The overarching objective of the thesis was to investigate the morphological changes in the corpus callosum (CC) in aging and dementia in relation to its role in cognitive and motor decline. The CC is the largest white matter tract in the brain, containing upwards of 200 million axons, and is believed important for communication and interaction between the two cerebral hemispheres.....To further elucidate the role of CC atrophy in dementia, we examined a population of 329 elderly subjects, and found that a higher rate of tissue loss in posterior CC is associated with an increased risk of dementia. This study represents the first to examine CC in elderly subjects longitudinally. In the same cohort, we investigated whether impairment in specific cognitive domains was associated with CC tissue loss. Previous studies had shown that processing speed and executive functions may be particularly reliant on the CC. Our findings indicated that CC tissue loss leads to selective impairment of processing speed but not memory or executive function deficits. Finally, CC tissue loss was also associated with impairment of motor function.....