Is there a connection between PTSD and combat blast exposure?

JUDY WOODRUFF: Tonight, we conclude our series War on the Brain. Special correspondent Soledad O'Brien reports on the efforts of researchers to find the cause of post-traumatic stress disorder. JACOB FADLEY, U.S. Army Veteran: Having PTSD is -- it's like being in a room where you have no control and everything's going wrong. Triggers are, for me, when I'm in traffic. SOLEDAD O'BRIEN: Jacob Fadley served 12 years and four tours in Iraq and Afghanistan.