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The gradually growing cohort of older citizens is one of the most important socio-economic problems in todays society. It is crucial that older individuals continue to live independently for as long as possible. Therefore a thorough knowledge of pathologies, that may hinder the cognitive and motor functioning of older adults, is absolutely necessary. However, even in clinically healthy older individuals significant effects of age-related brain atrophy are present, that eventually may influence performance of daily-life activities. In the current project, the changes of the central nervous system that form the basis of age-related changes in motor control were studied in detail. Medical imaging techniques, such as functional Magnetic Resonance Imaging (fMRI) and Diffusion Tensor Imaging (DTI), were used to visualize brain activation and brain structure respectively.In Phase I, the parameters of age-related brain activation were established in the context of (a) ipsilateral hand-foot coordination according to an easy and a difficult movement pattern, and (b) single limb drawing movements, reflecting eye-hand coordination. The spatiotemporal characteristics of these tasks required integration of cognitive and motor brain regions, allowing unique insights into the differentiated activation patterns as a result of brain aging. The results showed that healthy older adults exhibited a significantly altered brain function. This was accompanied by an overall increased variability of movements. For both hand-foot and eye-hand coordination, old as compared to young subjects showed enhanced processing of sensory information and increased attention to action, resulting in increased brain activity in frontoparietal networks. With regard to hand-foot coordination a decreased age-related recruitment of the contralateral motor cortex, supplementary motor area and bilateral putamen was also found. We speculate that this could represent early degeneration of the motor loop of the basal ganglia. Although no behavioral differences were observed between execution of the hand-foot coordination with the left as opposed to the right body side, fMRI data showed that movements with the left body side were associated with more ipsilateral cortical and more contralateral cerebellar activation in both young and old subjects. With respect to eye-hand coordination, older adults, despite their generally increased brain activity, were still able to increase their brain activation additionally when a second task had to be performed simultaneously with the primary task, i.e. a dual-task condition. This may imply that the old brain still has some form of flexibility when cognitive demands increase. Furthermore it suggests that the residual brain capacity allowed older adults to perform the dual-task with minimal performance decrements when compared to single-task performance.In Phase II of the project, postural control was linked to functional and structural brain aging, which can be of high importance with respect to the prevention of falls in older adults. More specifically, postural control was tested under conditions of increased cognitive/visuospatial load. Subjects performed mental rotations of abstract figures either in a sitting or standing position. Additionally, brain activation related to these mental rotations was measured using fMRI. The data showed that on average old as compared to young individuals had more body sway when standing on a stable surface and their body sway increased when performing the mental rotations. However, older subjects with high lingual gyrus activation preserved or even optimized their visuospatial performance when postural load increased, i.e. when shifting from the seated to the standing position, whereas no such association was found in the young group. Older adults with a high degree of early visual processing may have chosen to apply an improved rotation solving strategy when standing upright. Additionally, we examined older subjects postural performance under conditions that required sensory reweighting by using the modified Sensory Organization Test (Neurocom International Inc.). This test systematically manipulated sensory inputs (i.e. proprioception of the ankle joint, vision, or both) while measuring a subjects ability to maintain equilibrium. The results showed that old as compared to young subjects had significantly lower postural control when proprioception of the ankle alone, or both proprioception of the ankle and visual feedback were manipulated. These results argued for increased difficulties with reweighting sensory inputs with aging. More importantly, brain-behaviour correlations showed that a decrease in the integrity of the frontal forceps of the corpus callosum was associated with decreased postural control, when proprioceptive feedback of the ankle was manipulated. When both proprioception of the ankle and visual feedback were manipulated, the integrity of the occipital forceps was predictive of balance performance. This may imply that, apart from corticocerebellar and vestibular tracts, intracortical tracts play an equally important role in postural control.In summary, this PhD project has provided new insights into the neural substrates underlying age-related changes in motor coordination. Despite the overall increased brain function it seems that some form of neural flexibility is maintained even in old age. Nevertheless more research is needed to relate altered brain structure to functional and behavioural changes, as well as to determine changes in functional connectivity. A complete understanding of the age-related neural correlates of motor control will only be possible when multiple neuroimaging analyses are combined with neuropsychological screening, kinematic analyses and other techniques (for example genotyping). Nevertheless from our results as well as from the current literature, it seems clear that when developing intervention strategies to improve general motor function and postural control in advanced age, it is important to focus as much on the central processing, as on the musculoskeletal aspects of motor control
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It is generally known that the cortical motor system of each hemisphere controls the movements executed with the contralateral body side, such that left motor areas control the right body side, while the right motor areas control the left body side. Interestingly, the primary motor cortex (M1) seems to be also activated when movements are executed with the ipsilateral hand. In this doctoral project, we have used several transcranial magnetic stimulation (TMS) paradigms to investigate which potential mechanisms underlie this ipsilateral facilitation of motor areas. Our first study showed that the cortical excitability in M1 increased when the ipsilateral hand performed rhythmical flexion-extension movements. This facilitation increased further when movements were performed in the context of a complex coordination task requiring that wrist and ankle were moved rhythmically into opposite directions. Furthermore, the facilitation revealed a hemispheric asymmetry, such that more facilitation was seen in the left M1 when the left body side was activated than in the right M1 when a movement was executed with the right body side. We corroborated these findings with our second study using a 'virtual lesion approach'. This TMS technique allows to test a causal relationship of a specific cortical area and a specific behavior. Subjects had to produce an unimanual tapping task while disruptive TMS was applied to the ipsilateral M1. In right handed subjects, more disruptions of the tapping task were observed when left than when right M1 was stimulated, showing that particularly the left ipsilateral M1 is involved in unimanual digit control. However, in left handed subjects we observed two subgroups, such that half of the left handed subjects exhibited more disruptions when the ipsilateral left hemisphere was stimulated while the other half showed more disruptions when the ipsilateral right hemisphere was stimulated. Overall, we showed that the important role of the ipsilateral M1 in unimanual movements is influenced by handedness as well as (left) hemispheric specialization.The potential mechanisms underlying the functional lateralization of the ipsilateral M1 can emerge at different levels of the nervous system. Hemispheric asymmetries of the ipsilateral M1 could either reflect asymmetries in direct uncrossed corticospinal fibers or in transcallosal pathways suggesting that the left M1 has a stronger influence on the right M1 than vice versa. We tried to directly test the second hypothesis, but found no support for the notion that hemispheric asymmetries arise predominantly via transcallosal pathways. Based on our results, we speculate that hemispheric asymmetries might arise from areas upstream from M1, such as the premotor cortex. To test this hypothesis, we explored asymmetries of the premotor involvement in coordination movements using a virtual lesion approach. In our third study we tested lateralization of the dorsal premotor cortex (PMd) during bimanual movements, and found more disruptions of a complex bimanual coordination patternwhen the right PMd than when the left PMd was stimulated. Moreover, this asymmetry was influenced by handedness, as left handed subjects were more frequently disrupted when the non-dominant left PMd was stimulated. In the last study, we applied TMS in a virtual lesion approach over the ventral (PMv) and dorsal (PMd) premotor cortex while right-handed subjects performed either a bimanual coordination task or unimanual movement with the hand ipsilateral to the stimulated hemisphere. We found that particularly the PMv exhibited hemispheric asymmetries in ipsilateral unimanual movements, such that the left PMv was stronger involved in the control of movements executed with the left hand than the right PMv in right handed movements. Moreover, this asymmetry was not seen in the PMd, although we demonstrated the same lateralization as in the 3rd study (i.e. more disruptions when right than when left PMd was stimulated). Overall, we suggest that the left PMv primarily influences the ipsilateral M1 to control movements of the ipsilateral left hand, while the right PMd primarily regulates the transcallosal effects on the non-dominant right M1 to prevent unwanted mirror movements.In summary, the present PhD thesis provides novel insights into the role of the hemisphere ipsilateral to the moving hand. We have demonstrated that the involvement of the ipsilateral motor areas exhibits hemispheric asymmetries which depend on the task that is executed, the investigated motor area and the subject's handedness
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