By Fergus Walsh
BBC News medical correspondent
Virtual reality treatment for US troops
A virtual reality computer programme is being used to treat Iraq war veterans in the US.
The soldiers are able to relive the sights, the sounds and even the smells of warfare.
In a small windowless room a US marine puts on a 3D headset and picks up a dummy rifle.
Sergeant Robert Butler has been a marine for nearly 20 years and done two tours of Iraq.
After his last stint he returned with post traumatic stress disorder - what was once called shell shock.
Now he can finally deal with painful memories of the horrors of war.
Sergeant Butler believes his psychological problems stem from a patrol in 2005 where he witnessed the death of a father and his teenaged son who were killed after being caught up in a fire fight.
His son was about the same age as the boy who died.
"When I first came back I was just a complete recluse and avoided outside contact," he said.
Initially he was reluctant to join the virtual Iraq programme.
"I thought PTSD was something the doctors dreamed up for job security," he said.
"But I'd hit the point in my life where I felt I had zero control and was about to lose the one thing in my life that meant the most which was my family, so I was prepared to try anything."
Sergeant Butler demonstrates the computer scenario which was used to help him.
On a computer screen I can see the same image projected onto Sergeant Butler's visor.
He is in the front seat of a Humvee armed vehicle patrolling the streets of Iraq; each time he turns his head, the viewpoint on the screen changes.
Sights, sounds and smells
An explosion ahead cracks the front windscreen and you see that the virtual soldier sitting alongside him is wounded, blood streaming down his arm.
The platform, on which Sergeant Butler is sitting, vibrates, to add to the sense of reality.
And there are not just the sights, sounds and vibrations of war, there are also the smells.
Sergeant Butler has struggled with post traumatic stress disorder
These come from a machine which can release the scent of burning rubber, Middle Eastern spices, cordite, diesel fuel - even body odour.
Commander Scott Johnston, a clinical psychologist, runs the programme at the Naval Medical Center San Diego.
He said: "Our different senses are very powerful cues to our memory.
"Instead of allowing the person to continue to avoid these memories and haunt them, if we bring them out into the daylight and really face them we can decrease the negative effects on the individual."
This begins to explain how the programme is supposed to work.
The theory is that by repeatedly running the computer scenario it enables soldiers with PTSD to unlock and then discuss troubling wartime experiences which have been buried away.
After each thirty minute session on the computer, the soldiers have an hour of talking therapy with a psychologist.
"I'm a completely changed person", says Sergeant Butler.
"Am I 100%? No, because PTSD will always be part of my life; those memories never go away.
"But it definitely has helped me to take steps and file that information.
"It does come up, it gets processed like any other memory and I'm able to do the things a lot more now than before the war."
Commander Johnston says the preliminary results are exciting.
"We found that 30 out of 40 of our subjects were able to return to full duty so we are now starting to implement it across the different services for our returning warriors."
Many British as well as American troops have suffered psychiatric problems after serving in the Middle East.
But the Ministry of Defence in London has yet to be convinced by the virtual Iraq programme.
It says for some years it's been exploring the possible uses of virtual reality in treating mental health conditions, but this is still very much "work-in-progress".