BOSTON (Reuters) - Tests of 2,525 U.S. combat veterans after returning from Iraq have found that depression and post-traumatic stress disorder play key roles in determining who will suffer from health problems following a mild brain injury.

"We thought the symptoms would be related to concussion, but they turned out to be most strongly related to PTSD," said Dr. Charles Hoge, a psychiatrist at the Walter Reed Army Institute of Research.

The research, published on Wednesday in the New England Journal of Medicine, also suggests that the rate of such injuries is high.

"In this study, nearly 15 percent of soldiers reported an injury during deployment that involved loss of consciousness or altered mental status," Hoge and his colleagues reported.

More than 1.5 million Americans have gone to Iraq or Afghanistan since 2001, and some estimates of the rate of head and neck injuries as high as one in four.