Perceiving alterations in trajectories while throwing in a virtual environment

The trend in immersive virtual environments (VEs) is to include the users in a more active role by having them interact with the environment and objects within the environment. Studying action and perception in VEs, then, becomes an increasingly interesting and important topic to study. We chose to study a user's ability to judge errors in self-produced motion; more specifically, we had users throw a ball underhanded and, using a two alternative forced choice procedure, had them judge the fidelity of the trajectory of their throws. We chose throwing because it is a fairly common activity that many people have experience with and also so that we could model our study on previous research [Reitsma and Pollard 2003; Reitsma et al. 2008]. We changed the trajectories of the throws with three perturbations by altering both the vertical and horizontal components of the ball's initial velocity and also by changing the magnitude of the gravity acting on the ball over the course of its flight. All three perturbations were added in both positive and negative directions. Discrimination thresholds were determined for each subject using a maximum likelihood function and from these thresholds we were able to form some conclusions about users' ability to perceive errors in motion caused by their actions. Unlike Reitsma et. al. [2003; 2008], there was no difference between a user's ability to perceive alterations in the vertical and horizontal velocities of the ball's motion, regardless of direction. Also, lower gravities were marginally easier to detect than higher gravities.