UC San Diego

Speech is a highly-structured auditory signal that carries necessary and sufficient information for language comprehension among neurotypical, hearing populations. Speech perception, accordingly, involves the integration of both bottom-up auditory processing and top-down language knowledge. As explored in Chapter 1, however, while much is known about the brain's sensitivity to various acoustic and phonological manipulations, mechanistic transformative links between auditory sensation and linguistic perception remain poorly understood.

This dissertation contributes three studies to the ongoing pursuit of links between auditory and linguistic processing of speech in the brain. Intracranial stereo electroencephalography (sEEG) was recorded while participants listened to conversational English speech, and for each study, different sets of analyses were performed to target different aspects of these links.

Chapter 2 introduces a novel comparative technique for the isolation of phonological and morphological identity from the auditory speech signal. Through structured comparisons of the neural response to sounds in allophonic relationships with one another, sites that dissociate phonemic and morphological identity from acoustic similarity were identified. These results provide crucial evidence that the neural response to speech is sensitive to abstract sub-lexical structure.

Chapter 3 provides general models to account for the unique contributions of categorical phonemic information and spectrographic information to the neural response. In lower frequency bands, phonemic category information explained a greater proportion of the neural response variance than spectrographic information. These results indicate that the explanatory value of phonemic category features in neural encoding models is not reducible to the surface spectrotemporal features that correlate with those categories.

Chapter 4 proposes a model for the relationship between the speech stimulus and neural response that integrates both phonemic and spectrographic information. Inclusion of the stimulus covariance structure increased model accuracy only when categorical phonemic information was available, indicating that phoneme-related neural activity is likely mediated through the acoustic covariance structure of speech. Moreover, phonemic label information conferred no benefit to model fit when participants listened to speech in an unfamiliar language (Catalan).

Together these studies show that neural responses associated with categorical phonemic information are language specific and irreducible to speech acoustics, and they scout a path towards mechanistic, transformative links between auditory sensation and language cognition.