TWENTYNINE PALMS, Calif. — Two days before shipping off to war, Marine Pfc. Jesse Sheets sat inside a trailer in the Mojave Desert, his gaze fixed on a computer that flashed a rhythmic pulse of contrasting images.
Smiling kids embracing a soldier. A dog sniffing blood oozing from a corpse. Movie star Cameron Diaz posing sideways in a midriff top. Troops cowering for safety during an ambush.
A doctor tracked his stress levels and counted the number of times he blinked. Electrode wires dangled from near his left eye and right pinky finger.
Sheets is part of a military experiment to try to predict who's most at risk for post-traumatic stress disorder. Understanding underlying triggers might help reduce the burden of those who return psychologically wounded — if they can get early help.
PTSD can emerge after a terrifying event — car accident, sexual assault, terrorist attack or combat. It's thought to affect as many as one in five veterans returning from Afghanistan and Iraq.
Previously called shell shock, combat fatigue and post-Vietnam syndrome, PTSD was officially recognized as a mental disorder in 1980. Sufferers experience flashbacks, nightmares, sudden outbursts and social withdrawal and are sometimes haunted years after the trauma.
Military doctors have been mystified as to why certain troops exposed to bombings and bloodshed develop paralyzing stress symptoms while others who witness the same trauma shake it off.
Studies on veterans and civilians point to some clues. Childhood abuse, history of mental illness and severity of trauma seem to raise a person's risk. Having a social net and a coping strategy appear to offer some protection.
However, none of the factors explored so far are reliable predictors.
"Right now, we can't determine with certainty who will and who won't develop PTSD," said Paula Schnurr, deputy executive director of the Department of Veterans Affairs' National Center for Posttraumatic Stress Disorder. "Perhaps with better measures, we can get closer."
There's an urgency to detect early signs. Since the 2001 terrorist attacks, more than 1.8 million U.S. troops have fought in Afghanistan or Iraq. The Obama administration is weighing whether to increase forces in Afghanistan where violence has escalated in recent months.
One autumn morning, a throng of Marines squeezed into a trailer at the Marine Corps Air Ground Combat Center in Southern California before deploying to Afghanistan. They belonged to the 3rd Battalion, 4th Marine Regiment — nicknamed the "Thundering Third."
"We're doing this not to make you better prepared to go do what you have to do in Afghanistan. We're not doing this to make your health any better," said Dr. William Nash, a retired Navy psychiatrist and study co-investigator. "We're doing this so that we can learn more about how to protect Marines from stress injuries like PSTD."
Nash asked how many have heard of PTSD. A half dozen raised their hands.
Who wants PTSD? "Right, nobody," he answered rhetorically.
The trailer soon buzzed like a factory, with Marines rotating from one test station to another in an assembly line. They donated blood, urine and saliva samples so researchers can search for genetic biomarkers that might play a role.
Marines also underwent a blink test to gauge their startle response and neuropsychological screening. They filled out questionnaires and were interviewed by psychiatrists with a checklist to diagnose PTSD.
The work is funded by the Marine Corps, Veterans Affairs and Navy Medicine. Last year, about 1,000 Marines were recruited before leaving for Iraq.
This latest batch of 673 Marines who were tested during a two-week period in the fall headed to Afghanistan, where they're sure to see intense fighting. They will be followed up in the field by Navy corpsmen with special "stress first-aid" training to read early signals.
Researchers recently did six-month follow-up testing on some Marines who returned from Iraq. It will take time to analyze the results, said the study's lead investigator, Dr. Dewleen Baker of the VA San Diego Healthcare System.
Similar research is ongoing 1,300 miles away at the University of Texas at Austin's Laboratory for the Study of Anxiety Disorders, where scientists have collected detailed health data from 178 soldiers from nearby Fort Hood who recently came back from Iraq. The base was the scene of a massacre on Nov. 5. Army psychiatrist Maj. Nidal Malik Hasan is accused of opening fire, killing 13 people and wounding dozens more.
The decision to volunteer in the Marine experiment was personal for Lance Cpl. Jaecob Kyllo. His grandfather fought in Korea and Vietnam and two uncles served in Operation Desert Storm. Kyllo said his uncles were diagnosed with PTSD and suspects his grandfather had it, too.
"I've seen it before and it's not the most pleasant thing," said the 20-year-old from Seattle, who previously served in Iraq.
Sheets, 22, a private first class from Newark, Del., has never seen combat before. Before joining the Marines, he dabbled with college and was working a dead-end job hauling trash.
While in infantry school, he met a sergeant who suffered from PTSD. He's not exactly sure what it is, but has heard horror stories.
"A guy comes home from war and he's freaking out. He's beating his wife. He's drinking. He's doing everything he can. He'll go off and he'll hate the Marine Corps," Sheets said. "And it's just like, okay, is that going to be me when I come home?"