Le cerveau humain et les origines du langage.

Abstract

Pourquoi parlons-nous ? D' ou nous vient cette faculte remarquable et unique qui fait de l' homme le "sommet de l' evolution "? Pourquoi l' enfant acquiert-il en quelques mois des aptitudes linguistiques qui depassent largement ce que les ordinateurs actuels les plus puissants sont capables de produire? Autant de questions qui ont fait l' objet de debats parfois acharnes au cours du siecle qui vient de s' ecouler mais n' ont pas encore trouve de reelle reponse. Une certitude toutefois: c' est dans le cerveau humain qu' il faut chercher les cles du mystere, dans ses circonvolutions que la technologie moderne nous permet de mieux en mieux d' explorer et dans la connaissance de ses pathologies que la medecine decrit avec de plus en plus de precision. Cet article presente ainsi des elements de reflexion autour de ces deux axes de recherche: la neuroanatomie fonctionnelle et la pathologie du developpement du langage. Globalement, les donnees actuelles convergent pour faire du langage une aptitude complexe, profondement ancree dans notre patrimoine genetique mais susceptible d' evoluer encore radicalement en fonction des modifications de l' environnement auquel notre cerveau sera expose.

At the turn of the century, the issue of the origins of human language still remains one of the most attractive challenges for neuroscientists and neurologists. This article proposes an overview of various pieces of evidence, derived from - mostly recent - anatomical, physiological as well as clinical research. After a short recall of classical anatomical concepts about the brain language areas, and an illustration of advances to these classical conceptions provided by new imaging techniques, the central issue of brain lateralisation is taken as a basis for a reflection about the phylogenetic and paleontological origins of human language. One important conclusion to this section is that anterior and posterior parts of the language area, i.e. respectively its expressive and receptive components, probably differ in their origins. In particular, motor aspects of speech, as exemplified in recent brain imaging studies in deaf subjects, seem to be a necessary condition to a plain left lateralization of language. Development of brain/language relationship during the child's maturation is also a valuable source of information. Insights into the brain development have been derived from several approaches: the study of myelinogenesis, which offers a reliable timetable of the relative growth of different brain areas; development, mainly during late fetal life, of brain asymmetry, which is thought to be crucial to setting up the pattern of language lateralization. Finally, one brain structure, the corpus callosum, clearly measurable on magnetic resonance (MRI) pictures, appears as a very useful index of the potential role of various factors on brain maturation, including the effect of practice and training, now believed to be at least as effective as prenatal influences in determining the morphology of language-related brain regions. Finally, clinical and imaging studies of language-learning disorders in children has been one of the most fruitful approaches, in the recent years, yielding considerable insight into the cognitive, biological and genetic bases of human language. Functional brain imaging in these conditions has recently contributed in important ways to the knowledge of brain mechanisms involved in processing written language. [References: 48]