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 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 is a crippling condition that can emerge after a terrifying event -- car accident, sexual assault, terrorist attack or combat. It's thought to affect as many as 1 in 5 veterans returning from Afghanistan and Iraq.

Military doctors have been mystified as to why certain war fighters 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 Post-Traumatic Stress Disorder. "Perhaps with better measures, we can get closer."

Earlier this year, a quarterly publication from the national PTSD center found that studies to date have looked at only "a narrow band of the potential risk and resilience predictors" and that more work beyond surveys was needed.

New PTSD studies are using technology to try to get at the answer. Select Marine and Army units are undergoing a battery of physical and mental tests before deployment including genetic testing, brain imaging and stress exams. They are followed in war zones and upon return.

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.

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.