Two University researchers have determined for the first time how parts of the brain change at various ages, explaining why music is easier to learn in childhood, why foreign languages are more difficult to learn after age 12 and why some subjects, such as calculus, are better taught when children are older.

According to Peter Huttenlocher, Professor in Pediatrics, Neurology and the Committee on Neurobiology, the brain's ability to process different kinds of stimuli varies greatly from birth through adolescence. Huttenlocher and co-author Arun Dabholkar, Research Technician in Pediatrics, report their findings in "Regional Differences in Synaptogenesis in Human Cerebral Cortex," published in the Journal of Comparative Neurology.

By measuring the number of synapses -- connections between nerve cells -- Huttenlocher and Dabholkar were able to establish when the production of synapses reached its peak and when it began to decline in three portions of the brain: the auditory cortex, the primary visual cortex and the middle frontal gyrus (which controls higher-order thinking skills).

The production of synapses in the brain is strongly linked to the ability to learn. The later the production of synapses peaks in a particular portion of the brain, the more the learning related to that portion is influenced by such environmental factors as teaching and parental nurturing.

"We have always suspected that there may be a difference in the ways children learn at different ages, but until we did this study we didn't know for sure that those differences reflected development of synapses," Huttenlocher said.

The study was prompted by research on monkeys that showed that various portions of a primate's brain exhibited a similar pattern of synaptic development across a monkey's life span.

To determine if a similar situation exists among humans, Huttenlocher and Dabholkar used an electron microscope to measure synapses in sections of brains after autopsies. The three areas they examined are critical to learning. The auditory cortex helps process sound, the visual cortex helps process sight and the middle frontal gyrus controls motivation, as well as sophisticated thinking skills.

Huttenlocher found that the auditory cortex reaches its highest synaptic density at age three months and then begins to decline until age 12, when it levels out. For the visual cortex, the decline appears to begin even earlier.

In the case of the middle frontal gyrus, the peak in synaptic density is not reached until a child is three and a half years old. The slow decline of the number of synapses does not appear to stop until mid-adolescence, the researchers found.

By the time a person is an adult, the number of synapses in the various parts of the brain are relatively equal.

The findings have a number of implications for learning:

• The visual cortex appears least influenced by the impact of environment. People don't need to be taught to use their eyes in a binocular fashion to focus simultaneously on the same object, for example.

• Because the auditory portion of the brain develops during childhood, music is easier to learn at an early age, as are foreign languages.

• Because the middle frontal gyrus is the last of the three portions to develop, tasks that require higher-order thinking skills as well as those dependent on motivation are difficult to perform until a child reaches adolescence. The majority of students need to be in their teens before they can grasp the concepts of calculus, for instance.

"There has been a great deal of emphasis lately on the importance of early learning," Huttenlocher said. "That is important, but we need to realize what children are able to learn and not cram them with information they are not ready to handle.

"Similarly, we need to appreciate what adolescents can learn. They are able to perform higher-level thinking that is beyond most younger children.

"We also need to realize that because the portion of the brain controlling motivation develops last, we shouldn't be surprised if high school students have trouble making decisions about their life's work," Huttenlocher said. "It may very well be that their brains need to develop further, that their brains simply aren't prepared to make such decisions until early adulthood."