The vast amount of research on the rubber hand illusion
uses visuotactile synchronous stimulation to induce the illusion. This means that sight of the rubber hand being touched is
synchronous temporally and spatially with the tactile stimulation felt on the
corresponding (hidden) real hand. It has also been shown that the illusion can be induced with visuomotor stimulation
- meaning that (in this case) the
virtual hand moves synchronously with the movements of the corresponding
(hidden) real hand.
Which of these two methods of stimulation
is the most powerful in inducing the body ownership illusion - visuotactile or
visuomotor? In a recent paper
) we describe an experiment that addresses this
Participants were in a reclined position
and saw their full virtual body from a first person perspective through a
head-tracked, wide field-of-view head-mounted display. As they moved their leg
(A) the corresponding virtual leg would move synchronously or asynchronously
(B). When the experimenter tapped a leg with the wand (C) the participant would
see a virtual ball tapping the corresponding position on the virtual leg
(synchronously or asynchronously). Hence we had a 2 by 2 experimental design
(synchronous movement or asynchronous combined with synchronous tapping or
asynchronous) where these were delivered alternately. There were 60 participants in a between
groups design - hence each group of 15 experienced just one of the 4 combinations of these
The experiment was organised so that every
subject first experienced for a while the best possible setup - that is
visuomotor and visuotactile synchrony. Then after some questions had been
answered they experienced one of the 4 conditions. Then questions about body ownership and agency were again answered.
Based on the questionnaire responses
visuomotor synchrony outweighed visuotactile in producing the illusion.
However, unusually, we also attempted to
measure not only what generated the illusion but also what extinguished it.
Here we used a method first proposed in the study of presence in virtual reality (the sensation of being in the place depicted by the virtual
environment displays). This method is called ‘breaks in presence’. The assumption is
that the normal state is for the illusion of presence to occur, but
occasionally it breaks for various reasons (errors in rendering or tracking,
physical entanglement with cables, bumping into a Cave screen, or just spontaneous
switches in attention or perception). From an indication by participants about when each break occurs it is possible to estimate an overall probability of presence
(the proportion of time the participant had this illusion). Here we
adopted the same idea except that instead of presence we considered the
illusion of body ownership, and participants reported when the illusion
We found that a break in body ownership
could be caused equally by asynchronous visuomotor or visuotactile stimulation.
Hence while synchronous visuomotor was paramount in generating the illusion, the
number of breaks that occurred did not differ between visuomotor and
We also recorded skin conductance and heart
rate. This was in order to measure the response to a sudden event that took
place. The picture above shows that participants had their real (and virtual)
legs resting on a table. At the end of all the stimulation the table suddenly
pulled away. Participants tended to react with an involuntary response to stop
their legs from falling, and this showed up in both heart rate and skin
conductance changes. Moreover these changes were positively correlated with a questions
about how stressed they had felt at that moment. However, there were no
differences in the physiological changes amongst the four conditions of the
experiment - the event was equally arousing under all conditions. We believe that this is because seeing a virtual body that coincides spatially with your own body is already enough to produce a body ownership illusion. Additional synchronous multisensory stimulation only adds to this.
By looking at breaks in the body ownership
illusion we were able to assess subjective ownership through time as well as at
the end of all the stimulation. To obtain this information we used the same
method as in the ‘breaks in presence’ work - that is we only asked participants
to indicate when the illusion broke, and not when it started. For presence this
procedure makes sense, because if we ask people to report when they become
‘present’ in the virtual place the very requirement to report this may disrupt
it. Asking them to report when the illusion breaks is not the same, since, of
course, the illusion has already broken. However, with hindsight it is probably
possible to ask people to report when an illusion of body ownership kicks in
without disrupting the illusion. Indeed we did this in an earlier paper (PDF) (for
different reasons - we were interested in estimating time for the rubber hand
illusion to start). Although based only on reporting breaks our statistical
method can estimate the probability of being in the illusion of ownership
state, in future work we will also try out the idea of asking participants to
report when the illusion starts as well as when it ends.
Finally our method also includes an
approach that may overcome some problems in subjective assessment of the body
ownership illusion. Normally researchers ask participants in experiments under
different conditions to report things like ‘How much did you have the feeling
that the (virtual / manikin) body was your body?’. But participants naïve to
this idea (as they should be) have no real clue what we are talking
about. In everyday life we do not go around thinking "Oh my body feels like it belongs to me." As is the case with presence, the special qualia attached to a 'body ownership illusion' is to have that feeling of ownership even knowing that it is an illusion - that the virtual body is obviously not really your body.
Now especially in control conditions (e.g., asynchronous) we are asking them
to report on something that they do not know about - yet of course they will always give some answer to a questionnaire.
This is especially problematic in within-group studies where we ask people to
report the strength of the illusion in both an experimental (e.g. synchronous)
condition and in a control (e.g., asynchronous condition). But these are not balanced in the sense that the order of the conditions does matter. Experiencing first an asynchronous condition and then a synchronous
one is really very different from the other way around - since when the
synchronous condition is experienced first participants know what you are
talking about with respect to ‘body ownership’ and therefore can more
appropriately evaluate the asynchronous condition. No amount of counter
balancing can overcome this, and anyway it violates a fundamental assumption
behind the statistical analysis (by ANOVA) of within-group designs - that all orders
of stimuli delivery are equivalent.
Here what we did is give all participants
the experience of the best setup that we could offer (within the constraints of
that experimental design) to induce body ownership. Hence when later we ask
questions about their responses to the experimental conditions they have
already experienced the ‘best’ setup, so that they have an experience against
which they can compare.
Questionnaires alone are never the best
method of measurement. But they can be improved through asking people to
compare their responses to different setups; but first demonstrating as a
baseline the closest we can get to inducing the strongest response.
Kokkinara, E., and Slater, M. (2014).
Measuring the effects through time of the influence of visuomotor and
visuotactile synchronous stimulation on a virtual body ownership illusion.
Perception 43, 43 – 58. (PDF).
This research was conducted as part of the VR-HYPERSPACE project.