[Neurobiological bases of language and its disorders].

AIMS

The purpose of this study is to offer an update on the anatomofunctional bases of language and the theories that explain its normal and pathological development.

METHOD

Language is a clear example of one of the higher functions of the brain, the development of which is carried out, on the one hand, in a genetically determined anatomofunctional structure and, on the other hand, by the verbal stimulus provided by the environment. Several systems and subsystems are at play within the anatomofunctional structure and these operate in series and in parallel. A large amount of the knowledge we have about the neurophysiological bases of language come from observations carried out in adult patients with circumscribed lesions and their clinical consequences (aphasias). Findings from recent studies involving functional imaging in volunteers submitted to linguistic tests have added more data. According to Damasio, three functional systems are at work in language: 1. The instrumental system, which corresponds to the perisylvian region of the dominant hemisphere where phonological processing takes place; 2. The mediation system, which includes temporal, frontal and parietal areas that surround the anterior region and are where lexical items are organised in a modular fashion, in terms of categories, actions and functional or connecting words; 3. The semantic system, which includes extensive areas of the cortex in both hemispheres and is the seat of concepts and meanings. Throughout a child s development, the evolution he or she follows to reach the neurolinguistic organisation of the adult brain requires the integrity and proper functioning of these structures that are for the most part located in the dominant hemisphere. We review the different theories that appear in the specialised literature concerning the causes and fisiopathogenic mechanisms behind dysphasias in early childhood.

CONCLUSIONS

Among the numerous functions that take part in the complex language system, some are essential for its normal development. From the work of Tallal et al. it has been seen that the sequential and fast phonological processing of consonant vowel shifts is altered in dysphasic and dyslexic children. This finding is related with the disorders in the normal asymmetry of the temporal planum (the left is larger than the right) that has been observed in these patients and with the neuropathological findings of Galaburda et al. in dyslexic patients who had previously been dysphasic, in whom cytoarchitectural anomalies (heterotopias) were found, above all in the left perisylvian region. Obviously not all forms of dysphasia are the result of this alteration. According to Chevrie Muller s classical chart adapted by us in accordance with Damasio s scheme it is possible to locate the dysphasic syndromes in different loci and distinguish a different physiopathological mechanism for each of them.