Summarizing from the article of A Way to Reduce Stress. . :))

According to bartholow 2006, the researches 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 diminished response. People who play a lot of violent video games didn’t see them as much different from neutral. They become desesitised. However their responses are still normal for the non-violent negative scenes. Many have argued over scenes of graphic violence.