Reading in the Brain

"Brings together the cognitive, the cultural, and the neurological in an elegant, compelling narrative. A revelatory work."--Oliver Sacks, M.D.

The act of reading is so easily taken for granted that we forget what an astounding feat it is. How can a few black marks on white paper evoke an entire universe of meanings? It's even more amazing when we consider that we read using a primate brain that evolved to serve an entirely different purpose. In this riveting investigation, Stanislas Dehaene, author of How We Learn, explores every aspect of this human invention, from its origins to its neural underpinnings. A world authority on the subject, Dehaene reveals the hidden logic of spelling, describes pioneering research on hiw we process languages, and takes us into a new appreciation of the brain and its wondrous capacity to adapt.Stanislas Dehaene is the director of the Cognitive Neuroimaging Unit in Saclay, France, and the professor of experimental cognitive psychology at the Collège de France. He is the author of Reading in the Brain, Consciousness and the Brain, and How We Learn.

PENGUIN BOOKS

READING IN THE BRAIN

French scientist Stanislas Dehaene was trained as a mathematician and psychologist before becoming one of the world’s most active researchers on the cognitive neuroscience of language and number processing in the human brain. He is the director of the Cognitive Neuroimaging Unit in Saclay, France; professor of experimental cognitive psychology at the Collège de France; and a member of both the French Academy of Sciences and the pontifical Academy of Sciences. He has published extensively in peer-reviewed scientific journals and is the author of several books, including The Number Sense.

 

Praise for Reading in the Brain

A Washington Post Best Science Book of 2009

A Library Journal Best Sci-Tech Book of 2009

“In his splendid Reading in the Brain, French neuroscientist Stanislas Dehaene reveals how decades of low-tech experiments and high-tech brain-imaging studies have unwrapped the mystery of reading and revealed its component parts. . . . A pleasure to read. [Dehaene] never oversimplifies; he takes the time to tell the whole story, and he tells it in a literate way.”

—The Wall Street Journal

“Fascinating . . . By studying the wet stuff inside our head, we can begin to understand why this sentence has this structure, and why this letter, this one right here, has its shape. . . . Eloquent . . . Provide[s] a wealth of evidence.”

—Jonah Lehrer

“Dehaene’s masterful book is a delight to read and scientifically precise.”

—Nature

“Combining research and narrative, Dehaene weaves a fascinating explanation of how the prefrontal cortex co-opted primeval neurological pathways to learn a uniquely human skill.”

—Seed

“The transparent and automatic feat of reading comprehension disguises an intricate biological effort, ably analyzed in this fascinating study. . . . This lively, lucid treatise proves once again that Dehaene is one of our most gifted expositors of science; he makes the workings of the mind less mysterious, but no less miraculous.”

—Publishers Weekly

“Richly rewarding.”

—Kirkus Reviews

“[Dehaene] is that rare bird: a scientist who can write.”

—The Globe and Mail (Toronto)

“Inspire[s] a sense of wonder at the complexity at the task readers are performing just by scanning from page to page.”

—A.V. Club.com

“We are fortunate that Stanislas Dehaene, the leading authority on the neuro-science of language, is also a beautiful writer. His Reading in the Brain brings together the cognitive, the cultural, and the neurological in an elegant, compelling narrative. It is a revelatory work.”

—Oliver Sacks, M.D.

“In a moment when knowledge about the reading brain may be the key to its preservation, Stanislas Dehaene’s book provides the next critical rung of that knowledge. He does this through insights gained from his own prolific research, through his comprehensive grasp of the neurosciences, and through his unique combination of common sense and wisdom that shines through every chapter.”

—Maryanne Wolf, author of Proust and the Squid: The Story and Science of the Reading Brain

“Stanislas Dehaene takes us on a journey into the science of reading. We travel past firing neurons in monkeys, brain activation patterns in humans, people with brain damage, and culture as a whole. It’s a proactive and enjoyable synthesis of a tremendous amount of information, with just the right balance between getting the facts right and making them accessible to lay readers.”

—Joseph LeDoux, University Professor, New York University, and author of Synaptic Self and The Emotional Brain

“Reading in the Brain isn’t just about reading. It comes nearer than anything I have encountered to explaining how humans think, and does so with a simple elegance that can be grasped by scientists and nonscientists alike. Dehaene provides insight about the neurological underpinnings of the spectacular cognitive skills that characterize our species. Students of human evolution are not the only ones who will find Reading in the Brain fascinating. Parents, educators, and anyone else who nurtures the intellectual development of children cannot afford to ignore Dehaene’s observations about the best methods for teaching them to read!”

—Dean Falk, author of Finding Our Tongues: Mothers, Infants, and the Origins of Language

“The complicated partnership of eye and mind that transforms printed symbols into sound, music, and meaning, and gives rise to thought, is the subject of this intriguing study. It’s a wondrous journey: like that of stout Cortez, like H. M. Stanley’s search for Dr. David Livingstone, like the next stunning probe into outer space.”

—Howard Engel, coauthor of The Man Who Forgot How to Read

Reading in the Brain

The New Science
of How We Read

STANISLAS DEHAENE

INTRODUCTION

The New Science of Reading

Withdrawn into the peace of this desert, along with some books, few but wise, I live in conversation with the deceased, and listen to the dead with my eyes

—FRANCISCO DE QUEVEDO

At this very moment, your brain is accomplishing an amazing feat—reading. Your eyes scan the page in short spasmodic movements. Four or five times per second, your gaze stops just long enough to recognize one or two words. You are, of course, unaware of this jerky intake of information. Only the sounds and meanings of the words reach your conscious mind. But how can a few black marks on white paper projected onto your retina evoke an entire universe, as Vladimir Nabokov does in the opening lines of Lolita:

Lolita, light of my life, fire of my loins. My sin, my soul. Lo-lee-ta: the tip of the tongue taking a trip of three steps down the palate to tap, at three, on the teeth. Lo. Lee. Ta.

The reader’s brain contains a complicated set of mechanisms admirably attuned to reading. For a great many centuries, this talent remained a mystery. Today, the brain’s black box is cracked open and a true science of reading is coming into being. Advances in psychology and neuroscience over the last twenty years have begun to unravel the principles underlying the brain’s reading circuits. Modern brain imaging methods now reveal, in just a matter of minutes, the brain areas that activate when we decipher written words. Scientists can track a printed word as it progresses from the retina through a chain of processing stages, each of which is marked by an elementary question: Are these letters? What do they look like? Are they a word? What does it sound like? How is it pronounced? What does it mean?

On this empirical ground, a theory of reading is materializing. It postulates that the brain circuitry inherited from our primate evolution can be co-opted to the task of recognizing printed words. According to this approach, our neuronal networks are literally “recycled” for reading. The insight into how literacy changes the brain is profoundly transforming our vision of education and learning disabilities. New remediation programs are being conceived that should, in time, cope with the debilitating incapacity to decipher words known as dyslexia.

My purpose in this book is to share my knowledge of recent and little-known advances in the science of reading. In the twenty-first century, the average person still has a better idea of how a car works than of the inner functioning of his own brain—a curious and shocking state of affairs. Decision makers in our education systems swing back and forth with the changing winds of pedagogical reform, often blatantly ignoring how the brain actually learns to read. Parents, educators, and politicians often recognize that there is a gap between educational programs and the most up-to-date findings in neuroscience. But too frequently their idea of how this field can contribute to advances in education is only grounded in a few color pictures of the brain at work. Unfortunately, the imaging techniques that allow us to visualize brain activity are subtle and occasionally misleading. The new science of reading is so young and fast-moving that it is still relatively unknown outside the scientific community. My goal is to provide a simple introduction to this exciting field, and to increase awareness of the amazing capacities of our reading brains.

From Neurons to Education

Reading acquisition is a major step in child development. Many children initially struggle with reading, and surveys indicate that about one adult in ten fails to master even the rudiments of text comprehension. Years of hard work are needed before the clockwork-like brain machinery that supports reading runs so smoothly that we forget it exists.

Why is reading so difficult to master? What profound alterations in brain circuitry accompany the acquisition of reading? Are some teaching strategies better adapted to the child’s brain than others? What scientific reasons, if any, explain why phonics—the systematic teaching of letter-to-sound correspondences—seems to work better than whole-word teaching? Although much still remains to be discovered, the new science of reading is now providing increasingly precise answers to all these questions. In particular, it underlines why early research on reading erroneously supported the whole-word approach—and how recent research on the brain’s reading networks proves it was wrong.

Understanding what goes into reading also sheds light on its pathologies. In our explorations of the reader’s mind and brain, you will be introduced to patients who suddenly lost the ability to read following a stroke. I will also analyze the causes of dyslexia, whose cerebral underpinnings are gradually coming to light. It is now clear that the dyslexic brain is subtly different from the brain of a normal reader. Several dyslexia susceptibility genes have been identified. But this is by no means a reason for discouragement or resignation. New intervention therapies are now being defined. Intensive retraining of language and reading circuits has brought about major improvements in children’s brains that can readily be tracked with brain imaging.

Putting Neurons into Culture

Our ability to read brings us face-to-face with the singularity of the human brain. Why is Homo sapiens the only species that actively teaches itself? Why is he unique in his ability to transmit a sophisticated culture? How does the biological world of synapses and neurons relate to the universe of human cultural inventions? Reading, but also writing, mathematics, art, religion, agriculture, and city life have dramatically increased the native capacities of our primate brains. Our species alone rises above its biological condition, creates an artificial cultural environment for itself, and teaches itself new skills like reading. This uniquely human competence is puzzling and calls for a theoretical explanation.

One of the basic techniques in the neurobiologist’s toolkit consists of “putting neurons in culture”—letting neurons grow in a petri dish. In this book, I call for a different “culture of neurons”—a new way of looking at human cultural activities, based on our understanding of how they map onto the brain networks that support them. Neuroscience’s avowed goal is to describe how the elementary components of the nervous system lead to the behavioral regularities that can be observed in children and adults (including advanced cognitive skills). Reading provides one of the most appropriate test beds for this “neurocultural” approach. We are increasingly aware of how writing systems as different as Chinese, Hebrew, or English get inscribed in our brain circuits. In the case of reading, we can clearly draw direct links between our native neuronal architecture and our acquired cultural abilities—but the hope is that this neuroscience approach will extend to other major domains of human cultural expression.

The Mystery of the Reading Ape

If we are to reconsider the relation between brain and culture, we must address an enigma, which I call the reading paradox: Why does our primate brain read? Why does it have an inclination for reading although this cultural activity was invented only a few thousand years ago?

There are good reasons why this deceptively simple question deserves to be called a paradox. We have discovered that the literate brain contains specialized cortical mechanisms that are exquisitely attuned to the recognition of written words. Even more surprisingly, the same mechanisms, in all humans, are systematically housed in identical brain regions, as though there were a cerebral organ for reading.

But writing was born only fifty-four hundred years ago in the Fertile Crescent, and the alphabet itself is only thirty-eight hundred years old. These time spans are a mere trifle in evolutionary terms. Evolution thus did not have the time to develop specialized reading circuits in Homo sapiens. Our brain is built on the genetic blueprint that allowed our hunter-gatherer ancestors to survive. We take delight in reading Nabokov and Shakespeare using a primate brain originally designed for life in the African savanna. Nothing in our evolution could have prepared us to absorb language through vision. Yet brain imaging demonstrates that the adult brain contains fixed circuitry exquisitely attuned to reading.

The reading paradox is reminiscent of the Reverend William Paley’s parable aimed at proving the existence of God. In his Natural Theology (1802), he imagined that in a deserted heath, a watch was found on the ground, complete with its intricate inner workings clearly designed to measure time. Wouldn’t it provide, he argued, clear proof that there is an intelligent clockmaker, a designer who purposely created the watch? Similarly, Paley maintained that the intricate devices that we find in living organisms, such as the astonishing mechanisms of the eye, prove that nature is the work of a divine watchmaker.

Charles Darwin famously refuted Paley by showing how blind natural selection can produce highly organized structures. Even if biological organisms at first glance seem designed for a specific purpose, closer examination reveals that their organization falls short of the perfection that one would expect from an omnipotent architect. All sorts of imperfections attest that evolution is not guided by an intelligent creator, but follows random paths in the struggle for survival. In the retina, for example, blood vessels and nerve cables are situated in front of the photoreceptors, thus partially blocking incoming light and creating a blind spot—very poor design indeed.

Following in Darwin’s footsteps, Stephen Jay Gould provided many examples of the imperfect outcome of natural selection, including the panda’s thumb.1 The British evolutionist Richard Dawkins also explained how the delicate mechanisms of the eye or of the wing could only have emerged through natural selection or are the work of a “blind watchmaker.”2 Darwin’s evolutionism seems to be the only source of apparent “design” in nature.

When it comes to explaining reading, however, Paley’s parable is problematic in a subtly different way. The clockwork-like brain mechanisms that support reading are certainly comparable in complexity and sheer design to those of the watch abandoned on the heath. Their entire organization leans toward the single apparent goal of decoding written words as quickly and accurately as possible. Yet neither the hypothesis of an intelligent creator nor that of slow emergence through natural selection seems to provide a plausible explanation for the origins of reading. Time was simply too short for evolution to design specialized reading circuits. How, then, did our primate brain learn to read? Our cortex is the outcome of millions of years of evolution in a world without writing—why can it adapt to the specific challenges posed by written word recognition?

Biological Unity and Cultural Diversity

In the social sciences, the acquisition of cultural skills such as reading, mathematics, or the fine arts is rarely, if ever, posed in biological terms. Until recently, very few social scientists considered that brain biology and evolutionary theory were even relevant to their fields. Even today, most implicitly subscribe to a naïve model of the brain, tacitly viewing it as an infinitely plastic organ whose learning capacity is so broad that it places no constraints on the scope of human activity. This is not a new idea. It can be traced back to the theories of the British empiricists John Locke, David Hume, and George Berkeley, who claimed that the human brain should be compared to a blank slate that progressively absorbs the imprint of man’s natural and cultural environment through the five senses.

This view of mankind, which denies the very existence of a human nature, has often been adopted without question. It belongs to the default “standard social science model”3 shared by many anthropologists, sociologists, some psychologists, and even a few neuroscientists who view the cortical surface as “largely equipotent and free of domain-specific structure.”4 It holds that human nature is constructed, gradually and flexibly, through cultural impregnation. As a result, children born to the Inuit, to the hunter-gatherers of the Amazon, or to an Upper East Side New York family, according to this view, have little in common. Even color perception, musical appreciation, or the notion of right and wrong should vary from one culture to the next, simply because the human brain has few stable structures other than the capacity to learn.

Empiricists further maintain that the human brain, unhindered by the limitations of biology and unlike that of any other animal species, can absorb any form of culture. From this theoretical perspective, to talk about the cerebral bases of cultural inventions such as reading is thus downright irrelevant—much like analyzing the atomic composition of a Shakespeare play.

In this book, I refute this simplistic view of an infinite adaptability of the brain to culture. New evidence on the cerebral circuits of reading demonstrates that the hypothesis of an equipotent brain is wrong. To be sure, if the brain were not capable of learning, it could not adapt to the specific rules for writing English, Japanese, or Arabic. This learning, however, is tightly constrained, and its mechanisms themselves rigidly specified by our genes. The brain’s architecture is similar in all members of the Homo sapiens family, and differs only slightly from that of other primates. All over the world, the same brain regions activate to decode a written word. Whether in French or in Chinese, learning to read necessarily goes through a genetically constrained circuit.

On the basis of these data, I propose a novel theory of neurocultural interactions, radically opposed to cultural relativism, and capable of resolving the reading paradox. I call it the “neuronal recycling” hypothesis. According to this view, human brain architecture obeys strong genetic constraints, but some circuits have evolved to tolerate a fringe of variability. Part of our visual system, for instance, is not hardwired, but remains open to changes in the environment. Within an otherwise well-structured brain, visual plasticity gave the ancient scribes the opportunity to invent reading.

In general, a range of brain circuits, defined by our genes, provides “pre-representations”5 or hypotheses that our brain can entertain about future developments in its environment. During brain development, learning mechanisms select which pre-representations are best adapted to a given situation. Cultural acquisition rides on this fringe of brain plasticity. Far from being a blank slate that absorbs everything in its surroundings, our brain adapts to a given culture by minimally turning its predispositions to a different use. It is not a tabula rasa within which cultural constructions are amassed, but a very carefully structured device that manages to convert some of its parts to a new use. When we learn a new skill, we recycle some of our old primate brain circuits—insofar, of course, as those circuits can tolerate the change.

A Reader’s Guide

In forthcoming chapters, I will explain how neuronal recycling can account for literacy, its mechanisms in the brain, and even its history. In the first three chapters, I analyze the mechanisms of reading in expert adults. Chapter 1 sets the stage by looking at reading from a psychological angle: how fast do we read, and what are the main determinants of reading behavior? In chapter 2, I move to the brain areas at work when we read, and how they can be visualized using modern brain imaging techniques. Finally, in chapter 3, I come down to the level of single neurons and their organization into the circuits that recognize letters and words.

I tackle my analysis in a resolutely mechanical way. I propose to expose the cogwheels of the reader’s brain in much the same way as the Reverend Paley suggested we dismantle the watch abandoned on the heath. The reader’s brain will not, however, reveal any perfect clockwork mechanics designed by a divine watchmaker. Our reading circuits contain more than a few imperfections that betray our brain’s compromise between what is needed for reading and the available biological mechanisms. The peculiar characteristics of the primate visual system explain why reading does not operate like a fast and efficient scanner. As we move our eyes across the page, each word is slowly brought into the central region of our retina, only to be exploded into a myriad of fragments that our brain later pieces back together. It is only because these processes have become automatic and unconscious, thanks to years of practice, that we are under the illusion that reading is simple and effortless.

The reading paradox expresses the indisputable fact that our genes have not evolved in order to enable us to read. My reasoning in the face of this enigma is quite simple. If the brain did not evolve for reading, the opposite must be true: writing systems must have evolved within our brain’s constraints. Chapter 4 revisits the history of writing in this light, starting with the first prehistoric symbols and ending with the invention of the alphabet. At each step, there is evidence of constant cultural tinkering. Over many millennia, the scribes struggled to design words, signs, and alphabets that could fit the limits of our primate brain. To this day, the world’s writing systems still share a number of design features that can ultimately be traced back to the restrictions imposed by our brain circuits.

Continuing on the idea that our brain was not designed for reading, but recycles some of its circuits for this novel cultural activity, chapter 5 examines how children learn to read. Psychological research concludes that there are not many ways to convert a primate brain into that of an expert reader. This chapter explores in some detail the only developmental trajectory that appears to exist. Schools might be well advised to exploit this knowledge to optimize the teaching of reading and mitigate the dramatic effects of illiteracy and dyslexia.

I will also go on to show how a neuroscientific approach can shed light on the more mysterious features of reading acquisition. For instance, why do so many children often write their first words from right to left? Contrary to the accepted idea, these mirror inversion errors are not the first signs of dyslexia, but a natural consequence of the organization of our visual brain. In a majority of children, dyslexia relates to another, quite distinct anomaly in processing speech sounds. The description of the symptoms of dyslexia, their cerebral bases, and the most recent discoveries concerning its genetic foundations are covered in chapter 6, while chapter 7 provides an insight into what mirror errors can tell us about normal visual recognition.

Finally, in chapter 8, I will return to the astonishing fact that only our species is capable of cultural inventions as sophisticated as reading—a unique feat, unmatched by any other primate. In total opposition to the standard social science model, where culture gets a free ride on a blank-slate brain, reading demonstrates how culture and brain organization are inextricably linked. Throughout their long cultural history, human beings progressively discovered that they could reuse their visual systems as surrogate language inputs, thus arriving at reading and writing. I will also briefly discuss how other major human cultural traits could be submitted to a similar analysis. Mathematics, art, music, and religion might also be looked on as evolved devices, shaped by centuries of cultural evolution, that have encroached on our primate brains.

One last enigma remains: if learning exists in all primates, why is Homo sapiens the only species with a sophisticated culture? Although the term is sometimes applied to chimpanzees, their “culture” barely goes beyond a few good tricks for splitting nuts, washing potatoes or fishing ants with a stick—nothing comparable to the seemingly endless human production of interlocking conventions and symbols systems, including languages, religions, art forms, sports, mathematics or medicine. Nonhuman primates can slowly learn to recognize novel symbols such as letters and digits—but they never think of inventing them. In my conclusion, I propose some tentative ideas on the singularity of the human brain. The uniqueness of our species may arise from a combination of two factors: a theory of mind (the ability to imagine the mind of others) and a conscious global workspace (an internal buffer where an infinite variety of ideas can be recombined). Both mechanisms, inscribed in our genes, conspire to make us the only cultural species. The seemingly infinite variety of human cultures is only an illusion, caused by the fact that we are locked in a cognitive vicious circle: how could we possibly imagine forms other than those our brains can conceive? Reading, although a recent invention, lay dormant for millennia within the envelope of potentialities inscribed in our brains. Behind the apparent diversity of human writing systems lies a core set of universal neuronal mechanisms that, like a watermark, reveal the constraints of human nature.

CHAPTER 1

How Do We Read?

Written word processing starts in our eyes. Only the center of the retina, called the fovea, has a fine enough resolution to allow for the recognition of small print. Our gaze must therefore move around the page constantly. Whenever our eyes stop, we only recognize one or two words. Each of them is then split up into myriad fragments by retinal neurons and must be put back together before it can be recognized. Our visual system progressively extracts graphemes, syllables, prefixes, suffixes, and word roots. Two major parallel processing routes eventually come into play: the phonological route, which converts letters into speech sounds, and the lexical route, which gives access to a mental dictionary of word meanings.

The existence of the text is a silent existence, silent until the moment in which a reader reads it. Only when the able eye makes contact with the markings on the tablet does the text come to active life. All writing depends on the generosity of the reader.

—ALBERTO MANGUEL, THE HISTORY OF READING

At first sight, reading seems close to magical: our gaze lands on a word, and our brain effortlessly gives us access to its meaning and pronunciation. But in spite of appearances, the process is far from simple. Upon entering the retina, a word is split up into a myriad of fragments, as each part of the visual image is recognized by a distinct photoreceptor. Starting from this input, the real challenge consists in putting the pieces back together in order to decode what letters are present, to figure out the order in which they appear, and finally to identify the word.

Over the past thirty years, cognitive psychology has worked on analyzing the mechanics of reading. Its goal is to crack the “algorithm” of visual word recognition—the series of processing steps that a proficient reader applies to the problem of identifying written words. Psychologists treat reading like a computer science problem. Every reader resembles a robot with two cameras—the two eyes and their retinas. The words we read are painted onto them. They first appear only as splotches of light and dark that are not directly interpretable as linguistic signs. Visual information must be recoded in an understandable format before we can access the appropriate sounds, words, and meanings. Thus we must have a deciphering algorithm, or a processing recipe akin to automatic character recognition software, which takes the pixels on a page as input and produces the identity of the words as output. To accomplish this feat, unbeknownst to us, our brain hosts a sophisticated set of decoding operations whose principles are only beginning to be understood.

The Eye: A Poor Scanner

The tale of reading begins when the retina receives photons reflected off the written page. But the retina is not a homogeneous sensor. Only its central part, called the fovea, is dense in high-resolution cells sensitive to incoming light, while the rest of the retina has a coarser resolution. The fovea, which occupies about 15 degrees of the visual field, is the only part of the retina that is genuinely useful for reading. When foveal information is lacking, whether due to a retinal lesion, to a stroke having destroyed the central part of the visual cortex, or to an experimental trick that selectively blocks visual inputs to the fovea, reading becomes impossible.6

The need to bring words into the fovea explains why our eyes are in constant motion when we read. By orienting our gaze, we “scan” text with the most sensitive part of our vision, the only one that has the resolution needed to determine letters. However, our eyes do not travel continuously across the page.7 Quite the opposite: they move in small steps called saccades. At this very moment, you are making four or five of these jerky