1 INTRODUCTION
If you query Google about “rewiring your brain,” its autocomplete function will give you a list of the most popular search terms using that phrase. You can, according to the results of such a search, rewire your brain for love and for happiness, to become more successful at work, and even to find meaning in your life. Scrolling down the search results brings up yet more options: rewire your brain to think positively, cultivate self-confidence, sleep better, and avoid procrastination. If the Internet is to be believed, you can rewire your brain to improve just about any aspect of your behavior, and so the power to transform your life lies in your ability to consciously change that 1.4-kilogram lump of meat inside your head.
But what does “rewiring your brain” actually mean? It refers to the concept of neuroplasticity, a very loosely defined term that simply means some kind of change in the nervous system. Just 50 years ago, the idea that the adult brain can change in any way was heretical. Researchers accepted that the immature brain is malleable, but also believed that it gradually hardens, like clay poured into a mold, into a permanently fixed structure by the time childhood has ended. It was also believed that we are born with all the brain cells we will ever have, that the brain is incapable of regenerating itself, and, therefore, that any damage or injuries it sustains cannot be fixed.
In fact, nothing could be further from the truth. The adult brain is not only capable of changing, but it does so continuously throughout life, in response to everything we do and every experience we have. Nervous systems evolved to enable us to adapt to the environment and determine the best course of action in any given situation, based on what has been learned from past experiences. This is the case not just for humans, but for all organisms that have a nervous system. That is to say, nervous systems evolved to change, and so neuroplasticity is an intrinsic and fundamental property of all nervous systems.
The adult brain is not only capable of changing, but it does so continuously throughout life, in response to everything we do and every experience we have.
The concept of neuroplasticity therefore pervades every branch of brain research, and neuroscientists take it for granted that any experiment they perform will induce some kind of change in the nervous system of the organism they are studying. Different researchers define neuroplasticity in different ways, depending on exactly which aspect of brain and behavior they are studying, and the term is so vague that it has become virtually meaningless when used alone and without further explanation of exactly what type of plastic changes are taking place. Nevertheless, the idea that we can willfully shape our brains to change ourselves is an attractive one, and so the concept has captured the public imagination.
Today, neuroplasticity is a buzzword in many different realms. “Rewire your brain” has become something of a mantra for motivational speakers and self-help gurus, and the concept is being evoked by educationalists and business managers in their attempts to enhance learning and improve leadership skills. Misconceptions abound, however, and in these contexts neuroplasticity is usually ill-defined and often misunderstood. Some believe it has miraculous healing powers, and others say they can harness it with products or New Age therapies; but such claims are often hugely exaggerated and sometimes completely unfounded.
A Brief History of Neuroplasticity
Neuroplasticity is often portrayed as a revolutionary new discovery, but the concept has existed in one form or another for over 200 years. In the early 1780s, correspondences between the Swiss naturalist Charles Bonnet and the Italian anatomist Michele Vincenzo Malacarne discuss the possibility that mental exercise can lead to brain growth, and mention various ways to test the idea experimentally. Malacarne then did so, using pairs of dogs from the same litter and pairs of birds from the same clutch of eggs. He trained one animal from each pair extensively for several years then examined their brains, and claimed that the cerebellum was significantly larger in the trained animals than in the untrained ones.
Shortly afterward, the German physician Samuel Thomas von Sömmerring entertained the idea in an influential anatomy textbook published in 1791: “Does use and exertion of mental power gradually change the material structure of the brain,” he wrote, “just as we see, for example, that much used muscles become stronger and that hard labor thickens the epidermis considerably? It is not improbable, although the scalpel cannot easily demonstrate this.”
In the early nineteenth century, Johann Spurzheim, one of the founders of phrenology, suggested that development of the mental faculties and the brain structures associated with them could be stimulated by exercise and education. And Jean-Baptiste Lamarck, an opponent of Charles Darwin who argued that evolution occurs by the inheritance of acquired characteristics, believed that specialized brain regions develop through proper use of the related faculties.1
In the 1830s, the physiologist Theodore Schwann and the botanist Matthias Schleiden developed cell theory, which stated that cells are the basic structural units of all living things. The microscopes available at the time were not powerful enough to resolve the finer details of nervous tissue, however. It was still not clear whether cell theory also applied to the nervous system, and throughout the nineteenth century there was debate about the fine structure of the brain and spinal cord. Researchers were split into two camps: the neuronists, who believed that the nervous system must, like all other living things, be made of cells, and the reticularists, who argued that it is made up of a continuous sheet of tissue.
The debate was finally settled in the 1890s, thanks largely to the work of the Spanish neuroanatomist Santiago Ramón y Cajal. Exploiting advances in microscopy and new staining methods, Cajal examined and compared nervous tissue from different species, including humans, and, being an accomplished artist, documented his observations in beautiful drawings. Drawing on his own work, and that of various others, he amassed sufficient evidence to convince the scientific community that nervous tissue is made of cells called neurons, which form contacts with each other. In so doing, he established modern neuroscience as a discipline in its own right, and today is considered to be its founding father.2
Figure 1 (A) Schematic diagram showing the main structures of a nerve cell (https://commons.wikimedia.org/wiki/Neuron#/media/File:Neuron_-_annotated.svg, CC BY-SA 3.0). (B) Pyramidal neurons from different layers and regions of the cerebral cortex, based on drawings by Cajal.
Darwin speculated about neuroplasticity in The Descent of Man, published in 1874. “I have shown that the brains of domestic rabbits are considerably reduced in bulk, in comparison with those of the wild rabbit or hare,” he wrote, “and this may be attributed to their having been closely confined during many generations, so that they have exerted their intellect, instincts, senses and voluntary movements but little.”
But the term “plasticity” first appears in an 1890 textbook called The Principles of Psychology by William James. Here, James defines plasticity as “the possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once,” and explains habit formation in terms of strengthening of synapses and the formation of new connections: “If habits are due to the plasticity of materials to outward agents, we can immediately see to what outward influences, if to any, the brain-matter is plastic... and it is to the infinitely attenuated currents that pour in through [the sensory nerve-roots] that the hemispherical cortex shows itself to be so peculiarly susceptible. The currents, once in, must find a way out. In getting out they leave their traces in the paths which they take. The only thing they can do, in short, is to deepen old paths or to make new ones.”
In 1894, Cajal suggested that plasticity occurs at the junctions between nerve cells and that mental exercise leads to growth of new nerve fiber branches. “The theory of free arborization of cellular branches capable of growing seems not only to be very probable but also most encouraging,” he said in a lecture at the Royal Society in London. “A continuous pre-established network—a sort of system of telegraphic wires with no possibility for new stations or new lines—is something rigid and unmodifiable that clashes with our impression that the organ of thought is, within certain limits, malleable... especially during the developmental period... We could say that the cerebral cortex is like a garden planted with innumerable trees—the pyramidal cells—which, thanks to intelligent cultivation, can multiply their branches and sink their roots deeper, producing fruits and flowers of ever greater variety and quality.”
Three years later, the British neurophysiologist Charles Sherrington named these junctions “synapses,” from the Greek words syn, meaning “together,” and haptein, meaning “to clasp,” and stated that synapses are probably the sites at which learning takes place. He referred explicitly to synaptic strengthening: “Shut off from all opportunities of reproducing itself, the nerve cell directs its pent-up energy towards amplifying its connections with its fellows, in response to the events which stir it up.”
Others challenged the notion that learning could induce new nerve fiber branches, pointing to evidence that there is far less variation in the size of the brain than in that of any other organ, and that brain volume appears to remain constant throughout much of life. Cajal preempted this objection by suggesting a “reciprocal diminution of the cell bodies or a shrinkage of other areas... whose function is not directly related to intelligence.”
Less than 10 years later, however, Cajal appears to have changed his mind. “Once development was ended, the founts of growth of the axons and dendrites dried up irrevocably,” he wrote in his 1913 textbook, Degeneration and Regeneration of Nervous System. “In the adult centers, the nerve paths are something fixed, ended, and immutable. Everything may die, nothing may be regenerated.” This view quickly became one of the central dogmas of neuroscience, and researchers came to the general consensus that the brain is not materially affected by learning, experience, or training.3
A Revolution in Modern Neuroscience
This dogma persisted well into the mid-twentieth century. In the early 1960s, however, the physiologists David Hubel and Torsten Wiesel made a series of seminal discoveries about how sensory experiences affect the developing brain, and the neuroscientist Paul Bach-y-Rita provided evidence that the adult human brain is not so fixed after all, using a “sensory substitution” device that enabled blind people to “see” with their sense of touch. Several other researchers reported that they had seen new cells being born in the brains of adult animals of various species, but were largely ignored, or ridiculed.
Then, in 1973, Tim Bliss and Terje Lømo reported the discovery of long-term potentiation (LTP), a physiological mechanism by which synapses could be strengthened for prolonged periods of time. This was another seminal discovery. Today, synaptic modification is widely regarded as the cellular basis of learning and memory, and as such, LTP is by far the most intensively studied and best understood mode of neuroplasticity. Since the initial discovery, researchers have accumulated a wealth of knowledge about the molecular mechanisms underlying LTP and related processes. Ironically, though, the work tells us very little about how learning and memory might be enhanced.
In the late 1990s, more direct evidence for neuroplasticity emerged, with the discovery of neural stem cells in the adult brain. This, more than anything, convinced the scientific community: the consensus shifted once again, and neuroplasticity was hailed as a revolutionary new discovery that overturned everything we thought we knew about the brain. Now, with more advanced technologies at their disposal, neuroscientists can visualize the brain in unprecedented detail and manipulate neuronal activity with great precision. These new methods have uncovered numerous other modes of neuroplasticity and also elucidated some of the underlying mechanisms.
Neuroplasticity can be seen in various forms at every level of nervous system organization, from the lowest levels of molecular activity and the structure and function of individual cells, through intermediate levels of discrete populations of neurons and widespread neuronal networks, to the highest level of brain-wide systems and behavior. Some occur continuously throughout life, others only at specific periods of life, and different types can be both induced separately and together.
Neuroplasticity can be seen in various forms at every level of nervous system organization, from the lowest levels of molecular activity to the highest level of brain-wide systems and behavior.
Broadly speaking, there are two main types of neuroplasticity. Functional plasticity involves changes in some physiological aspect of nerve cell function, such as the frequency of nervous impulses or the probability of release of a chemical signal—both of which act to make synaptic connections stronger or weaker—or changes to the degree of synchronicity among populations of cells. Structural plasticity includes volumetric changes in discrete brain regions and the formation of new neural pathways, brought about either by the formation of new nerve fiber branches and synapses or by the growth and addition of new cells.
These different modes of plasticity occur over a wide range of timescales. Modification of synapses can occur on a timescale of milliseconds, synapses and dendrite branches are created or destroyed in the space of several hours, and new cells may be born or killed over periods of days. Other forms of neuroplasticity occur over even longer time frames—for example, brain maturation involves a protracted period of heightened plasticity that persists from late childhood into early adulthood, and losing one’s sense of sight or hearing or sustaining brain damage induces gradual changes that occur in subsequent weeks, months, and years.