For years, many scientists have assumed that explosive blasts affect the brain in much the same way as concussions from football or car accidents. Perl himself was a leading researcher on chronic traumatic encephalopathy, or C.T.E., which has caused dementia in N.F.L. players. Several veterans who died after suffering blast wounds have in fact developed C.T.E. But those veterans had other, nonblast injuries too. No one had done a systematic post-mortem study of blast-injured troops. That was exactly what the Pentagon asked Perl to do in 2010, offering him access to the brains they had gathered for research. It was a rare opportunity, and Perl left his post as director of neuropathology at the medical school at Mount Sinai to come to Washington.