EXPLORING THE EFFECT OF HAND APPEARANCE AND TACTILE FEEDBACK ON THE VIRTUAL HAND ILLUSION
Current virtual reality (VR) technologies allow users to not only observe virtual environments but also interact within them by performing a variety of tasks more efficiently and intuitively. In addition, humans experience VR content not only through their visual and auditory systems but also through the somatosensory system. Therefore, we decided to perform three different studies regarding Virtual Hand Illusion (VHI).
We conducted our first 3 x 2 study (abstract, mannequin, and realistic x tactile and no tactile) both in our lab setting and remotely to investigate the effects of virtual hand appearance and tactile feedback on ownership, external appearance, and tactile sensation embodiment dimensions when participants were instructed to perform an assembly task in a virtual environment. As a result, we observed that the mannequin hand had a greater effect size on ownership, and the realistic hand had a greater effect size on tactile sensation and external appearance. We also found that the tactile feedback condition had a greater effect size on external appearance than the no-tactile feedback condition, and the realistic hand appearance in conjunction with the tactile feedback had a significant effect size on the perceived tactile sensation. Participants in the lab setting rated the external appearance of the realistic hand model higher than the remote participants.
We conducted a second virtual reality study to explore the virtual hand illusion through three levels of appearance (Appearance dimension: realistic vs. pixelated vs. toon hand appearances) and two levels of tactile feedback (Tactile dimension: no tactile vs. tactile feedback using another virtual assembly task. We asked the participants to provide self-reported ratings on a survey that captured presence and five embodiment dimensions (hand ownership, touch sensation, agency and motor control, external appearance, and response to external stimuli). The results of our study indicated that (1) tactile feedback was associated with  a stronger sense of presence, touch sensation, and response to external stimuli; (2) pixelated hand appearance was associated with the least hand ownership and external appearance; and (3) in the presence of the pixelated hand, prior virtual reality experience of participants  was associated with their agency and motor control and their response to external stimuli ratings.
For our third study, we conducted a VR study to further explore the just noticeable difference (JND) of tactile feedback to understand humans’ perceptions of tactile stimuli. Our VR study examined the JND in terms of the intensity, duration, and frequency of tactile feedback provided through commercially available vibrotactile motion controllers, the Oculus Quest 2 controllers. We instructed participants to report whether they perceived a difference between a reference (variation) and a testing stimulus at each point in the experiment for a different property (intensity, duration, and frequency) of tactile feedback. We report both positive and negative JND values for the three properties of tactile feedback. We discuss our findings and limitations at the end of each chapter and provide future study directions in the final chapter.
In the end of the dissertation, we list out our contributions of all three studies in two different directions. One is regarding the design considerations in VR assembly applications. We consider specific hand models, hand and finger animations and assembly task indicators should be applied. The other direction is virtual hand design guideline. We suggest specific rendering style, activation of tactile feedback and JND values for adjustment in VR applications should be developed for stronger sense of embodiment.