The basic design of the brain and what bits do

Neuroscientists have an impressive set of words to describe where in the brain various parts lie. Inferior, medial, dorsal, rostral, caudal, anterior, posterior, superior. We are just going to work with two directions: back to front, and outer to inner.

The basics:

The brain is has a layered structure. You can think of it a bit like the layers of the Earth, from the crust, to the mantle, down to the burning, ancient core. The outer layers of the brain process information without caring too much about goals or emotions. Some call it ‘cold cognition’. The inner layers increasingly process information in terms of goals and emotions, so-called ‘hot cognition’. The innermost layers coordinate with the functioning of the rest of the body. When I see a spider, cold cognition recognises the visual pattern, hot cognition gets worried, the body is informed that its heart should race in preparation for fight-or-flight action, and cold cognition prepares the instructions to jump. The layers worked together as an integrated whole.

Let’s try that with a little more detail:

We’re in the outer layer, called the cortex. As we’ve seen, it’s big in humans compared to other animals. The back and the front do different things.

The back part of the cortex houses regions involved in sight (vision), hearing (audition), and the processing of space. Senses are processed along two routes. One route, called the ‘what’ pathway, tries to identify what things are. The other route, the ‘where’ pathway, processes where things are in space. You might want to combine this information: catch a cricket ball (howzat!) but don’t catch a snowball (duck!).

The motor areas are towards the front. At the boundary is an area for sensing the body, and the motor circuits for controlling parts of the body. Further towards the front are areas involved in planning, decision-making, and control. As we’ll see, these are still sort of motor circuits.

The cortex, the outer layer, is a sheet of neurons for processing information. The sheet of neurons, 2 millimetres thick, is just a bit smaller than a sheet of A3 paper, and it needs to be crumpled up to fit it in the skull.

The sheet processes information without caring too much about the results. Where you are on the sheet doesn’t radically change how the information is processed, it just changes what is processed. From back to front: sensory, motor, planning, decision making. Where you are is what you do. ¹

Sensory and motor systems are organised in hierarchies, moving from simple to complex. You can think of these hierarchies as being like a tower with many floors, with a separate tower for each sense. Each floor combines the work done below, and each floor has a farther view than the floor below. The lowest floor spots patterns in sensory information. The next floor up spots patterns within patterns. The next floor, patterns within patterns within patterns. Sensory and motor systems are trying to see patterns within patterns within patterns – and then make connections between the patterns.

After a while, the upper floors of the towers might know a thing or two about what patterns are likely. Based on their knowledge, the upper floors like to make suggestions to the lower floors on what they may be perceiving (just to help out, mind). The upper floors of the towers for the different senses talk to each other, across cables strung between the upper floors, to see if they can agree what’s out there in the world. The upper layers are connected to the frontal parts of the brain, to pass on conclusions and see if their view fits with expectations. It goes something like this:

In the Visual tower:

Lowest floor: I’m getting light and dark and some coloured pixels

Lower floor: From that, I’m getting edges and curves

Middle floor: From that, I’m getting legs and a body

Upper floor: You know, put those together and you get a spider.

Visual tower to front of brain: Were you expecting a spider?

Front of brain: I wasn’t expecting a spider. Are you sure it’s a spider, not some fluff?

Top of the Auditory tower: I’m getting scuttling sounds. And someone next to me is screaming. Sounds like spider to me.

Front of brain: True, that fits with spider. Wasn’t expecting one though.

Visual tower: Definitely looks like a spider. I’m seeing fangs.

Emotions: Eeeeek! It’s a spider!

Front of brain: Okay, I think we should do something about this. The emotions don’t seem to like spiders.

Emotions: Aargh!

Front of the brain: Motor systems, can we, I don’t know, leap up or something?

The motor system has a hierarchy too, but its higher levels are different. They’re about patterns more distant in time. The lowest levels are about immediate actions. The higher levels are about more complex sequences of actions, further forward in time. The lowest level says ‘Do it!’ (primary motor cortex). The next layer says, ‘Prepare to do it’ (supplementary cortex). The next layer up says, ‘You may want to do it sometime in the future’ (prefrontal cortex). A complex sequence of motor actions to be carried out at some future point in time can be described as a plan. Pre-frontal cortex, the planning and decision-making part of the brain, can also be seen as the top of the motor system hierarchy, looking the furthest forward in time.

We saw in the section on evolution that humans have more cortex. This means that humans can build their towers higher than other animals. In their senses, humans can discern more patterns within patterns, more complicated concepts; and in their motor systems, they can build further forward, creating plans into the more distant future.

[1] For naming buffs, the brain is split into lobes: frontal (at the front), temporal (at the side, by the temples and ears), parietal (back top), and occipital (back, bottom). But as I said, this resource isn’t about naming bits of the brain.