A framework is presented for comprehending partly participants' spatial perception in virtual environments. Specific hypotheses derived from that framework include: simulator sickness should be reducible through visual background manipulations; and the sense of presence, or of ``being in'' a virtual environment, should be increased by manipulations that facilitate perception of a virtual scene as a perceptual rest frame. Experiments to assess the simulator sickness reduction hypothesis demonstrated that congruence between the visual background and inertial cues decreased reported simulator sickness and per-exposure postural instability. Experiments to assess the presence hypothesis used two measures: self-reported presence and visual-inertial nulling. Results indicated that a meaningful virtual scene, as opposed to a random one, increased both reported presence and the level of inertial motion required to overcome perceived self-motion elicited by scene motion. The simulator sickness research implies that visual background manipulations may be a means to reduce the prevalent unwanted side-effects of simulators. The presence research introduces a procedure, possibly based on brain-stem level neural processing, to measure the salience of virtual environments. Both lines of research are central to developing effective virtual interfaces which have the potential to increase the human-computer bandwidth, and thus to partially address the information explosion.