The Triune Brain
Neuroscientist Paul D. MacLean formulated a model of the brain in the 1960s, detailed in his 1990 book The Triune Brain in Evolution, describing the brain in terms of three distinct structures that emerged along an evolutionary path. The idea became more popular through Carl Sagan’s book The Dragons of Eden – a speculative look at the evolution of human intelligence and through writings such as Jaac Panksepp’s 2003 book The Evolutionary Neuroethology of Paul MacLean: Convergences and Frontiers.
Although this model provides a highly simplified explanation of brain activity and organisation, and provides an easy-to-understand approximation of major brain functions, it lacks credibility in contemporary neuroscience. The evolutionary evidence for this 3-tiered model is not sound and the model belies the complexity and integrative nature of neural networking that runs through the entire brain. It is more precise and useful to conceive of the brain as a collection of neural networks like the salience network, the executive network, the default mode network, to name a few large functional systems across the entire brain. Nevertheless, MacLean’s model and terminology remain popular in much of the literature about brain development and function.
The model proposes 3 major divisions of the brain:
- Reptilian Complex (or what some call the “lizard brain”)
- Paleomammalian Complex (limbic system or “emotional brain”)
- Neomammalian Complex (the neocortex or “smart brain”)
The Primitive Brain (Reptilian Complex)
This system of the brain is responsible for the most basic survival functions, such as heart rate, breathing, body temperature, and orientation in space. Needless to say, functions like heart rate and breathing are rather important, and the control mechanisms in this part of the brain are rather consistent.
It is important to recognise that the functions of this part of the brain will take precedence over other brain activity. For example, if you try to hold your breath (a prefrontal cortex-initiated activity), you will find that as carbon dioxide builds up in your bloodstream, this primitive part of your brain is going to want to take over and make you breathe again. Through training you may be able to increase your resistance to the basic urge to breathe, but inevitably you will eventually give in and take a breath.
Such threats to survival are first addressed by the primitive brain—as illustrated in “peripheral shut-down”, where blood vessels are constricted on the periphery of the body in anticipation of physical trauma—and take precedence over other brain functions.
The Limbic System (Paleomammalian Complex)
Sometimes referred to as the “emotional brain”, the limbic system is the reactive part of us that initiates the “fight or flight” response to danger. Key areas of interest to psychotherapy are the hippocampus, the amygdala and the hypothalamus. These form a very fast subconscious evaluation and response system designed to keep us safe.
The amygdala is like an early-warning system, with the motto “safety first”—put that safety plan into effect before consulting the executive brain (the new cortex). Picture yourself jumping out of the way of a snake-like object before closer examination reveals it to be just a hose in the grass. This is a very important first response, because if it were left to the prefrontal cortex to initiate, for example, a leap out of the way of a bus you had inadvertently stepped in front of, then it might be too late: that evaluation system is too slow. The amygdala makes very fast, albeit not always accurate, evaluations and has a fast track from the thalamus (incoming information) through to the hypothalamus that can initiate a stress response to forestall impending doom. The hippocampus plays an equally important role by encoding events in time and space and consolidating them from short-term to long-term memory.
Of particular interest to therapists is the case where the limbic system gets the cues wrong—where there is no actual danger but the body is thrown into stress response anyway. From chronic low-grade stress to full-blown panic attacks, a maladaptive limbic system may be the key to what’s troubling your client.
The New Cortex (Neomammalian Complex)
The new cortex is our “smart” brain, the executive part of our system that is responsible for all higher-order conscious activity such as language, abstract thought, imagination, and creativity, to name just a few. It also houses much of our memory—not just our biographical memory, but all of the automatic memories essential to talking, writing, walking, playing the piano, and countless other familiar activities (keep in mind, however, that the division of the brain into three large parts is a highly simplified conception: functionally the connectivity between all these regions greatly blurs the boundaries).
Of special interest to therapists is the prefrontal cortex—the part of the brain right behind our forehead—which may be slower in responding to incoming information than the limbic system, but is much more sophisticated in its processing. Such “slow” thinking is the hallmark of our human intelligence. Complex and new thinking on technical, emotional, social, and logical planes takes place here. It is where we can be rational and logical, creative and inventive. But, significantly, the prefrontal cortex can be “hijacked” by the limbic system in the event of a perceived threat (whether imagined or real). Our prefrontal can “go offline” as blood flow is directed to the deeper limbic system, the first responder on a priority one mission to keep us safe.