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The Role of the Right Hemisphere in Language After Left Hemisphere Damage; Evidence from Inter-Hemispheric Connectivity
The role of the right hemisphere (RH) in compensating for Left hemisphere (LH) damage during language processing in patients with aphasia, has been called into question in the last decade. According to the hypothesis the increase in RH activation is due to a release from LH inhibition and the RH may play a maladaptive role in language recovery by exerting transcallosal suppression on homologues LH regions. I will present two studies that directly examine interhemispheric connections in patients with aphasia using fMRI, to shed light on the role of the RH in recovery from aphasia.
In study #1 we assessed effective connectivity using dynamic causal modelling during sentence comprehension in 7 post-stroke patients with aphasia. The results showed that all changes in inter-hemispheric connection in patients compared to controls were associated with poor language performance, inconsistent with a compensatory role for the RH. However, no transcallosal suppression was evident between homologous regions in neither patients nor controls, and the maladaptive involvement was a result of both excitatory and inhibitory right-to-left influences.
In study #2, a chronic patient with aphasia following LH damage received melody based intervention. The patient’s improvement in the production of trained and untrained phrases was accompanied by an increase in resting state functional connectivity between right frontal language areas and the supplementary motor area involved in motor planning.
Altogether these results demonstrate the complex role of RH regions in recovery from aphasia which depends on the performed task, among other factors, and can be gleaned from the analysis of interhemispheric connectivity. While an increase in both excitatory and inhibitory right-to-left connections in patients may be detrimental for a sentence comprehension task, an intervention that emphasizes intact RH functions may recruit right frontal areas for compensation in a sentence production task.