Doctoral Dissertation Research: Cross-language transfer in voice onset time: A window into perceptual adaptation in brain and behavior

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Of all languages acquired as a second language, English has both the largest number of users (978 million) and the broadest geographic reach. In the U.S., approximately 22% of the population speaks both English and another language. Consequently, “foreign-accented” varieties of English are common. Foreign-accented varieties differ from “native-accented" varieties in several ways. This doctoral dissertation focuses on how speech sounds are pronounced by different speakers who speak English as a first or second language. Previous research has shown that listeners are able to adapt to the speech sounds of foreign-accented speech within a minute of exposure. However, how listeners acheive this rapid adaptation is still unknown. Understanding how listeners achieve this level of flexibility in speech perception is critical for informing not only linguistic theories of speech perception but also technological advances in automated speech recognition systems like the popular voice assistants Alexa and Siri. The present dissertation investigates the interface between second language speech production and first language speech perception in a series of behavioral and electrophysiological experiments.The doctoral dissertation research is guided by three research questions: (1) How does experience with Spanish-accented English facilitate later comprehension of Spanish-accented English speech? (2) How specific does this experience need to be? (3) What can time-sensitive neurocognitive measures of language processing such as electroencephalography--the measure of electrical activity at the scalp--illuminate about second language-accented speech perception? The ideal adapter framework provides a strong theoretical foundation from which to investigate the interface between second language (L2) speech production and first language (L1) speech perception. Under this framework, listeners are sensitive to the probability distributions between acoustic cues and phonetic categories. Listeners can learn the specific cue-category mappings that characterize a particular group of talkers in order to achieve robust perception. The present dissertation uses the cue-category mapping between voice onset time (VOT) and the voiceless stop consonants /p/, /t/, and /k/ as a test case for this hypothesis. Perceptual ambiguity between these voiceless stops and their voiced counterparts /b/, /d/, and /g/, respectively, due to cross-language transfer in Spanish-accented English provides a window into the adaptation process. The present dissertation uses cross-modal priming to measure adaptation to Spanish-accented /p t k/ both behaviorally and neurocognitively with electroencephalography (EEG/ERP). The researchers implement an innovative experimental design to compare the factors of (A) exposure to variability across multiple talkers with the same accent and (B) the level of similarity between the accented features encountered during the exposure phase and those encountered during the test phase. To date, these factors have not been compared directly, limiting our understanding of adaptation to L2-accented speech. In addition, the researchers use a single group of talkers across exposure conditions to control the type and amount of experience with each individual talker. Finally, this study is the first to investigate perceptual adaptation to L2-accented speech with EEG. Using this fine-grained, time-sensitive measure reveals the relative contributions of phonetic, phonological, and semantic information in resolving perceptual ambiguity in L2-accented speech.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.