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Learning to produce new speech sequences (such as those in an unfamiliar word) is a central component of speech rehabilitation. However, the efficacy of therapy is limited by current gaps in our understanding of the brain mechanisms involved in speech sequence learning, particularly how phonological and motor contributions to learning interact. The aim of this dissertation is to apply neurocomputational models of speech to generate and test hypotheses regarding the neural mechanisms underlying speech motor programming and speech motor sequence learning in neurological disorders.
The first study applies the DIVA and GODIVA neurocomputational models to apraxia of speech, a motor programming disorder. This review identifies several brain regions where damage or dysfunction would give rise to the speech features observed in apraxia, including left ventral premotor cortex (vPMC) and left posterior inferior frontal sulcus (left pIFS), and generates model-based hypotheses as to exactly which facets of motor programming would be impaired based on lesions to these model locations.
The second study identifies behavioral and neural correlates of speech motor sequence learning using a non-native consonant cluster learning task in a cohort of patients with primary progressive aphasia (including progressive apraxia of speech) and related neurodegenerative disorders. Primary progressive aphasia is a category of neurodegenerative disease in which the location and degree of brain atrophy is closely tied to specific speech-language impairments in each individual, thus allowing for relatively precise localization of speech-language function. Results indicated a dissociation between speed and accuracy improvements, with distinct neural and behavioral correlates associated with each measure. Specifically, accuracy improvements were more strongly correlated with phonological working memory networks, including with cortical thickness in left pIFS. Duration improvements were instead dominated by sensorimotor contributions. However, left vPMC was associated with both speed and accuracy.
In combination, this work supports further investigation into the role of these two critical speech planning regions in speech motor sequence learning, and particularly how dysfunction in these regions may impact treatment outcomes in motor programing disorders like apraxia of speech.
