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EHESS - ENS - UNIVERSITE PARIS DESCARTES

Laboratoire Psychologie de la Perception, Université René Descartes Paris V

SENSE OF BODY OWNERSHIP AND THE

PERCEPTION OF PAIN

Dissertation Master in Cognitive Science Presented by Camila Valenzuela Moguillansky

Under the direction of Kevin O’Regan

June 2009

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Table of Contents

Acknowledgments 3

Summary 4

1 Introduction 5

1.1 Sense of Body-ownership............................................................................................................................................51.2 The Rubber Hand Illusion ...........................................................................................................................................51.3 The present study ........................................................................................................................................................112 Method 13

2.1 Participants ....................................................................................................................................................................132.2 General Procedure.......................................................................................................................................................132.3 Measurements...............................................................................................................................................................16

2.3.1 Proprioceptive drift ................................................................................................................................................162.3.2 Pain and heat intensity .........................................................................................................................................162.3.3 Localisation of pain ...............................................................................................................................................162.3.4 Questionnaire ...........................................................................................................................................................16

3 Results 17

3.1 Occurrence of the RHI ..............................................................................................................................................183.1.1 Proprioceptive Drift...............................................................................................................................................183.1.2 Questionnaire ...........................................................................................................................................................19

3.2 Perception of pain .......................................................................................................................................................203.3 Perception of warmth.................................................................................................................................................213.4 Locus of pain. ...............................................................................................................................................................234 Discussion 24

4.1 Proprioceptive drift.....................................................................................................................................................244.2 Questionnaire................................................................................................................................................................254.3 Perception of pain intensity .....................................................................................................................................274.4 Perception of warmth intensity...............................................................................................................................284.5 Localisation of pain ....................................................................................................................................................304.6 Methodological Remarks..........................................................................................................................................325 Conclusion 33

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Acknowledgments

In the first place I would like to specially thank Professor Kevin O’Regan who carefully guide

this project, for his great support, long discussions, his great patience, and trust.

Many thanks to the Pain Treatment Department of the Ambroise Paré Hospital, specially to

Dr. Didier Bouhasira and Françoise Morain.

Thanks to my friends, especially to Andrea Desantis for his great support, comments and

inspiring discussions, to Sergiu Popesco for his corrections and to Francisca Gutierrez for her

helpful comments.

Thanks to Professor Claire Petitmengin for her helpful ideas.

Thank to RISC for their nice people, their hospitality, great support and coffee supply.

And always special thank to my family, for their unconditionally support and trust.

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Summary

A recent study suggests that when, by use of the so-called "Rubber Hand Illusion" (Botvinick

& Cohen 1998), a person is induced to feel that a fake rubber hand is his or her own hand, the

temperature of the real hand is reduced (Moseley et al. 2008). This result suggests the

surprising fact that transfer of perceived body ownership of a limb can have the effect of

modifying the limb's physiological body parameters.

Similarities in neural pathways involved in pain and temperature perception and regulation

lead us to hypothesise that a transfer of body-ownership might also have the companion effect

of decreasing the intensity of perceived pain. Furthermore, we conjectured that the

localisation of pain sensation would shift towards the rubber hand.

We tested these hypotheses with an experimental procedure based on the standard Rubber

Hand Illusion protocol, but including a method to measure pain perception by thermal

stimulation. Our results show that participants’ estimation of pain indeed decreases when they

feel ownership of the rubber hand. Pain sensation is also localised closer to the rubber hand.

An additional finding shows on the other hand that there is no significant effect of body-

ownership on estimations of warmth (rather than pain), suggesting that pain and warmth

sensations may involve two separate systems.

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1 Introduction

1.1 Sense of Body-ownership

The sense of body-ownership is a concept that comes from the field of cognitive science and

philosophy of mind dealing with the sense of self. This concept refers to the experience of

being the owner of one’s body and the fact that it is one’s own body that is undergoing

experiences affecting the body (Gallagher 2000) 1. Although it is difficult to imagine the

possibility of feeling a sensation that does not belong to one's own body, there are

pathological and non-pathological situations were this occurs. Within the clinical literature for

example there are several disorders linked to a deficit in feeling of body ownership (entirely

or partially) which can be caused by different kinds of damage: neurological damage,

peripheral nerve disruption, deafferentation and local anaesthesia (de Vignemont 2007).

Asomatognosia for example is a case caused by neurological damage where a patient feels a

tactile stimulus applied to her hand but does not attribute the hand as belonging to her.

It is also possible to alter the sense of body ownership by creating illusory experiences in

healthy individuals (Testavin 1937, Botvinick and Cohen 1998, Holmes and Spence 2005,

Lenggenhager et al 2007). In this work I will concentrate in this kind of manipulation of

body-ownership modification. Specifically I will use the experimental paradigm of the

"Rubber Hand Illusion" (RHI) introduced by Botvinick & Cohen (1998) to modify the sense

of ownership of participants’ hands.

1.2 The Rubber Hand Illusion

In the RHI, a person watches a rubber hand being stroked in the same place and at the same

time that they feel their own hidden hand being stroked. After a few minutes, the person gets

the peculiar impression that the RH belongs to them. The most common measures used to

account for this phenomenon are a quantitative measure consisting in a displacement of the

felt position of the hidden hand in the direction of the RH, termed proprioceptive drift, and a

qualitative measure consisting in the person's report assessed by a questionnaire. The illusion 1 Within the categories of the self that has been established by Gallagher, the sense of body-ownership has been distinguished from the sense of agency. The sense of agency involves the awareness that I am the cause of an action or a thought; in the context of an action it would correspond to –“I am moving my body". On the other hand, the sense of body-ownership would correspond to – “I know that my body is moving”. This conceptual distinction does not suggest that these two aspects are dissociated; in our ordinary life they merge together and give rise to a coherent experience of self.

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works in approximately 60-80% of the people tested (Kammers et al 2009, Durgin et al

2007). It has been shown that the illusion is effective only when the stroking of the real and

the rubber hand is synchronous. When the real and the RH are not stroked synchronously, a

much smaller proprioceptive drift is observed and the subjects do not report having the

impression that the RH belongs to them (Botvinick and Cohen 1998, Ramachandran 2003,

Tsakiris and Haggard 2005, Tsakiris and Haggard 2006, Ehrsson et al 2004, Ehrsson et al

2005, Ehrsson et al 2006, Moseley et al 2008, Schütz-Bosbach et al 2009).

In addition to this first condition of synchronicity, another condition for the illusion to be

established is that the hand has to be in an anatomically plausible position and it has to

resemble the real hand (Graziano 2000, Tsakiris and Haggard 2005, Tsakiris and Haggard

2006, Costantini & Haggard, 2007 Ehrsson et al 2004). This fact has been interpreted in terms

of resemblance to a previous mental body representation that has to be fulfilled in order for

the illusion to work. Some authors refer to this mental representation as the body schema

(Tsakiris and Haggard 2005), others as the body image (Ramachandran 2003). But frequently

these concepts are used without carefully distinguishing them (Paillard 1999). According to

the classical definition, the body schema is an unconscious, dynamically modifiable model of

the topographic configuration of our body (Bernard 1995, Paillard 1999, Constantitni and

Haggard 2007). The body image on the other hand involves a conscious and conceptual body

representation. It encompasses ideas, attitudes and beliefs toward one's own body (Gallagher

2005; Costantini and Haggard 2007).

This second condition of coherence between the rubber hand (or the object involved) and the

mental representation of our body has opened a discussion in relation to the possible

components of the illusion in terms of bottom-up and top-down mechanisms (Tsakiris and

Haggard 2005, Tsakiris and Haggard 2006, Schütz-Bosbach et al 2009, Constantitni and

Haggard 2007). This discussion will be further explained when presenting the conditions for

the illusion to work.

The fact that individuals undergoing the RHI have the feeling that the rubber hand belongs to

them has been considered as an alteration in the sense of body-ownership. For this reason it

has been widely used as experimental paradigm to study different aspect of the sense of body-

ownership (e.g., Armel& Ramachandran, 2003; Costantini & Haggard, 2007; Durgin, Evans,

Dunphy, Klostermann, & Simmons, 2007; Ehrsson, Holmes, & Passingham, 2005; Ehrsson,

Spence, & Passingham, 2004; Ehrsson, Wiech, Weiskopf,Dolan, & Passingham, 2007; Farnè,

Pavani, Meneghello, &Làdavas, 2000; Holmes, Snijders, & Spence, 2006; Pavani, Spence, &

Driver, 2000; Press, Heyes, Haggard,& Eimer, 2008; Tsakiris & Haggard, 2005; Tsakiris,

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Hesse, Boy, Haggard, & Fink, 2007; Tsakiris, Prabhu, & Haggard, 2006; Walton & Spence,

2004, Peled, Ritsner, Hirschmann, Geva, Modai, 2000)

Let us consider now the mechanism that has been proposed to explain the RHI.

The illusion has been described in terms of a multi-sensory correlation. Within the multi-

sensory correlation theory there are different views that explain the phenomenon in slightly

different terms (see Botvinick and Cohen 1998 for connectionist model, Ramachandran 2003

for Bayesian mechanism, Makin et al 2008 for peri-personal space framework, Pavani 2000

for visual capture mechanism), yet it is widely accepted that integration of patterns of multi-

sensory activity underlies the illusion.

The covariance of the spatial and temporal pattern of visual stimuli (coming from the RH) and

tactile stimuli (coming from the subject’s hand) is perceived as if the seen stimulus is coming

from the person’s hand. A mechanism of this sort is suggested to underlie the ability to

identify our body as belonging to us, in other words to the sense of body ownership (Ehrsson

2004, Botvinick 2004). Evidence from brain activity studies corroborates this proposal.

Thus, Graziano et al (2000) replicate the RHI protocol in monkeys. They measured single cell

activity of multimodal neurons in area 5 of the parietal cortex. Graziano and collaborators

showed that in the case of synchronous stroking of the RH and the monkey’s hidden hand, the

pattern of activity of area 5 cells were similar to that when the monkeys looked at its own

hand. On the contrary, when the RH and real hand were stroked asyncrhonously, the cells

were no longer sensitive to the position of the RH. Furthermore these visuo-tactile neurons

have visual receptive fields that are bodily centered and not retinotopically centered, meaning

that their response to visual stimuli depends on the position of the body part to which their

tactile receptive field corresponds (Graziano 2000, Farnè 2000).

In the brain imaging study performed by Ehrsson and colleagues (2004) where they

investigated the fMRI pattern of subjects that were induced with the RHI, they found that the

illusion was accompanied by activation of the cerebellum and pre-motor cortex. The

activation correlates with the strength of the illusion and is synchronous with the illusion

onset. Later, in order to test if the RHI was induced without visual input, Ehrsson et al (2005)

performed an experiment where they induced a “somatic” RHI. In their study the

experimenter moved the blindfolded participant’s left index finger so that it touched the fake

hand, and simultaneously, the experimenter touched the participant’s real right hand,

synchronizing the touches as perfectly as possible. After some seconds, this stimulation

elicited an illusion that the subject was touching her or his own hand. The authors observed

that even in the absence of visual input their previous results were replicated: the pre-motor

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cortex and cerebellum were activated in the presence of the RHI and the strength of the

activation correlated to the intensity of the illusion. They interpreted their results by

suggesting that neural activity in the cerebellum and pre-motor cortex probably reflects the

feeling of body-ownership of the hand rather than a visual representation of the hand being

brushed. (Ehrsson 2005).

Another interesting brain imaging result comes from a study done by Tsakiris and

collaborators (2007). They used positron emission tomography (PET) to measure the brain

activity linked to the RHI. Their results showed activation in the ipsilateral posterior insula

that correlates to the experience of ownership of the rubber hand. On the contrary, when

subjects failed to experience the RHI, there was activation of the contralateral somatosensory

cortex. They distinguished between brain activity linked to the causes and brain activity

linked to the effects of the RHI. They claim that the activity in the pre-motor cortex found by

Ehrsson underpin the process of multi-sensory correlation that causes the RHI, whereas the

activity in the posterior insula correlates with the effects of the illusion, namely the

experience of ownership of the RH. Pre-motor cortex receives strong inputs from parietal

regions that integrate visual, tactile and proprioceptive inputs; the cerebellum is also known

for having inputs from different sensory modalities (Graziano 2000, Ehrsson 2005). Thus

these findings confirm the theory, which proposes multi-sensorial correlation as a mechanism

for identifying a body part as belonging to the self.

It has been maintained that the correlation processes underlying the sense of ownership are

asymmetrical: a small transformation of what the subject sees reduces the illusion more than

the equivalent transformation of what they feel (Costantini & Haggard 2007).

Costantini & Haggard (2007) performed an experiment where they studied the sensitivity of

the RHI to mismatches between visual and somatic experience. They compared the spatial

mismatch between the stimulation of the two hands, and equivalent mismatches between the

postures of the two hands. They created the mismatch either by adjusting the stimulation or

the posture of the subject’s hand, or by adjusting the stimulation or the posture of the rubber

hand. When the subject’s hand posture was slightly different from the rubber hand’s posture,

the RHI remained as long as stimulation of the two hands was congruent in a hand-centered

spatial reference frame, even though the altered posture of the subject’s hand meant that

stimulation was incongruent in external space. Conversely, the RHI was reduced when the

stimulation was incongruent in hand-centered space but congruent in external space. They

concluded that the visual–tactile correlation that causes the RHI is computed within a hand-

centred frame of reference, which is updated with changes in body posture and they claimed,

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“Current sensory evidence about what is ‘me’ is interpreted with respect to a prior mental

body representation”.

This brings us to the question of body representation, which is addressed by the second

condition required for the RHI to work. I will present three studies that illustrate the debate in

relation to this issue.

The first study is that by Ramachandran et al (2003). The authors carried out three

experiments where they recorded skin conductance response (SCR) and phenomenological

report of participants undergoing the RHI. They tested to what extent the subjects

incorporated the RH through ostensibly threatening the RH by bending its fingers. They also

studied to what extent this reaction persisted when presenting a non-hand like object and a

hand object but in an anatomically implausible position. The first experiment addressed the

question of whether the finger of the fake hand seemed painful, and whether the participant

registered an SCR. The phenomenological reports and SCR revealed that subjects felt the

threat of the RH significantly more in the synchronous than in the asynchronous condition --

suggesting that the RHI caused the participants to believe that was their real hand that was

being threatened. The second experiment aimed to test if participants would experience the

illusion in the case that the form of the external object was manipulated. The experimenter

synchronously tapped the bare table and the participants' hidden hand. Phenomenological

reports indicated hat the RH was more effective in inducing the RHI than the bare table. On

the contrary there was no significant difference in SCR between these two conditions. The

third experiment aimed to answer the question of whether the participants would still

experience the illusion if the location of the external object were manipulated. They placed

the fake hand 0.91m further forward than the real hand and a fake finger was bent back for the

painful stimulus. The results showed that subjects identified with the fake hand and the distant

fake hand, as measured by both self-report and SCR, more in the conditions when touch was

synchronized than when it was not synchronized. They concluded that image of our body is

transitory internal construct, easily modified by multisensory correlation mechanism, which is

resistant to “top-down” modulation.

The second study is performed by Tsakiris and Haggard (2005) and argues against

Ramachandran’s conclusion by claiming that there is a top-down influence of the body

schema on the size of the illusion.

In their first experiment participants watched the rubber hand in a position congruent with

their own hand (0°) or in an incongruent position, rotated 90° with respect to their own hand.

The stimulation between the real and the rubber hand was either synchronous or

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asynchronous. In a second condition the authors wanted to test the hypothesis put forward by

Ramachandran (2003) in the study described above. In this condition they replaced the rubber

hand by a wooden stick, and used asynchronous stroking as control. Differences between

synchronous and asynchronous conditions were significant only when participants saw a

rubber hand at a posture congruent with their own hand. This suggests that the mere

correlation of tactile and visual stimulation of one’s own hand and a neutral object is not

enough to induce the illusion. In a second experiment they manipulated the handedness of the

rubber hand. They found that the incongruent rubber hand did not elicit the illusion. In the

discussion of their results, they present the interplay of multi-sensory correlation and

constraints of anatomical congruency and physical similarity, in terms of bottom-up and top-

down processes. They argue that although a bottom-up process of visuotactile correlation

underlies the build-up of the illusion, it is modulated by top-down mechanisms originating

from the representation of one’s body. The authors suggest that these two conditions for the

RHI to work reflect the interplay between both sensory input and conceptual interpretation in

the construction of the cognitive representation of the body.

The third study argues against the hypothesis that conceptual interpretations play a role in the

RHI. Schütz-Bosbach et al (2009) performed a study that aimed at investigating whether

conceptual interpretation of the sensory quality of visual and tactile sensations influences the

occurrence of the illusion. Participants watched a rubber hand being stroked by a piece of soft

material or by a piece of rough material while they received tactile stimulation by a material

that was either congruent or incongruent with respect to the sensory quality of the material

touching the rubber hand. They hypothesised that if the RHI implied a conceptual

interpretation, the RHI would occur only in those conditions in which the tactile and visual

input were congruent not just with respect to the timing but also with respect to the tactile

property of the material with which the stimulation was being done. Their results show that

the congruency between the seen and felt quality of the stimuli did not affect the occurrence

of the RHI either measured by subjective report or by the proprioceptive drift. They conclude

that interpretations of hand sensations are resistant to the RHI, and that the RHI is not affected

by conceptual interpretations.

Although up to now, there is neither a clear theoretical framework nor a clear mechanism to

explain this “second, top down condition”, it is widely accepted that multisensorial

correlation is not sufficient to induce the RHI. If there is participation of top-down

mechanisms such as conceptual interpretations, this needs to be confirmed.

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1.3 The present study

The experiment which provides the motivation for the present work, is a recent study done by

Moseley et al (2008). In a series of six experiments they show that sense of ownership of a

rubber hand (RH) generates a decrease in temperature of the hidden real hand and a decrease

in the ability to do temporal discrimination of pairs of tactile stimuli given to the hidden hand.

Moseley suggests that the RHI could induce a sort of “replacement” of the subject’s hand.

This replacement would cause a modification of the base-line values of parameters defining

the self. The feeling of body-ownership of the RH could imply not just rearrangement in the

hand’s spatial reference frame as evidenced with the proprioceptive drift, but also in baselines

of physiological thresholds.

If the RHI implies a sort of “abandonment” of the real hand as suggested by Moseley, we

could speculate that the effects found in the real hand’s temperature might extend to other

parameters. An interesting possibility would be that temperature sensation and perhaps

thermal pain sensation might be lowered too.

An argument to consider this possibility comes from the fact that temperature and pain

sensations are thought to belong to the same system. This view considers pain as a

homeostatic indicator and modulator, a behavioural drive caused by physiological imbalance

(Craig 2002). A further argument comes from experience. Pain sensation quickly brings our

awareness to our body, to the experience that we own a body. Pain drives us to relieve painful

sensation in a body part. Thus, it is possible that modification of a body-part’s ownership

might be accompanied by modification of the felt pain in that body part.

In this study we address the question whether modification in the sense of body-ownership

alters the perceived intensity and localisation of pain.

To this end we carried out an experiment where we combined the classic RHI protocol with a

method to measure pain elicited by thermal stimulation.

Participants watched the RH being stroked while their own unseen hand was stroked

synchronously. After two minutes a thermic stimulus was given to the real hand. Participants

were asked to evaluate the quality (painful or warmth), intensity of the pain and warmth

sensations, and the position of the felt pain. This procedure was repeated for 6 different

temperatures (ranging from 38°C to 48°C). The felt intensity and location of each stimulus

was compared with a control condition where the RH was not stroked.

We also registered the perceived warmth intensity for each stimulus. Even though painful and

warmth sensations share common substrates in terms of peripheral and central structures and

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organization, there is experimental (Han et al 1998, Davis et al 1998, Iannetti et al 2003) and

experiential evidence to consider them as different systems. For this reason we asked

participants to evaluate, on two different scales, the felt intensity of both warmth and pain for

a given stimulus.

We hypothesised that in the presence of the RHI, the thresholds of pain perception of the real

hand would be modified, and that as a result the intensity of the perceived pain would be

reduced. Furthermore, we conjecture that the localisation of the pain sensation should shift

towards the rubber hand.

To our knowledge, no one has investigated these questions. Positives results would shed light

on new aspects of the conscious experience of owning a body and the unconscious processes

that uphold this body. In addition, showing that the RHI diminishes pain, opens new

possibilities in the field of pain research and treatment.

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2 Method

2.1 Participants

Following approval by a local Ethics Committee, the experiments were performed on paid

healthy participants who were carefully briefed about the experimental procedures and who

gave informed written consent. The volunteers were informed that they would feel various

non-painful or painful sensations but they were not informed about the illusion. A total of 21

subjects participated in the experiment.

2.2 General Procedure

Each participant took part in an experimental session that lasted about 75 minutes.

Participants’ hand temperature was controlled at the beginning of the experiment with an

infrared thermometer. In the case that participant’s hand temperature was lower than 30°C,

we asked them to rinse them with hot water and then we waited for stabilization of the hand

temperature before starting the experiment. There were no cases where participants’ hands

needed to be cooled down before the beginning of the experiment.

Participants seated in front of a table; the stimulated arm was placed inside a specially

constructed cardboard box (Figure 1). Subjects were able to place their hand through a hole

cut in the front of the box; another hole on top of the box allowed the participant to see the

rubber hand; most of the back of the box was removed, allowing the experimenter to

stimulate both hands. In the inside of the box a small cardboard marker indicated where the

tip of the subject’s index finger should be placed. A cardboard cover was placed on top of the

box. When the cover was removed, the participants saw the rubber hand; when the cover was

placed over the box, the participants were not able to see the rubber hand. Ruled lines were

drawn in the cover to allow participants to indicate the position of their index.

Each session was composed of six blocks: three RHI condition, and three CONTROL

conditions. In the RHI condition, the rubber hand and the participant’s hand were stroked

simultaneously and in synchrony. In the CONTROL condition the rubber hand was not

stroked. The CONTROL and RHI conditions were undertaken in an alternate order, half of

the participants began with the CONTROL condition and the other half with the RHI

condition.

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Figure 1. Cardboard box. Participants could see the RH but not their real (left) hand.

Cardboard box roofed by the cover with the ruler on the top (right).

Each block began with the cover of the box closed, participants were asked to place their right

hand index finger on the cardboard marker inside the box.

Then, participants were asked to indicate the position of their right index finger by means of

the ruled lines marked on the top of the box. This measure was called pre-test proprioceptive

evaluation. Participants were requested not to move their hand throughout the duration of the

whole experimental block, particularly during the judgement of their index position.

Subsequently, participants were asked to attentively look at the rubber hand and the

temperature of their right hand was measured with an infrared thermometer. In the RHI

condition, the experimenter stroked the rubber and the participant’s right hand in a

synchronous manner. The stroking was delivered during two minutes using two identical

brushes. Subsequently, a modified peltier-type thermal stimulator was used to deliver the

warm and hot stimuli. One out of six temperatures (38° C, 40 °C, 42° C, 44° C, 46° C, 48° C)

was assigned for a given stimulus. The order of the temperatures was balanced between the

blocks. The stimulus was applied with the electrode on the dorsum of the right hand during 6

seconds. At the same time the rubber hand was “stimulated” with an electrode of the same

kind. After the thermal stimulation, participants were asked to evaluate the quality of the

stimulus: “warm” or “painful” and the intensity of the “warmness” and “painfulness” on a

subjective scale from 0 to 10, 0 being the minimum and 10 the maximum of either warmth or

pain. This procedure was repeated five more times in order to complete the six temperatures

(38° C, 40 °C, 42° C, 44° C, 46° C, 48° C), except now the duration of the stroking of the

remaining five trials lasted only one minute instead of two minutes in the first trial. When the

judgement of the last thermal stimulation was completed, the cover was placed over the box

and the participant was asked to indicate, using the ruled lines, the location of the felt pain.

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The offset of the cover was changed randomly and the participant was then asked to judge the

position of the hidden index finger, measure that we called post-test proprioceptive evaluation

(Figure 2). Once the judgement of location of pain and the right index were finished,

participants were allowed to take their right hand out of the box. Participants were asked to

freely describe their experience and then they were asked to evaluate, using a scale from 0 to

10, ten statements (Figure 4) designed to estimate the degree to which they felt ownership of

the rubber hand. In the cases where participants’ hand temperature decreased or increased

more than 4° C away from the temperature that they had at the beginning of the block, we

heated up or cooled down their hand with hot water or a cold compress. There was no

manipulation of the participant’s hand temperature within the blocks.

In the CONTROL condition the same procedure was performed with the single difference that

the rubber hand wasn’t stroked.

Figure 2. General procedure. The sequence shown in the figure represents one experimental block. Each

block was composed of 6 trials, one trial for each temperature. The order of the temperatures was

balanced between each block. Each participant went through three CONTROL and three RHI blocks.

Here we show an example of a RHI block. The CONTROL block was identical except that the RH was not

stroked by the brush nor stimulated by the electrode.

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2.3 Measurements

2.3.1 Proprioceptive drift

At the beginning of each block, participants were asked to indicate the position of their right

index finger using the ruler that was on the cover of box, this measure was called “pre-test

proprioceptive judgement”. The cover of box was larger than the box and numbers on the

ruled lines were placed in a random order so participants could not guide themselves by hints

of previous judgements. The number given by the participant was paired with a ruler that was

hidden to him that indicated the actual participant’s hand position. The position of the index

finger was counted as zero, positions toward the rubber hand counted as positive values and

positions away from the rubber hand as negative values. At the end of each block participants

were asked again to indicate the position of their right index finger in relation to the ruled

lines. The offset of the visible ruled lines was changed between each measurement. We called

this measure the “post-test proprioceptive judgement”. We called “proprioceptive drift” the

difference between the post-test and pre-test proprioceptive judgements.

2.3.2 Pain and heat intensity

After each heat stimulus, participants were asked if they felt pain. If the participants felt the

pain, they were asked to evaluate the pain on a scale from 0 to 10 (0 being the minimum and

10 the maximum) and the heat on a different scale from 0 to 10. If they did not feel pain, they

were asked to give a score 0 to pain and to evaluate the heat on a scale from 0 to 10.

Participants were allowed to build their own criteria of evaluation, but they were asked to

keep the same criteria throughout the all experiment.

2.3.3 Localisation of pain

After the last thermal stimulus of each block, the cover was placed over the box and

participants were asked to use the ruled lines to respond to the question “where did you feel

the pain?” The zero was taken at the place were the electrode was placed to give the heat

stimuli. Positions towards the rubber hand were taken as positive; positions away from the

rubber hand were taken as negative.

2.3.4 Questionnaire

In order to quantify and categorise the experience of participants relative to the RHI, at the

end of each block, they were asked to indicate their degree of agreement with ten statements

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(Figure 4). Participants were asked to utilize a scale from 0 to 10, 0 being not agree at all, and

10 totally agree. The questionnaire was based on Botvinick and Cohen (1998) and Kammers

et al. (2009) but modified according to the specific issues addressed in the present study.

During our pilot experiments we had noted that the phenomenon of the RHI was experienced

differently according to the kind of stimuli that was given and the corresponding sensation

(tactile, heat, pain). Therefore in addition to the standard statement (statement 5) used in

previous studies to measure the RHI (Botvinick and Cohen 1998, Ramachandran 2003, and

M.P.M Kammers et al. 2009), we included three new statements that indicated the

phenomenon of the RHI related to each sensation (statements 6, 7 and 8).

Question 4 has no relationship with the RHI. We included it as a control expecting no effect

of the RHI over the responses to this statement.

Statement 1 points to the qualitative aspect of the rubber hand incorporation. Statements 2 and

3 try to elucidate the nature of the possible modifications in body representation that underlies

the RHI. In the study of Schütz-Bosbach et al. (2009), they ask the question of “how and to

what extent does the presence of the RHI alter the representation of the bodily self?” They

give three alternatives a) incorporation of an extra limb as an additional part of the body, b)

incorporation of the rubber hand as a tool (as would be the case for a walking stick) c) the

rubber hand displaces or substitutes the participant’s hand. Longo et al (2008) had also

suggested that one component of the experience of the RHI was “loss of own hand”,

suggesting that the rubber hand may in some sense displace the participant’s actual hand,

which is in line with the last alternative. Statements 2 and 3 correspond to alternatives a) and

c) respectively. Statements 9 and10 relate to the spatial location of the sensations of heat and

pain.

3 Results

A total of 21 healthy participants recruited from volunteer’s pool of the Ambroise Paré

Hospital took part in the psychophysical experiment (12 females and 9 males; aged 20-53

years). Participants’ results were retained if the variation of their hand temperature within

each block did not surpass 2°C. Two subjects were excluded.

18

3.1 Occurrence of the RHI

First of all we assessed whether under our experimental conditions the RHI was induced. In

order to answer this question, we looked at two measures; a) the proprioceptive drift, which is

the difference between the proprioceptive post-test evaluation and the proprioceptive pre-test

evaluation and b) subjective judgement, which is the level of agreement with the statements

linked to the experience of ownership of the RH.

3.1.1 Proprioceptive Drift

The means and standard deviation of the proprioceptive drift for the CONTROL and the RHI

are 1.20 cm ±2.93 and 3.68±1.86 respectively. A two tailed paired t-test shows that the RHI

has the significant effect of displacing the proprioceptive drift further towards the RH than the

CONTROL condition, t (18) = 2,54 p<0,05. Figure 3 shows the means and standard errors of

the proprioceptive drift in the CONTROL and RHI conditions.

Figure 3. Means and standard error of proprioceptive drift measured in cm.

19

3.1.2 Questionnaire

A paired t-test shows significance difference between the CONTROL and RHI conditions of

statements 1 t (18)= 3.32, p<0,01; 2 t (18)= 3.36, p<0,01; 5 t (18)= 8.07, p<0,01; 6 t (18)=

8.46, p<0,01; 7 t (18)= 5.57, p<0,01; 8 t (18)= 5.32, p<0,01; 9 t (18)= 5.01, p<0,01 and 10 t

(18)= 4.74, p<0,01. The highest t values are for statements 6 and 5.

1 « I felt as if my right hand was turning

“rubbery” »

2 « I felt as if my right hand have been

disappeared »

3 « I felt as if I had two right hands»

4 « I felt as if my right hand had melted»

5 « I felt as if the rubber hand was my hand »

6 « I felt as if the rubber hand was my

hand when I felt the brushes»

7 « I felt as if the rubber hand was my

hand when I felt the heat»

8 « I felt as if the rubber hand was my

hand when I felt the pain»

9 « I felt the heat over the RH»

10 « I felt the pain over the RH»

Figure 4. Questionnaire consisting in ten statements that participants evaluate after each block (top).

Means of agreement scores for the CONTROL condition (blue) and RHI condition (red) (middle).

20

3.2 Perception of pain

In Figure 5 we can see the means and standard errors of the pain scores for the imposed

temperatures.

In order to determine the effect of the two conditions on pain intensity, we entered the pain

ratings into a repeated measures analysis of variance with three factors: condition (control v/s

RHI), temperature (38°C, 40°C, 42°C, 44°C, 46°C and 48°C) and trials (first second and

third), resulting in a 2 X 6 X 3 repeated measures ANOVA. The analysis shows a main effect

of condition (F (1,18) = 5.234, p<0.034), a main effect of temperature (F (2.4,43.7) =

130.789, p<0.001), and main effect of trials (F (1.7, 30.6) = 6.831, p<0.003). We obtain an

interaction between condition and temperature (F (4.4, 61.3) = 2.957, p<0.16) and between

trial and temperature (F (4.8, 86.5) = 2.207, p<0.019). No other interactions are found.

As we observed a greater difference of pain scores for the higher temperatures, we performed

a second analysis equivalent to that described above but considering only the temperatures

44°C, 46°C and 48°C. The analysis showed a significant main effect for condition (F (1,18) =

6.302, p=0.02), a main effect of temperature (F (2,36) = 75.259, p<0.01), and a main effect of

trials, (F (1.8, 32.5) = 6.758, p<0.04). No significant interaction between factors was found.

Figure 5. Means and standard errors by temperature of the perceived pain intensity.

21

3.3 Perception of warmth

We wanted to examine if warmth perception was also modified by the RHI. We looked at the

warmth ratings in the CONTROL and RHI conditions by temperature. We observed a general

lower difference between the CONTROL and RHI ratings of heat compared to the pain

ratings. In Figure 6 we plot the means and standard errors of the scores by temperature. To

determine the effect of the RHI in warmth perception we entered warmth ratings into a

repeated measures analysis of variance of three factors: condition (control v/s RHI),

temperature (38°C, 40°C, 42°C 44°C, 46°C and 48°C) and trials (first second and third),

resulting in a 2 X 6 X 3 repeated measures ANOVA. The analysis shows a main effect for

temperature (F (5, 39.44) = 191.86, p<0.01), but not significant main effect for condition or

trial was found. No interaction is found. We repeat the analysis for the higher temperatures as

we did for perception of pain intensity. A main effect of temperature was found (F (1.5,

25.58) = 86.48, p<0.01). No other main effects or interactions were found.

We performed a third analysis taking in account solely the trials were participant’s pain score

was zero aiming to examine warmth perception only and avoid the possible interaction with

pain sensation. In the three higher temperatures there were many missing values due at the

high “non zero” responses, resulting in a great diminution of the number of data entered to

calculate the means (Figure 7). Therefore we performed the analysis of variance over the

temperatures 38°C, 40°C and 42°C. The analysis showed no significant effect for condition

neither for trial, as expected, it showed an effect for temperature (F (2, 43.78) = 62.92,

p<0.01).

22

Figure 6. Means and standard errors of perceived warmth intensities scores by temperature including all

data.

Figure 7. Means and standard errors of intensities scores by temperature including data when pain scores

were equal 0 (top). Number of subject from whom the mean of warmth intensity was calculated (bottom).

23

3.4 Locus of pain.

We wanted to answer the question if, in the case of the occurrence of the RHI, the location

pain sensation induced by the electrode, would be shifted towards the RH. With the purpose

of answering this question we look at two measures: a) we compared the evaluation of the

location of the pain by the means of the ruler in the RHI and CONTROL conditions, and b)

we look at the agreement scores of statement 10, “I felt the pain over the rubber hand”.

The means and standard deviation of pain localisation are 5.9±3.9cm in the CONTROL

condition and 9,1±6.1 in the RHI condition. A paired t-test shows that participants locate their

pain closer to the rubber hand in the RHI condition that in the CONTROL condition, t (18) =

3.34, p< 0.01. The scores for statement 10 (figure 4) show a significant difference between

the CONTROL and RHI conditions, t (18)= 4.74, p<0,01. Figure 8 shows the means and

standard error of the location of pain in the CONTROL and RHI conditions.

Figure 8. Means and standard errors of pain localisation by means of the ruler (top).

24

4 Discussion

4.1 Proprioceptive drift

In this study we found a significant difference between the proprioceptive drifts in the

CONTROL and RHI conditions, showing a stronger bias towards the RH in the RHI

condition. This result confirms previous findings and suggests that the RHI was

effective in the RHI condition.

The correlation that would be expected between the proprioceptive drift and the

questionnaire statements related to the experience of ownership of the RH does not

appear clearly (Figure 9). This curious fact was also noticed by Holmes et al 2006 and

commented by Schutz-Bosbach.

We observed a baseline tendency to locate the right index finger more towards the

body midline, confirming previous findings (Longo et al 2008).

Figure 9. Correlation between statement 5 and proprioceptive drift for each experimental block.

25

4.2 Questionnaire

The questionnaire was intended to evaluate participants' general experience of

ownership of the rubber hand, and also to verify whether that experience differed

while feeling the sensations of warmth and pain.

Statements 2 and 3 were included to consider the nature of the possible modifications

in body representation produced by the RHI. They assess two of the three possibilities

(see procedure) proposed by S. Schütz-Bosbach et al (2009): Statement 2 referred to

the possibility that the rubber hand displaced or substituted the participant’s own hand

and statement 3 concerned the option of incorporation of the rubber hand as an extra

limb.

However it became clear that statement 2 (« I felt as if my right hand has disappeared

») did not in fact assess what had been aimed for: it is not the same to feel that our

hand has been replaced and to feel that it has disappeared. Some participants reported

that this question was confusing: they felt that their real hand had somehow been

taken over by the RH, but they did not feel that their hand had “disappeared”.

Consequently, the results of this statement did not allow us to answer the question that

we had intended.

Statement 3 (« I felt as if I had two right hands») assessed more directly the option

that the possible modifications in body representation caused by the RHI included the

experience of incorporating the rubber hand as an extra limb. The results indicate that

the RH did not have an effect on the participants' response to this statement. We are

inclined to think that it is not this kind of modification that is in place during the

illusion.

Statement 4 (« I felt as if my right hand had melted») is not related to the experience

of ownership of the RH; we included it as a control statement. The fact that we do not

observe an effect of the RHI on the response to this question confirms that the

experience of participants under the RHI condition involved a specific

phenomenology that was linked to the sense of body-ownership and not to qualitative

changes of their hand.

We included statement 5 « I felt as if the rubber hand was my hand » as a direct

indicator of the RHI. Several studies have used this statement to indicate the

experience of body-ownership of the RH. (Botvinick and Cohen 1998, Ramachandran

et al, 2003, Longo et al 2008, Kammers et al. 2009). In addition, as we described in

26

the procedures, we included three statements that referred to the experience of

ownership of the RH, in relation to the types of sensation that our stimuli evoked:

tactile evoked by stroking during induction, warmth and pain evoked by electrode

stimulation during the test phase (statements 6, 7 and 8). The mean score obtained for

the statement referring to tactile stroking stimulation (statement 6) was higher than

those referring to the warmth and painful sensations (statements 7 and 8). The

difference of scores between statements 7 and 8 was not significant. These findings

suggest that the RHI was experienced differently during the tactile stimulation during

induction compared to the stimulation causing the warmth and pain sensations during

the test phase. It is yet to be determined if this was due to the type of the sensations

themselves or to the way the stimulation was applied: the thermal stimulation did not

have the spatial and temporal richness of the tactile stimuli, it lasted 6 seconds and it

was always applied on the same part of the hand. Nevertheless, we observed a big

difference between the CONTROL and the RHI for statements 7 and 8 evaluating the

presence of the RHI while feeling warmth and pain. Importantly for this study, this

suggests that the experience of the RHI was maintained while participants felt pain.

Statements 9 and 10 refer to the localisation of the felt warmth and pain. They show

significantly higher values in the RHI compared to the CONTROL condition,

confirming that during the thermal stimulations the RHI was maintained. They

suggest that the RHI had the effect of relocating the sensations of warmth and pain

towards the RH.

The questionnaire allowed us to obtain a quantifiable measure of certain specific

aspects of participants’ experience in relation to the two different conditions that our

experiment included. Nonetheless, the questionnaire imposes the response to certain

specific statements and that fact impoverishes the potential of participants' accounts,

does not allow participants to give a detailed and precise account of their experience

neither does it allow the experimenter to uncover possible experiential aspects that the

questionnaire does not probe. In order to understand the phenomenological

counterpart of the RHI and of the RHI while feeling pain, it would be very interesting

to find a method that allows us to access the qualitative aspects and structure of

participants' experience. Longo et al 2008 attempted to tackle this issue by proposing

a psychometric approach to introspective reports of the RHI. They used a Likert scale

to rate participants' agreement with 27 statements relating to the subjective experience

of the illusion and used principal component analysis to investigate the latent

27

structure of participants' experience. They found that in fact, a systematic and

measurable structure emerge in the experience of the RHI. However their method is

still based on preset statements, and may not reveal certain interesting aspects of their

experience.

An interesting possibility would be to use first person methods to gather and analyse

the structure of this experience. Such methods provide interview and analysis

techniques. The interview is designed to guide the process of introspection aiming to

seize the structure of a given experience (Petitmengin 2006, Vermersch 2003). The

further analysis of the interview allows to identify the components, which characterise

an individual experience, as well as to assess the similarities between individuals.

This could be especially relevant to the domain of pain research. As Price (2002)

remarks “Understanding the nature of pain at least partly depends on recognizing its

inherent first person epistemology and on using a first person experiential and third

person experimental approach to study it. This approach may help to understand some

of the neural mechanisms of pain and consciousness by integrating experiential

phenomenological methods with those of neuroscience”.

4.3 Perception of pain intensity

The results of this study confirm our hypothesis: the RHI decreases participants’

estimation of pain. To our knowledge this is the first evidence of such a phenomenon.

The fact that participants' estimation of pain intensity is significantly affected just for

higher temperatures, confirms the specificity of this effect for pain and not for warmth

sensation. Even if participants assigned non-zero pain scores for temperatures ranging

between 38°C and 44°C, it should be mentioned that it seemed to me that only at

46°C and 48°C did the subjects show a visible behaviour of feeling pain and scored

the pain sensation higher.

The main result of our study is in line with the hypothesis that the RHI implies a

“replacement” of the real hand by the RH. As mentioned in the introduction, Moseley

suggests that this phenomenon of replacement explains the decrease of participants’

hand temperature and the decrease in their temporal discrimination of tactile stimuli

while experiencing the RHI. In addition, in the study of Longo et al (2008), the

28

authors find that one of four major components of the subjective experience during

the RHI is the “loss of own hand”.

It is not easy to conjecture what mechanisms explain the relation between the RHI

and pain perception; pain perception is a multidimensional experience, pain-

modulating mechanisms are manifold and complex.

We can imagine two possible mechanisms that would explain the relationship

between the RHI and the decrease in the perceived pain. One possibility is that multi-

sensorial correlation itself relates to pain modulation processes. The second option is

that the experience of body-ownership itself, meaning having the feeling that the RH

belongs to our body and “replaces” our real hand, relates to pain modulation

processes. We could imagine that having the impression of going away from a source

of pain, having the feeling that one’s hand is actually somewhere else and not next to

the source of pain: would induce a decrease of intensity of the perceived sensation.

A possible way to test the first, more sensory, hypothesis is to use one of the control

conditions from Moseley et al 2008. They proved that it was not the mere multi-

sensory correlation that caused the temperature decrease of the real hidden hand, by

measuring participants' hand temperature while they were looking at their real hand

when stroked: in this condition, although a multi-sensory correlation was present, no

temperature decrease was found.

To test the second hypothesis, we could include a control condition where we

measure the estimation of pain of a body part that is not involved in the illusion. If the

decrease of pain estimation found in our experiment is due to the experience of body-

ownership of the rubber hand, we should not find an effect of the RHI on pain

estimation in that body part: the effect should be specific to the real hand involved in

the ownership transfer.

4.4 Perception of warmth intensity

Our result shows that the intensity of warmth sensation is not significantly modified

by the RHI. Nevertheless the similarity of the sets of curves for pain and warmth

(figure 10) makes this conclusion rather tentative. It might be that we don't find a

significant difference for lack of statistical power.

29

Another possibility is that the similarity of the curves is caused by the fact that in our

experimental protocol, participants gave scores for pain and warmth in every trial.

This could create a bias in warmth judgements as a result of the pain judgement

influence. To test this second possibility we removed the data belonging to the cases

where participants have non-zero scores for pain. As a result only the three lower

temperatures provided sufficient data to be statistically analysed. Thus we are left

with the tentative conclusion that only the pain system is affected by the RHI.

Figure 10. Means and standard errors of the perceived intensity for pain and warmth sensations

in the RHI and CONTROL condition.

Why could the RHI have an effect on pain perception and not on the perception of

warmth? How could we explain this difference?

Despite warmth and pain sensation sharing several anatomical substrates, there is

evidence that supports their anatomical and physiological specificity at different

levels: at the level of receptors (Han et al 1998, Iannetti et al 2003), conduction

velocities of afferent fibers (Iannetti et al 2003), and brain activation patterns (Davis

1998). Davis used high-resolution fMRI to study the involvement of discrete thalamic

and cortical sites during both painful cooling and heating of the skin of individual

subjects. He found that different patterns of thalamic and cortical activation could be

evoked during identical stimulation of different individuals. He argues that different

30

pain and temperature-related activations may help us to understand the complexities

of these sensory experiences and differences in human reports of pain and

temperature.

It could be that these anatomical, physiological and subjective differences between

warmth and pain sensation explain the difference that we observe between warmth

and pain sensations and their relationship with the sense of body-ownership.

If we understand pain sensation as a homeostatic indicator of the body/organism

integrity (Craig 2002, Melzack 1990), we could suppose that the experience of

owning a body is linked to perception of harm to that body. If we design an

experiment where we alter the sense of owning a part of this body, inducing

ownership for an external object and somehow a “disownership” of our own body

part, we could suppose that the perception of harm on this body part would be

modified. As warmth sensation might not be linked to a “guarding” self-integrity

mechanism, it might not be affected when altering the sense of ownership of our

body.

The possible ambiguity created by the fact that in our experimental protocol

participants gave scores for pain and warmth in every trial, could be solved in a future

protocol, by creating instead of two scales: one scale from 0 to 10 with five indicating

the beginning of pain sensation. In this way we could observe the effect of the RHI

along a continuum from warmth sensation to pain sensation.

4.5 Localisation of pain

Our results show that participants perceived that pain position was shifted towards the

rubber hand. The shift in the RHI condition was greater than in the CONTROL

condition and this difference was significant. These results mirror those obtained for

the proprioceptive drift: a baseline drift towards the rubber hand that is significantly

increased in the RHI condition. This result suggests that the illusion continued while

participants felt the pain, and confirms our hypothesis: participants experiencing the

RHI displace their felt pain towards the RH.

However, our results show greater drift towards the rubber hand in pain localisation

than in proprioceptive judgement and also a greater variance (see results and figures 2

and 7). This could be explained if we consider the following factors:

31

1. To determine the proprioceptive drift we asked participants: where is your index

finger in relation to a ruler? On the other hand, in order to judge pain localisation

participants were asked: where did you feel the pain in relation to the ruler? This

temporal difference between the two tasks adds an extra aspect related to

remembering; it could have an effect on the variance of the responses. This is

supported by participants’ reports. Some participants expressed that they felt

uncertain about localisation because they had to remember and recall the position of

the felt pain.

2. In the case of the proprioceptive drift, Subjects' index finger was always resting on

a little piece of cardboard, which, at the moment of judging about location, could

serve as a fairly precise indicator. On the contrary, when localising pain, the electrode

was no longer in contact with their hand; there was no reference that could be used.

This may also have affected the variance of participants’ responses.

3. In addition, the source corresponding to the proprioceptive sensation was smaller,

since it was constituted by the index finger, which had limited width. On the other

hand, the electrode was (how much?) wide, and covered a larger area of the hand.

This could in part explain the difference between pain position and proprioceptive

drift and their variability.

A protocol that considers these factors could allow us to uncover the cause of the

greater shift in pain localisation compared to proprioceptive drift.

32

4.6 Methodological Remarks

The method of thermal stimulation was decided in collaboration with Dr. Didier

Bouhassira, a specialist of pain treatment. In the process of designing the protocol of

the experiment we studied three possible methods to measure pain: a) Thermal

stimulation via Peltier electrode, b) Mechanical stimulation via Von Frey Filaments,

and c) Electrical stimulation measuring the RIII reflex.

Mechanical stimulation was discarded because the intensity of the mechanical

sensation is determined by the diameters of the filaments. The difference in diameters

could bias participant response: on one side, by confounding the filament diameter

with the elicited pain, and on the other side, the different diameters could serve as

indicator of stimulus identity. In addition, the sensation elicited by such filaments is

painful for patients suffering from pain-related pathologies, but is quite mild for

healthy subjects.

The RIII method seemed quite attractive to us since it gives an objective measure, the

electrical activity of the biceps femoris muscle, which correlates with participants’

perceived pain. Nonetheless, we have chose the thermal stimulation method as we

were interested in comparing the behaviour of warmth and pain perception in relation

to the RHI; this method is reliable, has been widely used in previous studies of pain

perception and corresponded to equipment availability constraint.

In choosing this method, we were particularly concerned with the measurement

aspect. Considering the results of Moseley showing that the RHI lowers hand

temperature, we wondered whether this decrease in skin temperature might introduce

a bias in pain perception. It could be that the reduction in skin temperature increases

or lowers the sensitivity to pain. This might mask or enhance any possible result,

complicating the analysis and interpretation of the data. However, the decreases of

temperature found by Moseley are quite small, ranging around 0.25 °C, on the other

hand, the temperature discrimination capabilities of normal individuals are around

2°C. We considered that this difference between scales would avoid possible

interactions. In addition, there is evidence that suggests that perceived intensity of

peripheral thermal stimuli is independent of internal body temperature (Mower 1976).

To control the possible effect of hand temperature in pain perception, we recorded

participants’ hand temperature between each trial and we controlled for it to be stable

33

throughout the experiment. We observed no relationship between participants’ hand

temperature and pain perception.

5 Conclusion

Our purpose in this study was to investigate the effect of changing perceived body-

ownership on the intensity and localisation of thermal pain sensations. By using the

protocol of the Rubber Hand Illusion and a method of administering thermal pain, we

showed that participants’ estimation of pain decreases on their own hand when they

feel ownership of a fake rubber hand. Pain sensation is also localised closer to the

rubber hand. On the other hand, no significant effect of body-ownership is observed

on estimation of warmth sensations.

These results suggest a specific relationship between the sense of body-ownership and

pain perception. From a neurophysiological point of view, the results are in line with

evidence that shows that the experience of body-ownership affects physiological

parameters associated with the homeostatic equilibrium of the body. This suggests the

existence of interactions between conscious processes, such as the experience of

ownership, and unconscious processes, such as the modulation of pain perception.

However, the mechanism that could explain the relationship between the experience

of body-ownership and the modulation of pain perception is still unclear. Also unclear

is the mechanism that explains the difference of the effect of the RHI on warmth and

pain sensations.

It would be interesting to investigate if the effect found on thermal pain extends also

to other kinds of pain: pain elicited by pressure or pain elicited by electric stimulation

for example. If this turned out to be true, this opens the road to possible uses of the

rubber hand illusion in clinical pain treatment.

34

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