Left Versus Right

The cerebrum of the human brain, in which most of our higher cognitive faculties appear to be located, is divided into two symmetric halves called hemispheres; these hemispheres are connected only by a small, hard bundle of tissue called the corpus callosum. It was discovered long ago that each hemisphere is responsible for one half of the body, but, because our remote acestors somehow contrived to evolve a 180° twist between the spinal cord and the barin, it is the left hemisphere of the brain which looks after the right side of the body, and vice versa. Hence, apart from possible aphasia, damage to the left hemisphere can produce such consequences as blindness in the right eye, deafness in the right ear, or paralysis of the right side of the body.

Language is not the only faculty located in the left hemisphere. In general, most of our analytical abilities appear to be concentrated there, too, such as the ability to do arithmetic, to solve an algebraic equation, or to decide the chronological order in qhich things have happened. Moreover, neurologists have managedto identify a number of well-defined areas in the left hemisphere with specific functions, like the language areas.

The right hemisphere is different. It appears to be more amorphous than the left, and there are few if any well-defined specialist areas in it. But the right hemisphere is the one we primarily rely on for recognition and association. For example, look at Figure 6.2. What do yo see? Almost certainly, you immediately perceive an English word. But why? There is in fact no word in the picture-only a jumble of straight lines. However, your right hemisphere has a powerful talent for spotting familiar patterns in just such a jumble of data, and it quickly finds one here. In fact, this is the sort of thing your right hemisphere is doing almost every moment. It sifts the mass of sensory data pouring into the brain and organizes this mass into comprehensible and familiar patterns.

At the moment, as I write, I am enjoying my magnificent view of the university’s car park. I can see the expanse of grey asphalt marked with neat white lines, and recognize it as a car park, as the same car park I see every day, and as the place I have parked my car. I can see two cars (today is Saturday, and things are quiet); I recognize them as cars, and note that neither belongs to anybody I know. Two people have just walked down the steps; I instantly realized that they were a man and a woman and both total strangers to me. Pretty humdrum stuff, apparently – humdrum, but absolutely vital to my existence. It’s my right hemisphere that’s working all this out for me. What happens when the right hemisphere is damaged?

One of the most famous cases of right-hemisphere damage is that of Dr.P., a skilled musician and music teacher who suffered an apparently mild stroke in his right hemisphere. In most respects, the stroke had no visible effects: he remained highly intelligent and cultured, able to speak fluently and elegantly, able to make subtle jokes, able to remember facts, able to play, sing and continue teaching. In fact, he was perfectly normal-except for one thing.

He had lost part of that humdrum ability I have just described. He could no longer assemble his visual data into recognizable patterns, and as a result he could no longer recognize things by looking at them. When an object was placed in his hands, he examined it carefully, and, using his healthy left-hemisphere analytical abilities, announced “It’s about six inches in length …. a convoluted red form with a linear green attachment.” When pressed with the question “But what do you think it is?”, he pondered in some perplexity “Not easy to say … it lacks the simple symmetry of the Platonic solids, although it may have a higher symmetry o its own.” Instructed to smell the object, he looked puzzled, but politely complied. Suddenly he came to life. “Beautiful! An early rose!”

Dr. P. Had lost neither the word rose nor the concept of a rose. He had instead lost the humdrum ability to put together a mass of red and green visual impressions into a coherent whole and assignt the result to the category of roses. His intact left hemisphere allowed him to construct a careful and detailed description of every part of the object, and to express that description in perfect English – but his damaged right hemisphere could do nothing with the visual data, and hence could not provide a unified concept to the left hemisphere for naming. However, it was only his visual processing that was defective, and as soon as useful non-visual information became available – in this case, smell – his right hemisphere functioned perfectly; Dr. P. Had no trouble in spotting the familiar smell of a rose and in interpreting what it meant.

One of the most striking and tragic consequences of Dr. P.’s disability was his total loss of the ability to recognize faces. Not only could he not recognize even the most familiar faces, he couldn’t tell whether something he was looking at was a face or not. When tested with photographs, he failed to recognize almost all the people in the pictures, even his brother, his wife and himself. Occasionally he could get one by spotting one or two particularly distinctive features: for example, by picking out the mane of hair and the bushy moustache, he correctly guessed that one picture was of Albert Einstein. But his disability was profound: Dr. P. Would pat parking meterson the head, assuming they were children, and try to strike up conversations with carved knobs on furniture, which he had mistaken for faces.

Dr. P.’s disability was not specifically linguistic, though of course it had substantial consequences for his ability to use language in the normal way. But the right hemisphere’s particular talents sometimes show up linguistically in more direct ways. One of these lies in the expression of emotions: some people with right-hemisphere damage lose the ability to express their emotions-although they can still feel emotions – and as a result their speech, though otherwise normal, comes out as flat, lifeless, mechanical, almost robotic, and their faces remain blank, no matter how elatedor furious they may feel. And even a victim of unusually severe left-hemisphere damage who has totally host the faculty of speech can still often sing songs, sometimes very well, and even learn new songs; apparently learning and producing the words of a song is something the right hemisphere handles, or can handle, in spite of its normal linguistic ineptitude. Engagingly, many such people retain the ability to swear like a marine sergeant – apparently the right hemisphere does have some vocabulary! (Actually, there is good reason to believe that swearing, like laughing, sobbing and screaming, is not controlled by the cerebral cortex at all, but by some deeper and more ancient part of the brain.)

Fascinatingly, when a user of sign language suffers that right-hemisphere damage which destroys the ability to register ordinary facial expressions, she can onen the less still make the facial expressions which form a part of the grammar of sign language – demonstrating that the linguistic use of expressions is governed by a different pat of the brain from that which handles non-linguistic expressions. Other types of right-hemisphere damage produce the strange disability called left neglect: the sufferer, though not technically blind, fails to see anything in the left visual field, fails to draw the left side when drawing pictures, and even fails to dress the left side of her body. But a signer with this impairment still uses the left side of the visual field normally when signing- again showing that the language faculties are largely independent of other, non-linguistic functions of the brain.

The standard model is clearly good enough to take us a long way toward understanding in the brain, but there is a great deal that we are far from understanding. One of the most puzzling problems is the way that words are stored in the brain. We believe that Wernicke’s area plays a crucial role in access to vocabulary, but that doesn’t mean the words themselves are stored in Wernicke’s area, and in fact we’re pretty sure they’re not. A number of fascinating and bewildering cases of language disability reported in the last few years have suggested tantalizing ideas but generally left us feeling more ignorant than ever about what’s going on. Most aphasics exhibit some degree of anomia, or the inability to find the words for things, and some of them, even with otherwise fairly normal speech, can hardly find any words at all. But certains patients have turned up with startlingly specific versions of anomia almost unaccompanied by any other symptoms. Here is a sample.

Several patients have been reported who have no trouble finding the words for inanimate objects like chair and road but who cannot find the words for living things such as woman and dog. Does this mean that we store words for living things in a separate place in our brain? Another man lost nothing but words for fruits and vegetables: he had no trouble of any kind with any other words, but he could not name any fruit or vegetable, whether he was given a verbal description, a picture, or even a real banana or cucumber, and he could not decide whether a particular item that was giving him trouble might be a fruit or a vegetable. It seems almost inconceivable that we should be storing words for fruits and vegetables in a special place, and few linguists would be happy to accept such a conclusion, yet how can we explain what has happened to this patient?

Perhaps the most striking cases are those of two women who had suffered left-hemisphere strokes. Both women proved to have no trouble with nouns, but to have severe trouble with verbs, even when the noun and the verb were the same word. They had no problem in speaking or writing down sentences like There’s milk in the glass or That’s a nice picture, in which milk and picture are nouns, but they couldn’t cope with sentences like She can milk a cow or Try to picture the scene, in which they are verbs. Even when the sentences were dictated to them, and they were asked to write down just the one word, they could write down the noun, but not the identical verb. These findings suggest that nouns and verbs are somehow stored differently in the brain. Such a conclusion is more acceptable to most linguists than special storage arrangements for names of fruits and vegetables, but it still doesn’t tells us tkhat things are compicated.

Even more perplexing are the patients who manage to lose entire languages. Elderly people who have learned several languages earlier in life sometimes, when afflicted with mind-rotting illnesses like Alzheimer’s disease, lose onel anguage after another, but they usually retain their mother tongue the longest. But a number of quite young people have turned up who speak two or three languages and who, after some form of brain damage, lose and regain all of their languages, one at a time, in rapid succession. One French Canadian man, bilingual in English, lost his French but kept his English, as a result of which he couldn’t speak to the doctors and nurses in the hospital. In some cases, this sort of thing can go on for months, and no one has any idea how to explain it.

In fact, aphasic patients have been discovered who exhibit almost every conceivable set of symptoms. There are sufferers who can speak but not read, read but not write, write but not read (even what they’ve just written!). The one conclusion which many specialists are prepared to draw from such bewildering observations is that our language capacity must be in some sense modular – that is, it cannot be a formless whole, but rather it must be divided up into a number of specialized subcomponents, each using a different set of neural fibres in the brain. While still controversial, this conclusion is not at all surprising, since we already know that other mental faculties are modular. As is shown by Dr. P.’s experience, vision is modular: different parts of the brain handle different aspects of vision. Why should we expect language to be different?

Indeed, we already know for sure that language processing is modular in certain respects. For example, when we hear someone speaking, our brains instantly decompose the speech sounds into at least three components: the identity of the voice, the tone of voice (angry, amused, or whatever) and the strictly linguistic content. Even when you hear a foreign language you don’t know, you can recognize individual voices and infer something about the tone, even though you can’t process the linguistic part. Clearly our brains must be separating these things out and processing them separately.

Bir Cevap Yazın

E-posta hesabınız yayımlanmayacak. Gerekli alanlar * ile işaretlenmişlerdir