Voluntary Self-tapping Induces Self-localization Shift
Masahiro Furukawa1 , Kouta Minamizawa2 and Susumu Tachi2

Abstract— Rubber Hand Illusion (RHI) is known to provide
a proprioceptive drift toward the direction where the rubber
hand is tapped. On the other hand, Out-of-Body Experiences(OBEs) induce an entire body shift from a third-person Point ofView (PoV) . These experiences are known to trigger external
passive tactile cues. Telexistence allows an entire body shift
from a first-person PoV, compared to RHI and OBE. However,the precise origin of the entire localization is not revealed
when we actively provide the internal voluntary tactile cue.
Therefore, in this paper, we focus on the effects of tactile cues
to determine self-localization during the action of self-tapping.
Participants viewed and tapped their own body on the back,and evaluated where on their body they perceived the tapping.
The experiments show that the localized position tends to shift
forward when an individual is tapped by a third-person; incontrast, the localized position tends to shift backward when
an individual is tapping his own back. The subjective report
indicates that participants perceived themselves as leaving
their own bodies, or that someone appeared in front of themsuddenly. Thus, we consider that self-produced tactile cue
induces telexistence experience.

I. INTRODUCTION
Synchronized visual and tactile cues can dramaticallyinduce localization shifting, such as the Rubber Hand Illusion(RHI) [1] and Out-of-Body Experiences (OBEs) [2]. RHI
is usually measured as the amount of proprioceptive drift
toward the location where an individual can see he is touchedfrom the First-Person View (FPV). In contrast, OBE is
known to induce an entire body shifting experience when
individuals can see where they are touched from a ThirdPerson View (TPV). In addition, both experiences require
the experimenter to provide tactile cues as an extraneous
stimulus. Compared to RHI and OBE, telexistence requires
FPV and an entire body shift. In such a case, the question
becomes one of determining where individuals perceive
themselves when the individuals touch themselves while
seeing themselves from TPV.
Body localization is divided into several groups in termsof experimental conditions: partial body or entire body, body
appearance, point of view (PoV), and passive or voluntarystimulus. The following sections describe our perspective interms of these four points.
1 Masahiro Furukawa is with the Faculty of Graduate School of Information Science and Technology, Osaka University, 2-1 Yamadaoka,
Suita, Osaka, Japan m.furukawa@ist.osaka-u.ac.jp 2 Kouta
Minamizawa and Susumu Tachi are with the Faculty of Graduate Schoolof Media Design, Keio University, 4-1-1 Hiyoshi, Kohoku-ku, Yokohama,
Kanagawa, Japan {kouta, tachi}@tachilab.org

II. RELATED WORK
A. Rubber Hand Illusion
RHI is one of the most famous illusions induced bymultisensory integration, such as synchronized visual andtactile cues. Botvinick et al. indicated three-way interaction
between vision, tactile, and proprioception [1]. Stroking arubber hand and a participant’s hand with a paintbrush foran extended period results in a dramatic proprioceptive shift.
That is, the perception of touch on the human hand shifts
towards the rubber hand. For this study, the experimenter
provided the tactile stimuli and the participants did not move,but saw the rubber hand. That is, the stimuli were provided
passively.
It is known that individuals can experience ownershipeven when the appearance and size of the body is relatively
different. Slater et al. showed that a computer-rendered arm
and an entire body in virtual space provide ownership [3].
In addition, Armel reported that individuals can experienceownership of artificial external objects [4]. Such ownershipto external objects is expressed as sensory projection. The
virtual arm and hand have the advantage that an experimentercan change their appearance in terms of size. Interestingly,
individuals can maintain body ownership and consciousness
even when the arm appears to be extremely long [5].
B. Body Ownership
The RHI procedure is used sometimes to study entire bodyownership. Petkova et al. showed that ownership towards
a life-sized mannequin can be obtained; moreover, theysucceeded in demonstrating body swapping experience [6].
Recently, BeAnotherLab by Bertrand et al. sensationallypublished artwork regarding entire body swaps [7]. For
the artwork, two participants wore Head-Mounted Displays(HMDs) that provided one another’s field of view, and theparticipants were asked to perform identical behavior, suchas viewing and stroking their body.
Proprioception has an important role in controlling our
posture, similar to strong visual cues. Petkova et al. reported
that proprioception is much more dominant in blind individuals [8]. In their study, Petkova et al. defined somatic rubber
hand illusion without vision, which uses somatosensory cues
instead of vision on sighted individuals. They also used a
self-touch procedure to compare dominance between visionand proprioception, similar to Davies et al. in a previous
study [9]. Rohde et al. indicated that frequent stimulationand measurements result in proprioceptive drift even underasynchronous conditions [10]. Therefore, Rohde et al. concluded that proprioceptive drift can be used for discussions
regarding only visual-proprioceptive integration.

C. Agency
Traditional RHI experimental procedures usually include
passive tactile stimuli. This means that, not only participants,
but also experimenters provide tactile stimuli. In other words,
the tactile stimuli is not produced as voluntary behavior.
Several works have attempted to use voluntary behavior to
produce not only afferent tactile sense, but also efferent
voluntary motor command.
Agency is another important topic in multisensory integration. Agency is obtained when executing body movementswith intention. Agency involves efferent motor command; incontrast, ownership is obtained through external sensations,in other words, afferent nerve signals. The related works
described in the previous sections focused on ownershipusing passive touch, such as an experimenter stroking aparticipant. In contrast, recent agency studies have used thevoluntary behavior of moving fingers, which we consider
as active touch. For instance, Tsakiris et al. [11] reported
that proprioceptive drift significantly decreases under active
movement conditions compared to passive conditions. Therefore, Tsakiris et al. concluded that agency is considered as
self-consciousness that arises from action, not from sensation. Kalckert et al. carefully investigated the interaction
between ownership and agency [12]. They indicated that
action that involves efferent motor commands contributes
to producing agency. Their study also demonstrated thatafferent sensation represents fragmented body parts, but
voluntary action provides coherent body self-consciousness.
D. Out of Body Experience
OBEs are known to provide dramatic proprioceptive shifting, although the proprioceptive drift is relatively small. Thatis, individuals perceive as though they existed outside of theirbodies.
Ehrsson succeeded in producing this sensation using an
HMD to provide an FPV and a video camera located behindthe participants [2]. Thus, the participants saw themselvesfrom behind. Ehrsson indicated that the synchronized visualtactile stimulation induced OBEs. Note that Ehrsson provided
a passive sensation to the participants under the condition of
FPV. In addition, Lenggenhager et al. requested participants
to walk where they felt in order to reveal a quantitative
amount of localization shifting [13]. Unlike Ehrsson’s first
OBE work [2], Lenggenhager used a computer-renderedvirtual body as well as a virtual object with the appearanceof a human-sized flat box as visual cue to disrupt spatial congruency. Similar to Ehrsson, Lenggenhager provided visualtactile stimuli such as stroking the participants’ back, andconcluded that synchronized stimuli reproduced OBEs eventowards a virtual body.
Watanabe et al. investigated OBEs under a self-tapping
condition using a long rod [14]. They used a questionnaire
similar to Ehrsson’s to obtain a qualitative impression interms of OBEs. In this study, a video camera embedded
in a robot head was placed 1.5 m behind the participants,
and the participants saw their backs in a manner similar to
Ehrsson’s study. After the passive tap condition, an active tap

condition was performed where the participants tapped theirback from behind. As a result of this experiment, the authors
concluded that the voluntary movement appears to contributein obtaining localization. However, qualitative studies on
where the participants perceive themselves or how frequently
they perceive them have not been conducted.
E. Telexistence
Telexistence refers to the concept and technology thatenables us to experience the real-time sensation of being
at a place other than where we actually exist, as well as
being able to interact with the remote environment, which
can be real, virtual, or a combination of both [15]. Tachi et al.
reported the design method of a visual display that allowed
users to view their surroundings through stereoscopy, thus
representing a proper distant sensation [16]. A robot headfollowed the users’ head movement in real time, thus allowing the users to move their head voluntarily. Consequently,
telexistence appears on the premise of voluntary behavior[17].
For telexistence, HMDs should be designed to provide
natural visual sensations. The visual ability to focus andto converge are popular parameters for providing proper
stereoscopic vision. It is important to design HMD optics
with less discrepancy to focus and for convergence [16][18]. Therefore, the HMD focal length is usually designed
approximately 1 m from the view position.
Telexistence systems usually consist of master-slave systems, which include a master cockpit to apply a user’s body
behavior to a virtual or real avatar robot. The first telexistence
vision system with one Degree of Freedom (DoF) [19][20]tracked a user’s head and followed the user’s voluntary
movement through a robot head. Tachi et al. mounted theone DoF telexistence robot head onto a vehicle in order toinvestigate the relationship between the direction the head
was facing and the direction the vehicle was following[21]. In their paper, Tachi et al. proposed construction of atelexistence system with the ability to explore, and comparedfeasibility in terms of smoothness and elapsed time throughthe exploration of corrosion avoidance.
The DoF of the telexistence avatar robot can be one of the
key indicators of the sense of immersion. The first DoF ishead movement. The head, including a binocular camera unit,
can have a maximum of six DoFs to determine the positionand orientation required to follow a user’s head movement.
Tachi et al. proposed a telexistence avatar with three DoFs
in the head DoF [16]. It is important to consider latency aswell as frequency width to be able to reproduce a user’s head
movement. Watanabe et al. constructed a six DoF robot headdesigned with a minimum of weight [18]. As a result, therobot head achieves much less latency and wide frequency
width when realizing a user’s voluntary head behavior. The
objective of this work is to be able to view a target object
from the front and from the side because of wide translational
movements.
The second DoF is limb, which is imperative in order toconsider manipulation in a remote place. When individuals

TABLE IC OMPARISON BETWEEN RHI, OBE S , AND T ELEXISTENCE
Experience

TargetPoV

AgencyStereo Camera HeadHMD
DC Motor
Wooden Stick

FoV

RHI

Partial Body

FPV

Not required

OBEs

Entire Body

TPV

Not required

Telexistence

Entire Body

FPV

Required

Wooden Stickview their hand in a telexistence system, the hand should
appear in the exact position where expected. Moreover, thehand should be voluntarily controllable in order to obtainagency. Tachi et al. were able to successfully prove thefeasibility of manipulating limbs in remote places using
a seven DoF arm and one DoF gripper hand [16]. Theirfindings indicate that strong agency is obtained even when it
appears that a robot as long as an arm represents a user’s
voluntary movements in real-time. The latest work on atelexistence anthropomorphic robot is TELESAR V, and it
achieved dexterous manipulation by transferring the haptic
sense using a full upper body, including a three DoF head,a six DoF body, both 15 DoF arms, and both seven DoF
hands [22]. Therefore, TELESAR V has 53 DoFs in total.
This higher DoF design was developed to investigate active
touch on telexistence, including dexterous behavior such as
stroking, tapping, and pouring. Thus, the fingertips havepressure, vibration, and thermal sensors to transfer the haptic
sense based on the factorial sensations of pressure-sense,
vibration-sense, and thermal sense. A user wears gloves
to obtain finger movement, which reproduces the haptic
sensation with a fingertip mounted haptic display.
F. Our Focus
In short, the question posed in the Introduction remains
open, where do individuals localize their entire body in selftouching situations? Unlike RHI, we focus on entire body
localization, such as OBEs and telexistence (see Table I).
That is, OBEs need to be investigated as the telexistence
setup, which can include OBE situations. In other words,
a telexistence setup requires that the PoV be located at
the same position where an individual perceives his body(FPV). In contrast, an OBE setup requires TPV. Self-touchcomprehends afferent somatosensory feedback and efferent
motor command as voluntary behaviors. Moreover, voluntary
behavior requires agency towards the artificial object, insteadof the actual body.
III. EXPERIMENT
A. Experiment Setup
The experiment requires switching the tactile cues on theparticipants’ back (provided passively) or hand (obtained
voluntarily) in a similar manner to a previous method [14].
In addition, it is necessary to follow a quantitative approachto describe the body localization shift. Finally, a telexistence
setup is required.
Thus, we developed a self-tapping machine as shown in
Figure 1, 2 (see Appendix A, B for details). Our self-tapping

1200

500

750

DC Motor

550

550
1000

Fig. 1.

unit: mmExperiment setup dimension

HMD

Stereo Camera Head

Wooden Stick

DC Motor

DC Motor

Wooden StickFig. 2. Participants observe their back through HMD in real time, and tap
their back on their own.

machine has a master-slave structure. That is, the stick placed
behind the participants follow the movement of the stickplaced in front of the participants when they move it.
The experiment setup of this study consists of an HMD
and a charge-coupled device (CCD) camera. The participants
wear an HMD and sit on a chair. The HMD has two 7.1inch LCDs, one for each eye, with a resolution of 1,024 ×
768 pixels. The horizontal field of view (FOV) is 42 degrees.
The pupillary distance is 65 mm. This setup is similar to theprevious research [18]. A black soft form shield is attachedto the HMD to cover all peripheral areas of the lens so thatparticipants cannot see their hand even when looking down.
To obtain FPV, we use a robot camera head. The robot headcalled TORSO has two small binocular cameras at a regularpupillary distance of 65 mm, which is 7 mm in diameter [18].
The horizontal FOV is 46 degrees and its resolution is 768
× 494 pixels. Note that these cameras can be replaced with
two simple regular cameras because we used TORSO at a
fixed position without allowing any joint movements in orderto maintain the view direction. These cameras capture realtime video and send it to the HMD so that the participants
can view themselves from behind. Their view is controlled
carefully so they can only see the stick.
B. Experiment Design
We designed a two-factor within-subject experiment to obtain quantitative data when judging relative position. The first
factor is the passive tactile cue provided on the participants’
back: it is provided or not (see Table II). The red triangle
indicates presence and position of tactile cue. The second

TABLE IIPASSIVE TAP C ONDITION
No response on the backEven though participants movethe stick back and forth, they do notreceive any taps on their back.
Response on the Back
Participants perceive tapping on their backas though the stick tapped them on the back.

TABLE III
ACTIVE TAP C ONDITION
No response on the Hand
Even though participants movethe stick back and forth, they do notreceive any responses from the stick.
Response on the Hand
Participants feel a response
as though the stick struck an object.

factor is the active tactile cue provided on the participants’hand with voluntary movement; similar to the ﬁrst factor, theconditions are : provided or not (see Table III).
The following paragraphs detail the participants’ experience during the experiment. When the participants pushed
the stick forward to tap, they perceived a force feedback asthough the stick tip had touched an object. The participants
might believe that someone was sitting in front of them,and that they had tapped that individual’s back. In contrast,
if the participants perceived a tap on their back at the sametime, they might believe that their body had moved suddenly
from their perceived position, or that their body had shiftedslightly or not at all.
The participants could observe the stick in the HMD FOV
during all experiment sequences. Thus, we consider that themaster-slave structure realized agency because both sticks
demonstrated identical behavior.
Under the no feedback condition, tactile feedback was
not provided to the back or to the hand. In this case, thestick behind the participants was stopped to prevent contact
with the participants’ back, although the stick in front of theparticipants moved further.
When feedback to the participants’ back was indicated,
the stick behind the participants continued to move basedon the movement of the front stick, until contact with theback occurred. Similar to the no feedback condition, theparticipants did not perceive any feedback to their hand. As
a result, they only perceived passive tapping to the back. We
used a unilateral control to isolate haptic feedback from thetapping to the back.
In contrast, when feedback only to the participants’ hand
was indicated, the front stick provided the sensation oftapping an object located in front of the participants. On
the other hand, the stick behind the participants was stopped

before contact with the back occurred. At this time, thestick that the participants can see looks tapping an object
in front them simply stops without tapping anything. The
front stick provided force feedback when the back stick was
supposed to reach the participants’ back, although the backstick did not actually reach the participants’ back. To provide
force feedback to the hand, we placed a virtual invisible
wall where the stick stopped. We used the penalty method
to determine the contact force. The PD gain was tuned toprovide the impression of contact against an object that wasneither considerably hard nor soft, such as a human body.
Another control condition was to provide tactile cues to theparticipants’ hand and back simultaneously. We consideredthat this condition is expected to reveal dominance of tactile
stimuli between the passive and the active tactile cues interms of agency. Needless to say, the stimuli appeared
extremely confusing because agency remained while moving
the stick voluntarily because of somatosensory cues combined with visual cues. In addition, the participants perceived
tapping feedback from the back simultaneously as though
they had tapped themselves on the back as described before.
This condition was implemented using a bilateral control
system that established a link between the two sticks.
C. Procedure
The experimenter requested participants to sit on a chairand place a black cloth over their shoulders to cover theirback. The experimenter explained the purpose of the experiment as such: I’d like to ask you to indicate the position
where you felt you were seated. After that, the experimenter
described the experiment ﬂow: Here is a stick in front ofyou, and you can move it.
Then, the participants wore the HMD and were able to see
the experimenter’s hand in front of TORSO, which simply
consisted of ﬁxed video cameras; the participants conﬁrmedthat they could see TORSO. The experimenter also moved his
hand away from the participants’ back, and the participants
conﬁrmed that the hand was not visible, but that the tip of the
stick was visible. He continued to explain the next condition
and moved the stick forward to demonstrate such condition.
The experimenter asked the participants to hold the stick
and move it forward and then back to its initial position; theparticipants were also instructed to remove their hand fromthe stick after one tap. After the participants experienced all
four condition combinations, the experimenter veriﬁed thatthe participants could perceive the feedback to their hand andback. The difference between each condition was noticeable
to the participants.
We employed a two-alternative force choice to obtain
quantitative results. Therefore, the experimenter asked theparticipants to evaluate where they felt situated compared tothe standard condition described in the next paragraph, andwhether they perceived their position as shifting backward
or forward.
The participants were trained to perform the experimental
sequence shown in Figure 3. The standard condition does
not provide any feedback to the hand or to the back. The

Second tap

Tapping Response

Answer
t

0.5s

1.5s

Fig. 3.

Experiment procedure in a trial

experimenter explained that the standard condition is thebaseline condition to compare with the target condition.
He asked participants to tap once after hearing a beeping
sound, and to indicate whether they perceived their location
as shifting forward or backward during the second (target)
condition compared to the ﬁrst (standard) condition.
The stick returned to its home position within 0.5 s before
every trial, and a 0.5 s beep (triangle wave at 1 kHz) was
sounded. During this term, the participants were asked not totouch the stick. In addition, the experimenter instructed theparticipants to move the stick within 5 cm when allowed tomove the stick. After the ﬁrst beep, the participants tapped
once to perform the standard condition. After the second
beep sounded for 1.5 s, the participants tapped again to
perform the target condition. After these two taps, theyjudged the shifting direction to push the “Top” or “Bottom”
button to choose “Forward” or “Backward” with the keypad
placed on their left hand. After pushing the button, the next
trial started immediately.
The participants learned to move the stick by attempting
several trials with the beep. After the training session, theexperimenter provided earphones to allow the participants tohear white noise.
The main session consisted of 80 trials that includeda factor of 2 × 2 = 4 conditions in total as described
previously. A short rest was provided after every 20 trials.
The participants heard white noise from earphones during
all trials, with the exception of the resting periods. Each rest
period lasted from approximately 30 s to a maximum of 2
min. The main session lasted approximately 20 min. The
condition order was counter-balanced in randomized orderfor each person. After all trials, the experimenter interviewed
the participants to obtain their subjective impression.
Six participants participated in the experiment (one female
and ﬁve males, average age 24.3 years, standard deviation 1.80). They were healthy and did not experience any
difﬁculty viewing real time video through HMD. Five of
the participants had previous experienced using HMD. All
participants were well trained so that they could continue
the task through all the trials.
IV. R ESULTS
A. Quantitative Result
As described before, each participant collaborated 80
times, which included 20 times per condition for all four
conditions. The focus is on the probability of localization
shifting. Therefore, we calculated the representative probability from each condition, including the 20 trials. This meansthat each participant provided four representative probability
values. Moreover, we hypothesized that the haptic cue affects
the values.

Tap on the backp<0.01

Shifting to the FRONT

Homing

No tap on the back
N=6

1

0.78

0.78

0.5
Shifting to the BACKFirst tap

Average Propability Reported Shift to the FRONT

Homing

0.22
0.08
0

No Response

With

Tapping Response on the HAND

Fig. 4.

Experimental results - probability distribution

The quantitative results are shown in Figure 4. The red
triangle indicates the presence and position of tactile cues
similar to Tables II and III. The horizontal axis shows
the feedback condition for the hand; that is, whether thetapping response is provided to the hand. “No response”
means that the feedback is not provided, and “With” meansthat feedback is provided. The plot of blank triangles andﬁlled circles show feedback conditions for the back; thatis, whether tapping occurred on the back. The vertical axis
shows the probabilities when the participants reported thatthey perceived as though their body shifted towards the back.
That is, if the participants indicated that they perceived as
though their body shifted towards the front, the probability
decreases and the plot is lowered in the graph, and vice versa.
The plots indicates the average probability of the report oneach condition. The error bar shows standard deviations.
We conducted statistical analysis by performing two-way
repeated measured analysis of variance (ANOVA). As aresult, the main effect of the feedback condition for theback is signiﬁcant (F (1, 5) = 113.75, p < 0.01). In contrast,
the main effect of the feedback condition for the hand is
not signiﬁcant (F (1, 5) = 0.65, n.s.). Moreover, interaction between the two factors is not signiﬁcant (F (1, 5) =1.43, n.s.). Note that the plot height does not indicate position, but the probability of answering.
B. Subjective Report
We obtained some subjective reports through an interview.
The representative reports are as follows. Participant A
indicated that he perceived tapping someone sitting behind
him when he perceived that there was an object in front ofhim. He explained that it seemed that someone had suddenly
appeared around him. He continued to say that there wereoccasions he perceived himself to be tapped, although heactually tapped the person sitting in front of him; moreover,he expressed that this was unfamiliar and strange. He addedthat he perceived himself to be tapped from a long distancewhen the tap was late. In addition, he expressed that thetwo sticks, connected with something rigid, had pinched his
body from front and back. Participant B indicated that sheperceived clearly to be tapping someone when she obtained a
response (from the stick), rather than perceiving herself to betapped. Participant B also said that she perceived her position
to be uncertain (“fuzzy”) when she obtained no response
from either the hand or back. Participant C reported that itwas easy to determine the answer and chose based on thetaps, not on what she saw.
V. D ISCUSSION
Localization significantly tends to shift towards the backfor the condition of no feedback to the hand or back (seethe bottom left of Figure 4). During this condition, theparticipants moved the stick back and forth once to compare
exactly to the condition. This means that the probability ratio
is 50 %. Therefore, it is feasible that the standard deviationis relatively high. However, the result tends to go towards
the back. As Kalckert indicated, voluntary behavior obtained
from artificial objects can provide agency [12]. This indicatesthat agency was strongly reproduced when the participants
saw their voluntary movements on the stick, which wasrealized exactly as their movement.
For the passive stimuli on the back condition (see the top
left of Figure 4), the answer significantly shifts towards thefront. Note that this passive stimuli is not provided from anexternal person such as the experimenter [2]. It is interestingthat the tactile cue provided to the participants’ back appears
to work as external passive stimuli even though agency
appears to be established according to the result described
previously. This estimation is supported by Lenggenhager’s
results [13] as well as Ehrsson’s results. According to thesubjective report, the participants perceived that they weresitting 1 m in front of their actual sitting position. It is important to consider that the participants perceived to be viewing
themselves from outside of their body’s consciousness. This
experience is a typical subjective report of RHI [2].
Passive stimuli with self-touch appears to induce OBEs.
The participants indicated that they experienced a relatively
large jump in terms of localization. Such a jump wasconsiderably large to consider proprioceptive drift [10]. We
consider that passive stimuli dominate voluntary active touch.
This result seems feasible because the internal forward
model predicts self-produced sensory inputs to be negated, as
Blakemore reported [23]. In other words, the external sensory
input of tapping on the back seems to have been clearly distinguished from the voluntarily movement, including motor
commands.
This localization shift is different from proprioceptive
drift, as Tsakiris et al. described [11]. Such shift is obtainedin terms of the entire body, rather than a part of the body(see Table I). Needless to say, the tactile stimuli was provided
with the stick, not onto it, so voluntary movement provides
agency only, and does not provide any ownership; in otherwords, sensation is not projected onto the stick, as Armel etal. described [4]. Above of all, the unilateral sensory loopthat includes voluntary behavior seems to indicate that tactile
sensation dominates agency. As a result, voluntary behavior
triggers OBEs. Thus, PoV also occurs out-of-body. However,the participants perceived the body to be in front of them. No
participant reported that the body was swapped or faced areversed direction. It appears that nobody perceived a bodyswapping experience [6].
The top right plot in Figure 4 was obtained for thecondition of feedback to both the hand and back. Similar
to the condition with tactile feedback only to the back,the result significantly tends towards the front. In contrast,
the localization significantly tends toward the front whencompared to the feedback only to the hand. Note that tappingmeans self-tapping under this condition. That is, the tap on
the back must be understood as self-generated sensory input,
and feedback to the hand must be considered as a result ofvoluntary tapping. Therefore, we expected the answer to be
distributed around the probability rate of 50 % because we
assumed no dominance between the tap on the back andfeedback to the hand. However, the result is as described
previously. Thus, the tap on the back seems to dominate thefeedback to the hand. This result can be explained similar tosensory suppression based on the prediction of self-generated
sensory input [23].
The last plot at the right bottom in Figure 4 is obtained forthe condition of feedback to the hand only. The plot shows
significant localization shifting compared to the condition
obtained by tapping on the back. In contrast, there is no significant difference from the standard condition without any
feedback. According to the subjective report, the participants
seemed to perceive as though someone were sitting in frontof them. This indicates that agency of the stick remained
for the conditions; moreover, ownership toward the artificial
object is not obtained similarly to the other conditions. The
PoV coincides with body consciousness because of stick
agency. In other words, the participants perceived simply
tapping something in front of them with the stick as theusual voluntary behavior from FPV. This indicates that theexperience is telexistence [19] because there is significant
localization shifting compared to proprioceptive drift in termsof entire body localization, rather than simply part of the
body [10].
Above all, tactile feedback during voluntary self-tapping
movement induces OBEs as well as telexistence as shown in
Figure 5. As mentioned previously, the PoV is discordant towhere the body is perceived during OBEs. In contrast, PoV
matches exactly where the body is perceived for the durationof the telexistence experience. Therefore, in this paper, we
can conclude that tactile feedback as sensory loop when selftapping voluntarily can induce these two OBEs in addition
to a telexistence experience. Thus, tactile feedback during
active touch enhances the localization of body consciousness
on telexistence [22], although the DoF is much diminishedin this paper.
Surprisingly, self-localization shifting is dynamically accepted. In many previous studies, adaptation requires as longas 30 min to obtain proprioceptive drift [10]. In contrast,in this study, the participants were required to experience
the set of two conditions and judge within several seconds.
It is noteworthy that sudden PoV shifting occurs triggered

: This mark indicates where participants feel tapping while self tapping.

Feel as if body is here
Out of Body Experience

Mismatch
PoV is here

Actual body is here

Telexistence
PoV is here
Camera is here

Feel as if body is here(a) Experimental Setup(b) Feel as if the back is tapped

Fig. 5.

PoV switch triggered by haptic cue

by haptic cue, although there is a large difference from theprevious PoV such that the visual cue is eliminated. Thisﬁnding strongly supports the signiﬁcance of haptic feedbackin telexistence because it can be considered that haptic cue
enhances the existence.
Previous research indicated that synchronized movement
provides the sensation of existence [16]. In this experiment,
the movement of the stick is well synchronized, as described
previously. In addition, all the participants reported that they
could not see their back. These facts indicate that the visualcue did not work to localize. Therefore, it seems that thesecond case, including out-of-body situations, rarely occur,even for the standard condition. Thus, it seems valid toconsider that the haptic cue provided on an individual’s hand
can allow the individual to perceive himself pushed backfrom their body.
The persons that seemed to appear to the participants
should be the participants themselves, but the fact that they
are not sounds feasible because their own back disappears,
thus becoming extremely difﬁcult for the participants to
conﬁrm whether the back was their own. This indicates thatvisual consistency maintains ownership, and it seems to be
the reason the participants perceived an existence different
from themselves. In addition, sensory feedback such as being
tapped is more dominant; in other words, sensory feedback
is the only feedback that can allow the participants toobtain ownership when visual cues relative to their body arelacking. In order to understand such a strange situation, theparticipants appear to have attempted to provide the previous
explanation. Note that they obtained some knowledge about
the experiment setup before the trials; moreover, the experimenter requested the participants to answer one of the two;
thus, the participants tended to understand that there was no“somewhere,” but simply forward movement. Nevertheless,it also seems valid to understand that the haptic cue provided
to the participants’ back caused them to perceive as though
they were outside their own body.
VI. CONCLUSION
In this study, we focused on the effect of haptic cues to determine self-localization in telexistence. To reveal the effect,(c) Feel as if tapping something
located in front of participants

we constructed a self-tapping device that allowed participants
to tap themselves on the back. The participants wore an
HMD and tapped their back using a stick to evaluate where
they perceived themselves to be. Unlike previous studies, we
designed a fast repetition sequence to measure quantitative
psychological data. We determined that the method validated
the result; moreover, the measurements were valid. It was
revealed that an individual’s localized position tends toshift backward when the individual is tapped; in contrast,
the position tends to shift forward when the individual is
performing the tapping. This quantitative result was also
supported by a subjective report that included statements toindicate that the participants perceived themselves to be outof their body, or that someone had appear suddenly. Thus, it
was determined that haptic cue switches the PoV dependingon active and passive states. For the next step, it is necessary
to consider balancing the visual cue and haptic cue to reveal
the dominant condition to obtain localization.
APPENDIX
A. Tapping Instrument
Each wooden stick is 50 cm in length and 9 mm in diameter,
and is placed on a linear slider on a 20 cm rack gear in front of theseated participant. The rack is driven by a pinion gear (the diameterof the reference circle is 30 mm) and is directly connected to a DC
motor (maxon DC motor RE40 series, 40 mm, Graphite Brushes,
150 W, 148866 / Encoder MR, 500 CPT, 228452) without anyother gear. The motor driver TITECH Driver PC-0121-2 has highpower output at a maximum of 450 W. This direct drive mechanism
provides force feedback through the stick to the participants’ handand allows us to move the slider with minimal force as well as
measure the distance travelled when the participant moves the stick.
The main mechanism around the linear slider, the rack gear, andthe pinion gear are covered.
The direct drive unit is placed 55 cm in front of TORSO
to produce the same view from the TORSO unit as from theparticipants’ position. The two units are located 1 m apart so that the
tip of the stick reaches the participants’ back. To avoid propagating
the tactile cue to the chair on which the participants sit, the directdrive units and the chair do not touch. In fact, the units stand on
the ground directly, and other ground plates are installed under thechair. The chair has a backrest to maintain the participants’ backat a constant distance from the tip of the stick behind them.

B. Visual Transfer System
The visual transfer system TORSO [18] is placed 1 m behindthe participants to provide visual feedback when they look atthemselves from behind. This distance was determined based on
several factors: the HMD FOV, the distance of the virtual screen,
and the length of the stick. When the participants look at their backfrom 1 m behind using the HMD, the FOV height extends from theparticipants’ neck to half of the stick. The FOV is sufficiently wide
to allow the participants to see their back. To tap their back, theideal length of the stick is approximately 50 cm. Several meters is
too long to tap, and 10 cm is too close for the participants to see
their entire back.
TORSO has two small video cameras installed 65 mm apart on alightweight head, which is designed to make fast head movements.
It has six DOFs with two rotational joints and one translational joint
for the torso, and with three rotational joints for the head. The top
axis acts as the panning axis to minimize the mechanical latency
between operator and TORSO.
In this study, the TORSO head was fixed at approximately 30
degrees so that the participants could see the tip of the stick at thebottom part of the FOV through the HMD.

C. Condition of Surrounding Environment
The room is an acoustic room with black walls. All lights are
off with the exception of a 40 W light bulb positioned next to
TORSO. A black cloth is placed on the participants’ shoulder
to cover their entire back and neck. The shutter speed of thebinocular camera embedded in TORSO is set to 1/250 s to decreasebrightness. The HMD parameters, such as brightness, contrast, andcolor temperature, are adjusted before the trial.

D. Control Parameters
The two motors are under master-slave control. The control
frequency is maintained at 10 kHz.
To maintain visual consistency, we use the same coefficient valueof 0.6 to diminish the moving distance of the back stick comparedto the front stick. The sticks are controlled with PD control. The
PD gains are sufficiently high to make contact noticeable, but
sufficiently low to avoid pain. In addition, the control block limited
its torque at a low level.
We multiplied the displacement by a coefficient value of 0.6 inorder to maintain visual congruency, as well as to maintain the same
movement with the back stick as the front stick. That is, if the front
stick travels approximately 10 cm, the back stick only travels 0.6 m
simultaneously. The coefficient value was determined through a preliminary study that determined that different displacements between
an individual and the target cause a pseudo force sensation [24].
However, this sensation seems less than other parameters becauseof changing inertia and because of the reset when conditions arechanged after every trial.
During the condition of feedback to both the back and hand,
we employ a position-based symmetrical bilateral control to link
the two sticks in terms of vision and haptic feedback so that theparticipants can perceive the contact force on the hand, as well as
the tap on their back, simultaneously. Both sticks move the same
distance, and the PD parameters are tuned sufficiently high to allow
the participants to perceive the force feedback and to allow the
sticks to maintain the same movement distance.

ACKNOWLEDGMENT
This work was supported by JSPS Grant-in-Aid for Scientific Research A (23240017).

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