| The overarching goal of the dissertation was to further understanding of the relationship between callosal organization, functional lateralization, and interhemispheric communication in the healthy, young adult brain. Three converging methodologies were employed: diffusion tensor imaging (DTI) to assess callosal organization, functional magnetic resonance imaging (fMRI) to assess functional laterality, and behavioral measures of interhemispheric transfer times (IHTT). The first set of studies (1A--1C) explored the relationship between callosal organization and IHTT. It was expected that fractional anisotropy (FA), a measure of relative axon organization derived from DTI data, would negatively correlate with IHTT within specific callosal subregions, indicating that callosal subregions are functionally specific and that greater callosal organization enables faster interhemispheric integration. Significant correlations between FA and IHTT were observed in the expected callosal regions for all three studies and additional significant correlations were found throughout the corpus callosum. The direction of these correlations varied by gender and task demands. Thus, successful interhemispheric integration may depend upon a particular callosal subregion, but parallel communication occurs throughout the corpus callosum in a manner reflecting the temporal dynamics of intra- and inter-hemispheric processing and individual differences in callosal organization. The second set of studies (2A--2B) focused on the relationship between callosal organization and activity in the nonspecialized hemisphere, and the impact of nonspecialized cortical activity on performance of a lateralized task. It was expected that low callosal organization would be associated with greater activity in the non-dominant hemisphere during memory encoding of words (left hemisphere dominant) and faces (right hemisphere dominant), thus impairing subsequent recognition. As expected, during word encoding, the low callosal organization group exhibited bilateral inferior frontal lobe (IFL) activity. However, right IFL activity impaired word recognition of only high callosal organization individuals, suggesting that high callosal organization is associated with stronger functional lateralization. These results did not hold for the more difficult face encoding task; bilateral IFL activity during encoding occurred regardless of callosal organization. Thus, individual differences in callosal organization determine rates and routes of interhemispheric integration and influence functional lateralization. Combining DTI, fMRI, and behavioral IHTT measures promises to be a powerful method for examining callosal function in the intact, living brain. |
