During the events of the Dead Space series, the protagonists will have many brief and often disturbing hallucinations. They are results of direct manipulations of the brain-nerves, by energy emanated either by Markers, or the Brethren Moons, the energy is completely invisible to the human eye and only marker containment units can block any and all marker signals. They are mainly for disrupting and manipulating humans (and presumably the other species consumed) to "help" the cause gain more biomass or new brethren moons.