The virtual reality world, Spira said, "triggers those thoughts, those feelings, those earlier reactions they had. What we're trying to do now is train them to have different reactions to those same stimulus and reminders."
For Frey, who served with the 3rd Marines 1st Battalion, the thoughts and feelings are still vivid. He says he was always on edge in Iraq from the constant threat of mortar attacks, roadside bombs and sniper fire.
"There never was a really secure place," he said, thinking it couldn't get much worse.
Then came Dec. 12, 2004.
Frey says the morning had been relatively quiet when a gunbattle suddenly broke out between insurgents and Marines in the city of Fallujah. Dickenson was shot in the head during the firefight and died. Frey, responding to a call of a man down, was hit a few minutes later.
The bullet ripped through Frey's right shoulder, shattering the bones in his arm and causing major nerve damage.
But when he got home, he found his mental wounds were at least as debilitating as the physical ones. He hopes the virtual reality treatments will help him get his life back on track.
"It gets me to control my breathing," Frey said. "I just stay in the moment and know I'm not over there."
Frey is experiencing a digital landscape developed by Dr. Skip Rizzo and his team at the University of Southern California's Institute for Creative Technologies. Rizzo and his colleagues created the simulation by recycling the virtual world designed for the combat video game "Full Spectrum Warrior."
A psychologist, Rizzo believes virtual reality therapy is so effective because it forces patients to recall painful memories rather than avoid them.
In traditional talk therapy, Rizzo said, "you're asking somebody who's been traumatized by an environment to imagine it in the great detail needed to produce a therapeutic effect. With virtual reality, we know what the person is seeing."