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Background: Altering the normal association between touch and its visual correlate can result in the illusory perception of a fake limb as part of our own body. Thus, when touch is seen to be applied to a rubber hand while felt synchronously on the corresponding hidden real hand, an illusion of ownership of the rubber hand usually occurs. The illusion has also been demonstrated using visuomotor correlation between the movements of the hidden real hand and the seen fake hand. This type of paradigm has been used with respect to the whole body generating out-of-the-body and body substitution illusions. However, such studies have only ever manipulated a single factor and although they used a form of virtual reality have not exploited the power of immersive virtual reality (IVR) to produce radical transformations in body ownership. Principal findings: Here we show that a first person perspective of a life-sized virtual human female body that appears to substitute the male subjects' own bodies was sufficient to generate a body transfer illusion. This was demonstrated subjectively by questionnaire and physiologically through heart-rate deceleration in response to a threat to the virtual body. This finding is in contrast to earlier experimental studies that assume visuotactile synchrony to be the critical contributory factor in ownership illusions. Our finding was possible because IVR allowed us to use a novel experimental design for this type of problem with three independent binary factors: (i) perspective position (first or third), (ii) synchronous or asynchronous mirror reflections and (iii) synchrony or asynchrony between felt and seen touch. Conclusions: The results support the notion that bottom-up perceptual mechanisms can temporarily override top down knowledge resulting in a radical illusion of transfer of body ownership. The research also illustrates immersive virtual reality as a powerful tool in the study of body representation and experience, since it supports experimental manipulations that would otherwise be infeasible, with the technology being mature enough to represent human bodies and their motion.
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First Person Experience of Body Transfer in Virtual
Reality
Mel Slater
1,2,3
*, Bernhard Spanlang
2,4
, Maria V. Sanchez-Vives
1,5
, Olaf Blanke
6
1Institucio
´Catalana Recerca i Estudis Avanc¸ats (ICREA), Universitat de Barcelona, Barcelona, Spain, 2Facultat de Psicologia, Universitat de Barcelona, Barcelona, Spain,
3Department of Computer Science, University College London, London, United Kingdom, 4Departament de LSI, Universitat Polite
`cnica de Catalunya, Barcelona, Spain,
5Institut d’Investigacions Biome
`diques August Pi i Sunyer (IDIBAPS), Barcelona, Spain, 6Brain-Mind Institute, Ecole Polytechnique Fe
´de
´rale de Lausanne (EPFL), Lausanne,
Switzerland
Abstract
Background:
Altering the normal association between touch and its visual correlate can result in the illusory perception of a
fake limb as part of our own body. Thus, when touch is seen to be applied to a rubber hand while felt synchronously on the
corresponding hidden real hand, an illusion of ownership of the rubber hand usually occurs. The illusion has also been
demonstrated using visuomotor correlation between the movements of the hidden real hand and the seen fake hand. This
type of paradigm has been used with respect to the whole body generating out-of-the-body and body substitution
illusions. However, such studies have only ever manipulated a single factor and although they used a form of virtual reality
have not exploited the power of immersive virtual reality (IVR) to produce radical transformations in body ownership.
Principal Findings:
Here we show that a first person perspective of a life-sized virtual human female body that appears to
substitute the male subjects’ own bodies was sufficient to generate a body transfer illusion. This was demonstrated
subjectively by questionnaire and physiologically through heart-rate deceleration in response to a threat to the virtual body.
This finding is in contrast to earlier experimental studies that assume visuotactile synchrony to be the critical contributory
factor in ownership illusions. Our finding was possible because IVR allowed us to use a novel experimental design for this
type of problem with three independent binary factors: (i) perspective position (first or third), (ii) synchronous or
asynchronous mirror reflections and (iii) synchrony or asynchrony between felt and seen touch.
Conclusions:
The results support the notion that bottom-up perceptual mechanisms can temporarily override top down
knowledge resulting in a radical illusion of transfer of body ownership. The research also illustrates immersive virtual reality
as a powerful tool in the study of body representation and experience, since it supports experimental manipulations that
would otherwise be infeasible, with the technology being mature enough to represent human bodies and their motion.
Citation: Slater M, Spanlang B, Sanchez-Vives MV, Blanke O (2010) First Person Experience of Body Transfer in Virtual Reality. PLoS ONE 5(5): e10564. doi:10.1371/
journal.pone.0010564
Editor: Mark A. Williams, Macquarie University, Australia
Received November 27, 2009; Accepted April 7, 2010; Published May 12, 2010
Copyright: ß2010 Slater et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was supported by the Integrated Project PRESENCCIA funded under the European Union’s Sixth Framework Program, Future and
Emerging Technologies (FET), Contract Number 27731. http://cordis.europa.eu/ist/fet/pr.htm. The work was also partially supported by MS’s European Research
Council grant TRAVERSE (227985). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: melslater@gmail.com
Introduction
Normally when something strikes our body we feel it at the same
place that we see it. When normal correlation between two sensory
streams is changed, for example, by seeing a plausibly located rubber
hand touched while simultaneously feeling the touch on our out-of-
sight real hand, the brain apparently engages in a re-evaluation of
probabilities and assigns ownership to the visible rubber limb [1,2].
These methods have also been used to produce illusions of body
morphing, adding supernumery limbs to the body [3,4,5,6], and out-
of-the-body experiences [7,8,9]. In conjunction with brain-imaging
techniques these manipulations can provide insight into the brain
areas involved in body representation, for example as in [10]. While
the vast majority of work in this field has shown that it is possible to
incorporate physical objects or video images of these into the body
representation, it has also recently been shown that the same methods
work with entirely virtual objects [11,12,13].
The examples of out-of-the-body experiences provide indirect
evidence that these illusions might apply to the whole body rather
than only to body parts. There is also evidence that ownership can
be attributed to a manikin that appears visually to substitute the
person’s real body as seen through head-mounted displays coupled
to a video camera oriented down at the manikin body [14]. These
out-of-the-body and the manikin experiments employed synchro-
nous visuotactile stimulation – the illusory visual body was seen to
be tapped or stroked in the same place as the real body was felt to
be stimulated. When there is asynchrony between felt and seen
touches changes in ownership do not occur or are less prominent
compared to the case of synchrony between both stimuli [13].
The experiment reported here is the first that shows that
ownership can be transferred to an entirely virtual body, using an
experimental design that separates perspective position from
visuotactile stimulation. We found that when perspective position
is included as a factor in the experimental design the importance of
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visual-tactile synchronization diminishes in comparison to what
would be expected from the literature.
Results
Overall Design
There were 24 male participants recruited for our study. They
were seated, and entered into the virtual reality through a wide
field-of-view head-tracked, head-mounted display and stereo
headphones. The scene in which they were located is shown in
Figure 1. They were asked to visually explore this scene for
2 minutes after which their viewpoint was transported to the
other side of the room to where two female virtual characters
were located, a seated young girl and a standing woman
(Figure 2).
What they experienced then depended on which of the
combinations of three binary factors they had been assigned by
the experimental design (Table 1). Perspective was either first person
(1PP, Figure 2A) or third (3PP, Figure 2F) with respect to the
seated girl. Movement refers to whether the observed head
movements of the virtual girl were synchronous with those of
the subject (MS, Figure 2D, 2F) or asynchronous (MS9). Touch
refers to whether the subject felt synchronously (TS) or
asynchronously (TS9) touched on his shoulder when the standing
woman stroked the shoulder of the seated girl (Figure 2C, 2D, 2F).
After almost 7 minutes of this period that included occasional
shoulder stroking, the viewpoint of the participant was lifted
upwards towards the ceiling, looking down on the scene below
(Figure 2G) during which time the shoulder stroking continued but
unaccompanied by physical sensations. Suddenly the standing
woman was seen to hit the seated girl around the face (Figure 2H).
After this the viewpoint translated downwards again, there were
some more (felt) shoulder strokes, and then the experimental trial
was terminated. The full sequence of events that occurred is shown
in Table 2.
Questionnaire Results
Immediately after the experience in the virtual reality, a 13-item
questionnaire was answered by the participants (Questionnaire
S1). Eight of these questions related to the issue of body ownership
(Table 3). Perspective gives the clearest set of responses (Figure 3A),
where the mean (and median) score for 1PP is always greater than
or equal that for 3PP on each of the questions. Movement appears
to have no particular effect, and synchronous touch has an effect
on some of the variables. From the fitted models estimates of the
probabilities of the questionnaire scores for four combinations of
the factors were obtained and are shown in Figure 3D. These data
show that the most important factors leading to the temporary
subjective illusion of ownership of the virtual body are the
participant’s perspective (i.e. in the girl’s body, 1PP) and touch
Figure 1. The scene. The scene that the participants entered was a room approximately the same size as the real room in which they were located.
(A) There were two female characters at the other end of the room, a standing woman who could be seen stroking the shoulder of a seated girl, and a
fireplace behind. (B) Looking down at himself a participant would see an empty chair. (C) To the participant’s left was a TV showing a real-time music
video. (D) To the right were a mirror frame and a door opening to a field.
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Figure 2. Participants were transferred to the other side of the room. (A) In the 1PP condition their body was substituted by that of the girl’s
(white shirt), and when looking down at themselves they would see her body. (B) Looking up they would now see that the woman (brown sweater)
was standing by them. (C) The woman stroked their shoulder. (D) Looking left they would see the reflection of the girl and the woman in a mirror. (E)
They were seeing the room and hearing the sounds from the TV from the perspective of the opposite side than in the first two minutes. (F) In the 3PP
condition they would be located to the right of the girl, and so see her and her reflection in the mirror – in the case shown with her head moves
synchronized with their own head moves. (G) Later the viewpoint shifted near to the ceiling and the woman continued to stroke the shoulder of the
girl, but the participant did not feel this. (H) Suddenly the woman struck the girl three times around the face - the wide-field-of-view in this image
corresponds more precisely to what the subject would have seen.
doi:10.1371/journal.pone.0010564.g002
Table 1. Allocation of Participants to the Experimental Factors.
Subject number:
Factor: 123456789101112131415161718192021222324
P 010110100101110010011010
M 111010001001001010001111
T 001001111001010011011100
For example subject 1 was allocated to the condition P9,M,T9; subject 9 to the condition P9,M,T.
doi:10.1371/journal.pone.0010564.t001
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(TS), concordant with a recent account of self-consciousness [15].
Our data also show that apparent head-movement synchrony was
least important for the body ownership illusion.
Heart Rate Deceleration
We measured heart rate deceleration (HRD) in response to the
woman slapping the girl, a parameter that has been associated
with reports of aversive stress in the context of picture viewing
[16]. We calculated the negative of the slope of heart rate change
during the first 6s after the event in question. The greater this
value the greater the initial deceleration and the greater the degree
of aversive stress (p588). We consider HRD for two pairs of events
(Figure 4). After the down transition (Table 2) the participants who
perceived from the girl’s perspective (1PP) showed a significantly
greater HRD than the participants who perceived the scene from
the displaced perspective (3PP). The same analysis was carried out
for a control period (across) and revealed no significant difference
between these groups of participants. Similarly, we found that
during the slap (duringS) the 1PP participants had a significantly
greater HRD than the 3PP participants, but for beforeS there was
no significant difference. Amongst the three factors considered in
this experiment only Perspective had a significant influence on the
HRD response.
There is, furthermore, consistency between physiological
responses (HRD) and the subjective questionnaire responses.
During the slap (duringS) and after the period of being in the
elevated position (down) the HRD was significantly positively
correlated with a feeling of the participant’s own body being
attacked (attack), the feeling that they might be hurt by the woman
(hurt) and body ownership (body). However, there were no
significant correlations between any of the questionnaire responses
and HRD for the control periods (beforeS and across). The full set of
correlations and significance levels is in Table 4.
Discussion
Our study extends earlier results that used simpler video and
virtual reality technology [7,8,9,14]. These suggested that
synchronous touch (TS) and 1PP [17,18] to be crucial factors for
the sense of ownership. Importantly, these previous studies only
manipulated a single one of the three factors (Perspective,
Movement, Touch). Our results suggest that when all three factors
are considered together that perspective, specifically first-person
perspective, clearly dominates as an explanatory factor for
subjective and physiological measures of ownership. The latter
provides a particularly powerful result, since participants were
responding to witnessing the girl being slapped while they were in
an elevated position even without any synchronous touch. The
1PP participants, i.e. those who earlier had been in the first person
perspective with respect to the girl’s body, had a significantly
greater physiological response than those who had earlier been in a
spatially close but distinct virtual perspective (3PP). Moreover,
stronger heart rate deceleration was positively correlated with the
feeling of body ownership and the feeling of being attacked or hurt.
Table 2. Sequence of the Events During the Experiment.
Variable Name Event Description Time to next event (s) Cummulative time (s)
Baseline: Experiment Starts with participant seeing the girl and woman across
the other side of the room
120 120
across Move across the room to enter the girl’s perspective (1PP) or to
the right of the girl’s perspective (3PP)
5125
stroke_1 First stroke by the woman on the girl’s shoulder 415 540
Perspective Shifts to the ceiling – girl and woman seen below 5 545
First arm stroke seen from ceiling position 45 590
duringS The woman slaps the girl* 50 640
down The perspective shifts back down to the girl 20 660
stroke_n The final arm stroke 31 691
The view moves back to the original perspective, and this continues for a final 30s. 30 -
*The event beforeS (before slap) was taken as 7s before the actual slap.
doi:10.1371/journal.pone.0010564.t002
Table 3. Questions relating to body ownership and their labels used in the text and figures.
body How much did you feel that the seated girl’s body was your body?
touch How strong was the feeling that the woman you saw was directly touching you on the shoulder?
woman How strong was the feeling that the touch you felt was caused by the woman that you saw?
cloth How strong was the feeling that you were wearing different clothing, from when you started the experiment, while you were in the part of the room
where the standing woman was located?
mirror How strong was the feeling that the body of the girl in the mirror was your body?
cnct When you were looking down from above how much did you feel a strong connection with the seated girl as if you were looking down at yourself?
attack When the standing woman hit the seated woman, how much did you feel this as if this was an attack on your body?
hurt After you returned from looking down from above how much did you feel that the standing woman might hurt you?
doi:10.1371/journal.pone.0010564.t003
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The minimal contribution of the specific type of agency that we
investigated (MS9compared to MS) seems to be in conflict with
previous studies that suggested the importance of motor cues for the
sense of self [19,20]. We note that previous studies have focussed
almost entirely on agency manipulations with respect to the upper
extremity and have been carried out in isolation from perspective and
touch manipulations (see [21] for an exception), making problematic
any direct comparison with our results. However, considering our
two questions that relate directly to body ownership, (body and mirror
from Table 3) participants in condition MS were more likely to give a
higher score to mirror than to body compared to those in MS9.Aplotof
the scores is shown in Figure S1. There are only 2 out of 12 cases for
those in condition MS where mirror ,body, and only 1 out of 12 cases
for those in condition MS9where mirror .body. The correlation
between these two sets of scores for those in condition MS (r = 0.91,
p,3.5610
25
) is greater than for those in MS9(r = 0.71, p,0.01).
Analysis of covariance of mirror on condition M with body as a
covariate suggests that the regression line of mirror on body has greater
slope in condition MS than in condition MS9(p = 0.06). The same
analysis has stronger support for there being two positively sloped
parallel lines with the one for MS having a greater intercept than for
MS9(p = 0.0093). This suggests that the synchronised head
movementdid,afterall,makesomedifference–resultinginthose
participants in condition MS giving higher scores to the question
mirror than to the question body.
It could be argued that the amount of synchronous visuo-tactile
stimulation was less than what is normally used to induce the
rubber hand illusion. Yet according to [22] the RHI can be
generated in about 80% of people with less than 15s of stimulation,
provided that the rubber hand and real hand are close to one
another (15–18cm). Moreover in our setup, unlike that of the RHI,
Figure 4. Means and standard errors of the Heart Rate
Deceleration data. The figure shows the means and standard errors
for HRD after four events: across: 0.5s after initially arriving at the other
side of the room; down: 0.5s after descending from above; beforeS:7s
before the slap; duringS: 2s into the slap sequence. 1PP was significantly
greater than 3PP on down (0.028) and duringS (0.034). The ANOVA fits
satisfied the requirement of normally distributed residual errors using
the Jarque-Bera test [37], except for 1PP on down, where a variable
transformation was found to obtain normality.
doi:10.1371/journal.pone.0010564.g004
Figure 3. Questionnaire responses for the main effects. (A–C) show the means and standard errors of the questionnaire responses by each of
P, M and T. Using proportional-odds cumulative logit models the notable significance levels are for P (body, p = 0.031; touch, 0.023; woman, 0.033;
cloth, 0.003; hurt, 0.046), and T (body, p = 0.095; touch, 0.085; woman, 0.024). The model fits were good, with the highest deviance being 29.8 on 25 d.f.
Panel (D) shows the estimated probabilities for the questionnaire responses for body, for four cases: for third person (P3, disembodiment) and for
asynchronous (TS9) and with synchronous touch (TS), and for first person (P1, embodiment) again comparing TS9with TS. In each case M = MS (the
graph is almost identical for M = MS9). There were no scores of 10 in these responses which accounts for the low estimated probability of ‘Very High’.
doi:10.1371/journal.pone.0010564.g003
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the participant was not required to continually look at the actual
point of contact between the virtual hand and shoulder. For most
of the time they looked up towards the virtual woman, and would
see her arm move up and down in synchrony with feeling of the
strokes, or see the same in the mirror reflection. Finally, we note
that continuous stroking may not be necessary to induce the
illusion. For example [23] showed that the RHI could be ‘topped
up’ by occasional sequences of stroking with periods of no stroking
in between. See Methods for further details of the stroking
sequences in our experiment.
Our experiment includes that of Lenggenhager et al. [7] as a
special case. The essence of their setup was to manipulate
ownership by a 3PP self-representation, that was touched
asynchronously or synchronously, comparable to our 3PP and
TS9compared with 3PP and TS. Figure 3D shows that with
respect to the questionnaire responses the estimated probability of
the response being in the Very Low category was much greater in
the asynchronous touch than in the synchronous touch condition.
Figure S2 gives the equivalent graph for the questions touch and
woman, which have greater probability estimates for the Medium to
High questionnaire response in the synchronous compared to the
asynchronous condition. There are no significant effects for T in
the case of the HRD.
Our experiment also includes that of Petkova and Ehrsson [14]
as a special case. Their main setup was to also manipulate
ownership by a 1PP self-representation (a manikin) that was
touched either asynchronously or synchronously – similar to our
1PP and TS9(girl’s perspective and asynchronous stroking) and
1PP and TS (girl’s perspective but synchronous stroking),
respectively. Figure 3D shows that the responses to the question
body supports the idea that synchronous stroking enhances
ownership of the 1PP perceived virtual body. The same is true
for the other questionnaire responses (Figure S2). However, HRD
is not affected by T.
In [14] there was a significantly different skin conductance
response when the manikin body was threatened in the
synchronous compared to the asynchronous touch condition.
However, the HRD measure that we use does not measure the
same type of response as skin conductance. The latter measures
arousal, the valence of which is unknown. HRD in particular has
been proposed to measure the degree of aversion to images [16].
In fact in our study there was no significant difference with respect
to skin conductance on seeing the slap between the 1PP and 3PP
conditions or TS and TS9conditions - it is simply an arousing
event to see someone slapped. However, there was significantly
greater aversion (as quantified by HRD) for those in the 1PP
condition, we speculate because they had a greater degree of
association with the body that was seen to be slapped.
Additionally there is a critical difference between our
experiment and that of Petkova and Ehrsson [14] where
cameras on the manikin’s head were in a fixed position, looking
down at the manikin’s body, and therefore the experimental
subjects had to have their head fixed in the same orientation. In
our setup the real-time head-tracking ensured that the act of
looking down involved motor acts and corresponding perceptual
changes comparable to physical reality. This may be why in
Petkova’s and Ehrsson’s setup synchronized touch was a critical
factor to achieve changes in ownership but it was less important
in our experiment. This also ties in with a recent observation
that the strong illusion of being in the place depicted by the
virtual reality [24] occurs when sensorimotor contingencies for
perception [25,26] are similar to those of physical reality, that is,
when a participant can use their body for perception in much
the same way as normally [27]. Sensorimotor contingencies
endow ‘place-ness’ to virtual space and the objects within it, and
a unique and highly special object is one’s own body. When the
virtual body is perceived to be in the same place as where the
real body should be, perhaps this provides very strong evidence
for the brain to generate the illusion that the virtual body is
onesown.Thisfindingwasunexpectedincomparisonwith
previous results that have emphasized the importance of
visuotactile synchronization.
The experiment had the unusual goal of attempting to generate a
body ownership illusion where the virtual body did not visually
resemble the real body of the participants, and was not even the
same gender. The reasoning was that if it were the case that the
illusion could be generated in these circumstances then it should also
be possible in a range of other less extreme situations - without
gender change, with less of a radical difference between the
participant’s own body and the virtual body. Gender categorization
is known to be persistent. For example, in [28] an experiment is
reported that shows that racial categorizations can be eliminated in
favor of categorization by membership of a cross-racial coalition.
When the same technique was applied to gender categorization the
effect of gender could not be extinguished. That experiment
therefore provided an illustration that top-down cognitive manip-
ulations could reduce the effect of race, but not of gender.
In contrast our experiment used mainly bottom-up sensory
stimuli–visual,tactile,vestibular and proprioceptive signals and
their correlations. This would lead us to think that with respect
to the issue of body ownership, bottom-up perceptual signals
play a more dominant role than top-down processing. Botvinick
andCohen[1]proposedthatvisuo-tactileintegrationwas
sufficient to generate the feeling that proprioception had shifted
to the rubber arm, while Armel and Ramachandran [2] went
further to postulate that such bottom-up sensory integration
between vision and touch was sufficient to generate the illusion
since it was shown to operate even with a neutral object such as
atable.However,thisresultwasnotreproducedbyTsakirisand
Haggard[29]whofoundthattheillusiondidnotoccurwhena
wooden stick was used in place of a rubber arm, nor did it occur
when there was postural incongruence (the left hand stimulated
with the rubber hand being a right hand). This led them to
conclude that bottom up influences provided a necessary but not
Table 4. Correlations Between Questionnaire Responses and Heart Rate Deceleration (significance levels in brackets).
Event cnct attack hurt body touch mirr woman clothing
across 20.02 0.06 20.35 20.35 20.33 20.38 20.35 20.36
beforeS 0.03 0.21 0.15 0.11 0.06 20.06 20.09 0.10
duringS 0.43(0.038) 0.43(0.04) 0.58(0.00) 0.39(0.06) 0.25 0.04 0.10 0.55(0.00)
down 0.20 0.40(0.05) 0.49(0.02) 0.61(0.00) 0.43(0.04) 0.25 0.35 0.31
doi:10.1371/journal.pone.0010564.t004
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sufficient condition for the illusion to occur. The discussion
about the relative influence of bottom-up and top-down factors
has continued with the introduction of the ideas of offline and
online representations [30] where the former refers to what our
body is normally like, and the latter refers to the temporal flow
of information that constructs how our body is right now
[31,32]. However, [30] refers to self recognition, which may not
bethesameasbodyownership.Wewouldarguethatownership
in the sense meant by the RHI illusion refers only to the feeling
that the seen rubber or virtual hand or body appears to be the
loci of proprioception and tactile sensation. Knowing that it is
an illusion, however, does not extinguish this feeling, which
appears to be an automatic response of the brain in dealing with
the conflicting sensory information. The results of our
experiment lend weight to the view that bottom up sensory
integration (or visual capture) can alter the sense of one’s body
by giving these powerful illusions of temporary transformation
in the form (female) and size (somewhat smaller) of the body. On
the other hand the virtual body does have a humanoid
appearance and we do not know whether the illusion would
break down if there were changes in the topology of the body.
The work described in [29], where it was shown that a non-
humanoid appearance of the arm [29] and left-right reversal
does not produce the illusion, suggests that there are clearly
limits imposed by top down processing on the type of body and
its configuration. The same was found with respect to the body
in [7].
Through an IVR a person can see through the eyes and hear
through the ears of a virtual body that can be seen to substitute for
their own body, and our data show that people have some
subjective and physiological responses as if it were their own body.
This virtual body may be seen perceived when looking directly at
oneself from a first-person perspective (or in a virtual mirror) and
the multisensory and sensorimotor contingencies involved in the
active process of looking down and seeing a virtual body where
one’s own body would be provides an important tool not just for
presence and virtual reality research [24], but also to understand –
eventually in conjunction with neurophysiology and neuroimaging
techniques - the neurobiology of self-consciousness.
Materials and Methods
Introduction
24 male subjects were recruited who were naı
¨ve to the purposes
of the experiment. The experiment was approved by the Comite´
E
´tico de Investigacio´ n del IDIBAPS (Hospital Clı
´nic, University of
Barcelona) and written informed consent was obtained from all
participants. A balanced between-groups design was used with the
three binary factors being Perspective, Movement and Touch as
discussed above (Table 1). Subjects were fitted with a wide field-of-
view light-weight (less than 1Kg) head-mounted display (HMD)
which was head tracked. This was a Fakespace Labs Wide5 HMD,
which has field of view 150u688uwith an estimated 160061200
resolution displayed at 60Hz, and the head-tracking was with an
Intersense 900. They were also fitted with a Nexus 4 physiological
recording device. Electrodes attached to this device were placed on
their left and right collar bones and the lowest left rib in order to
record the ECG (sampling rate 1024Hz).
Once they had entered the virtual environment they were asked
to look around the environment and report what they saw, for
purposes of acclimatization to the HMD and familiarization with
the environment, and especially to see the standing woman and
seated girl on the other side of the room. Then the experiment
started signaled by the virtual TV screen showing a music video,
and there was no further communication with the experimenters
until the completion of the trial. At the end of the experiment the
participants completed a questionnaire, and the critical questions
relating to the issue of body ownership are shown in supporting
information Questionnaire S1.
Supporting information can be found in Methods S1 and Movie
S1.
Subject Recruitment
Male participants were recruited by advertisement around the
campus. Recruitment continued until we had error free physio-
logical data for the target 24 participants. The mean age of the
retained 24 participants was 27.664.3 years, almost all were
students, researchers or employees of the University, who had no
prior knowledge of the experiment. Most of them had no prior
experience of virtual reality, and none of them had any experience
of our virtual reality system or laboratory. They were paid 10Jfor
their participation.
Procedures
After being fitted with the physiological recording equipment
and the HMD the subjects were instructed to look around the
virtual environment and report what they could see. This was in
order for them to become used to wearing the HMD and also to
become familiar with the scene, and especially to notice the two
female virtual characters on the other side. After this period of
acclimatization headphones were put on and the virtual TV screen
on the left hand side of the room now started playing a recorded
music video. The subjects had been instructed to continue to look
around the room, remembering also to look downwards.
After 2 minutes the first viewpoint transition occurred after
which the subject was on the other side of the virtual room. How
they experienced subsequent events depended on which combi-
nation of the experimental factors they had been assigned.
Experimental Factors
The experimental factors were:
Perspective. In the 1PP (first person) condition the participant
saw through the eyes of the body of the virtual girl (Figure 2A) and in
the 3PP condition (third person) the participant’s position was 1m to
the left of the girl’s body (Figure 2F).
Movement. In the synchronous condition (MS) the head
movements of the girl were displayed to be synchronous with those
of the participant (Figs 2D, 2F); in the asynchronous condition
(MS9) the head movements of the girl were displayed to move
asynchronously with respect to the participant, and were based on
pre-recorded head movements. Although the movements of the
virtual girl’s head were synchronous or asynchronous with respect
to the head movements of the participant, this was independent of
the visual field seen by the participant. This was always correct
based on his head position and orientation with respect to the
virtual scene, and determined wholly by the head-tracking.
Touch. In the synchronous condition (TS) when the woman
stroked the shoulder of the virtual girl the participant would feel
synchronous stroking on his shoulder (visuotactile correlation)
(Figure 2C, D,F, G); in the asynchronous condition (TS9)the
stroking felt on his shoulder would occur during the same time
interval as the stroking seen on the girl but the felt strokes themselves
would be asynchronous with respect to the visual strokes.
Sequence of Events
Variouseventswereprogrammed to occur during the course
of the 10 minutes after the viewpoint transition (Table 2).
Virtual Body Ownership
PLoS ONE | www.plosone.org 7 May 2010 | Volume 5 | Issue 5 | e10564
Almost immediately after ‘arriving’ in the location of the girl
(white shirt), the virtual woman (brown sweater) raised her left
arm and stroked the right shoulder of the girl. There were 22
stroking periods during the course of the subsequent 10 minute
experience, where there was visuotactile stimulation (either
synchronous or asynchronous) each consisting of between 1 and
5 strokes. There were 53 such strokes in total and the total
duration of all the stroking animations was 217s. There were 5
periods of stroking while the participant experienced the
elevated position looking down on the scene, where there was
no tactile stimulation associated with the visual stroking. A
critical event was that 415s after the first viewpoint transfor-
mation there was a second transformation, where the partici-
pant’s viewpoint was elevated to near the top of the ceiling and
oriented to be looking down on the scene below (Figure 2G).
The spatial coordinates of this viewpoint transformation were
chosen based such transformations in neurological patients with
out-of-body experiences [33,34]. As reported by many of these
patients this elevated location is not represented as embodied in
a person’s body and patients report not perceiving or having
reduced perception of somatosensory cues from their body.
Accordingly, although the visual arm strokes continued during
this period they were not accompanied by any actual stroking of
the participant’s arm. Then 45s into this elevated position the
woman suddenly slapped the girl around the face three times,
and the girl’s body swayed (Figure 2H) and there were
corresponding sounds. We rendered our animation in this way
because previous work on ownership has shown that threat-like
behavior is often associated with physiological changes
[2,9,14,35]. After this, the situation returned to as it had been
before the slapping, with the occasional arm stroking, and after
a further 50s the perspective shifted down again to the same
situation as it had been before the upward translation (location
of the girl).
Questionnaire
The questionnaire administered afterwards is shown in
Questionnaire S1. The questionnaire scores (between 0 and 10)
were recoded into ranges as Very Low (0), Low (1–3), Medium (4–
6), High (7–9) and Very High (10), based on the layout of the
questionnaire. Some of the questions were derived from previous
work [1,7,9] and others were introduced following interviews with
participants in extensive pilot trials. We note that questionnaires in
this area are problematic unless backed up by behavioral or
physiological evidence, see the discussion of this point with respect
to the virtual arm illusion in [11].
Statistical Methods
For the questionnaire responses we have chosen the
proportional odds cumulative logit model [36] since this
appropriately treats the responsesasordinaldata.Inorderto
use this we needed to group scores together, since the responses
are too sparsely distributed over the original range of scores
from 0 to 10.
However, for comparison we have also analysed the question-
naire responses using traditional analysis of variance. Three-way
analyses of variance with all two-way interactions were carried out
on the questionnaire responses. This is strictly not an appropriate
form of analysis since it assumes that the responses are at least on
an interval scale, and clearly this is not the case. Nevertheless this
approach is common, and it is presented for the sake of
completeness, but the results are not different from the previous
analysis. The significant results are presented in Table 5. The
residual errors of all models were compatible with normality,
based on Jarque-Bera tests.
A requirement of ANOVA is that the residual errors of the
model fit should follow a normal distribution. In most of the
analyses we carried out this was the case, as judged by a Jarque-
Bera test [37]. When this is not the case the standard solution is
to try to find a monotonic transformation of the response
variable so that the residual errors become normal. This was
accomplished systematically using the Box-Cox family of
transformations [38] yl, and finding the maximum likelihood
estimator for l.
Supporting Information
Figure S1 Scatter diagram of the questionnaire responses of
mirror by body, classified by M. Some of the coordinates occur
more than once so that the plotted points overlay one another.
Points above the diagonal line have mirror .body.
Found at: doi:10.1371/journal.pone.0010564.s001 (1.97 MB TIF)
Figure S2 Estimated probabilities for the questionnaire respons-
es on the touch related questions. These are shown for four
combinations of the factors, each with M = MS. The results are
almost identical for MS9. The left panel shows the results for touch
(Q5), and the right woman (Q10). There are no 0 scores for touch.
Found at: doi:10.1371/journal.pone.0010564.s002 (4.74 MB
TIF)
Questionnaire S1 The post experiment questionnaire.
Found at: doi:10.1371/journal.pone.0010564.s003 (0.02 MB
PDF)
Methods S1 Supporting Methods.
Found at: doi:10.1371/journal.pone.0010564.s004 (0.02 MB
PDF)
Movie S1 The video shows some extracts from the scenario,
mainly from the first person perspective position of the virtual girl,
and also from the 3PP condition. There is no sound on the video
although the subjects would have heard the music from the virtual
TV (spatialised). The sound is not included for copyright reasons.
Found at: doi:10.1371/journal.pone.0010564.s005 (20.46 MB
MOV)
Acknowledgments
We would like to thank Dr Henrik Ehrsson for commenting on an earlier
draft of this paper, Dr Doron Friedman for advice on physiological
recordings and Dr Vanessa F. Descalzo for help with the experiments.
Author Contributions
Conceived and designed the experiments: MS MVSV OB. Performed the
experiments: MS BS. Analyzed the data: MS. Wrote the paper: MS MVSV
OB. Designed and implemented the computer programming: BS.
Table 5. Significance Levels of Analysis of Variance of
Questionnaire Responses.
Questionnaire Variable P T
Hurt 0.041
Body 0.033 0.100
Touch 0.031
Cloth 0.0003
doi:10.1371/journal.pone.0010564.t005
Virtual Body Ownership
PLoS ONE | www.plosone.org 8 May 2010 | Volume 5 | Issue 5 | e10564
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Virtual Body Ownership
PLoS ONE | www.plosone.org 9 May 2010 | Volume 5 | Issue 5 | e10564
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