| Dyslexic individuals are impaired on a range of low-level sensorimotor tasks. Several theories have been proposed to account for these deficits, including abnormalities in temporal processing, the magnocellular system and cerebellar or parietal lobe function. Behavioral and functional neuroimaging studies described in this dissertation investigated sensorimotor function in dyslexic and non-dyslexic individuals. First, in a behavioral study of a remediation program designed to improve auditory temporal processing skills, language-impaired children were shown to improve on a measure of auditory temporal processing but did not demonstrate concomitant improvements on reading-related skills. This result does not support the theory that improvements in temporal processing ability lead to improved reading skill. Second, a functional neuroimaging study was conducted in normal volunteers in which unimanual thumb flexion rate varied parametrically. A network of motor areas was engaged by these tasks, and the medial premotor corticostriatal loop demonstrated rate-related changes during movement of the right, but not the left, hand. This finding supports the hypothesis that the left hemisphere is specialized for the control of voluntary movement. In addition to elucidating the neural underpinnings of motor control, this study served as a pilot study for an investigation of motor control in dyslexic and non-dyslexic individuals. In a third study, dyslexic and non-dyslexic subjects performed unimanual left and right and bimanual synchronous and asynchronous movements. Behavioral performance was matched between the two groups, but analysis of the functional neuroimaging data revealed significant between-group differences. Dyslexic subjects had significantly greater task-related signal change in the right posterior putamen and inferior parietal lobule and significantly less in the left post-central gyrus. These data support the theory of a parietal lobe dysfunction in dyslexia. These results are discussed in the context of current theories that have been put forward to explain the behavioral manifestations of dyslexia. Our results lend some support to the parietal lobe dysfunction theory of dyslexia. |
