| ZEN PHYSICS (what is this?) The Science of Death, the Logic of Reincarnation David Darling 
|
I have a splitting headache.
– Patient's comment following his cerebral commissurotomy
Redundancy and symmetry in the form of duplicated organs are not uncommon in the human body. We each have two lungs, two kidneys, two ovaries or testes. And we each, effectively, have two brains. The fact that these two brains normally work in perfect accord as the cooperating hemispheres of a single brain is made possible by the band of connecting tissue that provides a natural information superhighway between them.
The main bundle of (about two hundred million) nerve fibers linking the brain’s two halves, the corpus callosum (a structure present in all mammals except the marsupials, which are considered the most primitive of the class), is thought to have several functions, one of which is to allow memories and skills garnered during an individual’s lifetime to be laid down simultaneously in both hemispheres. Because of this replication, if part of one hemisphere is damaged through accident or disease (such as a stroke), there is a reasonable chance that the matching portion of the other hemisphere may eventually be able to take over some or all of the lost mental functions for the whole brain. Normally, the corpus callosum supports a heavy two-way flow of data traffic so that each half of the brain is kept almost immediately abreast of what its partner is doing or thinking. But what happens if this communications link is broken?
During the 1940s, the American surgeon William Van Wagenen severed the corpus callosums of about two dozen patients who were suffering from grand mal epilepsy. In some cases, the condition was so severe that the victims would experience several violent seizures an hour, every hour, without respite. Epileptic attacks originate in a specific, small region of one hemisphere that may differ from one patient to another. The attacks quickly spread, invade the opposite hemisphere, and thereafter overwhelm the victim. By decoupling the hemispheres, Van Wagenen hoped to confine the seizures to the hemisphere in which they started and so allow the other hemisphere to continue functioning as normal. To his disappointment, his patients showed little or no improvement.
Twenty years later, two surgeons in California, Joseph Bogen and Philip Vogel, suggested that the earlier operations had failed because Van Wagenen had not cut through all of the commissures (the bundles of nerve connections) between the two halves of the cortex. As well as the corpus callosum there are other, smaller cortical links, including the anterior and hippocampal commissures. To test their idea, Bogen and Vogel carried out complex commissurotomies on sixteen grand mal epilepsy patients in the 1960s and 1970s, and achieved remarkable postoperative success. The epilepsy was cured. But it was for another, very different reason that many psychologists were intrigued by the surgery. What effects, they wondered, had there been on the patients’ minds and behavior? To investigate this question, Roger Sperry of the California Institute of Technology and other researchers subjected several of the “split brain” patients to a battery of ingenious tests.
The split-brain studies made use of the well-known but bizarre (and unexplained) fact that the left side of the body and the left visual field are controlled by the right hemisphere, and the right side of the body and the right visual field by the left hemisphere. (Control of the auditory fields is more complex, while our sense of smell, the most neurologically ancient of the senses, is not crossed over at all, each nostril being “wired” to the hemisphere on the same side of the body.) Under normal circumstances, any thoughts or perceptions first registered in one half of the brain are quickly relayed to the other half through the cerebral commissures. But following complete commissurotomy this is no longer possible, and extraordinary conflicts can ensue. One of the split-brain patients, for example, found himself pulling on his pants with one hand while trying to take them off with the other. On another occasion, he attempted simultaneously to button and unbutton his jacket with opposite hands. With neither hand knowing what the other was doing, the patient was left bewildered as to why he seemed unable to perform so simple a task. In a particularly alarming moment, the same individual grabbed his wife with his left hand and shook her violently before his right hand could intervene. Another patient found each hand choosing different clothes to wear in the morning, and reported that every now and again her right hand would slap her awake when she (or, rather, her right hemisphere!) was in danger of oversleeping. Similar but even more startling behavior had been noted in a female patient in 1908 by Kurt Goldstein. He described how the woman had been taken to the hospital because her left hand would repeatedly close around her neck and start to choke her. On each occasion her right hand would come to the rescue and she would then be compelled to sit on her left hand, which she accused of being bad and beyond her control. While in bed, she would throw the pillows on the floor and tear the bedclothes – but only with her left hand. On the basis of his examination, Goldstein surmised that she had suffered damage to the corpus callosum, and that the two sides of her brain had thus been disconnected from each other – a diagnosis eventually confirmed after the woman’s death by a postmortem.
Given cases like these in which a patient’s hands act in direct opposition to one another, it is hard to avoid the conclusion that the two hemispheres of the brain, when separated, can function independently and without each other’s prior knowledge. In Sperry’s words:
Each hemisphere … has its own … private sensations, perceptions, thoughts, and ideas, all of which are cut off from the corresponding experiences in the opposite hemisphere. Each right and left hemisphere has its own private chain of memories and learning experiences that are inaccessible to recall by the other hemisphere. In many respects each disconnected hemisphere appears to have a separate “mind of its own.”One young woman, when asked whether she could feel her left hand, shouted, “Yes! Wait! No! Yes! No, no! Wait, yes!” Her face twisted as each of her two minds, only one of which could feel the hand, tried to answer. A researcher then handed her a piece of paper with the words “yes” and “no” written on it and asked her to indicate the correct answer. After a moment’s hesitation, the woman’s left forefinger jabbed at “yes” and her right forefinger at “no.” The psychologist Norman Geschwind has gone so far as to say it is misleading even to talk of “the patient” in the case of disconnection of the hemispheres. There are, he believes, really two people living inside the same skull.
Clearly, this idea flies in the face of those who believe, as Descartes did, that every human being has a unique seat of consciousness – a single mind that cannot be divided or fragmented whatever happens to its host brain. But one objection can still be raised from the Cartesian camp on the grounds of hemispherical asymmetry. Among the chief critics of the idea that separating the hemispheres inevitably leads to the creation of two separate minds was the English neurophysiologist Sir John Eccles, one of the few remaining dualists in his profession. Eccles pointed to the well-known fact that, in most people, the two halves of the human brain are not mirror images of each other. At a casual glance they may give this impression, but in fact they reveal subtle differences in structure and quite extensive differences in function. In particular, only one half of the brain normally houses the regions equipped to deal with language at a high level. In most people the left hemisphere is responsible both for speech and for the majority of language comprehension skills, whereas the right hemisphere is generally aphasic (speechless) and has only a childlike ability to understand words.
That language processing is both localized and lateralized within the brain has been known for more than a century. It is well established that damage to specific regions called Broca’s area and Wernicke’s area, which are normally found only in the left hemisphere, impairs a person’s speech and his ability to understand language, respectively. Because of the great importance of these regions, surgeons are especially anxious to avoid disturbing them if at all possible during brain surgery. A simple preoperative procedure known as the Wada test is carried out to determine in which hemisphere of a patient’s brain the main language centers lie. This test exploits the fact that each cerebral hemisphere is supplied independently with blood via a carotid artery running along the same side of the neck as the hemisphere. If sodium amytal or a similar drug is injected into the artery on the same side as the hemisphere controlling speech, the patient temporarily loses the ability to talk and cannot answer any questions for several minutes until the effects of the drug wear off. By contrast, there is generally no loss of speech if the opposite hemisphere is anesthetized.
Studies have shown that around 99 percent of right-handers have their primary speech and language centers in their left hemispheres (the same side of the brain which controls their dominant hand), while, surprisingly, 70 percent of left-handers are also linguistically left-hemisphere dominant. Of the remainder of left-handers, about 15 percent have their main language centers in the right hemisphere and 15 percent show evidence of speech control in both hemispheres.
Interestingly, research by Sandra Witelson, a psychologist at McMaster University in Hamilton, Ontario, has shown that left-handed and ambidextrous people have corpus callosums that are, on average, 11 percent larger than those of right-handers. This suggests a greater potential for communication between the hemisphere (assuming that the nerve fibers are equally dense) and, therefore, possibly a more equitable division of labor in many aspects of cognition, including language. It may be that we all start out potentially ambidextrous and with similar sized corpus callosums at birth but for some reason most of us rapidly lose twenty million or so neurons in the connecting body and develop a specialization in the brain that is associated with right-handedness. But why this should be is not clear.
Significantly, patients who are known to have suffered damage to the left side of their brain early in life are much more likely to show right-hemispherical or bilateral speech control. This backs up the view that, although we may have an innate, presumably genetic tendency to develop primary speech and language centers in just one hemisphere (usually the left), the other hemisphere can take over these functions if necessary. However, this plasticity of the brain falls off sharply with age so that by adolescence the capacity of one hemisphere to assume the specialized functions of the other is severely limited.
The objection of dualists, such as Eccles and his one-time colleague the late Sir Karl Popper, to the idea that commissurotomy creates two distinct minds or streams of consciousness rests on the grounds that one hemisphere – the dominant (and therefore usually the left) hemisphere – is overwhelmingly the better at handling high-level language. And it is the sophisticated use of language, the argument goes, that is the crucial ingredient in making us fully conscious. Eccles portrays the speechless right hemisphere as being primitive, bestial, and essentially unconscious. In his view, each human being has an indivisible self, a Cartesian ego or soul, that is associated exclusively with the left side of the brain. However, this argument does not bear up well under the weight of clinical evidence.
Dualists face a problem, for instance, in explaining the extraordinary postoperative recoveries of patients who have undergone hemispherectomy – the surgical removal of one entire brain hemisphere. This drastic procedure is now performed several dozen times a year in the United States, usually as a treatment for a rare condition known as Rasmussen’s encephalitis, which afflicts its victims with rapidly recurrent life-threatening seizures. Extraordinarily, it is found that if hemispherectomy is carried out in infancy or early childhood, the remaining hemisphere, whichever it happens to be, can take over all of the functions usually lateralized to the other half.
Typical is the case of Matthew Simpson, a young American boy who at about the age of four began to experience violent seizures. Rasmussen’s disease was diagnosed, and when medications failed to relieve his symptoms it was decided to proceed with a left hemispherectomy. With the removal of Matthew’s left hemisphere went his principal language centers. Yet today Matthew is doing well in school and, with the help of weekly language therapy sessions, is rapidly making up for lost ground – and lost cortex.
Apparently, the normally aphasic side of the brain has just as good a latent capacity to mediate speech and to understand language at an advanced level as does the side which, if undisturbed, becomes language-dominant. This fact is demonstrated, too, by patients who undergo hemispherectomy later in life. Those who have had their right hemispheres removed (and were preoperatively left-brain language dominant) display no obvious language problems. But, remarkably, patients who have undergone left hemispherectomy show much less linguistic impairment than do individuals who have simply suffered lesions to their speech and language centers. It seems that not only is language represented to a considerable extent in both hemsipheres but also that the left hemisphere normally exerts an inhibitory influence over the right. So, as far as language goes, we may actually be better off losing one entire side of our cortex rather than suffering localized damage to the language areas. Only when the left cortex is removed altogether can the full language potentialities of the right hemisphere be realized.
Patients who have had their language-dominant hemispheres taken out after early childhood do show some loss in the production of language, as would be expected, but are hardly impaired at all in comprehension. And even in language production, these people can still use the surviving half of their brain to confer messages in some oral form: they remain highly proficient at swearing (since the right cortex is largely in charge of emotional responses) and can continue to express themselves musically (again, a predominantly right-hemisphere skill). A female patient who underwent a left hemispherectomy could communicate much better in song than in speech after her operation. Some loss of oral skills may be inevitable, but those who are close to people who possess only a right hemisphere for a brain need no convincing that these individuals still possess a full, vigorous mind and a personality that can express itself in many different modes.
Of the split-brain patients studied, one, referred to as P.S., was unique in that both of his detached brain hemispheres had a pretty good command of language. He was sixteen years old when he had his commissurotomy, and was believed to have suffered damage to the left side of his brain when very young (causing the right side to compensate by becoming more language proficient than usual). For some time after his operation he could, like all of the other split-brain patients, communicate verbally only via his left hemisphere. But after two to three years, he acquired a useful language ability in the right side of his brain. Although his right hemisphere did not have access to the speech centers, P.S. could use his left hand to spell out words with Scrabble letters. This allowed researchers to establish a reasonable level of dialogue separately with each hemisphere – a breakthrough that was to lead to a striking confirmation of Sperry’s dual mind hypothesis. The separate “conversations” revealed that each hemisphere did indeed appear to house an independent stream of consciousness, to the extent that the two halves of P.S.’s brain would often respond in completely different ways to the same question. Asked on one occasion about future ambitions, for instance, the right hemisphere expressed a desire to become a race-car driver while the left hemisphere said it hoped to pursue a career as a draftsman. On a scale of one to five ranging from “like very much” to “dislike very much,” the right hemisphere frequently responded with a higher figure, though inter-hemispherical differences varied from day to day. Generally, it was found that on days when the two hemispheres gave more similar answers, P.S. was happy and relaxed, while at other times when the responses were in sharp disagreement, P.S. seemed more emotionally out of sorts.
Two of the researchers closely involved with studies on P.S., Joseph LeDoux and Michael Gazziniga, concluded:
Each hemisphere in P.S. has a sense of self and each possesses its own system for subjectively evaluating current events, planning for further events, setting response priorities, and generating personal responses. Consequently, it becomes useful to consider the practical and theoretical implications of the fact that double consciousness mechanisms can exist.In a similar vein, in his book The Bisected Brain, Gazzaniga wrote:
Just as conjoined [Siamese] twins are two people sharing a common body, the callosum-sectioned human has two separate conscious spheres sharing a common brain stem, head and body … A slice of the surgeon’s knife through the midline commissures produces two separate, but equal, cognitive systems each with its own abilities to learn, emote, think, and act.Only one observation, on the face of it, seems to pose a threat to the idea that commissurotomy results in a definite bifurcation of consciousness, the creation of two minds where previously there had been only one. Although on rare occasions, split-brain patients do, for instance, find their left and right hands working in opposition, most of the time their behavior is bafflingly normal. You could spend all day with a commissurotomy patient and never suspect the drastic change that had been wrought in his or her brain. In fact, as experience has shown, only sophisticated psychological testing can consistently reveal that anything unusual is going on. How can this be? How can someone have two hundred million neurons sliced clean through and continue to function more or less as if nothing had happened?
It is true that split-brain patients no longer have any direct connections between the two halves of their cortex. But the absence of cerebral commissures can apparently be circumvented to a surprising degree by other linking mechanisms that are not affected by the operation. For instance, there are commissures in the lower parts of the brain. One of these, the superior colliculus, is involved in locating objects and tracking their movements. So, even after the higher commissures have been cut, the two hemispheres can continue to tap into the same primitive “radar” system giving the whereabouts of things in the outside world (albeit that they lack the means to reach a consensus on what these things actually are). The brain stem, too, which is believed to play a vital role in generating basic emotional responses, continues to feed identical messages to the severed hemispheres.
Other ways of harmonizing the two disconnected brain halves are furnished by the senses. Each eye, for instance, projects to both hemispheres: the contents of the left visual field of each eye to the right hemisphere and the contents of the right visual field to the left hemisphere. As a result, eye movements initiated by one hemisphere to bring an object into direct view serve also to bring that object to the attention of the other hemisphere. A similar but more complex crossover of signals takes place with respect to hearing. And although the great majority of tactile signals are carried by contralateral nerve fibers (that is, fibers crossing over from the opposite side of the body) there are also a few ipsilateral fibers running from the left side of the body to the left hemisphere and from the right side of the body to the right hemisphere. All these subsidiary unifying mechanisms, which remain intact after complete commissurotomy, help the divided brain to continue to work as if it were a single unit.
That the brain should be so adept at keeping up the appearance of unity, even after it virtually has been sawn in two, is not really surprising. The brain has been exquisitely shaped and honed by evolutionary forces over many millions of years so that it is now perfectly tailored to protect and serve the interests of the organism in which it resides. It is the consummate survival machine. And one of the chief reasons for its phenomenal success is its ability to function so coherently. Despite the awesome complexity of activity among its two hundred billion neurons – a cell for every star in the galaxy – at the level of the whole organ it manages to act as a single, purposeful entity. In most situations, it presents to its owner and to the world at large a unified strategy. Why should we suppose, then, that it would start to behave differently, giving out conflicting messages, speaking discordantly with more than one voice, just because it had suffered some damage? Surely, the likelihood must be that whatever happens to the brain, whatever calamities it suffers, it will still make every attempt to stay true to its prime evolutionary directive. Like any effective committee, political party, or federation of states, it will continue to try to present a unified public face, to at least behave as if it harbored a single mind.
Apart from occasional instances when their left and right hands seem bent on different courses of action, split-brain patients don’t report being aware of an inner conflict. They don’t describe experiencing the presence of two different minds. But again this is hardly surprising. When a split-brain patient speaks, it is only his or her left hemisphere that is producing the speech and expressing itself. The right hemisphere, being mute, is compelled simply to listen. Even if it wanted to put forward an opinion of its own (and the right hemisphere seems much less strong-willed and focused than the left) it would be unable to. Therefore, asking a split-brain if he or she feels like two separate people living inside the same body resolves nothing. Even in the case of P.S., who has two “talking” hemispheres, one hemisphere is clearly more articulate than the other and, under normal circumstances, acts as the spokesperson for the whole brain.
Most of the time, then a split-brain patient appears to have just one mind. Moreover, when the presence of the two minds is overtly revealed, the language-dominated hemisphere, confused by something it cannot readily explain, tries to cover up its ignorance of what the silent mind is seeing and doing. It resorts either to educated guesswork (based on cross-cueing) or confabulation. For instance, in one of Sperry’s experiments a split-brain patient was shown a pencil in such a way that the image went only to his right hemisphere. When asked what he had seen, the patient (via his left hemisphere) replied “Nothing.” However, when offered a tray of objects and asked to pick up with his left hand (connected to the right hemisphere) the object previously shown, the patient immediately reached for the pencil. Upon being asked why he had picked up the pencil, the patient (that is, his left hemisphere) grew confused and, not wishing to appear foolish, invented a story about the pencil resembling one he had once owned. On another occasion, Sperry showed a series of innocuous pictures in the left visual field of a female split-brain patient, and then slipped in a photo of a nude. Immediately, the woman felt acutely embarrassed, but couldn’t explain why. Her talkative left hemisphere, not having seen the photo, had no idea why its owner was suddenly blushing and acting uncomfortably. So it contrived an answer to conceal its ignorance. When asked what the problem was, the woman complained that Sperry’s machine was “very peculiar.”
Why do we each feel single in spite of changing sensory impressions? Why don’t we ever feel literally in two or three or more minds? The answer is simple: there are no conceivable circumstances under which a person could feel double – for who would there be to feel the doubling? Logic and intuition dictate that you can be only one person at a time. And this is a conclusion fully vindicated by the experiences of both MPS victims and split-brain patients.
In the case of MPS, though several personalities appear to inhabit the same brain, typically only one is active at any given time – as if a single searchlight of consciousness illuminated each character in turn. And even in situations where one of the personalities (like Sally) claims to have access to what one of the personalities is thinking, the “supervisory” personality does not feel double. The situation is best thought of as a group of people sharing the same brain, with a few of the individuals being in a position to snoop on some of their cohabitants as if through a one-way mirror.
The case of split-brain patients is different (and less controversial) in that there are clearly two continuous, concurrent streams of consciousness in the same brain. Even so, there is no single entity that experiences dual awareness.
Consciousness, by its nature, is inevitably singular, unitary. Descartes gave the analogy of two one-eyed dogs fighting over a bone. The dogs would behave as if they saw one bone, not two! You cannot be two people at once. And no matter how hard you try to imagine your own stream of consciousness dividing, you always imagine the period after the split from the vantage point of one of the resulting streams.
In discussing such issues we quickly run into linguistic problems. We assume we know what words such as “you” and “person” mean. But it turns out that we normally use these terms in the Cartesian sense of being definite, indivisible objects. Our language therefore incorporates a philosophy of the self that fails to correspond with the reality science has now disclosed. What was one person can become two. What was “you” can become more than one you. Such bifurcations in the river of consciousness have happened. But the important fact is that when a “you” divides, or changes, or ends, or begins, it is not a problem for anyone concerned. From the point of view of the person you are now, change of any kind – especially death – seems to be a problem. But the only problem in fact is the advance fear of change itself. Once you actually stop being who you are now, the fear disappears, and you are free to continue life as someone else – a new you.