So what has changed in Dennis's brain to leave him so fearful? Doug Bremner exposes Dennis to vivid reminders of the Vietnam War. Pictures and sounds designed to frighten in order to study the brain circuits involved in the fear response. one current theory is that fear involves two separate pathways in the brain.

Any potential threat activates a specialized fear structure called the amygdala, which automatically sends out signals triggering responses like sweaty palms or increased heart rate. But a slower second circuit routes right through the cortex. This is how we weigh up a threat. And if it's a false alarm, the amygdala is shut down.

So it's a delicate balance. You need the permanent part of the brain to survive to turn on the fear response. But you don't want the fear response become so incapacitating that you can't think.

Could this balance have been upset in Dennis's brain?

Doug Bremner has looked at the brain activity in traumatized veterans compared with those who have come through unscathed. He's found there's far less activity in one particular area of Dennis's cortex than there should be. This area is part of the pathway which normally shuts down the automatic fear responses when they're not needed. So in Dennis's brain, even the slightest threat unleashes the full terror.

So why can't Dennis control his fear responses?

Doug Bremner thinks the best clue comes from the way patients like Dennis seem to dwell on their memories of the war.

One of the things that I found most interesting when I first began working with Vietnam veterans was that many of my patients, they could remember what happened in Vietnam as if it were yesterday. And they'd show me picture books of their friends from the war and talk about them as if they'd just seen them the day before. But they couldn't remember what they had for breakfast that morning. They had sort of a vague idea about maybe how long they'd been in the hospital. And these more recent things, they had a lot of problems with.

Bremner believes that Dennis's faulty fear response also involves a brain structure called the hippocampus, known to play a vital role in memory.

We did an MRI scan on Dennis. And this is his hippocampus here, this grey area there. And it's visibly— I can just look at this scan and tell you that it's small in volume compared to a normal individual where the hippocampus is much larger.

This change to Dennis's hippocampus is very exciting. Constant terror has changed the structure of his brain.

This is from the land we left so much at. It's incredible.

It's as if his experiences in Vietnam have worn such a deep groove that he's trapped in the past. Even a handful of soil from the Ho Chi tunnels is enough to reduce him to tears.