Functional imaging studies of the development of neural mechanisms for reading

Characterizing the neurobiological basis of cognitive development is critical to the investigation of developmental disorders of cognition. Functional neuroimaging techniques, such as positron emission tomography and functional magnetic resonance imaging, provide the opportunity to examine the neural mechanisms of cognitive development through study of adults, children, and subjects with neurological impairment. Reading, in particular, serves as an excellent model for cognitive skill acquisition. However, to date, variability between studies and methodological difficulties inherent in studying pediatric populations have precluded a complete rendering of the neural mechanisms of reading acquisition. This dissertation presents three complimentary studies which apply significant methodological advances to examine (1) the neural circuitry used by literate adults for reading, (2) the development of these neural systems in normal children, and (3) the neurobiological basis of precocious reading in a case of hyperlexia. These studies demonstrate that the mature adult circuitry for reading consists of a distributed network of cortical regions, including left superior temporal, infero-temporal, and inferior frontal cortices. Left posterior superior temporal cortex is engaged early in the course of reading acquisition, and its activity is modulated by children's phonological skills. Learning to read is associated with decreasing activation of right infero-temporal cortex and increasing activation of left inferior frontal and middle temporal cortices. A 9-year old hyperlexic subject demonstrated hyperactivation of the same left hemisphere inferior frontal and superior temporal phonological systems noted above, as well as the right infero-temporal areas which were developmentally disengaged by normal readers. Together, these findings suggest that the left superior temporal sulcus serves as a foundational area for phonological processes vital to reading acquisition. The left inferior frontal gyrus is recruited later in development for phonological aspects of reading, while right infero-temporal visual form processing mechanisms are disengaged. Despite its importance for reading in adults, no evidence was found for development of a left infero-temporal "visual word form area." These findings illustrate the importance of normative studies of cognitive skill acquisition for the interpretation of data from children with developmental cognitive disorders. Future investigations utilizing multiple imaging paradigms for longitudinal study of young children will significantly improve our understanding of the neural mechanisms of cognitive skill acquisition.