Anatomie des émotions

The study of emotion had been relegated to the fringes of neuroscience until quite recently because it was too subjective and irrational for the intellectual stiffness of modern science. Since about thirty years, the studies relating to emotions and underlying neural mechanisms have considerably multiplied utilising behavioural paradigms with normal subjects, clinical evaluation of patients suffering from neurological or psychic disorders, observations of patients with definite brain lesions and finally, using the most advanced and noninvasive technology available to capture revealing images of the living brain responding to emotions: functional brain neuroimaging. In this paper we describe the famous historical contributions to the comprehension of the links between brain and emotions. The oldest experimental demonstration of these links was carried out at the beginning of the twentieth century by Cannon and Bard who showed that the surgical ablation of a cat's neocortex induced nonmotivated aggressive behaviour called "sham rage". Klüver and Bucy described behavioural modifications generated by the ablation of portions of temporal lobes in monkeys. After lobectomy, animals developed a submissive and docile behaviour and hypersexuality as well as hyperorality. The case of Phineas Gage first described by Dr Harlow is also very famous as first description of personality changes after prefrontal damage. These case studies carried out on man and animal lead to the establishing of assumptions on the anatomical basis of emotions. Researchers such as Papez, Broca, MacLean and more recently Mesulam have successively contributed to the construction of the current neuroanatomical models of emotions where the cortico-limbic system constitutes the essential substrate of the emotional processes. The main anatomical structures of this system, namely the amygdala, the septal area, the hypothalamus and the prefrontal regions are then reviewed. The most relevant data supported the implication of the septal area being involved in pleasure and the amygdala in fear-related emotion. As interface between cognition and emotion, the prefrontal area has a more complex role, particularly in the regulation of social emotions. The contribution of both cerebral hemispheres is also discussed with the classical opposition "happy" left hemisphere and "pessimistic" right hemisphere. The final paragraphs provide recent data on the relations between emotional states and specific patterns of regional brain activity seized with the help of the various functional neuroimaging techniques. These techniques, which allow to study the emotional processes in healthy subjects "in vivo", challenge the hypothesis that specific brain regions have strictly specialised functions for the generation of emotions (e.g. the critical role of amygdala in fear-related processing). It is indeed likely that separate brain regions are involved in different aspects of emotion (e.g. the amygdala may not exclusively respond to threaten stimuli but to any emotionally relevant stimuli, regardless of valence). As in other domains of neurocognition we are moving from a localisationist perspective toward approaches where the mental functions are underlain by distributed neural networks. Although our knowledge of emotional mechanisms and their anatomical substrates is still fragmentary, the study of emotions constitutes a promising field of investigation. A particular interest is attached to the role of emotion in attentional functions, memory and decision making as well as the implication of cortico-limbic circuits in motivation.