Violent computer games may make people more likely to act aggressively, a study says.
Previous research has suggested a link between violence and game playing
Previous research has found people who play such games are more likely to be aggressive but some say this just shows violent people gravitate towards them.
But a team from the University of Missouri-Columbia said their study which monitored the brain activity of 39 game players suggests a causal link.
The findings were published on the New Scientist website.
The researchers measured a type of brain activity called the P300 response which reflects the emotional impact of an image.
When shown images of real-life violence, people who played violent video games were found to have a diminished response.
However, when the same group were shown other disturbing images such as dead animals or ill children they had a much more natural response.
When the game players were given the opportunity to punish a pretend opponent those with the greatest reduction in P300 meted out the severest punishments.
Psychologist Bruce Bartholow, the lead researcher of the study which will be published in full in the Journal of Experimental Social Psychology later this year, said: "As far as I'm aware, this is the first study to show that exposure to violent games has effects on the brain that predict aggressive behaviour.
"People who play a lot of violent video games didn't see them as much different from neutral.
"They become desensitised. However, their responses are still normal for the non-violent negative scenes."
The findings will back up what many have argued over recent years with the growth in games with scenes of graphic violence.
But some experts still remain unconvinced of a link.
Jonathan Freedman, a psychologist from the University of Toronto in Canada, said: "All we are really getting is desensitisation to images. There's no way to show that this relates to real-life aggression."
And Professor David Buckingham, an expert on the media and children at the Institute of Education, added there was still no consensus on whether violent games caused aggressive behaviour or were just played by violent people.
"The debate we are seeing is very similar to the one that has raged for years about TV. The truth is there are many factors that can lead to violence, such as being withdrawn and isolated, so it is hard to say it is because of one thing.
"In the absence of any proof, I think we have to be agnostic about it. However, I think there is an argument about the morality of some games.
"Some actually encourage amoral behaviour to win the game and I think parents should be talking to their children to make sure they realise this is a joke. Children are generally good at telling fantasy from reality, but parents should be discussing this."