But just what its activity means is not easy to interpret. As Marnie's awareness faded and Guardian took centre stage, activity throughout the hippocampus and in the surrounding temporal cortex ebbed. When Marnie pictured herself as Player, rather than a real alter ego, her brain activity didn't change. It's not multiple personalities in the same body, but the brain's reaction to damage. I think it's a form of brain damage from repetitive and overwhelming stress in early childhood or from overwhelming trauma in life.
Dissociation is a disruption in normally integrated consciousness.
Tsai speculates that Marnie's abnormally small hippocampus, almost certainly the result of the abuse she suffered early in life, may misfire in a way that makes possible the extreme dissociation required to switch personalities. He points out that everyone can dissociate, when they daydream, for example. We really don't know, but if the wiring is not as sound as it should be, it might easily get inhibited or over- excited, and that might tip the balance between the two personalities. David Healy, who heads the department of psychological medicine at the University of Wales College of Medicine in Bangor, finds the brain imaging results intriguing. You could describe this as a very graphic illustration of the idea that when a particular personality comes into play, it doesn't have access to the full range of brain resources.