Asura hands: Own and control two left hands
in immersive virtual reality environment
Asaki Kawaguchi, Yutaro Abe, Shogo Okamoto, Yuta Goto, Masayuki Hara, and Noriaki Kanayama
Abstract— Body ownership, which is the feeling that one’s
body part belongs to oneself, and agency, which is the sense ofbeing able to control one’s own body part, can be felt towards
fake body parts and those depicted by computer graphics. As
part of an attempt to transfer self-body awareness to fake bodysegments, we investigated whether body ownership and agency
are felt towards two visible left hands in an immersive virtual
reality environment. One of the two hands shown throughvirtual reality goggles spatially matched the unseen actual hand.
The other hand was fake and displayed at either the 10-cm
lateral or medial side of the position of the actual hand. These
two left hands moved synchronously with the actual left hand.
Participants completed a behavioral test and questionnaire
after adapting to the two left hands. In the behavioral test,
participants accessed randomly emerging spheres using the seen
hands as fast as possible. They used the fake hand to toucha sphere when it appeared near the fake hand 41% of thetime when the fake hand was displayed at the medial positionof the actual hand. The results of the questionnaire suggestthat agency was experienced for the two visible left hands forthis condition. By contrast, body ownership was felt mostly
against the displayed hand that was spatially consistent with theactual hand. These ﬁndings indicate that although agency canbe simultaneously felt for two seen left hands, body ownershipis felt only for either of the two visible left hands.

I. INTRODUCTION
Body augmentation is an attempt at extending human bodyfunctions by attaching additional mechanical or electronic
body parts to humans [1], [2], [3], [4], [5], [6]. For example, researchers have investigated a robotic ﬁnger thatcan be additionally worn on a human hand and controlled
to improve dexterous hand manipulation and understandthe extension of embodiment [1], [2]. In virtual reality
environments and in using computer-aided techniques, suchbody augmentation can be achieved using computer graphics
with relative ease [7], [8], [9]. For example, Asai et al.
projected an elongated human hand to provide an illusory
feel to reach for an object placed far from a human [9].
Body augmentation is partially based on the transfer of bodyownership to observable objects. This effect is known as therubber hand illusion [10], [11], which also holds in virtual
reality environments [12], [13], [14], [15], [16]. Realistic
human limbs depicted by computer graphics through virtual
reality goggles are perceived as parts of one’s body. For the
body-ownership transfer to effectively occur, some conditions should be met, such as spatiotemporal congruency [17],[18] in which visible fake and unseen actual human bodies
A. Kawaguchi, Y. Abe, S. Okamoto, and Y. Goto are with Tokyo
Metropolitan University. M. Hara is with Saitama University. N. Kanayama
is with AIST. This study is in part supported by MEXT Kakenhi #23H04360
and #20H04263.

appear and move synchronously from spatial and temporal
aspects.
To measure the intensity of the transfer of self-bodyawareness to visible fake body segments, the feelings of bodyownership and agency are investigated. Body ownership isthe feeling that a visible fake body part belongs to one’s
body. The sense of agency is the feeling that a visible
fake body part or tool can be controlled. These feelings areinvestigated by questionnaires and measurements of humanbehaviors. Researchers have long discussed the relationshipbetween body ownership and agency [19], [20], [21]. In
general, both types of feelings should be intense in immersive
virtual environments.
This study investigates whether body ownership andagency are experienced towards two visible left hands thatmove synchronously with an unseen actual left hand. Oneof the seen left hands is consistent with the position of theactual hand, whereas the other hand is positioned slightly
different from the actual hand. To the best of our knowledge,
similar investigations have yet to be reported. Earlier studies
on supernumerary ﬁngers and limbs tended to focus on thecontrol methods and functions of extended body parts forparticular tasks [1], [5], [6], [22], [23]. Conversely, threeresearch questions motivated this study. First, do humans
feel body ownership for the two visible left hands simultaneously? Second, do humans feel agency over the two visible
left hands simultaneously? Finally, what is the condition
that effectively elicits body ownership and agency for thetwo visible left hands? For this question, we compared two
conditions, for which a fake visible hand was located at thelateral or medial side of the position of the actual unseen
hand. Regarding the position and posture of the visible fake
hand, large inconsistencies with the actual hand deters aneffective body ownership transfer [24], [25], and the locationof the visible fake hand is an important factor. Answers to
the research questions provide a further insight into bodyaugmentation.
II. M ETHODS
A. Apparatus
An Oculus Quest2 (Oculus VR, LLC., CA) was used
for the virtual reality goggles. It displays an image with aresolution of 1832 × 1920 pixels per eye at 72 Hz. The left
wrists and ﬁngertips were tracked by the built-in cameras of
the goggles. The virtual environment was implemented with
Unity 2020.3.35.f1.

Fig. 1.
Two left hands seen in the virtual environment. During theadaptation phase, these hands are shown, and one matched the actual handin terms of its position and posture. Both hands moved in synchronouslywith the unseen actual left hand.

B. Participants
Eleven university students, who were unaware of the study
objectives, participated in the experiment. They provided
written informed consent before the experiment.

Fig. 2. Loci of spheres and actual and fake hands. The actual hand matched
the participant’s left hand in terms of its position and posture. Only one of
the six spheres randomly appeared at any moment. Four of the six spheres
were closer to the actual hand, and the others were closer to the fake hand.
The ﬁgure shows the case in which the fake hand was displayed at themedial side of the actual hand.

C. Stimuli: Multiple visible hands
As shown in Fig. 1, participants could see two left forearms and hands. One was displayed at the position where
their actual unseen hand existed referring to the 18 featurepoints of the actual left hand. The other, i.e., the fake
hand, was displayed at a displaced position. The fake hand
was positioned displaced 10 cm from the medial or lateralside of the actual hand. The two visible hands moved insynchronization with the participants’ actual hand.
D. Experiment procedure
The participants sat on chairs in a space where they could
fully move the left hand. They then conducted three tasks.
First, a 1-min adaptation task was performed, in which participants become familiar with the visible left hands. Second,a reaching task was conducted, in which participants toucheda sphere appearing at random positions, to investigate how
the two left hands were used. Finally, subjective intensities ofthe body ownership and agency were asked. Each participant
conducted these tasks for two conditions. In one condition,
the fake hand was displayed at the lateral side of the positionof the actual left hand. In the other condition, the fake hand
was at the medial side. These conditions were randomly
ordered.
In the adaptation task, one of the left computer-graphics
arms was displayed at the same position as the participant’s
actual left hand, and the other was displayed displaced 10 cm
inside or outside of the actual hand position. The two hands
and ﬁngers followed the motion of the participant’s actual
left hand and ﬁngers. During the experiment, participants
were instructed to place their right hand on their lap andmaintain their face and body orientation, while refraining
from touching with their left hand. In the adaptation phase,for 1 min, participants moved their left hand as if stroking

Fig. 3. Reaching task. The participants must touch the sphere that suddenly
appears using their ﬁngers as quickly as possible. The participants did notreceive any instructions regarding which hand to use.

the surface of a sphere as large as the hand. The hand
was in constant motion during this period. Participants wereencouraged to stare at the two hands for an equally long
period of time.
After the adaptation phase, participants performed a taskof reaching for a sphere of 3 cm in diameter. As shownin Figs. 2 and 3, a sphere appeared randomly at one of six
positions on the x-z plane set 10 cm in front the actual hand.
Either of the six spheres was 10 cm from the neighboring
spheres in the x and z directions. Four of the six positions
were closer to the actual hand than to the fake hand. Whenthe sphere was touched, it disappeared and the participant
placed their left hand to the home position in front of thebreast. A new sphere then appeared in a different location.
The sphere’s position referred to the actual left hand. Hence,
the home position was not strictly controlled. This process
was repeated 30 times with ﬁve replications for each sphere
position.
After each experiment, the participants completed ﬁve

TABLE I
Q UESTIONNAIRE ITEMS
Question
I felt as if I owned two left hands.
I felt as if the medial hand was mine.
I felt as if the lateral hand was mine.
I could control the medial hand.
I could control the lateral hand.
Which hand best matches the actual hand?

Category
Ownership
Ownership
Ownership
AgencyAgencyProportions of usage
to access spheres

***

5

n.s.

n.s.

3

When the medial hand When the lateral handmatched the actual hand matched the actual hand(lateral hand was fake) (medial hand was fake)

Error bar: S.E.
Fig. 5. Means and standard errors of the scores for Q1 (I felt as if I
owned two left hands), which is a question regarding body ownership. (a)When the lateral hand matched the actual hand. (b) When the medial handmatched the actual hand. n.s. indicates not signiﬁcant.

50%

***
0%
Actual hand
(Lateral)Dummy hand(Medial)(b) When the medial hand matched the actual hand
100%
Proportions of usage
to access spheres

71(a) When the lateral hand matched the actual hand
100%

Intensity

Q1
Q2
Q3
Q4
Q5
Q6

9

*

Error bar: S.E.

50%

***
0%
Actual hand(Medial)Dummy hand
(Lateral)Fig. 4. Means and standard errors of the proportions of cases in which thehand closer to the sphere was used to touch it. (a) When the lateral handmatched the actual hand. (b) When the medial hand matched the actual
hand. * and *** indicate signiﬁcant differences at p < 0.05 and 0.001,
respectively.

questions on a 9-point Likert scale (1: disagree, 5: neutral, 9:strongly agree). Table I lists the questionnaire items. Threequestions (Q1–Q3) concerned the sense of body ownership.
Two questions (Q4 and Q5) concerned the sense of agency.
Participants were also asked to indicate which hand, themedial or lateral hand, matched their actual left hand.
III. R ESULTS
Fig. 4 shows the means and standard errors of the proportions of cases in which the hand closer to the sphere wasused to touch it.
Fig. 4 (a) shows the results when the lateral hand matched
the actual hand. The lateral hand was used to touch spheres
closer to the lateral hand at a proportion of 0.85 ± 0.04 (themean and standard error among the participants). If only thevisible hand corresponding to the unseen actual hand wasused, this proportion would be 1.0; however, 0.85 is signiﬁcantly less than 1.0 (z = 6.23, p = 4.66 × 10−10 ). In sometrials, participants reached for the sphere displayed near theiractual hand using a relatively distant fake hand. Moreover,the mean proportion for the fake hand used to reach for the
nearby sphere was 0.41 ± 0.06, which is signiﬁcantly higher
than 0 (z = 8.74, p < 0.001). Participants occasionally used
the fake hand to touch the sphere when it was closer to thefake hand than to the actual hand.
Fig. 4 (b) shows the results when the medial hand matched
the actual hand. For this case, the actual hand at the medial
side was mostly used to touch spheres at 0.91 ± 0.06 when
they were nearer to it. This value is slightly less than 1.0(z = 2.33, p = 0.020). The fake hand at the lateral side wasused at a proportion of 0.25 ± 0.03 when the spheres were
nearer to it. This proportion is greater than 0 (z = 4.28,
p = 18.7 × 10−5 ). The fake lateral hand was also used totouch spheres at nearly a quarter of all the cases where thespheres were closer to the fake hand.
Figs. 5–7 show the results of the questionnaires. If thescore was signiﬁcantly above 5, body ownership or agency
was considered to have been felt.
Fig. 5 shows the mean scores and standard errors for Q1(I felt as if I owned two left hands). The scores were notsigniﬁcantly different from 5 irrespective of the position of
the visible fake hand. This suggests that participants did not
feel that they owned two left hands.
Fig. 6 shows the mean scores for Q2 (I felt as if the medial
hand was mine) and Q3 (I felt as if the lateral hand wasmine). Fig. 6 (a) shows the result for the condition when
the lateral hand matched the actual hand. The mean score
for Q2 was similar to 5 (t = −0.97, p = 0.18), whereasthat for Q3 marginally exceeded 5 (t = −2.0, p = 0.036).
These results suggest that ownership was felt for the visible
lateral hand that was spatially congruent with the actual hand.
Fig. 6 (b) shows the results for the condition when the medial
hand matched the actual hand. The mean score for Q2 wassigniﬁcantly higher than 5（t = −7.1, p = 1.6×10−5 ）．Themean score for Q3 is signiﬁcantly lower than 5 (t = 2.3,
p = 1.6 × 10−5 ). Body ownership was felt for the medial

(a) When the lateral hand matched the actual hand(a) When the lateral hand matched the actual hand
Error bar: S.E.

n.s.

*

Q2
(seen medial hand
was my own)

Q3(seen lateral hand
was my own)(b) When the medial hand matched the actual hand

Intensity

Intensity

Error bar: S.E.
9
7
5
3
1

***
Q2
(seen medial hand
was my own)

Q3(seen lateral hand
was my own)

Fig. 6. Means and standard errors of the scores of Q2 and Q3. (a) Whenthe lateral hand spatially matched the actual hand. (b) When the medial
hand spatially matched the actual hand. * and *** indicates a signiﬁcantdifference from 5 at p < 0.05 and 0.001, respectively. n.s. indicates not
signiﬁcant.

hand that matched the actual position, but not for the lateralfake hand. A signiﬁcant difference is observed in the scores
between Q2 and Q3 (t = 4.47, p = 0.0012). Participants feltthat they owned the visible medial hand when it spatiallymatched the actual hand.
Fig. 7 shows the results for Q4 (I could control the medial
hand) and Q5 (I could control the lateral hand). Fig. 7 (a)shows the mean scores for the condition where the lateral
hand matched the actual hand. The mean scores for Q4 andQ5 were signiﬁcantly higher than 5 (Q4: t = 2.6, p = 9.2 ×
10−3 , Q5: t = −1.8, p = 0.047). Agency was felt for both
visible hands, and the agency score was marginally greaterthan 5 for the lateral hand. Fig. 7 (b) shows the results forthe condition when the medial hand matched the actual hand.
The mean score for Q4 was signiﬁcantly higher than 5 (Q4:t = −13.8 p = 3.9 × 10−5 ). Agency was felt for the medial
hand when it matched the actual hand. However, the agency
was not felt for the fake hand at the lateral side (Q5: t = 0.53
p = 0.30)．
Q6 asked for which visible hand spatially matched theparticipant’s actual hand. When the lateral hand matched theactual hand, 9 of the 11 participants selected the lateral hand.
Two participants believed that the medial hand corresponded
to their actual hand. When the medial hand matched the*

Q4(seen medial handin control )Q5(seen lateral handin control )Error bar: S.E.
Intensity

Intensity*****(b) When the medial hand matched the actual hand

Error bar: S.E.
9
7
5
3
1

9
7
5
3
1

9
7
5
3
1

***

Q4
(seen medial handin control )

n.s.

Q5
(seen lateral handin control )Fig. 7. Means and standard errors of the scores for Q4 (I could control the
medial hand) and Q5 (I could control the lateral hand), which are questions
regarding agency. (a) When the lateral hand matched the actual hand. (b)When the medial hand matched the actual hand. *, **, and *** indicatesa signiﬁcant difference from 5 at p < 0.05, 0.01, and 0.001, respectively.
n.s. indicates not signiﬁcant.

actual hand, all the participants selected the medial hand.
IV. D ISCUSSION
In this study, we examined whether body ownership andagency were experienced simultaneously for the two visible
left hands. Our experiments showed that body ownership was
not felt towards the two left seen hands simultaneously. In
contrast, the agency was felt towards the two visible left
hands when the additional hand was located at the medial
side of the actual left hand. In this condition of the fake handbeing shown at the medial side, two of the eleven participants
believed that the medial seen hand matched their actual left
hand. Furthermore, the medial fake hand was used to reachthe target sphere for 41% of the time when a target sphereappeared near the fake hand. This proportion is signiﬁcantly
greater than 0%. These results collectively suggest that thetwo visible left hands were used as tools to reach for the
random spheres by participants.
Hence, tentative answers to the three questions of thisstudy are as follows. The ﬁrst question was “do humans feel
body ownership towards the two visible left hands simultaneously?” Our answer to this question is no. The second
question was “do humans feel agency over the two visible left
hands simultaneously?” Our answer to this question is yes.

The third question was “what is the condition that effectively
elicits body ownership and agency for the two visible left
hands?” Our answer to this question is that the additional
hand should be placed at the medial side of the actual handto increase the sense of agency.
We believe that the ﬁrst research question remains inconclusive; that is, whether body ownership over supernumeraryhands is experienced. Subsequent research must consider
two major points before reaching a conclusion. First, thequestionnaire may require revision. Q1 was “I felt as if I
owned two left hands,” which disagrees with common sense;humans have only one left hand. Participants might haveanswered Q1 referring to their common sense rather thanreﬂecting how they actually felt during the experiment. For
example, Q1 could be revised to “sometimes I felt that I
possessed the medial hand, and sometimes the lateral hand.”
Second, the reaching task may require a redesign. In one
condition, the sphere was placed 10 cm in front of the actual
left hand position. In the other conditions, the sphere wasplaced 10 cm left or right from the position of the actual left
hand. Hence, for two thirds of all cases, the spheres werecloser to the actual left hand. Consequently, participants usedthe visible hand displayed at the position of the actual left
hand more frequently than the fake hand. This design may
have biased the senses of ownership and agency experienced
by the participants.
Some aspects of our experimental design should be improved in the future. The introspective reports from theparticipants after the experiment indicated that participants
were uncomfortable with the posture of the visible lateral
hand when the fake hand was shown at the lateral side. In
this experiment, the lateral fake hand was simply positioned
10 cm laterally from the actual hand. Naturally, its apparent
direction should change accordingly.
V. C ONCLUSION
This study investigated the bodily self-consciousness towards two visible left hands in an immersive virtual reality
environment; earlier studies mostly focused on how supernumerary hands or ﬁngers can be controlled or leveraged
for some tasks. As a result of the user study involving
seven participants, body ownership was not simultaneously
experienced for the two hands; however, only one of themwas considered as the participant’s own limb. Most participants preferred the visible hand whose position matchedthat of the participants’ unseen actual left hand. However,in a task in which participants were required to quickly
touch a sphere randomly appearing in front of them, both
visible left hands were utilized. In the questionnaire survey,
agency was experienced over the two visible left hands
when the fake hand was at the medial side of the actual
left hand. Hence, our study suggested that agency can beexperienced for supernumerary hands with relative ease,unlike body ownership. Nonetheless, we intend to continue
to pursue conditions for which body ownership transfers tosupernumerary hands.

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