Les fonctions cérébrales complexes

Nonobstant l’énorme difficulté technique et conceptuelle associée à leur étude scientifique, beaucoup de progrès a été fait dans l’analyse structurelle et fonctionnelle du système nerveux central concernant le comportement, l’articulation du langage et la compréhension de la parole, les états de sommeil et de veille, les sentiments et les émotions, les différences sexuelles et la mémoire. En effet, l’accumulation d’observations cliniques les cent dernières années et l’avènement des études d’imagerie cérébrale ont été particulièrement importants pour la localisation corticale de plusieurs des fonctions supérieures. Ensuite, l’introduction des techniques d’imagerie non invasives a permis l’observation de l’activité neurale locale associée aux divers aspects de ces fonctions chez des êtres humains sains et chez les malades. Et finalement, des expériences chirurgicales et électrophysiologiques complémentaires récentes sur des animaux de laboratoire ont permis de commencer à élucider l’organisation structurelle et fonctionnelle de certaines des parties du cerveau impliquées dans les diverses manifestations de ces fonctions.

Les aires corticales dites d’association comprennent la plupart de la surface du cerveau humain et participent principalement à la transformation des divers signaux sensoriels dans des mouvements spécifiques. Les neurosciences en général ont groupé l’ensemble des habiletés associatives nécessaires à cette transition sous le nom de cognition ( comprise au sens le plus large du mot ),

120 _{« An obvious obstacle to exploring change in the brains of humans and other mammals is the enormous}

number of neurons and the complexity of synaptic connections. As a consequence, it is difficult to unambiguously attribute a behavioral modification to changes in the properties of specific neurons or synapses. One way to circumvent this dilemma is to examine plasticity in far simpler nervous systems. The assumption in this strategy is that plasticity is so fundamental that its essential cellular and molecular underpinnings are likely to be conserved in the nervous systems of very different organisms. One of the most

signifiant les différents processus par lesquels l’animal parvient à connaître son monde et à interagir avec lui121_{. Pourtant, dans le cas de la neurobiologie, ce mot} est utilisé plus précisément en référence à la capacité de l’animal de s’apercevoir d’une stimulation externe ou d’une motivation interne, d’identifier leur signification vitale et de répondre à celles-ci avec un comportement adéquat.

Outre l’approfondissement du travail inauguré par Broca et par Wernicke concernant l’identification des régions corticales qui participent à la production et à la compréhension du langage122, la recherche est parvenue dernièrement à associer certaines aires des lobes pariétaux à l’attention123 et à l’interprétation de

successful examples of this approach has been that of Eric Kandel and his colleagues at Columbia University using the marine mollusk Aplysia californica. » ( Purves et al., p. 574 ).

121 _{« "Cognition" is perhaps not the best word to indicate this wide range of neural functions, but it has become}

part of the working vocabulary of neurologists and neuroscientists. » ( Purves et al., p. 613 ).

122 _{« Studies of patients with damage to specific cortical regions and normal subjects studied by functional}

brain imaging indicate that linguistic abilities of humans depend on the integrity of several specialized areas of the association cortices in the temporal and frontal lobes. In the vast majority of people, these primary language functions are located in the left hemisphere: the linkages between speech sounds and their meanings are mainly represented in the left temporal cortex, and the circuitry for the motor commands that organize the production of meaningful speech is mainly found in the left frontal cortex. Despite this left-sided predominance for the “lexical” aspects of language, the emotional (affective) content of speech is governed largely by the right hemisphere. Studies of congenitally deaf individuals have shown further that the cortical areas devoted to sign language are the same as those that organize spoken and heard communication. The regions of the brain devoted to language are therefore specialized for symbolic representation and communication, rather than for heard and spoken language as such. Understanding functional localization and hemispheric lateralization of language is especially important in clinical practice. The loss of language is such a devastating blow that neurologists and neurosurgeons make every effort to identify and preserve those cortical areas involved in its comprehension and production. The need to map language functions in patients for the purpose of sparing these regions of the brain has provided another rich source of information about the neural organization of this critical human attribute. » ( Purves et al., p. 637 ).

123 _{« In 1941, the British neurologist W. R. Brain reported three patients with unilateral parietal lobe lesions in}

whom the primary problem was varying degrees of attentional difficulty … Since Brain’s original description of contralateral neglect and its relationship to lesions of the parietal lobe, it has been generally accepted that the parietal cortex, particularly the inferior parietal lobe, is the primary cortical region (but not the only region) governing attention. » ( Purves et al., p. 619 et 620 ).

la signalisation provenant des différents sens124_{. En particulier, la découverte que} le lobe frontal, le plus complexe de tous les lobes cérébraux, est profondément relié à l’établissement et au maintien du comportement et de la personnalité a été d’un intérêt général pour l’ensemble des neurosciences pour des raisons manifestes125.

Qui plus est, on sait aujourd’hui que l’organisation du comportement moteur somatique et viscéral associé aux émotions est gouvernée par des circuits nerveux situés dans le système limbique, qui comprend l’hypothalamus, l’amygdale et

124 _{« Clinical evidence from patients with lesions of the association cortex in the temporal lobe indicates that}

one of the major functions of this part of the brain is the recognition and identification of stimuli that are attended to, particularly complex stimuli. Thus, damage to either temporal lobe can result in difficulty recognizing, identifying, and naming different categories of objects. These disorders, collectively called agnosias ( from the Greek for “not knowing” ), are quite different from the neglect syndromes. As noted, patients with right parietal lobe damage often deny awareness of sensory information in the left visual field ( and are less attentive to the left sides of objects generally ), despite the fact that the sensory systems are intact ( an individual with contralateral neglect syndrome typically withdraws his left arm in response to a pinprick, even though he may not admit the arm’s existence ). Patients with agnosia, on the other hand, acknowledge the presence of a stimulus, but are unable to report what it is. These latter disorders have both a lexical aspect (a mismatching of verbal or other cognitive symbols with sensory stimuli; see Chapter 26 ) and a mnemonic aspect ( a failure to recall stimuli when confronted with them again; see Chapter 30 ). One of the most thoroughly studied agnosias following damage to the temporal association cortex in humans is the inability to recognize and identify faces. This disorder, called prosopagnosia ( prosopo, from the Greek for “face” or “person” ), was recognized by neurologists in the late nineteenth century and remains an area of intense investigation. After damage to the inferior temporal cortex, typically on the right, patients are often unable to identify familiar individuals by their facial characteristics, and in some cases cannot recognize a face at all. Nonetheless, such individuals are perfectly aware that some sort of visual stimulus is present and can describe particular aspects or elements of it without difficulty. » ( Purves et al., p. 622. Termes en italiques par l’auteur ).

125 _{« The functional deficits that result from damage to the human frontal lobe are diverse and devastating,}

particularly if both hemispheres are involved. This broad range of clinical effects stems from the fact that the frontal cortex has a wider repertoire of functions than any other neocortical region ( consistent with the fact that the frontal lobe in humans and other primates is the largest of the brain’s lobes and comprises a greater number of cytoarchitectonic areas ). The particularly devastating nature of the behavioral deficits after frontal lobe damage reflects the role of this part of the brain in maintaining what is normally thought of as an individual’s “personality”. The frontal cortex integrates complex perceptual information from sensory and motor cortices, as well as from the parietal and temporal association cortices. The result is an appreciation of self in relation to the world that allows behaviors to be planned and executed normally. When this ability is compromised, the afflicted individual often has difficulty carrying out complex behaviors that are appropriate to the circumstances. These deficiencies in the normal ability to match ongoing behavior to present or future demands are, not surprisingly, interpreted as a change in the patient’s “character”. … All these observations are consistent with the idea that the common denominator of the cognitive functions subserved by the frontal cortex is the selection, planning, and execution of appropriate behavior, particularly in social contexts. Sadly, the effects of damage to the frontal lobes have also been documented by the many thousands of frontal lobotomies (“leukotomies”) performed in the 1930s and 40s as a means of treating mental illness (Box B). The rise and fall of this “psychosurgery” provides a compelling example of the frailty of human judgment in medical practice, and of the conflicting approaches of neurologists, neurosurgeons, and psychiatrists in that era to the treatment of mental disease. » ( Purves et al., p. 623-624 et 626 ).

plusieurs régions du cortex cérébral frontal126_{. Mais en dépit de l’abondance des} connaissances concernant l'anatomie du système limbique et la biochimie de ses neurotransmetteurs, on ignore encore la manière dans laquelle cet ensemble complexe de circuits neuraux est engagé dans les différents états émotionnels, et l’on commence à peine à noter l’importance de la participation de ces circuits à des activités telles que la prise de décisions et les comportements sociaux127.

D’autre part, plusieurs études récentes accomplies sur des animaux de laboratoire ont permis d’associer certaines différences comportementales du mâle et de la

126 _{« Although everyday emotions are as varied as happiness, surprise, anger, fear, and sadness, they share}

some common characteristics. All emotions are expressed through both visceral motor changes and stereotyped somatic motor responses, especially movements of the facial muscles. These responses accompany subjective experiences that are not easily described, but which are much the same in all human cultures. Because emotional expression is closely tied to the visceral motor system, it entails the activity of the central brain structures that govern preganglionic autonomic neurons in the brainstem and spinal cord. Historically, the higher order neural centers that coordinate emotional responses have been grouped under the rubric of the limbic system. More recently, however, several brain regions in addition to the classical limbic system have been shown to play a pivotal role in emotional processing, including the amygdala and several cortical areas in the orbital and medial aspects of the frontal lobe. This broader constellation of cortical and subcortical regions encompasses not only the central components of the visceral motor system but also regions in the forebrain and diencephalon that motivate lower motor neuronal pools concerned with the somatic expression of emotional behavior. Effectively, the concerted action of these diverse brain regions constitutes an emotional motor system. The same forebrain structures that process emotional signals participate in a variety of complex brain functions, including rational decision making, the interpretation and expression of social behavior, and even moral judgments. » ( Purves et al., p. 687 ).

127 _{« Although a good deal is known about the neuroanatomy and transmitter chemistry of the different parts of}

the limbic system, there is still a dearth of information about how this complex circuitry mediates specific emotional states. Similarly, neuropsychologists, neurologists and psychiatrists are only now coming to appreciate the important role of emotional processing in other complex brain functions, such as decision- making and social behavior. A variety of other evidence indicates that the two hemispheres are differently specialized for the governance of emotion, the right hemisphere being the more important in this regard. The prevalence and social significance of human emotions and their disorders ensure that the neurobiology of emotion will be an increasingly important theme in modern neuroscience. » ( Purves et al., p. 709 ).

femelle à des différences dans la structure de certains de leurs circuits neuraux, le cas flagrant étant certainement celui des comportements reproductifs128_.

Par exemple, chez la femelle de la souris le nombre de synapses et la densité dendritique des circuits neuraux de la partie antérieure de l’hypothalamus sont considérablement plus élevés que ceux que l’on trouve dans la même partie du cerveau du mâle, tandis que chez ce dernier il y existe un certain noyau nerveux qui est pratiquement inexistant chez la femelle et qui paraît être associé au comportement moteur d’accouplement qui lui est propre. En général, les études sur la souris ont montré que la partie antérieure de l’hypothalamus est impliquée tant dans l’activité proprement reproductive que dans les comportements associés à l’élevage de la progéniture, au désir sexuel et à l’orientation sexuelle.

D’autres différences sexuelles dans la structure nerveuse de certains noyaux du cerveau ont été observées aussi par rapport à certaines fonctions cognitives, particulièrement dans le cas des oiseaux, où les deux sexes sont capables d’apprendre le chant caractéristique de leur espèce, mais où seulement le mâle se montre capable de le reproduire129_{. Des comportements plus complexes, tels la} construction du nid, la recherche de nourriture et l’élevage de la progéniture,

128 _{« Sex-related differences in the phenotypic expression of genotype are called sexual dimorphisms. While}

some of the behavioral distinctions involved may be rooted in cultural or social norms, sexual dimorphisms arise primarily because the brains of females and males are in some respects different. In the rat ( the animal in which most experimental work has been done ), several structures in female and male brains clearly differ in the number, size, and connectivity of their constituent neurons. In humans and other primates, structural differences are less obvious but nonetheless present. In both rats and humans, sexually dimorphic brain structures tend to cluster around the third ventricle in the anterior hypothalamus and are an integral part of the system that governs visceral motor behavior. Other sexual dimorphisms are apparent in cerebral cortical structures, implying differences in more complex regulatory and other behaviors. The development of these structural differences depends primarily on the early effect of gonadal steroid hormones on maturing brain circuits, an influence that apparently continues to some extent throughout life. The functional consequences of sexual dimorphisms in rodents are beginning to be well understood. Although the significance of such differences in humans is less clear, they provide a plausible neural basis for the wide variety of human sexual behavior. » ( Purves et al., p. 711 ).

129 _{« Some sexually dimorphic behaviors are simply part of the reproductive repertoire, whereas others are}

associated with cognitive functions. An example of dimorphic behavior related to reproduction is apparent in songbirds. In many species, the male produces complex song, but the female does not. Song production arises from the activity of specific brain nuclei; as described in Chapter 23, these nuclei are much larger in males than in females. The size of song control nuclei increases in females treated with testosterone or estradiol during development, and these “masculinized” females sing. » ( Purves et al., p. 711 ).

manifestent aussi des différences qui relèvent aussi de ces différences structurelles à des niveaux spécifiques du cerveau.

Plusieurs de ces différences structurelles ont été observées aussi chez les primates, mais la complexité de leurs comportements rend beaucoup plus difficile la tâche de les identifier. Ceci est particulièrement vrai de l’être humain, chez qui les comportements sexuels participent aussi d’une dimension socioculturelle, ainsi que de la perception subjective de soi130.

En général, les expériences de laboratoire ont démontré que ces différences structurelles résultent principalement de la régulation hormonale exercée sur les cellules nerveuses au cours du développement de l’organisme131, mais il existe aussi des comportements adaptatifs provenant de la plasticité neurale adulte qui sont caractéristiques de l’un ou de l’autre sexe de l’espèce. Un exemple de ces derniers nous l’avons dans la production de lait, que dans le cas de la souris peut

130 _{« Roughly speaking, the concept of sex can be subdivided into three categories: chromosomal sex,}

phenotypic sex, and gender. Chromosomal sex refers specifically to an individual’s sex chromosomes. Most humans have either two X chromosomes or one X and one Y chromosome, with XX being a chromosomal female and XY a chromosomal male. Phenotypic sex refers to an individual’s sex as determined by their internal and external genitalia, the expression of secondary sex characteristics, and their behavior. In the prototypical case, during development the XX genotype leads to an individual with ovaries, oviducts, uterus, cervix, clitoris, labia, and vagina—i.e., a phenotypic female. The XY genotype leads to a person with testicles, epididymis, vas deferens, seminal vesicles, penis, and scrotum—a phenotypic male (Box A). Gender, as the term is most often used, refers to an individual’s subjective perception of their sex and their sexual orientation, which is harder to define than chromosomal or phenotypic sex. It should also be apparent that some people consider gender to be a political and social construct. For present purposes, however, gender entails self-appraisal according to the traits most often associated with one sex or the other ( called gender traits ), which are influenced by societal expectations and cultural norms as well as by biology. Sexual orientation also entails self-appraisal in the context of culture. To understand the neurobiology of sex, it is helpful to think of chromosomal sex as largely immutable; phenotypic sex as modifiable by developmental processes, hormone treatments, and/or surgery; and gender as a more complex social and cultural construct that an individual may or may not want to accept. Clearly, chromosomal sex, phenotypic sex, and gender will not always be aligned. » ( Purves et al., p. 712-713 ).

131 _{« The development of sexual dimorphisms in the central nervous system is ultimately an outcome of}

chromosomal sex. Chromosomal combinations usually determine the phenotype of the gonads; the gonads, in turn, are responsible for producing most of the circulating sex hormones (see Box A). Differences in circulating hormones lead to a variety of differential effects on the individual’s development, including their physical appearance, response to pharmacological treatments, susceptibility to certain diseases, and brain development … Gonadal steroids—whether estrogens or androgens—stimulate sexually dimorphic patterns of development by binding to estrogen or androgen receptors. These receptors, which are transcription factors activated by hormone binding, influence gene transcription and, ultimately, the development of an array of targets, including sexually dimorphic neural circuits. » ( Purves et al., pp. 715 et 718 ).

être provoquée même chez des femelles vierges en les exposant à l’odeur caractéristique des nouveaux-nés de l’espèce132_.

Les dernières études cliniques accomplies sur des êtres humains ont commencé à fournir l’évidence de l’existence de dimorphismes sexuels au niveau du cortex cérébral. Pourtant, ces différences paraissent représenter des manières différentes dont le cerveau accomplirait les diverses fonctions corticales supérieures plutôt que des différences dans les fonctions mêmes. En tout cas, le grand nombre de facteurs participant à l’expression de chacune de ces fonctions chez l’être humain

132 _{« As mentioned earlier, there is growing evidence that some brain circuits continue to change over the}

course of an individual’s life, depending on both experience and hormonal milieu. For example, changes in the brain circuits of adult rats occur in conjunction with parenting behavior … Another example of adult plasticity under hormonal control is the altered connections between cells of the female rat hypothalamus after giving birth. In females prior to pregnancy, the relevant hypothalamic neurons are isolated from each