To investigate how the brain recovers from injury, researchers surgically created a small lesion on one side of a rat's brain, in a region that controls one forelimb. In the weeks following the surgery, neurons on the opposite side of the brain, which control the nonimpaired forelimb, sprouted new branches that could link to other brain cells. These new connections compensated for the brain injury, helping the rat regain use of its leg.

But when Timothy Schallert, Ph.D., and Theresa Jones, Ph.D., immobilized the non-impaired limb for 15 days following surgery by outfitting the rat with a plaster cast, no new growth occurred. Using the healthy leg in the days after the injury was crucial for the rat's brain to forge new neural networks.

The idea that the timing of activity influences recovery from brain injury has profound implications for patients who need physical therapy. Often, Schallert notes, patients don't begin rehab until their condition has stabilized. But for some types of brain damage, that initial post injury period might be when therapy is most effective at stimulating recovery. Due to the brain's complexity, however, early rehab will not be a panacea for all head injuries.

Schallert, a psychologist at the University of Texas, speculates that, by a similar process, staying mentally active could slow brain degeneration from old age and Alzheimer's disease. Imagine that an aging brain cell dies. The rat experiments suggest that there may be a brief window of opportunity for neighboring cells to pick up the slack. An inactive brain, says Schallert, "might be passing by tiny periods of rehabilitation."