trojedan mozak?

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Američki neuroznanstvenik Paul D. MacLean predložio je 1960-ih jedan trostruki model evolucije mozga sisavaca nazvan triune brain.

Model nije općeprihvaćen, ali nije ni odbačen, mada se čini da ima više poklonika među psihijatrima nego među neuroznanstvenicima. Iz zaključka članka Detlev W. Ploog, The place of the Triune Brain in psychiatry, iz 2003.:

MacLean’s sophisticated work culminating in the concept of the triune brain is the single and most useful concept we have for linking evolutionary psychiatry and neuroscience with concepts of the social sciences. …

MacLean was the first in neuroscience who showed concerns for the neural substrate of subjective experience, which is central to psychoanalysis. The limbic system derives subjective information in terms of emotional feelings that guide behavior required for selfpreservation and preservation of the species. To take this one step further, Jung, the Swiss psychiatrist and Freud’s adversary who developed the idea of archetypes, assigned the brainstem to the site of origin of archetypical dreams, that is, dreams of impersonal content dealing with species-specific behaviors and experiences, so-called great dreams. In MacLean’s concept, it is the striatal complex, the protoreptilian brain.

O tom modelu je 1986. pisao Carl Sagan u knjizi The Dragons of Eden: Speculations on the Evolution of Human Intelligence. Meni je, naravno, zanimljiv zbog analogije s Platonovom trostrukom psihom, a koja nije promakla ni Saganu.

One of the most engaging views of the subsequent evolution of the brain is a story of the successive accretion and specialization of three further layers surmounting the spinal cord, hindbrain and midbrain. After each evolutionary step, the older portions of the brain still exist and must still be accommodated. But a new layer with new functions has been added.

The principal contemporary exponent of this view is Paul MacLean, chief of the Laboratory of Brain Evolution and Behavior of the National Institute of Mental Health. One hallmark of MacLean’s work is that it encompasses many different animals, ranging from lizards to squirrel monkeys. Another is that he and his colleagues have studied carefully the social and other behavior of these animals to improve their prospects of discovering what part of the brain controls what sort of behavior. …

From experiments such as those with squirrel monkeys, MacLean has developed a captivating model of brain structure and evolution that he calls the triune brain. “We are obliged,” he says, “to look at ourselves and the world through the eyes of three quite different mentalities,” two of which lack the power of speech. The human brain, MacLean holds, “amounts to three interconnected biological computers,” each with “its own special intelligence, its own subjectivity, its own sense of time and space, its own memory, motor, and other functions.” Each brain corresponds to a separate major evolutionary step. The three brains are said to be distinguished neuroanatomically and functionally, and contain strikingly different distributions of the neurochemicals dopamine and cholinesterase. …

MacLean has distinguished three sorts of drivers of the neural chassis. The most ancient of them surrounds the midbrain… We share it with the other mammals and the reptiles. It probably evolved several hundred million years ago. MacLean calls it the reptilian or R-complex. Surrounding the R-complex is the limbic system, so called because it borders on the underlying brain. (Our arms and legs are called limbs because they are peripheral to the rest of the body.) We share the limbic system with the other mammals but not, in its full elaboration, with the reptiles. It probably evolved more than one hundred and fifty million years ago. Finally, surmounting the rest of the brain, and clearly the most recent evolutionary accretion, is the neocortex. Like the higher mammals and the other primates, humans have a relatively massive neocortex. It becomes progressively more developed in the more advanced mammals. The most elaborately developed neocortex is ours (and the dolphins’ and whales’). It probably evolved several tens of millions of years ago, but its development accelerated greatly a few million years ago when humans emerged. …

triune brain
triune brain

[W]e should expect the R-complex in the human brain to be in some sense performing dinosaur functions still; and the limbic cortex to be thinking the thoughts of pumas and ground sloths. Without a doubt, each new step in brain evolution is accompanied by changes in the physiology of the preexisting components of the brain. The evolution of the R-complex must have seen changes in the midbrain, and so on. What is more, we know that the control of many functions is shared in different components of the brain. But at the same time it would be astonishing if the brain components beneath the neocortex were not to a significant extent still performing as they did in our remote ancestors.

MacLean has shown that the R-complex plays an important role in aggressive behavior, territoriality, ritual and the establishment of social hierarchies. Despite occasional welcome exceptions, this seems to me to characterize a great deal of modern human bureaucratic and political behavior. I do not mean that the neocortex is not functioning at all in an American political convention or a meeting of the Supreme Soviet; after all, a great deal of the communication at such rituals is verbal and therefore neocortical. But it is striking how much of our actual behavior-as distinguished from what we say and think about it- can be described in reptilian terms. We speak commonly of a “cold-blooded” killer. Machiavelli’s advice to his Prince was “knowingly to adopt the beast.” …

I want to be very clear about the social implications of the contention that reptilian brains influence human actions. If bureaucratic behavior is controlled at its core by the R-complex, does this mean there is no hope for the human future? In human beings, the neo-cortex represents about 85 percent of the brain, which is surely some index of its importance compared to the brainstem, R-complex and limbic system. Neuro-anatomy, political history, and introspection all offer evidence that human beings are quite capable of resisting the urge to surrender to every impulse of the reptilian brain. There is no way, for example, in which the Bill of Rights of the U.S. Constitution could have been recorded, much less conceived, by the R-complex. It is precisely our plasticity, our long childhood, that prevents a slavish adherence to genetically preprogrammed behavior in human beings more than in any other species. But if the triune brain is an accurate model of how human beings function, it does no good whatever to ignore the reptilian component of human nature, particularly our ritualistic and hierarchical behavior, On the contrary, the model may help us to understand what human beings are about. (I wonder, for example, whether the ritual aspects of many psychotic illnesses-e. g., hebephrenic schizophrenia-could be the result of hyperactivity of some center in the R-complex, or of a failure of some neocortical site whose function is to repress or override the R-complex. I also wonder whether the frequent ritualistic behavior in young children is a consequence of the still-incomplete development of their neocortices.)

In a curiously apt passage, G. K. Chesterton wrote: “You can free things from alien or accidental laws, but not from the laws of their own nature. . . . Do not go about . . . encouraging triangles to break out of the prison of their three sides. If a triangle breaks out of its three sides, its life comes to a lamentable end.” But not all triangles are equilateral. Some substantial adjustment of the relative role of each component of the triune brain is well within our powers. …

The limbic system appears to generate strong or particularly vivid emotions. This immediately suggests an additional perspective on the reptilian mind: it is not characterized by powerful passions and wrenching contradictions but rather by a dutiful and stolid acquiescence to whatever behavior its genes and brains dictate.

Electrical discharges in the limbic system sometimes result in symptoms similar to those of psychoses or those produced by psychedelic or hallucinogenic drugs. In fact, the sites of action of many psychotropic drugs are in the limbic system. Perhaps it controls exhilaration and awe and a variety of subtle emotions that we sometimes think of as uniquely human.

The “master gland,” the pituitary, which influences other glands and dominates the human endocrine system, is an intimate part of the limbic region. The mood-altering qualities of endocrine imbalances give us an important hint about the connection of the limbic system with states of mind. There is a small almond-shaped inclusion in the limbic system called the amygdala which is deeply involved in both aggression and fear. Electrical stimulation of the amygdala in placid domestic animals can rouse them to almost unbelievable states of fear or frenzy. … Malfunctions in the limbic system can produce rage, fear or sentimentality that have no apparent cause. …

There are reasons to think that the beginnings of altruistic behavior are in the limbic system. Indeed, with rare exceptions (chiefly the social insects), mammals and birds are the only organisms to devote substantial attention to the care of their young-an evolutionary development that, through the long period of plasticity which it permits, takes advantage of the large information-processing capability of the mammalian and primate brains. Love seems to be an invention of the mammals. …

Much in animal behavior substantiates the notion that strong emotions evolved chiefly in mammals and to a lesser extent in birds. The attachment of domestic animals to humans is, I think, beyond question. The apparently sorrowful behavior of many mammalian mothers when their young are removed is well-known. One wonders just how far such emotions go. Do horses on occasion have glimmerings of patriotic fervor? Do dogs feel for humans something akin to religious ecstasy? What other strong or subtle emotions are felt by animals that do not communicate with us? …

Chief among the neocortical abstractions are the human symbolic languages, particularly reading and writing and mathematics. … Not all symbolic languages are neocortical however; bees- without a hint of a neocortex-have an elaborate dance language, first elucidated by the Austrian entomologist Karl von Frisch, by which they communicate information on the distance and direction of available food. It is an exaggerated gestural language, imitative of the motions bees in fact perform when finding food-as if we were to make a few steps towards the refrigerator, point and rub our bellies, with our tongues lolling out all the while. But the vocabularies of such languages are extremely limited, perhaps only a few dozen words. The kind of learning that human youngsters experience during their long childhood seems almost exclusively a neocortical function. …

Despite the intriguing localization of function in the triune brain model, it is, I stress again, an oversimplification to insist upon perfect separation of function. … Nevertheless, while bearing these caveats in mind, it seems a useful first approximation to consider the ritualistic and hierarchical aspects of our lives to be influenced strongly by the R-complex and shared with our reptilian forebears; the altruistic, emotional and religious aspects of our lives to be localized to a significant extent in the limbic system and shared with our nonprimate mammalian forebears (and perhaps the birds); and reason to be a function of the neo-cortex, shared to some extent with the higher primates and such cetaceans as dolphins and whales. While ritual, emotion and reasoning are all significant aspects of human nature, the most nearly unique human characteristic is the ability to associate abstractly and to reason. Curiosity and the urge to solve problems are the emotional hallmarks of our species; and the most characteristically human activities are mathematics, science, technology, music and the arts-a somewhat broader range of subjects than is usually included under the “humanities.” Indeed, in its common usage this very word seems to reflect a peculiar narrowness of vision about what is human. Mathematics is as much a “humanity” as poetry. Whales and elephants may be as “humane” as humans. (str. 34.-54.)

Osim opisa samoga modela i odgovarajućeg ljudskog ponašanja, zanimljive su i paralele koje Sagan povlači s Freudovim i Platonovim trostrukim koncepcijama psihe.

The triune-brain model derives from studies of comparative neuroanatomy and behavior. But honest introspection is not unknown in the human species, and if the triune-brain model is correct, we would expect some hint of it in the history of human self-knowledge. The most widely known hypothesis that is at least reminiscent of the triune brain is Sigmund Freud’s division of the human psyche into id, ego and superego. The aggressive and sexual aspects of the R-complex correspond satisfyingly to the Freudian description of the id (Latin for “it”-i. e., the beast-like aspect of our natures); but, so far as I know, Freud did not in his description of the id lay great stress on the ritual or social-hierarchy aspects of the R-complex. He did describe emotions as an ego function-in particular the “oceanic experience,” which is the Freudian equivalent of the religious epiphany. However, the superego is not depicted primarily as the site of abstract reasoning but rather as the internalizer of societal and parental strictures, which in the triune brain we might suspect to be more a function of the R-complex. Thus I would have to describe the psychoanalytic tripartite mind as only weakly in accord with the triune-brain model. …

A superior agreement is found in the metaphor for the human psyche in the Platonic dialogue Phaedrus. Socrates likens the human soul to a chariot drawn by two horses-one black, one white-pulling in different directions and weakly controlled by a charioteer. The metaphor of the chariot itself is remarkably similar to MacLean’s neural chassis; the two horses, to the R-complex and the limbic cortex; and the charioteer barely in control of the careening chariot and horses, to the neocortex.

In yet another metaphor, Freud described the ego as the rider of an unruly horse. Both the Freudian and the Platonic metaphors emphasize the considerable independence of and tension among the constituent parts of the psyche, a point that characterizes the human condition…

Because of the neuroanatomical connections between the three components, the triune brain must itself, like the Phaedrus chariot, be a metaphor; but it may prove to be a metaphor of great utility and depth. (str. 54.-56.)



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