“If the human brain were simple enough that we could understand it, it would be so simple that we couldn’t”

From where our collective human understanding was 500 years ago, science has advanced our knowledge in so many areas.

Physics can explain how the universe started, how large it is, the major forces which control movements of objects, what energy is, what determines night and day, seasons and the weather.

Chemistry has told us that everything is made of building blocks called atoms, how they react with each other and combine to form all matter in the universe.

Biology has told us that life occurs due to replicating molecules such as DNA, and that all living things are made of cells. We know lots about how our bodies work and can cure many of its diseases.

But one area of human understanding which still holds so much mystery is the brain. We know bits and pieces about how it works on a small scale – it is made of specialised cells which send each other electrical impulses. And on a large scale – it processes incoming information from our senses to generate actions. But we are struggling to understand how the first translates into the second.

Here are a few possible reasons why we haven’t got there yet…

The scale

It is often said that the human brain is the most complex structure in the known universe. It is made up of around 100 billion nerve cells. Each of these cells can be connected to thousands of others, making there at least 100 trillion nerve connections in total – more than the number of stars in the Milky Way. And the number of possible configurations and possible strengths of those connections is another huge number incomprehensibly larger again.

We know the brain is made of cells, which we call neurons, which send each other electrical impulses. In simple organisms, we can map out the connections between neurons like a wiring diagram. Humans are on another scale altogether but whatever the huge number of parts, it is finite. So maybe we could gradually try and map out all the neurons and their connections to each other?

I’m not sure how long it took Ramón y Cajal to draw the three neurons above but I’m pretty sure a lifetime would not be enough for him to draw 100 billion of them. Luckily, according to Moore’s law, computer processing power doubles every two years, and we can set computers to this task. This is the aim of the Human Connectome project.

One criticism of this enterprise is that is uses dead post-mortem brains. The nervous system is dynamic and all this would give us is the structure. But the “connectomists” would argue that their circuit diagram could be simulated on a computer system and various “sensory” inputs entered to see what would happen.

Secondly, despite what I said earlier, the brain is not just made of neurons. Neurons cannot function without supportive cells called glia, or without a blood supply, and immune cells surrounding them. And they are affected by signals coming from the rest of the body, and hormones travelling to them in the blood.

Thirdly, even if all this could be simulated in some way, we don’t know if this would tell as all we need to know. Nobody knows whether all phenomena produced by the brain such as conscious experience would be explained by these circuit diagrams.

Despite all these counter arguments, i’m sure that having a map of all the neuronal connections in the nervous system would advance our knowledge hugely. But this is some years away; and even when we’ve done it, it may turn out to be so huge that it is impossible to comprehend as a whole.

In the meantime, you could argue that you do not need to know all parts of a system to understand how processes occur – you do not need to know about each and every financial transaction in the world to understand how global economic systems work. If we know the general rules and pathways brain functions follow, and neural circuits work in clear patterns, we can just multiply these processes to form the whole thing…

We do know about some of the pathways in the brain and parts of the brain which seem involved in various tasks. We have worked these out by seeing what functions people lose when they have damage to part of their brain. For example – somebody has a stroke in their occipital lobe – then they become partially blind – therefore the occipital lobe must be involved in vision.

Other ways we did this in the past was by neurosurgeons cutting people’s heads open, prodding parts of the brain and seeing what happened. And more recently we can do it the opposite way round in healthy people by putting them in a functional MRI scanner (fMRI), ask them to do a particular task, and see which parts of the brain have increased blow flow to them (which we assume means those parts are working hard).

The complexity

It would be nice to think we could identify a “general theory” on the brain, where it could be explained by very few basic laws. We have been able to identify and name various parts of the brain. But anybody who has tried to learn the anatomy of the human brain is confronted by something like the image below.

As you can see, even on a macroscopic level the brain is difficult and messy. You can identify some structures, and if you manage to learn enough neuroanatomy, there are certain things that seem to make sense. But there are certain things that if you had to create it from scratch you would certainly not design. For example there seems to be very little logic to way the various pathways in the brainstem between the brain and spinal cord cross backwards and forwards for seemingly no good reason.

If you think about the pathway for vision. Firstly, the photoreceptors in the retina face the wrong direction, the nerves and blood vessels that supply the retina pass infront of them (presumably blocking some of the light). Then the light from half of the visual field crosses over to the opposite side of the brain, and half of it stays the same side, the information then passes right the way through the brain and is processed right at the back of the head in the occipital lobe.

The reason it is so messy is that the human brain has been formed by millions of years of evolution on top of previous versions of brains. Evolutionary biologists would claim that to understand how the brain functions and why it functions how it is it is essential to understand in what steps and in what conditions it evolved.

There’s nothing quite like it

One way we understand complex things is with metaphor. The heart is like a pump. The lungs are like bellows. The kidneys are like a filter. The brain is like a …?

We understand a whole lot more about all our other organs than the brain – their function, as well as their diseases. It remains a stereotype that (e.g. compared with cardiologists or orthopaedic surgeons), neurologists and psychiatrists do not have any real treatments for their patients.

Many neuroscientists with computer science background would argue that the brain works like a computer. It is made up of units (neurons) which can be in an off (not firing) and an on (firing) state, which in some sense make calculations. This metaphor has also been used from the other direction. There are many people trying to create artificial brains which exist only on computer software in artificial neural networks.

While it is tempting to think of the brain as a computer, we should not fall into the trap of assuming that it is. The brain is organic material, none of its cells are identical units, and there are all sorts of influences on each cells function – not just signals from other cells, but varying expression of genes, and varying concentrations of different chemicals and hormones.

People have historically always used the latest technology as an analogy for the brain. In ancient Greece many people thought it was similar to a hydraulic or pneumatic pump. In the 17th century Descartes thought it worked similarly to mechanical clockwork. In the 19th century Helmholtz compared it with telegraph networks. Freud compared the brain with steam engines.

And since computers were invented in the mid 20th century it has been assumed by many that the brain made calculations exactly like a one. So there are good historical reasons to predict that the computer metaphor will be discarded like those that came before.

The final reason (for now, I’m sure there are many others) why other sciences have had more success…

It is us

Throughout our experience of life – the way we perceive things, or remember things, or move around, seems easy and familiar. It seems so obvious, it takes no effort. But our subjective experience is not a good guide for what is objectively going on in there. And we can’t get out of our brains – everything happens through them. There are no sensory receptors in the brain itself. This is why brain surgery can be done with the patient awake – it cannot feel pain.

Using the example of vision again. Our visual experience of the world seems so seamless and unified – we see objects with colour, shape, and meaning. Intuitively it seems that the eye works like a camera. Only when cracks appear in our perception do we realise that our brain is putting everything together. There are medical conditions where the eye and retina are completely intact, but specific parts of the brain cortex are damaged which result in the inability to perceive movement, the inability to perceive colour, or the inability to perceive faces.

And we’re also not built to talk about it. Language is credited with the explosion of human population and our collective human abilities. But human language is predominantly designed/evolved for communicating about things in the external world, rather than turning itself on the human mind. Some essential concepts which seem to be functions of the brain such as “memory”, “thought” and “consciousness” are so hard to define – although not for want of trying by both philosophers and neuroscientists.

“If the human brain were simple enough that we could understand it, it would be so simple that we couldn’t”

This paradox may be true, but the understanding does not have to be done all by one brain. There are now thousands of people around the world working on it, aided with research tools, computer power, and money.

And a few hundred years ago people would not have been able to comprehend understanding complex mathematical concepts, what a quark is, or what is going on a star millions of light-years away. So why shouldn’t we crack the brain too?