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plete the connections that make up our neural circuitry (a journey of such daunting scale and precision that Bruer likens it to fi nding one’s way clear across the United States to a waiting partner on the opposite coast, not unlike Kit Carson’s mission to President Polk for General Fremont). It’s this circuitry that enables our senses, cognition, and motor skills, including learning and memory, and it is this circuitry that forms the possibilities and the limits of one’s intellectual capacity.
The number of synapses peaks at the age of one or two, at about 50 percent higher than the average number we possess as adults. A plateau period follows that lasts until around puberty, whereupon this overabundance begins to decline as the brain goes through a period of synaptic pruning. We arrive at our adult complement at around age sixteen with a stagger-ing number, thought to total about 150 trillion connections.
We don’t know why the infant brain produces an overabundance of connections or how it subsequently determines which ones to prune. Some neuroscientists believe that the connections we don’t use are the ones that fade and die away, a notion that would seem to manifest the “use it or lose it”
principle and argue for the early stimulation of as many connections as possible in hopes of retaining them for life. Another theory suggests the burgeoning and winnowing is determined by ge ne tics and we have little or no infl uence over which synapses survive and which do not.
“While children’s brains acquire a tremendous amount of information during the early years,” the neuroscientist Patricia Goldman- Rakic told the Education Commission of the States, most learning is acquired after synaptic formation stabilizes. “From the time a child enters fi rst grade, through high school, college, and beyond, there is little change in the number of synapses. It is during the time when no, or little, synapse formation occurs that most learning takes place” and we
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develop adult- level skills in language, mathematics, and logic.3
And it is likely during this period more than during infancy, in the view of the neuroscientist Harry T. Chugani, that experience and environmental stimulation fi ne- tune one’s circuits and make one’s neuronal architecture unique.4 In a 2011 article, a team of British academics in the fi elds of psychology and soci-ology reviewed the evidence from neuroscience and concluded that the architecture and gross structure of the brain appear to be substantially determined by genes but that the fi ne structure of neural networks appears to be shaped by experience and to be capable of substantial modifi cation.5
That the brain is mutable has become evident on many fronts.
Norman Doidge, in his book The Brain That Changes Itself, looks at compelling cases of patients who have overcome severe impairments with the assistance of neurologists whose research and practice are advancing the frontiers of our understanding of neuroplasticity.
One of these was Paul Bach- y-Rita, who pioneered a device to help patients who have suffered damage to sensory organs.
Bach- y-Rita’s device enables them to regain lost skills by teaching the brain to respond to stimulation of other parts of their bodies, substituting one sensory system for another, much as a blind person can learn to navigate through echolo-cation, learning to “see” her surroundings by interpreting the differing sounds from the tap of a cane, or can learn to read through the sense of touch using Braille.6
One of Bach- y-Rita’s patients had suffered damage to her vestibular system (how the inner ear senses balance and spatial orientation) that had left her so unbalanced that she was unable to stand, walk, or maintain her in de pen dence. Bach-y-Rita rigged a helmet with carpenters’ levels attached to it
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and wired them to send impulses to a postage- stamp- sized strip of tape containing 144 microelectrodes placed on the woman’s tongue. As she tilted her head, the electrodes spar-kled on her tongue like effervescence, but in distinctive patterns refl ecting the direction and angle of her head movements.
Through practice wearing the device, the woman was gradually able to retrain her brain and vestibular system, recovering her sense of balance for longer and longer periods following the training sessions.
Another patient, a thirty- fi ve- year- old man who had lost his sight at age thirteen, was outfi tted with a small video camera mounted on a helmet and enabled to send pulses to the tongue.
As Bach- y-Rita explained, the eyes are not what sees, the brain is. The eyes sense, and the brain interprets. The success of this device relies on the brain learning to interpret signals from the tongue as sight. The remarkable results were reported in the New York Times: The patient “found doorways, caught balls rolling toward him, and with his small daughter played a game of rock, paper and scissors for the fi rst time in twenty years. [He] said that, with practice, the substituted sense gets better, ‘as if the brain were rewiring itself.’ ”7
In yet another application, interesting in light of our earlier discussions of metacognition, stimulators are being attached to the chests of pi lots to transmit cockpit instrument readings, helping the brain to sense changes in pitch and altitude that the pi lot’s vestibular system is unable to detect under certain fl ight conditions.
Neural cell bodies make up most of the part of our brains that scientists call the gray matter. What they call the white matter is made up of the wiring: the axons that connect to dendrites of other neural cell bodies, and the waxy myelin sheaths in which
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some axons are wrapped, like the plastic coating on a lamp cord. Both gray matter and white matter are the subject of intense scientifi c study, as we try to understand how the components that shape cognition and motor skills work and how they change through our lives, research that has been greatly advanced by recent leaps in brain imaging technology.
One ambitious effort is the Human Connectome Project, funded by the National Institutes of Health, to map the connections in the human brain. (The word “connectome” refers to the architecture of the human neurocircuitry in the same spirit that “genome” was coined for the map of the human ge ne tic code.) The websites of participating research institutions show striking images of the fi ber architecture of the brain, masses of wire- like human axons presented in neon colors to denote signal directions and bearing an uncanny resemblance to the massive wiring harnesses inside 1970s super-computers. Early research fi ndings are intriguing. One study, at the University of California, Los Angeles, compared the synaptic architecture of identical twins, whose genes are alike, and fraternal twins, who share only some genes. This study showed what others have suggested, that the speed of our mental abilities is determined by the robustness of our neural connections; that this robustness, at the initial stages, is largely determined by our genes, but that our neural circuitry does not mature as early as our physical development and instead continues to change and grow through our forties, fi fties, and sixties. Part of the maturation of these connections is the gradual thickening of the myelin coating of the axons. Myelination generally starts at the backs of our brains and moves toward the front, reaching the frontal lobes as we grow into adulthood. The frontal lobes perform the executive functions of the brain and are the location of the pro cesses of high- level
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reasoning and judgment, skills that are developed through experience.
The thickness of the myelin coating correlates with ability, and research strongly suggests that increased practice builds greater myelin along the related pathways, improving the strength and speed of the electrical signals and, as a result, per for mance. Increases in piano practice, for example, have shown correlated increases in the myelination of nerve fi bers associated with fi nger movements and the cognitive pro cesses that are involved in making music, changes that do not appear in nonmusicians.8
The study of habit formation provides an interesting view into neuroplasticity. The neural circuits we use when we take conscious action toward a goal are not the same ones we use when our actions have become automatic, the result of habit.
The actions we take by habit are directed from a region located deeper in the brain, the basal ganglia. When we engage in extended training and repetition of some kinds of learning, notably motor skills and sequential tasks, our learning is thought to be recoded in this deeper region, the same area that controls subconscious actions such as eye movements.
As a part of this pro cess of recoding, the brain is thought to chunk motor and cognitive action sequences together so that they can be performed as a single unit, that is, without requiring a series of conscious decisions, which would substantially slow our responses. These sequences become refl exive. That is, they may start as actions we teach ourselves to take in pursuit of a goal, but they become automatic responses to stimuli. Some researchers have used the word “macro” (a simple computer app) to describe how this chunking functions as a form of highly effi cient, consolidated learning. These theories about chunking as integral to the pro cess of habit formation
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help explain the way in sports we develop the ability to respond to the rapid- fi re unfolding of events faster than we’re able to think them through, the way a musician’s fi nger movements can outpace his conscious thoughts, or the way a chess player can learn to foresee the countless possible moves and implications presented by different confi gurations of the board.
Most of us display the same talent when we type.
Another fundamental sign of the brain’s enduring mutability is the discovery that the hippocampus, where we consolidate learning and memory, is able to generate new neurons throughout life. This phenomenon, called neurogenesis, is thought to play a central role in the brain’s ability to recover from physical injury and in humans’ lifelong ability to learn. The relationship of neurogenesis to learning and memory is a new fi eld of inquiry, but already scientists have shown that the activity of associative learning (that is, of learning and remembering the relationship between unrelated items, such as names and faces) stimulates an increase in the creation of new neurons in the hippocampus. This rise in neurogenesis starts before the new learning activity is undertaken, suggesting the brain’s intention to learn, and continues for a period after the learning activity, suggesting that neurogenesis plays a role in the consolidation of memory and the benefi cial effects that spaced and effortful retrieval practice have on long- term retention.9
Of course, learning and memory are neural pro cesses. The fact that retrieval practice, spacing, rehearsal, rule learning, and the construction of mental models improve learning and memory is evidence of neuroplasticity and is consistent with scientists’ understanding of memory consolidation as an agent for increasing and strengthening the neural pathways by which one is later able to retrieve and apply learning. In the words
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of Ann and Richard Barnet, human intellectual development is “a lifelong dialogue between inherited tendencies and our life history.”10 The nature of that dialogue is the central question we explore in the rest of this chapter.
Is IQ Mutable?
IQ is a product of genes and environment. Compare it to height: it’s mostly inherited, but over the de cades as nutrition has improved, subsequent generations have grown taller.
Likewise, IQs in every industrialized part of the world have shown a sustained rise since the start of standardized sampling in 1932, a phenomenon called the Flynn effect after the po liti cal scientist who fi rst brought it to wide attention.11 In the United States, the average IQ has risen eigh teen points in the last sixty years. For any given age group, an IQ of 100
is the mean score of those taking the IQ tests, so the increase means that having an IQ of 100 today is the intelligence equivalent of those with an IQ 60 years ago of 118. It’s the mean that has risen, and there are several theories why this is so, the principal one being that schools, culture (e.g., tele vision), and nutrition have changed substantially in ways that affect people’s verbal and math abilities as mea sured by the subtests that make up the IQ test.
Richard Nisbett, in his book Intelligence and How to Get It, discusses the pervasiveness of stimuli in modern society that didn’t exist years ago, offering as one simple example a puzzle maze McDonald’s included in its Happy Meals a few years ago that was more diffi cult than the mazes included in an IQ test for gifted children.12 Nisbett also writes about “environmental multipliers,” suggesting that a tall kid who goes out for basketball develops a profi ciency in the sport that a shorter kid with the same aptitudes won’t develop, just as a
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curious kid who goes for learning gets smarter than the equally bright but incurious kid who doesn’t. The options for learning have expanded exponentially. It may be a very small ge ne tic difference that makes one kid more curious than another, but the effect is multiplied in an environment where curiosity is easily piqued and readily satisfi ed.
Another environmental factor that shapes IQ is socioeconomic status and the increased stimulation and nurturing that are more generally available in families who have more resources and education. On average, children from affl uent families test higher for IQ than children from impoverished families, and children from impoverished families who are adopted into affl uent families score higher on IQ tests than those who are not, regardless of whether the birth parents were of high or low socioeconomic status.
The ability to raise IQ is fraught with controversy and the subject of countless studies refl ecting wide disparities of scientifi c rigor. A comprehensive review published in 2013 of the extant research into raising intelligence in young children sheds helpful light on the issue, in part because of the strict criteria the authors established for determining which studies would qualify for consideration. The eligible studies had to draw from a general, nonclinical population; have a randomized, experimental design; consist of sustained interventions, not of one- shot treatments or simply of manipulations during the testing experience; and use a widely accepted, standardized mea sure of intelligence. The authors focused on experiments involving children from the prenatal period through age fi ve, and the studies meeting their requirements involved over 37,000 participants.
What did they fi nd? Nutrition affects IQ. Providing dietary supplements of fatty acids to pregnant women, breast- feeding women, and infants had the effect of increasing IQ by any-
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where from 3.5 to 6.5 points. Certain fatty acids provide building blocks for nerve cell development that the body cannot produce by itself, and the theory behind the results is that these supplements support the creation of new synapses. Studies of other supplements, such as iron and B complex vitamins, strongly suggested benefi ts, but these need validation through further research before they can be considered defi nitive.
In the realm of environmental effects, the authors found that enrolling poor children in early education raises IQ by more than four points, and by more than seven if the intervention is based in a center instead of in the home, where stimulation is less consistently sustained. (Early education was defi ned as environmental enrichment and structured learning prior to enrollment in preschool.) More affl uent children, who are presumed to have many of these benefi ts at home, might not show similar gains from enrolling in early education programs. In addition, no evidence supports the widely held notion that the younger children are when fi rst enrolled in these programs the better the results. Rather, the evidence suggests, as John Bruer argues, that the earliest few years of life are not narrow windows for development that soon close.
