Paul MacLean:

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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 brain stem 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 limbic system 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".

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