How the Brain Works: It’s all about layers part 1

The brain 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’.

spider-1403889352yvkThe innermost layers coordinate with the functioning of the rest of the body. When I see a1280px-man_running_scared_cartoon_vector-svg 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 work together as an integrated whole.

This week’s blog explores how the outer layer, the cortex, works.

The Outer Layer (aka The Cortex)

As we’ve seen, the cortex is big in humans compared to other animals. The back and the front do different things.

The cortex, 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.

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.

Between the back and the front, the 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 screenshot-2019-03-13-at-18-30-44patterns.

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.

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.

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How the Brain Works: The brain had to evolve

This week, we’re bringing you another tasty snippet from the CEN resource on How the Brain Works.  Here we reflect on the consequences of the often forgotten but completely fascinating fact that our brains weren’t designed or made, but have slowly and organically evolved over millions of years…






The instructions to build the brain – written in DNA – turn out to be more detailed for parts of the brain with a longer evolutionary history of retaining the same function. There’s been more time for biological trial and error to figure out the instructions to build specific structures tuned to specific functions.


Longer evolutionary history also tends to mean these instructions (or similar versions) are shared with other similar animals lying on the same branch of evolution’s family tree. In our case, that means in the branch of: vertebrates (backboned animals); sub-branch: mammals; sub-branch: social primates. A lot of the build plans are similar across similar species.

The general plan for building a body with a brain goes back a long way, perhaps 450 million years; the plans for brain cells – neurons, with their connections, electrical properties, and neurotransmitters – still further. As we’ll see, one bit of the brain is particularly enlarged in humans – the outer layer or cortex. In evolutionary terms, this greater amount of cortex is a recent addition. Therefore instructions to build components within the cortex are less detailed, and they depend more on development for their sculpting. By contrast, other bits of the brain, such as those responsible for the emotions, look almost identical in humans and chimpanzees.



First, evolution tends to innovate at the periphery. The new things that mark out a new species tend to be in the structure and function of its body (including the movements it can perform), its organs, and its sensory equipment. When it comes to the brain, the innovations are less specific, but tend to involve tweaking the general build plan used in this branch of the family tree: some parts of the brain grow bigger, some smaller, but the types of structure are the same. Evolution modifies the existing plan, more here, less there; it doesn’t build a new brain piece and add it in.


Take bats. They can navigate in the dark using sound. But there is no special new part of the bat-brain for navigating using sound. The ability to emit ‘ping’ noises, and enhanced hearing to differentiate the echoes that come back, are innovations. The bat brain uses similar types of brain structures to other mammals, but develops in them the ability to combine sound information to guide flight and not bump into cave walls.

What are the specific innovations in humans, separate from other social primates? We stand upright. We have hands and vocal articulators (lips, tongue, vocal cords) evolved to allow precise movements and speech production. Rib muscles that allow us to generate a smooth flow of air over the vocal cords to produce speech (this works fine so long as we’re not laughing). One particular part of the brain, the cortex, has grown bigger, giving us more thinking power. More on this later.



Second, evolution only improves what it already has. Over generations, the build plans for the body and brain can be changed and improved, but they are modifications of previous plans. This means solutions aren’t necessary the best ones. As the joke goes, a tourist asks, ‘How do I get to London?’ and gets the reply ‘Well, I wouldn’t start from here.’ A desktop computer may be better suited for solving some problems the human brain faces, but evolution has committed to doing things with neurons, with whatever limitations that involves. Evolution can only improve what neurons can do; it can’t switch to silicon.



Third, you don’t have to use something for what it was made for. The visual system was designed to recognise the physical world (objects, scenes) and social stimuli (faces, bodies) 1. But – once human culture had invented reading – each new generation of humans could then use the visual system to learn to read. This point has a second part: but if you use it for something different, it may not work perfectly.

screenshot-2019-02-12-at-14-51-09So, the visual system has been designed to recognise objects from whatever viewpoint: a coffee cup is a coffee cup if seen from the left, the right, or upside down. But in English, we asked children to learn that p, q, b, and d are all different letters, corresponding to different speech sounds. To the young child’s visual system, these all look like the same object (a round bit with a tail) viewed from different angles. It takes months, maybe years of learning to overcome the brain’s preference to interpret what it sees in terms of movable objects, and this is why children learning to read in English often mix up their b’s and d’s, and their 6’s and 9’s.

Lastly, when it comes to big changes in brain structure evolution always takes longer than you think. The brains of humans 5,000 years ago, even 50,000 years ago, looked pretty much the same as our brains now.