Damn those awesome guys at Ars Technica, they've gone and done an article that we've wanted to do for years: MRI scans of the gamer's brain. The results suggest that three areas of the brain are more active when the subjects thought they were playing against a human, an effect which is more pronounced in men than in women.
The human brain appears to try to parse the intentions of others by engaging its own decision-making process; in short, it appears to model another person's mind by seeing what it would do if it were in that other person's skull. The three areas of the brain that the authors identify are involved, in part, in making executive decisions for that brain's owner, in addition to evaluating other people's executive decisions. So, the fact that they're busier when a person thinks they're playing another human could also be interpreted as them focusing harder on an identical decision making process.