3- The Triune Brain Revisited

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You are probably familiar with the triune brain model  –

Brainstem – “reptilian brain” – Most of the brain in fish, amphibians, and reptiles.  Controls basic sensations, body functions, and instinctive drives.

Limbic System – “old mammalian brain” – Well-developed in all mammals, including humans.  Our “emotional brain,”  especially important in social emotions.

Cerebral Cortex – “new mammalian brain” – Well-developed in primates, especially humans.  Underlies thinking, language, and other cognitive functions.

That’s the triune brain.  It is the basis for our five-part overview map,  expanded to also include the reticular system and cerebellum.

But first, the triune brain itself  –

You probably learned about it in a class or program somewhere.  It was introduced by neurophysiologist Paul MacLean in 1970,  quickly gained wide popularity, and became a dominant brain-mind paradigm.  It has been called “the best concept we have” for understanding how brains structure minds.


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Like most paradigms, the triune model has become so familiar it’s usually taken for granted, its basic importance overlooked or dismissed.   But it has strongly shaped our views.  Before 1970, the idea of evolutionary brain structure was not well known outside the neurosciences.  For most people, the brain was a mysterious black box  —  unmapped terra incognita.  Today terms like “reptilian brain” and “limbic system” (both introduced by MacLean) are familiar, commonsense acknowledgements of our evolutionary heritage.

Of course the triune model is over-simplified, but so are the continents on a globe map of the world.  Both are roughly accurate, attractive in their wholeness, and useful as educational tools and orienting references.  Carl Sagan called the triune model  “a metaphor of great utility and depth,”  and like other recent thinkers, used it as a foundation for his own views of human nature.

According to MacLean, we have “three brains”  –  brainstem, limbic system, and cerebral cortex  –  underlying “a primal mind, an emotional mind, and a rational mind.”

“The three formations are remarkably different in chemistry and structure and in an evolutionary sense eons apart. Extensively interconnected, [they] represent an amalgamation of three-brains-in-one, or what may be appropriately called a triune brain… Stated in popular terms, [we have] three interconnected biological computers, each [with] its own special intelligence, its own subjectivity, its own sense of time and space, and its own memory, motor, and other functions.”

The Brainstem

A human brainstem is remarkably similar to a reptile’s brain.  And it still controls the same instinctive functions  –  basic animal drives, visceral regulation, simple sensory and body awareness.   Here are programs for physical survival (fight and flight), hunger, thirst, elimination, sexual drives, and other body functions.  MacLean said

“lizards and other reptiles provide illustrations of… patterns of behavior commonly seen in mammals, including man.  One can quickly list more than 20 such behaviors that primarily involve self-preservation or the survival of the species.”


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In humans, higher brain levels modify these instinct patterns.  That’s why, for example, our eating habits and courtship patterns differ from those of lizards.  But the basic feeding, sexual, and other drives are still programmed in our brainstems.

Brainstem programs are mostly hard-wired.  There’s little room for learning, other than simple conditioning, at this level.  Behaviors are routine and repetitive – rigid instinct patterns, just slightly modified by habit formation.  MacLean said

“The reptilian brain appears to be a slave to precedent…it appears to have inadequate neural machinery for learning to cope with new situations.”

Again, the reptilian brain controls basic sensations, body functions, and instinctive drives.  It underlies our basic animal nature.

And as you know, it has a core  –Core Consciousness Reticular System

Running lengthwise through the center of the brainstem is the reticular system.  MacLean included it as part of the “reptilian brain.”  But we inherited it from the first vertebrate animals  –  wormlike sea creatures, ancestors of reptiles and all other vertebrates.

“You may be familiar with Paul MacLean’s metaphor in which a reptilian brain is overlain by an old mammalian brain which is in turn overlain by the new mammalian brain. Well the reticular formation is, in effect, the chordate (worm) core of the reptilian brain. It is the oldest of the old.”

– Philosopher Bill Benzon

With the evolution of fish, amphibians, and reptiles, specific brainstem pathways and systems evolved from the diffuse core.  These systems are distinct and well-differentiated, except for their inner borders, which connect with core networks.  Neuroanatomist David Bowsher described a typical system:


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“[It] shows a clear-cut peripheral margin, but internally appears to merge with the reticular core…there is no clear internal boundary…It is as though the specific [systems] on the edge of the reticular core had been dragged away from it and differentiated out of it.”

These specific systems, activated by core networks, control our instinctual nature.

“There can be no doubt that this reticular activation…constitutes the central core of instinctual behavior. It provides the active source of continued excitation [known] as ‘motivation’ [or] ‘drive.'”

– Neurophysiologist P. Dell

The Limbic System

“In 1952, I suggested the term limbic system as a designation for [this region] which…represents an inheritance from lower mammals…The limbic system derives information in terms of emotional feelings.”

– Neurophysiologist Paul MacLean

MacLean was a pioneering limbic system researcher.  He coined that term, and also called it the “emotional brain.”  He emphasized its primary role in social emotions.

The limbic system is small and rudimentary in reptiles, but is well-developed in all mammals living today, including humans.  It determines how we interact with other members of our species.  In humans, it underlies our feelings about others, positive and negative, and their expression in our personal relationships.

On the positive side, we have caring, sharing, and cooperation.  We experience warm, affectionate feelings and enjoy the company of others.  We want to belong, to fit in with the group.  We play, help each other, imitate and conform.  We share these limbic emotions with other mammals.

MacLean particularly emphasized the importance of parent-offspring bonds  –  our strongest social emotions and the basis of the family.


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“Earlier, in considering lizards, I mentioned the cannibalism of the young. The evolution of the mammals seemed to bring with it a primal commandment against cannibalism. Also of momentous significance in mammalian evolution is… protracted parental care. Indeed, one might say that the history of the evolution of mammals is to a large extent the history of the evolution of the family.”

On the negative side, we have social aggression and anxiety, various offensive and defensive emotions.  Territorial competition and the will to power  are part of our mammalian heritage.  Like other mammals, we have bosses and subordinates, seek status, and play politics.  And like them we experience the social pain of separation and loss.  These negative patterns are also programmed in our limbic systems.

Rudimentary social (e.g  parental or territorial) emotions are present in some reptiles, but the social emotions blossomed with mammals’ evolution.  In humans, our cerebral cortex provides yet another level of social programming, while our limbic system still contains the basic programs.

Of course, the reticular system is central to limbic functions.  In the upper brainstem, the core splits in two major branches, one projecting diffusely throughout the limbic system.  It has been called “the activating core of emotion.” It activates and deactivates limbic pathways in ever-changing emotional patterns.

And information funnels back into the core from all limbic regions. Someone said  “all limbic roads lead to the reticular system.”  Someone else called it “the hub of the limbic wheel.”  Your emotions and feelings are rooted in your core.

The Cerebral Cortex

The cortex is rudimentary in reptiles and remains small in most mammals.   It expanded with the evolution of monkeys and apes, then mushroomed in human evolution.


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It’s where we do our thinking.  It generates images and ideas – imagination and intellect.  It contains programs for language, abstraction, calculation, and other cognitive functions.  It makes plans and predictions, solves problems and achieves goals.  It’s the basis of all our arts and sciences.

Memory and learning are important in shaping cortical functions.   While there’s no “memory system” in the brain  –  all systems have their own memory patterns  –  the memory capacity of the cortex far exceeds that of lower brain regions.  In the brainstem, it’s mostly simple conditioning and habit.  On the limbic level, learning plays a larger role, but much less than in the cortex.  No part of the brain really functions as a blank slate, but in some ways the cortex comes close.

MacLean called the cortex

“…a vast neural screen for the portrayal of symbolic language and the associated functions of reading, writing, and arithmetic. Mother of invention and father of abstract thought, it promotes the preservation and procreation of ideas.”

“[The] cortex provides the neural substrate for language and speech…we owe to it the infinite variety of ways in which we can express ourselves.”

And of course, the reticular system is the functional core of the cerebral cortex.  From the upper brainstem, a major reticular branch (ERTAS – described in Essay 1) projects diffusely throughout the cortex, activating it and its thought patterns.

In 1958, neurosurgeon Wilder Penfield commented on these reticular- cortical connections:

“The neurophysiologists… have made recent delighted excursions within the reticular formation of the brain. Electrophysiologic procedures have enabled them to [trace] the functioning of this cerebral pith upward and outward to the cerebral cortex… In this busy, so-called nonspecific system of ganglionic currents, from the cerebral cortex as well as to it, may well lie the pathways of cortical organization.”


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Sixty years later, we know he was right.  Your cerebral cortex and all its functions are deeply rooted in the “ganglionic currents” of your reticular “pith.” Your reticular core is the energizing source of your intelligence.

Five Functional Units

So the reticular system is the functional hub of  all “three brains”  – brainstem, limbic system, and cerebral cortex.  It deserves equal emphasis in our overview map.  All four systems play major roles in human nature.

But these systems function together.  Their interactions are more or less smooth, and our minds are more or less unified.  The cerebellum has a strong balancing, coordinating, and integrating influence on all brain functions.  It also deserves equal emphasis.

The cerebellum’s evolutionary structure is illustrated in the triune brain diagram (page 1).  It evolved along with our “three brains,” and is strongly interconnected with them at each level.

MacLean agreed that the cerebellum modulates all brain functions, motor and mental, but had little to say about that.  His own research focused on the limbic system, and during most of his career little was known about the cerebellum’s non-motor functions.  MacLean barely mentioned the cerebellum in his many publications about the limbic system and triune brain.

An exception was his 1990 book, in which he devoted several pages to cerebellar modulation of emotion and cognition.  He particularly focused on the “extraordinary size” of the new cerebellum in humans, its strong interconnections with the cerebral cortex, and its probable role in coordinating thought processes.


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But in general, MacLean did not address the cerebellum’s influence on our “three brains.”  That’s interesting, as he often commented on their “inadequate coordination” as a likely cause of some of mankind’s major problems.  The reptilian or old mammalian brains, he said, sometimes take control of individuals or even societies.  Our primitive animal natures can lead us into all kinds of irrational and destructive behaviors.  The solution, he suggested, may be better coordination between those old brain regions and the new, more rational cerebral cortex.

(MacLean was one of several thinkers to speculate about possible design problems in our brain’s evolution and their possible implications for mankind’s future.  More on that in another essay.)

So our brains have five major functional units  –  reticular system, brainstem, limbic system, cerebral cortex, and cerebellum.  Each plays a major role in human nature, and all deserve equal emphasis in an overview brain map.

Of course, this five-part model is just a refinement of MacLean’s model, and therefore is subject to similar criticisms.  Let’s take a quick look at them  –

Criticisms of the Triune Model

The triune brain has arguably become our dominant brain-mind paradigm.  It has certainly been important in shaping our view of ourselves.  But despite its success, it has received its share of criticism.

First, the non-scientific criticisms come from both ends of the ideological spectrum.  Many religious conservatives don’t believe in evolution, and aren’t buying the idea of evolutionary brain layers.   End of that discussion.


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On the other extreme, some New Age thinking views basic human nature as only positive, so rejects the idea of innate human aggression.  This contrasts with MacLean’s view, which locates aggression (along with other positive and negative instincts and emotions) in the reptilian and old mammalian brains.  In the idealistic New Age view, the triune model perpetuates a mistaken negative view of  humanity  –  “the myth of the beast within.”  The reptilian brain is rejected as a modern version of the biblical serpent-demon, and instinctive aggression as a kind of “original sin.”

Scientific criticisms of the triune model are mostly related to its over-simplification of complex brain structures and functions.  Dividing the brain into three     (or five) parts hardly does justice to its complexity.  Such generalizations necessarily sacrifice some accuracy of detail, and valid criticisms have been raised about some details of the triune model (see Notes).  From a fine-grained technical perspective, those shortcomings are significant, but much less so from an overview perspective.

The triune model is widely embraced in the psychological and social sciences, by educators from elementary to college levels, and by the general public.  A group that has not found it so useful is brain scientists themselves, as it does oversimplify the neurological complexities they deal with daily.  That has been used as an argument against the model, but in fact that’s no reason to discard it.     A globe map of the world may have no practical value for land surveyors, but it’s generally accurate and, for many people, a useful orienting reference and educational tool.

Again, most shortcomings of the triune model seem insignificant from an overview perspective.  One shortcoming, however, does not.  The strongest and most valid criticisms of MacLean’s model deal with his ideas about emotion. In fact, our understanding of the brain and emotion has changed significantly in recent decades.  That’s the focus of Essay 4.


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The triune model isn’t perfect, but it represents a major step forward in understanding our brains and ourselves.  It’s a “bridging concept” –  it makes neuroscientists’ findings accessible to people outside that discipline.  It provides an essential brain framework for psychology and the social sciences.  It’s a good educational tool, and helps inform the wider public about basic human nature.

That’s important, considering how many of mankind’s major problems involve confusion or disagreements about basic human nature.  The better we can understand ourselves, the better chance we have to solve our problems and achieve our goals, as individuals and as societies.

Neuropsychologist Gerald Cory described MacLean as “a pioneer, a trailblazer, a scientist, and thinker well ahead of his time.”  The triune model, he said, “is the most useful bridging link, thus far articulated, between neuroscience and the larger and pressingly critical questions of humanity’s survival.”

“The triune brain concept may need modification, then, as the body of neuroscience grows…but certainly not outright rejection. With appropriate clarifications, it is still by far the best concept we have for linking neuroscience with the larger, more highly generalized concepts of the social sciences. This is true even if its level of generality has limited utility for some neuroscience researchers…I think that it will continue to be influential, and with appropriate modifications as research progresses, provide an important underpinning for interdisciplinary communication and bridging.”

NOTES

Page 3 – The term “limbic system” has been appropriately criticized, as it is not one system, but actually many systems performing various related functions.  It might more accurately be called a macrosystem, or perhaps the “limbic region.”  Some have suggested dropping the term altogether, but it’s now well-established and that seems unlikely.

Page 7 – The cerebellum’s major evolutionary divisions are illustrated schematically in MacLean’s triune diagram.  However, as the cerebellum evolved, those evolutionary divisions became highly folded and wrinkled;  they are not as neatly layered as shown in the diagram.

Page 8 – The notes above provide two examples of minor inaccuracies in MacLean’s simplified model.  Here’s another:

The hippocampus is a small “limbic” region that is actually an ancient part of the cerebral cortex.  But it’s surrounded by limbic pathways, and wired into limbic circuitry, so it’s usually considered part of the limbic system.

The triune model isn’t perfect, but again, from an overview perspective, most of its shortcomings are not too significant.  Neuropsychologist Gerald Cory again:

“The triune brain concept may be wrong in some of its particulars, right in others, but still be very useful and valid in its more general features.”

REFERENCES

  • Benzon, Bill. The Anarchic Brain. www.new-savanna.blogspot.com/2011/10/anarchicbrain.html.
  • Bowsher, David. Brain, Behavior, and Evolution. Brain Behav Evol, 1973.
  • Cory, Gerald. MacLean’s Triune Brain Concept: In Praise and Appraisal. In The Reciprocal Modular Brain in Economics and Politics. Klower Academic/Plenum Publishers, New  York, 1999.
  • MacLean, Paul. A Mind of Three Minds: Educating the Triune Brain. In Education and the Brain. National Society for the Study of Education, Chicago, 1978.
  • MacLean, Paul. Evolution of the Psychencephalon. In Primate Brain Evolution: Methods and Concepts, ed E. Armonstrong and D. Falk. Plenum Press, New York, 1982.
  • MacLean, Paul. The Imitative-Creative Interplay of Our Three Mentalities. In Astride the Two Cultures: Arthur Koestler at 70, ed H. Harris. Random House, New York, 1976.
  • MacLean, Paul. The Triune Brain in Evolution: Role in Paleocerebral Functions. Plenum Press,  New York, 1990.
  • Penfield, Wilder. Designated Discussion. In Reticular Formation of the Brain, ed. H. Jasper, L. Proctor, R. Knighton, W. Noshay, and R. Costello. Little, Brown, and Co, Boston, 1958.