The Sense of Embodiment in Virtual Reality

Abstract

What does it feel like to own, to control, and to be inside a body? The multidimensional nature of this experience together with the continuous presence of one's biological body, render both theoretical and experimental approaches problematic. Nevertheless, exploitation of immersive virtual reality has allowed a reframing of this question to whether it is possible to experience the same sensations towards a virtual body inside an immersive virtual environment as toward the biological body, and if so, to what extent. The current paper addresses these issues by referring to the Sense of Embodiment (SoE). Due to the conceptual confusion around this sense, we provide a working definition which states that SoE consists of three subcomponents: the sense of self-location, the sense of agency, and the sense of body ownership. Under this proposed structure, measures and experimental manipulations reported in the literature are reviewed and related challenges are outlined. Finally, future experimental studies are proposed to overcome those challenges, toward deepening the concept of SoE and enhancing it in virtual applications.

Full text

Konstantina Kilteni*
Raphaela Groten
Event Lab
Facultat de Psicologia
Universitat de Barcelona
08035 Barcelona, Spain
Mel Slater
Event Lab
Facultat de Psicologia
Universitat de Barcelona
Barcelona, Spain
and
ICREA
Universitat de Barcelona
Barcelona, Spain
and
Department of Computer Science
University College London
London, United Kingdom
The Sense of Embodiment in
Virtual Reality
Abstract
What does it feel like to own, to control, and to be inside a body? The multidimen-
sional nature of this experience together with the continuous presence of one’s biolog-
ical body, render both theoretical and experimental approaches problematic. Never-
theless, exploitation of immersive virtual reality has allowed a reframing of this
question to whether it is possible to experience the same sensations towards a virtual
body inside an immersive virtual environment as toward the biological body, and if so,
to what extent. The current paper addresses these issues by referring to the Sense of
Embodiment (SoE). Due to the conceptual confusion around this sense, we provide a
working definition which states that SoE consists of three subcomponents: the sense
of self-location, the sense of agency, and the sense of body ownership. Under this pro-
posed structure, measures and experimental manipulations reported in the literature
are reviewed and related challenges are outlined. Finally, future experimental studies
are proposed to overcome those challenges, toward deepening the concept of SoE
and enhancing it in virtual applications.
1 Introduction
One of the central questions in cognitive science is how we experience
ourselves inside a body that interacts continuously with the environment. We
experience our self as being inside a body and more specifically a body that feels
‘‘ours,’’ which moves according to our intentions, obeying our will. In everyday
life, these sensations are normally coupled together, perceived as emerging from
only one body, the biological one, giving coherence to our self and our body
representation.
Experimental manipulation of this embodied experience is problematic, since
one’s body is always present and seemingly cannot be dissociated from one’s
self. However, studies of body perception reveal an alternative way to approach
this experience by manipulating the identity of a body part. In a now classical
experiment, the participant sits comfortably at a table with his or her left hand
resting on it. A left rubber hand is put on a table aligned with and close to the
real one. An occluding screen prevents the sight of the real left hand and arm.
Both the rubber hand and corresponding real hand receive synchronous tactile
stimulation from two paintbrushes at the same relative positions. After a few
seconds of such synchronous stimulation, the participant will probably experi-
ence a profound illusion known as the rubber hand illusion (RHI)—the rubber
Presence, Vol. 21, No. 4, Fall 2012, 373–387
ª2012 by the Massachusetts Institute of Technology *Correspondence to konstantina.kilteni@ub.edu.
Kilteni et al. 373

hand feels as if it were his or her real hand (Botvinick
& Cohen, 1998). Additionally, when asked to indicate
where the real hand is located, with eyes closed, the
participant will typically mislocalize it toward the rub-
ber hand in comparison to a similar measurement
taken before the synchronous visual-tactile stimulation
(Botvinick & Cohen; Costantini & Haggard, 2007;
Tsakiris & Haggard, 2005). The difference between
the pre and post experimental position estimation is
widely considered a perceptual correlate of the illusion
and is known as proprioceptive drift. Furthermore,
asynchronous stimulation of the real and rubber hand
has been shown to inhibit both the illusion and the
mislocalization (Armel & Ramachandran, 2003; Botvi-
nick & Cohen; Tsakiris, Carpenter, James, & Fotopou-
lou, 2010).
Although the RHI has provided an easy and replicable
way to address the identity of a body part, revealing at
the same time the role of multimodal input in the
embodied experience, the question of how we experi-
ence ourselves inside a body could not be addressed in
its entire complexity due to the experiment limitations.
The employment of virtual reality (VR) can be used to
reframe the main research question to address instead:
How, and to what extent, can we experience a virtual
body representation as our own body within a virtual
environment (Slater, Perez-Marcos, Ehrsson, & San-
chez-Vives, 2009)? Such use of virtual reality techniques
is encouraged by its unique advantages to easily manipu-
late the perceived scenario, but more importantly to vary
in a controlled way factors associated with the embodied
experience that would hardly be possible in physical real-
ity (Bohil, Alicea, & Biocca, 2011). For example, VR
makes possible in a relatively easy manner the manipula-
tion of the body representation in terms of structure,
morphology, and size, dissociating the egocentric visual
perspective from the body, and exploiting the role of
multimodal information in spatiotemporal terms for
body perception. Indeed, recent studies using VR or
similar techniques have made the first steps to approach
the multidimensionality of the embodied experience by
inducing whole body illusions analogous to the RHI
(Normand, Giannopoulos, Spanlang, & Slater, 2011;
Petkova & Ehrsson, 2008; Slater, Spanlang, Sanchez-
Vives, & Blanke, 2010).
The current paper presents a review and elucidation of
the concepts and mechanisms involved in these types of
illusions 5 by focusing on the phenomenology of embodi-
ment. The term embodiment, however, has been referred
to differently in various contexts due to its multidiscipli-
nary use and its various application areas, and for this
reason its conceptualization depends on the viewpoint
from which the issue is considered. From the philosophi-
cal perspective it is a part of the general discussion on
how one defines and experiences one’s self (Blanke &
Metzinger, 2009; Metzinger, 2008). For cognitive neu-
roscience and psychology, it is concerned with the ques-
tion of how the brain represents the body (Berlucchi &
Aglioti, 1997; Graziano & Botvinick, 2002) and how
this representation is altered under certain neurological
conditions (Lenggenhager, Smith, & Blanke, 2006;
Metzinger, 2009). In contrast, in robotics, the concept
is employed to distinguish ways through which artificial
forms of intelligence are represented contrasting those
virtual agents and robots that have a real physical repre-
sentation compared to those that do not (Foster, 2007;
Holz, Dragone, & O’Hare, 2009; Wainer, Feil-Seifer,
Shell, & Mataric, 2006). Embodiment has been also dis-
cussed in relation to presence in virtual environments
(Biocca, 1997), especially as there is evidence to suggest
that a virtual body in the context of a head-mounted dis-
play based virtual reality is a critical contributor to the
sense of being in the virtual location (Slater, Spanlang,
& Corominas, 2010). Furthermore, the role of embodi-
ment in one’s self-representation was also addressed
under the concept of self-presence introduced by Biocca
and restated further by Lee (2004).
The existence of multiple meanings of the term
embodiment can cause confusion in the research commu-
nity of the same type that has dogged the study of pres-
ence (Lee, 2004). For this reason, throughout the rest
of the paper, the term Sense of Embodiment
1
(SoE) will
be used to refer to the ensemble of sensations that arise
in conjunction with being inside, having, and controlling
1. For a conceptual differentiation between embodiment and sense
of embodiment see de Vignemont (2011).
374 PRESENCE: VOLUME 21, NUMBER 4

abody
2
especially in relation to virtual reality applica-
tions. The aim of the present paper is to provide a work-
ing definition for SoE, to discuss the corresponding
measures used in the literature and propose new ones,
and finally to review factors that potentially enhance the
SoE in VR, relating to existing experimental studies.
Research questions for future studies in the context of
SoE are pointed out aiming toward a deeper understand-
ing of this complex and multidimensional experience.
2 Sense of Embodiment (SoE)
2.1 Working Definition
Understanding and defining the SoE toward an ar-
tificial body can draw on ideas from recent proposals
concerned with the embodiment of artificial body parts
(i.e., specific limbs), by extending these ideas to artificial
whole bodies. According to de Vignemont (2011, p. 3),
an object ‘‘E is embodied if and only if some properties
of E are processed in the same way as the properties
of one’s body.’’ This definition is in line with that of
Blanke and Metzinger (2009, p. 7) who state that em-
bodiment includes the ‘‘subjective experience of using
and ‘having’ a body.’’ Therefore, the following defini-
tion is adopted:
SoE toward a body B is the sense that emerges when
B’s properties are processed as if they were the proper-
ties of one’s own biological body. (Definition: D)
2.2 Underlying Structure
Defining SoE in this manner, the conceptual clari-
fication still remains vague, because the properties and
the associated experiences from one’s biological body
are not specified further. Nevertheless, everyday experi-
ence concerning the biological body can manifest itself
in at least three main classes of such properties with the
corresponding phenomenology. First, one’s self-repre-
sentation in a body is driven and highly characterized by
its spatial attributes; for example, one’s self is located
inside a body. Furthermore, this spatial representation is
always self-attributed; that is, the body where one per-
ceives one’s self is one’s own body. Finally, this body also
obeys the intentions of one’s self; for example, one is the
author of one’s body’s actions. Indeed, in reviewing the
literature, the term embodiment has frequently been
associated with the concepts of sense of self-location (e.g.,
Arzy, Thut, Mohr, Michel, & Blanke, 2006), the sense of
agency (e.g., Newport, Pearce, & Preston, 2010) and the
sense of body ownership (e.g., Lopez, Halje, & Blanke,
2008). Therefore, the properties of one’s biological
body could be described under the conceptual umbrella
of these three terms.
2.2.1 Sense of Self-Location. Self-location is a
determinate volume in space where one feels to be
located. Normally self-location and body-space coincide
in the sense that one feels self-located inside a physical
body (Lenggenhager, Mouthon, & Blanke, 2009).
However, this collocation can break down when people
have out-of-body experiences (OBE) in which they per-
ceive themselves outside of their physical body (Leng-
genhager et al., 2006).
The sense of self-location refers to one’s spatial experi-
ence of being inside a body and it does not refer to the
spatial experience of being inside a world (with or with-
out a body); for example, the experience of presence—
specifically, Place illusion (Slater, 2009) or the sense of
‘‘being there.’’ Self-location and presence are psycholog-
ical states that refer to different issues. Whereas self-loca-
tion is concerned with the relationship between one’s
self and one’s body, presence refers to the relationship
between one’s self and the environment. If there is a
body representation where the self is perceived to be,
then the latter issue also includes the issue of the rela-
tionship between the body and environment. To better
illustrate this distinction between self-body and self-envi-
ronment or body-environment, an example of self-loca-
tion could be the feeling that one’s self is located inside
the biological body or an avatar’s body; whereas the
analogous feeling for presence would be the feeling of
one’s self being located in a physical or virtual room,
even if this does not require a body representation in the
form of an avatar. By considering a dichotomy of space
2. In this paper, we will make use of the term ‘‘body’’ as a container,
which can be any object in the context of virtual reality, and we will
make a special distinction when we refer to one’s biological body.
Kilteni et al. 375

between the self-body space (the space occupied by the
body B in which the self is perceived as located) and the
external/environmental space (the space-environment-
world where the self is perceived as situated even if this
does not include a body). The view makes the distinction
become more clear. A more extended approach to em-
bodiment could potentially include the presence sub-
component; however, here we focus on the relationship
between self and body. Although self-location and pres-
ence address different spatial questions, they can be con-
sidered as complementary concepts that together consti-
tute one’s spatial representation.
Self-location is highly determined by the visuospatial
perspective given that this is normally egocentric. Indeed,
it has been shown that where one feels located can be
influenced by the origin of visuospatial perspective
(Blanke & Metzinger, 2009; Ehrsson, 2007). Other stud-
ies on the role of perspective (e.g., Petkova, Khoshnevis,
& Ehrsson, 2011; Slater, Spanlang, Sanchez-Vives, et al.,
2010), showed that physiological responses to a threat
given to an artificial body were greater for first-person
perspective than for third-person perspective. The impor-
tance of egocentric visual perspective for self-location was
also highlighted by Lee (2004, p. 40) referring to self-
presence: ‘‘In the case of a psychologically assumed virtual
self (...) a virtual environment reacts to users as if they
were in there (e.g., first-person viewpoint game, other
people greeting you by name).’’ Although the present pa-
per is concerned with highly immersive virtual reality sys-
tems, the statement is in line with the proposed subcom-
ponent, since first-person perspective given from the
position of the virtual body serves as sensory evidence to-
ward one’s self-localization inside the virtual body.
Vestibular signals are also considered to play a signifi-
cant role in one’s self-localization (Lopez et al., 2008).
These signals contain information with respect to the
‘‘translation and rotation of the body in addition to ori-
entation with respect to gravity’’ (Blanke & Metzinger,
2009, p. 10). Interestingly, vestibular dysfunction was
observed in patients with OBE who experience them-
selves inside an illusory body while the origin of visual
perspective is perceived as coming from a position out-
side the bodily boundaries (Blanke, Landis, Spinelli, &
Seeck, 2004).
The tactile input also influences self-location since the
border between our body and the environment is our
skin. This is also related to the brain’s different encoding
of the space dependent on its proximity to the body.
According to this criterion, personal space is the space
our body occupies, peripersonal space is the space adja-
cent to the body that is within arms’ reach, and extraper-
sonal space is the far nonreachable space
(Halligan, Fink, Marshall, & Vallar, 2003; Vaishnavi,
Calhoun, & Chatterjee, 2001). Relevant to this is the
extension of peripersonal space by tool use that results
in tool embodiment (Giummarra, Gibson, Georgiou-
Karistianis, & Bradshaw, 2008). In addition, self-local-
ization inside a body with a different volume compared
to one’s biological body’s, results in a differently per-
ceived personal space as addressed in the study of
Normand et al. (2011). The study of Lenggenhager
et al. (2009) revealed that the position of seen tactile
stimulation when accompanied by congruent physical
stimulation can dominate the visual perspective and thus
determine our self-location.
2.2.2 Sense of Agency. The sense of agency
refers to the sense of having ‘‘global motor control,
including the subjective experience of action, control,
intention, motor selection and the conscious experience
of will’’ (Blanke & Metzinger, 2009, p. 7). Agency is
present in active movements. An example where agency
is disturbed is in patients with anarchic hand syndrome,
who reject the notion that they are controlling the
actions of their own limb and claim that their limb acts
according to its own intentions (David, Newen, &
Vogeley, 2008).
The sense of agency has been proposed to result from
a comparison between the predicted sensory consequen-
ces of one’s actions from the efference copy and the
actual sensory consequences (for a review see, e.g., David
et al., 2008). When the predicted consequences of the
action and the actual consequences of actions match by,
for example, the presence of synchronous visuomotor
correlations under active movement, one feels oneself to
be the agent of those actions. This also applies for the
embodiment of tools when these are under the control
of the user. The development of agency depends on the
376 PRESENCE: VOLUME 21, NUMBER 4

synchronicity of visuomotor correlations. Several studies
have shown that discrepancies between the visual feed-
back of the action and the actual movement negatively
affect the feeling of agency (Blakemore, Wolpert, &
Frith, 2002; Franck et al., 2001; Sato & Yasuda, 2005).
In the study of Franck et al., a discrepancy of more than
150 ms was found to reduce agency.
2.2.3 Sense of Body Ownership. Body owner-
ship refers to one’s self-attribution of a body (Gallagher,
2000; Tsakiris, Prabhu, & Haggard, 2006). It has a pos-
sessive character and it implies that the body is the
source of the experienced sensations. For example body
ownership is disturbed in patients with somatoparaphre-
nia who deny the ownership of their limb (Vallar & Ron-
chi, 2009).
The sense of body ownership has been proposed to
emerge from a combination of bottom-up and top-down
influences (Tsakiris, 2010; Tsakiris & Haggard, 2005).
Here, bottom-up information refers to the afferent sen-
sory information that arrives to our brain from our sen-
sory organs; for example, visual, tactile, and propriocep-
tive input, whereas top-down information consists of the
cognitive processes that may modulate the processing of
sensory stimuli; for example, the existence of sufficient
human likeness to presume that an artificial body can be
one’s body.
In terms of bottom-up influences, several studies on
the RHI have investigated the role of the synchronous
visuotactile correlations between the real and the rubber
hand. For the RHI, illusory ownership of the rubber
hand emerges only when the seen and the felt stimula-
tion follow the same spatiotemporal pattern (Botvinick
& Cohen, 1998; Shimada, Fukuda, & Hiraki, 2009;
Tsakiris & Haggard, 2005). Analogous to visuotactile
correlations, synchronous visuoproprioceptive correla-
tions during passive movements were also
found to induce ownership (Dummer, Picot-Annand,
Neal, & Moore, 2009; Tsakiris, Prabhu, & Haggard,
2006).
Other studies have focused more on the cognitive
influences in the induction of the illusion and have
revealed that the strength of body ownership depends
on the degree of morphological similarity between a real
biological arm or hand and the external object to be
incorporated (Armel & Ramachandran, 2003; Ehrsson,
Spence, & Passingham, 2004; Tsakiris et al., 2010;
Tsakiris, Costantini, & Haggard, 2008; Tsakiris &
Haggard, 2005) or the similarity of spatial configuration
between them (Costantini & Haggard, 2007; Ehrsson
et al.; Tsakiris & Haggard). These studies showed that
the illusion of ownership diminishes when the external
object does not resemble or is in a different spatial con-
figuration to the real arm or hand. However, in line with
these top-down influences, several studies have shown
that the illusion of ownership of a fake hand can be
induced when there is morphological similarity to a real
hand and arm; for example, rubber arm and hand (Botvi-
nick & Cohen, 1998), screen images of the real hands
(IJsselsteijn, de Kort, & Haans, 2006; Tsakiris et al.,
2006), other people’s hands (Schutz-Bosbach, Mancini,
Aglioti, & Haggard, 2006), or virtual hands (Sanchez-
Vives, Spanlang, Frisoli, Bergamasco, & Slater, 2010;
Slater, Perez-Marcos, Ehrsson, & Sanchez-Vives, 2008).
These two together imply that in order to induce owner-
ship toward an external object, a basic morphological
similarity with the real body part is needed.
Recently, it has been shown that body ownership is
not exclusive to artificial body parts but can also be felt
for artificial whole bodies; for example, avatars or man-
nequins (Normand et al., 2011; Petkova & Ehrsson,
2008; Slater et al., 2009; Slater, Spanlang, Sanchez-
Vives, et al., 2010).
The concept of SoE is regarded as having these three
underlying subcomponents: self-location, agency, and
body ownership. Evidence in favor of such a structure
was recently presented in the study of Longo, Schu¨u¨r,
Kammers, Tsakiris, and Haggard (2008) investigating
SoE in the RHI. A principal component analysis of the
results of a set of questionnaire items revealed that the
phenomenology of embodiment breaks down into these
three proposed subcomponents. However, there are
some limitations in the interpretation of these results;
for example, only the presumed agency was measured
(there was no actual agency for the rubber hand) and the
induced embodiment was for an artificial body part and
not for an artificial whole body. Despite these con-
straints, the analysis of Longo et al. revealed that the
Kilteni et al. 377

phenomenology of embodiment includes the sense of
self-location, interpreted there as body part location, the
sense of agency, and the sense of body ownership.
2.3 The Scale of the SoE
Under normal conditions, these three senses are
always experienced and without any doubt with respect
to the biological body. Nevertheless, in experimental
manipulations of these senses, such as in the RHI, partic-
ipants are required to express their illusory sensations on
a continuous scale (Botvinick & Cohen, 1998) rather
than a two-valued/binary/forced choice one; for exam-
ple, ‘‘I felt versus I didn’t.’’ The possible variance in the
intensity of ownership feelings toward the rubber hand
would imply that sense of ownership is on a continuous
scale, although in normal life such in-between values are
never considered. (Of course, the continuous scale—or
the ordinal scale typically employed in questionnaires—is
actually only a measuring device used by experimenters
and we cannot assume from this that the underlying phe-
nomenon is itself experienced on a continuous scale.)
This continuous scale has been applied for the sense of
self-location (Ehrsson, 2007) and agency (Longo et al.,
2008). Therefore, in this framework, the intensity of
experiencing the three subcomponents could be consid-
ered to vary continuously from none to a maximum
degree. The fact that SoE is considered as the synthesis
of these three senses, implies that SoE is also expressed
on a continuous scale from no to full degree, as also
stated by de Vignemont (2011). In the special case of
one’s biological body, all senses are experienced in the
maximum degree and one feels fully embodied.
2.4 Propositions of the SoE
Given the definition D and the proposed scale for
SoE, the following propositions (P) can be derived as
shown in Table 1.
From Table 1, P5 is true for one’s biological body but
it remains unknown whether such full embodiment can
be induced toward an artificial one. Minimal intensity
and maximum intensity here refer to the possible
strength of the induced sense(s). These values depend
both on the evidence toward the sense(s) that the spe-
cific experiment provides to participants; and on the par-
ticipants’ perceptual mechanisms.
2.5 Relationship Between SoE and its
Subcomponents
Having conceptualized the SoE as consisting of
these three subcomponents, evidence in favor of a domi-
nant component or information related to each subcom-
ponent’s contribution to the overall concept would serve
both future experimental manipulations of SoE as well as
theory. Nevertheless, the literature does not provide ei-
ther enough information or converging information to-
ward one single interpretation. Concerning self-location,
some authors treat embodiment as synonymous to this
(Arzy et al., 2006; Blanke & Metzinger, 2009; Leng-
genhager, Tadi, Metzinger, & Blanke, 2007), which
could further imply that self-location is the dominant
subcomponent in SoE or just a sufficient condition (P1).
On the other hand, body ownership has been proposed
to be unnecessary, an argument motivated by tool em-
bodiment (De Preester & Tsakiris, 2009) which does
not manifest feelings of body ownership (de Vignemont,
Table 1. Propositions for SoE Toward a Body B
One experiences SoE
toward a body B, if one feels self-located inside B at least in a minimal intensity (P1)
if one feels to be an agent of B at least in a minimal intensity (P2)
if one feels B as one’s own body at least in a minimal intensity (P3)
if and only if one experiences at least one of the three senses at least in a
minimal intensity (P4)
One experiences full
SoE toward a body B, if one experiences all of the three senses at the maximum intensity (P5)
378 PRESENCE: VOLUME 21, NUMBER 4

2011), also consistent with P4. Furthermore, perceived
agency has been proposed to be an important factor
which gives coherence to one’s body representation
(Tsakiris et al., 2006), consistent with P2. Moreover,
perceived lack of agency was shown to inhibit embodi-
ment (Newport et al., 2010). Even though these
approaches to the concept of embodiment do not
include all the proposed subcomponents, they are not in
conflict with the proposed definition and propositions.
On the contrary, they can be considered to focus on a
subset of sensations associated with particular subcom-
ponents and not to the entire complexity of the SoE
experience.
The current state of knowledge in literature on SoE
does not enable further specification concerning the
weights of the three subcomponents in the totality of
the embodied experience. These weights may not be
constant but time-varying (e.g., related to participants’
perceptual and attentional mechanisms), or experiment-
specific (e.g., related to the provided sensory informa-
tion that the particular scenario offers, especially for each
sense or for the task that participants are asked to do).
Furthermore, there is not enough systematic experimen-
tal evidence in favor of independence of the three sub-
components.
Probable dependencies or correlations between the
subcomponents have been suggested in the literature.
First, concerning the relationship between self-location
and body ownership, a body inside which one feels self-
located is very likely to be one’s own body. In the study
of Petkova et al. (2011) the visual perspective (related to
one’s self-location) was found to affect the induced body
ownership. The main research question, however, was
about body ownership and not about self-location.
Hence, it remains unclear whether visual perspective is
necessary for body ownership or whether breaking the
self-location weakened the induced body ownership. Sec-
ondly and similarly, a correlation between agency and
body-ownership can exist; for example, a body that obeys
one’s intentions will probably be one’s body and vice
versa. Tsakiris, Schutz-Bosbach, and Gallagher (2007)
stated that ownership does not imply a sense of agency
(self-generated movements are not necessary for owner-
ship), but that the sense of agency normally implies own-
ership. Studies that provide participants with agency to-
ward a fake hand in order to induce a body ownership
illusion (Dummer et al., 2009; Sanchez-Vives et al.,
2010; Tsakiris et al., 2006; Yuan & Steed, 2010) provide
evidence for such a relationship. However, especially in
technically mediated scenarios such as in telepresence,
this is not necessarily the case. A robot can be controlled
from a remote location as an advanced tool, which can
thus be embodied based on agency, but where sensory
evidence for self-location and body ownership is given to-
ward the physical body. In line with this, in the experi-
ment of Kalckert and Ehrsson (2012), ownership and
agency were found to be double dissociated. On the
other hand, in the study of Longo et al. (2008) even
though participants did not actually move their hands,
they nevertheless reported a sense of agency toward the
rubber hand. Moreover, Sato and Yasuda (2005), when
investigating agency in relation to the predicted (through
the efference copy) and actual feedback of actions, pro-
posed that agency is independent of felt body ownership.
This is based on the findings that with increased delay
between action and feedback, the felt agency was nega-
tively affected, but not the felt self-ownership. Finally,
van den Bos and Jeannerod (2002) reported that partici-
pants in their study had difficulty judging the ownership
of a hand when they did not perform the actions that
they saw it do, thus, contradicting the total independence
of these two concepts. Finally, concerning the relation-
ship of agency and self-location, the study of David et al.
(2006) suggested that the egocentric visuospatial per-
spective (related to self-location) and sense of agency are
independent components of one’s self-consciousness.
Despite the lack of sufficient systematic experimental
evidence in favor of or against interdependencies
between the subcomponents—self-location, agency, and
body ownership—the use of virtual reality techniques
makes it possible to induce, to a certain extent, the sense
of each subcomponent toward a different artificial body.
Sensory evidence such as visual perspective can be given
from body A such as the participant feels self-located
inside A. At the same time, synchronous visuotactile cor-
relations of the same spatiotemporal pattern between the
participant’s body and another body B of the same
appearance, but in a different position, can be used to
Kilteni et al. 379

induce body ownership (although the induced sense
might be weak, see Petkova et al., 2011). The physical
movements of the participant can also be registered with
the seen movements of a third body C inducing the
sense of motor control over C. Under such a setup, the
sensations associated with embodiment could be theo-
retically dissociated as coming from three separate
bodies; self-location from A, body ownership from B,
and agency from C. However, although self-location
may be provided with respect to A, it is quite possible
that there are effects of this on the other subcompo-
nents; for example, only providing self-location might
also lead to a sense of ownership. It is possible using
careful experiment design to separate out the unique
influences of each subcomponent. This is clearly not easy
to do, but it is possible.
In conclusion, little is known about the individual
contribution of each component to SoE or whether
there is a dominant contribution. The same lack of ex-
perimental evidence is apparent when addressing the
actual relationship between the three subcomponents;
that is, if positive/negative feedback in one of them
enhances/inhibits the experience in another. Future
studies should aim toward clarifying these open topics
systematically.
2.6 Measures of the SoE
It is essential to be able to operationalize the con-
cept of SoE for the purposes of measurement. This
becomes especially important when the effects of differ-
ent factors on SoE need to be assessed in an experiment.
This is very similar to the concept of presence in virtual
environments—a topic that has provoked significant
research over many years. Measurement usually relies
on questionnaires or physiological responses to
anxiety-provoking events in the virtual environment
such as Meehan, Insko, Whitton, and Brooks (2002),
but where new approaches are also in development
(Slater, Spanlang, & Corominas, 2010). However,
because of its composite phenomenology, there is not an
explicit measure of SoE. Nevertheless, induced SoE to-
ward an artificial body can be approached through meas-
uring it at the level of its subcomponents. An overview
of measures used in the literature for addressing SoE is
given in Table 2.
2.6.1 Self-Location. Recent experimental studies
have shown that the normally given coincidence between
self and body location can break in experimentally
induced out-of-body experiences. These studies used
both qualitative (questionnaires) and quantitative (per-
formance in motor or cognitive tasks, physiological
responses) measures to investigate illusory self-location
and possible deviations from the location of the biologi-
cal body. In the study of Ehrsson (2007), participants
were given a visual perspective from a point in space
behind their physical body through a pair of head-
mounted displays. Tactile stimulation seen from that
position was congruent to felt stimulation on the physi-
cal body. Participants experienced the sensation of being
located outside their biological body, as reported in
questionnaires, and showed higher physiological
responses in response to a threat toward this perceived
self-location compared to the control condition. Addi-
tionally, Lenggenhager et al. (2007) conducted a study
where the visual perspective was given from the normal
viewpoint. The felt stimulation on the physical body,
however, was congruent with the seen stimulation on an
artificial body that was in front of the participants. Partic-
ipants, when passively removed from the scene and asked
to return to their initial position, showed a significant
drift toward the artificial body. An alternative way to esti-
mate one’s perceived self-location through a mental ball-
dropping task was proposed by Lenggenhager et al.
(2009).
2.6.2 Agency. The measurement of agency has
not been addressed to the same extent as self-location
and ownership. Although there are studies that induce
illusory embodiment by providing participants with
actual control of the fake limb or body (Dummer et al.,
2009; Sanchez-Vives et al., 2010; Tsakiris et al., 2006;
Yuan & Steed, 2010), agency in itself toward the
embodied object was not the main focus of these experi-
ments. Instead, the control of the fake limb was used in
order to generate other illusory sensations in another
subcomponent, e.g., ownership. In Kalckert and Ehrs-
380 PRESENCE: VOLUME 21, NUMBER 4

son (2012), agency over the rubber hand was addressed
qualitatively; for example, by questionnaire. The same
measure was used in Longo et al. (2008), a study that
was, however, concerned purely with sensation since no
actual agency was provided.
2.6.3 Body Ownership. SoE at the level of body
ownership has been extensively addressed using both
qualitative (with questionnaires) and quantitative meth-
ods (performance in localization tasks such as proprio-
ceptive drift, performance in body part estimation, par-
Table 2. Overview of Measures of SoE in Terms of its Subcomponents
Sense of self-location Questionnaire items; for example, ‘‘I experienced that I was located at some distance
behind the visual image of myself, almost as if I was looking at someone else’’
(Ehrsson, 2007, p. 6) [supplemental material].
Estimation of body position: ‘‘...passively displacing the blind-folded participants
immediately after the stroking and asking them to return to their initial position...’’
(Lenggenhager et al., 2007, p. 1097) or ‘‘...imagine dropping the ball that they were
holding in their hand (mental ball dropping task, MBD).’’ The participants ‘‘...were
instructed to indicate with a first button press when they imagined releasing the ball
from their hand and with a second button press when the ball would hit the
ground...’’ (Lenggenhager et al., 2009, p. 112).
Physiological response in view of a threat toward the perceived self-location; for
example, Skin Conductance Response (SCR; Ehrsson, 2007).
Sense of agency Questionnaire items; for example, ‘‘it seemed like I was in control of the rubber hand’’
(Longo et al., 2008, p. 984), ‘‘I felt as if I was controlling the movements of the
rubber hand’’ (Kalckert & Ehrsson, 2012, p. 4).
Sense of body ownership Questionnaire items; for example, ‘‘I felt as if the rubber hand were my hand.’’
(Botvinick & Cohen, 1998, p. 756), ‘‘I felt as if the virtual body was my body’’
(Aspell, Lenggenhager, & Blanke, 2009, p. 4), ‘‘How much did you feel that the
seated girl’s body was your body?’’ (Slater, Spanlang, Sanchez-Vives, et al., 2010,
p. 4), ‘‘It seemed like the rubber hand belonged to me’’ (Longo et al., 2008, p. 983).
Proprioceptive estimations: through intermanual movements; for example, ‘‘both
before and after the viewing period (...) with eyes closed, the right index finger was
drawn along a straight edge below the table until it was judged to be in alignment with
the index finger of the left hand’’ (Botvinick & Cohen, 1998, p. 756; IJsselsteijn et al.,
2006), or verbal estimation, ‘‘Participants saw a ruler reflected on the mirror. (...)
they verbally reported a number on the ruler’’ (Tsakiris & Haggard, 2005, p. 81).
Estimation of body parts’ size; for example, participants ‘‘were told to adjust the
virtual belly size until they perceived it to be the size of their own real belly’’
(Normand et al., 2011, p. 3).
Physiological responses to threat; for example, SCR (Armel & Ramachandran, 2003;
Honma, Koyama, & Osada, 2009; Petkova & Ehrsson, 2008; Petkova et al., 2011;
Yuan & Steed, 2010), Heart Rate Deceleration (Slater, Spanlang, Sanchez-Vives,
et al., 2010).
Changes in physiological signals; for example, temperature (Hohwy & Paton, 2010;
Moseley et al., 2008).
Kilteni et al. 381

ticipants’ reactions under threat of the perceived body,
or changes in physiological measurements with or with-
out a threat event). More exploratory studies focused ei-
ther on exploiting any brain activity changes under body
ownership illusions using electrophysiological
(Kanayama, Sato, & Ohira, 2009; Peled, Pressman,
Geva, & Modai, 2003; Press, Heyes, Haggard, & Eimer,
2008), or hemodynamic (Ehrsson et al., 2004; Ehrsson,
Wiech, Weiskopf, Dolan, & Passingham, 2007) methods
or on the effect of temporary brain function disruption
to the illusion (Kammers et al., 2009; Tsakiris et al.,
2008).
The measurements in Table 2 of course do not directly
assess the SoE, but act as surrogates. Additionally, one
should be aware of conceptual overlaps when using these
measures, because the operationalization of the subcom-
ponents is not necessarily mutually exclusive. For
instance, the use of proprioceptive drift as a measure of
body ownership can be considered to address two under-
lying subcomponents of embodiment: self-location
interpreted as body location, since it is a localization task
of a body part; and body ownership, since the task
demands the localization of one’s own hand. Indeed, in
the study of Rohde, Di Luca, and Ernst (2011), proprio-
ceptive drift and ownership were dissociated. Addition-
ally, reactions to a threat are related both to self-location,
since such an event contains spatial information (e.g.,
the proximity of the threat to the perceived self-
location) but also to body ownership, because one
would manifest strong reactions to a threat in the case
where it refers to one’s own body, compared to when
there is no sensation of ownership. Future studies should
correlate both qualitative and quantitative data in order
to get a fuller picture of their association with the sub-
components of embodiment (e.g., Ehrsson, 2007; Slater
et al., 2008; Slater, Spanlang, Sanchez-Vives, et al., 2010).
Motivated by the lack in the literature of measures for
the sense of agency, task performance in motor tasks
(including kinematic and physiological analysis) could
provide valuable insights into this subcomponent. Based
on the assumption that motor tasks are performed more
successfully if the artificial body executing the task is
under finer control, presumably a high sense of agency
should correlate with high task performance. The use of
such a measure is in line with the study of Nielsen
(1963), where motor performance was used to reveal the
mechanisms of body recognition.
Apart from measuring the SoE in the level of its sub-
components, more indirect measures can be outlined
from a higher-level perspective concerning its potential
psychological, emotional, and behavioral consequences.
Such an approach would be in line with the concept of
self-presence as firstly depicted by Biocca (1997) who
posed the Cyborg’s Dilemma. Self-presence (Lee, 2004)
considered as ‘‘a psychological state in which virtual [...]
self/selves are experienced as the actual self in either sen-
sory or nonsensory ways’’ is actually the alteration of the
user’s behavior or emotional state because of induced
SoE toward the given virtual body representation. As
proposed by Lee, ‘‘intense feelings of self presence dur-
ing virtual experience (...) might create some types of
identity or reality confusion.’’ More specifically, if one
feels embodied in a virtual body, insults or praise regard-
ing this body, referring to properties that would not be
true for the biological body, should cause emotional
arousal. For example, in Normand et al. (2011), it was
shown that participants could be given the illusion that
they were much fatter than their real biological body. It
would have been interesting to investigate the correla-
tion between questionnaire items addressing body own-
ership and those referring to the emotional state; for
example, when other virtual characters criticize them for
being too fat. Relevant to this is a questionnaire pro-
posed recently by Ratan and Hasler (2009) applied
though for less immersive scenarios. Similarly, in Slater,
Spanlang, Sanchez-Vives, et al. (2010) male participants
experienced the virtual world through the eyes of a girl.
However, emotional or behavioral correlates of such vir-
tual representation were not investigated. Generally, a
virtual body representation with different morphology
with respect to one’s own biological properties (e.g.,
morphological appearance, number of limbs, size),
would probably have psychological or even motor conse-
quences. In the studies of Longo, Schuur, Kammers,
Tsakiris, and Haggard (2009) and Tsakiris (2008), such
consequences were indeed reported. Additionally, a vir-
tual body with characteristics associated with certain
social stereotypes but different from those of the biologi-
382 PRESENCE: VOLUME 21, NUMBER 4

cal body (e.g., with respect to race, gender, or age),
could result in the participant engaging in behaviors
associated with those stereotypes. All these hypotheses
could be addressed by future studies in immersive virtual
reality systems.
2.7 Enhancing the SoE
An approach to enhancing the SoE would be to
enhance each of its three subcomponents. We consider
each in turn.
2.7.1 Enhancing the Sense of Self-Location. Self-
location is influenced by the origin of the visual perspec-
tive and the associated vestibular and tactile information.
Clearly a fundamental requirement is for there to be
first-person perspective with respect to the position of
the eyes of the artificial body. Additionally, synchronous
visuotactile correlations can further strengthen this,
where the tactile event is seen visually on the body from
the first-person perspective position of the eyes. Lopez
et al. (2008) proposed that exposure to caloric and gal-
vanic vestibular stimulation could be used for experi-
mental manipulation of self-location. Therefore, in vir-
tual applications, these various types of manipulation
should be considered in order to examine their impact
on induced self-location within a virtual body represen-
tation. The necessity of such varied multimodal feedback
may increase in importance the more the virtual body
volume varies from that of the real biological body.
2.7.2 Enhancing the Sense of Agency. The
sense of agency is sensitive to any temporal discrepancies
between the execution of a self-generated movement
and visual feedback. Hence, visuomotor correlations
should be provided maintaining critical time boundaries
(see Franck et al., 2001). The sense of agency is easily
provided in virtual reality when the motion of the partic-
ipant is mapped to the virtual body in real-time or near
real-time. This can be achieved either by tracking rigid
bodies (by rigid reflective markers) attached to the par-
ticipant’s limb and computing the avatar’s movement by
an inverse kinematics method (Yuan & Steed, 2010), or
by tracking the full-body movements of the participant
with a real-time motion capture system and applying the
resulting motion to the avatar (Slater, Spanlang, &
Corominas, 2010).
2.7.3 Enhancing the Sense of Body
Ownership. The sense of body ownership, from the
perspective of bottom-up influences, can be enhanced by
increasing the sensory correlations between the physical
stimulation of the biological body and the seen stimula-
tion on the avatar’s body. Such synchronous sensory cor-
relations can be either visuotactile (e.g., with the use of
appropriate haptic feedback), or visuoproprioceptive
(e.g., by the participant’s passive movements and appro-
priate avatar animation). On the other hand, the virtual
body should obey certain structural and morphological
constraints in order to appear human-like. By maximiz-
ing the morphological similarity between one’s biologi-
cal body and the virtual one, top-down influences favor
the perception of ownership of the virtual body. In con-
trast to the previously mentioned categories, the sense of
body ownership may be highly susceptible to individual
differences; for example, the similarity in appearance
between the participant and the avatar. In line with this,
individualized avatars could strengthen ownership since
this would also promote body and self-recognition.
3 Conclusions
The present review has discussed a working defini-
tion for SoE by relating it to the normal embodiment we
experience toward our biological body. An underlying
structure was proposed, consisting of three subcompo-
nents: sense of self-location, sense of agency, and sense of
body ownership. Measures of the SoE as used in the liter-
ature were grouped and organized under this conceptual-
ization. Moreover, new possible measures based on the
psychological and emotional consequences of embodi-
ment and methods for the enhancement of the SoE in vir-
tual reality through its subcomponents were suggested.
Throughout the paper, several challenges became
apparent. Possible directions for future studies that aim
toward a deeper and more precise conceptualization of
SoE were outlined. Whereas SoE may consist of three
subcomponents, their relationship is far from under-
stood. Although these may be conceptually independ-
Kilteni et al. 383

ent, it is quite probable that there is an empirical correla-
tion between them. Future studies are needed to detect
the existence, if any, of such dependencies. Additionally,
there is no evidence for an equal contribution of each
subcomponent to the overall concept of embodiment. If
embodiment is a weighted combination of these, future
studies with experimental conditions that manipulate
different subcomponents could shed light on the poten-
tial dominance of a subcomponent.
The present paper focused on artificially induced em-
bodiment and more especially on using immersive virtual
reality for this purpose, a technology that seems ideally
suited to tackle research in this area. Based on the cur-
rent overview and proposed future studies, new insights
into this complex experience might be found and precise
guidelines could be provided toward the enhancement
of the sense of embodiment.
Acknowledgment
This work is funded under the European Research Council
Project TRAVERSE (#227985) (K. Kilteni and M. Slater)
EU FP7 project BEAMING (#248620) (R. Groten).
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