ISASE 2024

Minimal Impact of Tangible Objects on Body Ownership Transfer
in Immersive Virtual Reality
Yutaro ABE*, Asaki KAWAGUCHI*, and Shogo OKAMOTO** Department of Computer Science, Tokyo Metropolitan University,6-6 Asahigaoka, Hino-shi, Tokyo 191-0065, Japan

Abstract: Researchers have discussed the impact of tactile stimuli on the embodiment within virtual reality environments. We
conducted experiments under conditions akin to the rubber hand illusion, where tactile stimuli are known to enhance illusory bodily
awareness, within an immersive virtual reality setting. Participants were required to touch spheres that appeared randomly under twoconditions: with and without tactile stimulation. The condition incorporating tactile stimuli provided cotton balls at locations that werespatially congruent with the virtual spheres. Although we anticipated that tactile stimulation would enhance embodiment, no significant
differences in reported embodiment were observed between conditions with and without tactile objects. The results imply that when
visual stimuli alone are sufficient to generate embodiment, tactile stimuli do not significantly affect embodiment.
Keywords: Body-ownership, sense of agency, virtual reality, embodiment.

illusion experiments, a rubber-made artificial hand isperceived as the actual hand [7]. Within this framework,
the effectiveness of haptic stimuli is widely acknowledgedby many researchers [8, 9]. We conducted an experimentanalogous to the rubber hand illusion within an immersive
VR environment, where the embodiment is experienced
with a computer graphics (CG) hand. In such a condition,
we anticipated that the presence of haptic stimuli would
effectively enhance the embodiment of the CG image.

1. INTRODUCTION
One of the key concepts in current virtual reality (VR)techniques is embodiment, whereby the observed avatar
or virtual body parts are perceived as part of the operator’s
own body. Identifying conditions that effectively facilitate
embodiment is crucial for developing immersive VRenvironments. Furthermore, there has recently been ademand for the representation of physical stimuli inimmersive VR environments, leading to the increased useof haptic stimuli and interfaces in many VR applications,such as [1, 2].
Previous studies have shown inconsistent results
regarding the necessity of haptic stimuli for enhancing
embodiment. For instance, Hanashima and Ohyama [3]found that tactile stimuli were crucial for increasing thesense of ownership felt for an avatar in third-person
perspective conditions. Slater et al. [4] observed thathaptic stimuli, when congruent with visual stimuli,slightly enhanced the sense of ownership for the avatar.
Conversely, Maselli and Slater [5] demonstrated thatwhen the avatar featured realistic skin and body segments,
incongruent haptic stimuli did not impair the transfer ofbody ownership. Therefore, the impact of haptic stimuli
on embodiment varies depending on the experimentalconditions. It is essential to identify the conditions underwhich haptic stimuli effectively influence embodiment.
Experiments on body ownership transfer in VR
environments are often compared to those involving therubber hand illusion in real space [6]. In rubber hand

2. METHODS
2.1 Apparatus and systems
The Oculus Quest 2 (Oculus VR, LLC., USA) wasutilized as the virtual reality headset. The display
resolution was 1832 1920 pixels per eye, with a refresh
rate of 72 Hz in the environment we used. The motion ofthe left hand, from the wrist to the five fingertips, wasFigure 1: Adaptation task in the virtual reality
environment. Participant’s view from the headset.

1

Table 1: Questionnaire items to investigate theembodiment to the CG hand. Each item was answeredby using a 10-graded scale.
Question
Category
Q1 The seen hand was felt as if it
Ownership
were my own left hand.
Q2 I felt the presence of the seen
Ownership
hand.
Q3 The seen hand could beAgency
controlled as I wanted.
Q4 I felt as if the hand was
Dummy
controlled by someone else.
Q5 I felt as if my left hand had
Dummy
disappeared.
Table 2: Means and standard errors among theparticipants for Q1–Q5 and significant probabilities forthe differences between conditions with and without
tangible spheres.
Without
Withptangible spheres tangible spheres value
Q1
0.56
7.28 ( 0.71)
7.85 ( 0.55)
Q2
7.42 ( 0.40)
7.42 ( 0.34)
1.00
Q3
1.00
8.14 ( 0.95)
8.14 ( 0.43)
Q4
0.86 ( 0.59)
2.29 ( 1.13)
0.17
Q5
1.57 ( 1.11)
1.00 ( 0.53)
0.49

Figure 2: Reaching task. Top) Participant’s view.
Bottom) Arrangement of tangible spheres coveredwith cotton.

participants performed an adaptation task. In this task,
they moved their left hand along a long bar and across a
deformed plate suspended in the air, spending 30 seconds
on each activity.
After the adaptation task, as illustrated in Figure 2,
participants performed a reaching task. Each participant
positioned their hand 10 cm in front of their chest as astarting or home position. They then touched spheres thatappeared randomly, aiming to do so as quickly as possible.
After touching a sphere, the participant returned theirhand to the home position and awaited the appearance ofanother sphere. In a single set, a sphere appeared
randomly 15 times. Each sphere was 3 cm in diameter andappeared at one of three random locations in the air. Eachlocation was 10 cm away from the others
The above reaching task was conducted under two
conditions in a random order. The one condition was withtangible spheres, where cotton balls were arranged at thepositions of virtual spheres such that the participant would
feel their sense of touch. The other condition was without
the tangible balls.
After the reaching task completed in each condition,
the participant answered to the five questionnaire items
listed in Table 1 using 10-graded scales from 0 to 9.
Zero indicated that the question item would not beapplicable to the experiment at all. The greater scalevalues indicated the item was applicable with thetracked by cameras integrated into the goggle’s chassis.
The virtual space was developed using Unity
2020.3.35f1. As illustrated in Figure 1, the computer
graphics (CG) representation of the left hand, including
the parts extending from the elbow, was displayed in thesame position as the actual left arm. The fingers and handof this CG model moved synchronously with the real ones,utilizing a motion tracking function. A similar setup wasalso employed in [10, 11].
2.2 Participants
Seven university students in their 20s participated in thestudy after providing a written informed consent. They
were unaware of the objectives of the study.
2.3 Procedures
During the experiments, participants maintained thepositions and orientations of their head, body, and righthand. After launching the software, participants
positioned their left hand approximately 30 cm in front of
their chest. They ensured that the left hand did not touch
their body. The camera system then recognized the left
hand, and the CG hand was rendered in a position spatiallyconsistent with the actual hand. Throughout these
initialization processes, participants were instructed toclose their eyes. Subsequently, as depicted in Figure 1,27.86, 8.14, and 7.43 for Q1, Q2, and Q3, respectively,
which are already high and approach the maximum
score of 9.
Some participants reported a noticeable discrepancy
between the positions of the virtual and real spheres intheir introspective reports after the experiment. This
discrepancy could have led to a discernible visuotactile mismatch, rendering the tangible spheres
ineffective in enhancing embodiment. Moreover, wefound no evidence suggesting that this mismatch
negatively impacted embodiment.
Some participants noted that in the tangiblecondition, the physical presence of the seen spheres
was more pronounced than in the non-tangiblecondition. Their comments indicate that while thetangible condition may have enhanced the sense of
immersion in the virtual space, it did not increase theembodiment of the seen hand. Regrettably, we did notinclude questionnaire items about the presence of thespheres in our experiment, and thus, we lack data to
support this observation.

greater intensity. Q1 and Q2 were about the bodyownership. Q3 was about the agency. The bodyownership and agency are factors composing theembodiment [12, 13]. Q4 and Q5 were items toexamine how well the experiments were controlled,
and the scores for these items should not be as high as
those for Q1–Q3. These questionnaire items arecommonly used in the study of body-ownershiptransfer [5, 6].
2.4 Analysis
For each questionnaire item, we compared the meanscores between the conditions with and without tangible
objects by using t-tests with one sample.
3. RESULTS

Table 2 lists the means and standard errors of thescores for Q1–Q5 under the conditions with andwithout the tangible objects. The significanceprobabilities for the differences between the twoconditions are also listed.
Regarding Q1, the means and standard errors were
7.28 0.71 and 7.85 0.55 for the conditions without
and with tangible objects, respectively. For Q2, thesevalues were 7.42 0.40 and 7.42 0.32, respectively.
For Q3, they were 8.14
0.95 and 8.140.43,respectively. For these questionnaire items, i.e., Q1–Q3,
the mean values were not significantly different
between the two conditions.
Regarding the control items, the values for Q4 were0.86 0.59 and 2.29 1.13, respectively, and those forQ5 were 1.57 1.11 and 1.00 0.53, respectively. The
mean values were small and not significantly different
between the two conditions, indicating that theparticipants were not suggested the objective of theexperiment.

5. CONCLUSION

In this study, we were unable to confirm a positiveeffect of tactile stimulation on the operator’s handregarding embodiment with the CG hand in the virtual
reality environment. These experimental results
suggest that under typical conditions of use, bodyownership of the CG hand is sufficient without tactile
stimulation. However, we hypothesize that undercertain conditions, the presence of tangible objects mayenhance embodiment in virtual spaces. We intend toexplore these conditions in future research.
ACKNOWLEDGMENT
This study was in part supported by MEXT Kakenhi(23H04360 and 20H04263).

4. DISCUSSION
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The experiment aimed to assess whether tactile
stimulation enhances embodiment. However, theresults revealed no significant enhancement in thesubjective responses to questionnaire items related toembodiment. A likely explanation for the minimal
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