１章 x 脳の設計は欠陥だらけ？
１章 x 脳の設計は欠陥だらけ？
脳の作りは、案外いい加減スイッチが入ったままの小脳脳はアイスクリームコーンのようなもの人間の脳を賢くしているもの２章 x 非効率な旧式の部品で作られた脳
燃費が悪い脳軸索は水漏れする庭のホースのよう脳はサイコロを振る出来の悪い部品に、なにが素晴らしい能力を与えるか？３章 x 脳を創る
遺伝子ではできないこと発達中の脳は戦場のようなもの環境の豊かさはビタミンに似ている細かい配線を環境に任せた理由４章 x 感覚と感情
脳はなぜ「物語」を作るのか？Ｐ細胞とWhat経路、Ｍ細胞とWhere経路情報の隙間を埋める脳痛みを避ける学習５章 x 記憶と学習
人類は変質者！？脳が決めた性の特徴脳の性差とはなにか？愛とセックスの仕組み歯磨きで起こるオーガズム性的指向と脳の関係７章 x 睡眠と夢
８章 x 脳と宗教
体内時計の進化なぜ夢を見るのか？空飛ぶ夢とコリン作動性ニューロン怖い夢が記憶の統合を促す９章 x 脳に知的な設計者はいない
謝辞 参考文献 解説
The accidental mind / David J. Linden.
PrologueBrain, ExplainedOneThe Inelegant Design of the BrainTwoBuilding a Brain with Yesterday's PartsThreeSome Assembly RequiredFourSensation and EmotionFiveLearning, Memory, and Human IndividualitySixLove and SexSevenSleeping and DreamingEightThe Religious ImpulseNineThe Unintelligent Design of the BrainEpilogueThat Middle Thing
Further Reading and ResourcesAcknowledgmentsIndex
The large brain, like large government, may not be able to do simple things in a simple way.--- Donald O. HebbNow, the president says that the jury is out on evolution. . . Here in New Jersey, we're countin' on it.
THE BEST THING about being a brain researcher is that, in a very small number of situations, you can appear to have the power of mind reading. Take cock-tail parties. Chardonnay in hand, your host makes one of those introductions where he feels compelled to state your occupation: "This is David. He's a brain researcher." Many people are wise enough to simply turn around at this point and go looking for the bourbon and ice. Of those who stay behind about half can be counted on to pause, look heavenward, and raise their eyebrows in preparation for speech. "You're about to ask if it's true that we only use 10 percent of our brain, aren't you?" Wide-eyed nodding. An amazing episode of "mind reading."
Once we get past the 10-percent-of-the-brain thing (which, I should mention, has no basis in reality), it becomes clear that many people have a deep curiosity about brain function. Really fundamental and difficult questions come up right away:
"Will playing classical music to my newborn really help his brain grow?"
"Is there a biological reason why the events in my dreams are so bizarre?"
"Are the brains of gay people physically different from the brains of straight people?"
"Why can't I tickle myself?"
These are all great questions. For some of them, the best scientific answer is fairly clear and for others it is somewhat evasive (me, in my best Bill Clinton voice: "What exactly do you mean by "brain"?). It's fun to talk to non-brain researchers about these kinds of things because they are not afraid to ask the hard questions and to put you on the spot.
Often, when the conversation is over, people will ask, "Is there a good book on brain and behavior for a nonspecialist audience that you can recommend?" Here, it gets tricky. There are some books, such as Joe Le Doux's Synaptic Self, that do a great job on the science, but that are rough sledding unless you've already got a college degree in biology or psychology. There are others, such as Oliver Sacks's Man Who Mistook His Wife for a Hat and V. S. Ramachandran and Sandra Blakeslee's Phantoms in the Brain that tell fascinating and illuminating stories based on case histories in neurology, but that really don't convey a broad understanding of brain function and that largely ignore molecules and cells. There are books that talk about molecules and cells in the brain, but many of them are so deadly dull that you can start to feel your soul depart your body before you finish the very first page.
What's more, many books about the brain, and even more shows on educational television, perpetuate a fundamental misunderstanding about neural function. They present the brain as a beautifully engineered, optimized device, the absolute pinnacle of design. You've probably seen it before: a human brain lit dramatically from the side, with the camera circling it as if taking a helicopter shot of Stonehenge and a modulated baritone voice exalting the brain's elegant design in reverent tones.
This is pure nonsense. The brain is not elegantly designed by any means: it is a cobbled-together mess, which, amazingly, and in spite of its shortcomings, manages to perform a number of very impressive functions. But while its overall function is impressive, its design is not. More important, the quirky, inefficient, and bizarre plan of the brain and its constituent parts is fundamental to our human experience. The particular texture of our feelings, perceptions, and actions is derived, in large part, from the fact that the brain is not an optimized, generic problem-solving machine, but rather a weird agglomeration of ad hoc solutions that have accumulated throughout millions of years of evolutionary history.
So, here's what I'll try to do. I will be your guide to this strange and often illogical world of neural function, with the particular charge of pointing out the most unusual and counterintuitive aspects of brain and neural design and explaining how they mold our lives. In particular, I will try to convince you that the constraints of quirky, evolved brain design have ultimately led to many transcendent and unique human characteristics: our long childhoods, our extensive memory capacity (which is the substrate upon which our individuality is created by experience), our search for long-term love relationships, our need to create compelling narrative and, ultimately, the universal cultural impulse to create religious explanations.
Along the way, I will briefly review the biology background you will need to understand the things I am guessing you most want to know about the brain and behavior. You know, the good stuff: emotion, illusion, memory, dreams, love and sex, and, of course, freaky twin stories. Then, I'll try my best to answer the big questions and to be honest when answers are not at hand or are incomplete. If I don't answer all of your questions, try visiting the book's website, accidentalmind.org. I'll strive to make it fun, but I'm not going to "take all the science out." It will not be, as you might find on a label at Whole Foods, " 100 percent molecule free."
Max Delbriick, a pioneer of molecular genetics, said, "Imagine that your audience has zero knowledge but infinite intelligence." That sounds just about right to me, so that's what I'll do. Let's roll.
The Inelegant Design of the Brain
WHEN I WAS IN middle school, in California in the 1970s, a popular joke involved asking someone, "Want to lose 6 pounds of ugly fat?" If the reply was positive it would be met with "Then chop off your head! Hahahaha!" Clearly, the brain did not hold a revered place in the collective imagination of my classmates. Like many, I was relieved when middle school drew to a close. Many years later, however, I have been similarly distressed by the opposite view. Particularly when reading books or magazines or watching educational television, I have been taken aback by a form of brain worship. Discussion of the brain is most often delivered in a breathless, awestruck voice. In these works the brain is "an amazingly efficient 3 pounds of tissue, more powerful than the largest supercomputer," or "the seat of the mind, the pinnacle of biological design."What I find problematic about these statements is not the deep appreciation that mental function resides in the brain, which is indeed amazing. Rather, it is the assumption that since the mind is in the brain, and the mind is a great achievement, the design and function of the brain must then be elegant and efficient. In short, it is imagined by many that the brain is well engineered.
Nothing could be further from the truth. The brain is, to use one of my favorite words, a kludge (pronounced "klooj"), a design that is inefficient, inelegant, and unfathomable, but that nevertheless works. More evocatively, in the words of the military historian Jackson Granholm, a kludge is "an ill-assorted collection of poorly matching parts, forming a distressing whole."
What I hope to show here is that at every level of brain organization, from regions and circuits to cells and molecules, the brain is an inelegant and inefficient agglomeration of stuff, which nonetheless works surprisingly well. The brain is not the ultimate general-purpose supercomputer. It was not designed at once, by a genius, on a blank piece of paper. Rather, it is a very peculiar edifice that reflects millions of years of evolutionary history. In many cases, the brain has adopted solutions to particular problems in the distant past that have persisted over time and have been recycled for other uses or have severely constrained the possibilities for further change. In the words of the pioneering molecular biologist Frangois Jacob, "Evolution is a tinkerer, not an engineer."
What's important about this point as applied to the brain is not merely that it challenges the notion of optimized design. Rather, appreciation of the quirky engineering of the brain can provide insights into some of the deepest and most particularly human aspects of experience, both in day-to-day behavior and in cases of injury and disease.
SO, WITH THESE issues in mind, let's have a look at the brain and see what we can discern about its design. What are the organizational principles that emerge? For this purpose, imagine that we have a freshly dissected adult human brain before us now (Figure 1.1). What you would see is a slightly oblong, grayish-pink object weighing about 3 pounds. Its outer surface, which is called the cortex, is covered with thick wrinkles that form deep grooves. The pattern of these grooves and wrinkles looks like it might be variable, like a fingerprint, but it is actually very similar in all human brains. Hanging off the back of the brain is a structure the size of a squashed baseball with small crosswise grooves. This is called the cerebellum, which means "little brain." Sticking out of the bottom of the brain, somewhat toward the back end is a thick stalk called the brainstem.
We've lopped off the very bottom of the brainstem where it would otherwise taper to form the top of the spinal cord. Careful observation would reveal the nerves, called the cranial nerves, which carry information from the eyes, ears, nose, tongue, and face into the brainstem.
One obvious characteristic of the brain is its symmetry: the view from the top shows a long groove from front to back that divides the cortex (which means "rind"), the thick outer covering of the brain, into two equal halves. If we slice completely through the brain, using this front-to-back groove as a guide, and then turn the cut side of the right half toward us, we see the view shown in the bottom of Figure 1.1.
Looking at this image makes it clear that the brain is not just a homogeneous blob of stuff. There are variations in shape, color, and texture of the brain tissue across brain regions, but these do not tell us about the functions of these various regions. One of the most useful ways to investigate the function of these locations is to look at people who have sustained damage to various parts of the brain. Such investigations have been complemented by animal experiments in which small regions of the brain are precisely damaged through surgery or the administration of drugs, after which the animal's body functions and behavior are carefully observed.
The brainstem contains centers that control extremely basic regulation of
FIGURE 1.1. The human brain. The top shows the intact brain viewed from the left side. The bottom shows the brain sliced down the middle and then opened to allow the right side to face us. Joan M. K. Tycko, illustrator.
the body that are not under your conscious control, including vital functions such as regulation of heart rate, blood pressure, breathing rhythm, body temperature, and digestion. It also contains the control centers for some important reflexes, such as sneezing, coughing, and vomiting. The brainstem houses relays for sensations coming up the spinal cord from your skin and muscles as well as for command signals coming from your brain and destined for muscles in your body. It also contains locations involved in producing feelings of wakefulness versus sleepiness. Drugs that modify your state of wakefulness, such as sleeping pills or general anesthetics on the one hand and caffeine or amphetamines on the other, act on these brainstem regions. If you get a small area of damage in your brainstem (from an injury, tumor, or stroke), you could be rendered comatose, unable to be aroused by any sensation, but extensive damage in the brainstem is almost always fatal.
The cerebellum, which is richly interconnected with the brainstem, is involved with coordination of movements. In particular, it uses feedback from your senses about how your body is moving through space in order to issue fine corrections to the muscles to render your movements smooth, fluid, and well coordinated. This cerebellar fine-tuning operates not only in the most demanding forms of coordination such as hitting a baseball or playing the violin, but also in everyday activities. Damage to the cerebellum is subtle. It will not paralyze you, but rather will typically result in clumsiness in performing simple tasks that we take for granted, such as reaching smoothly to grasp a coffee cup or walking with a normal gait; this phenomenon is called ataxia.
The cerebellum is also important in distinguishing sensations that are "expected" from those that are not. In general, when you initiate a movement and you have sensations which result from that movement, you tend to pay less attention to those sensations. For example, when you walk down the street and your clothes rub against your body, these are sensations that you mostly ignore. By contrast, if you were standing still and you started to feel similar rubbing sensations on your body, you would probably pay a lot of attention. You would probably whirl around to see who was groping you. In many situations, it is useful to ignore sensations produced by your own motion and pay close attention to other sensations that originate from the outside world. The cerebellum receives signals from those brain regions that create the commands that trigger body motion. The cerebellum uses these signals to predict the sensations that are likely to result from this motion. Then the cerebellum sends inhibitory signals to other brain regions to subtract the "expected" sensations from the "total" sensations and thereby change the way they feel to you.
This may all sound a bit abstract, so let's consider an example. It is well known that you can't tickle yourself. This is not just true in certain cultures; it is worldwide. What's different about having someone else tickle you, which can result in a very strong sensation, and self-tickling, which is ineffective?
When researchers in Daniel Wolpert's group at University College, London, placed people's heads in a machine that can make images showing the location and strength of brain activity (called functional magnetic-resonance images, or fMRI) and then tickled them, they found strong activation in a brain region involved in touch sensation called the somatosensory cortex and no significant activation in the cerebellum. When people were then asked to tickle themselves on that same part of the body, it was seen that there was a spot of activation in the cerebellum and reduced activity in the somatosensory cortex. The interpretation of this experiment is that commands to activate the hand motions in self-tickling stimulated the cerebellum, which then formed a prediction of the expected sensation and sent signals encoding this prediction to inhibit the somatosensory cortex. The reduced activation of the somatosensory cortex was then below the threshold necessary to have the sensation feel like tickling. Interestingly, there are now reports that some humans who sustain damage to the
FIGURE 1.2. Force escalation in a tit-for-tat finger-tapping task. The white circles show the force of finger taps delivered by one subject, the black circles the force from the other subject. In 9 tit-for-tat exchanges, the force increased almost 20-fold. Adapted from S. S. Shergill, P. M. Bays, C. D. Frith, and D. M. Wolpert, Two eyes for an eye: the neuroscience of force escalation, Science 301:187 (2003); copyright 2003 AAAS. JoanM. K. Tycko, illustrator.
cerebellum cannot generate predictions of expected sensations and therefore can actually tickle themselves!
Daniel Wolpert and his colleagues at University College, London, have also devised a simple and elegant experiment to explain the cerebellum's involvement in the escalation of a shoving match (Figure 1.2). When a shoving match starts between two people the force of the shoving tends to escalate, often to the point of a full-blown brawl. Typically, we have thought of this solely in terms of social dynamics: neither participant wants to show weakness by backing down. That may explain why the conflict continues, but it does not necessarily shed light on why the force of each shove increases in a tit-for-tat exchange.
What Wblpert and his colleagues did was have two adult subjects face each other, each resting the left index finger, palm up, in a molded depression. A small metal bar on a hinge was then rested lightly on top of each subject's finger. The hinge was fitted with a sensor to measure the force delivered when the bar was pressed down. Both subjects were given the same instructions: exactly match the force of the tap on his finger that he receives with an equivalent tap when his turn comes. Neither subject knew the instructions given to the other.
Despite explicit instructions to the contrary, when the subjects took turns pressing on each other's fingers, the force applied always escalated dramatically, just as it does in schoolyard or bar-room confrontations. Each person swore that he matched the force of the other's tap. When asked to guess the instructions given to the other person, each said, "You told the other guy to press back twice as hard."
Why does this happen? Several clues point to the answer. First, it is not specific to social situations. When a person is asked to match the force of a finger tap which comes from a machine, he or she she will also respond with greater force. The second line of evidence comes from modifying the tit-for-tat experiment so that the tap is produced not by pressing on a bar but rather by moving a joystick that controls the pressure by activating a motor. The important difference between these two situations is that when the force is generated by bar pressing, making a stronger tap requires generating more force with the fingertip. When the joystick is used, however, the motor does the work and there is only a weak correlation between the force generated by the tapping finger and the force produced on the upturned finger of the other subject. When the tit-for-tat experiment is then repeated with joysticks there is very little force escalation. The interpretation here is similar to that offered for self-tickling: The cerebellum receives a copy of the commands to produce the finger tap (using the bar) that are proportional to the force applied. It then creates a prediction of the expected sensation that is sent to the somatosensory cortex to inhibit feedback sensations from the fingertip during tapping. To overcome this inhibition, the subject presses harder to match the force perceived from the last tap he or she received, thus escalating the force applied.
So, in most situations, the cerebellar circuit that allows us to pay less attention to sensations that result from self-generated movement and more attention to the outside world is a useful mechanism. But as any 8-year-old coming home with a black eye and a tale of "But Mom, he hit me harder!" will tell you, there is a price to pay for this feature. This is a common brain design flaw. Most systems, like the cerebellar inhibition of sensations from self-generated movement, are always on. They cannot be switched off even when their action is counterproductive.
Moving up and forward from the cerebellum, the next region we encounter is called the midbrain. It contains primitive centers for vision and hearing. These locations are the main sensory centers for some animals, such as frogs or lizards. For example, the midbrain visual center is key for guiding the tongue-thrust frogs use to capture insects in flight. But in mammals, including humans, the midbrain visual centers are supplemented and to some degree supplanted by more elaborate visual regions higher up in the brain (in the cortex). Even though we make only limited use of a frog-like visual region in our brains (mostly in orienting our eyes to certain stimuli), this evolutionarily ancient structure has been retained in human brain design and this gives rise to the fascinating phenomenon called blindsight.
Patients who are effectively blind owing to damage to the higher visual parts of the brain will report that they have no visual sense whatsoever. When asked to reach for an object in their visual field, such as a penlight, they will say, "What can you possibly mean? I can't see a thing!" If however, they are told to just take a guess and try anyway, they can usually succeed at this task at a rate much higher than would be due to pure chance. In fact, some patients can grasp the penlight 99 percent of the time, yet will report each time that they have no idea where the target is and they are guessing randomly. The explanation seems to be that the ancient visual system in the midbrain is intact in these patients and guides their reaching, yet because this region is not interconnected with the higher areas of the brain, these people have no conscious awareness of the penlight's location. This underscores a general theme that is emerging here.
The functions of the lower portions of the brain such as the brainstem and the midbrain are generally performed automatically, without our conscious awareness. As we continue our tour to those parts of the brain that are both literally and metaphorically higher, then we will begin to make the transition from subconscious to conscious brain function.
Furthermore, the midbrain visual system is a lovely example of brain kludge: it is an archaic system that has been retained in our brains for a highly delimited function, yet its action can be revealed in brain injury. As an analogy, imagine if your present-day audio electronics, let's say that sleek handheld MP3 player, still contained a functional, rudimentary 8-track tape player from the 1960s. Not too many of those would get sold, even with a really urban-hip, edgy ad campaign.
Moving a bit upward and forward, we reach two structures called the thalamus and the hypothalamus (which just means "below the thalamus"). The thalamus is a large relay station for sending sensory signals on to higher brain areas and also relaying command signals from these areas out along pathways that ultimately activate muscles. The hypothalamus has many smaller parts, each of which has a separate function, but one general theme of this region is that it helps to maintain the status quo for a number of body functions, a process called homeostasis. For example, when you get too cold, your body begins to shiver reflexively in an attempt to generate heat through muscular activity.
The shivering reflex originates within the hypothalamus.
Perhaps the most well-known homeostatic drives are those that control hunger and thirst. Although the urge to eat and drink can be modulated by many factors, including social circumstances, emotional state, and psychoactive drugs (consider the well-known phenomenon of "the munchies" from smoking marijuana and the appetite-suppressing action of amphetamines), the basic drives for hunger and thirst are triggered within the hypothalamus. When tiny holes are made surgically in one part of the hypothalamus of a rat (called the lateral nucleus; a "nucleus" in the brain is just a name for a group of brain cells), it will fail to eat and drink, even after many days. Conversely, destroying a different part of the hypothalamus (the ventromedial nucleus) results in massive overeating. Not surprisingly, a huge effort is under way to identify the chemical signals that trigger feelings of hunger and fullness, with the hope of making a safe and effective weight-loss drug. So far, this has proven to be much more difficult than anticipated because multiple, parallel signals for both beginning and ending feeding appear to play a role.
In addition to its involvement in homeostasis and biological rhythms, the hypothalamus is also a key controller of some basic social drives, such as sex and aggression. I will talk about these functions in detail later. A point that must be made here, though, is that the hypothalamus exerts some of its effects on these drives by secreting hormones, powerful messenger molecules that are carried in the bloodstream throughout the body to cause many varied responses. The hypothalamus secretes two types of hormones. One type has direct actions on the body (such as the hormone called vasopressin, which acts on the kidney to limit the formation of urine and thereby increase blood pressure), and the second type, the so-called master hormones, directs other glands to secrete their own hormones. A good example of the latter is growth hormone, secreted by the pituitary gland in growing children and adolescents but stimulated by a master hormone released by the hypothalamus. After much careful scientific thought, this master hormone was given the compelling name "growth hormone releasing hormone" (endocrinologists, like many scientists, are not known for their literary flair).
Up to this point, we have been looking at the brain sliced exactly down the middle. Many areas inside the brain are revealed with this view, but others are buried deep within the tissue and are not visible either from the outside surface or from the cut surface at the midline. Particularly important are two deeply buried structures called the amygdala ("almond") and the hippocampus ("sea-horse") that constitute part of a larger circuit in the center of the brain called the limbic system (which also contains portions of the thalamus, cortex, and other regions). The limbic system is important for emotion and certain kinds of memory. It is also the first place in our bottom-to-the-top tour where automatic and reflexive functions begin to blend with conscious awareness.
The amygdala is a brain center for emotional processing that plays a particular role in fear and aggression. It links sensory information that has already been highly processed by the cortex (that guy in the ski mask jumping out of that dark alley at me can't be up to any good) to automatic fight-or-flight responses mediated by the hypothalamus and brainstem structures (sweating, increased heart rate, dry mouth). Humans rarely sustain damage to the amygdala alone, but those who do often have disorders of mood and appear to be unable to recognize fearful expressions in others. Electrical stimulation of the amygdala (as sometimes occurs during neurosurgery) can evoke feelings of fear, and the amygdala also appears to be involved in storing memories of fearful events.
The hippocampus (which, when dissected out of the brain, actually looks more like a ram's horn than the seahorse for which it is named) is a memory center. Like the amygdala, it receives highly processed sensory information
from the cortex lying above it. Rather than mediating fear, however, the hippocampus appears to have a special role in laying down the memory traces for facts and events, which are stored in the hippocampus for a year or so but are then moved to other structures. The most compelling evidence for this model comes from a small number of people who have sustained damage to their hippocampus and some surrounding tissue on both sides of the brain. The most famous of these cases is called H.M. (initials used to protect privacy), a man who in 1953 underwent surgical removal of the hippocampus and some surrounding tissue on both sides of his brain in order to control massive seizures that had not responded to other treatments. The surgery was successful in controlling his epilepsy and did not impair his motor functions, language, or general cognitive abilities, but there were two disastrous side effects. First, H.M. lost his memory of everything that occurred 2―4 years before the surgery. He had extensive, detailed, and accurate recall of earlier events, but his memory of his life in the years just before the surgery is lost forever. Even more devastating is that since the surgery H.M. has been unable to store new memories for facts and events. If you were to meet him on Monday, he would not remember you