How Memory Changes With Age

For many people, memory loss becomes noticeable after about age 50. However, changes in aspects of memory function are detectable with neuropsychological testing as early as the 20s and 30s. This is similar to other physical traits, such as athletic performance, which also peaks in early adulthood. But one of the myths surrounding the term “age-related memory loss” is that all memories slip with the passing years. In fact, while some information may become harder to recall — and new memories may be harder to lay down in the brain — other memories will remain as accessible as ever.

In particular, there is truth in the old saying that “you never forget how to ride a bicycle.” Procedural memory — by which you remember processes and skills such as how to ride a bicycle, serve a tennis ball, or accomplish routine tasks — does not fade with age. In fact, it’s so resilient that it remains intact even in people with early- to mid-stage Alzheimer’s.

Visual memories, on the other hand, may be less resilient. While some research shows that older people’s recall of images such as faces is comparable to that of young people’s, other studies suggest a significant decline in older people’s ability to remember new images. In one, adults of different ages were asked to look at 18 detailed colored pictures; three days later, they were shown several of these pictures as well as others, and asked which ones they’d seen before. Between 60% and 70% of older participants’ memories were inaccurate, compared with just 25% to 35% of younger people’s memories.

 

Why memory fades

Brain regions involved with memory processing, such as the hippocampus and especially the frontal lobes, undergo age-related structural and neurochemical changes. These changes can undermine the encoding, consolidation, and retrieval of new information. Different kinds of memory can decline with age, including

  • the episodic form of declarative memory (e.g., which stock you sold last year from your retirement account)
  • the semantic form of declarative memory (e.g., facts, such as the exact year World War I started)
  • spatial memory (e.g., the directions to a new location).

It’s not just that you learn this sort of information more slowly; you may have more trouble recalling it because you hadn’t fully learned it in the first place.

Some of what scientists know about age-related memory loss comes from studies of animals. In one such study, older mice took longer to learn to escape from a maze than younger mice. These results are consistent with what scientists observe in people — and what people notice about themselves as they age. If you and your child or grandchild learn a new computer game together, chances are that the next day the child will remember more of the details of how to play the game than you do.

Willpower and effort can overcome some age-related difficulties with learning. Researchers now know that in many instances, if you make the effort to learn something well, you will be rewarded — you’ll be able to recall it as well as a younger person can (see “Behavioral strategies”).

 

When brain cells die

For years, the scientific view of an adult’s brain was anything but encouraging. Experts believed that your brain produced new brain cells only early in life and that once you reached adulthood, the growth of new neurons ceased and existing neurons began to die off. You may have heard the oft-repeated “fact” that you lose 10,000 brain cells a day. The idea was that your brain was shrinking, and that could mean only one thing: as you lost neurons, you also lost some of your capacity to learn, think, and remember.

Researchers now know that this brain cell degradation is less pronounced than previously thought. Still, the effects may be significant: in many older people, the loss of neurons affects the activity of neurotransmitters, chemicals that provide the means for communication among cells in the brain and nervous system. The aging brain seems to lose neurons in structures deep within the brain that produce neurotransmitters, such as dopamine, acetylcholine, and serotonin, all of which are important for learning and memory (see Figure 4).

Animal research suggests that the age-related decline in the ability to learn new information may, in part, reflect a decline in the level of dopamine in the brain’s frontal cortex. For example, young rabbits learn new tasks better than older rabbits, and older rabbits have half the dopamine activity in several key brain regions. Similar results have been found in monkeys. When dopamine-rich areas of monkeys’ brains are damaged, the animals exhibit a significant impairment in attention and vigilance. Finally, when dopamine-producing neurons are transplanted into the brains of aging rats, their cognitive function improves. In a small human experiment, researchers concluded that dopamine enhances the strength of connections between neurons related to learning, and decreases the “excitability” of the connections between other types of neurons. This function could be one way dopamine improves memory and learning.

Perhaps of greater importance, some receptors may cease to function normally. Receptors are the points on neurons where neurotransmitters attach themselves. These receptors play a major role in helping the neurotransmitters involved in learning and memory move from one neuron to another. The effects of these age-related changes are especially noticeable in regions of the brain involved in attention and memory, such as the frontal lobes. The result is that as you age, it takes longer to absorb new information as well as to form new memories.

In addition, the loss of neurons and receptors may make it harder to concentrate. The ability to perform tasks involving attention and executive function (see “Testing executive function”) declines with age. Thus, when people of all ages encounter new information, they may all take in the big picture, but those who are older may not absorb as much detail. For instance, after listening to a presentation, a 25-year-old and a 75-year-old may both remember the overall subject and basic ideas, but the 25-year-old may be able to recall more of the specifics.

These changes may sound disturbing, but neuroscientists actually consider them relatively minor as long as they’re strictly a sign of aging and not of an illness such as Alzheimer’s disease. In other words, age-related changes in the brain may slow down your learning and your recall and may make it harder for you to apply strategies for learning. But ultimately, they don’t impair your ability to function effectively. For example, they have no effect on your ability to make sense of what you know or to form reasonable arguments and judgments. Your wisdom gained from experience remains unscathed. Brain scientists have found that people can compensate for the slowdown in information processing and diminished ability to concentrate if they work harder at paying attention when they encounter something new and consolidate the new information by repeating it in their minds and using it — for example, by talking about it with friends (see “Memory-enhancing techniques”).

Figure 4: A wide web of memories

Wide Web of Memories

A vast network of interconnecting neurons (brain cells) delivers and permanently stores messages along pathways in the brain, primarily in the cerebral cortex, the large, domed outer layer of the brain. Scientists now know that memories are not stored in a single area but in a network of different areas of the brain. Brain cells communicate from one cell to the next across spaces called synapses, by way of chemical substances. These neurotransmitters activate the receptors on the neighboring cell.

 

Growing new brain cells

Another surprise in our concept of brain aging concerns the growth of new neurons, a process known as neurogenesis. In the early 1970s, researchers found that adult rats and guinea pigs did in fact grow new neurons. The same proved true for cats, chickadees, tree shrews, and marmoset monkeys. But most scientists clung to the assumption that adult humans were different from these animals until 1998, when they found compelling evidence to the contrary.

The evidence came from a study of five people who had died of cancer. Before their deaths, their brains had been injected with a chemical that helped doctors count the number of new cells. The intent was to count the number of new cancer cells, but the chemical also revealed new healthy cells. Sure enough, all five patients had recently sprouted new healthy neurons, and these neurons were in the hippocampus. This finding was nothing short of revolutionary. It transformed the way neuroscientists think about the aging brain and memory.

But many questions remain unanswered about neuron growth in the human brain. Although researchers now know that the brain normally creates thousands of neurons each day, most of these cells die in the first weeks of their existence. And the mechanisms behind the birth and death of these cells are still largely a mystery. However, research on rodents may point to answers for some of these questions. A 2006 study in The Journal of Neuroscience hinted that when rodents were taught tasks that engaged several areas in the brain, more new hippocampal cells survived. Plus, there was a direct relationship between how much the animals learned and the number of surviving cells. Even cells that were born before the animals began the learning exercises had a better chance of survival. In another study in The Journal of Neuroscience in 2007, researchers demonstrated that when brain-injured mice were treated with grafted stem cells, they recovered their memory function to the point where it equaled that of healthy mice. Finding ways to use stem cells to treat neurodegenerative diseases, such as Parkinson’s disease, remains a hot topic for research, but translating the biology of stem cells into a therapy for patients is likely many years away. In a review paper on the subject of stem cell therapy and neurodegenerative diseases published in 2011 in the Annals of Neurology, the authors wrote that getting the therapy “from bench to bedside may take well over a decade.”

 

Changes in brain structure

Brain tissue can be divided into two categories — white matter and gray matter. The gray matter contains the neurons, while the white matter contains the insulated “wires” (axons) that run between the neurons, connecting them to each other. Both “colors” are important. Many studies have shown that loss of or damage to white matter in people who are aging normally (meaning those without Alzheimer’s disease or mild cognitive impairment) is associated with a decline in executive function, working memory, and speed of processing. Cerebrovascular disease (damage to the blood vessels leading to the brain that reduces blood flow to the brain and can cause stroke) may be at the root of these changes, but this is a matter of debate. Although uncertainties remain, it seems prudent to reduce cardiac risk factors (see “Cardiovascular disease and its risk factors”).

Other possible biological causes for a decline in mental capacity as a person ages are reduced connections between brain cells, diminished gray matter volume, and less availability of neurotransmitters like dopamine.

 

 

 

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