Journal of Psychosomatic Research 71 (2011) 240–244

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Journal of Psychosomatic Research

Medically unexplained symptom reports are associated with a decreased response tothe rubber hand illusion

Eleanor Miles ⁎, Ellen Poliakoff, Richard J. BrownSchool of Psychological Sciences, University of Manchester, Manchester, M13 9PL, United Kingdom

⁎ Corresponding author at: Department of PsychoSheffield, S10 2TP, United Kingdom. Tel.: +44 114 222

E-mail address: e.miles@sheffield.ac.uk (E. Miles).

0022-3999/$ – see front matter © 2011 Elsevier Inc. Aldoi:10.1016/j.jpsychores.2011.04.002

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 30 November 2010Received in revised form 9 March 2011Accepted 5 April 2011

Keywords:Medically unexplained symptomsRubber hand illusionBody perception

Objective: Medically unexplained symptoms (MUS) have been hypothesized to result from a distortion inperception, whereby top-down factors influence the process of body representation. Perceptual illusionsprovide a novel method of investigating this hypothesis. This study aimed to investigate whether self-reported unexplained symptoms are associated with altered experience of the rubber hand illusion (RHI).Methods: A non-clinical MUS group with high scores on the Somatoform Dissociation Questionnaire (SDQ),and a control group with low scores on this scale, were exposed to the RHI. Illusion experience was measuredby self-reports and by proprioceptive alteration.Results: After controlling for somatosensory amplification and trait anxiety, the low-SDQ group responded

significantly more strongly to the RHI on both questionnaire and proprioceptive measures of the illusion. Incontrast, the high-SDQ group scored significantly higher on the Perceptual Aberrations Scale, a measure ofbodily distortions in daily life.Conclusion: These findings support the proposed link between MUS and disturbances in body representation,and suggest that a decreased reliance on current sensory inputs may contribute to symptom experience insusceptible individuals.

© 2011 Elsevier Inc. All rights reserved.

Introduction

Medically unexplained symptoms (MUS) are potentially disablingphysical symptoms that arise in the absence of medical pathology.They have been estimated to account for at least a third of thesymptoms seen in primary care [1]. In general, explanations of MUShave focused on the interaction between cognitive, behavioural andphysiological factors, which are hypothesized to form a self-perpetuating vicious circle of symptom experience [2]. Most modelsassume that MUS arise when relatively benign physical events in thebody are misinterpreted as evidence of serious illness [3–5]. Althoughsuch models can account for many MUS, they are less able to explainsymptoms such as unexplained blindness and paralysis, where moreprofound perceptual distortions seem to be in evidence. One modelthat does address these symptoms explicitly is that of Brown [6],which identifies them as distortions in somatic awareness arisingfrom overactive symptom representations in memory. In this view,MUS reflect the fact that bodily experience is an interpretation ratherthan a direct representation of sensory input, subject to ‘top-down’ aswell as ‘bottom-up’ factors. In this paper, we aim to experimentallytest this hypothesis through the use of a perceptual illusion.

logy, University of Sheffield,6623.

l rights reserved.

There are many perceptual illusions that illustrate how bodilyexperience can be distorted by relatively simple manipulations.Vibration of the biceps tendon, for example, can give rise to aperceived extension of the elbow joint and even illusory elongation ofthe nose if it is held during the procedure (the so-called ‘Pinocchio’illusion) [7]. In the rubber hand illusion (RHI), stroking a rubber handin synchrony with the participant's hidden hand can induce thesensation that the fake hand has become part of their body [8]. Theillusion does not occur when stimulation is asynchronous, demon-strating a necessary bottom-up condition for this effect; neither doesit occur when the rubber hand is in an implausible position or whenan object other than a hand is used [9], demonstrating the influence oftop-down factors. There are often large individual differences in theexperience of such illusions [10,11], and evidence suggests that someof this variation might reflect a trait-like dimension associated withbody experience more generally. For example, one study [10] found acorrelation between the strength of illusory arm extension (as in the‘Pinocchio’ illusion) and scores on the Perceptual Aberration Scale(PAS) [12], which asks about other experiences of body distortion ineveryday life.

If responsiveness to bodily illusions is a reflection of individualdifferences in everyday bodily experience, these illusions may providea method by which body distortion in clinical populations can beassessed, with the advantage of being more objective than retrospec-tive self-reports. Indeed, experience of the RHI has been found tocorrelate positively with self-reported bulimic symptoms in

Table 1Demographic information and dependent measures for the two SDQ groups. Allnumbers represent means and standard deviations unless otherwise specified

Low SDQgroup

High SDQgroup

(n=20) (n=20)

Age 19.8 (1.6) 19.6 (1.5)Percent female (n) 70.0 (14) 85.0 (17)Clinical questionnaires

Somatoform Dissociation Questionnaire 20.3 (0.5) 36.0 (9.2)State-Trait Anxiety Scale (state subscale) 32.3 (8.0) 40.5 (9.4)State-Trait Anxiety Scale (trait subscale) 33.8 (10.8) 43.9 (10.4)Somatosensory Amplification Scale 25.8 (7.8) 31.5 (5.4)Perceptual Aberration Scale 3.3 (2.9) 10.8 (4.4)

Perceptual drift towards the rubber hand (mm)Experimental condition 40.0 (31.6) 22.6 (28.6)Control condition −9.6 (28.0) 5.8 (29.4)

Reported experience of sensations relevant to illusionExperimental condition 1.5 (1.4) 0.9 (1.7)Control condition −0.5 (1.4) −1.0 (1.6)

Reported experience of sensations not relevant to illusionExperimental condition −0.8 (1.2) −0.3 (1.1)Control condition −1.1 (1.3) −0.7 (1.3)

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university students [13], while patients with anorexia nervosa appearless susceptible to the illusion than controls [14]. This experimentalevidence both implicates perceptual processes in these disorders, andsuggests that these body distortions may be specific to differentconditions. In order to study the role of such processes in unexplainedsymptom reporting, we compared susceptibility to the RHI in a groupwith high scores on a measure of self-reported MUS (SomatoformDissociation Questionnaire; SDQ-20) [15] and a control group withlow scores on the measure, using both self-report and proprioceptivemeasures of the illusion. If, as suggested by Brown [6], the tendency toexperience MUS is related to a disproportionate reliance on top-downfactors during the process of body representation, high MUS reportersmay be less susceptible to the illusion, which ultimately relies on‘tricking’ this process with discrepant sensory input. If MUSexperience represents a more general tendency to experienceperceptual distortion, however, high MUS reporters may be moreprone to experience the RHI than controls. In this case, theymight alsoexperience distortion during the control condition (where the illusiondoes not typically arise), or report experiencing sensations unrelatedto the illusion.

Method

Participants

Undergraduate students at the University of Manchester weregrouped according to self-reported MUS experience on the SDQ-20[15]. The SDQ-20 asks participants to rate their experience of 20physical symptoms (e.g., "My body, or a part of it, feels numb”; ‘I havean attack that resembles an epileptic seizure’) during the past year, ona scale from 1 (this applies to me not at all) to 5 (this applies to meextremely), and to state whether the symptom has been given anexplanation by a physician. In order to compute a score representingonly unexplained symptom experience, we rescored symptoms withan identified cause to 1 before adding up the total score. Using thismethod, the range of potential scores (20 to 100) remained the same,with the minimum score of 20 representing a participant whoreported zero unexplained symptoms, and with symptoms onlycontributing towards an increased score if theywere undiagnosed. Forthe MUS group, we selected twenty participants with relatively highscores compared to population norms (≥28) [16] and comparable tothose reported in somatoform disorder patients [17]. For the controlgroup, we selected twenty individuals with low scores on the scale(20 or 21; see Table 1 for demographics). Sixteen participants withscores between 22 and 27 also took part, but were excluded.

Measures

Clinical questionnaires

State-Trait Anxiety Inventory. As negative affectivity is known to co-vary with MUS experience [18,19], the trait subscale of the State-TraitAnxiety Inventory (STAI-T) [20] was used to assess and control forbetween-group differences. The STAI-T asks participants to rate 20statements (e.g., ‘I feel nervous’; ‘I amworried’) according to how theygenerally feel, on a scale of 1 (not at all) to 4 (very much so), giving atotal score of between 20 and 80. The STAI-T has good reliability andvalidity [20].

Somatosensory Amplification Scale. The Somatosensory AmplificationScale (SSAS) [21] asks participants to rate 10 statements (e.g., ‘I hateto be too hot or cold’; ‘Sudden loud noises really bother me’) on a scaleof 1 (not at all true) to 5 (extremely true), giving a total score ofbetween 10 and 50. This measure is thought to assess the tendency tofind sensations unpleasant and disturbing [3], and has been shown torelate to symptom experience [22–24]. Given that the rubber hand

illusion has an affective component, whereby ratings of pleasantnessof touch and enjoyment are related to illusion experience [25], thismeasure was included as a covariate in order to ensure thatdifferences between groups were not attributable to differences inaffective response to the tactile sensations. The SSAS has goodpsychometric properties [21].

Perceptual Aberration Scale. Following previous research [10], we gaveparticipants the Perceptual Aberration Scale (PAS), [12] whichmeasures the frequency of other bodily alterations in everyday life,to enable us to relate symptom experience to individual differences inself-reported bodily experience more generally. This scale asksparticipants to rate 35 statements (e.g., ‘I have had the momentaryfeeling that my body has become misshapen’; ‘I have sometimes feltthat some part of my body no longer belongs to me’) as either true orfalse, giving a total score of between 0 and 35. The PAS has beenshown to demonstrate good internal consistency and moderatestability [26,27].

Rubber hand illusion

The experiment was set up and conducted in a similar manner toother RHI studies (Fig. 1). Participants wore an orange rubber glove ontheir left hand throughout the experiment, and rested this hand on atable beneath a black box. Their right handwas placed underneath thetable. A black cape extending from their neck to the box obscuredtheir view of their arms throughout the experiment. During theillusion, a stuffed left-handed rubber glove was placed on the box,with its index finger 20 cm to the right of the participant's indexfinger, approximately 7 cm above their real hand. Two paintbrusheswere used to stroke the participant's and rubber hand's index fingersduring the illusion. Stimulation was given between the first andsecond knuckles of the left hand, in an irregular pattern consisting oflong and short strokes. In the experimental condition the paintbrushstrokes on the two hands were given at the same time (synchronous),and in the control condition the rhythm of the strokeswas varied in anirregular fashion between the two hands (asynchronous). In theexperimental condition, the paintbrushes were attached with a hingeto ensure that stimulation on the real hand and rubber hand wassynchronised. A black screen blocked the participant's view of thepaintbrush touching their hand.

The first dependent measure of the illusion was perceptual drifttowards the hand, that is, the degree towhich the participant's perceived

1 For both types of dependent measure, we also performed ANOVAs without covariates.For the perceptual drift measure, there was a significant interaction between condition andSDQ group [F(1,38)=6.30, P=.016]; differences between the high and low SDQ groupsremained non-significant in the control condition [F(1,38)=2.83, P=.101] and becamenon-significant in the experimental condition [F(1,38)=2.94, P=.094]. For the ques-tionnairemeasure, therewas a significant interactionbetweenquestion typeandSDQgroup[F(1,38)=5.33, P=.026]; differences between the high and lowSDQgroups remainednon-significant for the control questions [F(1,38)=1.87, P=.179] and became non-significantfor the relevantquestions [F(1,38)=1.37, P=.249]. Given that the lowandhigh SDQgroupsdiffered significantly in their scores on the two covariates, both of which relate to affectiveexperience, we suggest this change in the pattern of results may be because the covariatescorrect for these group differences in trait affective experience and thus reduce the noiseassociated with differences in the affective components of the illusion [25].

B

A

PaintbrushScreen used toobscure view

Rubber hand

Real hand

Fig. 1. The experimental set-up for the illusion. Fig. 1A shows the set-up during theproprioceptive measurements; Fig. 1B shows the set-up during the illusion.

242 E. Miles et al. / Journal of Psychosomatic Research 71 (2011) 240–244

hand location changed during the illusion. To measure this, a larger boxwas placed on top of the apparatus and a sliding arrowwasmoved acrossthe box until the participant indicated that the arrow was above wherethey felt their index finger to be. A scale viewable only to theexperimenter was used to measure the difference between theparticipant's perceived and actual hand position in millimetres. Thismeasurementwas performed immediately before and immediately afterthe illusion. Perceptual drift towards the hand was calculated as thedifference between these two measurements, with a positive valueindicating that perceived hand location had shifted towards the rubberhandduring the illusion, andanegative value indicatinga shift away fromthe rubber hand [9]. As individual differences in proprioceptive abilitymight influence this measure, two additional baseline measurements ofaccuracywere also takenbefore eachpresentationof the illusion. For eachof these measurements, the participant's hand was moved to twodifferent locations, and the sameprocedurewas followed. Themagnitudeof error across all of the pre-illusion estimates was used to calculate aperceptual accuracy measure.

The second measure of the illusion was a short questionnaire usedin the original RHI study [8], where participants were asked to agreeor disagree with various statements about their experience during theillusion. This questionnaire contains three questions relevant to theexperience of the illusion, such as ‘I felt as if the rubber hand were myhand’, and six questions on irrelevant sensations, such as ‘it felt as ifmy real hand were turning rubbery’, designed to ensure that positiveresponses to the relevant questions are not merely a result ofsuggestibility or demand characteristics.

Procedure

Participants were asked to keep their left hand and fingers stillthroughout the experiment. On the basis of pilot data, participantswere first given 30 s of synchronous and 30 s of asynchronousstimulation (in the same order as the experimental conditions), toremove any confounds resulting from the order in which the twoconditions were experienced. The experimental and control condi-tions were then presented in a counterbalanced order. Each period ofstimulation lasted for 5 min, during which participants were told tokeep looking at the rubber hand, while the index finger of the rubberhand and the index finger of the participant's left hand were strokedwith paintbrushes at a rate of approximately 1.5 Hz. After 5 min, theproprioceptive measures were taken and the questionnaire measureof the illusion was completed. Following every period of stimulation,participants removed their hand from the box and touched each of itsfar corners in order to ‘reset’ the proprioceptive representation oftheir hand, and thus control for the proprioceptive changes that occurover time when a body part is occluded [28]. After the secondcondition, the SDQ, SSAS, PAS, STAI-T and STAI-S were completed in arandom order.

Results

Clinical questionnaire data

A between-subjects MANOVA revealed a significant effect of SDQ group for allquestionnaires [Pb .009]. As previous research would predict, the high SDQ group scoredsignificantlyhigher on trait anxiety andsomatosensory amplification [t(38)N2.81, Pb .008].The high SDQ group also had significantly higher scores on the PAS [t(38)=6.76, Pb .001].

Perceptual drift measure of the RHI

Mean perceptual error was 24.6 mm (S.D.=16.0), and did not differ between the firstand second measurement [t(39)=.20, P=.84], or between the SDQ groups [t(38)=1.24,P=.22].

A mixed-model ANCOVA1 with the within-subjects factor of condition (exper-imental, control) and the between-subjects factor of SDQ group (high, low) wasperformed on perceptual drift scores, controlling for STAI-T and SSAS scores (correctedfor repeated measures by mean-centring; see [29] for rationale). Order of presentation(control condition first, experimental condition first) was added to this analysis and didnot interact significantly with condition [F(1,34)=2.16, P=.15], so was not consideredfurther. A significant effect of condition was found [F(1,36)=26.15, Pb .001], indicatingthat overall, participants' estimates of hand location shifted further towards the rubberhand during the experimental (i.e. synchronous) condition. The interaction betweencondition and SDQ group was also significant [F(1,36)=4.18, P=.048]. To follow upthis interaction, two univariate ANCOVAs were performed (with SDQ group as abetween-subjects factor and STAI-T and SSAS scores as covariates), first on perceptualdrift in the experimental condition, and then on perceptual drift in the controlcondition. We found that the low SDQ group exhibited higher perceptual drift in theexperimental condition as compared to the high SDQ group [F(1,36)=5.62, P=.023;Fig. 1A]; in other words, the low SDQ group experienced a greater shift in theirperceived hand position during the experimental condition. Perceptual drift in thecontrol condition was not significantly different between the two groups [F(1,36]=.20,P=.66] (Fig. 2).

Questionnaire measure of the RHI

Initial analyses confirmed that order of presentation had no effect on our findings,and that questionnaire scores were higher overall in the experimental condition (dataavailable from the authors). Therefore, a mixed-model ANCOVAwas performed only on

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data from the experimental condition, controlling for STAI-T and SSAS scores (correctedfor repeated measures as above), with question type (relevant, irrelevant) as arepeated-measures factor and SDQ group (high, low) as a between-subjects factor. Asignificant effect of question type indicated that the items assessing relevant illusorysensations were endorsed more highly than those assessing irrelevant sensations in theexperimental condition [F(1,36)=64.34, Pb .001]. This effect interacted significantlywith SDQ group [F(1,36)=10.87, P=.002]. To follow up this interaction, twounivariate ANCOVAs were performed (with SDQ group as a between-subjects factorand STAI-T and SSAS scores as covariates), first on experience of relevant sensations,and then on experience of control sensations. We found that the low SDQ group agreedmore with the relevant questions than the high SDQ group [F(1,36)=6.31, P=.017]; inother words, the low SDQ group reported stronger illusion experience than the highSDQ group. There was no significant difference in how the two groups responded to thecontrol questions [F(1,36)=.361, P=.361].

Discussion

When controlling for trait anxiety and somatosensory amplifica-tion, the low-SDQ group had significantly higher rubber hand illusionexperience than the high-SDQ group, as measured both by self-reported experience and by proprioceptive alteration. Thus, wedemonstrate that the tendency to report MUS is associated withdecreased experience of this bodily illusion. Why might this be?Experience of the RHI is thought to arise because the cognitive systemtends to interpret sensory inputs that are correlated in time and spaceas originating from the same object [8]. As discussed earlier, theunified percept that results from this visuo-tactile integration (i.e. oftouch coming from the rubber hand) is in part a “bottom-up”phenomenon – that is, driven by incoming sensory information [30]–but illusion experience is also influenced by “top-down” factorswithin the individual, such as expectations, beliefs, andmotivations. Itis likely that individual differences in illusory experience are partlydetermined by the relative weight that is placed on these top-down

Fig. 2. A perceptual drift (mm) towards the rubber hand, separated by condition(A), and questionnaire scores for the experimental condition only (B) in the high andlow SDQ groups. Bars represent estimated marginal means (±S.E.M.) when controllingfor somatosensory amplification and trait anxiety. In (A), zero indicates no change infelt hand position during the illusion, positive scores indicate drift towards the hand,and negative scores indicate drift away from the hand. In (B), positive scores indicateagreement with the questions, and negative scores indicate disagreement.

and bottom-up factors when bodily experience is generated by thecognitive system. One explanation for the current findings is thatduring the experimental condition, the low-SDQ group were moreinfluenced by external sensory input than the high-SDQ group, whoseexperience was more reliant on top-down knowledge about the taskand their body.

Such an account would explain why the high-SDQ group had anexperience that was more in keeping with “actual” sensory realitythan the low-SDQ group in the current study. In other situations,however, an increased reliance on top-down information may renderthe individual susceptible to perceptions that do not correspond withcurrent sensory input. It is normal, to some extent, for our perceptionto be influenced by top-down factors such as beliefs and previousexperience; this effect has been observed across a wide variety ofphenomena, from delusions to the placebo effect. If this process wereto be weighted more heavily toward top-down information, as wesuggest it is in people with MUS, this might result in perceptions thatare more consistent with cognitive information than with actualsensory input (as proposed by Brown [6]). This could explain why thehigh-SDQ group had higher scores on the PAS, whichmeasures amoregeneral tendency to experience perceptual distortions not consistentwith sensory reality.

This interpretation is consistent with recent research [31] showingthat high SDQ scorers reportmore touch sensations in the absence of atactile stimulus during the Somatic Signal Detection Task (SSDT; [32]),which also indicates distorted perceptual decision-making. While theSSDT is very different to the RHI, the findings of both studies suggest areliance on top-down information over sensory input in somaticperception. More generally, our findings suggest that the processesunderlying body representation may play an important role in theexperience of MUS, and thus that many models of this phenomenonare missing an important component. As our findings are similar tothose observed in patients with anorexia [14], the same processesmay well be involved in that disorder. Indeed, it has been suggestedthat instability in body perception may be an important negativepredictor of clinical outcomes in anorexia [33], consistent with ourhypotheses.

One limitation of the study is that all of our measures of bodilyexperience relied on subjective report. Our assessment of perceptualdrift was intended to provide a measure that was more objective andless subject to demand characteristics than simple self-report, butother methods of behaviourally assessing RHI experience may be adirection for future research in this area. Reaction times to tactilestimuli on the participant's real hand can be influenced by experienceof the RHI [34], for example, and may be a more objective measure.We also used a self-report measure of MUS and an analogue sample;although the average SDQ-20 score of ourMUS groupwas comparableto that found in somatoform disorder patients [17], it is unclearwhether the findings would generalise to confirmed and clinicallysignificant cases of MUS. Further research addressing this questionmay lead to a better understanding of the processes underlyingclinical MUS, and inform possible treatment avenues.

In conclusion, we have shown that self-reported unexplainedsymptom experience is associatedwith increased experience of bodilydistortions in everyday life, but with decreased experience of anexperimentally induced bodily distortion. We suggest that thispattern of results can be explained through a stronger reliance upontop down information, such as beliefs and knowledge, in the processof body representation in people with MUS. A possible implication ofthis idea is that, contrary to the typical assumption that attentiontowards the body is a pathogenic factor in MUS, training people to paymore attention to sensory inputs (e.g., using techniques such asbiofeedback) may actually help to alleviate unexplained symptomexperience. Another implication is that people withMUSmay bemoresusceptible to illusions in which beliefs about the body, rather thansensory inputs, are manipulated. Given that treatment options for

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MUS are relatively few and in many cases are ineffective [35], furtherstudies investigating the nature of distortions in body representationin MUS may have a valuable role to play in the development ofeffective treatments.

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