Since Operation Iraqi Freedom began two years ago, the Department of Defense reports that more than 1,500 U.S. soldiers have died and more than 11-thousand American troops have been wounded. Among the most debilitating wounds are traumatic brain injuries. One of the nation's leading brain treatment centers is the Veteran's Affairs hospital in Palo Alto, California. Within the confines of the sprawling VA campus, veterans of World War II and the Vietnam war work on rehabilitation alongside returning soldiers from Iraq and Afghanistan.

In today's warfare, the silver lining, if there can be one, is that new body armor and better emergency medical techniques are keeping more soldiers alive. But one area that can't entirely be protected is the face and parts of the skull.

That's where Erik Castillo was wounded. "They say I got injured by four mortar rounds landed nearby, close to me, and exploded and the shrapnel hit my head," he explains. The 21-year-old Army Specialist was injured in July in Baghdad. The right side of his forehead was shattered, and now sinks sharply inward. His right eye droops and can't align with the left. Speaking from his hospital bed, he repeatedly strokes the hair above his injury. He has physical injuries, too. "My left side of the body doesn't work so well," he says. "So I can't move my arm or my leg like normal. So that means I can't do regular stuff. Like I used to walk, grab things with my left arm." Doctors say his symptoms resemble those of a very severe stroke. He works daily with occupational and physical therapists.

Most patients take 18 to 24 months to recover from a traumatic brain injury. The severity of the injury determines how much function can be regained. It's a slow process because brain cells are unique. In other parts of the body, healing is done by replacement cells. For example, if you cut yourself, a few days later there's new skin being formed underneath the scab. But Harriet Zeiner, a clinical neuro-psychologist at the Palo Alto hospital, says the brain heals differently. "It's designed to," she explains, "because it's a repository for all the information you're ever going to have. And so if it got replacement cells, one of the main problems is, they'd be dumb, they wouldn't have access to all the information you've acquired over a lifetime."

Over the past three years, 440 U.S. troops who served in Iraq and Afghanistan have been treated for traumatic brain injuries; However, Dr. Zeiner believes many veterans with less severe brain damage are not getting the medical attention they need. The gaping hole in Erik Castillo's skull left no doubt about the severity of his injury. But for others, the diagnosis is not so obvious. Dr. Zeiner says just being near a blast can rattle someone's brain. And since there are no outward signs of injury, the soldiers are often unaware that their brain has suffered physical trauma. "The individual looks perfectly okay," she says. "They sound the same, they look the same, there's no injury. They simply think more slowly, they have memory lapses, they don't read the emotional signs from their partners very well. They appear to have changes in empathy."

Too often, she says, soldiers who complain of irritability or changes in concentration are treated for psychiatric problems, rather than neurological issues. Ideally, she says, she'd like to see the U.S. military screen all combat soldiers for signs of brain trauma before they return to civilian life.