MacLean's triune brain theory: what it is and what it proposes

MacLean's triune brain theory He proposes that the human species throughout evolution has been changing in terms of its brain morphology, but instead of seeing it as a process of global change and unified, describes it as a process in which new and independent brain structures arose, each operating with its own characteristics individual.

In this article we will review what exactly Paul MacLean's triune brain theory is, and we will see why it is not valid for the scientific community.

• Related article: "Parts of the human brain (and functions)"

What is MacLean's triune brain theory?

According to MacLean, in his book The Triune Brain in Evolution, our current brain is the sum of a process of superposition of layers, which were appearing consistently with the passing of the millennia and settled one on top of the other, but without the previous ones ceasing to exist.

Thus, MacLean's triune brain theory states that each of these structures has a logic of operation. its own individuality, and very different from that of the other layers, taking into account that the upper layers are the most evolved.

Let's see below what these layers are according to the theory of the triune brain.

1. the reptilian brain

This would be the first of the three layers that exist in our brain, and represents the most basic instincts of the human species; hunger, sexual reproduction, sleep, survival instinct and fight.

This brain would be composed of the first structures to appear, which are the basal ganglia, the brainstem and the cerebellum. All these structures They are located in the lower part of our brain..

The followers of this theory affirm that the reptilian brain contains a large amount of information in its individual memory; This information would be made up of ancestral rites and esoteric beliefs without any type of scientific foundation.

2. The limbic system

This second structure, or second brain, according to the theory, would be the one in charge of all the sensations that we experience when we carry out an activity. For example, when we eat something that we really like, when we have sex with a person that we like, or when we enjoy a beautiful landscape.

These emotions are in charge of the limbic system, which supposedly, according to the triune brain theory, comes from being the next structure of the nervous system to evolve after the appearance of the brain reptilian.

This layer is composed of the cerebral amygdala, the septum, the hypothalamus, the cingulate cortex, and the hippocampus.

3. the neocortex

This layer is the most recent in terms of human evolution, it is also known as the modern brain. It is exclusive to mammals.

the neocortex is in charge of everything related to abstract thought, logical and rational thoughtapart from the complex communication processes we use to communicate in modern society.

It is made up of the cerebral cortex, which is made up of gray matter, where there are a large number of neurons that are in a constant process of connection with each other.

• You may be interested in: "Neocortex (brain): structure and functions"

Considerations about the theory

Taking into account what has been seen so far, it can be said that MacLean's triune brain theory assumes that our current brain is nothing more than an addition process between individual layers that appeared in our species making us increasingly rational.

Each of these layers, as we have already seen, has its own characteristics; Even though the functions of one have to do with the functions of another structure, they operate autonomously. In other words, neither layer has control over the other, but they could, for example, make the subject aware of her desires and thus make her channel them properly.

This theory has not had a good receptivity in the world of the scientific community, and most of specialists in the field of neuroscience have expressed their disapproval of said postulate. Let's see why.

What does science say?

For experts in neuroscience, the theory of the triune brain has too many inconsistencies in its approaches to be considered valid.

It is now known that the specific functions that this theory grants to the mentioned structures are not properly of the most evolved vertebrates, that is to say, that in other species behaviors Similar.

For example, birds, without having to possess the limbic system, have a great protective instinct towards their breeding, which is a characteristic attributed to the second brain (limbic system) according to the theory of MacLean.

In addition, there are intelligent mollusks such as octopuses, whose brain has nothing to do with that of vertebrates, capable of having social behaviors and adjusted to rationality.

More recent discoveries also debunk the theory that the neocortex arose as the last stage in the evolution of the modern mammal. Today it is known that the first signs of gray matter in the upper areas of the brain were found in primitive mammals.

In other words, these structures did not appear in an ascending order as the author of the brain theory suggests. triune, but that they had all already existed in the same brain, which evolved in a general way, and not part by part.

Furthermore, the way evolution works is not about an accumulation of traits that do not influence each other. In other words, the changes produced by mutations that result in new traits in the structure of the brain do not occur individually, but rather in interaction with the rest, so that a part that previously specialized in certain mental processes can take over others if another new set of cells appears. nervous.