Abstract
Virtual reality is used to manipulate sensorimotor interactions in a controlled manner. A critical issue is represented by the extent to which virtual scenarios must conform to physical realism to allow ecological human–machine interactions. Among the physical constraints, Earth gravity is one of the most pervasive and significant for sensorimotor coordination. However, it is still unclear whether visual perception is sensitive to the level of gravity acting on target motion displayed in virtual reality, given the poor visual discrimination of accelerations. To test gravity sensitivity, we asked participants to hit a virtual ball rolling down an incline and falling in air, and to report whether ball motion was perceived as natural or unnatural. We manipulated the gravity level independently for the motion on the incline and for the motion in air. The ball was always visible during rolling, whereas it was visible or occluded during falling before interception. The scene included several cues allowing metric calibration of visual space and motion. We found that the perception rate of natural motion was significantly higher and less variable when ball kinematics was congruent with Earth gravity during both rolling and falling. Moreover, the timing of target interception was accurate only in this condition. Neither naturalness perception nor interception timing depended significantly on whether the target was visible during free-fall. Even when occluded, free-fall under natural gravity was correctly extrapolated from the preceding, visible phase of rolling motion. Naturalness perception depended on motor performance, in addition to the gravity level. In sum, both motor and perceptual responses were guided by an internal model of Earth gravity effects. We suggest that, in order to enhance perceptual sensitivity to physical realism, virtual reality should involve visual backgrounds with metric cues and closed-loop sensorimotor interactions. This suggestion might be especially relevant for the design of rehabilitation protocols.
Highlights
When controlled manipulations of sensorimotor interactions are required, virtual reality tools are a preferred choice in both basic research and rehabilitation (e.g., Sveistrup, 2004; SanchezVives and Slater, 2005; Bohil et al, 2011; Cano Porras et al, 2018)
We found that perception rate of natural motion (PR) was significantly higher and less variable in G0, the only condition in which ball kinematics was congruent with Earth gravity during both the rolling phase and the free-falling phase, than in all other conditions (Figure 5)
Both perceptual and motor responses are guided by internal models of physics that allow the prediction of the forthcoming dynamics of events unfolding under gravity acceleration
Summary
When controlled manipulations of sensorimotor interactions are required, virtual reality tools are a preferred choice in both basic research and rehabilitation (e.g., Sveistrup, 2004; SanchezVives and Slater, 2005; Bohil et al, 2011; Cano Porras et al, 2018). One would expect that observers should be sensitive to physical invariants, which humans are exposed to since birth. One such constraint is given by Earth gravity. The issue is especially relevant in rehabilitation applications requiring visuomotor interactions of the patients with the virtual reality setup. It becomes critical to know whether or not human observers are able to detect the congruence or incongruence of the stimuli with physical gravity. We first review evidence in favor and against the hypothesis that humans take gravity effects into account in sensorimotor interactions
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