The Custom-Made Brain

Cerebral Plasticity, Regeneration, and Enhancement

Jean-Didier Vincent, Pierre-Marie Lledo

• Publisher: Columbia University Press
• Year: 2014
• ISBN:
  9780231534215
• Topic: Medicina
• Subtopic: Neurologia

Two leading neuroscientists examine how the brain is in flux and how this applies to addressing neurological, cognitive, and emotional health. Beginning with a survey of the fundamental scientific developments that led to our current understanding of the regenerative mind, the authors elucidate the breakthrough neurobiological studies that paved the way for our present understanding of the brain's plasticity and regenerative capabilities. They then discuss the application of these findings to such issues as depression, dyslexia, schizophrenia, and cognitive therapy, incorporating the latest technologies in neuroimaging, optogenetics, and nanotechnology. Their work shows the brain is anything but a static organ, ceasing to grow as human beings become adults. Rather, the brain is dynamic, evolving organically in relation to physical, cultural, historical, and affective stimuli, a plasticity that provides early hope to survivors of trauma and degenerative disorders. "Highly informative, up to date, and entertaining, The Custom-Made Brain emphasizes that the brain is constantly being constructed during an individual's lifetime, like a medieval cathedral that is never finished and yet ages without reaching 'maturity.' The brain, like the cathedral, is forever being repaired and restored. A stimulating read." —Israel Rosenfield, City University of New York, author of The Invention of Memory: A New View of the Brain "This short account succeeds in an original and thought-provoking manner. It will become a valuable resource for clinicians who manage the care of those disabled by stroke, brain injury, or dementia. Many young scientists will be encouraged to take up the challenge of brain repair in the face of decades of therapeutic pessimism." —Lawrence Whalley, University of Aberdeen, author of The Aging Brain

1

INTRODUCTION

Man and his brain, no big deal!

—ANONYMOUS

ONE OUGHT REALLY TO SAY, “Man and his brain, what a marvel!” A man does not realize his brain is there, just as he does not feel a well-fitting suit: he forgets about it. Our head may feel heavy or painful, but, paradoxically, the brain, our organ of sensation, is without feeling, a soft mass painless even to the surgeon’s scalpel. It is carved from the same pattern for an entire species, but it expresses each individual’s self, in other words, his mind.

For long the structure of these 1,500 grams (3.3 lbs.) of soft, pinkish-yellow matter escaped analysis. Bishop Niels Stensen, or Nicolas Steno (1638–1686), was a great scientist, anatomist, and geologist, as well as a theologian.¹ He denounced “the pretension of these people [Descartes and others] who are so prompt to affirm and give you the history of the brain and the disposition of its parts with the same assurance as if they had been present at the composition of this marvelous machine and as if they had penetrated all the designs of its great architect.”² Today, after three centuries of anatomy and sixty years of modern neuroscience,³ many veils have been lifted concerning the structure of the brain and its functions. Thanks to Darwin and his theory of the evolution of species, it is no longer necessary to think of a design by the great architect. Fans of the mysterious can be assured, however, that the time has not yet come when we understand where all the secrets of the human mind are hidden. Builders of machines to read minds will be disappointed when they find they may be nothing more than Ubu Roi’s disembraining machines.⁴

The history of the human brain is intimately related to the odyssey of our species, which appeared about 200,000 years ago with a modern brain that has not evolved since. Who are these people who appeared toward the end of the Quaternary? Whether male or female, they had naked skin but hair on their heads. “He” soon learned to hide his penis and protect it from thorns along his path. “She” exhibited her pendulous breasts and fat buttocks as marks of her femininity. They showed neither shame nor modesty; clothing came later, together with morality and law. They experienced emotions but above all felt compassion, a characteristic born of natural selection and fundamental to the development of their species. These creatures with short canines and no claws could never have survived without mutual aid and the capacity to read one another’s minds, to guess their intentions and feelings. They walked in step, and their thoughts passed freely from one brain to another. Their eyes lit up by day and chased solitude away. At night they slept under the stars as if they rose from the flames of their fires.

What every gardener knows is true for all men: they are a product of the earth. So it is natural that their fate be linked to that of the soil that bore them. Six or seven million years ago, East Africa underwent a drought that led to the disappearance of the tropical forest and its replacement by woodland savanna. Moving from branch to branch was difficult, and this favored the appearance of bipedalism among the great apes that began to move at ground level.⁵ They were the forebears of humanity with a brain of only 400 grams (just under a pound). The famous Lucy was the jewel of the genus Australopithecus, which inhabited the earth at the end of the Tertiary. Later came the genus Homo. Homo habilis populated Africa, and another species, Homo erectus, colonized Europe two million years ago.

At the beginning of this period of prehistory we only find a handful of strange individuals. They stood upright with their heads squarely on their shoulders. They walked straight ahead, their eyes watching the world with amazement.⁶ They were dispersed⁷ over a territory corresponding to the southern part of modern Ethiopia, in the region of the lower Omo valley near Lake Turkana.⁸ These Homo habilis were masters of techniques for fashioning stone and flint into blades and other tools. Two million years before our modern era, these pilgrims had slowly but successfully conquered the earth. This “handy” man was not yet an intellectual: for that he had to wait until his brain doubled in size.⁹ Then his priority became to learn how to use his head. To face climatic change and find food far from his land of origin, Homo chose the intellectual argument, to develop his brain, but he also adopted a varied diet, which included meat. This omnivorous lifestyle and opportunism were as important as the intellectual approach. In fact, they were linked to it. As an adaptation to carnivorism, the canine teeth of Homo shortened, gradually to be replaced by tools and by an increased tolerance to fellow man: he preferred to spend his time perfecting his hunting technique and sharing his food rather than biting others.

With 900 grams (2 lbs.) of brain, Homo erectus invented society by sharing work, domesticating fire, cooking food, and fashioning clay. Once again, the nature of the earth and the climate produced changes in human development. There were nine ice ages between 900,000 years ago and 15,000 years ago, and they have left traces even today of the slow climatic oscillations that have marked the surface of our planet.¹⁰ The first men who arrived in Europe, including the famous Cro-Magnons, were trapped by glaciations. These long ice ages had profoundly modified the countryside, the contours of the earth, and sea level; across the world, a land bridge more than a thousand kilometers (six hundred miles) long linked the continents between Alaska and Siberia.

Paleolithic archeology has shed light on these periods, which were so rich in the fundamental events that sealed the destiny of modern man. Paleoanthropologists report that we must have had four common ancestors belonging to the hominins.¹¹ Their classification has been made possible by the development of anthropological genetics. For instance, we can easily distinguish Neanderthal man from modern Homo sapiens by their genes, but do they really represent two separate species?¹² The question remains open, given that certain modern Europeans possess up to 2 percent of Neanderthal genes. This Neanderthal contribution to the modern human genome is small but undeniable and shows that interbreeding was possible between Neanderthals and modern man.¹³ Are we in fact Homo neandersapiens?

Homo floresiensis was discovered in September 2003 in a cave on the island of Flores in Java, Indonesia. It complicates the issue. This hominin has a small brain (380 cc; 23 cu in), closer in volume to that of a chimpanzee than that of modern man. Yet this primitive man was already capable of making tools as complex as those of his more modern cousin. The mystery remains. Finally, a girl who lived some 50,000 years ago was discovered at the Denisova archeological site in southern Siberia.¹⁴ Is she a new species of hominin or a subspecies of Neanderthal? Speculation about the existence of Denisovans is rife. Some see here the proof of the existence of an Asiatic Neanderthal quite distinct from Homo. On the other hand, others see evidence of a new species living before the Neanderthals, a sort of pre-Neanderthal. Once again, genetic analysis provides the surprises. Denisovans apparently mated with Homo sapiens, for genetic analysis of modern Melanesians and New Guineans have demonstrated the presence of Denisovan genes. We must accept that human evolution did not follow only one branch but rather that after several dead ends only one led to Homo sapiens.

From this complex evolutionary tree we shall concentrate on two species (or varieties?): Homo sapiens, modern man, and Homo neandertalensis, often imagined as something of a beast. Indeed, the latter had a strange appearance. They were short but had a large head protruding backward; their nose was protuberant, their cheeks flattened; their low forehead ended with a bony ridge overhanging their orbits; their imposing frame was broad and muscular, bearing witness to their strength. These features earned them this reputation for bestiality; on the contrary, their graves and the relics they contain suggest a deep spirituality. Their brain weighed some 1,600 grams (3.5 lbs.), more even than that of modern man, suggesting well-developed intellectual capacities, as is confirmed by their mastery of tool making and the presence of manufactured objects such as jewelry and vessels. Neanderthals disappeared, and we do not know the reason for their extinction. But for the human brain modern times had arrived, and man had to make the most of this organ that evolution had bequeathed. So 35,000 years ago, Homo sapiens took over the world and rid it of all other hominin groups, including Neanderthals.

The average size of the adult human brain varies according to individuals and sex but, in spite of recurrent ideological polemics, we must emphasize that there is no significant correlation between size, ethnic origin, and individual intellectual faculties. On the other hand, this marvel of complexity, with its billions of cells, is in no way immutable or fixed, like the components of a computer. Its matter is built to change and only exists because of change. This means that it incarnates the future. It confers the faculty to accomplish tomorrow operations that we are incapable of realizing today or do things today of which we were incapable yesterday. All our particular skills, manual or intellectual, that conspire to make each of us a unique specialist are, to a great extent, fashioned during the first stages of the brain’s development in childhood and adolescence.

The growth of the modern human brain has two important characteristics that we do not find in other mammals, even in other primates. First, the brain needs some two decades to be complete. This slow growth offers the possibility for a long period of education, during which a central feature is instruction. The second is illustrated by the late development of the brain in the newborn. At birth the brain is hardly a quarter of its adult size. So man is born with a double paradox: an immature brain that is in no hurry to make up for lost time.¹⁵ We call this property secondary altriciality.¹⁶ During this long growth period the child receives signals from the outside world, interacts with its social group, and can acquire a new function: articulate speech. Nonhuman primates develop according to very different modalities. For example, the volume of the chimpanzee’s brain at birth is already some 50 percent of that of the adult, and its growth is over quickly, at around two years of age.

Thanks to the traces left by our environment on our neural circuits, each of us can develop a unique character. Our evolutionary heritage is a nervous system fashioned by the double action of experience and environment. This is the central theme of our story, and we shall discuss it in more detail in chapter 4.

The history of species is of capital importance in understanding how our brain works. The nervous system is contingent on the development of the animal kingdom. We shall see in chapter 2 that the first vertebrates, driven by hunger, perfected predation thanks to adopting a new “head,” which emerged from the front of their body and enabled them to move efficiently to seize prey. According to this principle it was locomotion, not sensation, that drove the development of the vertebrate nervous system, which became “central.” An efficient perception-action loop linking sensory receptors to muscles was the vital function on which evolution exerted pressure for our brain to develop.

To build a human brain required almost one and a half billion years of evolution. During three-quarters of this time the elaboration of a primitive nervous system enabled animals to acquire a greater degree of sensory and motor autonomy. Until the Jurassic, animals could only move to hunt prey or fight predators.¹⁷ Only much later did cognitive functions like language and symbolic thought appear to enable the immense qualitative leap that paved the way for the emergence of the brain of modern man and its unique qualities for abstraction. Such new mental faculties required a flexible, “plastic” nervous system, not a prewired one. Indeed, acquiring our manual and intellectual skills depends on cerebral machinery in perfect order and properly organized. However, at the same time this organization must be, at least in part, adaptable and reconfigurable at all times and at all ages.

This cerebral plasticity is unquestionably at its most spectacular in the child, but it does not disappear in the adult.¹⁸ We consider that there are two major periods of cerebral adaptability. The first, the critical period, corresponds to a window in time during which neural wiring is established so that the brain can acquire the components necessary for its activity and its final structure. At this time, sensory experience is crucial. By demonstrating that the visual cortex develops very early, under the influence of visual experience, the Nobel laureates David Hubel and Torsten Wiesel provided the first neurobiological evidence for the existence of a critical period, even if ethologists like Konrad Lorenz and Nikolaas Tinbergen had invoked it since the 1930s.¹⁹

During this particular period of brain development, the duration of which depends on the specific function (such as vision, audition, walking, language, or mathematics), the brain is the seat of intense changes that manage this immense construction site. During a very early period, the brain is passive; its circuits are waiting to be fed by sensory or motor stimulation provided by the environment in specific affective contexts. The developmental delay in the human fetus leaves the brain highly vulnerable to conditions in the world in which it is growing. François Truffaut’s film The Wild Child, inspired by the story of Victor de l’Aveyron, bears witness to the damage caused when essential experience does not materialize. Based on “The Memorandum and Report on Victor de l’Aveyron” by Jean Itard (1806), this film relates the capture of a deaf and dumb boy, who was running on all fours in the forests of Aveyron in France. Once captured by villagers, this young “savage” was taken to an institution for deaf-mutes in Paris, where he became an object of curiosity for numerous visitors. The neurologist Philippe Pinel considered Victor as an incurable idiot and even tried to intern him in the mental asylum at Bicêtre. The young Dr. Itard, from the deaf-mute institution, saved him from confinement. He persuaded Pinel to entrust him with care of the child, whom he believed capable of education. Unfortunately for him, despite developing surprising mental faculties, Victor never learned to speak. This true story illustrates to what point the human brain, so immature at birth, remains vulnerable to th...

Table of contents

• Cover 
• Title Page
• Copyright
• Dedication
• Contents 
• Acknowledgments
• Translator’s Note
• 1. Introduction
• 2. And Then There Was Shape
• 3. The Masterpiece
• 4. The Workshop of the Brain
• 5. The Brain Under Repair
• 6. The Enhanced Brain
• Epilogue
• Notes
• Index