Sections include: perspectives on "normality," taking advantage of new findings and new thinking about the brain, disorders in the adult brain, thinking about memory, the brain at midlife and beyond, habits for long-term health, the myth of the older brain, grateful aging
You might expect that, after spending 20 or so years in continuous self-construction, the adult brain would rest on its laurels for two or three decades. And so it could, if you were to let it, but the years from your early 20s through your 50s are called the prime of life for a good reason. Your fully formed brain is prepared to take in stride anything you care to challenge it with, short of toxins, traumas, and unhealthy stress.
If the hallmark of the child’s brain is “wiring up,” and the greatest feature of the adolescent brain is frontal lobe development, the most important trait the brain brings to adulthood and through the end of life can be summed up in one word, plasticity. Plasticity is the term neuroscientists have coined to describe the brain’s biological adaptations in response to new experiences or change. Scientists say the word in tones of awe in the context of child development, hope in the context of brain damage, frustration when the subject is certain mental and nervous system disorders, and admiration in the context of normal adult life and aging. It is plasticity that underlies our transit of all life’s major passages.
Plasticity allows us to learn, to form new habits, to adjust to new circumstances—whether as simple as remembering to make enough morning coffee for two after marriage, or as complicated as learning to use information technology when your employer decides to carve out a place in the “new economy.” Consider how nearly limitless may be the number of new things an adult brain must contend with: the first full-time, permanent job, and then, in future years, career changes; marriage, the birth and rearing of children, shopping and obtaining a mortgage for, and getting used to, a new home, and then doing it all over again in future years; helping offspring with schoolwork that looks nothing like what you remember, using tools that didn’t exist when you were a student; forming new circles of friends and colleagues from time to time and acquiring the social behavior common to those circles; taking adult classes in new topics, picking up a hobby, traveling to another country with your rusty high school foreign language skills; reordering your days or seizing on a new occupation for your retirement. Effortlessly (or sometimes not), you modify everyday routines to reflect the changing world around you and to seek out and involve yourself in new experiences. But everything would remain forever raw, difficult, and uncertain, day after day, if you could not count on your brain to swing into action through a steady stream of change and make it all familiar and easy.
Your brain allows you to become familiar with new circumstances through the process of “habituation,” in which its response to a sensory stimulus (pictures, music, the feel of a new pair of shoes) gradually decreases in intensity as the stimulus continues. In general, the brain is primed to focus on what changes, rather than what remains in a steady state. This is why, for example, city dwellers can truthfully say they don’t hear the 24-hour traffic roaring past their windows but may be awakened during a night in the country by a single cicada. Being alert to new or unexpected sensations is not only essential for our survival, it’s thrifty as well. By turning down the volume of signaling in response to things that happen steadily and consistently, the brain sets a fine example of energy conservation. At the same time, though, the brain invests considerable resources in paying attention to novelty and change, especially in the realm of sound. As if to underline once again the importance of this function, the brain carries it out by a circuit that redirects our attention to strange noises involuntarily, eliminating any hesitation or choice in the matter.
The brain also reallocates the precious resources of space and energy when a stimulus that once was novel becomes familiar; for example, recognizing the faces of people we have met recently is a job handled primarily in the frontal lobes, but once we’ve recognized them several times, the job is distributed along a larger neural circuit that even recruits some visual areas at the back of the brain for memory storage. And some well-practiced skills stake out additional territory in the brain, as has been seen in musicians: the cerebellum, which is thought to control a variety of sensory, motor, and cognitive functions, has a volume about 5 percent larger in musicians than in nonmusicians of the same age.
Perspectives on “Normality”
Given the reasonable constraints on opening the skulls of people who are alive and well, it is not a simple matter for scientists to find out how the normal brain carries out even the most ordinary tasks. The more expertly and unobtrusively the brain performs a particular function, the more difficult it is for researchers to track down all its component tasks to the brain sites, synapses, or signals where these tasks originate. For instance, attention, a faculty that seems to run on its own without any mental direction from us, is actually the output of a wide-ranging network that is partly built into the brain before birth and partly developed with deliberate effort by the individual. The ability to sustain attention is only one element of this feature, yet children attain it only after years of effort. The ability to shift attention from one object or task to another develops even more slowly, and the feat of not paying attention, of ignoring potential distractions, relies on a sophisticated filtering circuit that must be unique for each person and must even be able to change moment by moment as the person’s circumstances require.
With memory, too, it’s easy for us to think of a great many capabilities as a seamless (although very extensive) whole, but the brain doesn’t really work this way. The main aspects of memory will be described later in this chapter; suffice it to say here that remembering who Grandma is, how she looks, her phone number, when you’re going to visit her again, howshe makes that special noodle pudding, and how it smells when it comes out of the oven are all memories that draw some of their information from a wide variety of brain sites. This means researchers must focus their investigations very narrowly in order to be sure they’re looking at exactly what they want to observe rather than at their research target obscured by other features. The result of narrowing the research focus can sometimes be a study that seems geared toward proving the obvious—for instance, that it hurts to dip your fingers into extremely hot water. But the goal of such a study is simply to clear the ground for the real inquiry: such as, does the anticipation of pain reduce or increase the actual pain when it occurs? Before investigators can produce, say, functional magnetic resonance imaging (fMRI) scans that address this question, they must have images that establish a “baseline”—in this case, brain images of the volunteers’ reactions during the first dip, before they knew the water would be painfully hot.
The normal brain at work can also be studied in terms of any of the innumerable functions it carries out each moment: directing the body’s movements, for example, or selectively inhibiting them; producing language or interpreting someone else’s utterances; retrieving a long-stored memory and actively bringing it into association with a new experience. While researchers continue their efforts to add detail to the composite picture of the normal brain, each finding must be evaluated carefully in terms of its own particular, and necessarily limited, perspective.
Generally, the combination of two or more perspectives gives clinicians and scientists a working consensus on what the normal brain looks like. At the same time, however, the consensus view must allow for considerable variation. Just to take one example, normal brains are not all the same size. The average male brain is larger than the average female brain, and this difference is generally thought to correspond to the larger muscle mass of the average male, which requires additional nerve cells and signal-transmitting fibers. Brain size can also vary greatly among individuals and is not an index of intelligence. It is not the volume of the brain but its large surface area—the famous wrinkles and folds of the cerebral cortex—that allow our neurons to build trillions of connections into the signaling pathways within which take place all the things that make each of us unique.
In time, neuroscience will discover all the circuitry, neurotransmitters, and hormones we use to achieve the astonishing performance of the adult brain, but two other interesting examples of what researchers are studying may give some sense of the elegant orchestration at work within this complex organ. One example is that of parenthood, a decades-long endeavor that includes in its early years exhaustion, frustration, logistical challenges, tedium, nagging fears, and occasional bewilderment—and later the stresses of piloting children through adolescence. If we were to depend only on objective, cognitive skills to be loving parents, we might choose easier ways to spend our prime years. But since survival of a species means rearing the next generation to maturity, some form of brain plasticity must help us give up our carefree ways in order to undertake this most important of all human ventures.
Scientists exploring parenthood in the animal kingdom have seen intriguing clues about how that self-surrendering plasticity might be summoned, in the activity of a hormone, oxytocin, during mating and parenting. Oxytocin is produced in the hypothalamus, the brain site that maintains the body in a steady state by regulating such features as temperature, blood pressure, thirst, and hunger. Some researchers have been studying oxytocin and a closely related hormone, vasopressin, in a small, monogamous rodent known as the prairie vole, in hopes of finding “the neuroendocrine substrates for love.” In the prairie vole, the brain’s release of oxytocin at the time of mating appears to induce the female to develop a strong preference for the one male she is with; in the male it is vasopressin that seems to encourage this “partner preference.” (Neither hormone produces this effect in another species, the montane vole, which is polygamous.) These hormones are also significant in the voles’ new parenthood. With the birth of offspring, vasopressin floods the brains of both female and male prairie voles, along with extra oxytocin in females, and both sexes start behaving like parents: cleaning, feeding (exclusively the mother’s job), and sheltering their young. With the release of oxytocin, even female montane voles show more caring behavior during this period than at any other time in their lives. This transformation appears to be possible because the distribution of receptor sites in the brain for oxytocin actually changes within 24 hours of a vole’s giving birth—plasticity at work for the survival of the species.
If it is perhaps too fanciful that hormones can ever explain how we develop parental love, it is beyond question that brain plasticity allows us to acquire expertise in a tremendous variety of areas beyond our basic survival skills. A case in point is music, which exists in all cultures, has been found in evidence going back millions of years, and yet cannot be said to be “hard-wired” into the brain like the regulation of breathing or eating or sleep. Listening to music may seem like a simple, low-key function, but it requires the participation of a number of sites throughout the brain, each performing its own specialized task. Research in this field suggests that the left hemisphere handles the perception of rhythm, while the right hemisphere focuses on pitch and the discrimination of single notes and melodies. Both the “language areas” in the left hemisphere and their right-hemisphere counterparts are active during music listening. Broca’s area, which analyzes syntax when we hear someone speaking aloud, performs a similar function with music: it analyzes harmonic sequences. Wernicke’s area, and particularly its right-hemisphere counterpart, appears to deal with temporal analysis (in music as well as in speech).
And, in yet another example of plasticity at work, a study using electroencephalograms (EEGs) revealed a subtle difference in brain activity between people with musical training and those without. Notwithstanding the work of specific brain sites, in musically trained people the left hemisphere was dominant overall during music listening, whereas in nontrained music listeners the right hemisphere was dominant.
Taking Advantage of New Findings and New Thinking About the Adult Brain
Recent research on the brain has established two great principles. First, far from remaining static in adulthood, as we had long assumed, the human brain continues to grow and develop throughout our entire life span. This development takes place in two ways: by ongoing adjustments in signaling pathways and by the addition of new brain cells. Knowing this means that you should try, as you would with any fine, high-powered machine, to practice good maintenance to give it the best chance to provide peak performance. This means faithfully practicing basic brain care: plenty of rest, good nutrition, and good health habits. (See more in “Basic Brain Care.”) But the brain offers a priceless opportunity that no man-made machine can provide: in many respects we can make a material difference in how it ages, and even induce it to perform better over time.
This is thanks to the second, equally powerful principle, that brain development in adulthood, as in childhood, is shaped largely by outside stimuli rather than by specifications within the brain cells themselves. Here is rich food for thought: beyond simply letting our brain carry us, we can consciously decide in what ways we would like our brains to grow. Just as we may choose to strengthen our muscles with challenging workouts, we can encourage brain growth by keeping engaged in many different mental activities.
Good “workouts” for the brain can be found in almost any area of life. Productive, satisfying work—whether in paid employment, volunteer programs, or a challenging hobby—provides exercise for the brain on a regular basis. Socializing with old and new friends and visiting with family in person or by long-distance communication; analyzing new information (current events, for example, or the nitty-gritty of building a retirement portfolio) in the light of what is already known; and maintaining old skills or practicing a new one (sports, gardening, bird-watching, playing a musical instrument) all stimulate the brain in various ways.
When these activities include mild physical exertion as well, the brain receives a bonus; numerous studies now show that physical exercise at all ages makes a major contribution to the overall health of the brain. This link was suggested as long ago as the mid-1960s, when scientists compared the skull development of rats raised in an “enriched environment” (including running wheels and plenty of space for exercise) with that of rats raised in stark, “impoverished” conditions. More than 30 years later, another research team showed that not just the skull but whole populations of neurons benefited from an enriched environment. Researchers have established that even after its explosive growth during gestation and early development, the brain continues throughout life to give rise to new neurons, not only in rats and mice, but in humans as well. Strikingly, when normal adult mice are housed in an environment that is more complex than the standard laboratory setting, with more living space, greater social interaction, and more physical activity, the new neurons tend to survive at a higher rate, producing more brain growth. Of all the factors in this experiment, physical activity appears to be the most important: voluntary running, on a running wheel, led to the survival of as many new neurons as all the other enrichment conditions combined.
Exactly what takes place within and between brain cells to produce this effect is not yet clear, but studies in both animal and human subjects have produced evidence for several appealing possibilities. In Hannover, Germany, a small group of research volunteers displayed a higher velocity of blood flow in the middle cerebral artery after brief exercise of the arms or legs. The brain’s metabolism, or use of energy—as measured, for example, by oxygen saturation—rose along with the velocity of blood flow. Of course, much of this additional energy goes into producing the brain signals that allow us to move our muscles; whetherit provides the wherewithal for extra cognitive work as well is not yet clear. In middle-aged rats (fourteen months old), a daily one-hour swim improved the animals’ memory, possibly by reducing the buildup of oxidatively damaged proteins in the brain. Conversely, when rats bred to run long distances are abruptly stopped from doing so, their brains show a sharp decrease in brainderived neurotrophic factor (BDNF), a substance that is crucial for the nourishment of new brain cells.
In a study that assigned previously sedentary men and women, aged 60 to 75, to one of two exercise groups for six months—one group performing aerobic exercise (walking) and the other anaerobic exercise (stretching and toning)—those carrying out the aerobic exercise showed significant improvement in “executive” (frontal cortex) skills, such as working memory, planning, and scheduling, as well as in the speed with which they could switch between executive tasks. The increased oxygen consumption that comes with aerobic fitness may be the physical basis for the improvements, but the question is still open.
Back in the laboratory, scientists are working to identify drug compounds that may support new cell growth in the brain at the molecular level; but this approach is expected to supplement, rather than substitute for, the important benefits of continually challenging the brain with new activities.
Disorders in the Adult Brain
As impressive as the adult brain is, it is not invulnerable. Almost any of the disorders to which the brain and nervous system are subject can occur in the prime of life, and some illnesses that strike in young adulthood or early middle age are a major challenge to the general picture of strength that the typical adult brain presents. The most incapacitating include schizophrenia and bipolar disorder, which often emerge with the beginning of adulthood, multiple sclerosis, which typically strikes in the forties, and Parkinson’s disease, which in many cases appears in late middle age.
Other, more common disorders typically appear in adulthood rather than in childhood or advanced ages. These particularly widespread problems—headache, migraine, back pain, depression, anxiety, and alcoholism—can all become chronic or recurring and can be major impairments or even disabilities if they are not treated seriously as medical or psychiatric problems. Yet even in these disorders, the plasticity of the adult brain has a significant role—two roles, in fact. Sometimes it contributes to the problem, and sometimes, when treatment is undertaken, it can be recruited on behalf of recovery. The new “functional” imaging techniques that allow scientists and clinicians to observe the brain at work have now produced clear pictures of physical changes in the brain that take place as patients with major depression start to respond to treatment.
Two studies published together, one using PET (positron-emission tomography) and the other SPECT (single-photon-emission computed tomography, in which the image is formed from many photos taken by a gamma camera that revolves around the patient), showed changes in glucose metabolism and in blood flow (an indirect measure of brain activity) after 6 to 12 weeks of treatment. While it is reasonable to expect that in the course of recovering from an illness the brain would undergo some changes, it is still somehow startling to see them clearly. But the most intriguing observation from these studies is that the changes associated with psychotherapy and those associated with antidepressant drugs appear very similar. Does this mean the two forms of treatment are interchangeable, that one might as well take a pill as schedule a session? This is a question beyond the range of brain imaging. When it comes to exploring how physical changes in the brain may translate into changes in an individual patient’s behavior or understanding, the research has so far just scratched the surface.
The possibility of enlisting plasticity in the treatment of disorders is one of the newest directions in neuroscience research. Hopes for its potential are implicit in most of the brain “repair” strategies being investigated for damage ranging from trauma to stroke to spinal cord injury. But, like the kindred mystery of stem cell development, the biology of plasticity is a story just beginning to be told. For example, can plasticity in the adult brain combat conditions present since childhood? In some circumstances, perhaps.
An informal report published in a major scientific journal late in 2001 reported an intriguing observation, concerning a mathematician who had been born with cataracts, opaque areas on the lens of the eye that prevent light from reaching the retina. He took eyedrops for 40 years to dilate his pupils around the cataracts, giving him limited vision. When he decided to have the cataracts removed, he invited two scientists with expertise in vision and perception to study his eyes before and after the operation. The scientists took baseline measurements before the surgery and conducted various tests for 56 days following the operation.
During one test, as the researchers moved a light slowly across the pupil, the mathematician mentioned that the light seemed brighter at the far side of his eye. Startled by the comment, the researchers turned their attention to the patient’s photoreceptors. These cells, located in the cones on the surface of the retina, usually point toward the center of pupils, where light is brightest. The researchers theorized that the mathematician’s photoreceptors might be aligned to the side, where, because of lifelong squinting, his dilated pupils had been largest before the operation. To test their theory, they took a particular measurement, called SCE-I peak function, which indicates where a light shined into someone’s pupil appears brightest. From this, they inferred the position of the mathematician’s cones. In the first ten days after surgery, the patient’s SCE-I peak function shifted to the center. In the left eye, peak function moved 1.6 mm, indicating a 4-degree shift in the cones. In the right, it moved 2.6 mm, indicating a 6.5-degree shift. Describing this unexpected adaptation by the retina, the researchers speculated that a simple feedback mechanism may control the orientation of photoreceptors in the human eye, allowing the receptors, like sunflowers in a field, to turn to the light.
Thinking About Memory
If plasticity is the name of the game in adulthood, the principal player and captain of the team is memory. In the healthy adult brain, memory is a lifelong resource that supports virtually all our cognitive abilities. Even in the most ordinary day, it is difficult to think of a single moment in which we are not using some combination of declarative memory, implicit memory, visual memory, emotional memory, and even auditory and olfactory memory, all at once. We test new experiences against what we have stored away in these memory forms, and we use memory to guide everything from routine actions to major decision making.
Human memory is a reference library in constant use, and scientific research on memory at work bears out the vast flexibility and responsiveness that we count on in this everyday function. In one study, when 20 expert chess players underwent magnetic resonance imaging while they played chess against a computer, their brains showed the most activity in the areas of long-term memory storage. By contrast, inexperienced players showed the most brain activity in a region known for analyzing new information and forming long-term memories. The experts’ brains were also seen to give off “focal gamma bursts” of signaling, a sign of memory-related activity, for several seconds after every move by the computer—a pattern explained by the theory that expert memory relies on large amounts of information available in “chunks,” rather than numerous small items dispersed throughout the brain.
To appreciate the current scientific understanding of memory in terms of everyday experience, it helps to be familiar with several different but related aspects of this function. First, whatever we recall of our childhood, of a recent presidential election, or even of yesterday’s lunch menu, is considered part of long-term memory. The small structure known as the hippocampus, deep in the middle of the brain, plays a major role in forming these memories, which are then stored diffusely throughout the brain. By contrast, a short-term memory, or working memory, such as a particular phone number that we are about to dial, is held in the hippocampus only as long as it is needed.
Second, memories can be divided another way as well. The human brain contains both declarative memory (information, descriptions, memories that can be stated verbally) and nondeclarative, or “implicit,” memory (sensations, skills, procedures, and actions that can more easily be demonstrated than described, such as how to ride a bicycle).
Third, the feat of remembering actually consists of three steps. The memory must be formed, or encoded, with a unique pattern of nerve signals; it must be stored or maintained in its original state, whether for a few hours or a lifetime; and when needed, it must be actively retrieved.
Recognizing the three steps that are involved in remembering now adds an interesting new wrinkle to the discussion of memory problems: Do difficulties tend to arise during encoding, storage, retrieval, or some combination of the three? Recent work suggests that slight problems with memory in healthy older brains are mostly due to slower processing at the retrieval stage.
None of us will recall a story or commit a name to memory in exactly the same way at age 76 as we did at age 16, because the mental context in which we perform these functions may have changed entirely over half a dozen decades. But for the most part, what we have stored from life in our minds—the experiences and emotions, perceptions and information—remains there in safekeeping. It is the ease or speed with which we’re able to retrieve something from memory that changes with the passing of time.
As early as age 20, before we have even begun to think about adapting to age-related changes in brain activity, signal transmission throughout the entire central nervous system begins to slow down very slightly, by just a few milliseconds each year, in a trend that will continue throughout adulthood. This is part of normal aging, and it takes decades to lead to a noticeable effect on everyday perceptions or movements. But the retrieval of a memory is likely to involve more signals and more intricate signal pathways, and then the numerous small slowdowns can amount to an unwelcome delay.
Not all kinds of memories are equally affected, however: oft-repeated tasks that involve both physical and mental exertion, such as playing a musical instrument, are apparently less vulnerable to this age-related slowdown. Meanwhile, although our memory for the meaning and use of language is generally very well preserved, the slowdown may cause us to wonder why we can’t remember the three or four grocery items we have just been asked to buy—when in fact the problem is not a mind full of holes but simply difficulty in processing rapid speech.
Almost everyone has a lapse of memory now and then, but often we worry more about these lapses as we grow older. Although many adults in midlife and later life fear they will eventually, and maybe inevitably, lose their memory completely, these fears are unfounded: years of study have established that the normal adult brain need not suffer a major loss of memory at any age. The notion of senility, or of an unavoidable loss of mental functions, dates back to a time when the diseases that attack such functions (Alzheimer’s disease, in particular) were impossible to diagnose in living patients; thus people with Alzheimer’s disease were counted among the healthy, and their condition was considered a form of normal aging. Today the trained clinician can usually diagnose Alzheimer’s disease at an early stage so that patients may benefit from treatment, and worried potential patients can learn about the difference between this serious disease and normal, benign age-related memory impairment.
It is true that the first stages of Alzheimer’s disease can produce lapses similar to those of age-related memory impairment in a healthy person. The essential difference is that while the Alzheimer’s patient will go on to develop more serious memory deficits, cognitive impairment, and finally dementia, memory impairment in the healthy individual will remain at the level of mere annoyance. While there is still no quick or foolproof way to distinguish between age-related memory impairment and early Alzheimer’s disease, the doctor’s general rule of thumb is this: if you take longer than before to learn a new piece of information, a new spatial structure, or a new skill, but then you remember it as well as anyone else, you do not have Alzheimer’s.
The attention paid to mild memory slips common in the second half of life tends to overshadow the more important ability of the brain to compensate. In preparing to carry out a new task, for example, older adults may miss a point or two or may need to repeat the information about the chore once or twice. But having handled many smaller, larger, and equivalent tasks before, they are often able to fill in missing details for themselves— and to bring seasoned judgment to bear on the assignment as well. A similar power of compensation fills in if the normal older brain becomes less adept at carrying out a number of mental operations simultaneously: when it becomes more difficult to do many things at once, an older person may adapt by focusing on one thing at a time. For some individuals, this can also lead to greater awareness of the present moment and appreciation for what is important and what is not. “Depth of experience” becomes the watchword with increasing years.
Nevertheless, both clinical and research scientists are looking for ways to minimize the normal age-related memory snags and to supplement the brain’s powers of compensation. The hormone estrogen, already well known for its crucial role in reproduction, has received considerable attention from brain researchers as well. Many studies have suggested estrogen may protect the brain to some degree from age-related memory decline, and may even bolster memory in the normal brain. The antioxidant vitamin E, which removes the highly reactive oxygen molecules that are believed to cause age-related damage in the brain, is also a center of interest, but the scientific jury is still out on its value in treatment or in memory enhancement. Another dietary supplement that has been proposed as a memory aid is ginkgo biloba, but the scientific study of this substance is still in progress.
The Brain at Midlife and Beyond
Like the body, the brain undergoes some predictable changes with age, but most of them are less intimidating when their basis in natural processes is understood. These natural biological processes include a small but significant decrease in the rate of cerebral blood flow from young adulthood to midlife, which may contribute in a minor way to the age-related slowing in signal transmission and a decrease in the amount of “white matter,” the nonsignaling cells that sheathe and insulate the signal-transmitting fibers of the neurons around them. Oddly, the proportion of “gray matter” (the neurons themselves) in the brain shows no significant difference for individuals under age 40 and those over age 69, hovering around 48 percent for both these groups, but it drops and then slowly rises again during the decades in between. The loss of the insulating white matter, meanwhile, probably accounts for most of the age-related slowdown. Also, sometime in midlife the DNA of the brain’s mitochondrial cells, which supply essential energy, start to show alterations that may interfere with the brain’s ability to burn as much energy as it once did in sustained intense work or in meeting an impressive number of demands all at once.
A popular belief is that large numbers of brain cells die throughout the adult years. But to paraphrase that wise old author Mark Twain, the accounts of this death have been greatly exaggerated. Recent studies suggest that neuron death is restricted in normal aging, and physical evidence for great numbers of dead neurons in otherwise healthy brains has proven very hard to find. Since actual counts of neurons cannot be performed by even the most sophisticated imaging techniques but require a tangible slice of brain tissue, such a count can be performed only once on any individual, at the time of his or her autopsy. It isn’t possible, therefore, to track one person’s neuronal population at various ages.
To measure the loss or survival of neurons, all that can be compared is the average count of one individual or group against another at autopsy. And when we take into account another finding, that the total number of neurons in the healthy brain can vary among healthy individuals by as much as 100 percent, any “average” extent of neuronal loss becomes even more fugitive. Instead, techniques such as magnetic resonance imaging are used to estimate change over time, and these show a consistent decrease in the volume of the brain after about age 40. It is likely that this “shrinkage” derives much more from the loss of white matter, as discussed above, than of neurons.
One specific area known to lose size with advancing age is the corpus callosum, the thick bundle of nerve fibers through which the brain’s two hemispheres communicate with each other. This seems logical, since insulating white matter would account for a large amount of the corpus callosum’s bulk. The gradual loss of white matter here takes a toll: as patterns of signaling shift very slightly and unpredictably, the two hemispheres lose a certain amount of electrical coherence—that is, the simultaneousness of their signaling. Just as in traffic, every driver lined up at a stoplight has a slightly different reaction time when the light turns green, and these slight differences add up to a long delay for the driver at the end of the line, so the small decrease in coherence between hemispheres can blunt the immediacy of the brain’s responses.
Some adults in midlife or later also find themselves more easily distracted from a task or a line of thought than they used to be. Does the brain’s intricate system for maintaining attention also change with age? The scientific answer to this question is both no and yes, depending on which aspect of attention we are discussing. The deliberate fixing of our attention on a specific object or task appears to be an ability that remains well into late adulthood. However, the ability to shift our attention efficiently begins to falter sometime after middle age, which may explain the annoying sense of distractability when a new demand or perception presents itself in the midst of an ongoing task. This age-related deficit must have a biological basis rather than a psychological one because it has been observed in studies of young and older adult monkeys as well as in people. It’s not that our minds wander but that they may become less nimble than before at directing various circuits and brain functions toward a new target while keeping the previous target “on hold.”
On the other hand, the ability not to pay attention, to ignore a large variety of environmental stimuli that our brains have judged unimportant—the ticking of a clock, the familiar view of a busy street, or a waving tree branch outside the window—is not permanently wired into the brain. It is a mental skill that we all had to learn in early development, and one that may lose a little of its strength in later adulthood. Losing some of the ability to filter out extra stimuli can add to the sense of being easily distracted. At the same time, the brain’s creative powers of compensation allow older adults to draw on their experience and thereby anticipate and ward off potential distractors, or to reorient themselves quickly when they return to a previous task after an interruption.
Slowing reflexes, another common complaint, may be traced to the age-related decrease in the speed of signal transmission. We worry more about eyeblink-long lags in retrieving a memory, as discussed in the previous section, than about a slight loss of speed in our movements; but the two effects arise from much the same cause, the slight pull of age on the speed of nerve signal propagation.
Another familiar age-related change occurs in sleep patterns. Sleep changes, not just because decades of rising early for school, work, child care, or all three, have left the brain ready to rebel, but because our inborn circadian rhythms seem to loosen their control somewhat with the passing of years. In general, we begin to need less sleep, but insomnia and other sleep disturbances also become more common after age 50 or so, starting at a prevalence of about 27 percent in the general population and gradually increasing to almost 40 percent after age 80. Problems such as difficulty falling asleep and repeated waking in the night have been reported in both sexes and across a number of cultures, so the condition evidently arises from biological factors at least as much as from habits or lifestyle. In people who are subject to sleep disturbances, the direct trigger may be something as straightforward as snoring (your own or that of a fellow sleeper) or periodic leg or arm movements.
Finally, although it is not as freely discussed among adults as the occasional stiff knee or lapse of memory, a very serious but avoidable health threat to the brain in later years is depression. Why this risk should increase with age is an unanswered question, but researchers are studying many possible contributing factors. The brain’s gradual loss of white matter, as discussed above, may cost most people only a small amount of speed in the transmission of nerve signals, but it may tip the balance of mood (of “affect,” to use the proper professional term) in a system already under stress. One study has linked depression with illness elsewhere in the body, and not just because of sorrow about bodily illness, but because bodily illness may place an extra physiological burden on the brain. One particular kind of illness, heart disease, does seem to bring on a higher than usual risk of depression, but again, how this works in physiological terms is not yet known. Absent any other illness, clinicians and researchers of all stripes agree that lower levels of physical exercise bring a higher risk of depression, in older adults as in all age groups. The intensity of the exercise may also matter: mild activity brings the risk down, but not by as much as intense exertion.
Menopause has often been blamed as a trigger for depression, but this does not hold true for very many women. A study of more than 1,500 women in Melbourne, Australia, showed that menopausal status had no direct effect on well-being. Factors that did have an impact were general health, relations with other people, attitudes toward aging (and menopause), and lifestyle choices, such as smoking and physical exercise. Preliminary research on the male side of the picture reaffirms that hormonal shifts do play a role, if not a solo, in the risk of depression: in older males, those with depression were found to have the lowest levels of testosterone, and treating some of these men with testosterone relieved their depression.
Habits for Long-Term Health
In the maintenance of our well-being, long-term habits and lifestyles produce real results. The choices we make and the habits we keep, day in and day out, add up to about half of what determines how well we will be in our later years. Apart from luck in the genes we receive and the prudence to avoid fatal accidents, nothing counts more toward the well-being of our brain and body than what we ingest and how active we are.
The first important choice concerns cigarette smoking. Smoking damages the brain in ways that may take decades to appear. In a study of men aged 18 to 39 and 64 to 81, heavy smoking reduced the electrical coherence of several different “wave types” of signaling between the brain’s two hemispheres. Even in the younger group, heavy smoking reduced the coherence of one type of wave. Long-term smokers were found to have significantly lower cerebral blood flow, independent of whether or not smoking had also contributed to hardening of the arteries in the brain. Smoking had a mixed effect on the consolidation of memories; low- and mid-intensity smokers (those smoking cigarettes yielding 0.8 to 1.3 mg of nicotine each) performed worse than nonsmokers on a memorization task after a half-hour delay but better than nonsmokers after a delay of one month. Long-term exposure to cigarette smoke and its by-products even makes itself felt in our ears: a study from a rural village in Kuala Lumpur revealed that long-term smoking nearly doubled the risk of some degree of hearing loss. In men aged 40 and older, the average rate of hearing impairment, about 30 percent, rose to 51 percent among smokers. Thus, for the senses and the mind as well as the body, not smoking is an obvious safeguard of well-being.
A related but less clear picture is that of alcohol use. While moderate drinking (one or two five-ounce glasses of wine per day is the usual example) has been found in a number of studies to reduce the risk of heart disease, excessive amounts of alcohol may increase the risk of brain hemorrhage. In any case, at any point along the spectrum, age is a factor in the effects of alcohol on the brain and body. Older adults have a declining ability to metabolize alcohol (to break it down into its component molecules) along with less water in the body to dilute it, allowing alcohol to act more rapidly and more intensely on the older brain. In later life, alcohol may also contribute to depression and to sleep disorders, which then take their own toll.
The health of our brain also bears the lasting effects of what we eat every day. It now appears that certain fruits and vegetables can do more than keep the body well—they may also help ward off the subtle but pervasive slowdown of nerve signaling that affects the brain in later years. In preliminary results from an animal model, spinach and an extract taken from strawberries were found to protect against the signaling slowdown; spinach was more effective overall, but in both cases the effect was substantial. The brain-friendly element in these foods may be vitamin E, an antioxidant—that is, a compound that shields cells from potentially destructive electrically charged molecules. Other studies of vitamin E, in humans as well as in animals, have found apparent protective effects. Although the formal medical jury is still out, enough evidence so far has led many doctors to admit they take vitamin E supplements for brain health, just in case.
It has often been said before, here as elsewhere, but still bears repeating: regular, habitual physical activity makes for a healthier brain. When undertaken as a conscious choice (even by rats voluntarily using a running wheel), exercise is proving in study after study to nourish brain cells, reduce the risk of many age-related brain illnesses, and generally buffer the effects of daily wear and tear on this extraordinarily busy organ.
The Myth of the Older Brain
In the last decade, the concept of “senility” has undergone a dramatic change: as the understanding of Alzheimer’s disease and the ability to diagnose it have grown, the number of new cases of “senility” has shrunk to almost nothing. This is because once the evident cases of Alzheimer’s disease and other specific disorders are weeded out, there is simply no general condition that can be called senility. Previous generations, unable to recognize the very gradual development of Alzheimer’s in healthy-looking elderly people, had to assume that what they were observing was the typical course of old age.
Dementia, too, has long had a distorted meaning as a fearful combination of insanity and feeblemindedness. But in medical discourse, dementia has nothing to do with insanity. Instead the term very specifically applies to a set of cognitive problems including memory impairment, loss of judgment, and inability to think in abstract terms, often accompanied by some changes in personality. Thus a person may be diagnosed with an ailment that may eventually bring about dementia, but no one need fear the sudden onset of dementia as a process in aging all by itself.
Neurodegenerative diseases, if they are going to occur at all, do tend to appear at a more advanced age. But although the diseases most people worry about developing in old age—Alzheimer’s, Parkinson’s, and Huntington’s diseases and ALS (amyotrophic lateral sclerosis)—take a considerable toll, they are not typical risks of aging. By current estimates, Alzheimer’s disease affects about 4 million Americans, or about 1 in 75, though it will become more common—affecting up to 1 in 20—as our population ages. Currently, Parkinson’s disease has developed in about 1 million people, a rate of fewer than 1 in 250; about 50,000 people have Huntington’s disease, 1 in 5,000; and ALS afflicts about 25,000 people, or fewer than 1 in 10,000.
For people diagnosed with these still incurable illnesses, and for everyone who cares about them, what matters is not the prevalence but where medicine and medical research stand in finding answers for them. The most encouraging news has been the demonstration that each of these illnesses almost certainly has a strong genetic basis. This offers the eventual prospect of decisive interventions, as genetic research develops into a more precise and systematic hunt for disease processes and points at which to interrupt those processes than has ever been possible. (It is important to remember that for most diseases a certain combination of genes does not mean an illness will definitely appear; it means only that a person may have more of a chance of developing it. Most of these diseases need other as yet unknown, biological events or triggers before they appear.)
Just as aging brings risks and predictable changes to the brain, it also opens new opportunities and consolidates the gains of a lifetime, a reason for gratitude. Our inclination to look to older adults for such desirable traits as patience, forbearance, and responsibility has a solid basis in reality. A study that followed more than 200 people over a 50-year period found that psychological health—not simple happiness but the qualities of being dependable, responsible, and productive, and of having good relationships with other people—increased steadily from age 30 onward. This happy flowering suggests still other ongoing processes in the brain, either deliberate or unconscious, have yet to be discovered, and many carefully designed studies and in-depth interviews have begun to search for such processes.
Other research, begun in the late 1990s and early 2000s, may even help us understand how the accumulation of experience and the passage of time can work together in the mind to produce one of the most highly prized qualities of all—wisdom. In fact, seen in terms of lifelong development, the subtle attrition of neurons with age may represent not a sad loss but a progressive fine-tuning of cerebral networks.
Midlife and beyond is the optimal time of life to undertake challenges that require, above all, a larger perspective, as is assumed by the respected and esteemed role the elderly enjoy in many non-Western societies. In the Western context, this may translate to a spectrum of possibilities.
■ More abstract or “philosophical” work in one’s field or in a new profession altogether. The car salesperson who becomes a schoolteacher, the retired surgeon who creates a body of work as a painter, and other such second-career choices may be the natural and agreeable trajectory for the older brain.
■ Volunteer work in the community or far from home (for example, in the Peace Corps). Keeping the brain’s extensive library of experience and judgment in regular, vigorous use is a form of mental exercise that comes with obvious psychological benefits as well.
■ Appreciating recreation and the company of family and friends. The gentler sides of life are too often dismissed because their value is hard to measure. But the older brain thrives on social contact and offers a constant resource to loved ones, whether by maintaining its fine-tuning in engagement with others, as in playing chess, where speed counts for nothing and years of experience add up to mastery, or finding thoughtfulness in solitary pursuits such as gardening, which rewards the ability to savor day-to-day efforts and rewards while also envisioning and planning for seasons far in the future.
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