Image caption The longer the boys watched the videos, the less they responded to the violence within them

Repeated viewing of violent scenes in films, television or video games could make teenagers behave more aggressively, US research suggests.

The National Institutes of Health study of 22 boys aged 14 to 17 found that showing dozens of violent clips appeared to blunt brain responses.

Dr Jordan Grafman said it might make aggression feel more "acceptable".

However, a UK expert said the reasons behind violence were too complex to be explained by laboratory research.

The effect of violent imagery on young people has been debated from the early days of television, and, more recently, that debate has expanded to include video games.

Various studies have suggested that exposure appears to have an effect on the way that the brain processes emotional responses, yet it is unclear whether this can have a direct impact on behaviour.

The US study, published in the journal Social Cognitive & Affective Neuroscience, involved 60 violent scenes from videos being collated, mostly involving street brawling and fist fights.

The suggestion is that, over a period of time, people can develop a kind of tolerance to these images - but another word for that is just boredom Professor David Buckingham, Director of the Centre for the Study of Children, Youth and Media

The violence was ranked "low", "mild" or "moderate", and there were no "extreme" scenes.

The response of the boys as they watched the clips was measured in a number of ways.

They were asked to rate whether they thought each clip was more or less aggressive than the one which preceded it, and were brain scanned using functional magnetic resonance imaging, which shows in real time which areas of the brain are active.

In addition, electrodes attached to the fingers detected increasing sweat - a sign of an emotional response.

The longer the boys watched videos, particularly the mild or moderate ones, the less they responded to the violence within them.

In particular, an area of the brain known as the lateral orbitofrontal cortex, thought to be involved in emotional processing, showed less activity to each clip as time went on.

'Social problem'

Dr Grafman said: "Exposure to the most violent videos inhibits emotional reactions to similar aggressive videos over time and implies that normal adolescents will feel fewer emotions over time as they are exposed to similar videos."

He said that this could actually produce more violent reactions from the teenager.

"The implications of this include the idea that continued exposure to violent videos will make an adolescent less sensitive to violence, more accepting of violence, and more likely to commit aggressive acts since the emotional component associated with aggression is reduced and normally acts as a brake on aggressive behaviour."

However, another academic said it was almost impossible to explain violence in these terms.

Professor David Buckingham, the director of the Centre for the Study of Children, Youth and Media, said that violence was a "social problem" with many contributing factors, not simply a matter of looking at how the brain worked.

"The suggestion is that, over a period of time, people can develop a kind of tolerance to these images - but another word for that is just boredom.

"This debate has been going on since before we were all born. In the 19th Century people were panicking about the effect of 'Penny Dreadfuls'.

"If we are truly interested in violence and aggression, rather than blaming the media for everything wrong in the world, we need to look at what motivates it in real life."