Veriﬁcation of out of body sensations, attribution and localization byinteraction with oneself
Kouichi Watanabe∗

Susumu Tachi†

Keio University

Keio University

A BSTRACTTo enhance the realistic sensation and the human presence in a
telepresence or a telexistence system, it is important not only tomatch the audio-visual sensation of the operator with the robot but
also to match the embodiment of the operator with the robot by reﬂecting the somatic sensation of the operator. Presently, however,in telexistence, the key factors for matching the embodiment of theoperator with the robot are unclear and lack established evaluation
methods. In this paper, we experiment with out-of-body sensations
in a telexistence system on the basis of the rubber hand illusion. We
construct a system to self-interact with and evaluate the stimulationinﬂuence of the self-attribution and self-localization of the body.
Index Terms: H.5.1 [Information Interfaces and Presentation]:
Multimedia Information Systems—Artiﬁcial, augmented, and virtual realities
1

I NTRODUCTION

We have been researching the body sensation of humans in telexistence or telepresence. Telexistence or telepresence is a technology
by which we can experience advanced realistic sensations of being in a remote environment through remote operation and remotecommunication. In a telexistence or a telepresence system, there isa master-slave relationship between an operator and a remote robot,
and information is transmitted from the operator to the remote robot
and back. The master-slave system for achieving these objectives
is under development [1]. Even though the presence and realistic
sensations have improved, in several of the existing master-slave
systems, they remain inadequate. Therefore, it is necessary to examine how high presence and realistic sensations can be transferred.
In addition, the standard index for measuring these sensations suchas presence and existence should be deﬁned. In [2], the author(s)mentioned several factors for the presence; however, it is necessary to reveal the key factors for the applications. In a telexistence
master-slave system, the operator must be able to exist in a remoteenvironment through the slave robot. In a remote environment, weneed to look at the presence and existence in order to offer to theoperator the existence sensation.
We focus on the body sensation of humans as a very importantfactor for the presence. We think that the presence and existence
will be created by building the body sensation of being present.
Therefore, it is necessary to establish the localization and attribution of the body in the remote place and to make that body ours
without doubt. If there is a method to control the body sensation
intentionally, it is possible to enhance the quality of telexistence. In
this paper, we look at the presence and existence from the viewpointof body sensation.
There is research that has succeeded in extracting the body sensation from the body using the out-of-body sensation experience.
∗ e-mail: kouichi@tachilab.org† e-mail:tachi@tachilab.org

There is also research on the out-of-body sensation. In [3, 4], it isshown that it is possible to extract the sensation of the body fromthe physical body. The theory of rubber hand Illusion (RHI) is usedin both studies. RHI is a phenomenon when a person feels the rubber hand as his own hand by synchronous stimulation. Researchersof [3, 4] extended this phenomenon to the entire body. On the basisof the concept of RHI, they tried to localize and attribute the bodysensation from the physical body to the virtual body by touchingboth bodies.
In this paper, we examine whether the localization and attribution of the body sensation accrue in an environment with a robot,
on the basis of previous research methods. The main aim of thisresearch is to acquire knowledge in order to reveal the method ofcontrolling the ”body sensation” in humans. First, we experimentwith previous research setups under our experimental conditions;
we change the actor that holds the two rods. Next, we experimentwith the actor changing from a certain actor to oneself in a telexistence environment. Thus, we consider whether past research applies to the telexistence system. In other words, we study where
and how the localization and the attribution of body are caused by
the presence or absence of stimulation.
2

R ELATED WORKSIn this section, we introduce two related researches.
In the past, [3, 4] experimented with the illusion of the out-ofbody sensation to provide and apply to the body the perception of
the RHI [5]. In the RHI, the rubber hand, which resembled the human hand, was set in front of the participant and the hand of theparticipant was hidden. The experimenter touched the rubber hand
and the participant’s hand (hidden from the participant’s view) simultaneously. Then, the participant could see only the rubber handthat was touched by the experimenter. As a result, the participant
felt the rubber hand as his own. [3, 4] also extended this perception
to the entire body using a part of the body.
In [3], the participants wore a pair of head-mounted displays thatwere connected to two video cameras placed behind their backs,
which enabled them to see their backs. The experimenter stood beside a participant and used two plastic rods to simultaneously touch
the participant’s actual chest and the chest of the “illusory body” bymoving one rod toward a spot just below the cameras in view. After 2 min of stimulation, the participants were asked to respond toa questionnaire (10 statements, 3 illusion statements, and 7 control
statements) with a seven-point visual analog scale. The participants
reported the experience of sitting behind their physical bodies andlooking at them from this spot. To provide an objective evidence forthe illusion, they registered the skin conductance response (SCR)
as a measure of the emotional response when the illusory body was”hurt” when it was hit with a hammer after a period of stimulation. They reported signiﬁcantly greater threat-evoked SCRs after
the illusion condition. From these results, they concluded that body
localization and attribution are better constructed by multisensory
correlation (visual information and tactile information). The authorof this research also examined several other phenomena of bodysensation [6].
In [4] the author examined whether the body localization and attribution occurred by synchronous/asynchronous stimulation within60 sunder similar conditions. The difference with [3] is that thestimulation point is the back and not the chest of the participant. In
addition, the motion shift of the subject was measured for quantitative veriﬁcation. Motion shift is the distance between the initial
position and the ﬁnal position of the subject when the subject returns back to his/her initial position after he/she has been movedby the experimenter. They also used a questionnaire of ten statements, and they reported that strong illusion only occurred in thesynchronous stimulation mode based on the questionnaire and motion shift results. Furthermore, they examined the difference of avisual body form and suggested that strong illusion occurred when
the subjects saw a human-like image.
From these results, it will be possible to control the localization
and attribution of the body, between the real and the virtual body, byadding the synchronous/asynchronous stimulation to the visual andauditory senses of a third person under an environment in which
one can see himself/herself.
3

E VALUATION PROCEDURE OF EMBODIMENTIn this paper, we built upon past research in order to examine
whether the localization and attribution of body sensation accrue
in an environment with a robot. Thereby, we considered whether
the past research results could be applied to the telexistence system. That is, we studied where and how the localization and theattribution of the body are caused by the presence or absence ofstimulation. In particular, we focus on the actor who applies thestimulation to the physical body and virtual body and experimentwith the condition that the robot replaces the actor as the experimenter. Next, we change from the PAHNToM to oneself in a telexistence environment.
4

Two styrene foam rods (participant’s side rod: 2 cm in diameter and 40 cm in length; robot’s side rod: 2 cm in diameter and
60 cm in length) were used to repeatedly touch the participant’s
physical chest, which was out-of-view and just below the cameras.
The two rods were attached to two haptic PHANToM Omni (SensAble technologies) devices. The two PHANToM devices moved tofollow preset motion patterns. The motion command for the two
PHANTOM devices was issued from a personal computer to avoid
the delay between the master and the slave.
The participants were seated in a relaxed position and were instructed not to move. They wore an HMD that was connected tothe stereo cameras behind the participant’s back at the same heightas the person’s eyes. This arrangement meant that the person saw
his or her back from the perspective of a person sitting behind him
or her with stereoscopic vision and without noticeable delay (depending on the LCD refresh rate of 30 ms). The experimenterstands besides the participant, holds the rod at the participant’s side,
and operates two rods periodically. As an experimental condition,
the rods stimulate the participant’s physical and virtual chest constantly. After the participant puts off the HMD, he or she answers
ten questions of a prepared list.
experimenter
subject
PHANToM

robot head
PHANToM

E XPERIMENTS

4.1 Preliminary experiment
In this section, we explain the preliminary experiment on the basis of the past research of the out-of-body experience. From theseresults, we judge that this phenomenon can apply to telexistence.
One aspect of our experiment is to examine the response of the
body when a machine stimulates the participant. In conventional research, the participant can see the experimenter; therefore, the participant feels that the experimenter who is a person touches him/her.
However, supposedly in telexistence or telepresence, the operator
causes the localization and attribution of the body without any person in the master and slave sides. Moreover, in previous research,
the localization and attribution of the body are decided by the synchronous or asynchronous stimulation. Then, we review the effectof the localization and attribution of the body when the actor is
not a person or there is no other person except the participant. In
addition, we examine the difference between the effect of the synchronous and that of the asynchronous stimulation. In this experiment, we selected the PHANToM Omni as the robot because of its
simplicity and functional force feedback.

1. 5m

Figure 1: Experimental system environment.

4.1.1 Experimental environment of preliminary experiment
The arrangement of this experiment is shown in Figure 1. In addition, Figure 2 shows the actual environment.
Two cameras are connected to head-mounted displays (HMDs)
with a wide ﬁeld-of-view and high resolution (resolution = 1024 ×
768 pixel, diagonal ﬁeld of view = 42.0 °for each eye, and refresh
rate = 30 ms). This HMD was developed in our laboratory, and its
viewpoint distance adjusts to match the distance between the twocameras; therefore, the participants can feel the remote environment
rather naturally. The cameras and the HMDs are placed side-byside 1.5 m behind each participant’s back. The slave robot headof the visual telexistence system, which can express a 6 degrees of
freedom motion of the upper body, acts as the stereo camera system.

Figure 2: Experimental system environment (actual).

4.1.2 Experimental condition of preliminary experiment
The motion constraints of the rods are categorized to six modes:• Sync 2 passive rods,

Sync/Async, 2 passive rods

Sync/Async, 1 passive / 1 active rods

Sync/Async, 2 active rods

Figure 3: Experimental modes.

No.

Question

Q1

I experienced that I was located at some distance behind the visual image of myself, almost as if I was looking at someone else.

Q2

I felt as if my head and eyes were located at the same place as the cameras, and my body just below the cameras.

Q3

I experienced that the hand I was seeing approaching the cameras was directly touching my chest (with the rod)

Figure 4: Illusion statement.

No.

Question

Q4

I felt that I had two bodies.

Q5

I experienced that my (felt) body was located at two locations at the same time.

Q6

I experienced a movement-sensation that I was floating from my real body to the location of the cameras.

Q7

I felt as if my head and body was at different location, almost as if I had been decapitated.

Q8

I did not feel the touch on my body but at some distance in space in front of me.

Q9

I could no longer feel my body, it was almost as if it had disappeared.

Q10 The visual image of me started to change appearance so that I became (partly) transparent.
Figure 5: Control statement.

• Async 2 passive rods,• Sync 1 passive / 1 active rods,• Async 1 passive / 1 active rods,• Sync 2 active rods,• Async 2 acrive rods.
In Sync 2 passive rods and Async 2 passive rods, the experimenterstands beside the participant, holds “two rods” and touches the participant’s physical chest and the virtual chest. In Sync, two rods
move synchronously, and in Async, two rods move asynchronously,
which generates additional random phase signals to the rod at the
robot side. These modes are presuming the same motion as in past
research. In Sync 1 passive/1 active rods and Async 1 passive/1 active rods, the experimenter stands beside the participant and holds
only ”one rod” at the participant side and touches the participant’s
physical chest. Then, the PC-controlled PHANToM automaticallymoves the rod at the robot side and touches the virtual chest of
the participant. The participant can see the rod moving in frontof him/her while seeing his/her body touched by the experimenter.
Similarly, to above two modes, the rods move synchronously in
Sync and asynchronously in Async. In Sync 2 active rods andAsync 2 active rods, the experimenter does not stand beside theparticipant. The PC-controlled PHANToM moves the two rods at
the robot and participant side and touches the physical chest andvirtual chest. The difference between Sync and Async is the sameas above. Figure 3 shows the difference among the above threepatterns.
The duration time of the stimulation is 60 s, and the period of thestimulation is 1 Hz in the synchronous mode. In the asynchronousmode, it is 1 Hz with a random phase signal to PHANToM.
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chronous.

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Yϵ3. The experimenter gives instructions to the participant to takethe same posture as the robot head
4. The experimenter touches the participant’s physical chest andvirtual chest, placed just below the cameras, according to the
motion of PHANToM (controlled by the PC in response tomodes)
5. After 60 s, the participant answers ten questions.

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Procedures 2- 6 are executed in each of the 6 modes. Six students
participated in this experiment (ﬁve males and one female, in their
twenties). All participants were healthy.
After a stimulation of 1 min, the participants were required toanswer a questionnaire. The ten questions required a rating of thestrength of sensation. The questions were presented in a random
sequence. The ﬁrst three questions are illusion statements, whichrate high when the participant feels his/her body sensation in the
robot. On the other hand, the other seven questions are control
statements, which rate low when the participant did not feel his/herbody sensation in the robot. These questions are the same as in [3],making it easy to compare the results.

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Figure 7: Experimental result of preliminary experiment: Asynchronous.

4.1.3 Results of preliminary experiment
The results of the preliminary experiments are shown in Figures 6
and 7. Figure 6 shows the result of the synchronous stimulationof each mode, and Figure 7 shows the result of the asynchronous
stimulation of each mode.
From the results, the rating of questions Q1-Q3 is high and therating of the other questions (Q4-Q10) is low in synchronous stimulation. In contrast, in the asynchronous stimulation, the rating of all
the questions is low. The trend of these results is similar to that observed in the previous research. Moreover, the difference between
the ratings of the questions of each mode is not large. Therefore,
it shows that the localization and attribution of the body occur regardless of the kind of actor that provides the stimulus.
Next, we look at the individual results. In synchronous stimulation, the score of “Sync, 2 active rods mode” is high all around.
This trend is good for the illusion statement; however, it is noteworthy that the score of questions 5 and 6 are also high. These twoquestions ask whether the person feels his/her body at two places.
Therefore, it seems that the person loses sight of the localization
and attribution of oneself. In asynchronous stimulation, the scoresof question 1, question 2, and question 3 are low; however, the scoreof question 1 is higher than those of the other two questions. We assume that this result is attributed to the experimental environment.
In our environment, the condition of the robot head and HMD is optimized (view angles and distance between eyes are equal in both);therefore, a very natural stereoscopic view is achieved. These results indicate the possibility that a realistic sensation is high, regardless of the synchronous of stimulation. Thus, the score of question
1 could increase by a stereoscopic effect of the image, in spite of
the asynchronous stimulation. In addition, the score of question 8 ishigher than that of the rest. In the asynchronous stimulation, the rod
touches the actual body before the apparent rod touches the apparent body. The timing between the touch of the actual and apparent
rod differs; therefore, we assume that the subject feels the stimulation at a point distant from the actual body. On the other hand, in
synchronous stimulation, the score of question 8 is low. Therefore,
we think that the person will tangibly recognize the apparent rod
position.

are placed side-by-side 1.5 m behind each participant’s back. Similar to the stereo camera system, the slave robot head of the visualtelexistence system can perform a 6 degrees of freedom motion of
the upper body. As in the preliminary experiment, two styrene
foam rods (participant’s side rod: 2 cm in diameter and 40 cm inlength; robot’s side rod: 2 cm in diameter and 60 cm in length)were used. The two rods were attached to the two haptic PHANToM Omni (SensAble technologies) devices. The difference withthe preliminary experiment is that the direction of the two PHANToM devices with the rod follows the direction of the participant.
The two PHANToM devices are in a master-slave relationship, andthe control method changes according to the experimental conditions. To avoid delays between the master and the slave, as in the
preliminary experiment, a personal computer directs the motion of
the two PHANToM devices. Moreover, the hand of the participant
was covered to hide its view by the camera. Responding to the experimental conditions, instead of the human’s back, we placed theobject in front of the PHANToM at the participant’s side. In thisexperiment, the advantage of using the PHANToM is the ease insetting up the experimental conditions.
As in the preliminary experiment, the participants are seated ina relaxed position. They wear an HMD that is connected to stereo
cameras behind their back. The participants use the PHANToM
with the rod at its side and touch their back, which they have in theirview. During the experiment, the participant wears a earphone andhears the sound of a metronome. The participant receives at his/herback the stimulation by the rods of the robot side constantly. Afterthe participant puts off the HMD, he or she answers ten questions
from a prepared list.

subject
PHANToM

robot head
PHANToM

4.2 Main experiment
From the results of the preliminary experiment, we represent theenvironment of previous research and ﬁnd that it does not depend on
the actor of the stimulation. Next, we verify this result by probing
and looking for the kind of elements that inﬂuence the out-of-bodysensation.
The feature of our experiment is to use the active tactile stimulation by telexistence, and not the passive stimulation by a third person. By interaction with one’s body, active stimulation is achieved.
Thereby, a person can become both a giver and a receiver of tactile
stimulation. Therefore, in this section, we evaluate the factors that
decide the localization and attribution of the body.
We also use rods for tactile stimulation, and we examine the difference in the presence or absence of the stimulation to the arm andthe body.
4.2.1 Experimental environment of main experiment
The arrangement of this experiment is shown in Figure 8. In addition, Figure 9 shows the actual environment. Similar to the preliminary experiment, two cameras are connected to head-mounted
displays (HMDs) with wide ﬁeld-of-view and high resolution (resolution = 1024 × 768 pixel, diagonal ﬁeld of view = 42.0 °for eacheye, and refresh rate = 30 ms). This HMD was developed in ourlaboratory, and its viewpoint distance can adjust to match the distance between two cameras; therefore, the participants can feel the
remote environment in a natural way. The cameras and the HMDs

1. 5m

Figure 8: Experimental system environment.

4.2.2 Experimental conditions of main experiment
The motion constraints of the rods are grouped into four modes.
We grouped by the existence or nonexistence of the stimulation to
the subject’s arm, which has the rod at the subject’s side, and thestimulation to the subject’s back, which is touched by the rod at the
robot side.
• Arm + Back
• Arm
• Back• None“Arm + Back” refers to the stimulation to the subject’s arm andback. “Arm” refers to the stimulation to the subject’s arm, whereas
the stimulation to the subject’s back is nonexistent. “Back” refers to4. The participant touches the participant’s physical back in view
5. After 60 s, the participant answers ten questions.
Procedures 2-6 are executed in each of the four (4) modes. Fivemale students participated in this experiment (all ﬁve were in their
twenties). All participants were healthy.
We assume that the stimulation is the key factor. If the stimulation is applied to the arm of the subject, then the subject will feel as
if he/she is the actor. If the stimulation is applied to the back of thesubject, then the subject will feel as if he/she is the
4.2.3 Results of main experiment

Figure 9: Experimental system environment.

the nonexistent stimulation to the subject’s arm, whereas the stimulation to the subject’s back is present. “None” refers to the lackof stimulation to the subject’s arm and back. These differences areshown in Figure 10.

Arm+Back

Back

Arm

None

Figure 10: Experimental modes.

After a stimulation period of 1 min, the participants were required to answer a questionnaire comprising eleven questions that
rated the strength of the sensation. The questions were presented ina random order. The ﬁrst three questions were illusion statements,
which were rated high when the participant felt his/her own bodysensation in the robot. In contrast, the other eight questions were
control statements, which were rated low when the participant did
not feel his/her own body sensation in the robot.
The duration time of the stimulation is 60 s. The experimenter
directs the participant to operate the rod using a period of 1 Hz.
The details of the experimental procedure are provided below:
1. The experimenter explains the system conﬁguration to theparticipant
2. The participant wears the HMD and earphone
3. The experimenter instructs the participant to take the same
posture as the robot head

The results of the main experiments are shown in Figure 13.
From the results, we ﬁnd that the score of question 1 is positive
and the scores of questions 7-11 are negative. These trends are similar to the results of the preliminary experiment. Question 3 andquestion 4 are important statements. They decide where localization and attribution occur in the subject’s body. The “Arm” modeis positive for question 3 and negative for question 4. Therefore,
when the stimulation to the subject’s arm is present, the subject
feels as the actor, that is, the subject feels in the robot position.
In our expected result, the “Back” mode is positive for question
4 and negative for question 3. However, the actual results showthat the “Back” mode is positive for both questions 3 and 4. The
score of question 4 is larger than that of question 3, and it is thelargest. Therefore, the “Back” mode is the most effective in question 4. Why the score of question 3 is positive? As one reason,
we think that the experimental environment is not appropriate. We
used PHANToM as the actor to provide stimulation to the subject’sback. PHANToM can exert force but its power is small. Therefore,
it might not stimulate sufﬁciently enough. Moreover, PHANToM
has three degrees of freedom, which is a large value. For thesereasons, we must reconsider the experimental environment. Alternatively, there is a possibility that the score of question 3 increases
owing to the activity of the subject’s arm. That is, the activity ofthe person may be dominant in the case of body localization andattribution. Thereby, the subject would feel both as the actor andas the receiver. In fact, because the score of questions 5 and 6 ispositive in the “Back” mode, the subject would feel as having twobodies.
We did not obtain the expected results in this case; however, theresults show that the existence or nonexistence of stimulation on thesubject’s arm and back can change the feeling of body. In particular,
the activity of the person may be important to body localization andattribution in a remote place. In the next step, we will reconstruct
the experimental environment and examine the body sensation indetail.
5

C ONCLUSIONSIn this paper, we examined whether the localization and attributionof the body sensation accrue in an environment with a robot, on
the basis of methods of past research. First, in a preliminary experiment, we examined the out-of-body experience on the basis ofpast research. From the results, we concluded that this phenomenon
could be applicable to telexistence. As a result, the localization andattribution of the body occur regardless of the kind of actor thatprovides the stimulus.
Next, we veriﬁed this phenomenon in a probing situation andlooked for the kind of elements that inﬂuence the out-of-body sensation. We used the active tactile stimulation by telexistence, andnot the passive stimulation by a third person. Consequently, the activity of the person may dominate the localization and attributionof the body. As a next step, we will reconstruct the experimental
environment and examine the body sensation in detail.

No.

Question

Q1

I experienced that I was located at some distance behind the visual image of myself, almost as if I was looking at someone else.

Q2

I felt as if my head and eyes were located at the same place as the cameras, and my body just below the cameras.

Q3

I experienced that my hand was directly touching other back (with the rod).

Figure 11: Illusion statement.

No.

Question

Q4

I experienced that my back was touched by other hand (with the rod).

Q5

I felt that I had two bodies.

Q6

I experienced that my (felt) body was located at two locations at the same time.

Q7

I felt that I had three bodies.

Q8

I experienced that my (felt) body was located at three locations at the same time.

Q9

I experienced a movement-sensation that I was floating from my real body to the location of the cameras.

Q10 I felt as if my head and body was at different location, almost as if I had been decapitated.
Q11 I did not feel the touch on my body but at some distance in space in front of me.
Figure 12: Control statement.

Q11 : I did not feel the touch on my body but at some distancein space in front of me.

Avg None
Avg Back
Avg Arm
Avg Arm+Back

11

Q10 : I felt as if my head and body was at diﬀerent location,
almost as if I had been decapitated.

10

Q9 : I experienced a movement-sensation that I was ﬂoatingfrom my real body to the location of the cameras.

9

Q8 : I experienced that my (felt) body was located at threelocations at the same time.

8

Q7 : I felt that I had three bodies.

7

Q6 : I experienced that my (felt) body was located at two
locations at the same time.

6

Q5 : I felt that I had two bodies.

5

Q4 : I experienced that my back was touched by other hand(with the rod).

4

Q3 : I experienced that my hand was directly touching otherback (with the rod).

3

Q2 : I felt as if my head and eyes were located at the same placeas the cameras, and my body just below the cameras.

2

Q1 : I experienced that I was located at some distance behindthe visual image of myself, almost as if I was looking at someone
else.

1
-3

-1

Figure 13: Experimental result.

1

3

ACKNOWLEDGEMENTS
This work was partly supported by SCOPE: Strategic Information
and Communications R&D Promotion Programme.
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