Bimanuele Coördinatie en Switching Gedrag: de rol van veranderingen inhersenstructuur in achteruitgang van prestatie bij veroudering

Aging of the world population is a fact. Consequently, there is an increasing number of relatively healthy older adults that may need help during the execution of their daily tasks because of a loss of motor as well as cognitive skills. These older adults have partly lost their functional independence which can lead to a decreased quality of life. In the current thesis, it is investigated how alterations in brain structure with increasing age contribute to declines in specific skills. Brain structure was investigated using specific MRI (magnetic resonance imaging) and analysis techniques. The first part focuses on the relations between brain structure and performance on a motor oriented skill, namely bimanual coordination. The second part focuses on a cognitively oriented skill, namely switching between different tasks. Both skills are essential for many daily life activities. For example, imagine a typical morning: combing your hairs; buttoning your clothes; preparing your sandwiches. Besides, these bimanual skills are often interrupted by answering your phone or answering some questions of your relatives, thus requiring switching between tasks. The first part of this thesis comprises two experimental studies in which the associations between brain structure and bimanual coordination performance were investigated. The apparent simplicity of bimanual tasks masks their underlying complexity. Because of this complexity, declines in bimanual coordination are often observed in older adults. Accordingly, they often show slower and less accurate performance. During bimanual tasks, the coordination between the hands in the brain mainly occurs via the corpus callosum. This is a large bundle of white matter tracts that enables the connections between both hemispheres. In the first study, the microstructural organization of 7 subregions of the corpus callosum was investigated and related to performance on different bimanual tasks with specific task characteristics in a group of young and older adults. Accordingly, diffusion tensor imaging (DTI) was used to divide the corpus callosum into these subregions, based on connectivity with specific cortical regions. Results indicated that declines in microstructural organization in specific subregions of the corpus callosum were related to deterioration of bimanual performance on different tasks within the older adults. In a second study, it was investigated whether age-related atrophy in a selection of subcortical structures also contributes to deterioration of bimanual performance with aging. This was achieved by collecting structural scans from participants between 20 and 79 years old. Results of this study indicated that age-related atrophy of specific thalamic subregions contributed to bimanual performance declines. Interestingly, these thalamic subregions subserve connectivity with some key cortical regions in bimanual coordination, i.e., the premotor, the primary motor and the somatosensory cortical regions. Also task switching performance, i.e., the ability to switch between different tasks, has been shown to deteriorate with increasing age, thereby affecting daily functioning. In the second part of this thesis, the associations between white matter structural properties and task switching performance were investigated by means of an extensive literature overview and a third experimental study. To start, the associations between microstructural properties of different white matter connections and task switching performance as reported in different age and patient groups were discussed in the literature overview. Also the third (experimental) study was included in this overview. In this study, it was investigated how white matter structural alterations contribute to the slowing of switching performance with increasing age. In this regard, two white matter measures were selected that have been proposed to be related to myelin. The first measure was a radial diffusivity measure obtained using DTI, for which higher values could be indicative of less myelin content. The second measure was a magnetization transfer ratio measure obtained using MTI (magnetization transfer imaging), for which lower values could be indicative of less myelin content. From this study it was concluded that lower magnetization transfer ratio values in the superior corona radiata, a white matter tract that connects frontal and subcortical areas, partly explained the observed slowing of switching performance in older adults. In conclusion, in this thesis it was investigated how alterations in brain structure contribute to deterioration of performance with increasing age. It was found that both microstructural changes in the corpus callosum (white matter) and local atrophy in subcortical regions (gray matter) are related to deterioration of bimanual performance in older adults. Additionally, it was found that mainly structural alterations in a specific white matter tract, i.e., the superior corona radiata, seem to contribute to slowing of task switching performance in older adults. We express our hope that this thesis will contribute to a greater understanding of the aging process, with the ultimate aim of promoting prolonged independent functioning of older adults.status: publishe