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Intonation Units in spontaneous speech evoke a neural response beyond speech acoustics
Poster C38 in Poster Session C, Wednesday, October 25, 10:15 am - 12:00 pm CEST, Espace Vieux-Port
Maya Inbar1, Shir Genzer1, Anat Perry1, Eitan Grossman1, Ayelet N. Landau1; 1Hebrew University of Jerusalem
Spontaneous speech is produced in chunks called Intonation Units (IUs; Chafe 1994). IUs are defined by a set of auditory cues, including changes in syllable delivery rate, resets in pitch level, resets in volume and pauses. IUs are presumably universal in human language, and we recently found that they have a consistent temporal structure across six languages with different grammatical and socio-cultural profiles (Inbar, Grossman and Landau, 2020). Linguistic theory suggests that IUs pace new material relative to the discourse, and as such serve as a window onto the dynamic focus of attention in speech processing. In this study, we identify a neural response unique to the boundary defined by the IU. We characterize the distinct contributions to this response of acoustic variation, on the one hand, and linguistic structuring, on the other. We measured the EEG of participants (N=50) who listened to different speakers recounting an emotional life event in Hebrew. We analyzed the stimuli prosodically into IUs, and categorized each word as either ending an IU or not. Additionally, we quantified an acoustically-based measure of prosodic boundary strength at each word. We modelled the neural response at each EEG channel and time point relative to word offset using a hierarchical GLM approach. We find that the EEG response to IU-final words differs from the response to IU-nonfinal words over and above the expected response for a given acoustic boundary strength. The EEG response at IU closure includes a negative deflection at right-anterior electrodes, starting as soon as the last word in the IU ends and lasting circa 200 ms. IU closure is further characterized by a centroparietal positive deflection between 150-500 ms after the last word in the IU. In addition, we find that stronger acoustic boundaries elicit a larger anterior negativity between 100-400 ms. This pattern of results resembles a known ERP component, the Closure Positive Shift (CPS; Steinhauer, Alter and Friederici, 1999). The CPS was found at prosodic phrase boundaries in several other languages using isolated, constructed sentences. To the best of our knowledge, this is the first study to characterize a CPS-like response to spontaneous speech during naturalistic listening conditions. Moreover, we accomplish this within the framework of functional linguistics, in which there is an established connection between the prosodic chunking of speech and the flow of information during communication. In addition, we address a debate in the CPS literature considering the extent to which the CPS reflects a linguistic structuring of the input beyond the bottom-up response to acoustic boundary cues. By explicitly modelling both contributions to the CPS, acoustic and linguistic, we find what appear to be two different components within the classical CPS. Finally, we discuss our findings in light of the body of research on rhythmic brain mechanisms in speech processing, and delineate the ways in which IU-related neural activity contributes to the previously characterized delta-band neural speech tracking.
Topic Areas: Prosody, Speech Perception