The Tell-Tale Brain

works without also understanding how it evolved. As the great biologist Theodosius Dobzhansky said, “Nothing in biology makes sense except in the light of evolution.” This stands in marked contrast to most other reverse- engineering problems. For example when the great English mathematician Alan Turing cracked the code of the Nazis’ Enigma machine—a device used to encrypt secret messages—he didn’t need to know anything about the research and development history of the device. He didn’t need to know anything about the prototypes and earlier product models. All he needed was one working sample of the machine, a notepad, and his own brilliant brain. But in biological systems there is a deep unity between structure, function, and origin. You cannot make very much progress understanding any one of these unless you are also paying close attention to the other two.

You will see me arguing that many of our unique mental traits seem to have evolved through the novel deployment of brain structures that originally evolved for other reasons. This happens all the time in evolution. Feathers evolved from scales whose original role was insulation rather than flight. The wings of bats and pterodactyls are modifications of forelimbs originally designed for walking. Our lungs developed from the swim bladders of fish which evolved for buoyancy control. The opportunistic, “happenstantial” nature of evolution has been championed by many authors, most notably Stephen Jay Gould in his famous essays on natural history. I argue that the same principle applies with even greater force to the evolution of the human brain. Evolution found ways to radically repurpose many functions of the ape brain to create entirely new functions. Some of them—language comes to mind—are so powerful that I would go so far as to argue they have produced a species that transcends apehood to the same degree by which life transcends mundane chemistry and physics.

And so this book is my modest contribution to the grand attempt to crack the code of the human brain, with its myriad connections and modules that make it infinitely more enigmatic than any Enigma machine. The Introduction offers perspectives and history on the uniqueness of the human mind, and also provides a quick primer on the basic anatomy of the human brain. Drawing on my early experiments with the phantom limbs experienced by many amputees, Chapter 1 highlights the human brain’s amazing capacity for change and reveals how a more expanded form of plasticity may have shaped the course of our evolutionary and cultural development. Chapter 2 explains how the brain processes incoming sensory information, visual information in particular. Even here, my focus is on human uniqueness: Although our brains employ the same basic sensory-processing mechanisms as those of other mammals, we have taken these mechanisms to a new level. Chapter 3 deals with an intriguing phenomenon called synesthesia, a strange blending of the senses that some people experience as a result of unusual brain wiring. Synesthesia opens a window into the genes and brain connectivity that make some people especially creative, and may hold clues about what makes us such a profoundly creative species to begin with.

The next triad of chapters investigates a type of nerve cell that I argue is especially crucial in making us human. Chapter 4 introduces these special cells, called mirror neurons, which lie at the heart of our ability to adopt each other’s point of view and empathize with one another. Human mirror neurons achieve a level of sophistication that far surpasses that of any lower primate, and appear to be the evolutionary key to our attainment of full-fledged culture. Chapter 5 explores how problems with the mirror-neuron system may underlie autism, a developmental disorder characterized by extreme mental aloneness and social detachment. Chapter 6 explores how mirror neurons may have also played a role in humanity’s crowning achievement, language. (More technically, protolanguage, which is language minus syntax.)

Chapters 7 and 8 move on to our species’ unique sensibilities about beauty. I suggest that there are laws of aesthetics that are universal, cutting across cultural and even species boundaries. On the other hand, Art with a capital A is probably unique to humans.

In the final chapter I take a stab at the most challenging problem of all, the nature of self-awareness, which is undoubtedly unique to humans. I don’t pretend to have solved the problem, but I will share the intriguing insights that I have managed to glean over the years based on some truly remarkable syndromes that occupy the twilight zone between psychiatry and neurology, for example, people who leave their bodies temporarily, see God during seizures, or even deny that they exist. How can someone deny his own existence? Doesn’t the denial itself imply existence? Can he ever escape from this Godelian nightmare? Neuropsychiatry is full of such paradoxes, which cast their spell on me when I wandered the hospital corridors as medical student in my early twenties. I could see that these patients’ troubles, deeply saddening as they were, were also rich troves of insight into the marvelously unique human ability to apprehend one’s own existence.

Like my previous books, The Tell-Tale Brain is written in a conversational style for a general audience. I presume some degree of interest in science and curiosity about human nature, but I do not presume any sort of formal scientific background or even familiarity with my previous works. I hope this book proves instructive and inspiring to students of all levels and backgrounds, to colleagues in other disciplines, and to lay readers with no personal or professional stake in these topics. Thus in writing this book I faced the standard challenge of popularization, which is to tread the fine line between simplification and accuracy. Oversimplification can draw ire from hard-nosed colleagues and, worse, can make readers feel like they are being talked down to. On the other hand, too much detail can be off-putting to nonspecialists. The casual reader wants a thought-provoking guided tour of an unfamiliar subject—not a treatise, not a tome. I have done my best to strike the right balance.

Speaking of accuracy, let me be the first to point out that some of the ideas I present in this book are, shall we say, on the speculative side. Many of the chapters rest on solid foundations, such as my work on phantom limbs, visual perception, synesthesia, and the Capgras delusion. But I also tackle a few elusive and less well-charted topics, such as the origins of art and the nature of self-awareness. In such cases I have let educated guesswork and intuition steer my thinking wherever solid empirical data are spotty. This is nothing to be ashamed of: Every virgin area of scientific inquiry must first be explored in this way. It is a fundamental element of the scientific process that when data are scarce or sketchy and existing theories are anemic, scientists must brainstorm. We need to roll out our best hypotheses, hunches, and hare-brained, half-baked intuitions, and then rack our brains for ways to test them. You see this all the time in the history of science. For instance, one of the earliest models of the atom likened it to plum pudding, with electrons nested like plums in the thick “batter” of the atom. A few decades later physicists were thinking of atoms as miniature solar systems, with orderly electrons that orbit the nucleus like planets around a star. Each of these models was useful, and each got us a little bit closer to the final (or at least, the current) truth. So it goes. In my own field my colleagues and I are making our best effort to advance our understanding of some truly mysterious and hard-to-pin-down faculties. As the biologist Peter Medawar pointed out, “All good science emerges from an imaginative conception of what might be true.” I realize, however, that in spite of this disclaimer I will probably annoy at least some of my colleagues. But as Lord Reith, the first director-general of the BBC, once pointed out, “There are some people whom it is one’s duty to annoy.”

Boyhood Seductions

“You know my methods, Watson,” says Sherlock Holmes before explaining how he has found the vital clue. And so before we journey any further into the mysteries of the human brain, I feel that I should outline the methods behind my approach. It is above all a wide-ranging, multidisciplinary approach, driven by curiosity and a relentless question: What if? Although my current interest is neurology, my love affair with science dates back to my boyhood in Chennai, India. I was perpetually fascinated by natural phenomena, and my first passion was chemistry. I was enchanted by the idea that the whole universe is based on simple interactions between elements in a finite list. Later I found myself drawn to biology, with all its frustrating yet fascinating complexities. When I was twelve, I remember reading about axolotls, which are basically a species of salamander that has evolved to remain permanently in the aquatic larval stage. They manage to keep their gills (rather than trading them in for lungs, like salamanders or frogs) by shutting down metamorphosis and becoming sexually mature in the water. I was completely flabbergasted when I read that by simply giving these creatures the “metamorphosis hormone” (thyroid