Author:

Keywords:

laterlaization, brain

Abstract:

The brain is an exquisitely malleable organ undergoing many changes in its functional and structural make-up throughout development, maturation, learning, ageing and disease. These neuroplastic changes and their implications for learning, rehabilitation and pathological conditions have become an increasing source of fascination to researchers. Recently, the advent of Magnetic Resonance Imaging has provided the research community with state-of-the-art techniques to study neuroplasticity without the need for invasive procedures. For the project under study, we joined two ends of the neuroplasticity spectrum: on the one hand, we looked at neurodegeneration in the context of healthy ageing, and on the other, we studied functional remodelling in the context of learning. Focusing on the motor system, an in-depth knowledge on lateralisation and long-term plasticity was acquired. Due to the lateralised organisation of the motor system, studying motor plasticity comes with specific challenges. In chapter two, we investigated this hemispheric specialisation associated with a motor task, first, in the context of right-lateralised visuospatial attention, and, second, in the context of left-lateralised motor dominance. For the first, we questioned whether the currently described role for visuospatial attention in the right hemisphere can be extended to spatial attention during a motor task. As a result, we found that a right-lateralised ventral attention network was also engaged during our ongoing motor task in space, irrespective of which effector executes the task. This led us to expand the description of right-lateralised spatial attention to include goal-directed motor behaviour in space. For the second, we identified which areas exhibited a relatively increased neural response in the left hemisphere during ipsilateral left hand movements than in the right hemisphere during right movements. Further, we examined whether this relatively enhanced ipsilateral activation is caused by asymmetrical hand dexterity or handedness, task complexity or asymmetries in transcallosal information transfer as measured by TMS. As a result, an asymmetrical BOLD response was conveyed for left premotor and parietal regions. The ipsilateral response in these regions did not reveal a direct causal relationship with any of these factors. The grey matter degeneration which typifies ageing has received increasing interest in the last decades. Although this decline ultimately affects the entire brain, it shows a preference and different trajectory for certain regions. Similar to this topological heterogeneity, also the analysis procedures used to assess such tissue loss may differ widely. Therefore, in chapter 3, we applied a voxel-based and a label-based procedure for quantifying the neural degeneration which accompanies ageing. Both methods converged in finding grey matter loss for areas most saliently affected by healthy ageing such as the frontal lobe. By contrast, the voxel-based approach diverged from the volumetric approach when smaller structures were concerned or when grey matter was included as a covariate. Moreover, the voxelwise approach overestimated the grey matter degeneration in regions prone to spatial misregistration such as the insula. These results emphasise the need for a comprehensive mapping of the convergences and divergences across voxel-based and volumetric methods. We propose that, in order to capture the entire spectrum of changes associated with ageing, various methods are ideally applied in conjunction. For chapter 4, we aimed at acquiring a better insight into the principles governing long-term fine-motor skill learning of the nondominant hand. Despite a wealth of motor learning studies, research into the reorganisational effects across a few months still constitute a gap in the literature. Most studies focused on novel task acquisition rather than dexterous manual improvement and, also, the impact of effector-(in)dependency has often been overlooked with many studies lacking a consistent differentiation between hand and hemispheres. In order to fill this void, we investigated hand-related functional response patterns triggered by a three-monthly dexterous training of the nondominant hand. Our findings revealed that left-handed dexterous improvement coincided with a drop in neural activity within pericentral and inferior parietal regions of the right hemisphere. Finding these decreases regardless of the performing effector suggested that also unilateral motor activity relies on a bilateral motor network which is modifiable through practice. The nondominant hand writing practice from chapter 4 was presumed to induce structural plastic modulations at the level of grey matter. However, no reliable results could be generated due to an acquisition-related artefact. Thus, we could not conclude whether the amount of structural plasticity induced by our practice paradigm is sufficiently elevated to be picked up with the currently applied techniques. We therefore advocate, as a key endeavour for future morphological research, to render morphometry more robust against technical limitations. To summarise, this project employed a wide range of techniques and analysis approaches, combining functional as well as structural measures, cross-sectional and longitudinal paradigms, in order to shed more light on expansive versus refractive processes of plasticity. We have elucidated processes of asymmetric motor activations and have acquired novel insights into mechanisms of dexterous improvement of the nondominant hand. We have deepened our understanding of the contribution and pitfalls of voxel- and label-based techniques for morphometric investigations. On a concluding note, we underscore the need for incorporating information obtained from various modalities in order to surpass a mere description of lateralised, plastic or ageing processes in isolation and attempt a more systemic depiction of their principles.