Dr. Henry Krystal, who was forced into slave labor by the Nazis as a teenager and later focused his noted psychiatric career on the emotional scars carried by former concentration camp inmates, died on Oct. 8 at his home in Bloomfield, Mich. He was 90.

The cause was complications of Parkinson’s disease, his sons said.

In his decades of research, Dr. Krystal, as a professor of psychiatry at the Michigan State University College of Osteopathic Medicine, interviewed more than 2,000 survivors of the Holocaust, drew on his own wartime reflections and studied the responses of victims of the atomic bomb attack on Hiroshima, Japan.

He concluded that because traumatized individuals complained primarily of physical manifestations, like headaches, they overlooked potential treatment for depression and other emotional symptoms, leaving themselves vulnerable to self-medication, like addiction and other impulsive behaviors.

His work led to novel healing responses to post-traumatic stress disorder, like biofeedback and cognitive psychotherapy. The disorder was recognized in veterans after the war in Vietnam.