Speech processing under effortful listening conditions in a first and second language in younger and older bilinguals

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Despite an aging population and the importance of speech understanding, the factors that influence speech perception in bilinguals are understudied and virtually nothing is known about second language (L2) speech processing in older adults. In a first or native language (L1), speech understanding requires a complex set of processes that map degraded acoustic inputs onto stored representations of phonological and lexical information and integrate that information with long-term knowledge. When listening to an imperfect signal, additional cognitive resources (verbal working memory [WM], cognitive control, higher-order language information) are needed to understand speech. This task is even harder for a bilingual listener who has imperfect knowledge due to poorer phonological competence and lower vocabulary and syntactic knowledge in L2. Bilinguals vary in L2 competence, with factors such as L2 age of acquisition and proficiency playing important roles. My NSERC DG program will close critical gaps in the literature. I will examine L2 sentence understanding in large samples of younger and older English-French bilinguals with a range of L2 competence. I will use conditions that impede (phonological masking) or facilitate (context, visual cues) listening, and I will consider a full set of individual measures of top-down cognitive resources. I will measure behaviour, stages of processing using neural activity, and ratings of listening effort. Research Theme 1 will examine speech perception in noise using event-related brain potentials (ERPs; N400) and time-frequency EEG evoked by sentences varying in contextual constraint. I will determine the degree to which listeners use contextual cues to integrate words in L2, how this is influenced by listening condition, whether this differs with age, and whether patterns are moderated by individual differences in cognitive ability and listening effort. Research Theme 2 will examine the extent to which audiovisual (AV) speech cues facilitate sentence processing in L2. I will examine bottom-up (visual/hearing acuity, viseme and phoneme perception) and top-down factors (context, WM, processing speed). Study 1 will examine these factors as bilinguals perceive sentences in unimodal (visual- and auditory-only) and bimodal (AV) conditions in L1 and L2. I will determine the extent to which bilinguals benefit from AV speech cues in L2, over and above their ability to exploit these cues in L1. Study 2 will then test if AV speech cues benefit L2 verbal WM. This research will have a transformative impact by producing a comprehensive model of L2 speech understanding that accounts for complex listening conditions and individual differences. Given that many factors influence listening (noise, degraded speech during telecommunication, presbycusis, etc.), this work will have considerable relevance to the lives of everyday Canadians.