Narrator: This is Science Today. Many people believe a concussion means losing consciousness, but it's really any alteration in mental function that occurs after a blow to the head. Martin Holland, a clinical professor of neurosurgery at the University of California, San Francisco describes what happens when a concussion occurs.

Holland: The cell gets disturbed so much that it takes a heck of a lot of energy to try and reestablish the cell's normal state. The cell goes into a state of starvation where it can not produce enough energy to try to reestablish all its biochemical mechanisms.

Narrator: Symptoms may be subtle - a headache, dizziness, memory lapses and confusion or even a personality change.

Holland: For the most part, they are most severe immediately or right around the time of the concussion and with time they recover. Now, on occasion you'll find a patient that does not recover from the post-concussive syndrome and we really don't know why they don't. But for the most part, most people do and by six months, most people who have had a concussion that have suffered post-concussive syndrome no longer have anything.

Narrator: For Science Today, I'm Larissa Branin.