Posterior Perception, Anterior Action
Consider again the neural tube. In Brain Ontogeny, we learned how brains emerge from five little bumps in the neural tube.
The neural tube develops under several organizing principles. One of the most important is the Bell-Magendie law: in the nervous system, motor nerves & computations happen in the front of an animal (anterior action), perceptual nerves & computations are at the back (posterior perception). This principle applies to the entire nervous system, from the spinal cord to the neocortex.
From an evolutionary vantage point, this principle is fairly obvious. Because gravity is a thing, the vertebrate body plan features anterior limbs (movement requires limbs between body & ground) and posterior vision (because of the sun’s position relative to the Earth). Notice how bipedalism and its deformation of the human neuraxis obscures this principle. Our quadruped ancestry thereby illuminates the functional organization of the human nervous system.
The Abstraction Hierarchy
The nervous system is a complex organ. Sometimes, choices do not require much analysis. Often, however, a decision becomes clear only after contemplation.
As information travels through the brain, it becomes more abstract, and resultant behavior more flexible. This is the brain’s abstraction hierarchy.
Darker colors denote concrete representations, lighter colors denote abstraction.
In cognitive science, the following synonyms hold:
- Information is a synonym for memory.
- Sites of information processing is a synonym for modules.
The abstraction hierarchy can inform both levels of analyses. Let’s see an example.
Cognitive scientists like to distinguish between information encoding events (episodic memory) and information encoding concepts (semantic memory). The abstraction hierarchy explains the relation between the two: the concept of apple generalizes any number of events that contain apples. That is, semantic memory is situated at a higher level than episodic memory.
How do we know the brain is organized hierarchically? Brain damage provides one line of evidence.
- Severe damage to the brain stem is fatal.
- However, if a human loses everything but the brain stem, they cling to life (persistent vegetative state)
- If the midbrain is spared, such that a person loses only their cerebrum, they can spontaneously move, but their behavior is disorganized.
Renowned neuroscientist Charles Sherrington once described the brain as an enchanted loom:
The brain is waking and with it the mind is returning. It is as if the Milky Way entered upon some cosmic dance. Swiftly the head mass becomes an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of subpatterns.
The abstraction hierarchy teaches us: the higher you climb the enchanted loom, the more flexible your dance.
Biological Decision Making
We are now in a position to understand decision making.
Decisions are bridges between perception and action.
Given your brain’s abstraction hierarchy, people have invented many names for this singular concept:
Until next time.