Date Approved

12-17-2012

Date Posted

9-29-2014

Degree Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department or School

Psychology

Committee Member

Ellen Koch, Ph.D., Co-Chair

Committee Member

Renee Lajiness-O’Neill, Ph.D., Co-Chair

Committee Member

Susan Bowyer, Ph.D.

Committee Member

Thomas Schmitt, Ph.D.

Abstract

The aims of this dissertation are to 1) review the genetic, neurodevelopmental, structural, and functional brain imaging studies that are the foundations of our understanding of dyslexia and 2) investigate the pattern of activation and functional connectivity of neuronal networks critical in working memory in dyslexics by means of magnetoenchephalographic (MEG) coherence imaging. Dyslexics showed an early onset of activation in the precentral gyrus and the superior frontal gyrus, which differed from controls where activation was initiated in posterior cortical regions (supramarginal gyrus and superior temporal gyrus). Further, dyslexics showed lower normalized amplitudes of activation in the right superior temporal gyrus and right middle temporal gyrus than controls during a spatial working memory (SWM) task. In contrast, during a verbal working memory (VWM) task, dyslexics showed lower normalized amplitudes in the right insular cortex and right superior temporal gyrus and higher, likely compensatory, activation in the right fusiform gyrus, left parahippocampal gyrus, and left precentral gyrus. Dyslexics performing a SWM task showed significantly reduced MEG coherence and lower 1) right frontal connectivity, 2) right fronto-temporal connectivity, 3) left and right frontal connectivity, 4) left temporal and right frontal connectivity, and 5) left occipital and right frontal connectivity. MEG coherence by frequency band showed lower mean coherences in dyslexics than in controls at each frequency range and when the bands were combined during the SWM task. In contrast, during the VWM task, dyslexics showed a higher coherence in the low frequency range (1-15 Hz) and lower coherence in the high gamma frequency range (30-45 Hz) than controls. Logistic regression of the coherence by group membership was significant, with an overall predictive success of 84.4% (88.9% for controls and 77.8% for dyslexics). Coherence between the right lateral orbitofrontal gyrus and right middle orbitofrontal gyrus paired region substantially contributed to group membership. These findings deepen our understanding of the underlying pathophysiology of dyslexia, highlighting the importance of working memory circuits and prefrontal cortical dysregulation in this disorder. These results have far-reaching ramifications not only for prevention and early diagnosis, but also for the development of effective, evidence-based treatments and interventions.

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