This study is interesting.

In the study, published today in Molecular Psychiatry, lead author Greg West, an associate professor of psychology at UdeM, reveals that habitual players of action games have less grey matter in their hippocampus, a major part of the brain.
And the more depleted the hippocampus becomes, the more a person is at risk of developing brain illnesses and diseases ranging from depression to schizophrenia, PTSD and Alzheimer’s disease.

Gaming has been shown to stimulate the caudate nucleus more than the hippocampus; 85 per cent of players rely on that part of the brain to navigate their way through a game. The problem is, the more they use the caudate nucleus, the less they use the hippocampus, and as a result the hippocampus loses cells and atrophies, the new study shows.

Source: Playing action video games can actually harm your brain