Immersion and coherence in a stressful virtual environment

We report on the design and results of two experiments investigating Slater's Place Illusion (PI) and Plausibility Illusion (Psi) in a virtual visual cliff environment. PI (the illusion of being in a place) and Psi (the illusion that the depicted events are actually happening) were proposed by Slater as orthogonal components of virtual experience which contribute to realistic response in a VE. To that end, we identified characteristics of a virtual reality experience that we expected to influence one or the other of PI and Psi. We designed two experiments in which each participant experienced a given VE in one of four conditions chosen from a 2×2 design: high or low levels of PI-eliciting characteristics (that is, immersion) and high or low levels of Psi-eliciting characteristics. Following Skarbez, we use the term "coherence" for those characteristics which contribute to Psi, parallel to the use of "immersion" for characteristics that contribute to PI. We collected both questionnaire-based and physiological metrics. Several existing presence questionnaires could not reliably distinguish the effects of PI from those of Psi. They did, however, indicate that high levels of PI-eliciting characteristics and Psi-eliciting characteristics together result in higher presence, compared any of the other three conditions. This suggests that "breaks in PI" and "breaks in Psi" belong to a broader category of "breaks in experience," any of which result in a degraded user experience. Participants' heart rates, however, responded markedly differently in the two Psi conditions; no such difference was observed across the PI conditions. This indicates that a VE that exhibits unusual or confusing behavior can cause stress in a user that affects physiological responses, and that one must take care to eliminate such confusing behaviors if one is using physiological measurement as a proxy for subjective experience in a VE.