The triune brain in evolution: role
in paleocerebral functions
New York: Plenum
Press
Chez les différents types de
vertébrés, le système
nerveux central répond à un
plan général commun
d'organisation, et montre, des plus anciens
aux plus récents d'entre eux, une
complication graduelle en rapport avec des
niveaux de vie de plus en plus
indépendants et fonctionnellement de
plus en plus élevés.
Certaines formations correspondraient
à des activités nerveuses
rudimentaires propres aux
vertébrés inférieurs:
chordencéphale, centre de
réflexes locaux et régionaux
(tronc cérébral). Le
mésencéphale des reptiles
véritable cerveau primitif (voies
senitives centripètes, et motrices
centrifuges) devient le lieu de centres
secondaires chez les mammifères
à écorce ou pallidum
développé.
La théorie de Paul Mac Lean ( Les
3 cerveaux : 1971) explicite cela de
façon plus précise et
construite. Selon lui, le cerveau s'est
développé par "couches
successives", répondant ainsi
progressivement aux besoins de
l'évolution :
d'abord de purs instincts (cerveau
reptilien) qui ne possède pas de
mémoire, lieu d'origine du
bâillement ;
puis acquisition de réponses
émotionnelles contrôlées
(système limbique) ou
cerveau «paléomammalien»
interface synaptique et humorale commune
à tous les mammifères,
siège du bâillement
d'émotivité des singes (et des
bâillements sexuellement induits); il
possède une mémoire;
enfin cerveau
«néomammalien»
caractérisé par le
développement cortical chez l'homme,
en particulier des lobes frontaux
siège de la réplication du
bâillement.
Ces trois cerveaux cohabitent toujours en
nous. Un fonctionnement harmonieux des
différents cerveaux est
nécessaire en incluant des
émotions ou certaines formes de
comportement archaïques pour le profit /
contôle du néo-cortex
siège de la raison / pensée
consciente.
- Cerebral evolution and emotional
process: new findings on the striatal complex
PD MacLean Ann NY Acd Sci
1972; 193; 137-149
- The triune brain in conflict PD MacLean
Psychother Psychosom 1997;
28; 207-220
- Evolutionay psychiatry and the triune
brain MacLean Psychological
Medecine 1985; 15; 219-221
The American neurologist Paul MacLean is a
proponent of "microgenesis", the view that
the structure of our brain mirrors its
evolution over the ages.
Mac Lean believes that our head contains
not one but three brains: a "triune"
brain. Like the layers of an
archeological site, each brain corresponds to
a different stage of evolution. Each brain is
connected to the other two, but each operates
indivually with a distinct
"personality".
The neocortex does not control the
rest of the brain: all three parts interact,
although it is true that the neocortex
interacts in a more "cognitive" manner. But
the "brain" that interacts in a more
"instinctive" manner can be as dominant and
even more. And ditto for the "emotional" one.
The oldest of the three brains, the
"reptilian" brain, is a midbrain reticular
formation that has changed little from
reptiles to mammals and to humans. This
"brain" comprises the brainstem and the
cerebellum. It is responsible for
species-specific behavior: instinctive
behavior such as self-preservation and
aggression.
The cerebellum and the brainstem
constitute virtually the entire brain in
reptiles. The most basic life-sustaining
processes of the body, such as respiration,
heart beat and sleep, are controlled by the
brainstem. More precisely, the brainstem is
the brain's connection with the autonomic
nervous system, the part of the nervous
system that regulates functions such as
heartbeat, breathing, etc. that do not
require conscious control. It has always
active, even when we sleep. It endlessly
repeats the same patterns over and over,
mechanically. It does not change, it does not
learn. In the beginnings, this system was
basically most of the brain, and limbs and
organs were controlled locally.
Most mammals share with us the limbic
system, which MacLean believes was born
after the reptilian system and was simply
added to it. The earliest mammals had a brain
that was basically the reptilian brain plus
the limbic system. MacLean therefore believes
this to be the old mammalian (or
"paleomammalian") brain. The limbic system
contains the hippocampus, the thalamus and
the amygdala, which are considered
responsible for emotions and emotional
insticts (behaviors related to food, sex and
competition). These emotions are functional
to the survival of the individual and of the
species. This system is capable of learning,
because it contains "affective" memories,
which is emotion-laden memories. Ultimately,
the limbicsystem is about "pain" and
"pleasure": avoiding pain and repeating
pleasure.
The neocortex is the main brain of the
primates, which are among the latest
mammals to appear. All animals have a
neocortex but only in primates it is so
relevant: most animals without a neocortex
would behave normally. This "neomammalian"
brain is responsible for higher cognitive
functions such as language and
reasoning.
The oldest brain is located at the bottom
and to the back. The newest sits on top and
to the front. They all complement each other
to produce what we consider human behavior.
Each is an autonomous unit that could exist
without the others The elegance of MacLean's
model is that it neatly separates mechanical
behavior, emotional behavior and rational
behavior. It shows how they arose
chronologically and for what purpose. And it
shows how they coexist and complement each
other. They constitute three steps towards
modern "intelligence".
Brain evolution relating to family,
play, and the separation call.
MacLean PD.
Arch Gen Psychiatry 1985; 42; 4;
405-17
Mammals stem from the mammal-like
reptiles (therapsids) that were widely
prevalent in Pangaea 250 million years ago.
In the evolutionary transition from reptiles
to mammals, three key developments were (1)
nursing, in conjunction with maternal care;
(2) audiovocal communication for maintaining
maternal-offspring contact; and (3) play. The
separation call perhaps ranks as the earliest
and most basic mammalian vocalization, while
play may have functioned originally to
promote harmony in the nest. How did such
family related behavior develop? In its
evolution, the forebrain of advanced mammals
has expanded as a triune structure that
anatomically and chemically reflects
ancestral commonalities with reptiles, early
mammals, and late mammals. Recent findings
suggest that the development of the
behavioral triad in question may have
depended on the evolution of the
thalamocingulate division of the limbic
system, a derivative from early mammals. The
thalamocingulate division (which has no
distinctive counterpart in the reptilian
brain) is, in turn, geared in with the
prefrontal neocortex that, in human beings,
may be inferred to play a key role in
familial acculturation.
The Portuguese neurologist Damasio
thinks that consciousness is an internal
narrative. The "I" is not telling the story:
the "I" is created by stories told in the
mind ("You are the music while the music
lasts").
Damasio breaks the problem of
consciousness into two parts: the "movie in
the brain" kind of experience (how a number
of sensory inputs are trasnformed into the
continuous flow of sensations of the mind)
and the self (how the sense of "owning" that
movie comes to be). The former is a purely
non-verbal process: language is not a
prerequisite for consciousness. Nonetheless,
language is the source of the "I", a second
order narrative capacity.
Neurological research has proven that
distinct parts of the brain work in concert
to represent reality. Brain cells represent
events occurring somewhere else in the body
Brain cells are "intentional", if you will.
They are not only "maps" of the body: besides
the topography, they also represent what is
taking place in that topography.
Indirectly, the brain also represents
whatever the organism is interacting with,
since that interaction is affecting one or
more organs (e.g., retina, tips of the
fingers, ears), whose events are represented
in brain cells. The brain stem and
hypothalamus are the organs that regulate
"life", that control the balance of chemical
activity required for living. Consequently,
they also represent the continuity of the
same organism. Damasio believes that the self
originates from these biological processes:
the brain has a representation of the body
and has a representation of the objects the
body is interacting with, and therefore can
discriminate self and non-self and then
generate a "second order narrative" in which
the self is interacting with the non-self
(the external world). This second-order
representation occurs mainly in the
thalamus.
From an evolutionary perspective, we can
presume that the sense of the self is useful
to induce purposeful action based from the
"movie in the mind". The self provides a
survival advantage because the "movie in the
mind" acquires a first-person character, i.e.
it acquires a meaning for that first person,
i.e. it highlights what is good and bad for
that first person, a first person which
happens to be the body of the organism,
disguised as a self. This second-order
narrative derives from the first-order
narrative constructed from the sensory
mappings. In other words, all of this is
happening while the "movie" is playing. The
sense of the self is created while the movie
is playing by the movie itself. The thinker
is created by the thought. The spectator of
the movie is part of the movie.
