The Puzzle of Left-Handedness (21 page)
This changed everything. The localization of brain functions turned out to be possible after all, as the phrenologists had always said it was, if on the wrong grounds. While it wasn’t the case that entire personal characteristics or qualities such as creativity, mendacity or perseverance had a fixed location, far simpler, often rather abstract functions certainly did. Relatively primitive things such as registering the place where the light falls on the retina, sensing where you are being touched, bending and stretching an arm or finger, or moving the jaw muscles to create a chewing motion can today be traced to activity in precisely defined areas of the cerebral cortex. Complex character traits, by contrast, are the ultimate result, at a very high level, of brain processes working together, combined with our upbringing, the laying down of experiences in our memories and various external stimuli. There’s no way for us to recognize in the final result the basic components that go to make up aspects of our characters, any more than we can recognize in the graceful motion of a ballet dancer performing
Swan Lake
the nerve impulses that race through her body, stretching or contracting the muscles at precisely the right moment.
 | The left half of the brain, showing the areas identified by Broca and Wernicke that are involved, respectively, in speech production and language comprehension. At the top is the motor cortex, used to steer moveable body parts. Immediately behind it lies the somatosensory cortex, which processes signals sent from the skin to the brain by the sense of touch. |
The functional symmetry that phrenologists took for granted turned out not to exist, or to exist only in part. Broca’s area is located in the left brain only. The equivalent area in the right brain has nothing to do with speech, and the same goes for a good number of other areas that have since been discovered. This meant that the two halves of the brain could no longer be regarded as equal, which inevitably raised the question of which half was in charge.
One myth was thereby exchanged for another, since almost immediately the idea took hold that the left half of the brain was dominant. The mere fact that most people are right-handed was enough to suggest that the mechanisms controlling that hand, known to originate in the left brain, were more highly developed. This made it all too tempting to regard that cerebral hemisphere as superior in a general sense. Furthermore, all the newly discovered specialist areas that were found in only one half of the brain were in the left half, and they had to do with typically human and therefore highly esteemed capabilities such as linguistic and – later – arithmetical skills. The first and most important discoveries were Broca’s area, which is mainly involved with the production of speech, and an area described by German Carl Wernicke in 1874 that seemed to be engaged mainly in language comprehension. Although the motor cortex, responsible for controlling physical movement, is pleasingly symmetrical, occurring in both halves of the brain, German scientist H. Liepmann showed in the early years of the twentieth century that the left half nonetheless performs a unique function in coordinating complex actions.
Everything conspired to make the left cerebral hemisphere seem central to the human mind, richly endowed with high-level, typically human functions. It was exclusively responsible for the body’s more demanding physical acts and charged with the most important intellectual feats: the production and comprehension of language, arithmetical competence and, via the right hand, writing. For a long time the right half of the brain seemed to be there to make up the numbers. It put people in mind of an arid, sparsely populated desert, where nothing happened that was worthy of note, nothing at least that helped us with anything beyond the demands of bare survival.
For a while there was fairly general agreement that the left cerebral hemisphere was the overall champion, but as to the function of the other half, at least two different beliefs emerged. One described the right half of the brain as largely a fallow reservoir of brain capacity that offered possibilities as yet unknown. It could take over the functions of the other half if necessary, so it might be capable of almost anything. From an evolutionary point of view if no other, this is a rather problematic story. It would be quite extraordinary if such an energy-demanding, astonishingly complex organ had existed for millions of years without being used. Nevertheless, this belief prevailed for long enough to introduce the misconception, as optimistic as it is ineradicable, that we use only 2, 10 or 20 per cent of our brainpower.
The other belief was that the right hemisphere duplicated the left, serving as a kind of backup system. It was not empty but full of slumbering copies of the apparatus active in the left half, ready to jump into the breach on demand. It certainly is the case that our bodies have duplicate systems that provide us with spare capacity. We have two kidneys, two lungs and two Fallopian tubes, for example, mirror images of each other, even though we could get by perfectly well with one. Yet none of these duplicate systems behaves the way people imagined the right half of the brain to behave. Both work at 100 per cent capacity from the start; we don’t have a kidney or a lung that sits idly waiting to be called upon should the other cease to function. This alone makes it extremely improbable that we have brains largely made up of an idle reserve mechanism.
Not until the 1950s did it begin to become clear what was going on in the right half of the brain, initially as a result of work by researchers including Henry Hecaen in France and Britons Oliver Zangwill and Brenda Milner. A series of right-brain tasks came to light, including the interpretation of visual information and later the identification of people by their faces, musical abilities such as the recognition of melodies, and spatial orientation.
These new insights were brought to us by means of new research techniques. Previously almost all available knowledge had been drawn from what were known as natural experiments, a rather high-sounding way of describing random events. A person might be brain damaged in a fall, by a blow to the head, or after a stroke or cerebral infarction that incapacitated him or her in all sorts of different ways. Once the patient had died, sometimes many years later, researchers could open up the skull and see which parts of the brain had been destroyed. By studying the location of the damage and the symptoms that accompanied it, and comparing the effects in different patients, a picture gradually emerged of which bits of the grey matter were essential for which tasks.
It was better than nothing, but it did not do an enormous amount to help research along. Accidents, strokes and infarctions disable random and often quite large parts of the brain that don’t neatly coincide with a presumed arrangement of brain regions devoted to specific functions. They often produce a complex jumble of disabilities that is difficult to interpret, and they never strike in exactly the same way twice, which makes it extremely hard to compare one patient with the next. Many people are too badly affected to take part in studies, and on top of all this comes the eternal problem that dogs this kind of research: the fact that two things coincide does not mean there’s a direct causal connection between them. You cannot simply take the fact that with a specific form of brain damage a specific function is lost and conclude that the location of the damage indicates the place where that function resides. After all, if you take a wheel off a car it can no longer move, but that doesn’t mean the car travels on just that one particular wheel.
Until the middle of the twentieth century there were two other possible ways of learning about the structure of the brain. One was the surgical removal of part of the cerebral cortex. Extraordinary circumstances aside, this route was blocked in the case of human beings. Scientists therefore had to resort to animal experimentation, with all its limitations and uncertainties. The only alternative method consisted of the direct electrical stimulation of parts of the cortex during brain surgery. This technique did in fact lead to the discovery of a number of areas that have to do with voluntary movement and sensory perception, and it is still of great service to surgeons in the process of determining exactly where they need to make incisions, but until recently it was too crude to tell us anything about less concrete and primitive functions. A technique called deep brain stimulation is now yielding intriguing results, but the procedure involves a great deal of risk, so it can be applied only in the most serious cases of brain damage.
Eventually new techniques were discovered that made it possible to allow the two halves of the brain to work independently. They included the astonishing split brain operations carried out by American neuropsychologist Roger Sperry and a number of colleagues in the mid-1960s. The
corpus callosum
was severed, as were any other, smaller connections, so that the two cerebral hemispheres could not come into contact with each other except by a tortuous route through the brain stem.
This may sound like the work of a mad scientist, but Sperry was no Dr Frankenstein. He had been presented with patients affected by a severe form of epilepsy. They suffered terrible seizures caused by powerful, spontaneous waves of electrical impulses that bounced back and forth between the two halves of their brains via the
corpus callosum
, rather like water sloshing in a bowl that’s being shaken. The idea was that by breaking the connection the waves of impulses would be prevented from building up such devastating strength. Animal experiments showed no other detectable consequences, so Sperry began separating the two halves of his patients’ brains.
It worked. Sperry’s operations brought considerable relief to many hopeless epileptics, but at the same time they offered a unique opportunity to address each cerebral hemisphere separately, by supplying information to one half of the retina only. Research of this kind confirmed beyond doubt that essential tasks involved in the organization and production of language generally take place in the left half of the brain, while the recognition of images mainly happens in the right half. If one of his patients saw a house with the right side of his retina, and therefore with the right half of his brain, he often had no difficulty pointing with his left hand to things that are typically associated with houses yet was unable to say what he could see. Finding the word that refers to a house is a task of the left brain, and that half had no idea what the patient was seeing. Conversely, if the word ‘house’ was presented to the left side of the retina – and therefore to the left half of his brain – he was able to read it but in most cases could not point to the appropriate picture. In order to recognize an image of the thing referred to by the word he was reading, he would have had to engage the right half of his brain, which was impossible in any direct way because the two halves were no longer connected.
A less drastic though still deeply invasive treatment was what’s known as the Wada test, after its inventor the Japanese-Canadian brain surgeon Juhn Atsushi Wada. In the years immediately following the Second World War, Wada worked in Japan with brain-damaged ex-servicemen who were being treated with electric shocks. The therapy brought them some relief, but it also caused further damage to the men’s memories and linguistic abilities. After much trial and error, Wada hit upon the idea of using amobarbital (sodium amytal) to shut down the left cerebral hemisphere, where those functions were located, before the shocks were administered. Then fate decided to give Wada a helping hand. He was sent a young man who had been shot in the head and as a result had descended into an endless sequence of epileptic attacks. One fit immediately sparked the next. This no longer resembled uncontrolled sloshing back and forth in a bowl, it was more like a pan of water brought to a furious boil. The patient did not regain consciousness even for a moment. It seemed nothing could be done, until Wada tried breaking through the chain reaction by anaesthetizing the left half of the young man’s brain. The convulsions ceased.
Wada had not only chanced upon a way of reducing the damage inflicted by electroconvulsive treatments and gaining a grip on seemingly uncontrollable epilepsy, he had unintentionally invented the Wada test. Anaesthetizing one half of the brain gave him an unprecedented opportunity to take someone with an undamaged brain as his test subject and communicate with just one or other of the cerebral hemi -spheres. He was able to confirm and expand upon the findings of Sperry with his split-brain patients. If the left half of the brain was incapacitated, the test subject was temporarily unable to count or talk and had great difficulty understanding spoken or written instructions. If the right half was sent to sleep, the person in question proved unable, for example, to sing.
On the other side of the world, a couple of years before Wada set to work, a German neurologist had made an important discovery. Klaus Conrad nursed and performed tests on some 800 men with bullet wounds to their brains in a German field hospital in the final years of the war. It may sound harsh, but gunshot wounds are popular among brain researchers, since they cause neater, more clearly defined brain damage than accidents, strokes or infarctions. He discovered something extraordinary. Since Broca’s time almost a century earlier, scientists had believed that the language centres were located on the same side of the brain as was responsible for the control of the preferred hand, which in the overwhelming majority of right-handed people meant the left side. It was assumed that the brains of left-handers were reversed, so that both these functions were located in the right brain, an assumption that grew out of the old idea that the two sides of the brain were each other’s mirror image in some way. Echoes can be detected here of ancient ideas about left-handedness and
situs inversus
.
