The brain basis of arithmetic, reading and reading disability

Reading and arithmetic, both foundational skills essential for classroom success, are symbolic systems that were invented relatively recently in regard to evolutionary history; so recently that the human brain has not evolved equipped with the ability to visually process, comprehend and manipulate these symbols. However, brain plasticity allows recycling of regions designed for object recognition, magnitude processing, and verbal language in order to master these skills. Through a series of functional and structural MRI studies, this dissertation examines (1) the neural substrates that are common to reading and arithmetic in typically developing children and adults, (2) the brain basis of arithmetic processing in children diagnosed with developmental dyslexia, and (3) the neuroanatomical profile of this reading disorder. In the first study, we show that the left inferior frontal gyrus, a region known to support verbal language and phonological awareness, is co-activated during specifically in reading and addition in adults, but not in children. Since addition is typically solved through retrieval-based strategies (but subtraction through procedural), this overlap suggests that language processing supports the acquisition of arithmetic fact retrieval. In a second study, we show that addition (not subtraction) results in hypoactivity in the left superior temporal and inferior frontal gyri in dyslexic compared to typical children. In a third and final study, we show that females with dyslexia have a unique neuroanatomical profile characterized by reductions in gray matter volume in primary sensory and motor cortices. This is strikingly different from what we and others see in males: less gray matter volume in regions known to support verbal language processing. Our findings suggest that models of the brain basis of dyslexia, primarily developed through the study of males, may not be appropriate for females and suggest a need for more sex-specific investigations into dyslexia. This work informs future research in cognitive neuroscience, and has implications for both the typical and struggling student.