SCIENCE FILE: An exploration of issues and trends affecting science, medicine and the environment.

Q: At what age do we have our maximum number of brain cells? Does somebody in a very cerebral occupation, such as a mathematician, use up brain cells faster than somebody who performs primarily physical work?

A: The brain continues to expand very slowly after birth and the maximum number of brain cells occurs about the age of 5, said Dr. Richard Haier of UC Irvine. Between the ages of 5 and 20, superfluous brain cells are trimmed away in a process known as “neural pruning.” Smaller numbers of brain cells continue to be lost throughout the rest of our lives. Brain cells aren’t really used up. In fact, they are more likely to wither away if they are left fallow. As is the case with much of the rest of the body, those who don’t use it, lose it.