“I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”
Arthur Conan Doyle, A Study in Scarlet
Changes in the Nervous System with Aging
Normal aging is associated with a host of changes in the brain and nervous system, although these changes do not necessarily affect thinking and behavior.
From age 30 to 70 the blood flow to the brain decreases by 15 to 20 percent. The brain’s weight also decreases with age but this decline appears to be in a few specific places rather than overall and may be largely due to lower water content. Healthy older people experience a moderate loss of neurons in the gray matter, a loss that is much more extensive in people with dementia. There is also commonly a loss of neurons in the cerebellum and the hippocampus, which is involved in some aspects of memory function and spatial navigation. However, there is some indication that hippocampal size and function may be modifiable. A 2000 study of London cab drivers, for example, revealed that drivers had a larger right hippocampus than did the control subjects and that there was a direct correlation between the size of the hippocampus and the length of time on the job. In this case it seems the challenge of routinely memorizing complex routes may have had an actual physical impact on the cabbies’ brains.
Less dramatic neuron losses occur in deeper and more primitive brain structures such as the brain stem. For some nerves the density of their interconnections seems to be reduced with aging. However, the dendrites at the ends of nerves (and the connections between them) continue to grow, albeit slowly, even in advanced age, which suggests there is some degree of continuous re-patterning of the nervous system that occurs throughout life.
There are also some age-related changes in certain chemical messengers (neurotransmitters). For example, the enzymes that produce and activate the neurotransmitter acetylcholine decline significantly with age, a decrease that is most prominent in the area of the brain that is involved in learning, memory, language comprehension and falling in love. Changes in cell membranes can impair cells’ ability to send and receive chemical messages. For example, binding sites for serotonin in the frontal cortex and hippocampus are reduced with age, which could affect mood, cognition, learning, sleep and temperature regulation. There is also a decrease in dopamine-related receptors, which may have effects in motor activity, cognition, memory, motivation and reward. Similar aging changes have been observed in cortical and pineal beta-adrenergic receptors that may play a role in the sleep-wake cycle.
Although they may sound daunting, these changes are not necessarily detrimental to thinking or behavior. Language skills and sustained attention, for example, are not altered with aging. Some aspects of cognitive ability do seem to change, such as the ability to retain large amounts of information over a long period of time. These changes do not develop uniformly or inevitably and many older people continue to perform at levels that are comparable to, or even exceed, those of much younger people. Later chapters will delve into cognitive changes—and maintenance—in greater detail.
Changes in the Senses
We experience a host of changes that affect eye health and vision as we age. The most common aging-associated change in vision is presbyopia, a condition in which it becomes harder to focus on nearby objects. This is mainly due to a reduced elasticity of the lens and weakening of the ciliary muscle, which is the muscle that manipulates the lens shape to control focus. Presbyopia affects men and women equally and often begins in a person’s twenties, although it is typically not noticeable until age 40 or 50. Eye glasses usually correct the problem.
As we age our eyes also adapt more slowly to abrupt changes in light. This correlation is so consistent with age that you can estimate a person’s age to within three years based solely on this measure. This change is not trivial: it means that abrupt shifts from darkness to light, such as walking out of a garage onto a sunny driveway, can be temporarily blinding to an older person while the eyes adapt. Aging also reduces the ability to see in dark and semi-dark situations. After two minutes of reduced illumination, young people’s eyes are almost five times as sensitive as older people’s eyes; after 40 minutes there is a 240-fold difference.
Changes in the eyes also affect our appearance as we age. The tissues around the eyes naturally atrophy and lose fat, often causing the upper lid to droop and the lower lid to turn inward or outward. These changes, combined with a decreased production of tears, increase the risk of eye infection.
As we age we also become more prone to diseases of the eye, including glaucoma, cataracts and macular degeneration. Glaucoma, an increased pressure in the eye that can progress to blindness, becomes more likely as the iris becomes more rigid, the pupil becomes smaller and other changes occur in the lens. Cataracts, an extremely common condition caused by a progressive accumulation of various substances in the lens, cause blurred vision and change the way colors are perceived. Because the substances in cataracts are yellow, the lens becomes less transparent to the blue part of the color spectrum, making blue appear greenish blue. Because of this filtering people dyeing their hair white or silver often do not notice if their hair takes on a slight blue tint.
It is unclear whether the retina changes as a result of normal aging although blood vessel disease involving the retina is common. Changes in the blood supply of the retina and possibly in the pigmented layer of the retina can lead to macular degeneration, one of the most common causes of vision loss in older people. Changes in the cornea, the most superficial surface of the eye, also can occur although they usually are related to disease and not to aging.
It is difficult to tease out the hearing changes of normal aging from those that result from excessive noise exposure. Regardless of the distinction many older people experience significant changes in the shape and structure of the ear and declines in their hearing ability. As we grow older the walls of the ear canal become thinner, the eardrum thickens, the bones and joints in the inner ear often begin to degenerate and earwax production decreases. In the inner ear there is a loss of hair cells in the organ of Corti, loss of cochlear neurons, a thickening of capillaries and a degeneration of the spiral ligament. These all contribute to hearing loss.
Hearing loss for pure tones, called presbycusis, becomes more common with age in both men and women, though overall, the loss is slightly milder for women. Higher frequencies are more affected than lower frequencies. Aging is also associated with a decreased ability to distinguish between different pitches. Between 25 and 55 years of age pitch discrimination declines linearly, but after age 55 the declines are steeper, especially for very high and low frequencies. This is important because pitch discrimination plays a role in speech perception, even without pure tone hearing loss. Speech intelligibility declines less than 5% from age 6 to 59 but deteriorates rapidly thereafter, dropping more than 25% from peak levels after age 80. This decline is felt even more acutely in situations with ambient noise, such as in a restaurant.
The evidence regarding taste sensitivity is inconclusive and varies both among individuals and by the substance tested. The tongue atrophies with age, which may result in diminished taste sensation, but the number of taste buds remains unchanged and the responsiveness of these taste buds appears to be unaltered.
The sense of smell declines rapidly after age 50 for both men and women and the parts of the brain involved in smell degenerate significantly. By age 80 smell detection is almost 50 percent poorer than it was at its peak but complete loss of smell is a sign of illness such as Parkinson’s disease and not normal aging. Taste and smell work together to make the discrimination and enjoyment of food possible. Some people find they have trouble recognizing blended foods by taste and smell.
Broadly speaking, we become less sensitive to touch as we age although this occurs at different rates for different types of touch and different parts of the body. In general the response to painful stimuli is diminished with aging. Sensitivity of the cornea of the eye to a light touch declines after the age of 50 while touch sensitivity to the nose begins to decline by age 15. Pressure touch thresholds on the index finger and on the big toe decline more in men than in women.
Navigation-related structural change in the hippocampi of taxi drivers. Maguire EA, Gadian DG, Johnsrude IS, Good CD, Ashburner J, Frackowiak RS, Frith CD. Proc Natl Acad Sci U S A. 2000 Apr 11; 97(8):4398-403.