Abstract
Stereopsis and active motion parallax are two of the main perceptual factors provided by head mounted displays to create a sense of presence in virtual environments. However, their relative contribution to create the sense of presence is not clear and existing results are somewhat controversial. Here, we study the contribution of stereopsis and active motion parallax using variants of the classic pit room paradigm in two experiments. In the first, participants were required to cross a deep abyss between two platforms on a narrow plank. They completed the task under three experimental conditions: (1) Standard VR (both motion parallax and stereopsis were available); (2) The lateral component of motion parallax was disabled; (3) Stereopsis was disabled. Participants responded to a presence questionnaire after each condition and their electrodermal activity (EDA) was recorded as a measure of stress and anxiety in a threatening situation. Results revealed a main effect of condition and the importance of motion parallax (F[2,54] = 6.71; p< 0.005). Questionnaire results, on the other hand, did not show an effect of experimental condition (F[2,54]=0.04; n.s.). In the second experiment, we applied a similar paradigm in a less stressful context. Participants were standing on the ledge over the pit and dropped a ball trying to hit a target on the ground. Experimental conditions and dependent measures were similar to experiment 1. Both EDA (F[2,36] = 4.70; p< 0.05) and presence questionnaire (F[2,36] = 5.73; p< 0.01) revealed a main effect of condition. Motion parallax affected the EDA and questionnaire scores more than stereopsis. The results from this study suggest that in VR, motion parallax is a more efficient depth cue than stereopsis in terms of fear response which was measured by EDA. Presence questionnaires also showed the importance of motion parallax in the second experiment.