Triune brain

The triune brain is a model proposed by Paul D. MacLean to explain the function of traces of evolution existing in the structure of the human brain. In this model, the brain is broken down into three separate brains that have their own special intelligence, subjectivity, sense of time and space, and memory. The triune brain consists of the R-complex, the limbic system, and the neocortex.

The R-Complex
The R-complex, also known as the "Reptilian Brain", includes the brain stem and cerebellum. The term "Reptilian brain" comes from the fact that a reptile's brain is dominated by the brain stem and cerebellum which controls instinctive survival behavior and thinking. This is similar in humans. This brain controls the muscles, balance and autonomic functions (e.g. breathing and heartbeat) ; thus it is primarily reactive to direct stimuli.

The Limbic System
MacLean first introduced the term "limbic system" in a paper in 1952. This portion of the brain derives from "the old mammalian brain". The limbic system is the source of emotions and instincts (e.g.. feeding, fighting, fleeing, and sexual behaviour). When this part of the brain is stimulated, such as by mild electric current, emotions are produced.

MacLean observed that everything in the limbic system is either "agreeable or disagreeable." Survival is based upon the avoidance of pain (disagreeable) and the recurrence of pleasure (agreeable).

The limbic system comprises the amygdala, the hypothalamus, and the hippocampus. The limbic system must interact with the neocortex in some way. The limbic system cannot function entirely on its own. It needs to interact with the neocortex to process the emotions.

The Neocortex
The neocortex, also known as the cerebral cortex, is similar to the brain of higher mammals and controls higher-order thinking skills, reason and speech.