Tool Module: The Effects of Normal Aging on Our Cognitive Abilities
From birth to death, the human body suffers the ravages of time. Once we become adults, our bodies lose muscle strength and mass as the years go on. Bone loss reduces our skeleton’s mechanical strength. If we are injured, our tendons and ligaments take longer to repair themselves. Under constant assault from the oxidizing effects of free radicals, the elastin and collagen fibres in our skin deteriorate, gradually making it lose its elasticity. And yet, with the spectacular increase in human lifespan over the past century, it is no longer unusual to encounter people who are still sharp and alert well into their 80s and 90s.
But many of these people are loaded down with eyeglasses, hearing aids, and other sensory prostheses, because the performance of the human sensory organs diminishes markedly with age. The thresholds for sensing smells and flavours rise considerably. The sense of sight is affected in several ways: for example, the number of photoreceptors in the retinas decreases, making vision blurrier; the field of vision narrows; the lens becomes more opaque; and the iris becomes more rigid.
When it comes to the sense of hearing, things are not much better. Inside the ear, as people age, the tympanic membrane thickens, the number of hair cells in the organ of Corti decreases, and neurons in the cochlea die off, all of which makes the ear less responsive and makes it harder to distinguish a conversation from background noise. These reductions in sensory performance reduce the amount of relevant information that older people take in, thus making them a bit more isolated than younger people. The aging of their sensory organs prevents them from correctly receiving stimuli from the outside world and thus affects their overall cognitive state.
Like the sensory organs, the brain itself is not immune to aging. Mechanisms associated with aging of brain tissue include inflammation, the effects of free radicals, and hormonal changes. The brain is also subject to damage by factors such as cardiovascular disease, head trauma, unhealthy lifestyles, and chronic stress.
One effect that aging has on the brain is to reduce its mass. Although differences in brain size are a poor indicator of relative intelligence when it comes to comparing different species, or the two sexes of a given species, changes in brain size in a given individual do provide a fairly good idea of the corresponding changes in that person’s intellectual faculties. Hence it is interesting to note that the volume of the human brain peaks at about age 14, then gradually declines for the rest of a person’s life. On average, brain mass and volume decrease by about 2% every 10 years. Of course, there are some exceptions that have yet to be explained, where people over 100 years old have suffered little if any loss of brain tissue.
Contrary to what was once believed, this decline that is seen in most people does not accelerate after they reach age 50, but rather continues at the same pace into old age. Another past belief now being questioned is that this loss in brain mass and volume in the course of aging is attributable to the normal death of several tens of thousands of neurons every day. The methods that were used to quantify these losses of neurons have now been criticized, and a number of alternative explanations have now been advanced—for example, that the shrinking of the brain may be due mainly to a reduction in the volume of the neurons and their dendrites, and not to their death.
In some cases, however, the atrophy might be due largely to the death of neurons, caused by a developing neurodegenerative pathology. For example, a 2001 study using serial magnetic resonance imaging showed that the medial temporal lobe as well as two other areas of the brain were already showing subclinical atrophy five to eight years before Alzheimer’s-type dementia was diagnosed. Considering the high prevalence of reported cases of Alzheimer’s among people in their 80s and 90s, abnormal brain atrophy arising from Alzheimer’s might already be occurring in many people in their 60s or 70s who will be diagnosed with this condition later on.
And when atrophy does occur, it does not affect all parts of the brain uniformly. Two structures that are important for memory—the medial temporal lobe, where the hippocampus is located, and the frontal lobe—are especially affected. This damage to the neurons also leads to a significant drop in the concentrations of certain neurotransmitters, such as dopamine, a decrease in which is associated with a decline in motor functions.
Nor is the deterioration over time limited to the brain’s grey matter, where the neurons’ cell bodies are located: the white matter, including the axons, suffers damage as well. A 2008 study by David Ziegler used diffusion tensor imaging to obtain images of axon bundles in the brain and was the first to show that these nerve fibres degenerate as the brain ages, just like the grey matter does. Moreover, this thinning of the axon bundles was broadly distributed in the depths of the frontal, temporal, and parietal lobes, as well as in the corpus callosum, which connects the brain’s two hemispheres.
What is most remarkable, however, about Ziegler’s study is that he did not find any correlation between the thickness of the grey matter in the subjects’ cortexes and their performance in cognitive tests. But he did find several correlations between performance on these same tests and the integrity of the bundles of white matter. These findings suggest that cognitive performance is affected by the integrity of the connections between the neurons even more than by the condition of the neuron cell bodies in the grey matter.
There are some indications that the deterioration of the white matter is caused by damage to the myelin sheath. This sheath consists of glial-cell extensions that wrap around the axon and enable it to conduct nerve impulses more quickly. Discontinuities in the myelin sheath might play a significant role in the slowing of nerve transmission that is observed in the course of aging.
The brain maintains its plasticity throughout a person’s lifetime, so it has many mechanisms to compensate for the circuits that have deteriorated. But the cognitive functions that these circuits used to perform almost automatically will now be taken over by alternative neural pathways that are generally less efficient, and so some of these functions will be performed more slowly.
Many studies have shown, however, that on the whole, our cognitive abilities do not show any major decline until we are in our 50s or early 60s. And even then, in healthy people, performance diminishes very slowly. For example, in one study, at age 81, only 30 to 40% of the subjects showed any significant decline in their cognitive abilities. In other words, two-thirds of the participants in the study had only minimal decreases in their cognitive faculties—or rather, in some of their cognitive faculties, because several remained relatively intact while others even improved with age.
Among the cognitive functions that decline gradually with age, memory is one of those most often cited. But memory is a complex phenomenon that involves several different mechanisms, not all of which are equally affected by aging.
One of the forms of memory that is most affected is incidental memory, the kind that lets you remember things automatically and with practically no effort. This is the form of memory that you use, for example, to describe the scenes from a film in detail as you are coming out of the movie theatre. Incidental memory peaks during the teen years or early 20s and then decreases. As people age, it therefore becomes harder for them to recall numerous details unless they make a conscious effort to do so.
Working memory—the kind that lets us remember a telephone number for a few seconds, or follow the thread of a conversation—also is often affected by age. The slowdown in processing speed that results from aging of the brain seems to make information disappear from working memory before we have even been able to consolidate it in long-term memory. The decline in the performance of working memory might therefore also at least partly explain why long-term memory also declines with age.
Our ability to retrieve information from memory is also sometimes affected, which explains the phenomenon of having a word “at the tip of your tongue”. On the other hand, implicit memory—the kind involved in conditioning and in motor learning—appears to be less affected by aging, as is our semantic memory of knowledge that we use frequently in the course of our lives. As for our vocabulary, it continues to increase throughout our lives.
Over the decades, some people lose some of their ability to focus their attention on the relevant aspects of a set of information. Because these people get distracted by irrelevant details, their reasoning processes become slower.
Some authors break general reasoning ability down into two types of “intelligence": fluid and crystallized. Fluid intelligence is the kind in which we apply attention and memory to quickly process new pieces of information. In contrast, crystallized intelligence is associated more with the knowledge and vocabulary that we acquire over the course of our lives. Hence it is no surprise that these authors find it more common for fluid intelligence to decline with age than for crystallized intelligence to do so.
But the most important thing to remember about normal aging of the brain may be that the cognitive repercussions of brain aging vary tremendously from one individual to another. For most people, these impacts are typically minimal, but for some people, they can be so severe as to cause conditions such as Alzheimer’s-type dementia.
Thus, intellectual decline is not inevitable until what is often an advanced age. And even when such a decline does occur, often it is slight, does not interfere with daily life, and does not necessarily mean the start of dementia. Moreover, in addition to the brain’s own natural compensatory mechanisms, people can use artificial ones, such as mnemonic devices, lists, and calendars. Not to mention the legendary wisdom that is supposed to come with age—the ability to make good decisions in areas where we have accumulated a lot of specialized knowledge. But in a world where social and technological change is occurring ever more rapidly, it may still be the better part of wisdom to have a teenager on hand to troubleshoot your computer for you as you grow older.
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