Research: Video Game Violence Desensitize Brain to Violence

Psychologist Bruce Bartholow from the University of Missouri-Columbia and his colleagues have found that people who play violent video games show diminished brain responses to images of real-life violence, such as gun attacks, but not to other emotionally disturbing pictures, such as those of dead animals, or sick children. And the reduction in response is correlated with aggressive behaviour.

I can sort of understand it -- the first time you see something, you react strongly, and each sucessive viewing just makes that image more mundane. I agree with Jonathan Freedman, a psychologist from the University of Toronto, who says "All we are really getting is desensitisation to images. There-s no way to show that this relates to real-life aggression."
I think for a game player, a certain amount of desensitisation to images is necessary, given the amount of times things cycle through. Until technology gets to the point that unique visuals are created in real-time, it's impossible to separate out the images from the actual action.
New Scientist: Violent video games alter brain's response to violence