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Now, a new study says that may be because they don't have enough of a certain kind of receptor in the limbic system of the brain that’s involved in emotional processing.
These mu-opioid receptors are part of a neurochemical system that plays an important role in pain relief and pleasure responses.
Understanding the role of these receptors in a person's response to seeing awful images may lead to a better understanding of post-traumatic stress disorder (PTSD), which is suffered by soldiers, rescue workers and others who experience traumatic events, say the study authors.
This study makes "a link between areas of the brain that are responding to the unpleasant pictures and this neurochemical system -- the opioid system," says Dr. Stephan F. Taylor, one of the authors and an assistant professor of psychiatry at the University of Michigan.
"This is a target area to understand how responses to aversive stimuli are handled" by the brain, Taylor says.
The study appeared in the Proceedings of the National Academy of Sciences.
The researchers, using positron emission tomography (PET) scanning, looked at the relationship between the mu-opioid receptors and brain activity, indicated by blood flow, in the limbic system.
Disturbing pictures
The study involved 12 males who each underwent two PET scans. The first scan measured the number of available mu-opioid receptors in the men's brains. The second scan measured brain activity in the men as they were shown both neutral and unpleasant black-and-white images.
The unpleasant images included pictures of bloodied human faces or corpses. The other set included benign scenery and faces with neutral expressions.
All the men had increased limbic system activity when they looked at the unpleasant images. However, the amount of activity was greater in the men who had lower numbers of available mu-opioid receptors.
It appears those receptors may help inhibit the brain's response to emotional stimuli, the authors say.
Understanding PTSD
"If we understand the mechanism behind a person experiencing an extreme reaction to stress, we definitely have more knowledge that helps us design treatment" for people with PTSD, Taylor says.
An expert in PTSD says this particular opiate receptor has never been looked at before in relation to how people respond to unpleasant imagery.
"We have a general idea that the limbic system is involved in the processing of emotional information, and this study adds the idea that there may be a specific role of mu-opioid receptors," says Rachel Yehuda, director of the Traumatic Stress Studies Division at Mount Sinai School of Medicine and the Bronx Veterans Affairs Medical Center.
She says this study may help lead to a better understanding of PTSD. The next step would be to look at trauma survivors with and without PTSD, and compare their numbers of mu-opioid receptors.
It's possible that people who are more likely to develop PTSD have fewer mu-opioid receptors either before they are traumatised or after they develop PTSD.
"Since having fewer receptors is associated with increased responsiveness to something unpleasant, then this would help explain individual differences in why people who undergo the same event have different outcomes," Yehuda says.
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