing evidence for a merge operation that applies equally to linguistic objects of arbitrary complexity. More superficial models of language, based solely on sequential transition probability over lexical and syntactic categories, only captured activity in the posterior middle temporal gyrus. Formal model comparison indicated that the model of multiword phrase construction provided a better fit than probabilitybased models at most sites in superior temporal and inferior frontal cortices. Activity in those regions was consistent with a neural implementation of a bottom-up or left-corner parser of the incoming language stream. Our results provide initial intracranial evidence for the neurophysiological reality of the merge operation postulated by linguists and suggest that the brain compresses syntactically wellformed sequences of words into a hierarchy of nested phrases.ost linguistic theories hold that the proper theoretical description of sentences is not a linear sequence of words, in the way we encounter it during reading or listening, but rather a hierarchical structure of nested phrases (1-4). Whether and how the brain encodes such nested structures during language comprehension, however, remains largely unknown. Brain-imaging studies of syntax have homed in on a narrow set of left-hemisphere areas (5-16), particularly the left superior temporal sulcus (STS) and inferior frontal gyrus (IFG), whose activation correlates with predictors of syntactic complexity (6,7,10,13,14). In particular, core syntax areas in left IFG and posterior STS (pSTS) show an increasing activation with the number of words that can be integrated into a well-formed phrase (10,14). Similarly, magneto-encephalography signals show increasing power in beta and theta bands during sentence-structure build-up (17) and a systematic phase locking to phrase structure in the low-frequency domain (18).These studies leave open the central question of whether and how neural populations in these brain areas create hierarchical phrase structures within each sentence. To address this question, intracranial recordings with more precise joint spatial and temporal resolution may be necess...

Probabilistic language models in cognitive neuroscience: Promises and pitfalls

“…Similar to the interpretations by Brennan et al (2016) and Nelson et al (2017), the authors interpreted the results as showing that hierarchical linguistic structure is computed during comprehension because it improves the fit to empirical data over competitive non-hierarchical models.…”

“…Current applications of probabilistic language models in cognitive neuroscience show that probabilistic language models can be used with hemodynamic and electrophysiological methods and allow researchers to investigate and focus on spatial fingerprints for specific linguistic computations in cortical regions (Willems et al, 2016;Henderson et al, 2016) or to compare predictions of different models against each other on the basis of same neurobiological data, be it fMRI time courses , language event-related M/EEG components (Frank et al, 2015;Wehbe et al, 2015), or spectral contents of electrophysiological signals (van Schijndel et al, 2015;Nelson et al, 2017). The studies employed language stimuli in both auditory and visual modalities and, with the exception of the studies by Frank et al (2015) and Nelson et al (2017), used language stimuli in naturalistic, narrative contexts.…”

“…The studies employed language stimuli in both auditory and visual modalities and, with the exception of the studies by Frank et al (2015) and Nelson et al (2017), used language stimuli in naturalistic, narrative contexts. We now turn to a more detailed discussion of specific advantages and disadvantages of the approach.…”

“…The effects of surprisal are in part consistent with the results by Willems et al (2016) who similarly report word surprisal effect in the posterior temporal lobe. Nelson et al (2017) investigated modulations of average high frequency (70-150 Hz) power in intracranially recorded electrophysiological signals by hypothesized syntactic phrase-structure building operations during a word-by-word sentence reading task. In model-comparison analysis, they contrasted explanatory power of nonprobabilistic hierarchical syntactic predictors (counting the number of open syntactic nodes at the moment when each word was presented) and probabilistic language models.…”

Showcasing the interaction of generative and emergent linguistic knowledge with case marker omission in spoken Japanese

“…A recent neurophysiological brain activity study (Nelson et al 2017) claims to have demonstrated the neurophysiological reality of phrase structure. In the authors' opinion, their results "strongly motivate the view of language as being hierarchically structured and make it increasingly implausible to maintain the opposite nonhierarchical view" (Nelson et al 2017: E3676).…”