Brain structures and behaviour

A lot of people like to talk about the role of various brain regions in human behaviour. Fewer like to talk about brain structures. But in many ways, looking at the brain structures rather than all the different subdivisions into regions tells a cleaner story about behaviour. Let me show you how.

¶Why a brain

Before I talk about how all the brain bits contribute to behaviour, I think it’s worth talking about why we have a brain at all. This way, you won’t just come away with a random idea of structures that do things, but why structures do those things and why it’s all so confusingly blurry.

Perhaps the most important attribute of living creatures is the ability to transform the world into adaptive responses to the world. Take in what’s out there, and then act so that what’s out there becomes more good or less bad.

This is what bacteria do with cellular signalling pathways, or flowers do with photoreceptors and hormones. But more complex worlds require more complex architecture to transform the world into behaviour. So, we have the nervous system—a huge computer to do just that. And our nervous system has an obvious split. An older, peripheral system that takes in the information about the world and solves many transformations of world to action all by itself. Then also, a newer ‘central’ system—the brain—that takes much of that information and does even more stuff with it.

There are plenty of animals that never found it very useful to develop a centralised system, and remained using the older decentralised peripheral system. Plenty of complex behaviour happens in these systems, and doesn’t need a brain at all. In fact, having a centralised system can be quite a problem. If you cut a huge swath of nervous system out of a nematode, it will go on about its business just fine. The rest of the nervous system can compensate for the damage, because it all does basically the same job. If you take damage to your brain, you’re going to have a very hard time going on as you were before, because each bit of it does very specific jobs.

And that issue is actually the benefit we gain—fundamentally, the reason we sacrifice the flexibility of a decentralised peripheral nervous system by tacking on a centralised one (the brain) is that it acts as a unified decision system that can take in sensory information about the world from everywhere and responds by integrating it all together. So, your eyes might see something, and the brain can also use information from the ears and fingers and nose to figure out what’s going on. In a decentralised system, there is no such integration. Information comes in where it’s received, is processed right there, and then gets sent back out again, with no helpful additional information from elsewhere in the body. I remind you of the knee jerk reflex I drew in a previous article.

So, the brain is an integrator first and foremost. And as we’ll see, each structure of the brain seems to have developed to integrate fairly specific kinds of information based on where it’s located relative to your senses.

If you take nothing more than that away, you’ll be doing pretty well.

But, there is a corollary effect to this new development. We might have added a brain to make integrating information more efficient, but that doesn’t mean the brain remained obedient to its task. You might build a TV room as an extension to your house, but it doesn’t mean it’s just going to remain a TV room. You have all this new space, and you’ll use it for all sorts of new things. Hanging clothes, hosting parties, whatever. It’s the same with the brain—you have this new system, and as time goes on, evolution found all sorts of new things it could do with this new system.

So, each structure seems to be trying to do specific things, mostly, but also you’ll find that they moonlight in the most odd and confusing ways.

And the last little corollary is that, although it’s very easy to see the brain as an improvement—the latest and greatest evolutionary gift, this isn’t really quite the right way to think about it. A brain brings new opportunities, but it also comes with new demands and new limits. A decentralised architecture, like in animals with no brain, are intrinsically parallel—they can do lots of things all at once—everything is processed where it’s received which means the rest of the system can be processing other things at the same time. With a brain, everything needs to go through the brain first, and so it acts as a bottleneck—it can only manage so many things at once.

So, brains are integrators with specialised structures to integrate specific kinds of information. But this specialisation has long since become messy, making it hard to tell what is really responsible for what. And, very importantly, it’s made us quite restricted in how we go about integrating that information—we can’t integrate everything all at once.

That’s a very good starting point for understanding the brain.¹

¶The major brain structures

Entire textbooks are written that split the brain up into smaller and smaller pieces. And each textbook alters, or backtracks, or loops around the others, because the brain is, more than anything else, a very confusing object.

But there are some clearer distinctions than others, and the clearest distinctions come from looking at the different structures that make up the brain.

You’re probably familiar with the neocortex, or cerebral cortex. This is the wrinkly sheet of brain that’s crumpled up around the outside of the thing. But inside that wrinkly sheet, there are a handful of other, quite different looking, brain bits.

Here, I’m going to describe them, trying to link the whole thing together into a narrative. It won’t be the only narrative you could tell about them, but, for me, it’s the most useful when I’m thinking about how people behave.

¶The brain stem

At the very top of the spine is the brainstem—the medulla, the pons, and the midbrain. Sitting at the bottom of the brain, it acts as the conduit between the brain and the rest of the body. Almost all the information from the peripheral nervous system enters here, and arcs out into the other brain structures.² So, where better to process information most intimately related to survival than this central ingress? The brain stem seems to manage functions related to our heart rate, breathing, and blood pressure. It also holds the reticular activating system, which seems to manage aspects of your sleep-wake cycle as well as doing something very important for arousal and attention—taking the sympathetic responses from the body and transforming them into the focused attention you need to solve whatever problem you’ve detected. Why bother sending all this critical processing further up into the brain when it can be managed at the very start?

On the way out, we don’t have quite the same picture. Much of the information coming out of the brain does pass through the brainstem, but for voluntary movement there is the corticospinal tract—a direct pathway from the neocortex (the outermost layer of the brain, where movement is largely formulated) to the nerves that plug into your muscles.

¶The cerebellum

Hanging off the back of the brainstem is the cerebellum. It looks a little like a mini-brain all on its own. This is a mysterious little thing. It accounts for maybe 10% of the brain, but holds over half the neurons. Mostly, people will say that this has something to do with movement, because if you damage it, people start moving really weirdly.

Occasionally in the past though, and more recently too, we’ve found that the cerebellum appears to be involved in almost anything you care you name. Every functional parcellation of brain is connected to a part of the cerebellum too. So, some have asked whether the cerebellum does something more fundamental. Perhaps it takes the rough computations coming in and out of the brain via the brainstem and refines them. Uses all those neurons to make them even more precise, whatever they might be. Or maybe it does something a bit more abstract—taking the information being processed in the brain and calculating predictions about how the world might change if we acted this way or that way in response.

Whatever it’s doing, if you’re looking to do a brain science PhD, the cerebellum almost certainly has a lot of low-hanging fruit to be discovered.

¶The diencephalon (thalamus and hypothalamus)

As we move up, past the brainstem and the mysterious cerebellum, we hit the diencephalon—a binary system of thalamus and hypothalamus. The thalamus, famously, is called the ‘relay station’ of the brain. Sensory information comes here, and then gets directed to wherever it needs to go in the more central structures or on the neocortical layer wrapped around the outside. Obviously, such a highly-connected region of the brain is then found to be important for almost everything those other structures are involved in. Attention, consciousness, emotion, the thalamus likes to moonlight in them all. The question, of course, is: is the thalamus important because it’s relaying the information? Or is the thalamus important because it has come to do more than simply relay the information? It’s often very hard to tell, but it would be pretty odd if evolution hadn’t found more uses for it than simply acting as an email forwading service.

The hypothalamus, on the other hand, does clearly seem to do stuff. It plays a crucial role in homeostasis (keeping the body in balance) and the regulation of autonomic functions. It controls things like temperature regulation, thirst, hunger, sleep cycles, and the hormonal output of the pituitary gland. By integrating signals from the body and brain, the hypothalamus maintains equilibrium and coordinates the endocrine system’s response to internal and external changes to keep the body in balance.

Both functions are very sensible—again, most signals coming into the brain pass through these regions after passing through the brainstem, so they play very important roles in basic bodily functions and transmission of information.

¶The middle structures

In the middle of the brain, you have a cluster of structures that are often distinguished from each other. What they seem to be doing is adding colour to the transformations of sensory information into action that the cerebral cortex manages.

So you have the hippocampus, a little seahorse shaped structure that seems to be extraordinarily important for memory. Not all kinds of memories, but mainly those related to personal experiences and the context in which they occur. This includes episodic memory and spatial navigation.

Nearby, you have the amygdala—a little almond-shaped blob of brain that seems to charge our memories—essentially stored combinations of sensory input—with emotional significance. You walk into your old house, and feel all the old nostalgic pleasures, or you walk into a room where something bad happened, and you’ll feel the dread you once felt there—it was the amygdala that made that connection in the first place.³

In between, you have the basal ganglia—a stripy little cluster of brain bits sort of shaped like a seashell. This bad boy connects all over the brain, but particularly to the thalamus. Functionally, what it seems to do is choose what information gets through the thalamus or not. It inhibits things, and then selectively stops inhibiting things depending on what’s going on. But it has these looping connections through the cerebral cortex and the other middle structures too.

What is it doing with all these connections, and this important selective function? Well, we know that it’s incredible important in reward-based behaviour—indeed it is fundamental in addiction. This has us thinking that it determines the relative value of things. Some sensory inputs are more preferable than others, and this will change whether you’re hungry, or thirsty, or horny, and so perhaps its the basal ganglia that’s working out which sensory inputs should play a greater role in later processing over others by inhibiting some and letting others through.

¶The neocortex—limbic lobe and cerebral cortex

Wrapped around the middle structures are two wrinkly sheets of brain, one on top of the other. The bottom-most of these is the ‘limbic lobe’—named because once upon a time, we started calling various combinations of the middle structures and this layer of neocortex the ‘limbic’, or ‘emotion’ system.

That’s quite out of fashion now, because these middle structures are obviously doing much more than just emotional processing, and no one could agree on sensible ‘groups’ anyway. So it might better to think of this layer, and the one above it—the cerebral cortex—together as the ‘neocortex’, which is the technical term for them.⁴

Together, the neocortex accounts for a huge chunk of the brain. A set of big flat sheets, wrinkled because it means you can cram a bunch more brain into a bunch less space. Add to that the fact that each sheet can be separated into six sub-layers, and you have an enormous amount of processing power. A lot of computing space to connect all the combinations of sensory inputs we can sense into all the combinations of behaviours we can respond with.

Different parts of the neocortex appear to be responsible for different things, and those things seem to depend on where the senses plug in. So, the eyes are linked up to the back of the neocortex, and in that area you have bits of brain that respond to visual information—orientation, contrast, colour. At the sides, where the ears are linked up you have bits of brain that respond to tones and frequencies and language. At the top, where your body parts plug in, you have bits that respond to taste and pressure and whatnot. And possibly, we could consider the limbic lobe as the place where our sense of the body plugs in—all the visceral feelings of goodness and badness, arousal and the need for balance. These, some think, are a kind of sensory input of their own. Certainly these bodily feelings are heavily tied to emotions, and so it’s no surprise that the limbic lobe seems very involved in emotion too.

In the in-between places, where no one sense in particular links in, you have bits of brain that are more associative—responding to mixtures of sensory information. Or, particularly at the front and the extreme sides, far from all the sensory inputs, you have bits of brain that respond to more abstract stuff still—relative value, and preference, and the need to over-ride an automatic response to some sensory input.

And then, after taking in all this information, and integrating it in the in-between places, it sends it to areas that are linked to actions—regions that are responsible for moving your body, setting you up to start behaving.⁵

¶Putting it all together

This has become quite the treatise. But considered all together, it’s very simple. Information comes in—about your body, and the world outside your body. It runs up through the brainstem. Some basic functions are handled there, but mostly the information is passed up via the diencephalon, through the middle structures, where it collects ‘colour’—memories, emotions, the value of some things over others depending on how hungry or thirsty you are, things like this. At the same time, it’s routed into the neocortex, which integrates the information to transform all this information into action—what are you actually going to do about it? And of course, both in and out, it’s communicating with the cerebellum, which is probably doing something important at all times, but outside of refining movement, we’re not sure exactly what.

¶Outro

So what does this all mean for behaviour? How does knowing this help us?

Well, it helps us understand that the brain, fundamentally, is a bottleneck. All these structures, doing all this processing, manage most of the decision-making you do all together at once. They do it messily—descriptions of what one structure is involved in will usually overlap with others, and so any process your brain is managing is probably messily engaging lots of regions at the same time. And this processing happens in parallel—the ‘colour’ is being collected at the same time as the behaviour is being formulated.

Provide too much information to a system like this, and it could never manage such a mess. Try to prompt too many behaviours, and it could never coordinate everything in time.

So, the brain has to streamline. Much of the neocortex is dedicated to habitual responding. Mapping common inputs to common outputs, so it can free up processing when the information coming in isn’t so easy to integrate. Much of the ‘colour’ works similarly, memories and emotions prompted by mere fragments of the stimuli that made the brain create them in the first place. Much easier to let the emotions run freely than to spend valuable time and processing power being very precise.

Only very rarely does the brain spend the time trying to do new things, or with any great precision. Doing that all the time would make its job too hard.

Ideally, it also helps us understand a little of why the brain is still so poorly understood. If you’re trying to work out how one bit works, how do you isolate it and not all the bits that do the same thing as it? How can you tell if that’s its main job and not just one of the things it sometimes moonlights as?

So there. A very short course in neuroanatomy. And I hope that the tedious detail has made it quite clear why I much prefer to stick to sweeping generalisations. There’s very little in the detail of the brain that’ll tell you more about behaviour, just like there’s little in the detail of a carburettor that’ll tell you how to drive better. But at least now, when someone tells you that the ‘amygdala is the fear centre’, you can say ‘but what about the limbic lobe’? It’ll throw them for a loop. Try it.