People with post-traumatic stress disorder (PTSD) may be less sensitive to pain than others who don't have the condition, say Dutch scientists.
Why some are vulnerable to PSTD remains a mystery
PTSD patients report panic attacks, flashbacks, anxiety and depression following a traumatic event.
The Archives of General Psychiatry study suggests PTSD patients' brains may be wired up differently.
A UK expert said it was unknown why some people developed the condition after trauma, while others did not.
Researchers used brain scans to compare what happened when volunteers were given hot objects to hold.
The PTSD patients generally said the objects felt less hot.
Scans confirmed their brains were less active than those of their unaffected counterparts.
The researchers do not know why the processing of pain signals should be different in patients with the condition.
Many of those diagnosed with PTSD had taken part in conflicts as part of the armed forces, and the project was carried out jointly between the Central Military Hospital in Utrecht and the city's Rudolf Magnus Institute of Neuroscience.
A total of 24 military veterans, half with PTSD and half without, were chosen to take part.
The research used a functional magnetic resonance imaging (fMRI) scanner, which can show, in real-time, which areas of the brain are most active.
The veterans were placed in the scanner, then given objects heated to an uncomfortable temperature to hold. They were asked how painful it was to hold the object.
The level of pain reported by the PTSD-diagnosed veterans was significantly less than that reported by the other volunteers.
At the same time, the brain activity measured by the scanner in the parts of the brain which process pain signals was also far less marked in the PTSD group.
While other studies have suggested that patients with PTSD may have differences in the structure of certain parts of their brain compared with healthy people, the research team did not suggest a reason for the difference in pain regulation between the two groups they tested.
Dr Stuart Turner runs The Trauma Clinic, a private clinic aimed at patients with PTSD in London, and helped the National Institute for Clinical Excellence (NICE) produce recent guidelines on the treatment of the condition.
He said the underlying reasons why some patients developed PTSD but others didn't were still largely unknown.
He said: "Studies have shown that virtually every adult, during his or her lifetime, will experience an event sufficient to cause post-traumatic stress disorder, but only a few will actually go on to develop the condition.
"We do know that, as well as the traumatic event itself, there are other 'vulnerability factors' such as lack of social support after the event which make PTSD more likely."