Magnetic resonance imaging findings for dyslexia: A review

Developmental dyslexia is a brain disorder that is associated with a disability to read, which affects both the behavior and the learning abilities of children. Recent advances in MRI techniques have enabled imaging of different brain structures and correlating the results to clinical findings. The goal of this paper is to cover these imaging studies in order to provide a better understanding of dyslexia and its associated brain abnormalities. In addition, this survey covers the noninvasive MRI-based diagnostics methods that can offer early detection of dyslexia. We focus on three MRI techniques: structural MRI, functional MRI, and diffusion tensor imaging. Structural MRI reveals dyslexia-associated volumetric and shape-based abnormalities in different brain structures (e.g., reduced grey matter volumes, decreased cerebral white matter gyrifications, increased corpus callosum size, and abnormal asymmetry of the cerebellum and planum temporale structures). Functional MRI reports abnormal activation patterns in dyslexia during reading operations (e.g., aggregated studies observed under-activations in the left hemisphere fusiform and supramarginal gyri and over-activation in the left cerebellum in dyslexic subjects compared with controls). Finally, diffusion tensor imaging reveals abnormal orientations in areas within the white matter micro-structures of dyslexic brains (e.g., aggregated studies reported a reduction of the fraction anisotropy values in bilateral areas within the white matter). Herein, we will discuss all of these MRI findings focusing on various aspects of implemented methodologies, testing databases, as well as the reported findings. Finally, the paper addresses the correlation between the MRI findings in the literature, various aspects of research challenges, and future trends in this active research field.