BRAIN CONNECTIVITY, AGING AND MOTOR BEHAVIOR

Daily activities requiring simultaneous use of both hands are routine in human life. Buttoning a shirt, doing sports, playing a musical instrument, all require varying degrees of coordination expertise. In the absence of motor disorders, everyone is easily capable of buttoning their own shirt while only a minority is able to effortlessly play Beethoven's Moonlight Sonata. From birth, only a few innate motor behaviors are part of the human repertoire. Through practice and observation, this repertoire is remarkably expanded throughout the lifespan. As age increases, quality in motor performance noticeably decreases. Movements tend to be slower and less precise. Moving both limbs together - vital for successful execution of complex coordination - becomes increasingly more difficult. This decrease in motor performance is the result of many processes. In the past decades, a large body of evidence has been collected about brain changes as a function of aging and how they impact motor behavior. In spite of that, important questions about the aging motor system still remain. More specifically, the studies presented in this thesis investigated brain activity and functional network correlates of practice in the context of increasing age. Although age-related differences in motor learning have been previously investigated, past research focused mainly on simple tasks or short practice protocols. In the first study, a complex bimanual coordination task-set was employed to investigate the effects of extensive practice on age-related changes in motor performance and skill acquisition. After two weeks of practice, error reduction was evident and was even larger in older as compared to young adults. Although the performance gap decreased, error scores were still significantly higher in older adults following training. At the neural level, planning and execution of the bimanual task was associated with more neural recruitment in older in relation to younger adults. This difference in activation remained prevalent after the training intervention, despite overall training-induced reduction in brain activation. Finally, higher activity in prefrontal regions was associated with better subsequent performance gains in older adults. Thus, in older as compared to young adults, brain activity increased in terms of BOLD amplitude (larger amplitude) as well as extent (more brain regions active). In the second study, the temporal relationships among different brain regions were investigated using functional network analyses. We observed that cortical networks were less segregated in older relative to young adults during rest and actual performance of the bimanual task. In contrast to cortical networks, striatal and cortico-striatal connectivity was significantly weaker in the older group, especially during task execution. In networks which were more strongly interconnected in older adults, less connection rearrangements took place from rest to task execution (i.e. networks were less flexible). No significant associations between whole-brain connectivity or flexibility, and bimanual performance were detected. Nonetheless, flexibility and connectivity in prefrontal regions were associated with better performance, especially in older adults. Bimanual performance decreases in older age are more prominently observed in highly complex coordination modes. In other words, there is an interaction between age and task complexity. In the third study, functional network characteristics associated with the performance of bimanual tasks of varying complexity were examined in a group of participants of different ages (a lifespan perspective). Networks were broadly grouped according to task relevance (positive or negative). Aging was associated with reduced connectivity within task-positive and task-negative networks, and reduced segregation between them. Those changes in connectivity were observed to mediate age-related increase in inter-limb interference. Network rearrangements over task duration were less present with increasing age, though no strong association with behavior was detected. Altogether, we observed that increasing age was associated with decreased baseline bimanual performance, but not training-induced performance improvement. At the neural level, brain resource usage increased as a function of age. This extra resource usage was generally non-functional (i.e. did not lead to better performance), with the notable exception of prefrontal regions. In conjunction with age-related reduction of network flexibility, those findings indicate a reduced functional differentiation between various regions in the aging brain in relation to different task demands. This thesis provides valuable insights into age-related changes in the neural correlates task complexity and practice in the context of continuous bimanual coordination.status: publishe