The bodily self: Insights from clinical and experimental research

The

bodily

self:

Insights

from

clinical

and

experimental

research

Sebastian

Dieguez

a

,

Christophe

Lopez

b,

*

a

LaboratoryforCognitiveandNeurologicalSciences,Unite´ deNeurologie,De´partementdeMe´decine,Universite´ deFribourg,Fribourg,Switzerland

b

LaboratoiredeNeurosciencesInte´grativesetAdaptatives,UMR7260,CNRS,Aix-MarseilleUniversite´,CentreSaint-Charles,FR3C–CaseB,3,place

Victor-Hugo,13331Marseillecedex03,France

1. Introduction

In thisreviewarticle,wepresentanoverviewofclinicaland experimental approaches tothe study of body representations (also called body schema, corporeal awareness or bodily self), providinginsightintotheirneuralandcognitivebases.

Thefirstsectionprovidesanoverviewofunilateraland non-lateralizedneurologicaldisordersofthebodilyself,oftenfollowing damage to the right cerebral hemisphere. Here, we address hemiasomatognosia,feelingsofdisappearanceandtransformation ofbodyparts,supernumeraryphantomlimbs,somatoparaphrenia andautoscopicphenomenainvolvingtheentirebody.Disordersof thebodily selfhave been difficultto classifysystematically, as notedhistorically bypioneering researchers[1–5]and in more recentworks[6–8].Indeed,thebodilyselfisinitselfaconceptually complextopic,becauseitsexperienceis inherentlymultimodal, subjective,and global. Thebodily self arisesfromthe dynamic integrationof bodilyand environmental visual,tactile, proprio-ceptive,vestibular,auditory,olfactory,visceraland motor infor-mation[9].Unlikeotherworldlyobjects,thebodyisthesourceof itsownperception,asubjectandanobjectatthesametime.The

humanbraincomputesbodilyinformationviadifferentmapsand networks,notablyareasoftactile,proprioceptive,vestibularand interoceptiveprojectionintheprimarysomatosensory,aswellas unimodal and heteromodal cortices, providing an unified and global representation of the lived body, which allows for experiencing it as a unique self and agent. To these sensory components,recentresearchhasaddedspatialandsocialfactors involvedinself-otherbodilyinteractions,andego-andaltercentric perspectivesonbodilyandactionperceptions[10,11].

Verbalandhigher-levelcognitiveaspectsofbodilyknowledge aresometimessubsumedundertheconceptof‘‘bodyimage,’’said tobeaconsciousandabstractrepresentationofthebody,involving for instance the naming of body parts and general knowledge about human bodies. This classical distinction with ‘‘body schema,’’ which involves situated, directly experienced, uncon-scious,andnon-verbalaspectsofthebodilyselfhasbeenwidely discussedelsewhere (e.g.,[12,13]), andweadopttheviewhere thatasomatognosiapersepertainstothebody schemadomain

[14,15].Assuch,wethinkthatdisorderssuchasautotopoagnosia (impairednamingandpointingofbodypartsondemand[16,17]), Gerstmann’ssyndrome(amongothersymptoms,impairednaming andpointingoffingersondemand[18,19]),orideomotorapraxia (impairedproductionofgoal-directedgesturesondemand[20])– all involving damage to the left parietal lobe – are clinically, phenomenologicallyandconceptuallydifferentfromdisordersof

Keywords: Bodilyself Asomatognosia Illusions Delusions Virtualreality

Thisreviewarticlesummarizesneuropsychologicaldescriptionsofabnormalbodyrepresentationsin

brain-damagedpatientsandrecentneuroscientificinvestigationsoftheirsensorimotorunderpinningsin

healthyparticipants.Thefirstpartofthearticledescribesunilateraldisordersofthebodilyself,suchas

asomatognosia,feelingsofamputation,supernumeraryphantomlimbsandsomatoparaphrenia,aswell

asdescriptionsofnon-lateralizeddisordersofthebodilyself,includingAliceinWonderlandsyndrome

andautoscopichallucinations.Becausethesensorimotormechanismsofthesedisordersareunclear,we

focusonclinicaldescriptionsandinsistontheimportanceofreportingclinicalcasestobetterunderstand

thefullrangeofbodilydisordersencounteredinneurologicaldiseases.Thesecondpartofthearticle

presentstheadvantagesofmergingneuroscientificapproachesofthebodilyselfwithimmersivevirtual

reality,roboticsandneuroprostheticstofostertheunderstandingofthemultisensory,motorandneural

mechanismsofbodilyrepresentations.

* Correspondingauthor.

E-mailaddress:[email protected](C.Lopez).

http://doc.rero.ch

Published in "Annals of Physical and Rehabilitation Medicine 60 (3): 198–207, 2017"

which should be cited to refer to this work.

thebodyschemaandwillnotbeaddressedhere.Thus,thisreview focusesondisordersofthebodyschema,whicharepredominant afterrightbraindamage[21].

In the second section, we summarize current experimental investigations in healthysubjectsregarding thebodilyself. We focusonexperimentalparadigmsthathavecreatedmultisensory conflicts(ofteninvolvingvisualandtactilesignals)tomodulatethe bodyschema(e.g.,evokingavirtualAliceinWonderlandsyndrome

[22]orthesenseofhavingachildbody[23]),thesenseofowning thebody(e.g.,evokingtherubberhandillusion[24]),andthesense of embodied self-location (e.g., evoking an out-of-body–like illusion [25]). Theseapproaches have been foundpromising to betterunderstandthesensorimotormechanismsthatunderliea largerangeofbodilydisordersobservedfollowingbraindamageor duringepilepticseizuresormigraineepisodes(Table1).

Finally, we offer someconcluding remarks highlighting the importance of mergingapproaches from neuropsychology with modern neuroimaging techniquesand protocols fromcognitive neuroscience,immersivevirtualreality,roboticsand neuropros-theticsforestablishingamorecomprehensivemodelofthehuman bodilyselfanditsdisorders.

2. Disordersofthebodilyself

2.1. Unilateraldisordersofthebodilyself

2.1.1. Hemiasomatognosia

Theterm‘‘hemiasomatognosia’’wascoinedbyFrench neurol-ogistJeanLhermitte[3]torefertoaneglect,lackofinterest,or unawarenessofonepartorentirehalfofone’sbody.Suchpatients generallyignoretheirleftarmand/orleg;theybehaveandspeakas if thesedid notexist. One of Zingerle’s [26]patients, withleft hemiplegia,didnotpaytheslightestattentiontohisleftside,never lookedatit,neverspokeaboutit.Allorderstomovewereexecuted ontherightside,and,whenconfronteddirectly,thepatientdidnot see any absurdity in having only one body side. Zingerle and Lhermittesawinthisprofoundunawarenessforonebodysidethe source of other clinical phenomena such as anosognosia and unilateralneglect.IntheFrenchclinicalliterature,notionssuchas unawareness(me´connaissance)andlackofownership (de´sappar-tenance) werelater oftenusedtodescribehemiasomatognosia’s diverse manifestations. Frederiks [27] attempted to clarify the issuebyproposingadistinctionbetween‘‘conscious’’and ‘‘non-conscious’’ hemiasomatognosia. Theformer referredto patients whoperceivedtheir bodyasincompleteoramputated,yetfully realizedtheillusorynatureofthesefeelings.Thelatterreferredto thesubjective‘‘disappearance’’ofonehalfofthebody(mostoften theleftone), withoutthepatientbeingabletonoticethis very disappearance. Today, ‘‘non-conscious hemiasomatognosia’’ is conceived ofaspersonalneglect, motorneglect,or anosognosia for hemiplegia. In each of these cases, there is some kind of

indifferenceforabodypart.Personalneglectreferstotheclassical pictureofapatientwho‘‘forgets’’tocomb,shaveormakeupthe leftsidebecauseofanattentional,perceptiveorrepresentational disorder.Motorneglectreferstopatientswithoutobjectivemotor disorders who underutilize their left members [1,28]. These patients behave as if they werehemiplegic although they are not.Conversely,patientswithanosognosiaforhemiplegiabehave asiftheyarenotparalysed:importantly,theynotonlydenythat they are paralysed, they also tend to ignore their left side in general. Frederiks [27]summarized thesesymptoms as ‘‘atten-tionaldisordersforthehemibody’’.Garc_¸inetal.[29]wroteofa particularlystrikingcase:‘‘Theobservergetsthefeelingthatthe subjectbehavesasifheunderwentanamputationoftheleftsideof thebody’’.Thelossof lateralizedbody partrepresentations can occuratdifferentlevelsofmultisensory,sensorimotororcognitive integration.Forinstance,patientsmayrecognizetheirownbody partswhenpresentedvisuallybutcompletelyforgetaboutthem whentheyareoutofsight.

2.1.2. Feelingsofamputation,hemi-depersonalization

Hereweaddressdisorderslabelled‘‘conscious hemiasomato-gnosia’’byFrederiks[27].Suchpatientshavevividfeelingsthata partoftheirbodyhasdisappearedorfeelitstronglydiminishedor blurred.Conceptually,thesedisordersseemtobethereverseof phantom limbs after amputation (most amputees retain a sensationofcompletenessdespitehavingphysicallylostabody part).Inbothcases,patientsfullyappreciatetheillusorynatureof theirsensations.

Arelateddisorderisthefeelingthatabodypartisnolonger attachedtotherestofthebody,asifitwere‘‘floating’’atsome distance(sensation of disconnectionor splitting). Symptomsof absenceorseparationofbodypartsareusuallyofshortduration and appear mostly as part of seizures or migraine episodes

[2,4,27,30].Othercasesoccurduetocorticalorsubcorticalstrokes

[31]. Direct electrical stimulation at the righttemporo-parietal junctioncanalsoinducethistypeofillusioninthevisualmodality

[32]. These symptoms are not necessarily accompanied by hemiplegia, unilateral neglect or anosognosia. Sometimes, a sensationofstrangenessforan‘‘absent’’or‘‘disconnected’’body part,thenfeltas‘‘alien’’,‘‘numb’’or‘‘empty’’,isreported:theterm hemi-depersonalization,ordepersonalizationforabodypart,has beensuggested[1,3].Patientswithsuchsymptomsoftenfeelthe needtocontrolthesebodypartsbysightortouch,withoutsuch strategiesbeingalwaysabletorestorenormalbodilyfeelings[1]. Othersymptomscaninvolvedistortionsintheperceivedsizeof selectedbodypartsorhalfofthebody.Thesesubjectivealterations ofbodilysizearevividlyexperiencedbutareusuallyrecognizedas illusory.Torefertothesesymptoms,Frederiks[33]usedtheterms microsomatognosia and macrosomatognosia. Both terms are reminiscentoftheconceptsofhyposche´matie(i.e.,ashrinkingof thebodyrepresentation)andhypersche´matie(i.e.,anenlargement

Table1

Mainmodelsforneuropsychological,neuroscientificandneuroimaginginvestigationsofthebodyschema/imageandthesenseofbodyownershipandself-location/

embodiment.

Bodilyexperience Clinical(neuropsychological)model Experimental(neuroscientific)model Functionalneuroimagingmodel

Bodyschema/bodyimage Macro/microsomatognosia[33]

Supernumeraryphantoms[44]

Anorexia,bulimianervosa

Bodyidentityintegritydisorder[108]

Rubberhandillusion[24]

Immersivevirtualreality[22,23,104]

Shrinkingillusion[109]

Bodyownership Somatoparaphrenia[50]

Bodyidentityintegritydisorder[108]

Rubberhandillusion[24]

Virtualarmillusion[81]

Numbnessillusion[84]

Rubberhandillusion[85,87]

Virtualarmillusion[110]

Self-location/embodiment Autoscopichallucinations:

out-of-bodyexperience[67]

Heautoscopy[74]

Full-bodyillusion[92]

Out-of-bodyillusion[25]

Bodyswapillusion[98]

Immersivevirtualreality[103]

Full-bodyillusion[76]

Bodyswapillusion[86]

ofthebodyrepresentation) originallycoinedbyBonnier[34]in 1905todescribedistortionsofthebodyschema (asche´matie)in patients withsensory and central disorders [35]. In neurology, microsomatognosia(hyposche´matie)occurswhen abody partis experiencedassmallerthanusual(e.g.,somehemiplegicpatients perceive their hand as a child’s hand). In macrosomatognosia (hypersche´matie),a body partis perceivedas largerthanusual, oftentimesmuch heavier too. The illusion can expand to such degreesthatthebodypartisfeltasfillingtheroomorhittingthe roof.These disordersoccur most oftenduringmigraine [36]or epileptic seizures [2] but also after brain damage leading to sensoryormotorimpairment[33,37].

2.1.3. Supernumeraryphantomlimbs

‘‘Supernumerary phantom limbs’’ refer to the subjective experienceofhavinganadditionalbodypart,usuallyalimb,felt asanentitysharingpropertieswithaphysicalbodycounterpart andoccupyingadifferentplaceinspace.Somepatientsexperience thepresenceofa‘‘thirdlimb’’andclearlyidentifythisexperience as an illusion, whereas others report multiple arm or leg reduplicationsaspartofadelusionandseeminglybelieveintheir physicalexistence[38–40].Inthelattercase,theterm‘‘delusional reduplicationofpartsofthebody’’hasbeenapplied[39]. Super-numerary phantom limbs have been scarcely reported in the literature [40–42], yet they display striking diversity in their manifestations. In most cases, the‘‘additional limb’’ is a static somestheticperceptlocatedseparately,butonthesameside,ofa plegiclimb.Itissometimesfeltassmallerandindifferent,oreven awkward,positions,thanthephysicalcounterpartlimb.

Movementsofthesupernumeraryphantomareratherrareand mostoftenautomaticorinvoluntary(seeFig.1and[42]).Awoman with right fronto-mesial damage involving the supplementary motorarea andthecingulargyrusandno hemiplegiaormotor disordersreportedthefollowingpeculiarity:whenevershemoved her physical limb, a phantom seemed to occupy, after a few seconds,theplaceleftbythereallimb[43].Thus,theexistenceof thissupernumeraryphantomspecificallyreliedonmovementsof therealcounterpartlimb.Stillanotherpatientreportednotonly beingable to‘‘trigger’’ and moveher supernumerary phantom voluntarilybutalsoclaimedtoseeit(assomewhatwhitishand transparent)andsometimesevenfeltittouchherface.Inaddition, this complex phantom, as reported by the patient, could not coexistinthesameplacewithotherobjectsorbodyparts(inwhich casesit‘‘disappeared’’instantly)[44].Suchaclinicalpresentation is rare but neatly illustrates the diversity of supernumerary phantomlimbsintermsoftheinvolvedmodalities,motoraspects andsensitivitytofeedback.Thisdiversityisalsoreflectedinthe involved brain areas: most often, the right hemisphere is implicated,butthebasalganglia,parietallobe,thalamus,medial prefrontal cortex or supplementary motor area can also be involved[15].

2.1.4. Somatoparaphrenia

Gerstmann[45]coinedtheterm‘‘somatoparaphrenia’’torefer toclearlydelusionaldisordersofthebodilyself.Accordingtothis author,somerelatedcasesdeservedtobedistinguishedaccording to the mental frame of the patients. Most patients with hemiasomatognosia and anosognosia for hemiplegia remain indifferent or make rather limited claims regarding their impairment,whereasothersseem to‘‘standapart’’by virtueof their sheer bizarreness and exuberance. Here, is Gerstmann’s original definition of somatopraphrenia: ‘‘[A] specific psychic elaboration(markedbyformationofillusions,confabulationsand delusions)withrespecttotheaffectedmembers orside of the body, believed or experienced as absent’’ (p. 912). Somewhat confusingly,theterm‘‘verbalasomatognosia’’hasalsobeenused

torefertocasesinwhichpatientsmisidentifytheirownlefthand, presentedvisuallybytheexperimenterforsomeoneelse’shand

[46].Somatoparaphreniaismostly usedtorefer tofalsebeliefs regarding one’s body part or half body, the most frequently reportedbeingthebeliefthattheybelongtosomeoneelse.Tothat extent, current and classical definitions of delusional disorders directlyapplytosomatoparaphrenia[47–49].However, aswith supernumerary phantom limbs, the clinical presentation of somatoparaphrenia differs considerably among patients, which suggestsdifferenttypesofsomatoparaphrenia.

Themostcommontypeofsomatoparaphreniaisthe misattri-butionofone’sbodypart(usuallythelefthand,armorleg)tothe doctor,anurse,aneighbouringpatient,orsomeoneunspecified

[26,45,50].Somepatientsalsomisattributetheirparalysedlimbto someone absent or even long dead [51]. A sophisticated and delusionalaccountoftheeventcanbeprovided,oftentimeswitha

Fig.1.Supernumeraryphantoms.A37-year-oldwomanreportedthesensationof

havingathirdleftarmandlessfrequentlyreportedthesensationofhavingathird

leftleg.Thepatienthadnovoluntarycontrolofthephantomlimbmovement,but

thephantomfollowedthemotionofthereallefthand.Thepatientwasfullyaware

oftheillusorynatureofherexperience,althoughthephantomlimbsfeltsovivid

thatshehaddifficultiesdistinguishingthemfromthephysicallimbs.MRIrevealed

aninfarctionoftherightdorsomedialfrontallobeaswellasprenatallesionofthe

corpuscallosum.Notethatthepatient’sdrawingismirror-reversedsothatshe

representedherleftbodyontherightsideoftheimage.

ReproducedfromHarietal.[42]Threehands:fragmentationofhumanbodily

awareness.NeurosciLett1998;240(3):131–4,withpermissionfromElsevier.

persecutory flavour.For instance,a patient reportedby He´caen etal.[52],wenttothelengthofcomplainingbylettertoanurseshe accusedofhavingtakenawayherarmandthreateninglyaskedher to bring it back.The intensity of the delusion can vary:some patients acknowledge that their claims are bizarre, and others steadfastlyholdontotheirbeliefs.Bycloselyinspectingthearm from the shoulder down, patients can come to realize that it belongstothem,butnotalways[51].Onepatientthusdeclared: ‘‘my eyes and my feelings don’t agree, and I must believe my feelings.Iknowtheylooklikemine[theaffectedlimbs],butIcan feeltheyarenot,andIcan’tbelievemyeyes’’[5].

OnepatientreportedbyHe´caenanddeAjuriaguerraprovidesa goodillustrationofhowdifficultitissometimestocategorizethese disorders[2].Thispatientspontaneouslydeniedthathislefthand belongedtohimbutdidnotattributeittosomeoneelse(when shownhishand,hesimplysaid‘‘it’snotmine’’).However,upon seeing the doctor’s hand, he claimed he recognized his hand (‘‘there is mine’’). Thepatient held tothis idea even whenthe doctorproducedmovementswithhishandandtookafewsteps back.Hebegantodoubthisassertiononlywhenthedoctorwastoo faraway(‘‘I’mstartingtobelievethatitisnotmine’’).Thus,there are 2 typesof ownershipmisattributionin somatoparaphrenia: somepatientsidentifytheirlimbasfromanotherperson (self-as-othererror),whileothersidentifyanotherperson’slimbastheir own (other-as-self error, this type being much more rarely reported).

Brain damageleadingtosomatoparaphreniainvolvesa wide fronto-temporo-parietal network in the right hemisphere and morespecificallytheinsula,theprefrontalandorbitarycortex,the underlying white matter and subcortical structures (thalamus, basalgangliaandamygdala)[50,53–55]aswellasthe hippocam-pus[15].Theimplicationofmultisensoryregionsintheabnormal sense of owning a body suggests that coherent multisensory integrationisrequiredforelaboratingbodilyself-consciousness. Somatoparaphrenia, like many other alterations of bodily self-consciousness[9],mayinpartberelatedtomisintegrationofor conflicts between proprioceptive, interoceptive and vestibular signalsaboutone’sbodypositionandmotionwithvisualsignals fromthebody.Interestingly,sensorysignalsmodulationbycaloric vestibular stimulation [56–58] and visual inspection of the disowned hand in a mirror [59] can significantly decrease somatoparaphrenicdelusions.Therefore,bodyownershipisunder thecontrolofperipheralsensorysignals.

2.2. Non-lateralizedandbilateraldisordersofthebodilyself

The previous section provided an overview of lateralized (mostly on the left side)disorders of thebodily self. Here, we brieflyaddressnon-specificallylateralizeddisordersanddisorders extendingtotheentirebody.Forinstance,macrosomatognosiacan sometimesinvolvetheheadortheentirebody,inducingfeelingsof enormityor of‘‘fillingtheroom’’.Thiskindofsymptom occurs mostlyduringmigraineaurasandarecalledAliceinWonderland syndrome[60].Otherpatientscanfeeltheirentirebodyasabsent or unreal, typically during depersonalization [61] or, in more extreme cases, describe their body as dead or non-existent, a conditioncalledCotardsyndrome(whichLhermittetermed‘‘total asomatognosia’’ [3]). Moregenerally,suchsymptomsand those that follow can be conceptualized as ‘‘complete’’ forms of the unilateraldisordersreviewedintheprevioussections[62–64].

Themoststrikingdisordersinvolvingthemisrepresentationof theentirebodyarethosesometimeslabelled‘‘illusorydoubles’’, reminiscent of the ‘‘delusional reduplication of body parts’’ described above,andof the‘‘double’’anddoppelga¨ngermotives inromanticandgothicliterature[65].Suchstrikingalterationsof the global bodily self occur under varied circumstances, not

necessarilypathological,andaremostoftentransitory.Theyare currently described and studied under the term ‘‘autoscopic phenomena’’ [66–68]. This area includes multimodal illusions producingmoreorlesscompletedoublesofthebody.Subjectscan thusperceiveavisualprojectionoftheirownbodyinfrontofthem (autoscopichallucination),inwhichcasethe‘‘double’’appearsasa mirror reflection in external space,while the realself remains firmlytighttoitsphysicalbodyandlocation.Therefore,autoscopy assuchismainlyavisualphenomenon,althoughitcansometimes involvesomemotorresonancewhenthedoublemovesaccording tothesubject’sownmovements.

Inothercases,thesubjectreportsfeelingthelocationofthe ‘‘self’’atthesametimeintheprojecteddoubleandinthephysical body (heautoscopy)or alternatingbetween them.The ‘‘double’’ can then acquire some limited sensorimotor and psychological autonomyandisthusclosetotheliterarydoppelga¨ngerdescribed by Hoffmann, Dostoyevsky, Poe and Hogg. Heautoscopy has a visual component but also involves unstable vestibular and sensorimotoraspects[67,69]andastrongemotionalaffinity(or repulsion)towardsthe‘‘double’’.Whenthedoubleisnotactually seen but rather feltin close spatialproximity, thecondition is called ‘‘feeling of a presence.’’ The subject has a more or less fleetingsensation of someone’spresencenearby, withoutbeing abletoclearlyperceiveoridentifyitbutalsowithoutexperiencing itasadoubleofoneself[70].

Whentheperspectiveofthesubjectisentirelyrelocalizedinthe projecteddoubleandthusthesubjecthasthevividsensationof being‘‘out-of-the-body’’andcan‘‘see’’thephysicalbodyfroma distantandelevatedperspective,theconditioniscalled ‘‘out-of-body experience’’ [32,67,68]. During such episodes, widely popularized by their inclusion in the hallucinatory phenomena called ‘‘near-death experience’’ [71], there is a very strong vestibularinvolvement(feelingsoflightnessandfloating,reversal of the visuo-spatial perspective), and the purely autoscopic component(‘‘seeingoneself’’)canbelessprominentoraltogether absent[69,72,73].

In neurologicalcases,autoscopic hallucination, beingmostly visualdisturbances,involvesdamagetotheoccipitalcortex,often unilaterally [74]. Otherdisorders, beingmore complex, involve varied disturbances of multimodal and vestibular integration, notablyattheleftposteriorinsularegardingheautoscopy[74],the insulaandafronto-temporo-parietalnetworkforthefeelingofa presence[75],andtherighttemporo-parietaljunctionfor out-of-bodyexperiences[76].

In the next section, we describe experimental procedures developed to study mechanisms underlying the bodily self in healthy subjects, to better understand the disorders we have described.

3. Neuroscientificinvestigationsinhealthyparticipants

Inthissection,wesummarizerecentworkfromneuroscience andexperimentalpsychologythathasendeavouredtoinvestigate howthebrainrepresentssomeaspectsofthebodyandtheself.We focusonresearchofhealthyparticipantsrelatedtothesenseof owningthebody(alteredinsomatoparaphrenicpatients)and self-location/embodiment (altered during out-of-body experiences) (see Table 1), 2 experiences deemed crucial for establishing a minimalsenseofselfhood[77].

3.1. Investigatingbodypartrepresentations

Healthyparticipantsresearchhasextensivelyusedthe‘‘rubber hand illusion’’ (RHI) [24,78] to investigate the multisensory foundationsofbodyownershipaswellasitsneuralunderpinnings

(Fig.2A).TheRHIis evokedwhen a rubberhand placedin the participant’s field of view is touched in synchrony with the participant’shand(hiddentotheparticipant).Afteraminute of synchronous stimulation of the fake and real hands, some participantsreportthattherubberhandfeelsasifitweretheir own hand. Subjectivereports measured by questionnaires (i.e., visual analog scales) indicate that illusory ownership for the rubberhandissignificantlylargerforsynchronousthan asynchro-nousvisuo-tactilestroking.Inaddition,theRHIischaracterizedby amislocalizationoftheparticipant’shandinspace.Whenaskedto locatetheir handinthehorizontalplane(e.g.,bypointingwith their right hand toward the tip of their left index finger), participantstendtolocatetheirhandclosertotherubberhand thanitactuallyis.Thiserrorinself-handlocalizationtowardsthe rubber hand has been termed ‘‘proprioceptive drift’’ and is classically interpreted as a consequence of a ‘‘visual capture’’

[79,80].

SeveralvariantsoftheRHIdevelopedincludepresentationofa virtualhandinimmersivevirtualenvironments(i.e.,the‘‘virtual armillusion’’ [81];see Fig. 2B),orthe presentationof multiple hands[82].Ehrssonetal.[83]developedavariantoftheRHIduring whichtheexperimenterusestheparticipant’srightindexfingerto touch a left rubber hand while the experimenter touches the participant’slefthandwiththeexperimenter’sownfinger(Fig.2C). Thisprocedurecreatesthesensationthatparticipantsaretouching theirownhand(insteadofarubberhand)withtheirrightindex finger.Anotherparadigm,the‘‘numbnessillusion’’[84],replaces the rubber hand by a confederate’s hand. In this illusion, the participant and the confederate have the palm of their hands pressedagainsteachother.Participantsstroketheirownleftindex fingerusingtheirrightthumbandatthesametimestrokewith their rightindexfingertheconfederate’sindexfinger(Fig.2D). Participantsmayexperiencetheirleftindexasnumb(hencethe term‘‘numbnessillusion’’)orbigger,asifitwereencompassingthe confederate’sindexfinger.Thesesensationsoccuronlywhenthe participant’s and confederate’sindex fingers are synchronously stroked.

Several neuroimagingstudiesinvestigated theneural under-pinningsof body partownershipby usingthe RHI.Brain areas significantlymoreactivatedduringtheRHI(i.e.,during synchro-nousvisuo-tactilestimulation)weremostlylocatedintheinsula, cingulatecortex,premotorcortexandextrastriatecortex (extras-triate body area, EBA) [83,85,86]. Moreover, an increase in the BOLD signal in the insula was positively correlated with the magnitudeoftheproprioceptivedrift[87].Finally,onestudy[84]

recordedsomatosensoryevokedpotentialsduringthenumbness illusionwhileparticipantsreceived electricalstimulation ofthe median nerve. The numbness illusion was associated with increased amplitude of the N20 component of somatosensory evokedpotentials.Thisresult suggestssomatosensory enhance-ment in the primary somatosensory cortex, in keeping with increased evoked potentials over the somatosensory cortex reportedafterrealanaesthesiaoftheparticipant’shand[88].

Fig.2.Paradigmstoinvestigateownershipforbodyparts.A.Inthevisualvariantof

the‘‘rubberhandillusion,’’theexperimenteruses2identicalpaintbrushesto

synchronouslyorasynchronouslystroketheparticipant’shand(notvisibletothe

participant)andarealisticrubberhand(visibletotheparticipant).B.Inthe‘‘virtual

armillusion,’’spatialandtemporalsynchronyiscreatedbetweenthetouchapplied

totheparticipant’shand(toppartofthefigure)andthetouchobservedonthe

virtualarm(bottompartofthefigure:sceneshowninthehead-mounteddisplay).

ReproducedfromEvansandBlanke[110].Sharedelectrophysiologymechanismsof

bodyownershipandmotorimagery.Neuroimage2013;64:216–28.http://dx.doi.

org/10.1016/j.neuroimage.2012.09.027,withpermissionfromElsevier.C.Inthe

non-visualvariantoftherubberhandillusion,spatialandtemporalsynchronyis

createdbetweenthetouchappliedtotherubberhandbytheparticipant’sright

indexfinger(thatispassivelymovedbytheexperimenter)andthetouchappliedby

theexperimentertotheparticipant’slefthand.D.Inthe‘‘numbnessillusion,’’the

participant(hereontheleftpartofthepicture)strokeswithhisrightthumbthe

dorsalpartofhisleftindexfingerandatthesametimestrokeswithhisrightindex

fingerthedorsalpartoftheconfederate’srightindexfinger.

We conclude this section by emphasizing recent findings showingthatillusoryownershipoverafakehandchangesseveral aspectsoftheownbodyphysiologicalstates[89,90].Forexample, Moseley et al. [90] reported that illusory ownershipof a right rubberhandsignificantlyreducedthetemperaturerecordedover theparticipant’srighthand.Thistemperaturereductionwasnot found for the participant’s left hand and foot (not stimulated duringtheexperiment),sodisruptingthesenseofownershipfora givenbodypartmodifiestemperatureregulationatthelevelofthis samebodypart.Interestingly,bodyownershipabnormalitiesand changeintemperatureregulationareassociatedinseveralclinical conditionssuchasschizophrenia,neuropathicpain,anorexiaand bulimianervosa[90].Thus,therelationbetweenbodyownership/ disownershipandhomeostaticregulationmayshedlightonthe multisensorymechanismsofseveralconditionscharacterizedby abnormalbodyrepresentations.

3.2. Investigatingwhole-bodyrepresentations

InvestigatingtheRHIandrelatedillusionswouldnotbeenough tounderstandthesensorimotorfoundationsofhumanbodily self-consciousnessbecauseitaddressesonlybodypartrepresentations. Yet,in additionto representationsof multiple body parts, and perhapsmoreimportantly,thebodilyself,alsoentailsglobaland coherentwhole-body representation [9,77]. Accordingly,recent neuroscientific investigations intothe bodily selfhave endeav-oured to adapt theRHI to the entire body by using thesame principles,thatis,visuo-tactileorvisuo-motorsynchronybetween thephysicalbodyandtheseen(fake)body[91].

In the ‘‘full-body illusion’’ [92], participants wore a head-mounted display through which real-time videos of their own bodycanbeseenfromtheback(thevideorecordingsystembeing placedacoupleofmetersbehindtheirownbody),orvideosofa plasticmannequin ora virtualcharacter[92–96]. Thus, partici-pants standing upright observed their own body (or the mannequin’s body) as if it was seen from an external, third-person perspective (i.e., as if participants were located some metersbehindthisbody)(Fig.3A).Whenatactilestimulationwas appliedontheparticipant’sback,insynchronywithtouchapplied to the back of the body depicted through the head-mounted display,participantsself-identifiedwiththisbody. Self-identifica-tion(measuredbyusingquestionnaires)wassignificantlystronger aftersynchronousthanasynchronousvisuo-tactilestroking.Asfor the RHI, self-identification with the avatar decreased skin temperatureattheleveloftheparticipant’sbody[97]andreduced theparticipant’spainperception[95].Inaddition,thefull-body illusionwascharacterizedbyerrorsinself-localizationinspace.

Fig. 4 illustratesthe proceduresfor measuring a ‘‘whole-body’’ proprioceptivedriftandshowsthatparticipantslocalized them-selvesclosertotheobservedbodyaftertheyreceivedsynchronous visuo-tactilestroking.Acrossseveralvariantsoftheillusion,the meanproprioceptivedriftwas177cmforvisuo-tactilestroking and36cmforasynchronousstroking,whichdifferedsignificantly

Fig.3.Experimentalparadigmstoinvestigatewhole-bodyownershipand

self-location.A.Paradigmusedtoevokethe‘‘full-bodyillusion’’:identificationwiththe

bodyseeninthehead-mounteddisplayandapparentforwardshiftinself-location.

B.Paradigmusedtoevokethe‘‘out-of-bodyillusion’’:identificationwiththebody

seeninthehead-mounteddisplayandapparentbackwardshiftinself-location.

Reproduced from Ehrsson [25]. The experimental induction of out-of-body

experiences. Science 2007;317:1048, with permission from The American

Associationfor theAdvancementof Science.C. Paradigm usedtoevoke

self-identificationwithaplasticmannequin’sbodywithoutchangeinself-location.

ReproducedfromPetkovaandEhrsson[98].IfIwereyou:perceptualillusionof

bodyswapping.PLoSOne2008;3:e3832.http://dx.doi.org/10.1371/journal.pone.

0003832.D.Immersivevirtualenvironmentcoupledwithmotiontrackingsystems

toinduceself-identificationwithachildavatarseeninahead-mounteddisplay.

ReproducedfromBanakouetal.[23]Illusoryownershipofavirtualchildbody

causesoverestimationofobjectsizesandimplicitattitudechanges.ProcNatlAcad

Sciences U S A 2013;110(31):12846–51. http://dx.doi.org/10.1073/pnas.

1306779110.

(P<0.005)[11].Becausethemagnitudeofthisdriftisrathersmall,as fortheproprioceptivedriftobservedduringtheRHI,itcanbeseenasa compromise between the physical self-location and a full-blown disembodied self-location at the position of the avatar the participantsself-identifiedwith.

Avariantofthefull-bodyillusionisreferredtoasthe ‘‘out-of-bodyillusion’’[25](Fig.3B).Incontrastwiththefull-bodyillusion, participantswerestrokedontheirchestwhiletheyobservedina head-mounteddisplayareal-timevideooftheirownbodyseen fromtheback.Self-reportsincludedstatementssuchas‘‘Wow!I

feltasthoughIwasoutsidemybodyandlookingatmyselffrom the back’’ [25]. Self-identification with the seen body was measuredbyrecordingskinconductanceresponseswhile parti-cipantsobservedathreatdirectedtowardstheseenbody.Theskin conductance response waslargerafter synchronousthan asyn-chronousvisuo-tactilestroking.Inanothervariantoftheillusion, participantsbenttheirheadforwardasiftheywerelookingattheir own stomachand feet[86,98,99] (Fig. 3C). Theywore a head-mounted displayin which videos of a plasticmannequin were shownfromanelevatedviewpoint.Incontrastwiththefull-body illusionand theout-of-body illusion, the mannequin was seen from a first-person perspective (but seen froma disembodied, third-person viewpoint in the former illusions) and with a descendingviewpoint.Participantsand themannequinreceived eithersynchronousorasynchronousvisuo-tactilestroking,andthe degreeofidentificationwiththemannequinwasmeasuredboth subjectively (questionnaires)and objectively (skin conductance response). Synchronous visuo-tactile stroking evoked stronger self-identification withthe mannequin and stronger emotional responseswhenparticipantsobservedathreatdirectedtowards themannequin’sstomach.

OnlyafewfunctionalMRIstudieshaveanalysedbrainactivity duringthebodyillusionsdescribedabove.The‘‘full-bodyillusion’’ modulates theBOLDsignalatthetemporo-parietal junction,in closevicinityofareasthataremostfrequentlydamagedinpatients reporting out-of-body experiences [76]. In contrast, observing fromafirst-personperspectiveamannequin’sbodybeingtouched insynchronywithone’sbodyactivatedmostlythepremotorcortex butnotthetemporo-parietaljunction[100].

Finally,itisnotablethatimmersivevirtualenvironmentsand motion capturetechnologieshave fosteredinvestigationsofthe multisensory foundations ofbody representationsbecausethey allowpresentingrealistichumanavatarsandmanipulatingtheir shape,size,ageandskincolour[22,101–104].Ageneraloutcomeof thesevirtualrealitystudieswasthatspatio-temporalsynchrony betweentheparticipant’smotionandthatofanavatarembedded inavirtualenvironmentledparticipantstostronglyself-identify with the avatar. Recent studies indicate that sensorimotor synchrony between the physical and virtual body not only modulates the participants’ own body representation but also altersaspectsoftheiraffectiveandmoraljudgments.Forexample, synchronous motion between the participant and an avatar depicting a 4-year-old child modified the participant’s body schema,asmeasuredbytheirabilitytomanipulatevirtualobjects

[23](Fig.3D).Inthisstudy,participantsoverestimatedthesizeof objectsaftertheyidentifiedwitha4-year-oldchildavatar.Inthe samelineofresearch,self-identificationwithanoverweightavatar changed the perceivedsize of the participant’s body and their potentialactions in theenvironment [104]. Several therapeutic interventions in neurology and psychiatry have recently been derivedfromimmersivevirtualenvironmentsgiventheapparent ease toinduceself-identificationwithavatars and tomodulate bodily,socialandaffectiverepresentationsbyusingappropriate technologies[101,105].

4. Conclusionsandclinicaloutcomes

Clinicalcasesandrecentneuroscientificstudiessummarizedin thisreviewindicatethatrepresentationsofthebodyandselfare varied,multimodalandplastic.Regardingclinicalcases,onefact mustbehighlighted:ourcurrentunderstandingofthebodilyself historicallymostlydependsoncarefulexaminationofawidearray ofclinicalconditions.Asaprivateandsubjectiveexperience,the bodilyselfanditsdisturbancescanonlybeapproachedthrough systematicandempathicquestioningofpatientswillingtoshare

Fig.4.Illusoryself-locationevokedduringthefull-bodyillusion.Afteraperiodof

synchronous or asynchronous visuo-tactile stimulation, participants were

displacedbackwardsandaskedtowalkbacktowheretheythoughttheywere

locatedduringtheexperiment.Thepositionwhereparticipantsstoppedwastaken

asameasureofself-locationinthehorizontalspaceduringthefull-bodyillusion.

Thisprocedurewasconductedtomeasurea‘‘whole-body’’proprioceptivedrift,

similartothatmeasuredwiththerubberhandillusion.A.Participantswalked

significantly further towards the seen avatar after synchronous visuo-tactile

stroking (blue symbols) than after asynchronous visuo-tactile stroking (red

symbols). The zero position indicates the location of the participant’s body

duringthevisuo-tactilestrokingandpositiveandnegativevaluesindicatedriftsof

self-locationtowardsandawayfromtheseenavatar,respectively.Dataareplotted

accordingtoref.1aand1b[92],ref.2[94],ref.3[96],ref.4[95],ref.5[111],andref.

6[112].ReproducedfromLopezetal.[11]Inthepresenceofothers:self-location,

balancecontrolandvestibularprocessing.NeurophysiolClin2015;45(4–5):241–54.

http://dx.doi.org/10.1016/j.neucli.2015.09.001. Copyright 2015 ElsevierMasson

SAS.Allrightsreserved.B.DataaremeanSDproprioceptivedriftcalculatedacross

severalvariantsofthefull-bodyillusion.

their feelings and sensations. To further investigate this topic, investigatorsandcliniciansmustknowtheirsemiologyandpursue this purelyclinicalendeavour.Evenin thisdayandageofnew neuroimaging technologies, data analysis methods and virtual realitysettings,patientsandtheirstoriesremainthemostprecious avenuetodiscovermoreaboutoursenseofbodilyawarenessand ownership.Whenpossible,theclinicalexamshouldinvolveafull neurological, neuropsychological, psychiatric and neuroimaging investigation. The interview should let patients speak by themselves, describe their bodily feelings in their own words, andthenfocusonmoredetailedaspects,suchasthosedescribedin thesection‘‘Disordersofthebodilyself’’ofthisarticle[6].Some questionnairesandguidelineshavebeenpublished[6,106,107].It is highlyunlikelythatalldisordersofthebodilyselfhavebeen discovered, described and labelled:we thus insist that clinical investigation,case reportsandcase seriesarestillanabsolutely necessaryapproachforthestudyofthebodilyself.

However,aswehaveseen,experimentalresearchcanexploit multiple methods and paradigms tofurther investigateneural, cognitive, affective, spatial and social correlates of bodily awareness.Virtualreality,robotics,neuroprostheticsand increas-inglyothermethodswillcertainlyhelpdelineatethemechanisms formultisensoryandcerebralbodilyrepresentations.Suchstudies havealreadyrevealedthehighlyplasticnatureofthebodilyself, sensitive as itis torathersimple visuo-tactile andvisuo-motor conflicts. From these results, these experimental methods now seem to be very promising non-invasive approaches toward rehabilitation of patients with neurologic and psychiatric dis-orders.

Disclosureofinterest

Theauthorsdeclarethattheyhavenocompetinginterest.

Acknowledgements

Someoftheresearchleadingtotheseresultsreceivedfunding fromthePeopleProgramme(MarieCurieActions)oftheEuropean Union’s Seventh Framework Programme (FP7/2007-2013) (REA grant No. 333607) (BODILYSELF, vestibular and multisensory investigations of bodilyself-consciousness). C. Lopez’sresearch is alsosupportedbyagrantfromtheVolkswagenStiftung(grantNo. 89434: Finding Perspective: Determining the embodiment of perspectivalexperience).

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