Part 8 (2/2)

Well then, if the cingulate cortex is involved with extended consciousness, is it well connected too? During the performance of conscious tasks, connections from the cingulate cortex to brain areas supporting the five neural networks for memory, perception, motor action, evaluation, and attention activate. Something else is happening, too. While engaging in a wide a.s.sortment of conscious tasks that require different types of brain activity, another area of the brain also is always activated, along with the anterior cingulate cortex (ACC). That was the dorsal lateral prefrontal cortex (dlPFC). And it is no coincidence that these two areas have reciprocal connections-more loops. Moreover, in the ACC there is a particular type of long-distance spindle cell that is present only in the great apes.9 And, as you may have guessed, the dlPFC is also a hotbed of connections to the same five neural networks mentioned above.* Way back in chapter 1, we discussed the different layers of the cortex. These long-distance neurons originate mostly from the pyramidal cells of layers II and III. These layers are actually thicker in the dlPFC and inferior parietal cortex.

Extended Consciousness and Modularity.

We are now getting to areas in the brain that are more specialized. If they become damaged, the result is the loss of a specific ability, not con sciousness itself. Throughout this book, there has been much talk about modules in the brain and how each has its specific contribution. The idea of a module of neurons dedicated for such specific duties such as reciprocity or cheater detection is fascinating, and the modularity of the brain becomes even more apparent when lesions in the same specific part of different brains cause the same specific deficit, such as the inability to recognize familiar faces. The odd thing is, we don't feel that fractionated. That is one of the reasons why we find these modules so fascinating (and why the very idea of a modular brain can be difficult to believe). ”My brain is doing that? Crazy!” No, you didn't have any idea, because these modules are all working automatically, under cover, below the level of consciousness. For instance, if certain stimuli trick your visual system into constructing an illusion, consciously knowing that you have been tricked does not make the illusion disappear. That part of the visual system is not accessible to conscious control. We need to remember that all that nonconscious stuff is also contributing to and shaping what comes to the conscious surface. Another thing to keep in mind is that some stuff just cannot be processed nonconsciously. Unfortunately, your high school trig exam may have been an early reminder of this.

If consciousness requires the input of several modules, then the other problem we have to remember is connectivity. We learned in the first chapter that there are only a limited number of connections per neuron, and the more modules there are, the less they are interconnected. Even keeping this in mind, the sheer number of neurons and their connections, ah, well, boggles the mind. The human brain has approximately one hundred billion neurons, and each, on average, connects to about one thousand other neurons. A quick little conscious multiplication reveals that there are one hundred trillion synaptical connections. So how is all this input getting spliced and integrated into a coherent package? To put it anthropomorphically, how does one module know what all the other ones are doing? Or does it? How do we get order out of this chaos of connections? Even though it may not always seem so, our consciousness is rather kicked back and relaxed when you think about all the input with which the brain is being bombarded and all the processing that is going on. In fact, it is as if our consciousness is out on the golf course like the CEO of a big company while all the underlings are working. It occasionally listens to some chatter, makes a decision, and then is out sunning itself. Ah...is that why they call some types of brain processing executive functions?*

Beyond Modularity.

The modular crowd recognizes that not all mental activities can be explained by modules. Sometimes you have to step out of that cubicle and communicate with other cubicles. At some point along the processing route, the input from the modules needs to be synthesized, spliced together, and packaged-or ignored, suppressed, and inhibited. Here is the big mystery. How does it happen? Some controlled processing is going on, and there must be a mechanism that supports flexible links among these processing modules. Many theoretical models of this mechanism have been proposed, including the central executive,10 the supervisory attention system,11 the anterior attention system,12, 13 the global works.p.a.ce,14 and the dynamic core.15 What processes need to be brought together? There are certain components to human consciousness, which we can figure out simply by thinking about what general mental tools we are using. By doing this we are accessing our consciousness and are able to identify what we are conscious of. Let's just pretend you are still conscious while reading this paragraph, and I haven't flipped your arousal switch off. Or maybe your mind has begun to wander, wondering where you should go on vacation next summer or what color to paint the kitchen. Your conscious thoughts require some form of attention, either to these words or visions of the Cote d'Azur. You may be using short-term memory (working memory) to keep track of what you have read, or long-term memory to call into mind past vacations or the color of your friend's kitchen. You also are using your visual perceptions and language ability while reading this, and most likely while you are formulating your presentation of sun-drenched afternoons sipping pastis. You may be silently talking to yourself (known as inner speech), listing the reasons why this vacation is a good idea. Not only is all that contributing to your consciousness, but so are your emotions and desires. Once all these mechanisms are running, you end up being able to reason about what I have written and fit it in with what you already know, or to figure out how to talk your spouse into renting that villa. The good thing is, you are not thinking about your income taxes or picking up your dry cleaning...uh-oh, now you are. That is an example of top-down attention.

There are two phenomena we have to explain. One is that we feel like smoothly running, coherently thinking beings who are usually in control of our thoughts. We usually don't feel like police dispatchers with reports coming in from hundreds or thousands of different sources, deciding what is important or useful or not, or like triage nurses lining up incoming information in order of its importance, but somehow this is happening in our brains. Look around the room you are in and then close your eyes. Was it dusty? How many pencils and pens were on the table or desk? Were there any birds or flowers out the window? How about any dust on the screen? How many other books were in the room? Who wrote them? All this information is going in through your eyes, being perceived and processed and sorted unconsciously, but it is not all making it up to the level of consciousness (luckily) until you direct your attention to it. We also have to explain how we come out with a feeling of ourselves, with our own autobiography; and why, although our consciousness changes from minute to minute, our conscious sense of self does not. Somehow, information is being integrated into a nice package.

THE GATEKEEPER TO CONSCIOUSNESS: ATTENTION.

Only certain information makes it through to consciousness. It is a dog-eat-dog world in our brains. Experiments have shown that in order for a stimulus to reach consciousness, it needs a minimal amount of time to be present, and it needs to have a certain degree of clarity. However, this is not quite enough. The stimulus has to have an interaction with the attentional state of the observer. This can occur in two ways, which are referred to as either top-down or bottom-up processing. Just exactly what is going on here is not known, but Stan Dehaene, Jean-Paul Changeux, a neuroscientist at the Pasteur Inst.i.tute in Paris, and various collaborators suggest that the top-down mode, when you consciously direct your attention, may be a result of activity in the thalamocortical neurons, those loops that I mentioned earlier. In the bottom-up mode, they suggest, the sensory signals coming from nonconscious activity have so much strength that they can reorient top-down amplification to themselves.16 This is when your attention may be captured without conscious control. For example, you may be concentrating on a project at work, when all of a sudden you realize you are hearing the fire alarm.

You should note here an important point: Attention and consciousness are two separate animals. First off, cortical processors control the orientation of attention. Although there may be top-down voluntary control, there may also be bottom-up nonconscious signals of such strength that they can co-opt attention. We experience this all the time. You may be consciously thinking about the project that you are working on, when off go your thoughts to somewhere else, seemingly beyond your control. Second, although attention may be present, it may not be enough for a stimulus to make it to consciousness.17 You are reading that article about string theory, your eyes are focused, you are mouthing the words to yourself, and none of it is making it to your conscious brain, and maybe it never will.

SELECTIVE DISRUPTIONS OF CONSCIOUSNESS.

Brain lesions in the parietal lobe that affect attention can also affect consciousness. This is shown in a dramatic way in people who have lesions, usually caused by a stroke in the right parietal lobe, that cause disruptions of attention and spatial awareness. These people often behave as if the left side of their world, including the left side of their body, does not exist. If you were to visit such a person, and entered the room on the left, he would not realize you were there. If you served him dinner, he would eat from only the right side of the plate! He would have shaved only the right side of his face, (or if a woman, would have put makeup on only the right side), would read to you only the right page of a book or newspaper, and would draw only the right side of a clock, or half of a bicycle. But what is truly odd, they don't think there is anything wrong! They are not conscious of their problem.

This syndrome is known as hemineglect. It includes a lack of awareness for sensory events located toward the side opposite the side where the lesion is (e.g., toward the left following a right-hemisphere lesion), as well as a loss of other actions that would normally be directed toward that side.18 Some patients may neglect half their body, attempting to climb out of bed without moving their left arm or leg, even though they have no motor weakness on that side. Neglect can also be present in memory and imagination. One patient, when asked to describe the view from one end of a piazza from memory, described only the right half, but when asked to describe it from the other end looking back, described the other half with no reference to what had just been described from the other direction.19 This phenomenon indicates that our autobiographical self is derived from our conscious musings. If we are not conscious of it, it doesn't exist.

Many patients with hemineglect do not realize that they are missing any information. This is known as anosognosia. If their lesion has also caused paralysis, they remain unaware of it. They will tell you the limp arm next to them belongs to someone else. They can be aware that they have been diagnosed with a deficit, but may refuse to believe it. One patient stated, ”I knew the word 'neglect' was a sort of medical term for whatever was wrong, but the word bothered me because you only neglect something that is actually there, don't you? If it's not there, how can you neglect it? It doesn't seem right to me that the word 'neglect' should be used to describe it. I think concentrating is a better word than neglect. It's definitely concentration. If I am walking anywhere and there's something in my way, if I'm concentrating on what I'm doing, I will see it and avoid it. The slightest distraction and I won't see it.”20 As this patient hints, the odd thing about hemineglect is that although it can occur when there is actual loss of sensation or motor systems, it can also occur when all the sensory modalities and musculoskeletal systems are working. Neglect seems to be a loss of conscious awareness of these stimuli. Indeed, if you present a visual stimulus to both the right and left side at once, patients with left hemineglect report seeing only the right stimulus, and appear unconscious of the left stimulus. However, if you present the same left visual stimulus in isolation, so that it hits the same exact place on the retina, with no right visual stimulus at all, the left stimulus would be perceived normally. If there is no compet.i.tion from the normal side, then the neglected side will be noticed.

We were the first to study this phenomenon in a controlled study, over twenty-five years ago. Bruce Volpe, Joseph LeDoux, and I asked the question, ”Can information in the neglected field be used at a nonconscious level?” We presented pictures or words, one to each visual field. The only thing the patient suffering from hemineglect had to do was say if the two words or pictures were the same or different. Now remember, because they had neglect, when some sort of stimulus was presented to each visual field, they always verbally stated they conciously saw only the one stimulus, the one that was presented to their left (language) hemisphere. Nonetheless, when they were asked to judge if the words or pictures were the same or different, they responded very well. In short, somehow, somewhere in the brain, the information was combined, and a correct decision was possible, even though the patient was unable to say what the different stimulus was that had been presented to the right hemisphere. Needless to say, if they had guessed ”same,” they would have concluded in a post-hoc sort of way that the stimuli had been the same.

This experiment started a small cottage industry of experiments exploring what kinds of processes could go on subconsciously. For example, word-priming studies have also shown that even when a word is presented to the neglected field and the patient denies its presence, the information is still being processed unconsciously and would be used for word identification.21 So even if the information is there at the nonconscious level, in order for it to make it to consciousness, and for the person to become aware it is there, attention has to be directed to it. Furthermore, neglect is most apparent in compet.i.tive situations, in which information on or closest to the ”good” side comes to dominate information on the ”bad” side.18 Another odd thing is that when a patient is asked about the presence of the limp arm, instead of saying that he doesn't feel it, he goes so far as to say that it belongs to someone else. What's up with that? If asked to do something that requires the use of both hands, instead of replying that he is unable to, he will reply simply that he doesn't want to. And why don't these patients complain about the problem? If you couldn't see the left half of the room, wouldn't you complain?

This is where split-brain patients are going to help out with explaining this phenomenon and also shed some light on consciousness. The largest tract of neurons in the brain is called the corpus callosum, (CC), and it connects the two hemispheres, along with a smaller tract of neurons in the front part of the brain called the anterior commissure. The corpus callosum contains about two hundred million neurons that originate in which cortical layers? You guessed it: II and III,22 the layers where most of the long-distance neurons originate. The corpus callosum has not been the focus of much attention in the past, but in light of the growing significance of the modularity and lateral specializations of the brain, this connectivity can be seen in an evolutionary light, as we touched on in chapter 1.

SPLITTING THE BRAIN.

The surgical procedure to cut the corpus callosum is a last-ditch treatment effort for patients with severe intractable epilepsy for whom no other treatments have worked. Very few patients have had this surgery, and it is done even more rarely now because of improved medications and other modes of treatment. In fact, there have been only ten split-brain patients that have been well tested. William Van Wagenen, a Rochester, New York, neurosurgeon, performed the procedure for the first time in 1940, following the observation that one of his patients with severe seizures got relief after developing a tumor in his corpus callosum.23 Epileptic seizures are caused by abnormal electrical discharges that in some people spread from one hemisphere to the other. It was thought that if the connection between the two sides of the brain was cut, then the electrical impulses causing the seizures wouldn't spread from one side of the brain to the other. The great fear was what the side effects of the surgery might be. Would it create a split personality, with two brains in one head? In fact, the treatment was a great success. Most patients' seizure activity decreased 60 to 70 percent, and they felt just fine: no split personality, no split consciousness.24, 25 Most seemed completely unaware of any changes in their mental processes. This was great, but puzzling nonetheless. Why don't split-brain patients have dual consciousness? Why aren't the two halves of the brain conflicting over which half is in charge? Is one half in charge? Is consciousness and the sense of self actually located in one half of the brain?

Split-brain patients will do subtle things to compensate for their loss of brain connectivity. They may move their heads to feed visual information to both hemispheres, or talk out loud for the same purpose, or make symbolic hand movements. Only under experimental conditions, when we eliminate cross-cuing, does the disconnection between the two hemispheres become apparent. We are then able to demonstrate the different abilities of the two hemispheres.

Before we see what is separated after this surgery, we need to understand what continues to be shared. There are subcortical pathways that remain intact. Both hemispheres of the split-brain patient are still connected to a common brain stem, so both sides receive much of the same sensory and proprioceptive information automatically coding the body's position in s.p.a.ce. Both hemispheres can initiate eye movements, and the brain stem supports similar arousal levels, so both sides sleep and wake up at the same time.26 There also appears to be only one integrated spatial attention system, which continues to be unifocal after the brain has been split. Attention cannot be distributed to two spatially disparate locations.27 The left brain does not pay attention to the blackboard while the right brain is checking out the hot dude in the next row. Emotional stimuli presented to one hemisphere will still affect the judgment of the other hemisphere.

You may have been taught in anatomy lectures that the right hemisphere of the brain controls the left half of the body and left hemisphere controls the right half of the body. Of course, things are not quite that simple. For instance both hemispheres can guide the facial and proximal muscles, such as the upper arms and legs, but the separate hemispheres have control over the distal muscles (those farthest from the center of the body), so that, for example, the left hemisphere controls the right hand.28 While both hemispheres can generate spontaneous facial expressions, only the dominant left hemisphere can generate voluntary facial expressions.*29 Because half the optic nerve crosses from one side of the brain to the other at the optic chiasm, the parts of both eyes that attend to the right visual field are processed in the left hemisphere, and vice versa. This information does not cross over from one disconnected hemisphere to the other. If the left visual field sees something in isolation from the right, only the right side of the brain has access to that visual information. This is why these patients will move their heads to input visual information to both hemispheres.

It has also been known since the first studies of Paul Broca that our language areas are usually located in the left hemisphere (with the exception of a few left-handed people). A split-brain patient's left hemisphere and language center have no access to the information that is being fed to the right brain. Bearing these things in mind, we have designed ways of testing split-brain patients to better understand what is going on in the separate hemispheres. We have verified that the left hemisphere is specialized for language, speech, and intelligent behavior, while the right is specialized for such tasks as recognizing upright faces, focusing attention, and making perceptual distinctions.

Where attention is concerned, the hemispheres interact quite differently in their control of reflexive versus voluntary attention processes.30, 31, 32 There is a limited amount of overall available attention.33 The evidence suggests that reflexive (bottom-up) attention orienting happens independently in the two hemispheres, while voluntary attention orienting involves hemispheric compet.i.tion, with control preferentially lateralized to the left hemisphere. The right hemisphere, however, attends to the entire visual field, whereas the left hemisphere attends only to the right field.34, 35, 36 This can explain part of the problem of our hemineglect patients. When the right inferior parietal lobe is damaged, the left parietal lobe remains intact. However, the left parietal lobe directs its visual attention only to the right side of the body. There is no brain area paying attention to what is going on in the left visual field. The question that remains is, why doesn't this bother the patient? I'm getting there.

Breaking Up Is Not So Hard to Do.

The left hemisphere is specialized for intelligent behavior. Don't leave home without it!

After the human cerebral hemispheres have been disconnected, the verbal IQ of a patient remains intact,37, 38 and so does his problem-solving capacity. There may be some deficits in free-recall capacity and in other performance measures, but isolating essentially half of the cortex from the dominant left hemisphere causes no major change in cognitive functions. The left remains unchanged from its preoperative capacity, and the largely disconnected, same-size right hemisphere is seriously impoverished in cognitive tasks. Although the right hemisphere remains superior to the isolated left hemisphere for some perceptual* and attentional skills, and perhaps also emotions, it is poor at problem solving and many other mental activities. A brain system (the right hemisphere) with roughly the same number of neurons as one that easily cogitates (the left hemisphere) is incapable of higher-order cognition-convincing evidence that cortical cell number by itself cannot fully explain human intelligence.39 The difference between the two hemispheres in problem solving is captured in a probability-guessing experiment. We have subjects try to guess which of two events will happen next: Will it be a red light or a green light? Each event has a different probability of occurrence (e.g., a red light appears 75 percent of the time and a green 25 percent of the time), but the order of occurrence of the events is entirely random. There are two possible strategies one can use: frequency matching or maximizing. Frequency matching would involve guessing red 75 percent of the time and guessing green 25 percent of the time. The problem with that strategy is that since the order of occurrence is entirely random, it can result in a great deal of error, often being correct only 50 percent of the time, although it could result in being correct 100 percent of the time also, but it is fully dependent upon luck. The second strategy, maximizing, involves simply guessing red every time. That ensures an accuracy rate of 75 percent, since red appears 75 percent of the time. Animals such as rats and goldfish maximize. In Vegas, the house maximizes. Humans, on the other hand, match. The result is that nonhuman animals perform better than humans in this task.

The human's use of this suboptimal strategy has been attributed to a propensity to try to find patterns in sequences of events, even when told the sequences are random. George Wolford, Michael Miller, and I tested the two hemispheres of split-brain patients to see if the different sides used the same or different strategies.40 We found that the left hemisphere used the frequency-matching strategy, whereas the right hemisphere maximized! Our interpretation was that the right hemisphere's accuracy was higher than the left's because the right hemisphere approaches the task in the simplest possible manner with no attempt to form complicated hypotheses about the task.

However, more recent tests have yielded even more interesting findings. They have shown that the right hemisphere uses frequency matching when presented with stimuli for which it is specialized, such as facial recognition, and the left hemisphere, which is not a specialist in this task, responds randomly.41 This suggests that one hemisphere cedes control of a task to the other if the other hemisphere specializes in that task.42 The left hemisphere, on the other hand, engages in the human tendency to find order in chaos. The left hemisphere persists in forming hypotheses about the sequence of events even in the face of evidence that no pattern exists-in playing slot machines, for instance. Why would the left hemisphere do this, even when it can be nonadaptive?

The Left Hemisphere Is a Know-It-All.

Several years ago, we observed something about the left hemisphere that was very interesting: how it deals with behaviors we had elicited from the disconnected right hemisphere about which it had no information. We showed a split-brain patient two pictures: a chicken claw was shown to his right visual field, so the left hemisphere saw only that, and a snow scene was shown to the left visual field, so the right hemisphere saw only that. He was then asked to choose from an array of pictures placed in full view in front of him. From the array of pictures, the shovel was chosen with the left hand and the chicken with the right. When asked why he chose these items, his left-hemisphere speech center replied, ”Oh, that's simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” Here the left brain, observing the left hand's response without knowing why it has picked that item, had to explain it. It will not say, ”I don't know.” Instead it interprets that response in a context consistent with what it knows, and all it knows is: chicken claw. It knows nothing about the snow scene, but it has to explain pointing to the shovel with the left hand. It has to find reasons for the behavior. We called this left-hemisphere process the interpreter.

We also tried the same type of test with mood s.h.i.+fts. We showed a command to the right hemisphere to laugh. The patient began to laugh. Then we asked the patient why she was laughing. The speech center in the left hemisphere had no knowledge of why its person was laughing, but out would come an answer anyway: ”You guys are so funny!” When we triggered a negative mood in the right hemisphere by a visual stimulus, the patient denied seeing anything but suddenly said that she was upset and that it was the experimenter that was upsetting her. She felt the emotional response to the stimulus, all the autonomic results, but had no idea what caused them. Ah, lack of knowledge is of no importance, the left brain will find a solution! Order must be made. The first makes-sense explanation will do-the experimenter did it! The left-brain interpreter makes sense out of all the other processes. It takes all the input that is coming in and puts it together in a story that makes sense, even though it may be completely wrong.

THE RELATIONs.h.i.+P BETWEEN THE INTERPRETER AND CONSCIOUS EXPERIENCE.

So here we are, back to the main question of the chapter: How come we feel unified when we are made up of a gazillion modules? Decades of split-brain research have revealed the specialized functions of the two hemispheres, as well as providing insights into specialization within each hemisphere. Our big human brains have countless capacities. If we are merely a collection of specialized modules, how does that powerful, almost self-evident feeling of unity come about? The answer may lie in the left-hemisphere interpreter and its drive to seek explanations for why events occur.

In 1962, Stanley Schachter and Jerry Singer at Columbia University injected epinephrine into subjects partic.i.p.ating in a research experiment.43 Epinephrine activates the sympathetic nervous system, and the result is an increased heart rate, hand tremors, and facial flus.h.i.+ng. The subjects were then put into contact with a confederate who behaved in either a euphoric or an angry manner. The subjects who were informed about the effects of the epinephrine attributed symptoms such as a racing heart to the drug. The subjects who were not informed, however, attributed their autonomic arousal to the environment. Those who were with the euphoric confederate reported being elated and those with the angry confederate reported being angry. This finding ill.u.s.trates the human tendency to generate explanations for events. When aroused, we are driven to explain why. If there is an obvious explanation, we accept it, as did the group informed about the effects of epinephrine. When there is not an obvious explanation, we generate one. The subjects recognized that they were aroused and immediately a.s.signed some cause to it. We talked about this in the last chapter when we discussed looking over the edge of the Grand Canyon. This is a powerful mechanism; once seen, it makes one wonder how often we are victims of spurious emotional-cognitive correlations. (I am feeling good! I must really like this guy! As he is thinking, Ah, the chocolate is working!) Split-brain research has shown us that this tendency to generate explanations and hypotheses-to interpret-lies within the left hemisphere.

Although the left hemisphere seems driven to interpret events, the right hemisphere shows no such tendency. A reconsideration of hemispheric memory differences suggests why this dichotomy might be adaptive. When asked to decide whether a series of items appeared in a study set or not, the right hemisphere is able to identify correctly items that have been seen previously and to reject new items. ”Yes, there was the plastic fork, the pencil, the can opener, and the orange.” The left hemisphere, however, tends to falsely recognize new items when they are similar to previously presented items, presumably because they fit into the schema it has constructed.44, 45 ”Yes, the fork [but it is a silver one and not plastic], the pencil [although this one is mechanical and the other was not], the can opener, and the orange.” This finding is consistent with the hypothesis that the left-hemisphere interpreter constructs theories to a.s.similate perceived information into a comprehensible whole. By going beyond simply observing events to asking why they happened, a brain can cope with such events more effectively if they happen again. In doing so, however, the process of elaborating (story making) has a deleterious effect on the accuracy of perceptual recognition, as it does with verbal and visual material. Accuracy remains high in the right hemisphere, however, because it does not engage in these interpretive processes. The advantage of having such a dual system is obvious. The right hemisphere maintains an accurate record of events, leaving the left hemisphere free to elaborate and make inferences about the material presented. In an intact brain, the two systems complement each other, allowing elaborative processing without sacrificing veracity.

The probability-guessing paradigm also demonstrates why having an interpreter in one hemisphere and not the other would be adaptive. The two hemispheres approach problem-solving situations in two different ways. The right hemisphere bases its judgments on simple frequency information, whereas the left relies on the formation of elaborate hypotheses. Sometimes it is just a random coincidence. In the case of random events, the right hemisphere's strategy is clearly advantageous, and the left hemisphere's tendency to create nonsensical theories about random sequences is detrimental to performance. This is what happens when you build a theory on a single anecdotal situation. ”I vomited all night. It must have been the food was bad at that new restaurant where I ate dinner.” This would be a good hypothesis if everyone who ate what you ate became ill, but not just one person. It may have been the flu, or your lunch. In many situations, however, there is an underlying pattern, and in these situations the left hemisphere's drive to create order from apparent chaos would be the best strategy. Coincidences do happen, but sometimes there really is a conspiracy. In an intact brain, both of these cognitive styles are available and can be implemented, depending on the situation.

The difference in the way the two hemispheres approach the world can be seen as adaptive. It might also provide some clues about the nature of human consciousness. In the media, split-brain patients have been described as having two brains. The patients themselves, however, claim that they do not feel any different after the surgery than they did before. They do not have any sense of the dual consciousness implied by the notion of having two brains. How is it that two isolated hemispheres give rise to a single consciousness? The left-hemisphere interpreter may be the answer. The interpreter is driven to generate explanations and hypotheses regardless of circ.u.mstances. The left hemisphere of split-brain patients does not hesitate to offer explanations for behaviors that are generated by the right hemisphere. In neurologically intact individuals, the interpreter does not hesitate to generate spurious explanations for sympathetic nervous system arousal. In these ways, the left-hemisphere interpreter may generate a feeling in all of us that we are integrated and unified.

In his masterpiece, The Alexandria Quartet, Lawrence Durrell tells a story in four books, Justine, Balthazar, Mountolive, and Clea. Each of the first three books tells the story of a group of people living in Alexandria, Egypt, just before World War II, from the viewpoint of a different character. If you were to read only the first book, Justine, you would have a distorted idea of all that was going on. The second book, Balthazar, gives you more information, and the third even more. In all three, however, the reader is at the mercy of the narrators. Your interpretation of the story is dependent upon what they tell you: Your interpretation is dependent upon the supplied information. This is true for the interpretive system in the brain, also. The conclusions of an interpretive system are only as good as the information it receives.

Now, finally, we can consider our patients with hemineglect. First, let's start with an easy case. If a person has a lesion in the optic nerve that carries information about vision to the visual cortex, the damaged nerve ceases to carry that information; the patient complains that he is blind in the relevant part of his visual field. For example, such a patient might have a huge blind spot to the left of center in his visual field. No wonder he complains. However, if another patient has a lesion not in the optic tract but in the visual cortex (the area where the visual information is processed after it is received), and it creates a blind spot of the same size in the same place, he usually does not complain at all. The reason is, the cortical lesion is in the place in his brain that represents an exact part of the visual world, the place that ordinarily asks, ”What is going on to the left of visual center?” With a lesion on the optic nerve, this brain area was functioning; when it could not get any information from the nerve, it squawked-”something is wrong, I am not getting any input!” When that same brain area is itself damaged and no longer does its job, the patient's brain no longer has an area responsible for what is going on in that part of the visual field; for that patien

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