There's a saying that people can get used to anything. Unfortunately, it seems to apply to teenage boys watching violent media clips.

A recently published study measured the effect of showing a series of short video clips that varied in violence to teenage boys and measuring both their skin response and the activity in specific parts of their brain.

Not only did the researchers see imaging evidence of the violence becoming more mundane to the boys as time passed, they also saw that this effect was strongest in the boys whose questionnaires showed the most exposure to violent media in their daily life.

The results showed that the more clips the teenagers viewed, the less sensitive they were to the violence in those clips. Both skin conductance (sweat level) and activity in a portion of the brain called the LOFC decreased as they saw more and more clips. While they still showed a reaction to the least violent clips, clips with mild or moderate violence elicited less of a reaction the longer the study went on.

It's possible that similar results would have been seen had they been shown humorous clips. After all, by the 60th clip they might be all laughed out. But this study has some disturbing implications that would not have come from the teenagers viewing funny clips.

The LOFC (left lateral orbitofrontal cortex) is an area of the brain thought to influence aggressive behavior in individuals. Desensitization in the LOFC is thought to increase the likelihood of aggressive behavior. And adolescent brains are not yet fully developed. It's not clear how viewing so much violence affects future brain development, but there are many troubling possibilities.

The researchers suggest that the decrease they saw in teenage boys' LOFC activity upon repeated viewing of violent clips shows a blunting of emotional response that may increase the probability that these boys see aggression as acceptable behavior, leading them to behave more aggressively in real-life situations.

They point out that it is not known whether exposure to violent media does cause viewers to act more aggressively. What this study and others do show is that repeated exposure to violent media makes viewing the violence much less shocking. What that does to a viewer's real-life behavior is a subject of much heated debate, both among researchers and the general public.

In a pre-study, the researchers collecting 114 four-second video clips which they characterized as differing in their levels of violence but not in excitement. Twenty-two teenagers rated the violence level in these clips as low, mild or moderate, and 60 of these clips (20 of each degree of violence) were chosen for use in the actual study.

The types of violence shown in the clips included fist fights, street brawls and stadium violence.

For the actual study, a different group of 22 teenagers was chosen, aged 14-17. Each subject viewed three separate runs of all 60 videos, indicating by a button whether the video they were currently seeing was more or less violent then the previous video. Subjects' brains were scanned by fMRI (functional magnetic resonance imagery) while they viewed the videos. Additional information was collected from each subject about their background and their emotional state before viewing the videos, immediately after viewing the videos and one day and two weeks later.

When analyzing the results, not only did the researchers see imaging evidence of the violence becoming more mundane to the boys as time passed, they also saw that this effect was strongest in the boys whose questionnaires showed the most exposure to violent media in their daily life.

No one really knows what effect so much exposure to violent media is having on society as a whole. This study indicates that at the very least, it's making the idea of violence much less shocking than it otherwise would be.

An article on the study was published online by the journal Social Cognitive and Affective Neuroscience on October 7, 2010.