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What makes us a human? Walking on two legs? Language? Empathy? Our large prefrontal cortex, which enables our predictive and problem solving skills?
Many of the characteristics we cherish as distinguishing us from our more primitive evolutionary cousins often decline with age. We consider the pinnacles of our humanity to be social harmony, creativity, independence, cultural tradition, mobility, etc., but none of these are permanent within a human lifetime — they can all fade, leaving a shell of a once-robust and complex human. As this happens, what is left of the human?
To further explore the intricacies of aging, we can divide our human traits into two categories: Cognitive and physical. In the physical realms, we interact with the world by gathering sensory data, and converting that to motions and actions of our own. Within the cognitive realms we have powerful memory, imagination, and planning. All of these processes are mediated by systems in the body that can, and often do, deteriorate over time.
Let us first look at the purely physical behaviors of humans. All of us have 6 major senses (yes 6!): Sound, Sight, Smell, Taste, Touch, and another curious sense called Proprioception — a calculation of where our bodies are in space. The first sense that systematically loses function is hearing. A newborn is capable of detecting pitches anywhere from 20-20,000 Hz, or cycles per second. Over time, tiny specialized cells in the inner ear lose their ability to detect the highest pitches — like mosquitos and the eeee of a tv screen — leaving older individuals with a smaller range of hearing. Next to go is vision, affecting many people early in life, but in particular after age 40.
Interestingly, one’s sense of smell also declines in later years — in fact, loss of olfactory function may become a new diagnostic avenue for early Alzheimer’s Disease. When you lose your sense of smell, much of what we think of as taste also goes away. This may explain why older folks often have smaller appetites — the world may progressively become blander for them. Our sense of touch does not significantly degrade in sensitivity, but it does drop off in speed — nerve cells that once delivered information about pressure on the arm or a tickling at the knee now deliver the information after a delay. As the nerve impulses slow, so does overall movement. Combine signal slowing with muscle atrophy and loss of sensory information, and it is easy to see why broken hips from a fall are so common among the elderly.
On the cognitive side of things, the brain loses mass by about .5% per year (1% in Alzheimer patients). This is an average. Some parts of the brain don’t detectably lose volume at all (the cerebellum, visual cortex, touch cortex). These are generally the occipital and parietal lobe areas. On the other hand, the hippocampus (a major memory structure) and the frontal lobe show more marked loss of volume. This heavily affects memory, and older people have a harder time remembering details of events. This is partly due to frontal lobe atrophy, which affects one’s ability to focus attention, and partly because hippocampal cells for memory are disappearing at a rate of 5-8% every decade of one’s life. Interestingly, though it is more difficult for older people to form new memories, they retain facts and memories from their earlier years with high accuracy.
For the most part, we aren’t losing brain cells (with a few exceptions). Rather, the cells shrink and form fewer connections. You can compare this how many old friends you keep in touch with. The people themselves rarely disappear, but the relationship might. Some people meet this lower brain connectivity with greater computational efficiency — from their behavior, you might never notice that their brain is shrinking. Others show visible decline, losing their ability to keep mental lists, forgetting why they are in a medical clinic or what month it is. The Geriatric Mental Health Foundation estimates that “four million Americans currently suffer from Alzheimer’s Disease or a related form of dementia. Nearly 10 percent of all people over age 65 and up to half of those over age 85 are thought to have AD or another form of dementia…” Caring for these individuals is also expensive, costing the US about $100 billion every year.
Eventually, driving becomes unsafe, cooking becomes difficult, even basic speech can become a challenge. All of these age-related declines chisel away at our independence, at what makes us human. As a result, many older folks become depressed, less active, less engaged. It’s a scary prospect, but we needn’t fear all effects of aging. In fact, there are several domains in which humans tend to improve with age – or at least don’t decline noticeably. Our social skills become better over time, along with vocabulary and ability to discriminate relevant information from the irrelevant. In many cultures, older generations also care for the children, relieving those of working age from the task. In time, the children move into adulthood, and their parents become the oldest generation.
Caring for the elderly is costly, and the coming years will see further increase in elderly populations as Baby Boomers move into retirement. The United States isn’t the only country facing this demographic shift. China is considering lifting its one-child ban to counteract an ever-growing older population. In Japan, citizens now buy more diapers for adults than for children.
Among species it is less common to preserve and protect an aging population than to allow the sick and old to die. Humans are different. We constantly invent practices and technologies that will extend our life for as long as possible. We devote work, energy and resources to individuals beyond their productive and reproductive years. Wouldn’t it be more advantageous to focus on child-rearing and people in their prime? What better investment could be made for the future? And yet, we keep grandma around. So perhaps as individuals, we become less human, but in return we collectively increase our humanity by caring for our elderly.
Source note: All facts in this article come from a UC Berkeley lecture series by William Jagust, MD on the aging brain.