A psychophysical study on gravity perception in multiscale virtual reality

Abstract

In virtual reality, altering the perceived body size of a user can lead to a mismatch between subjectively realistic and physically accurate simulations. This study investigates whether humans judge gravity based solely on its physical value, or whether gravity perception is also influenced by visual cues such as their perceived body size in a virtual environment.

We scaled participants’ virtual bodies using ten different scales ranging from 0.1x to 1x. Participants were subjected to two gravity conditions. In the normal physics condition, objects behaved normally, using Earth gravity. In the movie physics condition, gravity was set to one tenth of normal Earth gravity, and objects behaved as they would under a decreased gravitational acceleration. Participants were presented with a virtual environment of constant size, and they were subjected to all gravity-scale combinations in a randomized order. In each trial, participants threw a ball towards a target and were prompted whether the ball used normal gravity.

We trained two Bayesian logistic mixed-effects models in an attempt to predict participants responses. The frst one uses gravity as its sole predictor and supports the idea that humans judge gravity based only on its physical value. The second one uses the interaction of gravity and size as predictors, and supports the idea that humans use a combination of both variables when judging gravity. We used Bayes factors to quantify the likelihood of our observed data under each model.

Aligning with previous studies, we confrmed that participants use both gravity and perceived body size as a guide when judging gravity. We observed that participants considered the normal physics condition as realistic when their bodies were close to normal size and the movie physics condition to be more realistic when scaled down, even if this is not physically accurate. Using multiple scales allowed us to show that this shift in preference is gradual, and not a binary shift.