Differential Contributions of Transcallosal Sensorimotor Fiber Tract Structure and Neurophysiologic Function to Manual Motor Control in Young and Older Adults.

Abstract

Consider tying your shoes, one of the most automatic movements an adult performs. Each hand works independently during this task to accomplish a unified goal. With advanced age comes a decline in motor control affecting the ability of older adults to perform such activities of daily living. Specifically, older adults show pronounced deficits in the ability to perform tasks with both hands. I investigated whether age-related declines in callosal microstructural integrity and inhibitory function contribute to age differences in the ability to perform such tasks. In the first study I determined the relationship between corpus callosum microstructural integrity and interhemispheric inhibition in young adults. I found a positive relationship between interhemispheric inhibition and microstructure of interhemispheric fibers that was specific to tracts connecting the primary motor cortices. My second study revealed that young adults with greater interhemispheric inhibition had reduced motor overflow during a unimanual force production task; however these same individuals had the poorest performance during a bimanual independent force production task. I suggest that a high capacity for interhemispheric inhibition from one motor cortex to another can effectively prevent motor overflow during unimanual tasks, however it also limits the ability for optimal control during independent bimanual tasks, possibly due to a reduced capability for interhemispheric cooperation. My third study determined whether age reductions in callosal structure and inhibitory function underlie impairments in independent bimanual control. I found that better microstructure of callosal tracts connecting the two primary motor cortices was positively related to bimanual task performance in older adults, but negatively related to performance in young adults. Further, increased interhemispheric inhibition was related to poorer bimanual task performance in older adults across all tasks, whereas this relationship was only observed in young adults for the independent bimanual task. Collectively, the results of my dissertation have identified age reductions in callosal structure and their resultant impact on neurophysiological function and manual motor control. These studies provide a mechanistic understanding that can be leveraged for the design of targeted training interventions that will allow individuals with dysfunction of interhemispheric inhibition, to maintain independence and improve their quality of life.