During the past year, a flood of articles in popular and professional publications have discussed the implications of brain science for education and child development. Although we should consider ideas and research from other fields for our professional practice, we must assess such ideas critically. This is particularly true when we look at how a vast, complex field like brain science might improve classroom instruction.
Three big ideas from brain science figure most centrally in the education literature, and educators should know four things about these ideas to make their own critical appraisals of brain-based education. My own assessment of recent articles about brain research is that well-founded educational applications of brain science may come eventually, but right now, brain science has little to offer education practice or policy (Bruer, 1997, 1998).
Three big ideas arise from brain science: (1) Early in life, neural connections (synapses) form rapidly in the brain; (2) Critical periods occur in development; and (3) Enriched environments have a pronounced effect on brain development during the early years. Neuroscientists have known about all three big ideas for 20 to 30 years. What we need to be critical of is not the ideas themselves, but how they are interpreted for educators and parents.
Early Synapse Formation
Synapses are the connections through which nerve impulses travel from one neuron to another. Since the late 1970s, neuroscientists have known that the number of synapses per unit volume of tissue (the synaptic density) in the brain's outer cortical layer changes over the life span of monkeys and humans (Goldman-Rakic, Bourgeois, & Rakic, 1997; Huttenlocher & Dabholkar, 1997; Rakic, Bourgeois, & Goldman-Rakic, 1994). Not surprisingly, human newborns have lower synaptic densities than adults. However, in the months following birth, the infant brain begins to form synapses far in excess of the adult levels. In humans, by age 4, synaptic densities have peaked in all brain areas at levels 50 percent above adult levels. Throughout childhood, synaptic densities remain above adult levels. Around the age of puberty, a pruning process begins to eliminate synapses, reducing synaptic densities to adult, mature levels.
The timing of this process appears to vary among brain areas in humans. In the visual area of the human brain, synaptic densities increase rapidly starting at 2 months of age, peak at 8 to 10 months, and then decline to adult levels at around 10 years. However, in the human frontal cortex—the brain area involved in attention, short-term memory, and planning—this process begins later and lasts longer. In the frontal cortex, synaptic densities do not stabilize at mature levels until around age 16. Thus, we can think of synaptic densities changing over the first two decades of life in an inverted-U pattern: low at birth, highest in childhood, and lower in adulthood.
This much is neuroscientific fact. The question is, What does this inverted-U pattern mean for learning and education? Here, despite what educators might think, the neuroscientists know relatively little. In discussing what the changes in synaptic density mean for behavior and learning, neuroscientists cite a small set of examples based on animal research and then extrapolate these findings to human infants. On the basis of observations of changes in motor, visual, and memory skills, neuroscientists agree that basic movement, vision, and memory skills first appear in their most primitive form when synaptic densities begin their rapid increase. For example, at age 8 months, when synapses begin to increase rapidly in the frontal brain areas, infants first show short-term memory skills for places and objects. Infants' performance on these tasks improves steadily over the next four months. However, performance on these memory tasks does not reach adult levels until puberty, when synaptic densities have decreased to adult levels.
Thing to Know No. 1: Neuroscience suggests that there is no simple, direct relationship between synaptic densities and intelligence. Increases in synaptic densities are associated with the initial emergence of skills and capacities, but these skills and capacities continue to develop after synaptic densities decrease to adult levels. Although early in infancy we might have the most synapses we will ever have, most learning occurs later, after synaptic densities decrease in the brain. Given the existence of the U-shaped pattern and what we observe about our own learning and intelligence over our life spans, we have no reason to believe, as we often read, that the more synapses we have, the smarter we are. Nor do existing neuroscientific studies support the idea that the more learning experiences we have during childhood, the more synapses will be "saved" from pruning and the more intelligent our children will be.
Neuroscientists know very little about how learning, particularly school learning, affects the brain at the synaptic level. We should be skeptical of any claims that suggest they do. For example, we sometimes read that complex learning situations cause increased "neural branching" that offsets neural pruning. As far as we know, such claims are based more on brain fiction than on brain science.
Critical Periods in Development
Research on critical periods has been prominent in developmental neurobiology for more than 30 years. This research has shown that if some motor, sensory, and (in humans) language skills are to develop normally, then the animal must have certain kinds of experience at specific times during its development.
The best-researched example is the existence of critical periods in the development of the visual system. Starting in the 1960s, David Hubel, Torsten Wiesel, and their colleagues showed that if during the early months of life, cats or monkeys had one eyelid surgically closed, the animal would never regain functional use of that eye when it was subsequently reopened (Hubel, Wiesel, & LeVay, 1977). They also showed that closing one eye during this time had demonstrable effects on the structure of the visual area in the animal's brain. However, the same or longer periods of complete visual deprivation in adult cats had no effect on either the animal's ability to use the eye when it was reopened or on its brain structure. Only young animals, during a critical period in their development, were sensitive to this kind of deprivation. They also found that closing both eyes during the critical period had no permanent, long-term effects on the animals' vision or brain structure.
Finally, they found that in monkeys, "reverse closure" during the critical period—opening the closed eye and closing the open eye—allowed a young deprived animal to recover the use of the originally deprived eye. If the reverse suturing was done early enough in the critical period, recovery could be almost complete. These last two findings are seldom mentioned in popular and educational interpretations of critical-period research.
Over the past three decades, hundreds of neuroscientists have advanced our understanding of critical periods. We should be aware of three conclusions about critical periods that these scientists generally endorse. First, the different outcomes of closing one eye, both eyes, and reverse suturing suggest that it is not the amount of stimulation that matters during a critical period. If only the amount mattered, closing both eyes should have the same effect on each eye as it had when only one eye was closed. Neuroscientists believe that what matters during critical periods in the development of the visual system is the balance and relative timing of stimulation to the eyes. What does this mean? For one thing, it means that more stimulation during the critical period does not necessarily result in a better-developed visual system.
Second, neuroscientists have learned that critical periods are quite complex and that different critical periods exist for different specific functions (Daw, 1995). For example, within the visual system are different critical periods for visual acuity, binocular function, and depth perception. For humans, even in an early developing system like vision, these periods can last until early childhood. For language, the critical period for learning phonology—learning to speak without an accent—ends in early childhood, but the critical period for learning a language's grammar does not end until around age 16.
Neuroscientists now also think that for each specific function of a sensory system, like vision, there are three distinct phases within the critical period. First, there is a time of rapid change during which a function, like depth perception, quickly matures. During the second phase, sensory deprivation can result in deterioration or loss of that function. After the period of sensitivity to deprivation, there seems to be yet a third phase of the critical period. During this phase, the system retains sufficient plasticity to compensate for deprivation and regains near-normal function if appropriate sensory experince occurs.
Given these complexities, neuro-scientists know that it makes little sense to speak of a critical period for vision or for any other sensory system, let alone of a critical period for brain development. Critical periods are simply windows of learning opportunity that open and then slam shut.
Finally, neuroscientists are beginning to understand why critical periods exist and why critical periods have adaptive value for an organism. They believe that as the result of evolutionary processes, highly sensitive neural systems, like vision, have come to depend on the presence of environmental stimuli to fine-tune their neural circuitry.
Relying on the environment to fine-tune the system results in neural circuits that are more sensitively tuned than they ever could be if they were hard-wired by genetic programs at birth. Relying on the presence of certain kinds of stimuli just at the right times would seem to be a highly risky developmental strategy, especially for a system like vision that is fundamental to survival. The reason it is not risky is that the kinds of stimuli needed during critical periods—patterned visual input, the ability to move and manipulate objects, noises, the presence of speech sounds—are ubiquitously and abundantly present in any normal human environment. Nature has made a bet that the stimuli will be present, but nature has placed its money on an almost sure thing. The brain expects certain kinds of stimuli to be present for normal development, and they almost always are, unless a child is abused to the point of being raised in a deprivation chamber. William Greenough and his colleagues (1992) have characterized the kind of brain modification that occurs as a result of critical periods "experience-expectant brain plasticity."
Thing to Know No. 2: If critical periods are a result of our evolutionary history and nature's bet on almost sure things occurring in a child's environment, then neuro-scientific research on critical periods is likely to have little relevance to formal education. From what we know to date about critical periods, they contribute to the development of basic specieswide abilities, like vision, hearing, or language. For this reason, despite what we read, the specifics of home or preschool environments matter little, if at all, to how children's sensory and motor systems develop.
For similar reasons, critical periods say little about formal education. Formal schooling instructs children about the social and cultural particulars, not about evolution-based, specieswide skills and behaviors. Currently, we have no reason to think that there are critical periods for the acquisition of culturally and socially transmitted skills, like reading, mathematics, or music, just to name a few of the favorite examples. As far as we know, people can acquire these skills at any age; can benefit from instruction at any age; and can increase their intelligence and expertise, given the right opportunities, at any age (Greenough, 1997).
The Effects of Enriched Environments
Neuroscientists have been studying the effects of enriched environments on rats' behavior and brain development for nearly 50 years. Some of the best and most current of this work is that of Greenough and his colleagues at the University of Illinois (1992). In this research, neuroscientists study how raising rats in different environments affects their brain structure. Typically, scientists study the effects of two contrasting environments. Some rats are raised alone in small cages with only food and water available. This "isolated environment" is the typical laboratory environment for a rat.
Other rats are raised in large, group cages that also contain novel objects and obstacles. Greenough calls these environments complex, rather than enriched. He points out that complex environments are enriched only in comparison with a rat's typical lab cage. Neuroscientists use complex environments to mimic the rats' wild or natural environment. They are not special, accelerated rodent learning environments. One should not think of them as high-quality infant care or Head Start for rats. One should think of them more as attempts to create New York City subway tunnel conditions in the laboratory.
In electron microscopic studies, Greenough and his colleagues found that young rats raised in complex environments have 25 percent more synapses per neuron in the visual areas of their brains than do rats raised in isolation. However, increases in synapses per neuron ratios do not occur to this extent in all brain areas, and some brain areas show no effects of complex rearing at all. On the basis of this research, we can see that it is definitely not the case, as we often read, that complex environments result in a 25 percent increase in brain connectivity.
More important, however, 15 years ago, Greenough and his colleagues established that the brains of adult rats also form new synapses in response to complex environments. Other studies in monkeys and humans have definitively established that the adult brain remains highly plastic and capable of extensive neural reorganization throughout life. The brain's ability to reorganize itself in response to new experiences is what makes it possible for us to learn throughout our lives. The ability of the mature brain to change and reorganize, a finding seldom mentioned in the education literature, is a new, exciting finding of brain science (Nelson & Bloom, 1997).
Thing to Know No. 3: Research on complex environments and related findings tells us that the brain can reorganize itself for learning throughout our lifetimes. This new insight runs counter to our current fixation on early development and critical periods. However, in thinking about how this research relates to educational practice and policy, we must be careful not to confuse complex with enriched. Neuroscientists use complex as a descriptive term for a laboratory simulation of a wild or natural environment. Education writers tend to use enriched as a value-laden term. In the popular and education literature, enriched environments tend to be culturally preferred, middle-class environments. These environments tend to include things that the writers value—Mozart, piano lessons, playing chess—and to exclude things that they scorn—video games, MTV, shooting pool. These writers tend to identify enriched environments with Cambridge, Massachusetts, and Palo Alto, California, and deprived environments with Roxbury and East Palo Alto.
As far as neuroscience goes, all these activities and environments are equally complex—and neuroscience says nothing about which are more or less enriched than others. In assessing claims about environments and the brain, we should be aware of how easy it is to slide from describing complexity to prescribing enrichment. We should be careful not to use neuroscience to provide biological pseudo-argument in favor of our culture and our political values and prejudices.
Educators should know one final thing.
Thing to Know No. 4: Research on early synapse formation, critical periods, and complex environments has a long history. Yet, we have little understanding of what this research might mean for education. Our appeals to this research are often naive and superficial. Other brain-related themes popular in the education literature—emotional intelligence, the social brain, the brain in the entire body, the intelligent immune system, down-shifting—have a much less reliable grounding in neuroscience. Educators seeking to base practice on the best science might want to assess recommendations stemming from these ideas even more carefully and critically.
