Visceral Games has posted an in-depth blog update over on the Dead Space 3 blog, discussing the Marker-induced dementia that plagues both Isaac Clarke and newcomer protagonist John Carver. Without spoiling too much, Isaac Clarke doesn't struggle with the madness brought on by the Markers, but Carver is not so lucky.
Having lived a difficult life before even arriving on Tau Volantis, Carver is more susceptible to the inner demons that the Markers can wreak havoc with. Having grown resentful of his family, Carver is crushed when they are taken from him during a Necromorph outbreak. The internal struggle and inability to express his grief leaves Carver vulnerable to the Marker-induced dementia. As hard as it might be, Carver has to confront his past before he can ever move on from his inner demons.
From a gameplay standpoint, this feeds into the cooperative element of Dead Space 3. If you are playing as Carver during these side missions, you will experience the world through a demented state, with constant reminders of the trauma that plagues Carvers past. On the other side of the coin, playing as Isaac Clarke is a completely different experience. Clarke sees the world as it is, and must watch as Carver struggles with beings that don't exist in the physical world.
Check out the full blog update at http://www.deadspace.com/. Dead Space 3 ships on February 5th, 2013 on Xbox 360, PC and PlayStation 3.