Imagerie cérébrale et maladies psychiatriques (schizophrénie).

Abstract

Les maladies psychiatriques ne s' expliquent pas par une anomalie de structure mais par un desordre du fonctionnement cerebral. Les nouvelles techniques d' imagerie fonctionnelle cerebrale, qui permettent de l' etudier directement, ont ete appliquees a l' etude des patients psychotiques. Il y a tout lieu de penser que l' etat de repos mental chez un sujet psychotique va dependre de son activite mentale immediate. La presence de symptomes negatifs, comme la pauvrete psychomotrice et la pauvrete de l' expression orale, est associee a une diminution de l' activite metabolique dans le cortex prefrontal gauche. A l' inverse, les hallucinations et les idees delirantes sont associees a une hyperactivite du cortex temporal gauche. Une approche differente consiste a etudier le patient dans une situation mentale controlee et induite par des instructions precises sous la forme de taches psychologiques. De nombreux arguments indirects suggerent que le probleme fonctionnel de la schizophrenie serait davantage celui d' une deconnexion fonctionnelle entre les regions cerebrales que d' anomalies restreintes a une seule region. Des arguments directs confirmant cette hypothese devraient voir le jour dans un proche avenir grace a de nouvelles methodes d' imagerie.

Since psychiatric disorders most probably result from abnormalities of brain function rather than structure the new functional brain imaging techniques are of enormous importance in the study of psychiatric patients. Three main approaches have been adopted. First, the pattern of brain activity has been observed in patients while at rest or when experiencing particular symptoms. The presence of negative features such as poverty of speech and social withdrawal tend to be associated with reduced activity in the frontal cortex, while patients experiencing hallucinations tend to show over-activity in sensory association cortex. Second, the pattern of brain activity has been observed while patients performs tasks designed to engage particular brain systems. Abnormal patterns of activity have been observed when patients perform complex problem solving tasks and when they are engaged in auditory imagery. Third, brain imaging has been used to examine hypotheses about the mechanisms underlying particular symptoms. For example, some symptoms, such as delusions of control, may arise from a failure of self-monitoring which results in sensations steming from the patients own actions being responded to as unexpected events of external origin. The observation of over-activity in relevant brain areas is consistent with this hypothesis. This over-activity may result from disconnections between frontal regions concerned with generating actions and posterior regions concerned with analysis of the sensory consequences of these actions.