It weighs about 3lb, has the consistency and appearance of cold porridge and is famous for being the most complex object in the known universe. The human brain is more than an organ: it houses all our thoughts, memories and emotions, and is responsible for that most human of all traits, consciousness.
Yet, for all of its importance in defining who we are and our awareness of the world around us, we know very little about what makes the brain tick. We don't understand how it can remember events that took place in early childhood and why it is able to perform feats of incredible intellectual invention and artistic creativity.
The latest research by Professor Eleanor Maguire gives us a tantalising insight into how "episodic" memories of events are recalled in one area of the brain, the hippocampus, but even this work pales when one considers the complexity of that inscrutable pulp of organic tissue which sits between our ears, encased in its protective shield of bony cranium.
Many of the facts of the brain are testament to its complexity. It consists of about 100 billion nerve cells, called neurons, and if all of them were connected end to end they would go around the world several times. Each is capable of transmitting an electric current along its fibre to one of up to 10,000 interconnections, or synapses, that link one neuron to another.
This means there are 100 trillion interconnections in the brain. Just counting them at the rate of one a second would take you 30 million years. And at each synapse there may be one or more of many dozens of chemical messengers, called neurotransmitters, controlling the all-important transmission of electrical signals from one neuron to another.
But the complexity does not end here. Scientists have discovered other "internal" chemicals that act as secondary messengers, controlling communications within a neuron. These can change the property of a nerve cell and are likely to be important for such vital functions as being able to form or recall a memory.
And then there are the so-called "housekeeping" cells of the brain, the glial. They sit alongside the electrically-active neurons and outnumber them by about 100 to 1. But few researchers who study them believe they are simple housekeepers. They too seem to be involved in controlling the important functions of the brain.
So we have a peculiar conundrum. We know the human brain is the most complex object there is, but we know it is the least-understood aspect of our biology. Yet the only thing we have to study the brain is the brain itself.
Many of the early insights into the human brain came from patients with brain damage. Phineas Gage, a 19th-century American railroad worker, suffered a terrible accident at work and lost a good part of his left frontal cortex. He survived, but his personality changed beyond recognition because he was no longer able to control his basic emotions.
The frontal cortex, the outer part of the brain above the eyes, is well developed in man and is viewed as being responsible for "higher" functions connected with being human. The evolutionary older parts of the brain, nearer the brain stem, handle life's more basic urges such as sex and hunger, but the frontal cortex essentially protects us from the uncontrolled release of these animal instincts, a civilised inhibition that poor old Phineas had lost.
Studying the brains of other animals has given some insight into the human brain. A sea slug called Aplysia, for instance, played a critical role in earning some scientists a Nobel prize for working out a neural mechanism involved in memory formation. They also found that serotonin, one of those neurotransmitters, can "strengthen" the synapse between two neurons – as when a memory is formed.
But perhaps the most intriguing insights into the brain have come from the sort of scans used by Professor Maguire and her colleagues. Her pioneering work on London taxi drivers, for instance, found their navigation skills were linked with an enlargement of the back part of the hippo- campus which seems to be involved in storing mental street maps.
Her latest work has shown it is possible to predict what someone is thinking in terms of episodic memory by studying which part of their brains "lit up" in the scanner. These regions are assumed to be working more actively because they have increased blood flow, which delivers oxygen and glucose to the parts doing the work.
However, this type of brain scanning, called functional magnetic resonance imaging (fMRI), has its limitations. It does not measure nerve activity directly, only the flow of blood to the nerves. Secondly, it cannot discern the more diffuse activity in other parts of the brain that may also be involved in the task in question.
And thirdly, for all its sophistication, fMRI is still a very blunt instrument to study such a complex object. It takes, for instance, the human brain just 300 milliseconds to recognise a human face. On the other hand, it takes several seconds for the brain's blood vessels to dilate enough to be detected by an fMRI machine – a monumental delay that belies the flashlight immediacy of how the brain works.
We know a lot more about the brain than we once did, but we are still a long way from truly understanding it.Reuse content