The Role of the Supplementary Motor Area in Bimanual Coordination and Aging: Insights into Neural Mechanisms and Motor Performance

Abstract

Aging is an inevitable part of life that affects bimanual coordination, which is crucial to successfully perform everyday activities such as getting dressed, preparing a meal or driving a car. Understanding how the brain changes with age is essential for preventing motor function decline and optimizing rehabilitation strategies. This PhD research aimed to offer new insights into the neural mechanisms underlying motor control and aging, with particular emphasis on the role of the supplementary motor area (SMA) in bimanual coordination. By employing a multimodal approach that combines magnetic resonance spectroscopy (MRS), functional magnetic resonance imaging (fMRI), and transcranial magnetic stimulation (TMS), this research advances our understanding of neurochemical excitatory–inhibitory (E–I) mechanisms, their relationship to brain activation, and the role of the SMA during bimanual motor performance in the aging brain. The first part of my PhD thesis showed that neurochemical E-I mechanisms seem to be altered in the SMA during aging and accompanied by bimanual coordination declines. Age-related differences of gamma-aminobutyric acid (GABA) and glutamate-glutamine complex (Glx) in the SMA were revealed, with lower metabolite concentrations in older adults compared to young adults. Yet, the relative E-I balance seemed to be preserved in aging. Despite no task-related modulation of metabolites in response to the bimanual tracking task (BTT), a positive relationship between excitatory mechanisms within SMA during task execution and successful motor performance was found in both age groups. These findings were complemented by the second part of my PhD, which demonstrated that task complexity affects bimanual coordination, with older adults showing greater performance declines. Nevertheless, both age groups exhibited a positive relationship between Glx modulation and brain activation, as measured by the blood-oxygen-level-dependent (BOLD) signal, during complex bimanual task demands. These results further underscore the role of excitatory, rather than inhibitory, mechanisms in the SMA across age groups, which may be driven by task complexity.