When Brain Cells Die - How Memory Changes With Age: Improving Memory Understanding Age Related Memory Loss

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When brain cells die

For years, the scientific view of an adult's brain was anything but encouraging. It was an unquestioned truth 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: Your brain functions were slipping away, too. The prevailing theory was that as you lost neurons, you also lost some of your capacity to learn, think, and remember. This is not the whole story.

Some of the old ideas about the brain have proved true. The typical adult brain does lose neurons with age, but far fewer than previously thought. This doesn't mean that the loss is insignificant: 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).

Figure 4: A 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 stored not 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 called neurotransmitters that activate the receptors on the neighboring cell's body or long tentacle-like dendrites.

Animal research suggests that the age-related decline in the ability to learn new information may, in part, reflect a decrease 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.

Perhaps of greater importance, some receptors for NMDA (N-methyl-D-aspartate) 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 memory. When scientists use imaging methods such as fMRI (see "Watching the brain at work") to observe the brain at work, they can see that the functioning of regions important in memory, such as the frontal cortex, changes with age. 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 functions (see "Testing executive functions") 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 many details. 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 details.

These changes may sound disturbing, but neuroscientists actually consider them relatively minor as long as they're strictly a sign of aging and not 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 remember. 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").

Last updated:	January 23, 2007