The Spark of Life: Electricity in the Human Body (30 page)

The enormous complexity of the human brain and the difficulty of identifying individual connections make construction of a similar circuit diagram for our own brains an almost insurmountable problem, never mind the fact that it would be different in each individual and would change as we learn new skills and have new experiences. Nevertheless, we are not entirely ignorant of how our brains work.

The idea that different bits of the brain are specialized for specific functions was first championed by Franz Joseph Gall in the early nineteenth century. By extensively examining the skulls of his friends, his patients and the inmates of local asylums and prisons, he came to the conclusion that different bits of the brain were associated with different mental attributes such as valour, cautiousness, ambition, wit and mechanical skill, and that this was reflected in the size and shape of the overlying skull. A charismatic speaker, he travelled throughout Europe delivering public lectures on his ideas, even giving a presentation to the German royal family. He also amassed a collection of 300 human skulls and over 100 plaster casts. But although phrenology – the practice of deducing a person’s character from the bumps on their head – enjoyed a brief vogue, it has no basis in science.

The first real clues to understanding what jobs different brain regions carry out came from studying people with brain damage caused by injury or disease. One of the most celebrated of these individuals was Phineas Gage. On 13 September 1848, Gage was the foreman of a gang of construction workers building the bed of a new railway line outside the town of Cavendish in Vermont. He was preparing to explode a large boulder and was using a long iron rod (one and a quarter inches in diameter, about four feet long and more than thirteen pounds in weight) to tamp down the blasting powder into a hole drilled in the rock. Alas, a spark caused by the iron striking the rock ignited the dynamite, which exploded, driving the rod straight through Gage’s skull. It entered through the left cheekbone, damaging his eye, and exited though the top of his head, landing some twenty-five metres away, smeared with his blood and brains. Gage ‘was thrown upon his back, and gave a few convulsive motions of the extremities’ but rather remarkably he spoke within a few minutes, was able to sit upright in the cart that transported him to his hotel and then even walked up a long flight of stairs. The first doctor to examine him was disinclined to believe his story until Gage got up and vomited and ‘the effort of vomiting pressed out about half a teacupful of the brain, which fell upon the floor’. A second physician, who arrived an hour and a half later, found Gage conscious and talking, but noted that both ‘he and his bed were covered in gore’.

Although Gage recovered physically, it soon became clear the accident had changed him. Previously of well-balanced mind, friendly, energetic, hard-working, and a great favourite with his colleagues, he became obstinate, vacillating, uncooperative and indulged in ‘the greatest profanity’. He was, said his friends, no longer the same man. What Gage’s story shows is that our personality and emotions are linked to the function of certain brain regions. The damage to his prefrontal cortex had led to his inappropriate behaviour and loss of social inhibitions.

Another unfortunate whose ailment provided information about where different functions are located in the brain was Monsieur Leborgne, who was unable to say anything other than ‘tan’ when Paul Broca examined him in 1861. When Leborgne died shortly afterwards, a post-mortem revealed that a small region of his left cerebral hemisphere was damaged. Immortalized as Broca’s area, this region of the brain is concerned with speech production. A few years later, Carl Wernicke discovered several patients with a different speech problem: although able to articulate words clearly and fluently, they simply spoke gibberish, a meaningless, incoherent rush of disconnected words, but with the syntax of the sentences more or less correct, such as in, ‘I can’t talk all of the things I do, and part of the part I can go alright, but I can’t tell from the other people.’ It is now recognized that this bit of the brain is involved in language comprehension. Wernicke’s area lies some distance from Broca’s area further towards the back of the brain.

For most purposes, the left and right sides of our brains are symmetrical. Language, however, is confined largely to the left side of the brain. A patient who has a stroke in their left cerebral hemisphere may therefore find themselves paralysed down the right side of their body and unable to speak. By contrast, a stroke on the right side of the brain can lead to paralysis of the left side of the body but usually has only a limited effect on speech. Fascinatingly, people whose Broca’s area has been damaged by a stroke are often able to sing words that they cannot speak – it seems that singing involves an entirely different bit of the brain.

All Fired Up and Ready to Go

Another means of determining what function a specific brain region performs is to stimulate it directly with a small electric current. One of the first scientists to do so in a systematic fashion was Eduard Hitzig, who in the mid-1800s experimented on Prussian soldiers whose skulls had been shattered by bullets, leaving part of their brain exposed. Hitzig noticed that when a small current was applied directly to the brain it caused involuntary muscle contractions in the subject. Subsequent studies on dogs revealed that a small strip of cerebral cortex – now known as the motor cortex – controlled the movement of specific parts of the body.

In a similar fashion, sounds, sights and even the sense of touch are mapped onto the cerebral cortex. At the top of the brain sits the somatosensory system. Here, inputs from sense organs in your skin are neatly arranged so that all the signals from one location on your skin go to the same bit of the brain: legs, feet, fingers and toes each get their own areas. The most sensitive parts of the body, like the lips, fingers and genitals, are assigned larger brain areas, with more neurones, than less sensitive parts of your skin like the small of the back. Similarly, inputs from your eyes are mapped onto the visual cortex at the back of the brain, with the signals received by the same part of your visual field going to the same place, while sounds are organized according to frequency in the auditory cortex. Indeed, it now seems that there may be several such maps for each of the senses: like all good machines, the brain may have some built-in redundancy. The information is not wired straight through, however: it passes through many relay stations and is highly processed en route.

The ability to evoke sensation and action simply by stimulating a specific region of the brain is of considerable clinical significance. It is often used in brain operations to ensure, for example, that a surgeon removing a tumour removes the correct bit and nothing else. During this operation the patient is awake and able to say what they feel: it does not hurt as the brain has no pain receptors and local anaesthetics are used to dull the pain fibres in the skin overlying the skull. Such operations can also yield useful information about where memories, words and information are stored.

Brain Waves

Early studies of the brain thus operated on much the same principle as a small boy with a new mechanical toy, who takes it to pieces to see how it works. More recently, non-invasive ways of looking at brain function have been devised, in which it is possible simply to watch what happens by recording brain activity.

The first of these techniques is the electroencephalogram (EEG), which is a record of your brain waves. Just as it is possible to record the electrical activity of your heart cells from electrodes attached to your chest, so it is possible to see what is going on inside your brain from multiple electrodes stuck to your scalp with conductive jelly. These pick up the minute voltage changes generated by the collective activity of millions of nerve cells in the surface layer of your brain. Your brain waves appear as oscillations in voltage that are constantly changing in frequency and amplitude as different regions of your brain surge into activity or fall silent. The EEG is much smaller and more difficult to record than the electrocardiogram and far more difficult to interpret. It’s a little like trying to understand the complex relationships between people living in a large city by listening in on all their telephone calls simultaneously: the multiple unconnected conversations make little sense and the vast number of them means it is impossible to pick out individual conversations.

All this means that the EEG has rather limited value as a research tool. Nevertheless, it does provide a glimpse of what the brain is doing and it has been particularly useful in studies of sleep and epilepsy, both of which are associated with marked changes in the EEG. The first recording of a human EEG was made in 1924 by Hans Berger, but it was not until some years later that its clinical value became apparent when it was observed that an epileptic seizure appears as a dramatic increase in brain activity – an electrical earthquake, in effect. It was later found that the EEG can be used not only to detect a seizure, but also to provide an indication of where it originates.

The EEG is also used to monitor the depth of anaesthesia, and to distinguish between whether an individual is in a coma or is dead. In most countries, death is defined as the cessation of brain electrical activity and legally you are dead when your brain waves stop, even through the rest of your cells may survive for many minutes, or even hours, after brain death. Such a definition is not only sensible, but clearly also of importance for organ transplantation. It means that the heart of a deceased individual can be kept beating by life-support equipment, so that most of the organs remain alive and can be used to save another person’s life.

Watching the Brain at Work

In the last few decades, new imaging techniques have transformed the way we can study the living brain. Brain-scanning methods can see deep within the brain and provide a far better picture of what is happening in different brain regions than the EEG. Unlike the EEG, however, they do not record the electrical activity of the brain directly. Instead, functional magnetic resonance imaging (fMRI) measures brain blood flow and positron emission tomography (PET) measures the metabolic activity of your brain cells. Both of these are believed to be related to the electrical activity of the brain, because the more active a nerve cell, the more energy it will consume and consequently its metabolism will increase to provide it. As nerve cells do not possess internal reserves of nutrients, the more active they are, the more glucose must be delivered via the bloodstream. Consequently, blood flow to that brain region is also enhanced.

fMRI has been invaluable for studying brain function, for it can be carried out on awake volunteers. It has revealed how the pattern of electrical activity in the brain changes during sleep, under anaesthesia, in migraine, epilepsy, and a multiplicity of everyday tasks such as learning, memory, speech – even thinking. Simply by scanning someone’s brain while they are asked questions, shown pictures, or asked to think about something, we can identify which bit of the brain is involved. Ask a person to imagine playing tennis and the blood supply to their motor cortex increases, as they think of smashing a high lob or a sizzling serve. Broca’s and Wernicke’s areas light up when you speak, confirming what was found from studying brain-damaged patients, and the reward centres of the brain spring into action when a smoker thinks of a cigarette.

Brain-scanning technology is transforming our understanding of how the brain works and what we think about ourselves. But it is worth remembering that the smallest brain region that can be distinguished in such scans still contains many hundreds or thousands of neurones, and what is detected (indirectly) is their summed activity. Thus there remains a huge gap between our highly detailed knowledge of what happens at the level of a single nerve cell, and how individual nerve cells are wired together to produce the electrical activity of our brains.

MRI and PET scanners are also invaluable clinical tools. Damaged areas of the brain can be easily identified and tumours or regions prone to epileptic seizures can be located. If an operation is needed, having a detailed picture of the precise location of the problem and its relation to crucial regions of the brain means it is far easier to avoid collateral brain damage.

Recently, a team of scientists of Cambridge and Liege universities have shown that it is possible to communicate with people’s brains directly, simply by asking them to answer ‘yes’ or ‘no’ to a question and then looking at their brain scans. Not that it is possible to determine if someone is simply thinking ‘yes’ or ‘no’, but if you are asked to envisage playing a game of tennis if your answer is affirmative, it is possible to detect a response in your motor cortex, and if you are asked to think of navigating around your house if your answer is negative, then a different region of your brain lights up. The patterns of brain activity are so distinctive that even an untrained observer can identify the subject’s response with almost 100 per cent accuracy. While it seems somewhat uncanny to be able to talk to someone this way, even more scary is the fact that four out of twenty-three patients who were believed to be in a persistent vegetative state were also able to give correct answers to questions, suggesting they may be at least minimally conscious and able to hear, but are totally cut off from the world because they cannot move at all, not even flicker an eyelid.

How the Brain Sees

All these studies serve to show that different regions of the brain are specialized for specific functions. The big mystery is how information is encoded and processed by the brain and how the different bits of the brain communicate. While this is far from understood, dramatic progress has been made in the last fifty years. Consider a single example – that of vision.