Cortical Representations of Body Parts in Children and in Individuals Born with Missing Hands

The present thesis addressed the cortical representation of body parts in normally limbed children and adults as well as in individuals born with missing hands. With the application of behavioral, neuropsychological and neuroimaging methodologies in different populations, i.e. healthy normally limbed adults and children as well as participants with limbs missing since birth, this issue could be examined in a broad scientific context. The four studies of my thesis investigated the following questions, each addressing different aspects of body representation: (1) Does a congenital absence of a limb (so-called amelia) cause somatotopical alterations of the body representation in the primary motor cortex; (2) do amelic patients present with measurable abnormalities in higher-order body representations (e.g. body schema) as measured by motor imagery tasks; (3) does lifelong experience of phantom sensations of these missing limbs relate to changes in the body schema; (4) is there a developmental effect of the body representation in the healthy brain, i.e. do children differ from adults in performing a motor imagery task assessing components of the body schema? The results of the first study revealed a significantly altered topological organization of the primary motor cortex in participants with unilaterally missing hands as compared to normally limbed controls. The second study showed that children's body schema is highly influenced by visual and, even stronger by proprioceptive inputs, whereas in adults, both, visual and proprioceptive sources affected the body schema in a comparably strong way. The results of study 3 and 4 are consistent with each other insofar as they suggested (1) that in subjects with one hand missing since birth the lifelong use of the other, existing hand facilitates motor imagery regarding the missing hand. (2) Furthermore, in participants with bilaterally missing hands, the presence of phantom sensations could be shown to favor motor imagery regarding congenitally absent limbs. These findings are highly relevant, clinically as well as for the basics of neurosciences, as they provide converging evidence for cerebral plasticity, which is in turn a precondition for neuro-rehabilitation. Moreover, the behavioral and anatomical research in amelic participants provides a unique, natural model to investigate the development and the topography of body representations. It could also be helpful for the understanding of the genesis of phantom sensations in congenitally or traumatically absent limbs.