Our ability to learn and remember is limited by the accuracy of our senses, our points of contact with the world. But vision, hearing, touch, smell, and taste are not just detection systems. The sense organs also comprise a primitive kind of memory, a temporary storage system or “buffer” for the brain. Much of the input to our sense organs reverberates in receptors, and that reverberation allows even weak stimuli—for example, images flashed so quickly that we have no conscious awareness of them—to impact decisions we make later on. Without the buffering ability of our sense organs, a great deal of information about the world would be lost to us. Unfortunately, as we age, our sensory systems deteriorate, and at the extreme, we become completely insensitive to a wide range of input. For example, high-pitched tones that we can detect at a mere 30 decibels when we are young have to be boosted to an earsplitting 90 decibels for the elderly to hear. (Physics buffs: That’s about a million times the energy intensity.) And pupil size decreases as we age, so when it is dim, the elderly person’s eyes pick up about a third as much light as people in their prime. Because the deterioration of sense organs limits our access to critical information—speech, text, music, street signs—thinking itself is impaired.
And loss of information is just part of the problem. Research by psychologist Monica Fabiani and her colleagues at the University of Illinois at Urbana-Champaign suggests that in older people the main problem might not be that the sense organ is rejecting input but rather that the brain itself is having trouble filtering out irrelevant information. In a recent study, Fabiani had people of various ages read a book while trying to ignore auditory tones piped through headphones. Overall, the older the individual, the more trouble he or she had ignoring the tones. “The background stimuli may flood your thinking with things that are irrelevant and that you cannot inhibit,” Fabiani says. As a result, “you basically lose the capacity to perform tasks.”
Most people think of human memory as a single system. But because different kinds of information are retained differently, experts speculate that distinct types of memory systems exist in the brain. Some information stays with us for only a short time—generally no more than a few seconds unless we do something with it. For example, if somebody tells you a phone number and you do not immediately repeat it, it will very likely disappear, never to return. Research suggests the existence of a short-term memory system, consisting in turn of two subsystems: immediate memory (the temporary storage system that holds on to information we don’t process in some way) and working memory (a system that allows us to retain information as long as we keep using it).
As we age, our ability to process new information in working memory is severely compromised. In a typical test procedure for evaluating working memory, cognitive aging researcher Timothy Salthouse of the University of Virginia asked people to perform arithmetic computations while also trying to remember the last digit in each problem. People in their twenties were typically able to solve four or five of these problems in a row and still recall the final digits without error. With each decade, performance deteriorated; people in their seventies could typically solve no more than two such problems in a row and still get the final digits right.
One of the simplest ways to assess memory is to read test subjects a list of words and ask them, after a short time has passed, to repeat as many as they can. In a 1990 study, Hasker Davis and his colleagues at the University of Colorado found that people in their twenties could typically recall 90 percent of a list of 15 words after a short delay. With each additional decade of age, the percentage of words recalled decreased. People in their eighties could recall only about half the words.
Some information in our short-term memory system is consolidated into a long-term storage system, where it remains available to retrieve for months or years. If a memory of anything from a good meal to a coworker’s name persists for 5 years, there is a good chance it will persist for another 40. But as we age, the degradation of sensory and working memory systems makes it increasingly difficult for us to transfer information into long-term storage. That’s why, if you are over 50, you are more likely to remember the lyrics to a Beatles song than to any song you have heard in the past 20 years. To put this another way, our ability to learn new things is extraordinary when we are young and peaks in our teens. We can learn after that, but it becomes increasingly difficult. In an early study by psychologist Jeanne Gilbert, English speakers of different ages were asked to learn Turkish vocabulary words. People in their sixties learned 60 percent fewer words than young adults in their twenties who spent equal time and effort on the task.
One of the most frustrating experiences we have as we age is accessing a particular word from long-term memory—the so-called “word-finding” or “tip-of-the-tongue” problem. Deborah Burke, a psychology professor at Pomona College who has studied this phenomenon for more than 20 years, explains that old people suffer from a disconnect between the meaning of a word—which
presumably tells you that it is the correct word to say right now—and the sound of that word. It is, she says, “the most irritating and disturbing cognitive problem” reported by older adults. We do not know what causes the disconnect.
We also get dumber as we age. iq remains fairly stable, but that is because it is a relative measure—a quotient (the Q) that shows where we stand relative to people our own age. The problem is that raw scores on intelligence tests actually peak in our teens, remain high for a few years, and then decline throughout life; iq remains fairly stable only because people decline at roughly the same rate. And yes, even geniuses decline. I recently asked Nobel Laureate James Watson, 84, when he reached his intellectual peak, and he replied, “Twenty, maybe 21—certainly before we found the dna structure.” That seminal work had been done when he was 25.
Intelligence, like memory, is divided into types that decline somewhat differently. Factual information is the basis of what is called crystallized intelligence, and much of the crystallized knowledge we acquire stays fairly strong at least into our sixties. However, fluid intelligence—our ability to reason—declines dramatically in most people, in large part because we get slow. Generally speaking, on tasks involving reasoning, what a 20-year-old can do in about half a second takes a healthy 80-year-old more than two seconds—if, that is, he or she can do it at all. As Douglas Powell of the Harvard Medical School puts it in his recent book, The Aging Intellect, “No other single mental ability declines as rapidly during the adult years as processing speed.”
Neuroscientists tackle the decline in reasoning and working memory under an umbrella concept called executive function. Somewhere in the brain there seems to be a coach: a system or structure that schedules and prioritizes, garnering resources, redirecting attention, or switching tasks as needed. Adam Gazzaley, a neurology professor at the University of California, San Francisco, has conducted research documenting how that coaching ability declines as we age. For example, older people are bad at multitasking, Gazzaley says, because they have trouble redirecting attention back to a task after it has been interrupted. On average, people in their seventies generally require twice as much time to do two things at once as do young adults, and they also make more errors on the tasks. That inability to focus takes its toll. “I would not be capable of doing groundbreaking work today,” renowned physicist Freeman Dyson, 88, told me recently. When he was young, Dyson said, he could focus on a single problem nonstop for a week. “Today,” he said, “I’m limited to two hours a day of serious work—which wouldn’t be enough.”
The deterioration of these four systems appears to be an inevitable part of normal, healthy aging, although the rate of decline varies among individuals (see “How Some Brains Stay Razor Sharp,” page 50). When you add disease to the picture, things truly look bleak. Half of Americans over 85 are suffering from Alzheimer’s disease, which eventually robs people of their memories, identities, and the ability to function even minimally. Alzheimer’s becomes increasingly common with age—so common that neurologist Gary Small of UCLA suggests that if we all lived to 110, we all would have it. These are the brutal truths we must face as we and our loved ones age.
Next Page: Tale of the Scans—A Mind in Decline