Intellectual capacity

In the first large studies to follow the same group of normal healthy humans for many years, scientists have uncovered unexpected results. They report declines in some mental functions and improvements in others. In one study, the speed of carrying out certain tasks became slower, but vocabulary improved. Several studies found less severe declines in the type of intelligence relying on learned or stored information, compared with the type that uses the ability to deal with new information.

This research is supported by animal studies in which scientists found that changes in mental function are subtle. For example, in rodents and primates in which only minor brain abnormalities can be detected, certain spatial tasks, such as navigating to find food, tend to become more difficult with age.

The aging brain is only as resilient as its circuitry. Scientists debate whether this circuitry is changed only by neuron atrophy or whether some neuron loss over time also is inevitable. In any event, when the circuitry begins to break down, remaining neurons can respond by expanding their roles.

Learning conditions may dictate what happens to brain cells. Studies of rats shed light on some of the changes that occur in brain cells when the animals live in challenging and stimulating environments. In tests of middle-aged rats exposed to these environments, researchers found that dendrites in the cerebral cortex, which is responsible for all conscious activity, developed more and longer branches when compared with rats housed in isolated conditions. Another study showed that brain cells in rats given acrobatic training had greater numbers of synapses per cell than rats given only physical exercise or rats who were inactive. The scientists concluded that motor learning generates new synapses. Physical exercise, however, improved blood circulation in the brain.

Other scientists report that rats reared in a stimulating environment made significantly fewer errors on a maze test than did similar rats kept in an isolated environment. Moreover, the stimulated rats showed an increase in brain weight and cortical thickness when compared with the control animals.

Older rats tend to form new dendrites and synapses as do younger animals in response to enriched environments. But the response is more sluggish and not as large. Compared to younger rats, the older rats have less growth of the new blood vessels that nourish neurons.

While much has been learned about the aging brain, many questions remain to be answered. For instance, does the production of proteins decline with age in all brain neurons? In a given neuron, does atrophy cause a higher likelihood of death? How does aging affect gene expression in the brain—the organ with the greatest number of active genes? Are there gender differences in brain aging that may be due to hormonal changes at menopause?

Neuroscientists speculate that certain genes may be linked to events leading to death in the nervous system. By understanding the biology of the proteins produced by genes, scientists hope to be able to influence the survival and degeneration of neurons. (From the book Brain Facts: a Primer on the Brain and Nervous System, 2002)

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