How does the brain work?

The media are full of stories about how this or that area of the brain has been shown to be active when people are scanned while doing some task. The images are alluring and it is tempting to use them to support this or that just-so story. However, they are limited in that the majority of the studies simply tell us where in the brain things are happening.

But the aim of neuroscience is to discover how the brain works. Yet the spatial resolution of these images is only of the order of millimetres. This means that within any one area that is shown to be active there are many millions of nerve cells or neurons. It is an awesome challenge to find out how these are connected and how their combined activity carries out particular operations. And just think: there are an estimated 86 billion neurons in the human brain as a whole!

One way of trying to find answers is to appeal to the operations that are carried out by computers. In other words, we can appeal to ‘computational’ theories. One of the pioneers of what we now call ‘Computational Neuroscience’ was David Marr. In his seminal book on Vision (1982), he suggested that we address the problem at three levels: 

The computational level – what is being computed?
The algorithmic level – what transformations are being carried out?
The level of implementation – how are these transformations implemented?

In a computer, the operations are carried out by silicon chips, but in the brain, by circuits of neurons. It is not so difficult to understand how silicon chips work because we have made them. The problem is that this is not the case for neuronal circuits.

…there are an estimated 86 billion neurons in the human brain as a whole.

So it is simply astonishing that when he was a graduate student in the late 1960s David Marr produced theories as to how three of the structures in the brain actually work. These were the cerebellum, involved in the automation of motor skills; the hippocampus, involved in retrieving memories from our life; and the neocortex, involved in categorising and classifying the sensory information that we receive from the world. At the time, the majority of scientists were addressing questions as to what each brain area did. Yet here was someone having the daring to ask what transformations were performed and how they were implemented in neuronal circuitry.

These theories turned out to have a very widespread influence in the scientific literature. And they were so far ahead of their time that it was not for ten years that the most basic prediction made in the cerebellar theory was tested and found to be correct. Since then, neuroscience has advanced rapidly, with the advent of many new methods. We can trace the anatomical connections of individual neurons or groups of neurons; we can record the activity of individual neurons or groups of neurons while animals or people are performing tasks; we can record the magnetic signals that are produced when neurons become active, and do so in people using detectors from outside the head; and we can even turn off the activity of particular neurons using light. So we are now in a position to ask how Marr’s early theories hold up in the light of the findings of today’s neuroscience.

Neuroscience is coming of age.

It is a tragedy that David Marr did not live to find out for himself. He died in his mid-thirties in 1980, though his wife, the neuroscientist Lucia Vaina, is still working. Had he lived he would have been 70 last year. Without question, there would have been a scientific gathering to congratulate him on his achievements.

As it is, there is no better way of celebrating one of the most influential scientists of the modern era than to use current knowledge to produce theories of how the cerebellum, hippocampus, and neocortex actually work. And there are scientists around the world who are now doing so. Neuroscience is coming of age.

Featured image credit: Computer by Lorenzo Cafaro. CC0 public domain via Unsplash.

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