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PhD Defense - Matthew Leonard

A growing majority of the world’s population is able to speak and understand more than one language, yet most of our knowledge of how the brain processes words comes from monolingual speakers of a small number of languages.  Furthermore, most studies of both monolingual and bilingual individuals have utilized brain imaging methods that provide either excellent spatial or temporal resolution, but not both.  In aggregate, these studies have demonstrated that information is encoded in both local and regional brain networks across time, so it is therefore necessary to understand the specific spatiotemporal dynamics of these processes.  
This dissertation seeks to advance our knowledge of the various stages of word processing across languages in bilinguals and across the various modalities in which language occurs.  Using a multimodal imaging approach that combines the temporal resolution of magnetoencephalography (MEG) with the spatial resolution of magnetic resonance imaging (MRI), we focus on an early word form encoding stage and a later lexico-semantic stage in both the visual and auditory modalities.  
In two studies with bilingual participants, we show that the less proficient language (regardless of whether it was learned first or second) recruits an extended network of brain regions beginning during the earliest stages of word encoding (~150 ms for written words and ~100 ms for auditory words) and continuing through lexico-semantic processing (~400 ms).  
In a third study, we investigate the properties of a newly discovered neural response that occurs at ~100 ms to auditory words.  This response represents the encoding of acoustic information into a phonemic code, which can then be passed onto the classical lexico-semantic brain areas (and the extended network in less proficient languages) for integrating the word into the surrounding semantic context.  
These studies help clarify the importance of proficiency and experience in evaluating how bilinguals process each of their languages, which provides a unique perspective on the general process of language acquisition.  Furthermore, understanding the spatiotemporal dynamics of word encoding in both the visual and auditory modalities allows us to identify fundamental neural mechanisms that are modality- and stimulus- independent.