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Neural correlates of auditory word processing in infants and adults


For the majority of people, words are first learned and are communicated in high proportions in the auditory modality. However, the neural dynamics underlying speech perception are poorly understood. Even more limited, is knowledge of the neurophysiological processes and neuroanatomical structures that afford developing language abilities in infants. This dissertation investigates these issues in a series of related studies that are aimed at characterizing the spatial and temporal neural dynamics of auditory word processing in both developing 12-19 month old infants and adults. The first study, performed in adults, reveals new evidence for a neural response that is selective for auditory words, relative to acoustically- matched control sounds. This response appears to index a stage in speech processing wherein an incoming word sound is translated from an acoustic signal into a linguistically relevant code. This information can then be passed along the speech processing stream so that eventually the appropriate meaning of a word can be selected amongst representations stored within associative left fronto-temporal networks. The second study, performed in both adults and 12-18 month old infants, demonstrates that the neural mechanism responsible for encoding lexico-semantic word information has similar spatial and temporal characteristics in infants and adults. Prior work has not been able to establish whether infants and adults share similar neural substrates for language, and these findings suggest that the neurophysiological processes important for word understanding reside within similar neural networks throughout the lifespan. Finally, to gain a better understanding of the regional neuroanatomical changes that take place in the developing cortex of 12-19 month old- infants, the third study examines age-related changes tissues signal properties assessed with magnetic resonance imaging. This a period in development that is pivotal for emerging linguistic, cognitive and sensorimotor behaviors, however, the maturational changes that occur brain structures are poorly understood at these ages. This study reveals large changes in structural measures within precisely the specific areas that were demonstrated to generate lexico-semantic activity in study two. Together, these studies help to advance current understanding of neurophysiological processing stages and neural structures involved in auditory word processing in both the developing and mature brain. These findings invite a host of new studies that will continue to further knowledge of how speech processing is instantiated within the brain. Finally, with the use of multimodal imaging techniques such as those described in the present studies, there is increasing potential for new research aimed at understanding the neurobiological underpinning of language and other cognitive behaviors

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