Structural and functional brain (re)organization in stroke and dyslexia

Abstract

Stroke is a leading cause of disability worldwide. Even in case of severe brain damage neuronal plasticity may contribute to functional recovery. Understanding the underlying mechanisms could improve prognostication and may reveal targets for novel rehabilitation techniques. First, recovery from aphasia, a disorder of language, was studied. Twelve aphasic patients with left hemispheric stroke were examined with functional MRI (fMRI) >1 year post-stroke. Language related inferior frontal gyrus (IFG) activity was associated with improvement of naming ability and comprehension, while the observed increase of activity in the right IFG may have been reflective of up-regulation of domain-general cognitive processing. A second study with twelve patients was conducted longitudinally over 1 year. Better language outcome was related to an increase of activation in both the left and right posterior inferior temporal gyrus, while an increase in activity in the right IFG was associated with worse outcome. The extent of left posterior temporal gyrus activation assessed with early functional neuroimaging could improve prediction of outcome. Variations in language activation over time were found in different nodes of the recovering language network related to actual language improvement over time and thus suggestive of recovery-associated plasticity. Variations in language activation were also related to hemodynamic responsiveness or with an unspecified origin. It is therefore important to control for these factors in longitudinal fMRI studies after stroke. Hemodynamic responsiveness was found an unlikely confounder in the interpretation of language fMRI results. White matter connections between language areas are important for optimal reorganization of the language network after stroke. Diffusion Tensor Imaging (DTI) was used at one year in five patients and 8 healthy subjects to investigate integrity of the left and right superior longitudinal fasciculus (SFL). Integrity of right SFL was similar in patients and controls and was related to better language outcome after one year reflecting the importance of pre-morbid connectivity within the right hemisphere. Besides aphasia, impairment of motor function is a cardinal feature of ischemic stroke. Increase of fMRI activation of the sensorimotor cortex in response to tactile stimulation of digit 3 of the affected hand in 10 patients correlated strongly to concurrent gains in motor function over 1 year after stroke. Increased responsiveness of the sensorimotor cortex over the subacute poststroke period may contribute to motor recovery and may provide a target for new therapeutic approaches. Finally, we studied the involvement of the cerebellum in dyslexia, a language disorder of reading and spelling not related to brain damage like stroke. In a study with 26 participants with dyslexia and 25 control subjects, behavioral evidence for (mildly) impaired cerebellar function was found to co-occur with reading impairments, but a causal relationship was not present. Besides, no differences in local grey matter volume, nor in structure-function relationships within the cerebellum were found between the two groups using Voxel based morphometry. Therefore, the so-called ‘cerebellar deficit hypothesis’ suggesting impaired cerebellar structure and function as a causal factor in developmental dyslexia needs to be reconsidered.