Language and hemispheric specialization

Language is a highly lateralized cognitive function and has been the scope of multiple studies on the different role of the left hemisphere (LH) and the right hemisphere (RH). Based on structural language's models, clinical, behavioral and imaging data confirm the advantage of the LH in nearly all aspects of language processing. On the other hand, dynamic models postulate the presence of hidden or latent right hemispheric competencies, which can appear in certain situations. In this chapter we confirm the existence of dominant phonological, graphemic and lexico-semantic processes in the LH, and discuss the potential role of some latent functions, particularly in the RH. Data from clinical, behavioral sciences and neuroimaging support the following statements: i) cortical representation of language are naturally lateralized in the left hemisphere, but some steps such as prosodic comprehension and expression, some reading strategies and some aspects of lexico-semantic processes can take advantage of complementary functions in both hemispheres. ii) A tonic inter-hemispheric interaction, excitatory and inhibitory, has been demonstrated at least in the lexico-semantic processing. iii) Despite these interactions, neural networks involved in language are stable, and are barely modified by experimental changes in stimulus presentation. However, the pattern of activation of this network depends on the presentation. iv) Following focal lesion of the LH, compensatory homotopic areas of the RH can be used for language relearning. These compensatory areas do not seem to replicate the lost function but to use and adapt language functions anteriorly developed in this hemisphere. These compensatory activations vary according to the localization of the lesion.