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:
Level 2:
Level 3:
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.
