“PAUL D. MacLEAN'S TRIUNE BRAIN HYPOTHESIS:

WHICH PLATONIC METAPHOR FITS AND WHICH DOES NOT?”

 

Charles Don Keyes

Department of Philosophy

Duquesne University

Paul D. MacLean, M.D., Senior Research Scien­tist, National Institute of Mental Health, argues that the present human brain retains the earlier stages of its evolutionary development.  The oldest are "reptil­ian" structures which survive in the human basal ganglia.  "Paleomammalian" structures inherited from non-primate mammals persist in the human limbic system, while the primate or "neomammalian" neocortex reaches its highest evolutionary develop­ment in our pre-frontal lobes.  The three structures are anatomically and chemically different, but since they function as a whole, the human brain is "triune."

 

Thesis

MacLean's hypothesis is a partial naturalistic restatement of Plato's tripartite metaphor of the human soul as a "many-headed, many colored beast" in the Republic (Book IX).  It is not, as Carl Sagan and John Durant claim, based on the bipartite soul metaphor in the Phaedrus.  They mistakenly treat the latter functional model as if it were constitutive but fail to notice the manifest "triune" constitutive nature of the former model.

 

MacLean's Influence

Durant claims that MacLean's triune brain concept is "probably the single, most-influential idea in brain science in the postwar period, at least in terms of public or popular perceptions as for what brain science has to say about the human condition" (Har­rington, 1991).  Durant (1985) attributes the remark­able influence of Mac­Lean's triune brain "metaphor" partly to its simplicity:  "Mac­Lean's work represents an island of accessible and wide-ranging generaliza­tions in an ocean of abstruse and arcane technicali­ties."

 

Pseudo-Simplicity

The same "simplicity" might also help explain why MacLean's concept has been misunderstood in a variety of ways.  Certain misinterpretations are also premature because they were formulated and became hardened before MacLean completely unfolded his project in a crucial essay (1985) and his most defini­tive book (1990).  One such premature misinterpreta­tion comes from the concept's captivity to certain ways of interpreting human behavior in the 1970's, which could be characterized as the "decade of the beast."

The triune brain concept is actually pseudo-simple, because its seeming epic clarity conceals a many-sided complexity.  We readily grasp the parts of the triune brain concept one by one, because they are all concrete.  But since their meaning is their intercon­nection, the whole of the concept is not contained in any of its parts.  We have to suspend judgment about each of the three structures until we have grasped their unity.  Yet we can't grasp their unity without a preview of the parts.  That requires looking at each separately as if in a series of one-dimensional still-life snapshots of a three-dimensional process, all the while trying to explain how each one fits into a three-dimensional dynamic process.  It is excruciat­ingly difficult to clarify what already looks simple.

 

Metaphor or Hypothesis?

Sagan suggests the triune brain concept "may prove to be a metaphor of great utility and depth," while Durant claims it is a "very powerful," but misleading, metaphor.  The triune brain concept is more than a metaphor; it also contains empirical hypotheses which are verifiable, at least in principle, because they can be tested experimentally.  Mac­Lean's basic metaphor seems to be two-sided.  One side inspires new empirical hypotheses.  The other requires us to interpret the meaning of the experi­ences it describes.  The triune brain concept is less a finished result than a starting point that provokes those who study it to emerge and go in new direc­tions.  The dynamic nature of MacLean's concept not only makes it hard to interpret, but there is also a surprising diversity of ways of taking hold of it without misinterpreting it.

 

MacLean Light and Dark

MacLean's writings contain both life-affirming claims about human capabilities and statements about the malaise of the human predicament.  The differ­ence between these two sides of the triune brain concept is partly intrinsic and partly historical.

Some interpretations of MacLean's hypothesis are pessimistic fixations on the misery of the human species.  Others are "life affirming" and focus on the fact that human beings are capable of altruism and other ennobling mental qualities.  I asked MacLean how he viewed these conflicting interpretations of his triune brain hypothesis and the fact that both seem to be based on different sides of it.  He answered, "I hope both sides exist."

His 1949 essay suggests that limbic paleomam­malian processes are irrational and that the frontal lobes might need to override them.  These lower processes have commonly come to associate with the id, the beast, or sin in man (e.g., gluttony, lechery, etc.).  In the light of this it is interesting that through the large uncinate fasciculus, the frontal lobes 'stand guard' over this region.  Could it be that feelings of guilt are fomented here?   Mac­Lean's more pessimis­tic interpreters especially appreciate this early text.

His 1985 essay and definitive book of 1990, however, challenge such one-sided interpretations that took the 1949 quotation out of context, especially during the 1970's.  MacLean's more recent works focus on the positive contributions that "paleo­psychic" processes make to the neocortex.  These processes include the "emotional mentation" of the paleomammalian limbic system, the "protomentation" of the reptilian stage, as well as the cerebellum.  The link that such primitive brain structures have with the prefrontal lobes, the newest products of brain evolu­tion, produces "rational" behavior.  The latest sub­phase of the evolution of limbic structures makes altruistic behavior possible because the medial dorsal nucleus links them to the prefrontal lobes.

 

Negative and Positive Emotions

Ruth Macklin (1978) recognized both the dark and light sides of the triune brain concept even before MacLean unfolded his entire project.  She argues that his concept can account for both negative and positive emotions.

Negative emotions are irrational in the sense that they lead to destructive behavior.  They come from the miserable condition MacLean calls "schizophys­iology," which cuts evolutionarily earlier structures off from a beneficial relation to the neocortical processes that might otherwise override them.  Some of MacLean's interpreters who focus on the darkest end of the interpretative spectrum cite this condition to argue that humankind is a freak of nature, misera­ble because its brain is a mistake of evolution.

Positive emotions are rational in the sense that they lead to beneficial behavior that springs from a state of internal happiness.  Macklin quotes a life-affirming text by MacLean that comes from the 1960's:  "A concern for the welfare of the species is based on sexuality.  In the complex organization of old and new structures under consideration, we presumably have a neural ladder, a visionary ladder, for ascending from the most primitive sexual feeling to the highest level of altruistic sentiments" (Mac­Lean, 1962).

 

Mistaking Function for Constitution

According to Sagan, MacLean's triune brain concept is remarkably similar to Plato's image of the soul in the Phaedrus as a charioteer and two horses.  Sagan mistakes Plato's driver with MacLean's late mammalian brain and the two Platonic horses with the two earlier brain structures when he quotes that dialogue about the soul (246A-B):

Let it be likened to the union of powers in a team of winged steeds and their winged chari­oteer.  Now all the gods' steeds and all their charioteers are good, and of good stock; but with other beings it is not wholly so.  With us men, in the first place, it is a pair of steeds that the charioteer controls; moreover one of them is noble and good, and of good stock, while the other has the opposite character, and his stock is opposite.  Hence the task of our chari­oteer is difficult and troublesome.

MacLean does not attribute as much sovereignty to the late mammalian brain as Sagan does.  Nor are his two older drivers always in conflict with one another.  MacLean's reptilian part is not always violent, as Sagan's comment implies:  "In all of us there is something like a crocodile" (Sagan, c 1980).

Something like Sagan's confusion about which Platonic metaphor fits the triune brain concept seems to lie at the base of Durant's claim that the lone driver inside the chariot is the genuinely human part of us and that the two horses pulling it are our bestial nature.  Using this view of the bipartite metaphor to interpret MacLean, Durant claims MacLean's concept means that the "beast within us" constantly threatens to break lose and do harm.

Durant (1981) views what he thinks is MacLean's bipartite split as a hostile dualism of "good and bad, rational and irrational, the controller and the con­trolled."  As a result, "Homo sapiens is two beings in one, beast and part man, and his destiny hangs on the outcome of the continual battle which they wage for the mastery of his life."  Durant claims that this division of human nature into two beings ultimately "goes back in Western culture to the doctrine of original sin" (Harrington, 1991).

The main underlying problem with this interpreta­tion is that MacLean's three-in-one model does not reduce to two-in-one.  We would have to do violence to the Phaedrus model to make it fit MacLean's concept by turning the lone driver inside the chariot with three drivers struggling to control the bipartite functional conflict of the two horses.

 

Plato's Tripartite Division

The Platonic metaphor of the soul in the Republic (Book IX) as a many-headed, many-colored beast describes the soul's constitution.  It represents the three parts of the soul as:  desiring, spirited, and learning.  These respectively love gain, honor, and wisdom.  He has Socrates invite us "to see" what we ourselves look like as a triune combination in beast terms (588C-D):

"Well, then, mold a single idea for a many-colored, many-headed beast that has a ring of heads of tame and savage beasts and can change them and make all of them grow from itself."

 

"That's a job for a clever molder," he said.  "But, nevertheless, since speech is more easily molded than wax and the like, consider it as molded."  "Now, then, mold another single idea for a lion, and a single one for a human being.  Let the first be by far the greatest, and the second, second in size."  "That's easier," he said, "and the molding is done.”

 

"Well, then, join them—they are three—in one, so that in some way they grow naturally together with each other."

 

"They are joined," he said.  "Then mold about them on the outside an image of one—that of the human being—so that to the man who's not able to see what's inside, but sees only the outer shell, it looks like one animal, a human being."

 

The Platonic beasts within us are miserable (unjust) when they lack harmony and are in conflict with one another.  This happens when reason loses control of the whole because we "make strong the manifold beast and the lion and what's connected with the lion, while starving the human being and making him so weak that he can be drawn wherever either of the others leads and doesn't habituate them to one another or make them friends but lets them bite and fight and devour each other"  (588E-589A).

All the same beasts within us are happy (just) when they are in harmony.  Reason produces the harmony when it takes charge "of the many-headed-beast—like a farmer, nourishing and cultivating the tame heads, while hindering the growth of the savage ones—making the lion's nature an ally and, caring for all in common, making them friends with each other and himself, and so rear them" (589B).

MacLean's triune brain concept could be viewed as a naturalistic restatement of this tripartite constitu­tional model of the soul provided we do not expect the content of the one to translate directly to the other.  For instance, MacLean attributes certain qualities to the limbic system which Plato distributes among the rational and desiring, not just the spirited, part, etc.  At the same time there are important similarities.  The tame and savage beasts partly resemble the protoreptilian brain, as the lion does the paleomammalian, and as the human head is neomam­malian.  Furthermore, the beasts of both models are neither treacherous nor beneficent in themselves.  The happiness and misery they cause depends on the presence or absence of harmony.

 

Plato's Bipartite Division

The Platonic metaphor of the soul in the Phaedrus (and certain parts of the Republic) is not constitutive, but functional.  It seems to represent the two-fold division as the conflicting ways of existing that lead to justice and injustice.  The good horse seeks the kind of knowledge that leads to reason's harmony, while the other neglects it on account of its gluttony for the food of opinion:  In the beginning of our story we divided each soul into three parts, two being like steeds and the third like a charioteer (253C-D):