The Hyperplasticity Hypothesis: Speech Encoding and Plasticity in Typical and Dyslexic Readers

Dyslexia is a prevalent developmental disorder that culminates in a reading impairment. Individuals with dyslexia fail to learn to read at grade level despite intelligence, motivation, adequate instruction, and regardless of socioeconomic status. There are many theories of dyslexia focused on impairments in vision, cerebellar function, sensory function, and deficits in automatization; notably the vast majority of dyslexia theories are focused on impairments in the use or access to phonological information. This dissertation focused on the role of auditory perception in adults with dyslexia. Theories of auditory impairment in dyslexia attempt to explain the results of static speech perception tasks. However, the adult perceptual system is dynamic: it must be flexible enough to handle variability from different talkers or accented speech, yet remain inflexible enough to segregate native speech from acoustic noise. The hyperplasticity hypothesis proposes that individuals with dyslexia have heighted auditory plasticity. The dyslexic auditory system reorganizes to a greater extent than a typical auditory system to accommodate auditory information. To test the theory of hyperplasticity in dyslexia, a dynamic speech perception paradigm was employed, namely the perceptual learning for speech task. Three overarching questions are addressed in this dissertation. First, how do individuals with dyslexia perform on a dynamic speech perception task? The hyperplasticity hypothesis predicted that dyslexics’ response to idiosyncratic speech stimuli would be excessively plastic, showing an increased acceptance of phonetic variation. Second, how does the auditory system of individuals with dyslexia adapt to accommodate a new variation in speech production (e.g. accent)? An electrophysiological measure, the complex Auditory Brainstem Responses (cABR) was used to predict outcome on the perceptual learning for speech experiment. Finally a byproduct of collecting the cABR was pre-exposure to the phonetic categorization tokens at the midpoint and endpoints. Therefore, it was crucial to ask: How does exposure alter the results of dynamic speech perception? Taken together, results of these experiments support the view that the auditory system of individuals with dyslexia may be hyperplastic, which could lead to pervasive differences in the way these individuals process speech