JUDY WOODRUFF:

Now to our special three-night look at the efforts to treat, diagnose and possibly find a cure to post-traumatic stress disorder.

One in five U.S. military personnel serving in combat will suffer some form of PTSD. But it often goes untreated because of the stigma associated with the disorder.

Tonight, special correspondent Soledad O'Brien begins our series, War on the Brain.

Almost every day, at some point, I would relive the IED attack.

It was like I couldn't turn my mind off, like I was being flooded, like a cyber-attack, reliving that explosion like a broken record. It just kept playing, and I couldn't stop it.

COL. GREG GADSON (RET.), U.S. Army: I remember I was driving, and I opened my window, and I turned my radio up, and I got it to stop. It was like it left an imprint on me, and I didn't even know where it came from.