It appears that "choice," even in animals, can impact brain function. According to Steven Arnold, a professor of pschiatrhy and neurology at the University of Pennsylvania and director of the Penn Memory Center, animal studies have shown that the brain makes new cells when animals exercise. However, this appears to happen only when the animals choose to exercise. For example, when a mouse is forced to run on a wheel, the effects to the brain are not as beneficial as when the mouse voluntarily chooses to run on the wheel just for fun. Find a form of physical exerecise that you enjoy and do it just for fun.