The Puzzle of Left-Handedness (23 page)
The areas of the brain that light up in left-handers and right-handers when verbs for actions involving hand movements are recognized. The meanings of such words seem to be stored not only in the areas of the brain that are concerned with motor functions but in the side of the brain that controls the preferred hand.
If the theory of embodied cognition holds water, then it would be logical to expect left- and right-handed people to differ in their ways of understanding and remembering those verbs that refer to actions carried out by the hand, such as ‘pinch’, ‘throw’, ‘pick up’ or ‘draw’, aided as they are by the pre-motor areas for their preferred hand. This turns out to be the case. Results of f
MRI
tests show that with right-handers the relevant areas in the left cerebral hemisphere are highlighted and with left-handers the corresponding areas in the right.
Add to this the remarkable phenomenon discovered in 2009 by Daniel Casasanto – that people respond more positively to things in the real world that appear on the same side as their preferred hand than they do to things on the other side – and little remains of the image we once had of the differences in how the brains of left- and right-handed people are laid out. Those differences can sometimes be far greater than we used to think and they seem to relate to a far broader range of functions, perhaps even to all functions. Strong lateralization, with tasks that are entirely reserved for one half of the brain, seems to be far less in evidence than we once imagined. Lateralization was once seen as a monument not only to our knowledge about our brains but to the difference between us and other species. That monument is now being shaken to its foundations.
Despite the apparently larger, more varied and more polymorphic differences that have been discovered between the brains of left-handers and those of right-handers, left-handed people remain as ordinary and inconspicuous as ever. The most important conclusion may therefore be that our brains are far more flexible than we once thought.
All this is not to say that the beliefs of the second half of the twentieth century were entirely incorrect. On the contrary. There’s no reason to doubt that in the majority of cases the right cerebral hemisphere is concerned with such things as spatial orientation, the creation and recognition of melodies and the interpretation of images. The left is usually better at counting, arithmetic, keeping track of time and processing language.
Yet we need to take into account that many of the functions we recognize as such are not single entities at all but the result of a meshing together of a wide range of smaller tasks and capacities. To think, to understand, to read and write – these are brief, simple terms for unfathomably complex processes. It would be quite strange if all the processes involved in such higher functions were located in either one cerebral hemisphere or the other.
The functions we can positively identify as located in a specific place in one or other half of the brain will tend to be relatively straightforward, abstract processes, dozens of which go to make up what we think of as higher functions. Perhaps the location of a given function is not even particularly important. The overall difference in approach between the two halves of the brain may be far more relevant. It may be that the difference between our cerebral hemispheres has less to do with the specific processes that reside in them than with how those processes happen. In other words, they may differ more in their way of working than in what they do.
To find one possible indication of this we need to turn our attention back to our hands. We can all make a fist, lay one hand flat on a table or drum a tabletop with our fingers. People with brain damage can do all these things too, so long as they aren’t hampered by severe paralysis. All of us, including brain-damaged people, can easily mimic those three acts in a given order after they have been demonstrated to us once: ball fist, lay hand flat, drum fingertips. Unless, that is, there is damage to the left side of the brain. The performance of each act still presents no difficulties, but getting them in the right order does. The revelatory thing here is that whether the patient happens to be left- or right-handed, this difficulty is manifested not only by his or her right hand, which is controlled by the damaged left brain, but to an equal degree by the left hand, controlled by the undamaged right brain.
Clearly the problem does not lie in hand-control. It seems those damaged parts of the left brain are involved not so much in the performance of the acts themselves as in making them happen in the required order, with either hand. Healthy people and people with damage to the right brain generally have no problem at all with simple sequencing tasks.
This suggests that the planning and organization of complicated undertakings, in other words the compiling of a programme of activities that must be carried out in a specific order, is a speciality of the left brain. It’s precisely this capability that lies at the root of William Calvin’s ideas about the causes of hand preference. Ever since Liepmann’s discovery in the early 1900s that the left cerebral hemisphere is involved in complex motion in both halves of the body, this breakthrough has been only a short step away. It also tallies with the likelihood that brain damage on the left side will cause linguistic problems, since the processing of language is all about putting together and plucking apart sentences and words, structures that have to be compiled in precisely the right order. In fact language processing involves a highly complex arrangement of levels and subsidiary tasks, each of which has its own strict rules about order and sequence that fall under the heading of grammar. Each phrase, whether spoken or heard and understood, requires the processing of a vast pile of sequence-sensitive data with such astonishing rapidity that it doesn’t delay us at all.
We can perhaps safely conclude that the left and right cerebral hemispheres both do their work in their own way, sometimes alone, sometimes in collaboration. The left brain is primarily skilled at aspects of tasks that have to do with counting, language production and comprehension, timing and consciousness of time – all tasks in which sequencing is important. The right brain seems to be more active in the recognition of situations and in associating them with each other. A competitor in a quiz who unfailingly recognizes any song from its first chord can probably attribute that talent primarily to the right side of the brain, which identifies the unique combination of timbre, pitch and so forth that occurs only in the opening bars of that particular number. If the competitor then taps out the rhythm, the left brain becomes fully involved, since he or she is engaging in an activity organized in time.
These differences in the way the two halves of the brain work seem to be little more than tendencies. Strong tendencies, certainly, but they don’t amount to an absolute distinction. There’s room for considerable variation, for a greater or lesser contrast, for more or less obvious lateralization. It seems left-handedness can somehow be a consequence of this variation, but it needn’t be. We shouldn’t forget that as far as the separation of functions goes, the great majority of left-handed people show the same pattern as almost all right-handed people. Nor should we forget that despite the effectiveness of modern technology, we still have little firm knowledge about the brain.
The two cerebral hemispheres differ not only in what they do and in how they function but anatomically, which is to say in their material form. A great many parts of the brain are larger on one side than the other in most people. Sometimes they differ in shape as well. In left-handers these differences seem on average slightly less pronounced, and in a few cases the proportions are actually the reverse of those found in most right-handers. When it comes to brain anatomy, left-handers also demonstrate rather more variation, but again this is no more than a tendency and the majority are no different from the average right-hander in the physical shape and composition of their brains. There may therefore be a weak connection between hand preference and brain shape, but there is not, to use phrenological terminology, a bump in the skull representing hand preference.
There is another possible anatomical cause, which strictly speaking lies outside the brain. It has sometimes been suggested that hand preference arises from differences in the nerve bundles that connect the brain with the arms and legs. The left half of the brain directly controls not only the right arm and hand but the whole of the right side of the body, and vice versa. In practically all respects, that is. The connections between one half of the brain and the muscles or, for example, the sense of touch on the opposite half of the body take the form of a thick bundle of nerves that runs from the brain down the spinal column and from there to all the extremities on that side. But there are also connections between each cerebral hemisphere and the body on the same side. This bundle of nerves is nowhere near as thick, although it does mean that if one side of the brain is damaged, the side of the body it controls won’t necessarily be completely paralysed and totally numbed.
| Vesalius, Fabrica (1543). The main features of the nervous system are shown here, including the loose bundles of nerves that lead to the arms and hands and the long connections that lead via the spine to the torso and legs. |  |
From each half of the brain, then, two bundles of nerves lead to the spinal column, a thick bundle that connects with the opposite side of the body and a thin one straight down. The thick bundles cross each other just below the brain, reaching the side they control immediately above the spinal cord. These two thick bundles, despite the fact that each has roughly the same amount of body to control, can differ in size considerably. The same goes for the thinner bundles. In eight out of ten people both the thick and the thin bundles that run to the right side of the body are thicker than their counterparts that serve the left side. Eight out of ten; a very suggestive proportion. You might almost suspect that hand preference arises out of a better infrastructure that makes traffic between the brain and that half of the body smoother and more precise. Almost, because this turns out not to be the case. There is no connection at all between hand preference and the difference in the thickness of these nerve bundles.
*
In 1865 Dax’s son succeeded in having his father’s work published, but by then the credit had gone to Broca. Not entirely unfairly, since Broca had a far more precise idea than Dax as to where speech functions were located.
26
Animal Crackers
When in the course of the nineteenth century it gradually became evident that in humans real differences exist between the two halves of the brain, a great many neurologists immediately hurled themselves upon the animals of the woods and fields in order to find out whether perhaps there was evidence of similar specializations. The crude neurological techniques of the time made it impossible to use healthy humans in research, so scientists were limited to examining people who happened to have suffered brain damage, or carefully tinkering with those who for one reason or another needed to undergo brain surgery. There were fewer difficulties with animals, which were enthusiastically ex -perimented and operated upon, sometimes with astonishing results. Contrary to expectations, animals often turned out to have a preference for one paw over the other and to stick to that preference with great determination.
In one experiment rats were put into a cage with a tube attached, in which lay a tasty treat. The tube was mounted in an extreme corner of the transparent front of the cage, up against the side, so that the rat had to use the paw on that side to reach into it. Some rats stubbornly kept trying to use the wrong paw. They clearly had a marked preference, so much so that it didn’t even occur to them that they could easily get at the treat with the other paw. There were less dogmatic rats too, for whom it made no difference where the tube was placed. They had no hesitation in using whichever paw was convenient.
Even more remarkable results were achieved when rats with a strong paw preference had the part of the brain controlling that paw delib -erately damaged. After a few days they were able to move their prefer -red paw again, but it functioned less well than before. Nonetheless, the rats continued to favour it. This seemed very similar to the way people respond. Whatever may cause hand or paw preference, once it’s become established there’s an extreme reluctance to switch.
Yet however surprising the results of experiments like these may be, they produce little of relevance to human hand preference. It turns out to be almost impossible, for example, to find a specialization in one half of the brain of an animal, whether ape or frog or anything in between, that resembles the kind of specialization we see in humans. In birds such as sparrows and canaries, the left brain does seem to be more involved in singing than the right, but bird brains differ so fundamentally from those of mammals – and therefore people – that this doesn’t tell us a great deal. Another problem is that hardly any criteria are imaginable that could be used to measure a bird’s preferences. In rats, reaching for treats serves as a relevant task, but how do you measure foot or wing preference in a sparrow or a seagull?
To a greater degree even than experiments involving people, animal experiments are hard to interpret. Appearances can be deceptive, increasingly so the more an animal differs from us. The renowned scientific journal
Nature
reported in 1996 that ‘hand’ preference had been found in an amphibian for the first time: the European common toad. Researchers had stuck pieces of wet paper over the animals’ noses and mouths, or pulled small balloons over their heads, and watched to see which of their front feet (‘hands’) the animals used in their initial attempts to get rid of the annoying obstruction.
The Italian-Australian research team reported sensational results from its toad-baiting. The creatures seemed more like people than any other species. No fewer than six out of ten were right-handed, while only one in six favoured the left. The remaining quarter had no clear preference. This was extraordinary, since if chance determined hand preference then a quarter would be left-handed, a quarter right-handed and the rest indifferent. Toads were demonstrating a distinctly uneven distribution, just like humans, such that the left-handed, at around 15 per cent, were proportionately almost as rare as left-handed people. For a short time it seemed as if lateralization, the presumed source of our hand preference, could be traced back to the common ancestor of humans and toads.
Unfortunately the scientists had spoken too soon. Tomio Naitoh of Shimani University in Japan and Richard Wassersug of Dalhousie University in Halifax, Canada, took up their pens to disabuse the readers of
Nature
. Toads, they wrote, have a habit of removing poisonous and indigestible food remains via their mouths. They do this by vomiting up their entire stomachs and wiping the stomach lining clean with a front hand. Since a toad’s stomach is asymmetrical, with a shorter membrane on one side, it always hangs out of the right side of its mouth, which makes it easier by far to reach with the right hand. It was very likely that the toads were reacting to foreign bodies on their faces as if they were cleaning their stomachs. You could still call this a hand preference, but its cause lies in the structure of a toad’s intestines, not in its brain. So much for the right-handed common ancestor of man and toad.
A few years earlier, in 1990, other researchers had claimed that rhesus macaques were right-handed, based on the fact that the bones in their right arms were usually slightly longer and stronger than those in the left. But oddly, the researchers added, the difference in males became smaller with age and the difference in females larger. They cautiously concluded that something else might be at work here. Two years later New Zealander Rachel Baskerville identified what it was. Changes to the skeleton that coincide with age and sex often have a hormonal cause. In humans, for example, asymmetries were found long ago in the vicinity of the shoulder, a result of the fact that testosterone acts slightly more on the bone on one side than the other. Testosterone levels fall in men over the course of a lifetime, whereas in women they increase slightly, a pattern neatly reflected in monkey bones.
The differences between limbs in rhesus macaques could also be a side effect of small irregularities in the symmetry of the torso as a whole. In people with back problems caused by malformations or by a tilted pelvis, one leg is often found to be slightly shorter than the other, by way of compensation. Whatever the precise reason for the difference found in the monkeys’ arms, it probably has nothing to do with paw preference.
A multiplicity of animal experiments has nevertheless produced some results. First of all it’s clear that paw preference does exist in mammals, but in a way consistently different from the picture in humans. In animals with a paw preference, the right-pawed group is always roughly similar in size to the left-pawed group, while around half of individuals lack a clear preference. We never see anything like the uneven distribution so characteristic of man.
An even more intriguing result is that breeding for paw preference, in contrast to breeding for colour and all kinds of other characteristics, has proven impossible. It was tried for many years with mice to no avail. In generation after generation the picture remained the same: there were always roughly as many left- as right-pawed individuals plus a large group that showed no preference. This proved, if nothing else, that paw preference is not passed down simply according to Mendel’s laws of inheritance. If we persist in thinking that paw preference in animals has something in common with hand preference in people, then the laboratory rat has only succeeded in making hand preference slightly more of a puzzle than it already was.
