Three metaphorical layers of the brain

Attention! These are NOT literal layers of the brain.

Let’s begin by examining the macro-organization of the brain. For this, we’ll use a model proposed in the 1960s by neuroscientist Paul MacLean. His “triune brain” model describes the brain as having three functional domains:

So, our brain is divided into three functional blocks, with the usual advantages and disadvantages that come with categorizing any continuum. The biggest drawback is its excessive simplification.

• Level 1: The ancient component of the brain — its foundation — present across various species.

• Level 2: A region that evolved later and developed in mammals.

• Level 3: The neocortex, which evolved relatively recently and is located on the surface of the brain.

• Anatomically, there is significant overlap between these three levels (for instance, one part of the cortex could be considered part of level 2 — more on this later).

• The flow of information and commands doesn’t just move top-down, from level 3 to 2 and 1. One strange and interesting example we’ll explore in Chapter 15: if a person is holding a cold drink (temperature processed by level 1), they are more likely to perceive a new acquaintance who approaches them as a “cold” person (level 3).

• The automatic aspects of behavior (in simplified terms — the domain of level 1), emotions (level 2), and thinking (level 3) are inseparable.

• The triune model gives the misleading impression that evolution simply stacked one level on top of another, without any changes to the ones that already existed.

Despite all its flaws — which MacLean himself acknowledged — this model will serve as a useful organizational metaphor for us.