In striving to understand the impact of unhealed trauma on the human brain, it's helpful to have a basic understanding of the brain. A simple model of the human brain is to envision it as three separate brains (the triune brain); each with its own separate functions and sense of time.
The triune brain (MacLean, 1990):
1. R-complex brain or brainstem,
2. The limbic brain; and,
3. The cerebral brain or neocortex.
The psychiatrist Dan Siegel, M.D., executive director of the Mindsight Institute, and internationally known educator on the human brain, often uses the "The Hand Model of the Brain," as a useful way to understand the triune brain. As Siegel (2012) explains, if you make a fist with your thumb on the inside (go ahead, give it a try) you can see the resemblance to a human brain. The base of your fist (at your wrist) represents the brain stem, the thumb represents the limbic brain, and the fingers wrapping around your thumb represent the neocortex. There are multiple videos of Siegel explaining his model online if you wish to do some searching on YouTube or other engines.
We will be working from this triune model throughout this section of the article.