The brains of US soldiers with post-traumatic stress disorder or traumatic brain injury remain a mystery to the US armed forces, according to a top US Army official.

"The issue is, we just don't know that much about the brain," Gen. Peter Chiarelli, the Army's vice-chief of staff, said in an interview on ABC's "This Week" Sunday.

He continued, "We automatically assume so many times that a person that's in a blast has a concussion. Many times, they don't have a concussion. Instead, they have posttraumatic stress."

Gen. Chiarelli told host Christiane Amanpour that understanding soldiers' wounded brains is the key to meeting America's "moral obligation" to help them transition into civilian life.

"You know, the problem with post-traumatic stress is that in the United States, the National Institute of Mental Health will tell you, for regular civilians, it is 12 years between the initiating event and when someone first seeks help," he said.

Chiarelli continued, "Now the issue there isn't that they finally seek help. It's all the things that happen in between."

The Army general stressed that giving the troops more time to rest at home before new deployments would go a long way in preventing divorce, homelessness, substance abuse, as well as suicide among servicemen and women and veterans.

Of 1,500 pilots deployed today, 62 percent were on their third deployment, 40 percent were on their fourth, he noted.

Roughly two million American servicemen and women have been rotated through the theaters of war in Iraq and Afghanistan over the past decade.

Chiarelli defended the Veterans' Administration, which has a million disability claims in a backlog that Amanpour noted could take years to process.

"But," he admitted, "the disability evaluation system is a World War II relic. It was designed for a whole different force coming out of World War II."

When asked when the system would be fixed, the general pointed to Congress.

"We need to fix it right now," he said. "We need to take this one on."

Chiarelli concluded, "We need to understand that a volunteer army is totally different from the Army we had in World War II, that it is a totally different kind of soldier in today's Army and throughout the services, and we've got to fix that system."

This video is from ABC's This Week, broadcast Dec. 26, 2010.