The NFL playoffs continue this weekend, with the San Francisco 49ers on a quest to reclaim Super Bowl glory. But once the ultimate bragging rights are settled, one thing remains unsettled, and unsettling. Concussions are endemic in football at every level, from peewees to the pros. Still, we don't know exactly how they're caused. Adding urgency, a study of brain tissue from former San Diego Chargers linebacker Junior Seau revealed this week he had degenerative brain disease, a possible factor in his suicide last year. Now, Stanford University is searching for answers -- and its laboratory is the football field itself.