The Wisdom Paradox Page 5
Production of neural structures is complemented by the elimination of excessive neurons, dendrites, and synapses. This process, known as pruning or apoptosis, occurs after birth and also unfolds at different time courses for different parts of the brain, the frontal cortex being the last. Pruning is akin to sculpting, a process that the great sculptor Auguste Rodin described as “eliminating everything that does not belong.” Pruning is not random, but rather is a consequence of reinforcing heavily used neural structures and letting go of those underused or not used at all. These competitive processes of the brain molding itself are somewhat akin to natural selection, which was captured in the term “neural Darwinism,” coined by Gerald Edelman.
Neurons are not the only type of cells found in the brain. In fact, they account for only about one-third of all the brain cells. The remaining two-thirds are taken up by the glial cells, which serve various supporting functions and come in two kinds: astrocytes and oligodendrocytes. At a certain point in development the process of myelination begins: Oligodendrocytes begin to wrap around long axons, forming a fatty protective coating called myelin. Myelin is white, which gave rise to the term white matter (composed of all the long pathways covered with myelin), as opposed to gray matter (composed of all the neurons and short local non-myelinated pathways). Myelin facilitates signal transmission along the axon, greatly enhancing and improving transmission of information within large coordinated neuronal ensembles. Dramatic increase in brain weight during the first years of life is largely due to myelination. The brain structures are not fully functional until the axons connecting them are insulated with myelin, and the time course of myelination varies vastly from structure to structure. As you can probably guess by now, myelination takes the longest in the frontal cortex, extending well into late adolescence and young adulthood, possibly until the age of thirty. The volume of the frontal lobe, and particularly of the prefrontal cortex, continues to grow at least until the age of eighteen and possibly longer, and this growth reflects an ongoing increase in white matter.
If nothing else, this brief review shows that brain development is the interplay among numerous processes unfolding at different time scales. This is a time of great flux in the life of the brain. This is also a time of great flux in the life of the mind—the time of learning, of accumulating the basic fund of mental skills and knowledge, and ultimately the time of forming our identities.
You may have noticed that the frontal lobes, the prefrontal cortex in particular, are the last to complete their biological maturation—only by young adulthood, sometime in the very end of the second decade and possibly even in the third decade of life. Modern society operates on the basis of certain tacit or explicit assumptions about the age of social maturity. This is the age of emergence of the cognitive and personality traits that we associate with social maturity, such as the capacity for impulse control, foresight, and critical self-appraisal. Like the biological maturation of the frontal lobes, these “adult” traits reach their full functionality sometime in the end of the second and the beginning of the third decades of life. Unsurprisingly, this age has been codified in virtually every modern society as the age of transition from social immaturity to social maturity. This is the approximate age (plus or minus a few years) when you are ready to assume a whole range of “mature” rights and responsibilities, such as driving, voting, getting married, buying alcohol, serving in the military, and finally being treated by the legal system as an adult and not a minor. What most people don’t realize is that the emergence of these “adult” traits is most likely caused by the maturation of the frontal lobes, a belief shared by an increasing number of neuroscientists. Thus, many neuroscientists find it useful to think of the completion of the maturation of the frontal lobes, particularly the myelination, as the watershed between the first and the second seasons of the brain: the stage of development and the stage of maturity.
Mature Brain
The second season, the season of maturity, is characterized by less neural flux and by greater stability of brain structures. This is the age of productive activity, when the emphasis gradually shifts from learning about the world to contributing to and molding the world around us through our individual professional and vocational activities. This is the most extensively studied season of the mind and of the brain. In fact, until a few decades ago our knowledge was limited to this stage. The standard texts of neuroanatomy, neurology, or neuropsychology, as well as dozens of books written for the general public, are mostly about this stage, so there is no point in restating much of this normative knowledge here. Suffice it to say, in our zeal for generalizations we have been treating the mature brain in rather generic terms. This is undoubtedly a useful enterprise, and a reasonable point of departure for any scientific inquiry, but only to a point. While perusing any standard text, you are not likely to encounter any reference to the gender differences in brain organization, let alone to the individual differences. But such differences do exist and we are only now beginning to understand them. From the aerial view of all humanity represented by a composite, we are gradually moving to the understanding of the neural foundations of individuality.
Aging Brain
Then comes the third season, the season of aging. What happens to the magnificent brain machinery as we move further through life? How golden is the “golden age”? Oddly, scientists did not endeavor to address this question until relatively recently. Hippocrates himself omitted the brain from the litany of old-age woes in his Aphorisms. About this, the leading neuroscientist of aging Naftali Raz observed:
... So overwhelming are the transformations of the aging body and so pervasive are the changes in its basic functions that it may not be particularly surprising that the most famous of the ancient servants of Aesculapius did not find the brain and higher cognitive functions sufficiently important to be included in his list of geriatric troubles.
But the brain is affected in aging, even in successful, healthy aging. It would be strange if it weren’t because, like every other organ, the brain is of the flesh. Extensive research has been conducted over the last few decades to understand such changes, and today we have a relatively comprehensive picture of what happens to the aging brain, even when the process is unencumbered by neurological illness or dementia. Much of the discussion that follows in this chapter is based on Naftali Raz’s own research and on his cogent reviews of the state of affairs in the field of brain aging research.
Some of the changes that occur as the brain ages are global. Both the weight of the brain and its volume shrink by about 2 percent every decade of adult life. The ventricles (cavities deep inside the brain containing cerebrospinal fluid) increase in size. The sulci (spaces between the walnut-like convolutions of the cortical mantle) become more prominent. All these changes suggest a modest amount of atrophy or shrinkage of brain tissue, even as part of normal aging. The connections between neurons become increasingly sparse (a process known as “debranching”), and so does the density of synapses (the sites of chemical signal transmission between neurons). Blood flow to the brain becomes less abundant, and the oxygen supply to the brain less generous.
Both gray matter and white matter are affected in aging. In the white matter, small focal lesions appear. They are sometimes called hyperintensities in the technical parlance of MRI radiology. In most cases, the aging-associated “hyperintensities” reflect vascular illness, but they may also reflect demyelination of pathways. They tend to accumulate with age. The relationship between these focal white-matter lesions and cognitive decline is not a simple linear one, but rather of a threshold nature. Up to a point, they remain benign, but once their total volume reaches a certain level, cognition begins to deteriorate. Some scientists believe that the white matter is more susceptible to the effects of aging than the gray matter.
Against the background of such global changes, certain parts of the brain fare better than others. A number of cortical and subcortical structures are affected, but to different deg
rees. In the neocortex, the classic neurological rule of “evolution and dissolution,” first introduced by John Hughlings Jackson, seems to operate: The phylogenetically (evolutionarily) youngest cortical subdivisions (which develop only at the later stages of “evolution”), the so-called heteromodal association cortex, are affected to the greatest extent by “dissolution” due to aging. These include inferotemporal, inferoparietal, and particularly the phylogenetically most recent prefrontal cortex. By contrast, the phylogenetically older cortical subdivisions, which include the areas involved in receiving raw sensory information and the motor cortex, are least affected. The prefrontal cortex, a subdivision of the frontal lobe in charge of complex planning and the organization of complex behaviors in time, is affected to the greatest extent by aging.
A similar relationship exists between ontogenetic (occurring during a lifetime) development and decay: The brain structures last to develop at the organism’s growth stages are the first to succumb to decline with age. In assessing the relative vulnerability of various brain structures, the fate of the pathways projecting from and to these structures is particularly instructive. Therefore, the chronology of pathway myelination is a useful marker both of development and of decline. By this light, the longer it takes for the pathways to myelinate, the more susceptible is the corresponding structure to the effects of aging. Again, the prefrontal cortex comes out as the most vulnerable, particularly its dorsolateral subdivision. The changes in the frontal lobes involve the deterioration of both the gray matter and the white matter, as well as the depletion of major neurotransmitters (chemicals in charge of signal transmission between neurons): dopamine, norepinephrine, and serotonin. As was the case in development, the fate of the frontal lobes serves as the watershed between the second and the third seasons of the brain, the stage of maturity and the stages of aging.
Outside the neocortex, the hippocampus and the amygdala are only moderately affected by aging, not nearly as much as the frontal lobes. Hippocampus is found on the inside aspect of the temporal lobe in each hemisphere and is important in the formation of new memories. The amygdala (the word means “almond” in Greek; reflecting its shape) is found right in front of the hippocampus on the inside aspects of the temporal lobes and is important for the experience and expression of emotions.
Interestingly, the hippocampus is not affected by aging in other mammalian species, such as monkeys and rodents. This may be merely a chance difference, but it is also possible that evolutionary pressures favored the human brain with the slightly decaying hippocampus. For those among us with an unbounded faith in the adaptive nature of evolution (but prudent enough not to lapse into an outright teleological frame of mind), what could the nature of such evolutionary pressures be? Just as an idle possibility to consider, it could conceivably be related to the fact that humans depend on previously acquired cognitive templates much more than do other species. Hence, an aging human brain, unlike an aging monkey or rodent brain, may benefit from dampening the formation of excess new information, which somehow competes with these templates.
FIGURE 5. Map of Brain Regions Affected in Aging. The darker the color, the more susceptible the brain structure is to the effects of normal aging.
Another interesting finding is the difference in the relative vulnerability of various brain structures in normal aging and in dementia. Unlike in normal aging, in Alzheimer’s disease the hippocampus and the posterior heteromodal neocortex of the temporal and parietal lobes deteriorate more rapidly than the frontal lobe. Thus, the disparity between the deterioration of the frontal lobes and the hippocampus evident in the MRI of an aging brain may tell us whether it is undergoing the process of normal aging, or whether it exhibits early features of Alzheimer’s disease.
The fate of various subcortical structures generally follows the same Jacksonian principle of “evolution and dissolution.” The basal ganglia and the cerebellum (both important for various aspects of motor control) are moderately affected, and so is the midbrain. The pons (the brain area responsible for basic arousal) and the tectum (the first station of sensory input processing within the brain) seem to be affected very little or not at all.
How do these profound changes in brain anatomy translate into the changes in brain function, into cognitive changes? Again, numerous studies have been conducted, painstakingly documenting the adverse mental changes that attend normal aging. It appears that the overall speed of mental operations declines, as do the sensory functions (the ability to receive inputs about the physical world outside). The functions that depend on the frontal lobes appear to falter in particular. These include mental inhibition, the capacity to refrain from distractions or from habitual, knee-jerk reactions to situations. They also include “working memory,” a loosely used term employed by most scientists to refer to the ability to hold certain information in mind while engaging in some cognitive processing for which this information is germane. Another function of the frontal lobes, mental flexibility (an ability to switch rapidly from one mental process to another and from one frame of mind to another), has also been found to decline with aging.
Certain forms of attention are also impaired, particularly selective attention (the ability to pick out salient events in the environment and to concentrate on them) and divided attention (the ability to shift attention back and forth among several activities unfolding in parallel). Memory is not spared either. This particularly concerns the ability to learn new facts (semantic memory) and to form memories about specific events (episodic memory). In fact, the erosion of new learning is among the earliest manifestations of cognitive aging.
Ageless Brain
This frightful litany of adverse cognitive changes has been documented by giving the subjects various laboratory neuropsychological tests, and by comparing their performance across age groups. Clearly, the cognitive woes parallel morphological and biochemical woes of the brain, and all this sounds like pretty bad news.
But a closer look at aging cognition leads one to conclude that the news is not as bad as it might appear to be. A puzzling phenomenon did not fail to escape the attention of numerous scientists. Despite this multifarious, well-documented neurological and cognitive decline, it is very common for aged individuals to perform quite competently in real-life situations, both in their everyday lives and on the job. This often includes the discharge of very high-level professional and executive responsibilities, and even world-class feats of artistic and scientific creativity and statesmanship.
Scientists commonly refer to this mysterious ability as “cognitive expertise” and its mechanisms have for years remained obscure. The examination of these mechanisms will be among the focal points of this book. Having faced up to the bad news, it is now time to consider the good news of aging! This mysterious cognitive expertise, which has the uncanny ability to resist the unwelcome effects of aging, resonates with two other highly prized traits commonly associated with mature age: competence and wisdom
There seems to be a paradox there. And since cognitive expertise, competence, and wisdom are not extracranial phenomena hovering over our heads like a saint’s halo, but rather very much products of our brain, this paradox becomes a question of neurobiology, a question for a neuroscientist to tackle. In the forthcoming chapters we will examine the phenomena of wisdom and competence and will then proceed with the tour of their neural machinery. But first, let us consider the paradox itself and see how very cogent cognition may be supported by brains touched by aging and neuroerosion. To that end, we will examine the lives of several historical personalities from various arenas of human accomplishment.
3
AGING AND POWERFUL MINDS IN HISTORY
Late-Blooming Achievers
Humans are among the relatively few species with an average life span extending far beyond the age of procreation. Why did evolution contrive (pardon the anthropomorphic and teleological turn of phrase) to prolong the lives of individuals who have nothing more to contribute to the propagation of th
e species through biological means? What were the evolutionary pressures leading to this odd phenomenon? One possibility is that the elderly make a critical contribution to the survival of the species through other means—particularly through the accumulation of knowledge and its transmission to the new generations via cultural means, such as language. While obvious to scholars, this point has been often overlooked in popular culture.
In our culture, mental vigor is often associated with youth, and mental decline with age. The creative potential of the aged is often dismissed. My friend’s nineteen-year-old son Jaan put it in a capsule emblematic of our cultural prejudice: “I am surprised when people your and my father’s age are capable of learning anything new at all!” That his father has been among the most compelling educational innovators in Europe, the head of a major university, a presidential candidate, and at the time of this writing a high-profile member of the Parliament in his North European country, seemed to have left the young man utterly unimpressed.
Today, Jaan’s dismissive thinking is challenged by numerous examples of successful and yes, innovative people of relatively advanced age—like his father, his father’s friend (I like to think), and many, perhaps most, of the readers of this book. This fact is too obvious, too widely accepted and supported by too many examples for me to belabor it at length in this book. By repackaging it slightly and presenting it as an earth-shattering revelation, I would be insulting your intelligence. So I will focus on two less obvious points, which, if anything, amplify the main premise.