Bimanual motor control: functional and structural brain measures in healthy subjects and traumatic brain injury patients

Summary In daily life, a large number of activities require the integrated use of both hands. For instance, bimanual motor control is needed to tie your shoelaces, to open a jar, to drive a car or to ride a bike. In order to accurately time and execute such movements, communication between both hemispheres via the corpus callosum (CC) is required. Therefore, an important element of the present thesis was the exploration of the specific role of the CC in bimanual coordination. In addition, bimanual motor control was studied in patients with traumatic brain injury (TBI). TBI patients often experience damage to the CC due to its specific location and composition, and generally demonstrate persistent motor control deficits for years following the injury. Here, we wanted to gain insights into injury-related bimanual coordination deficits as well as concomitant alterations in (sub)cortical motor systems. In part oneof the present thesis we first included a literature survey. This extensive overview revealed an essential role of the CC in bimanual coordination performance, and to some extent made it possible to map specific bimanual task characteristics onto different CC subregions, particularly regarding the type of sensory modality involved. In this literature reviewour first experimental study was included in which the microstructural organization of 7 CC subregions of healthy young adults was correlated with performance on a complex bimanual coordination task, performed either with or without augmented visual feedback. Diffusion tensor imaging (DTI) results demonstrated that bimanual coordination skills, particularlyin the absence of augmented visual feedback, were related to microstructural properties of the CC subsections connecting bilateral primary motor and bilateral occipital cortices. We concluded that in a group of healthy young adults, brain structure can predict variation in bimanual coordination performance. Subsequently, in the same group of healthy subjects, the added value of diffusion kurtosis imaging (DKI; providing a more complete picture of the white matter microstructural organization) relative to DTI was explored in relating white matter microstructure to bimanual performance. Although DKI resulted in higher absolute values of diffusion, and lower absolute values of anisotropy, associations between bimanual performance and white matter organization were comparable for bothdiffusion models. In the second part of the present thesis the focus shifted from the healthy brain to the traumatically injured brain. On the one hand we investigated TBI-induced grey matter volumetric alterations, and on the other hand TBI-induced functional (sub)cortical reorganization, both in relation to bimanual task performance. Structural imaging revealed enlarged ventricles and atrophy in overall and regional subcortical grey matter volume in TBI patients relative to controls. Behaviorally, TBI patients demonstrated poorer (i.e., slower and less accurate) bimanual performance than controls, which was significantly related to volume reductions in subcortical motor-related grey matter structures.Finally, we examined whether these bimanual coordination deficits were accompanied with altered brain function. Compared with controls, functional magnetic resonance imaging (fMRI) findings in TBI revealed reduced (sub)cortical activations during the planning phase of the movement, followed by more widespread and increased neural activity during the execution phase. It was suggested that TBI-induced altered planning-related activations might be interrelated with the nonselective recruitment of brain areas during execution, as well as with impaired motor control. In conclusion, the present doctoral thesis provided evidence for the important and rather specific role of the corpus callosum in bimanual coordination. With the introduction of new neuroimaging techniques, such as diffusion-weighted imaging, this brain structure-function relationship can now be described at a microstructural level, resulting in increased specificity. In addition, we demonstrated that TBI-induced persistent impairments in bimanual motor control were related to subcortical grey matter loss, and were accompanied by altered functional activations, as evidenced by fMRI. Ultimately, these findings may contribute indirectly to the improvement and establishment of new motor rehabilitation approaches following traumatic brain injury.status: publishe