Figure 1. The
experimental setup. Real:
the participant wears a high resolution, wide field-of-view, stereo,
head-tracked head-mounted display (NVIS SX111) and EEG cap (g.tec). Virtual top:
the virtual reality showing the gender-matched virtual body spatially
coincident with the participant’s actual body, and in the same posture. Virtual
bottom: The two experimental conditions seen by the participant when
looking towards the hand from a first person perspective: HAND - virtual
hand stabbed by the knife; TABLE - virtual table stabbed by the knife
When you wear a head-tracked wide
field-of-view stereo head-mounted display and you look down towards your body you can see a life-sized virtual one instead. You look around and you see a
reflection of that body in a mirror. In your whole life whenever you have
looked towards your body you have seen it, likewise towards a mirror. Hence the
simplest perceptual hypothesis for the brain to adopt is that this is your
In this study we looked at what happened when
the virtual body was threatened. When someone anticipates that a knife might
stab their hand that is resting on a table they would be likely to attempt to
move the threatened hand out of the way. They would expect to feel considerable
pain should the knife actually stab the hand. In this work we considered what
happens when a person’s real body is visually substituted by a life-sized
virtual body, and they see a threat or attack to a hand of this virtual body
seen from first person perspective. Our experiment investigated brain activity
in response to events that would cause pain to the observer were these events
to occur in reality. Our contribution has been to introduce a new technique for the study of pain observation,
by using immersive virtual reality (IVR) for the scenario and stimulation,
while recording brain activity with EEG.
experiments typically present a series of pictures with hands or other
extremities undergoing painful situations, and they compare the brain response
of the participants to the activation produced by pictures where the same
extremities do not undergo painful situations [1-4]. Many of these experiments
present scissors and needles perforating the extremities as painful stimuli. A
potential advantage of immersive virtual reality is that there is greater
ecological validity, going beyond the presentation of two-dimensional, static
stimuli. There is a life-sized, three dimensional virtual body seen in stereo,
that visually substitutes the obscured real body of the participant and results
show that this normally induces a whole body ownership illusion . Our
hypothesis was that harm to the virtual hand would be associated with positive
changes in P450 in line with previous studies, and that this would be enhanced
with illusory body ownership. We also investigated the mu band and readiness
While immersed in the
virtual reality the 19 participants (10 female, right-handed) repeatedly
experienced during 15 minutes two conditions in a within-group design: HAND
where the knife stabbed the virtual right hand, and TABLE where the knife
stabbed the table 15 cm away from the right hand (Figure 1). The experiment
consisted of 70 trials repeating the HAND and TABLE conditions (30 HAND and 40
Both EEG and
electromyography (EMG) were recorded using an gUSBamp amplifier with a resolution of 30nV; the electrodes were set
to cover the motor cortex area and surrounding: FC3, FC4, C3, C4, CP3, CP4
located according to the 10/20 standard EEG recording; the reference was set
with an ear clip on the left ear lobe; the ground was positioned on the
forehead; electrodes in the face measured ocular activity (EOG). Three EMG
electrodes were placed in the flexor carpi ulinaris muscle of the right arm to
measure whether participants moved their hand. All the electrodes were kept to
impedances below 10 kΩ. The data was recorded with a sampling frequency of 512
Hz. After the exposure participants answered a questionnaire on a 1-5 Likert
Scale where 1 was anchored to strong disagreement and 5 to strong agreement:
: I felt as if the hand I saw in the virtual world might be my hand.
: I had the feeling that I might be harmed when I saw the knife inside the hand.
: I had the feeling that I might be harmed when I saw the knife outside the hand.
: The hand I saw was the hand of another person.
Body Threat: I saw the knife as a threat to my body.
Figure 2. EEG Recordings. Left: Grand averaged stimulus locked ERPs for six
representative front, central and parietal electrode locations. A significant
increase in the amplitude of the P450 is observed in the HAND condition mainly
at C3 and CP3 locations. Baseline from [-200 ms to 0 ms], time 0 indicates the
stimuli onset; a low pass filter 12Hz half-amplitude cutoff was applied. Right:
(a) Time Frequency Evolution of the two conditions and the difference in the
spectral activity. (b) Grand averaged 1-s short time power spectra calculated
from EEG data (electrode C3) recorded. The baseline corresponds to the range
[-1 to 0] seconds before the stimuli and the activity period corresponds to the
range [0.7 to 1.7] seconds after the stimuli. Both the Baseline and TABLE
frequency spectra show a peak in the mu-rhythm that is attenuated in the HAND
condition. (c) Grand averaged Mu-rhythm (9-12Hz) Event Related
Desynchronization for the C3 electrode. (d) Grand averaged Readiness Potential
(C3-C4) subtraction between the brain activity in the two hemispheres shows
movement preparation effects. Low pass filter 8Hz, half-amplitude cutoff.
Figure 3. Box plots showing the responses to the questionnaire. The thick lines are the medians, and the boxes are the interquartile ranges (IQR). Wilcoxon matched pairs sign-rank tests show differences between Ownership and No Ownership (P < 0.0001); Harm Hand and Harm Table (P < 0.0002); Body Threat and Harm Table (P < 0.0003). Harm Hand and Body Threat ( P < 0.018).
• The results suggest that when a person is in an immersive virtual
reality and has body ownership illusion towards a virtual body that apparently
substitutes their own body, there are autonomic responses that correspond to
what would be observed were the events to take place in reality. Overall
automatic brain mechanisms –P450– were found in this variation of the classical
pain observation experiment, which is consistent with previously reported
• The results cannot be explained as participants experiencing empathy
towards another person since they witnessed attacks to their co-located virtual
body and both subjective and objective data suggest that they experienced this
as an attack on their own body.
• The results support our initial hypothesis that a threat to a virtual
hand, towards which the participant has an illusion of ownership, would significantly
produce a harm prevention effect (the Readiness Potential (C3-C4) and
oscillatory movement-related components, the mu-ERD), such as trying to move it
away from the source of the harm. The questionnaire also confirmed high levels
of ownership over the virtual body.
• The correlation between the automatic brain mechanisms –P450– and the
subjective illusion of ownership suggests
a potentially new measure of virtual embodiment.
, A., et al.,.
, I., et al., Cerebral Cortex, 2007.
M., et al., PLoS ONE, 2010. 5(5).
Funded by European Union FP7 IntegratedProject VERE (#257695); FI-DGR
predoctorate grant from the Catalan Government co-funded by the European Social
Found (EC-ESF); Spain MICIN (PSI2011-29219); ERC project TRAVERSE (#227985).