  An avatar mirroring the user's movement is commonly adopted in Virtual
Reality(VR). Maintaining the user-avatar movement consistency provides the user
a sense of body ownership and thus an immersive experience. However, breaking
this consistency can enable new interaction functionalities, such as pseudo
haptic feedback or input augmentation, at the expense of immersion. We propose
to quantify the probability of users noticing the movement inconsistency while
the inconsistency amplitude is being enlarged, which aims to guide the
intervention of the users' sense of body ownership in VR. We applied angular
offsets to the avatar's shoulder and elbow joints and recorded whether the user
identified the inconsistency through a series of three user studies and built a
statistical model based on the results. Results show that the noticeability of
movement inconsistency increases roughly quadratically with the enlargement of
offsets and the offsets at two joints negatively affect the probability
distributions of each other. Leveraging the model, we implemented a technique
that amplifies the user's arm movements with unnoticeable offsets and then
evaluated implementations with different parameters(offset strength, offset
distribution). Results show that the technique with medium-level and
balanced-distributed offsets achieves the best overall performance. Finally, we
demonstrated our model's extendability in interventions in the sense of body
ownership with three VR applications including stroke rehabilitation, action
game and widget arrangement.
