Shufflebrain
Page 3
Bogen and Vogel's first patient was an epileptic middle-aged World War II veteran. When he awoke from surgery, he couldn't talk. No doubt to the relief of everyone concerned, his speech did return the next day. His seizures could be controlled. And to outward appearances, he and others who have undergone the operation are "just folks," as Michael Gazzaniga, another former student of Sperry's, said during a lecture.
But the split-brain operation has profound effects, although it took careful observation to detect them. Recall that an object in the left visual field signals the right hemisphere, and vice versa. Taking advantage of this, and presenting visual cues in one field at a time, Gazzaniga discovered that most people who had undergone split-brain operations could read, write, and do arithmetic normally, but only with their left cerebral hemispheres. When tested in their right hemispheres, they seemed illiterate, unable to write, and incapable of adding simple sums. Addressing a symposium a few years ago, Gazzaniga described a typical experiment. He held up the word, HEART, in such a way that H and E, presented in the left visual field, signaled the nonreading right hemisphere, while the rest of the word cued the left hemisphere. "What did you see?" , Gazzaniga asked. His subject responded, "I saw ART." The right hemisphere seemed blind to words. But was the right hemisphere really blind? Worse, did it simply lack intelligence? Or even a human mind?
Gazzaniga soon found that the right side of the cerebrum functioned admirably in nonverbal situations. For instance, when shown a picture of a cup, in such a way that it cued the right hemisphere, the person could reach behind a screen, feel among a collection of objects, and find a cup. In fact, the right hemisphere could manifest profound intelligence and sardonic wit. When presented with a picture of a smoldering cigarette, one subject, instead of matching it with a cigarette, brought forth an ashtray.
Not only is the right side capable of humor, but various studies indicate that people tend to use this hemisphere to comprehend geometric form, textures, and music. It's as though, in most of us, the dominant left side does the mundane jobs of reading, writing, and arithmetic, leaving the right hemisphere free to create and appreciate art.
Lateralization, as hemispheric differentiation is called, need not be investigated with the knife.[3] The psychologist Victor Milstein showed me a visual field-testing rig that he and his colleagues use in screening for brain damage. In fact, some of the best evidence of musical tendencies in the right hemisphere came from a test used by Bogen's group prior to actual surgery. Called the amobarbital test, it was perfected by Bogen in collaboration with another member of Sperry's group, Harold Gordon. Amobarbital is an anesthetic. The test involves injecting anesthetic into either the left or the right common carotid artery in the neck, thus anesthetizing one hemisphere at a time. (Actually, blood from a carotid artery on one side will reach the other side of the brain, through a channel called the circle of Willis. But the volume of blood crossing over is small in relation to what flows to the same side.) Gordon compared audio tapes of Bogen's patients singing before and after either the right or left hemisphere had been put to sleep. With the left hemisphere unconscious and the right one awake, most people sang well. But, with some exceptions, the subjects sang flat and off-key when the right hemisphere was unconscious.
Laboratory animals display interesting behavior after split-brain surgery. Two disconnected hemispheres may learn to respond to what would otherwise be conflicting stimuli. The animals can even learn at a faster pace. (There are, after all, two intelligences instead of one.) One side of the brain may be taught to avoid a stimulus that the other side responds to favorably. A split-brain monkey, for instance, may lovingly fondle a toy doll with its right hand and angrily beat it with the left. (Arms are voluntarily controlled by opposite hemispheres.) Sperry has even reported that persons with split brains sometimes maintain two entirely different attitudes toward the very same object--simultaneously.
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At first glance, and when the results were new, split-brain research looked like a powerful case for a structural theory of mind-brain. (I used to refer to it in the classroom.) Language memory, for example, seemed to be housed in the dominant hemisphere (along with handedness). Music memories seemed to be stored over on the nondominant side. But as more facts emerged, and as all the evidence was carefully weighed, what seemed like such a clear-cut case became fuzzy again.
As I mentioned earlier, some people are born without a corpus callosum. Sperry's group studied one such young woman extensively.[4] Unlike persons who have undergone split-brain surgery, those born without a corpus callosum don't show lateralization: both hemispheres reveal similar linguistic ability. Children who have had split-brain operations show much less lateralization than adults. A few years ago, after I'd written a couple of feature articles on hemispheric differences, a student who had read one of them came to see me, puzzled. If the left side of the brain stores language, he asked, how do people taking an amobarbital test know the lyrics of a song when only the right hemisphere sings?
It was a perceptive question. Clearly, no natural law confines language to one and only one side of the brain. Otherwise, no one with complete separation of the cerebral hemispheres could handle language on the right side; and children would show the same degree of lateralization as adults. Nor would Bogen and Gordon have found individual variations in music or language during the amobarbital test.
Gazzaniga has conducted a great deal of research on children. Before the age of two or three, they exhibit little if any lateralization. Hemispheric differences develop with maturity. We are not born with lateralized brains. How do most of us end up that way?
Circuitries in the visual system can be altered by the early visual environment.[5] There's direct evidence about this for laboratory animals, and a good circumstantial case has been made for humans. Environment has a much more profound effect on even relatively uncomplicated reflexes than anyone had ever suspected. Maybe culture and learning play critical roles in lateralizing. Maybe as we mature, we unconsciously learn to inhibit the flow of information into one side of the brain or the other. Maybe we train ourselves to repress memories of language in the right hemisphere. Maybe the formation of language and the routines in arithmetic proceed more efficiently when carried out asymmetrically--unless we are singing.
Inability of a right hemisphere to read doesn't necessarily preclude memory there, though. Maybe the right hemisphere has amnesia. Or, relying on the left side to handle language, the right hemisphere may simply not remember how it is done. We do repress and do not remember all sorts of things, all the time. I cannot recall my third-grade teacher's name, although I'll bet I could under hypnosis. With regard to repression, consider something like functional amblyopia, for instance--blindness in an eye after double vision, even when there are no structural defects in the eye. It is as though the mind prefers not to see what is confusing or painful--as double vision can be. But with correction of the double view, that same blind eye sometimes regains 20/20 vision.
Thus, we really cannot turn the results of split-brain research into a conclusive argument in favor of a structural theory of mind. We do not know whether split brains show us the repository or the conduits of memory. We do not know if what is coming out flows directly from the source or from a leak in the plumbing.
But the split human brain raises still another question: What does the operation not do? Why didn't the knife create half-witted individuals? Why were both personalities "just folks," as Gazzaniga said? Why two whole personalities? Isn't personality part of the mind? Why doesn't personality follow the structural symmetry of the brain? If we split this page in two, we wouldn't have whole messages in both halves.
It's not that a structuralist cannot answer such a question. But the structuralist's thesis--my old argument--must be tied together with an embarrassingly long string of maybes.
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The mind-brain conundrum has many other dimensions and extends to virtually every level of organizat
ion and discourse, from molecules to societies of animals, from molecular biophysics to social psychology. Name the molecule, cell, or lobe, or stipulate the physiological, chemical, or physical mechanism, and somebody, someplace, has found memory on, in, around, or associated with it. And, in spite of the generally good to splendid quality of such research, there's probably someone else, somewhere, whose experiments categorically deny any given conclusion or contradict any particular result.
Among those who believe, as I did, that memory is molecular, there are the protein people, the RNA people, the DNA people, the lipid people. And they're often very unkind to each other. Why? Most scientists, consciously or unconsciously, practice the principle of causality--every cause must have one and only one effect, or a causal relationship hasn't been established. If you are an RNA person and somebody finds memory on fat, that's unpleasant news. For RNA and fat cannot both be the cause of memory.
Some investigators believe that memories can be transferred from animal to animal in chemical form; that it's possible to train a rat, homogenize its brain, extract this or that chemical, and inject the donor's thought into another rat or even a hamster. The disbelievers vastly outnumber the believers, for a variety of rational and irrational reasons. Not everyone has been able to reproduce the results;[6] but memory transfer is in the literature, implicating quite a variety of alleged transfer substances.
Some research on memory does not implicate molecules at all. And while some data suggest that memories depend on reverberating circuits to and from vast regions of the brain, other evidence places memory in individual cells.
Who's right? Who's wrong? As we shall see later in the book, this is not the question.
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Dynamics of the learning process have suggested to psychologists that two distinct classes of memory exist: short-term memory and long-term memory. Short-term memory is, for example, using the telephone number you look up in the directory and forgetting it after you have put through the call. Long-term memory operates in the recollection of the date of New Year's, or in remembering the telephone number, you don't have to look up. Can we find any physiological evidence to support the psychologists' claim? The reader probably knows that electroconvulsive shock (ECS) can induce amnesia. ECS can totally and permanently obliterate all signs of short-term memory, while producing only temporary effects on long-term memory.[7] Certain drugs also induce convulsions with results very similar to those produced in experiments with ECS. Taken together, the evidence does indicate that short-term memory and long-term memory depend on different physiological mechanisms.
Some investigators employ a very interesting theory in dealing with the two classes of memory.[8] According to this theory, short-term memory is the active, working memory, and it exists in an idealized "compartment." Long-term memory is stored memory, and the storage "depot" differs from the working compartment. According to the theory, the working compartment receives incoming perceptual data, which create short-term memories. The short-term memories, in turn, make the long-term memories. In other words, in the learning process, information from experience moves into the working compartment, becomes short-term memory, and then goes on to the storage depot. But what good would the memory be if it were confined to storage? According to the theory, the working compartment has two-way communication with the storage depot. In this theory, when we use long-term memory, we in effect create a short-term working memory from it. And there's more. Learning doesn't depend simply on what comes into the mind. The remembered past has a profound effect on what we're learning in the present. Cognition--understanding--can't be divorced from the learning process. The working-memory theory maintains that the active memory in the working compartment is a blend of perception from the senses and memory drawn from storage. When we forget, the active memory "goes off" in the working compartment.
But the concept of two classes of memory gives rise to imponderables in the mind-brain connection. If different physiological mechanisms handle short-term and long-term memories, how do we explain their informational identities? After all, Butterfield 8 is Butterfield 8 whether we forget it immediately or remember it to the end of our days. There are other problems. The useful working-memory theory requires a more general theory to link it to reality.
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Nevertheless, there is a great deal of empirical evidence of a piece of the human brain that is involved in short-term memory. This structure is known as the hippocampus. Shaped like a zucchini, but about the size of a little finger, the hippocampus (Greek for sea-horse) is buried deep within the cerebrum's temporal lobe. A person with a damaged hippocampus exhibits defective short-term memory, whereas his or her long-term memory shows sign of being intact. One clinical sign of a lesion in the hippocampus is when a person can't repeat a name or a short sequence of randomly presented numbers but can, for instance, recite the Gettysburg Address. I will say more about the hippocampus in chapter 10. For now, I want to make this point: If we take the holist's classical position, we will have to dismiss important facts about the hippocampus.
Well, then, why can't we consider the hippocampus the seat of short-term memory? I've been asked sophisticated versions of this very question by several persons who work with the brain. There are correspondingly sophisticated reasons why we can't. But let me indicate some simple ones.
First of all, there are entire phyla of organisms whose brains lack hippocampi. Yet these same creatures often have splendid working, short-term memories. I can give another example from my own laboratory. Salamanders whose cerebrums, and therefore hippocampi, have been amputated learn as well as normal animals. Perhaps salamanders and various other forms of life are simply too lowly to count? Later in the book, I will summarize experiments whose results show that cats can learn, and thus exhibit working memory, without their hippocampi. The point, once again, is that structuralism is no more enlightening than holism, in regard to the role of the hippocampus.
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I mentioned earlier that we humans require the visual cortex in order to see. But on a summer forenoon, when I look out my office window, I sometimes observe a hawk, perhaps 600 feet up, gliding in circles above the meadowed and hardwood-forested Indiana University campus, searching the ground for a target less than a foot long. Why doesn't the hawk dive after that discarded Hershey bar wrapper or the tail of that big German shepherd? The hawk is up there in the clouds doing complicated data processing with its visual system. It's certainly seeing. Yet that hawk, unlike a human being, doesn't employ a visual cortex. It doesn't even have one. For the visual cortex in the occipital lobe is a mammalian characteristic.
Birds process their visual sensations in what is called the midbrain tectum. (Barn owls, in addition, handle depth perception in a mound of brain, on the front of the cerebrum, called the Wulst, the German word for pastry roll. Indeed, the Wulst[9] does look like something on a tray in a Viennese bakery.)
Mammals, humans included, also have tectums, which they use in pupillary light reflexes. A human who has suffered complete destruction of both occipital lobes, and loses the entire visual cortex as a consequence, becomes blind, although some evidence indicates that this person may be able to sense very strong light. Firm evidence shows that rats, rabbits, and even monkeys can sense diffuse light following complete destruction of their occipital lobes. Do the tectum and the visual cortex (and the Wulst, too, of course) comprise the seat of vertebrate vision? If a vertebrate lacks some, but not all, of these structures, it may lack certain special features of vision. If the creature lacks a tectum, a visual cortex, and a Wulst, will it have no vision at all?
The argument works, up to a point. Specific lesions in a frog's tectum produce specific deficits in its visual perception. But let me tell you a little anecdote from my own laboratory in the days before my experiments with shufflebrain.
I was doing experiments with larval salamanders. For control purposes, I had to have a group of neurologically blinded animals. That would be a ci
nch, I thought, since the tectum is the seat of vision in animals below mammals (the function of the Wulst hadn't been worked out yet). All I had to do, I thought, was go in and remove the tectum, which I did. Was I in for a surprise when the animals came out of anesthesia! Every single animal could see! I didn't even consider publishing the results, feeling certain that I must have goofed up somewhere. But a few years later, the animal behaviorist, G. E. Savage, reported basically the same thing, except in adult fish.
It's not that the visual cortex and the tectum (or the Wulst) aren't important. And it's not that the vision of a squid is identical to yours and mine. The fact is that we really can't assign all vision to a single anatomical system. We can relate specific features of visual perception to certain structures in particular organisms. But we can't generalize. And if we can't generalize, we can't theorize. Which leaves mind-brain open to the holist--or the magician.
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I can't think of anyone who has contributed more to our knowledge of functional human neuroanatomy than the late Wilder Penfield. Yet the mind-brain question eventually forced him into mysticism. A neurosurgeon who began his career early in this century, Penfield developed and made routine the practice of exploring and mapping a region of the brain before cutting into it. Good doctor that he was, he was preoccupied by the question of whether the treatment would be worse than the disease.
The cerebrum doesn't sense pain directly. Painful sensations from inside the skull travel on branches of peripheral nerves. These nerve fibers actually leave the skull, join their parent trunks, reenter the cranium, and fuse into the brainstem, the lower region of the brain between the cerebrum and spinal cord. Local anesthesia deadens the skull, scalp and coverings of the brain; intracranial surgery can thus be performed on a fully conscious person, which is what Penfield usually did. With electrodes, he stimulated a suspicious area and found out, firsthand, the role it played in his patient's actions and thoughts. In this way, Penfield confirmed many suspected functions and discovered new ones as well.