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p70 [Talking about a surgical procedure to address severe cases of grand mal epilepsy] 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, [of the brain] 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 [severing all neurological connections to stop the grand mal seizures from passing from one hemisphere to the other] this is no longer possible, and extraordinary conflicts can ensue. One of the split-brain patients, for instance, 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 hands knowing what the other was doing, the patient was left bewildered as to why he seemed unable to perform so simple a task... 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. ..
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 [Roger] Sperry’s words:
Each hemisphere . . . has its own . . . private sensations, perceptions, thoughts, and ideas, all of which are cut off from 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.”
p72 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... 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 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... 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.
p73 That language processing is both localized and lateralized withing 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...
Would we know if groups within the human family varied it this respect? If 10 or 20% of the total population was switched right for left, how would we know? Wow! My question is answered in the same paragraph. Before brain surgery they test by anesthetizing just one hemisphere.
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 about 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 hemispheres (assuming that the nerve fibers are equally dense) and, therefore, possibly a more equitable division of labor in many aspects of cognition, including language. [My Richard Feynman side wants to be tested for this. I’m largely ambidextrous. Do the people who “think like me” share my hemispherical division of functionality... whatever it may be?] 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.
p74 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... 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. [Sir John] 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... 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 function usually lateralized to the other half.
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p75 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 a left hemispherrectomy 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 hemispheres 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. [That is strange.] Only when the left cortex is removed altogether can the full language potentialities [why not “potential?” ] 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 convey messages in some oral form: they remain highly proficient at swearing (since the right cortex is largely in charge of strong emotional responses) and can continue to express themselves musically (again, a predominantly right-hemisphere skill)...
Makes you wonder how Tourettes is related to this. Also, being personally deficient when it comes to both “strong emotional responses” and the performance of music, I wonder if my right-hemisphere is doing anything but taking up space up there.
[Back to split-brain patients. One, referred to as P.S. had “pretty good command of language” in both hemispheres. His right side could communicate by spelling 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... the right hemisphere expressed a desire to become a race-car driver while the left hemisphere expressed a desire to pursue a career as a draftsman... 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... 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.
p77 In a similar vein, in his book The Bisected Brain... [Michael Gazzaniga, one of the researchers from the previous quote.] wrote:
Just as conjoined {Siamese} twins are two people sharing a common body, the callosumsectioned 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 commissrotomy 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.
p78 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 of each eye 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 ipsolateral 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 unifying mechanisms, which remain intact after complete commissurotomy, help the divided brain to continue to work as if it were a single unit.
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p79 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. [Except again for swearing and, again, couldn’t Tourettes be a case of the right hemisphere attempting to put in its 2 cents? ] Therefore, asking a split-brain patient 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.
My Multiple Goddesses theory of female personality, which I’ve thought of several times in this chapter, makes all this even more complicated. What if hormones affect the balance of power between the two hemispheres? I don’t want to jump ahead too far, but what if a unified self is an illusion.
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-dominant hemisphere, confused by something it cannot readily explain, tries to cover up its ignorance of what the silent mind is seeing and doing. [Here’s a, random, but interesting thought: before the computer/smartphone age, most people expressed themselves by writing with their dominant hand. Now, people type or text using both hands and hemispheres in a cooperative exercise. Surely using both hemispheres to express yourself must “train” the brain differently than simply using the dominant side. Also, I first was aware of being ambidextrous (or I first became ambidextrous?) when I taught myself to write with my left hand. Most of my typos are the result of my hands being out of sync. That and my mind running ahead of my hands so that I start to toss in letters belonging to words I haven’t gotten to yet in the sentence.] 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, the left hemisphere) grew confused and, not wishing to appear foolish, invented a story about the pencil resembling one he had once owned. [This would seem to be very similar to the “blind-sight” that Sacks wrote about. And it even reminds me of “Jimmie” who couldn’t remember that Kennedy had been assassinated but always chose “Dallas” as the city that a hypothetical assassination would have happened in (I think I have that right). This actually brings up a more interesting problem for this whole discussion of self/brain: in this chapter we have been focused on the self conscious mind which, as Robert Persig suggested in Lila can be thought of as an application running on top of a lower level operating system the application is entirely unaware of. Trying to reverse engineer the brain (or at least to understand it) by quizzing the mind is going to be less than successful. Here’s an even more esoteric analogy: this is like trying to understand a computer that is actually a virtual machine running on another computer. At best, you could learn something about the virtual machine but probably next to nothing about the actual computer creating that simulation.] 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. [I wish he indicated the sex of the nude as this could be the perfect place for a Blade Runner reference, Rachel: “Is this testing whether I'm a replicant or a lesbian Mr. Deckard?”] Immediately, the woman felt acutely embarrassed, [See! it would work as a Voight Kampff test] 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 uncomfortable. So it contrived an answer to conceal its ignorance...
p80 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.
...even in situations where one of the personalities (like Sally) claims to have access to what one of the other 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 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.
I’m not sure I get the one-eyed dog analogy, or I don’t get what Descartes would have meant by it. If you think of the two hemispherical, split-brain personalities as the two one-eyed dogs then, yes, they would each see one bone -- you could probably even contrive it so that they saw different bones but were unaware of that fact.
The MPS version is more interesting if we think of good old “Sally” again. If she is aware of “Christine’s” perception of the bone as well as of her own, she, to some extent sees two bones. Even if the second is, for her, another person’s interpretation of a bone, it is still a different bone and brings with it Christine's associations. Though, to Darling’s point, she would have no doubt as to which was the “real” bone.
Actually I take it back about her seeing two bones since -- if she can’t understand what Christine is thinking when she hears French, how can she see what Christine sees when she sees the bone. And, with that last thought in mind, what does it even mean to say she is aware of Christine’s thoughts if she doesn’t know what she understands when she hears or thinks in French? And yet she is supposed to know her dreams. This is very puzzling. Even more puzzling than it is odd. Which is saying something.
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