22 juillet 2006

Film : Race - The power of an illusion

This documentary by the Public Broadcast Service can be found at http://www.pbs.org/race/. The first of the three episodes – “The difference between us” – does a good job of making a few facts clear about skin colour and genetics:

The measured amount of genetic variation in the human population is extremely small. Genetically, we aren’t very different. In fact, we are among the most similar of all species:

- only 1 out of every 1000 nucleotypes that make up our genetic code differs from one individual to another;

- penguins have twice our amount of genetic difference, yet they all look alike to us;

- and fruit flies, ten times; hence any two fruit fruit flies may be as different as a human is from a chimpanzee.

We have a long history of searching for racial differences, and attributing performance and behavior to them. It is frightening to see how many “scientific” studies were made on the subject. This was so true in the United States that 28 states passed laws forbidding intermarriage to safeguard white racial purity. The nazi propaganda machine pointed out that their eugenic policies were entirely consistent, and in fact derived from, ideas of american race scientists. Then, as we remember, Jesse Owens won four gold medals at the 1936 Berlin Olympics…

Why is skin colour differentiated according to the lattitude? A plausible hypothesis is that sunlight is essential to have active vitamin D. Therefore, in higher lattitudes, children who have lighter melanin might be healthier thanks to their capacity to absorb more light. Along those lines, it soon becomes clear that the best explanation for genetic differences is not rather, but rather geography.

In addition, environment along with opportunity combine with genes to explain individual performance. In the 1930’s, jewish teams dominated the american basketball. For a long time afterwards, as opportunities changed, basketball teams were almost african american. But today, 20% of NBA’s players are foreign-born. Who is the NBA’s top pick? Chinese!

Mitochondrial DNA does not code for any trait, and is inherited only from the mother. Therefore, it tells us something about one of our distant ancestors: our mother’s mother’s mother’s mother’s mother’s etc. An easy test proves that mitochondrial DNA differences are not correlated with skin colour. As for other genetic differences, Richard Lewontin did pioneering work to find out just how much variations occurred within, and across, the groups which were called races. He was using gel electrophoresis, and his findings were a milestone. For proteins, blood groups, as well as DNA sequencing, it alwas comes out the same: 85% of the variation between human beings is between any two individuals within the same local population. Of the small amount of variation in our genes, there is therefore likely to be almost the same average difference between two african american persons as between an african and a caucasian american.

Still, of the small remaining genetic variation, we know that some genes are found more often in some populations. For instance, the sickle cell disease, which can alter the shape of red blood cells, is thought to be an african american disease. In fact, this trait is not uncommon in people from the mediterranean region (up to 30% in Greece), and this is due to the relative resistance it confers to malaria. Therefore, sickle cell is not a racial trait, but rather the result of having ancestors who lived in malarial regions.

Today, we almost have genetic proof that all preexisting humanoid species died – Neanderthals in Europe, Homo Erectus in Asia. A second migration of the current Homo Sapiens from Africa occurred, and all modern humans are descendants of this second migration which is probably less than 100,000 years old. Therefore, because everything originated in subsaharan Africa, most of humanity’s genetic variation today can be found in Africa.

DVD : The nature of sex

This series is a documentary on sexual reproduction across numerous animal species. It serves as a good basis to corroborate the more abstract theories of The selfish gene of The extended phenotype.

  1. The primal instinct
  2. A time and a place
  3. The sex contract
  4. The human animal
  5. The mystery of mating
  6. The young ones

  1. The primal instinct

Courtship: Japanese crances dancing on ice, male birds of paradise showing off their plumage to the female who will have only one mate and is very picky, male fiddler crab with his giant claw getting in his way, male bower bird building a useless bower and decorating it with mostly blue items to seduce the female who will take care of the chicks alone. Only the largest and strongest, dominant male elephant seals will breed, and they will stop eating and live off their fat during the breeding season. Zebras harmless playfights used to practise male fights. Magellanic penguins of Patagonia mate with the same partner year after year, and at spring the female recognizes her mate’s call after months of separation before they get reacquainted. Digger bees of Arizona have no time for courtship, the male must find and dig out a female who has developed underground, before a fight with other males ensues and the strongest of them leaves with the prize leaving dead competitors. Two female whipped-tail lizards can mate together, there are no males in this species. Barnacles are both male and female and find a barnacle acting the other sex by probing their rock with a long penis-like tentacle. The male jawfish quickly comes out of his hole to display his presence for the female to join him at great risk. A male can become a prey while trying to attract a female, such as the singing male spade-foot toad who ends up eaten by a snake. The male of the black widow spider is much smaller than the female and gets eaten by her after mating, but he has had time to put a “plug” inside the female to make sure no other male can mate with her. In Australia, the marsupial mouse male’s unhealthy sexual behavior causes him to prolong mating for up to twelve hours, then moving along to another female until he dies of exhaustion, after doing everything he could to make sure he is the father. Giving birth is hazardous, as for a female gazelle who becomes a prey for hyenas and scavengers during the process. In bacteria, asexual reproduction can produce billions of clones a day. Similarily, the female aphid can give birth to clone females without needing to mate. So what is sexual reproduction for? Diversity! In colonies of naked mole-rats, only one female gives birth while other individuals have specific roles such as toilet-training the youths. The jewel wasp doesn’t raise its young, bith can give it a head start by paralyzing a cockroach, cutting its antennas, pulling it into her tunnel before laying a single egg on it for the larvae to have something to eat since it will never meet its parents. Like many other birds, the male scrub turky takes great care of the eggs and chases off a monitor lizard by throwing dust at it. The male jacana takes care of the chicks alone and has evolved to carry them under his wings away from a treatening crocodile. When the warblers leave their nest for a moment, a female cuckoo comes in, throws one egg out and lays her own egg which has evolved to look like the warbler’s. The south american male rhea incubates the eggs of several females and babysits up to fifty chicks. In Argentina, a rodent called mara raises the youngs in a communal nursery, but a female will only provide milk for her own.

  1. A time and a place

Annual rising of the pololo worms synchronized with the moon on the coral reefs of Samoa. The Pacific grunion fish mates by the moon to lay its eggs, for them to mature and wait for the next high tide to hatch. The horseshoe crab, a distant relative of the spider and an eon-old creature, rides the highest tide to lay its egg-laying beach, which gave its offspring an evolutionary advantage at times when there were no predators on dry land. For their eggs to hatch during early spring, birds must lay them during winter, when the only clue to the coming season is a slightly longer day. Once every three years in late summer, the green turtle heads for a warm tropical beach where she lays a clutch of eggs; the exact temperature of the sand will determine the sex of the offspring: the warmest eggs will give larger individuals, females. The newborn gazelle faon, instead of standing on its feet rightaway to run away from jackals, adopts another strategy: it stands so absolutely still that it cannot be seen. The year’s first year often starts a reproductive chain. When the spadefoot toad mates in the Arizona desert, the process must be conducted at double time for the tadpoles to mature and bury into the sand before the pool evaporates. In some cases, there is a precise rhythm for reproduction although no external event seems to be involved. After being buried, feeding and growing underground for 17 years, thousands of cicada nymphs emerge on the same day to climb up the trees, split their skin and let a cicada out; if it falls on the ground while drying its wings, it will have spent 17 years in the ground for nothing! If everything goes well, the female mates, then lays 30 eggs and dies; when the eggs hatch, the larvae fall on the ground and bury themselves again… A little african finch called the weaver bird has an interesting way to tell the female about the quality of his genes: it must weave an intricate nest before it mates, making it as green as possible to prove its dexterity at completing the nest before the leaves dry. The sea tiger is a sea slug which follows the trail of other sea slugs to eat them; when two encounter each other, they start eating each other but as they recognize their own kind their mating organs protrude on their sides and they start mating, each individual being both male and female both will become pregnant. The male ibex recognize the female’s breeding condition by sticking his tong out to detect a scent signal released by the female. Sexual signalling can take various forms, as in female chimpanzees whose genitals become large and swollen when they are ready to mate, every few years. The bolas spider captures moths by baiting them with a sticky thread scented with the essence of female moth. The density of the forest often limits the value of sight in signalling, giving birds a strong incentive to sing to attract a mate. The female elephant is in breeding condition every five years or more and sends her signal by ways of chemicals in her urine which the male will taste; when males are in mating condition, call “must”, liquid oozes from glands located above their eyes. The singing of male humpback whales can travel under water around the globe, and is so complex that half an hour can pass before it is repeated; in every mating season, all males sing the same universal song and change it simultaneously. Ritualized display by males to attract females can also be performed in the human species, as in Woodabe tribes of Sierra Leone where men must use make up, sing and dance to be chosen by women.

  1. The sex contract

In many societies, after a long courtship, man and woman celebrate their lifelong pledge to one another in a sex contract called marriage. Mountain goats are nomads and males don’t have a territory to defend; since females raise their young alone, monogamy is not necessary for these animals. The male baboon must groom the female or her baby before he can mate; some males even cheat by pushing the baby around until it whines to be able to comfort it afterwards and show how good a father they are! Competition between male giraffes is harsh because only the winner will be able to mate with all the females in the herd, and yet the ritual to establish dominance called “necking” is not a true fight and never draws blood. In some male snakes, no bites or wounds are inflicted during mating fights, and instead a male must hold the other’s head to the ground to establish its dominance. In the Arizona desert, when a female wasp digs herself out of the sand, several males are waiting and fight to reach her. A sneaky male beetle takes advantage of two larger males being busy fighting to mate with the female unnoticed. Sneaky, treacherous behaviors are common among males and females. In rhesus macaques, only the dominant alpha male has easy access to females; the females like to mate with the newcomers who have not yet climbed the hierarchical ladder, but they must do so secretly or the alpha male will punish the female – not the young male. The mating praying mantus female devours the head of the male, resolving the conflict between two nervous centers: the one in his head telling him that he is in danger, and the one in his abdomen propelling him towards sex. In Utah mormon crickets, it is the male who chooses the female he wants to mate with by lifting her body to measure her weight and estimate the number of eggs she might be carrying. During mating the male attaches to the female’s abdomen a sperm packet loaded with a large food reserve representing up to 25% of his weight, so his contribution to the next generation is larger than the female’s, which is why he sets the rules for the sex contract. Cheetah brothers can live together for life and suprisingly do not fight together when a female wanders into their territory and rather are willing to share her because in order to be able to mate they must have a territory, and they need each other to do so. Termites are all brothers and sisters from a single mother, so they can give up sex and take care of the newborns instead. Although monogamy is the most prevalent sex contract in human societies, sexual contracts are quite varied, going from to polygamy to polyandry in the western Himalayas where a married girl is shared between her husband and all his brothers.

  1. The human animal

Two million years ago in the great rift valley of East Africa, homo habilis – the fist species of human – climbed down the trees out of the forest and started travelling, hunting in the savannah. Since we know little about them, we can study the only other primate to have moved into the plain: the baboon. To survive the large predators of the plains, the first humans would almost certainly have lived like modern baboons, in groups, despite the stress and tension living together can generate. But unlike baboons, early humans became hunters as well as gatherers because of the scarcity of fruit and leaves in the plain. Hunting was impossible for a female who had to take care of her baby for years, thus making males important for them. As chimpanzees live in forests and are less exposed to predators, social bonding is loosened, and females mate with any male when they are in oestrus. It is the “multi-male system”. Males hardly ever fight over sex and are therefore almost the same size as females; instead, they compete through the size of their testies which allow them to copulate more often. The male-female size difference in humans is the same as in chimpanzees, but females have no oestrus display: how did this evolve? For 1 ½ million years, homo erectus travelled and left remains in various places from which we can make deductions about their life. Male skeletons are 20% larger than females; with a larger brain, babies had to be born less developed to be able to pass through the birth canal; the baby’s survival depended on both a father and a mother to be reared. Women are the only modern primate who has permanently swollen breasts; this could have evolved to attract men and display a woman’s ability to give birth. They do not signal when they are ovulating, which could keep the valuable male guessing, attentive all the time. These conditions can allow for strong pair bonding, perhaps heading towards monogamy. A pack of african wild dogs is one of the largest existing family groups, governed by strict hierarchical rules, sticking together for survival. Why are females of Borneo’s proboscis monkeys so attracted by pot bellies and protruding noses in males? Male ourang-otans are at least twice the size of females, lead solitary lives, do not help raise the youngs, and their only contribution to reproduction is through the passing on of strong healthy genes. Among humans, both sexes compete for the best mate. In a bride market in Morocco, men and women come for the honest purpose of shopping for a mate by evaluating them mainly on their looks and childgiving ability for women, or looks and wealth for men. There is a balance between the rules of male dominance, and the female’s ability to chose a mate. Social taboos against inbreeding can be seen in all human cultures. In a large study of israeli kibbutzim, it has been found that because children are raised like brothers and sisters in a large family-like lifestyle, no one marriage had occured between children raised in the same group, although they were not related. The aversion to brother-sister incest is so strong that it can be triggered simply by close familiarity.

  1. The mystery of mating

Bitterling fish lay their eggs inside musles for them to hatch in security doing no harm to the musle. But animals living on land had to evolve to adapt to internal fertilization. Male spiders have a leg adapted to placing a sperm packet inside the body of the female – without being eaten. In some dragonflies, the male’s penis has evolved to scoop the sperm of other males out of the females body, then fertilize her and hold on to her until she lays her eggs so no other male can get to her. In female bats, the genital tract has a special form to which the male’s penis has evolved, ensuring that copulation with another species does not take place. The sperm of male porcupines forms a plug inside the female, to make sure no other male can fertilize her. Male east african olive baboons have a lasting relationship with the female, even when she is not in heat, in order to be there when she is ready to mate. In a species of mediterranean worm, the female has a body the size of a walnut and a nose up to six feet long with which it can eat worm larvae; those which are not eaten become females, and those who are become microscopis males who will spend their life as sperm donors inside the female’s body. Crocodile eggs do not have a predetermined sex; this will be decided by the temperature of the surrounding sand. In a bee colony, on the contrary, the queen has collected sperm from several males in order to fertilize her eggs to produce the female workers for the colony. Otherwise she also lays male eggs depending on the type of cell the colony is building. The queen therefore obeys what the colony decides resulting in more females than males being produced; the males are thrown out of the colony to spread its genes outside. The siamese fighting fish male catches the female’s egg in his mouth, then gently spouts them in a nest of bubbles at the surface where he can keep an eye on them. In giant waterbugs, the male makes an even bigger sacrifice by letting the female glue her eggs on his back for him to take care of, making the male a sitting duck for predators as the female then swims away; in return, this allow him to make sure the eggs are not fertilized by another male. In Panama, the poison arrow male frog also babysits its tadpoles and carries them to a larger pool. In some african bloodsucking flies, as its maggot cannot suck blood, the female nurtures it inside her body until it occupies her entires abdomen and can be released, at which time it will promptly go from pupation to adulthood and suck blood for itself. In australian caves, food is so scarce for white rumped swiflets that the parents can rise only one at a time, so they manage to almost force feed the chick to grow very fast until it is large enough to incubate a second egg during the same short breeding season, leaving the parents enough time to fetch food for both chicks. The male mally fowl builds a large mound of ash and rotting vegetation for the female to lay her only egg inside and keep it warm, almost baking themselves an offspring. So much effort pays off giving birth to a mature chick ready to take care of itself. A very special kind of fish, the male seahorses has a pouch in which the female lays her eggs, making him pregnant for 50 days. The australian echidna female is a rare mammal which has a pouch like a kangaroo, but keeps an egg inside it until it hatches, giving birth to an immature embryo who will still need the protection of her pouch before it becomes large enough. The newborn wallaby embryo is still so underdeveloped that it does not even have hind legs, and yet can crawl up it mother’s belly into the pouch. The female chameleon gives birth not to an egg, but to a fully developed baby chameleon which can take care of itself. Female gray headed fruit bats have evolved to doing everything upside down, from mating to giving birth, so the baby needs to be born with strong and well-developed feet to hang on to its mother in this strange position until it can leave her.

  1. The young ones

Some animals are born orphans, as for squids in which the parents lays a large pouch containing hundreds of offsprings so a few can survive predators. Most bird parents, on the contrary, are little more than feeding machines for their chicks, and both parents must stick together to be up to the task, which explains why 90% of bird species are monogamous. Discous fish feed their fry by exuding a slimy nutritious substance from their skin. Australian bulldog ants produce “trophic eggs” that are only used to feed their larvae. The female amarobias (?) spider feeds her spiderlings by making one last large meal and then dying for them to feed on her body, making the ultimate sacrifice. In wild dogs societies, only the dominant female of the pack breeds, and therefore gives birth to – and must feed – a huge offspring. Desert bees bury a single egg in an underground “bomb shelter” with a large ball of pollen, but flies then fly above the nest a drop their eggs from high above; as the bee larva eats the pollen ball, the fly larvae will eat the bee. For eagle chicks, food is so scarce that parents can usually only one, so the first egg to hatch gives birth to an older and stronger chick which will often kill its younger sibling. Why do lions kill cheetah cubs? The jacana male uses the old broken wing trick, pretending to be injured and risking his own life to divert a snake away from his eggs. Pink flamingos take care of their young by placing them in daycare. In Israel, ibex females hide their kids on an unaccessible cliff where even they have trouble getting back to nurse them. The herd offers the calves a good protection for Serengeti’s wildebeast, unless one loses sight of his mother and wanders away from the herd.

Livre : La bosse des maths - Stanislas Dehaene

Comment comptons-nous ? Les possibilités du cerveau vont de l’estimation du nombre de personnes dans une salle aux mathématiques les plus abstraites en passant par le calcul mental. Les neurosciences, l’étude de patients atteints de lésions cérébrales et présentant des troubles liés au calcul (acalculie), ainsi que l’imagerie médicale, ont permis de progresser dans la compréhension de ces mécanismes. C’est la spécialité de Stanislas Dehaene, chercheur à l’INSERM et jeune membre de l’Académie des Sciences, qui présente dans La bosse des maths une synthèse de ce qui est connu aujourd’hui, et expose ses propres théories.

Depuis Hans, le cheval calculateur qui défraya la chronique au début du 20ème siècle, des expériences un peu plus sérieuses ont permis de montrer que les animaux sont bien capables de compter, de distinguer les quantités, et d’effectuer des opérations arithmétiques élémentaires. Des rats ont pu être entraînés à appuyer un nombre précis de fois sur un levier (4, 8, 12, ou 16 fois), puis sur un autre levier pour « clore » la série. Ils s’acquittent de cette tâche avec une marge d’erreur élevée mais qui met en évidence le fait qu’ils ont une assez bonne estimation des quantités. Un chimpanzé au Japon a pu apprendre les 9 premiers chiffres, sait dénombrer des ensembles avec 95% de réussite, sait ordonner des chiffres par ordre croissant. Un autre sait réaliser des additions telles que 4+2=6 en manipulant des symboles, et choisir le plus grand parmi plusieurs nombres compris entre 1 et 9.

Chez l’homme, la théorie de Piaget qui domine encore largement repose sur l’idée qu’à la naissance le cerveau est une page blanche, et que les notions abstraites telles que les quantités et les nombres sont acquises par l’expérience au cours des phases successives du développement. Pourtant il est possible de réaliser avec des nourrissons des expériences très astucieuses qui contredisent cette théorie. En l’absence de langage, la réaction d’un bébé est mesurée en chronométrant le temps qu’il passe à fixer une image plutôt qu’une autre, ou bien encore en mesurant l’intensité de la succion qu’il exerce sur une tétine quand on lui présente différentes configurations. Ainsi, un nourrisson d’une semaine semble déjà capable de distinguer 2 de 3, qu’il s’agisse de points, de dessins, ou de sons. Chez un enfant de cinq mois, lorsqu’on dissimule successivement une poupée puis une deuxième derrière un rideau, quelle n’est pas sa surprise lorsqu’on abaisse le rideau et qu’il ne voit plus qu’une poupée ! Ceci reste valable même lorsque les objets dissimulés sont en mouvement et posés sur un tourne-disque. Ainsi presque tous les bébés sont-ils capables de distinguer les quantités un, deux et trois, mais aucun ne parvient à distinguer quatre de cinq ni de six. Ceci laisse supposer que notre cerveau contient dès la naissance des circuits, programmés dans notre patrimoine génétique, qui nous permettent de manipuler approximativement des petites quantités.

On constate que dans toutes les civilisations, les chiffres 1, 2 et 3 ont un statut à part et sont généralement représentés par autant de traits ou de points, ou bien un symbole évoquant ces derniers (ce qui est le cas des chiffres arabes : 3 est une contraction de trois traits horizontaux, si, si !). Mais dès qu’on nous présente un nombre plus élevé de traits, comme IIIII, nous sommes incapables d’apprécier rapidement la quantité concernée et devons faire appel à de nouveaux symboles comme V. Cette faculté d’estimer en un coup d’œil un nombre d’objet s’appelle subitisation, et si elle est très efficace jusqu’à trois objets, le temps de réaction et le taux d’erreur commencent à augmenter à partir de quatre et au-delà. Inconsciemment, ce mécanisme influence notre aptitude à manipuler des quantités, même lorsqu’elles sont représentées sous forme de symboles tels que 8 ou 9, probablement parce que notre cerveau les convertit automatiquement en une estimation de quantité quasi-continue (« analogique »). Ainsi, le temps mis pour comparer deux chiffres est plus long lorsqu’il s’agit de 7 et 8 que lorsqu’il s’agit de 2 et 3 (et ce quelque que soit le niveau d’études du sujet !). Quand il s’agit de décider si un nombre est plus grand ou plus petit que 65, le temps de réponse est d’autant plus faible que le nombre présenté est éloigné de la référence ; c’est l’effet de distance. Nous n’associons pas seulement une estimation quantitative aux symboles que nous lisons, mais également d’autre caractéristiques plus surprenantes : sous l’effet de l’écriture de gauche à droite, les nombres croissants sont visualisés par la plupart des gens comme une ligne allant de gauche à droite et de bas en haut ; certaines couleurs sont associées préférentiellement à certains chiffres ; et la taille des caractères utilisés brouille notre perception de la quantité que nous sommes en train de lire.

Le système de numération chinois est plusieurs fois cité en exemple pour son efficacité :

- le nombre de mots nécessaires pour apprendre à compter est minimal : un mot pour chaque chiffre de 0 à 9 ; et quatre mots pour 10, 100, 1000 et 10000 ; il faut vingt-sept mots pour parvenir au même résultat en français ;

- les mots chinois sont très courts, donc rapides à prononder (0,25 s. par chiffre contre 0,3 s. en français) ce qui permet aux Chinois de retenir 9 chiffres dans leur mémoire auditive là où nous ne pouvons en retenir que 7 ;

- la syntaxe est simplifiée à l’extrême et ne souffre aucune irrégularité : 231 se dit deux cent trois dix un ; que l’on songe à notre cent quatre-vingt dix-sept ou au thirteen qui se confond avec thirty…

Ceci semble expliquer que, à âge égal, les petits Chinois savent compter plus loin que les Américains, et sont meilleurs en calcul.

Comment effectuons-nous les calculs ? Comme on l’a vu, les calculs très simples comme 2+1=3 sont à la portée d’un bébé, d’un chimpanzé, et même d’un rat, et font appel à la partie de notre cerveau qui estime les quantités. L’étape suivante consiste à mémoriser les tables d’addition et de multiplication jusqu’à 9, ce qui s’apparente à une poésie particulièrement difficile. Pour calculer 7x8 nous faisons donc essentiellement appel à notre mémoire long terme. Mais lorsqu’il s’agit de calculer 12x10 ou 620-21, nous avons mémorisé à l’école une biblothèque d’algorithmes parmi lesquel nous devons chercher lequel est approprié pour trouver le résultat ; il s’agit encore d’un troisième mécanisme.

Pour savoir quelle partie du cerveau est utilisée pour chacune de ces opérations, nous disposons de plusieurs méthodes. La première, appelée dissociation, consiste à examiner minutieusement des patients atteints de lésions cérébrales et chez qui une fonction cognitive est altérée mais une autre intacte. Ce type de diagnostic permet d’établir que les deux fonctions ont des localisations différentes dans le cortex. Parmi plusieurs exemples, on note M. Nau… à qui l’on présente le chiffre 5 et parvient à dire qu’il s’agit d’un chiffre et non d’une lettre, mais ne peut l’identifier qu’en comptant sur ses doigts. Après des tests approfondis il s’avère que M. Nau… demeure capable d’approximer les quantités avec une marge d’erreur importante, mais a perdu toute faculté de jugement exact. Ainsi, si on lui présente les chiffres 6, 7 et 8, après quelques instants il ne sait plus si il y avait un 5 ou un 9, mais est certain qu’il n’y avait pas de 1 ni de 3 qui sont bien trop petits.

La fonction qui permet d’estimer les quantités, est localisée dans le cortex pariétal inférieur (en haut à l’arrière du crâne) ; elle est présente dans les deux hémisphères, mais plus active du côté dominant (à gauche pour les droitiers). En revanche, le calcul mental, qui est associé au langage, est le monopole exclusif de l’hémisphère gauche (sauf cas exceptionnel de personnes chez qui l’organisation cérébrale est en miroir). Ainsi, un vétéran du Vietnam qui a eu une grande partie de l’hémisphère gauche emporté par une balle (et a survécu !) est aujourd’hui capable de lire les chiffres, de comparer les quantités, et d’estimer le cardinal d’un ensemble, mais ne parvient à réaliser que la moitié des additions à un chiffre qu’on lui présente tandis que les opérations plus complexes sont insurmontables pour lui. M. M. quant à lui a une lésion du cortex pariétal inférieur. Il est capable de réciter « trois fois neuf vingt-sept » mais il place 2 entre 3 et 5. Dans de très rares cas, des patients peuvent souffrir d’epilepsie localisée dans le cortex pariétal inférieur, et avoir des crises qui se déclenchent dès qu’ils tentent de faire un exercice de calcul mental trop soutenu. Le diagnostic est parfois rendu compliqué lorsque la compréhension, le langage ou l’écriture sont atteints, comme chez ce patient qui, confronté au problème 4+5 dit « huit », écrit 5, mais choisit toujours 9 parmi plusieurs chiffres proposés ! Ou encore cette patiente atteinte d’une petite lésion du cortex prémoteur, qui produit des gribouillis incompréhensibles lorsqu’elle tente d’écrire son nom, mais est capable de poser par écrit une multiplication à plusieurs chiffres de sa plus belle écriture.

Au vu de ces observations, la théorie que propose Stanislas Dehaene pour modéliser le fonctionnement du cerveau dans le domaine du calcul est la suivante :

- le cortex pariétal inférieur serait spécialisé dans la représentation des quantités approximatives ;

- le calcul mental faisant appel aux tables mémorisées s’appuierait sur les noyaux gris de l’hémisphère gauche, impliqués dans la mémorisation et la restitution de séquences motrices automatiques ;

- pour résoudre des problèmes plus complexes, le cortex préfrontal et le cortex cingulaire antérieur interviendraient pour orchestrer les opérations : identification des nombres, récupération des quantités, choix de la stratégie de résolution, exécution, détection d’erreurs éventuelles.

Reste à se demander si ces fonction sont ainsi localisées dès la naissance et pour toute la vie, ou si c’est là le résultat d’un apprentissage qui transforme une zone vierge en unité de calcul. Un début de réponse est apporté par l’étude d’enfants qui présentent des problèmes d’acalculie incurable dès les premières années d’école, et présentent une anomalie ou une lésion cérébrale localisée. Ceci remet en question l’étendue de la plasticité cérébrale de l’enfant. Le modèle privilégié serait le suivant : la lecture et le calcul n’existent que depuis quelques millieurs d’années, trop peu pour que l’évolution ait façonné des régions du cerveau génétiquement spécialisées dans ces opérations. Mais du fait que l’organisation de certaines connections est en partie déterminée génétiquement, certaines parties du cerveau seraient mieux adaptées que d’autres à ces fonctions, c’est pourquoi les opérations correspondantes s’y localiseraient préférentiellement. [NDR : Il existe des fonctions très évoluées dont la localisation n’est pas aussi rigide. J’ai été très frappé de découvrir qu’un enfant épileptique pouvait réapprendre à parler – quoique au prix d’immenses difficultés – après une ablation totale de l’hémisphère gauche à l’âge de 7 ans !]

Stanislas Dehaene a organisé et participé à bon nombre d’expériences d’imagerie cérébrale dans le but de tester cette théorie. La première famille de techniques d’imagerie repose sur le fait que, lorsqu’une aire cérébrale est sollicitée, cela déclenche en quelques secondes une augmentation de sa consommation en glucose et une dilatation locales des vaisseaux pour augmenter l’apport d’oxygène. La tomographie par émission de positrons, par exemple, consiste à injecter au patient un marqueur légèrement radioactif qui va se concentrer dans les régions actives et émettre des particules qu’il s’agit ensuite de localiser en 3D. Avec la résonnance magnétique nucléaire, cette famille de techniques atteint une très bonne résolution spatiale, de l’ordre du millimètre. En revanche, sa résolution temporelle est très mauvaise, puisqu’il faut faire une « pause » de plusieurs dizaines de secondes pour prendre un « cliché ». Si l’on veut observer la propagation du signal d’une aire cérébrale à une autre en temps réel, il faut faire appel à l’électroencéphalogramme, ou à la magnétoencéphalographie, qui ont à l’inverse une très bonne résolution temporelle et une très mauvaise résolution spatiale… Malgré ces limitations, ces techniques donnent des résultats qui vont dans le même sens que les observations cliniques citées plus haut, et confortent donc peu à peu le modèle proposé.