Inducing illusory ownership of a virtual body
http://www.ehrssonlab.se/pdfs/Slater-et-al-2009.pdf
-When the experimenter touched the real hand
of the subject with the Wand, the subject would
see the virtual ball touch the virtual hand, registered
in the same place on the virtual hand. In
this way synchronous visual and tactile stimuli
could be applied to the virtual and real hand
(Figure 1A). The asynchronous stimulation in
the control condition was achieved by using prerecorded
movements of the virtual ball. Using
this setup we compared the responses between
two groups of volunteers, with 21 participants
in the synchronous and 20 in the asynchronous
condition. The specifi c questions we used to
indicate the illusion were:
1. Sometimes I had the feeling that I was receiving
the hits in the location of the virtual arm.
2. During the experiment there were moments
in which it seemed as if what I was feeling was
caused by the yellow ball that I was seeing on
the screen.
3. During the experiment there were moments
in which I felt as if the virtual arm was my
own arm.
EXPERIMENT 2 – VISUAL–MOTOR SYNCHRONY
Having demonstrated that visuo-tactile correlations
can induce an illusion of ownership of a virtual
arm, we then explored whether this illusion
can be induced in the absence of tactile stimulation
– see also Dummer et al. (2009) and Tsakiris
et al. (2006). We carried out an experiment to
investigate whether the virtual arm illusion can
be induced by active movements of the fi ngers and
hand (Sanchez-Vives et al. in preparation with a
preliminary report by Slater et al., 2008b). There
were 14 male participants in this within-groups
counter-balanced experimental design. The illusion
related questions were:
1. I sometimes felt as if my hand was located
where I saw the virtual hand to be.
2. Sometimes I felt that the virtual arm was my
own arm.
Here the participants wore a data glove that
detects hand and fi nger positions and transmits
real-time data to the computer that controls the
display of a virtual hand (Figure 1B). Only when
the movement of the virtual hand was synchronous
with the movement of the participant’s real
hand there was an ownership illusion. This was
indicated by questionnaire response (the two
questions above) and proprioceptive drift (using
the method introduced by Botvinick and Cohen,
1998). The fact the illusion could be induced by
active movements and congruent visual feedback
is important for virtual reality applications where
participants will need to interact with environmental
objects.
EXPERIMENT 3 – USING A BRAIN–COMPUTER
INTERFACE
We carried out a further experiment but without
any tactile stimulation or overt movements
(Perez-Marcos et al., 2009). Here the participants
had the task to open and close their virtual
hand through a brain–computer interface
(BCI). This used a cued motor imagery paradigm
(Pfurtscheller and Neuper, 2001) on which participant
had been previously trained (Figure 1D).
There were two conditions – in the synchronous
one the hand opened and closed as a function of
the participant’s motor imagery. In the second –
asynchronous – condition the hand opened
and closed independently of the subject’s motor
imagery. In the synchronous condition, but not in
the asynchronous, there was a sense of ownership
of the virtual hand. After the 5 min of BCI control
(synchronous or asynchronous) of the arm, the
virtual arm and table suddenly fell and the EMG
recordings showed that there was greater muscle
activity in the arm compared to an earlier reference
period before the arm fell – but only for
the synchronous condition. However, there was
no proprioceptive drift in either condition. This
may suggest that actual sensory feedback (touch
or proprioceptive feedback) is necessary for recalibration
of position sense and the elicitation of
a full-blown virtual hand illusion. Alternatively,
mental imagery may not be as potent in inducing
the illusion as actual stimulation. Future experiments
are needed to clarify to what degree virtual
limbs can be owned by BCI control alone.
THE VIRTUAL BODY
To what extent can the multisensory correlations
employed to produce the virtual hand illusion
generalise to the whole body? The evidence
is beginning to point towards an affi rmative
answer to this question – that the illusion of
ownership of a virtual body may be generated.
There is both indirect and direct evidence for
this. In Ehrsson (2007) a setup was employed
to give people the illusion that they were behind
their real bodies. Subjects wore a set of headmounted
displays that displayed real-time stereoscopic
images from two cameras located behind
where they were actually seated – thus shifting
their visual ego-center to behind themselves.
The experimenter was standing just behind the
participant and the participant could see where
they were sitting in the room and identify the
experimenter standing behind them just next
to them. The experimenter then used a stick
to tap their chest (out of sight) while tapping
underneath the location of the cameras. The felt
tapping was either synchronous with the visual
jabbing movements towards a point beneath the
cameras, or asynchronous. In the synchronous
condition subjects reported a strong illusion of
being behind their physical bodies as judged
by the questionnaire responses, for example ‘I
experienced that I was located at some distance
behind the visual image of myself, almost as if I
were looking at someone else’ (Supplementary
Figure 1, Ehrsson, 2007). People also experienced
that the scientist was standing in front
of them, i.e. there had been a change in the perceived
self- location. This fi nding was reinforced
by skin conductance responses that correlated
with an attack on their ‘phantom body’ location
in the synchronous but not in the asynchronous
condition. Thus this is evidence that the sense
of one’s body place can be dislocated to a position
which is different from the body’s veridical
position, and is therefore indirect evidence for
the idea that a virtual body might become felt
as one’s own.
More direct evidence has come from Petkova
and Ehrsson (2008), who employed cameras
attached to the head of a manikin that was looking
down on the manikin’s body. Again the videosignals
from these cameras were presented in real
time to the participant who was wearing a set of
head-mounted displays. Now looking down at
themselves subjects would see the manikin body
in a similar location where their own body would
be. Synchronous tapping on the stomach of the
manikin and the real stomach resulted in a strong
illusion of ownership of the entire body (as evidenced
by the questionnaire responses), which
was again confi rmed by augmented skin conductance
responses in correspondence to physical
attacks on different body parts of the manikin in
the synchronous but not in the asynchronous tapping
condition. This suggests that entire bodies
can be owned and that ownership of one stimulated
body part automatically enhance ownership
of other seen parts of the body.
A similar full body experiment was reported by
Lenggenhager et al. (2007). In the critical experiment
the participants looked at a body presented
a few meters in front of their selves through a
head-mounted display. Thus the participants saw
the back of the body, and when the experimenter
stroked them on their back, they would see this
stroking on the back at the distant body location.
This resulted in the reported sense of being at the
location of the body in front, and a version of the
proprioceptive drift measure provided a further
verifi cation. In this case there was a reported projection
of the sense of touch and self-localisation to
a body observed from a third-person perspective,
which is different from the experiments by Ehrsson
(2007) and Petkova and Ehrsson (2008) where the
owned artifi cial body was always perceived from
fi rst person perspective. To what extent the reported
self attribution in these two set of experiments
engage common or different perceptual mechanisms
is still an open question (see Science E-letters
for further discussion2). However, Lenggenhager
et al. (2009) recently reported an experiment that
directly compared the two paradigms and found
evidence to suggest that self-localisation is strongly
infl uenced by where the correlated visual–tactile
event is seen to occur.
DISCUSSION
The experiments reviewed in this article strongly
suggest that virtual limbs and bodies in virtual
reality could be owned by participants just as
rubber hands can be perceived as part of one’s
body in physical reality. Furthermore, the experimental
fi ndings suggest that ownership of virtual
limbs and bodies may engage the same perceptual,
emotional, and motor processes that make
us feel that we own our biological bodies. To what
extent this ‘virtual body illusion’ works when the
movements of the simulated body are controlled
directly by the participants thoughts, via BCI
control, is an important emerging area for future
experiments.
The visual realism of the virtual arm and
the arm’s environment does not seem to play
an important role for the induction of the illusion.
In our laboratory we have seen the illusion
work well with many different types of simulated
hands. This is similar to the traditional rubber
hand illusion which does not seem to depend on
the physical similarity between the rubber hand
and the person’s real hand – anecdotal observations;
see also (Longo et al., 2009). Further,
adding realism to the simulation by adding shadows
(Figure 1C) did not enhance the ownership
illusion (Perez-Marcos et al., 2007), unpublished
results. These observations would fi t with physiological
properties of cells in premotor and
intraparietal cortices which are involved in the
fast localisation of limbs in space (Graziano,
1999; Graziano et al., 2000), but not involved
in visual object recognition and the fi ne analysis
of visual scenes. This realisation is important for
the development of virtual reality applications
because it means that one is not restricted to
ultra-realistic simulations and high defi nition
visual displays.
Virtual reality additionally provides power
to investigate these illusions at the whole body
level. In Figure 2 we show an example of what
can be seen when someone wears a tracked
head-mounted display, looks down, and sees a
virtual body in place of their real one. The very
act of looking down, changing head orientation
in order to gaze in a certain direction, with the
visual images changing as they would in reality
is already a powerful clue that you are located
in the virtual place that you perceive. We argue
elsewhere that multisensory contingencies that
correspond approximately to those employed
to perceive physical reality provide a necessary
condition for the illusion of being in the virtual
place (Slater 2009). Now imagine that you move,
and the virtual body moves in correspondence
with your movements, or you see something
touch your virtual body and you feel the touch
in the corresponding location in your real body.
These events add signifi cantly to the reality of
what is being perceived – not only are you in the
virtual place, but you also have the illusion that
the events occurring are real – therefore increasing
the likelihood that you would respond realistically
to virtual events and situations
FUTURE PERSPECTIVE
BCI control of owned virtual bodies will probably
have many important clinical and industrial
applications, for example in the development of
the next-generation BCI applications for totally
paralysed individuals. These people would in
principle be able to control and own a virtual
body and engage in interactions in simulated
environments. The fi rst attempt in this direction
(Experiment 3; Perez-Marcos et al., 2009)
suggests that this dream might have a chance of
success. When the motor imagery resulted in the
expected opening and closing of the virtual hand
then the ownership illusion and motor recruitment
occurred (but not proprioceptive drift).
The fundamental question here is whether a
correlation between intentions of movement
and pure visual feedback, in the absence of any
tactile or proprioceptive feedback, is suffi cient
to induce the rubber hand illusion and produce
recalibration of visual, tactile and proprioceptive
representations. If so, this would demonstrate
that multisensory recalibration could occur as
a result of internal simulation of action and its
sensory consequences. This issue is not fully
settled yet, given that in Perez-Marcos et al. the
illusion of ownership did not go along with proprioceptive
drift. Future experiments whereby the
participants can execute different types of virtual
hand movements via so called ‘un-cued’ BCI may
be a promising avenue for future experiments
of this sort