Paul MacLean, the former director of the Laboratory of the Brain and Behavior at the United StatesNational Institute of Mental Health, developed a model of the brain based on its evolutionary development. It is referred to as the "triune brain theory" because MacLean suggests that the human brain is actually three brains in one. Each of the layers or "brains" were established successively in response to evolutionary need. The three layers are the reptilian system, or R-complex, the limbic system, and the neocortex. Each layer is geared toward separate functions of the brain, but all three layers interact substantially.

The Reptilian Complex
 
       The R-complex consists of the brain stem and the cerebellum. Its purpose is closely related to actual physical survival and maintenance of the body. The cerebellum orchestrates movement. Digestion, reproduction, circulation, breathing, and the execution of the "fight or flight" response in stress are all housed in the brain stem. Because the reptilian brain is primarily concerned with physical survival, the behaviors it governs have much in common with the survival behaviors of animals. It plays a crucial role in establishing home territory, reproduction and social dominance. The overriding characteristics of R-complex behaviors are that they are automatic, have a ritualistic quality, and are highly resistant to change.

 
The Limbic System

        The limbic system, the second brain to evolve, houses the primary centers of emotion. It includes the amygdala, which is important in the association of events with emotion, and the hippocampus, which is active in converting information into long term memory and in memory recall. Repeated use of specialized nerve networks in the hippocampus enhances memory storage, so this structure is involved in learning from both commonplace experiences and deliberate study. However, it is not necessary to retain every bit of information one learns. Some neuroscientists believe that the hippocampus helps select which memories are stored, perhaps by attaching an "emotion marker" to some events so that they are likely to be recalled. The amygdala comes into play in situations that arouse feelings such as fear, pity, anger, or outrage. Damage to the amygdala can abolish an emotion-charged memory. Because the limbic system links emotions with behavior, it serves to inhibit the R-complex and its preference for ritualistic, habitual ways of responding.

        The limbic system is also involved in primal activities related to food and sex, particularly having to do with our sense of smell and bonding needs, and activities related to expression and mediation of emotions and feelings, including emotions linked to attachment. These protective, loving feelings become increasingly complex as the limbic system and the neocortex link up.

 
The Neocortex
 
       Also called the cerebral cortex, the neocortex constitutes five-sixths of the human brain. It is the outer portion of our brain, and is approximately the size of a newspaper page crumpled together. The neocortex makes language, including speech and writing possible. It renders logical and formal operational thinking possible and allows us to see ahead and plan for the future. The neocortex also contains two specialized regions, one dedicated to voluntary movement and one to processing sensory information.
 

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       We have mentioned that all three layers of the brain interact. The layers are connected by an extensive two-way network of nerves. On-going communication between the neocortex and the limbic system links thinking and emotions; each influences the other and both direct all voluntary action. This interplay of memory and emotion, thought and action is the foundation of a person’s individuality.

       The full extent of this interconnectedness is unclear. However, it is entirely incorrect to assume that in any situation one of our three "brains" is working and the others are not. What we can do, tentatively, is assume that at times one particular focus may be dominant while the rest of the brain acts in support and that education can influence which focus dominates.
 

Caine, Renate Nummela and Geoffrey Caine.  Making Connections:  Teaching and the Human Brain. Nashville, TN: Incentive Publications, 1990.