Body ownership affects visual perception of object sizeby rescaling the visual representation of external spaceBjrn van der Hoort & H. Henrik EhrssonPublished online: 8 May 2014#The Author(s) 2014. This article is published with open access at Springerlink.comAbstract Size perception is most often explained by a com-bination of cues derived fromthe visual system. However, thistraditional cueapproachneglectstheroleoftheobserversbody beyond mere visual comparison. In a previous study, weused a full-body illusion to showthat objects appear larger andfartherawaywhenparticipantsexperienceasmall artificialbody as their own and that objects appear smaller and closerwhen they assume ownership of a large artificial body (Bar-bie-doll illusion; van der Hoort, Guterstam, &Ehrsson, PLoSONE, 6(5), e20195, 2011). The first aim of the present studywastotest thehypothesisthat thisown-body-sizeeffect isdistinct from the role of the seen body as a direct familiar-sizecue. To this end, we developed a novel setup that allowed forocclusion of the artificial body during the presentation of testobjects. Our results demonstrate that the feeling of ownershipofanartificialbodycanaltertheperceivedsizesofobjectswithout the need for a visible body. Second, we demonstratethat fixationshiftsdonot contributetotheown-body-sizeeffect. Third, weshowthat theeffect existsinbothperi-personal spaceanddistant extra-personal space. Finally,through a meta-analysis, we demonstrate that the own-body-size effect is independent of and adds to the classical visualfamiliar-size cue effect. Our results suggest that, by changingbody size, the entire spatial layout rescales and newobjects arenow perceived according to this rescaling, without the need tosee the body.Keywords3Dperception .Cue integration .Multisensoryprocessing .Visual perception .IllusionIntroductionWhenadultsseeatoythat theyhavenot seensincetheirchildhood, they tend to be surprised by how small it appears,because they remembered it to be much larger. Anecdotes likethis suggest that objects are perceived to be larger to smallerobserversandsmallertolargerobservers. Inlinewiththisidea, Poincar (1952) proposed a thought experiment in whichtheentireworld, includingyourbody, becomesathousandtimeslargerduringsleep.Uponawakening, youwouldnotnotice this tremendous change because our body serves us asa system of axes of coordinates (Poincar, 1952, p. 100). Arecently published experimental study used a body illusion toconfirmPoincars notion that changing body size changes theperceivedsizeof objects (Barbie-doll illusion; vanderHoort, Guterstam, &Ehrsson, 2011). Thepresent studysought toexaminethepossiblemechanismsof thisown-body-sizeeffect byisolatingtheroleof themultisensoryexperience of owning a body from the mere use of classicalvisual cues. Because size perception and depth perception areclosely related (size is the distance between two sides of anobject, and depth is the distance between the object and theobserver), we will usetheterm space perceptionto refertoboth.Psychology textbooks typically explain the visual percep-tionofspaceonthebasisofthevisualcueapproach(e.g.,Cutting & Vishton, 1995; Goldstein, 1999; Schwartz, 2010).Accordingtothisapproach,theperceivedsizeofanobjectdependsonacombinationofitssizeontheretinaanditsperceiveddistance from theobserver.Amultitudeofvisualcuescanbeusedtoperceiveobjectdistance. Pictorialcues(e.g.,familiarsize,relativesize,texturegradient,andlinearperspective), motion-produced cues (depth from motion andmotionparallax),andbinoculardisparityareallvisualcuesderived fromretinal information. In addition, oculomotor cues(accommodation and eye convergence) are considered visualB. van der Hoort (*):H. H. EhrssonDepartment of Neuroscience, Karolinska Institutet, Retziusvg 8,17177 Stockholm, Swedene-mail: bjorn.van.der.hoort@ki.seAtten Percept Psychophys (2014) 76:14141428DOI 10.3758/s13414-014-0664-9distance cues because the muscles from which they derive arepart ofthevisual system. Thevisual cueapproachdepictsvisual perception as being much like a video camera, becauseit onlyemphasizesvisual cueswithout accountingfor thepresence of a body.However, recent psychological research has revealed thatvisual perception can also be altered by nonvisual cues that arerelated to the physiological state of the body (Proffitt, 2006;Proffitt & Linkenauger, 2013). For example, wearing a heavybackpack, fatigue, and poor health make hills appear steeper(Bhalla & Proffitt, 1999) and distances appear larger (Proffitt,Stefanucci, Banton, & Epstein, 2003). Another potential non-visual cue in visual space perceptionthe size of the observ-eris moredifficult totest experimentally(inawithin-subjectsdesign)becauseit wouldrequiremanipulatingthebody size of individuals in a laboratory setting. Until recently,the methods that came closest to this requirement altered theapparent size of ones hand either by zooming in and out onvideo-recorded displays (Marino, Stucchi, Nava, Haggard, &Maravita, 2010; Pavani & Zampini, 2007) or by using mag-nifying and minifying goggles (Linkenauger, Ramenzoni, &Proffitt, 2010). It has been shown that such visual distortion ofthe hand affects both the hand shape during object grasping(Marinoetal.,2010)andsizeperceptionasmeasuredbyasize-matchingtask(Linkenaugeretal.,2010).Interestingly,thelatter studyfoundthat someoneelseshanddoesnotinduce this effect, which suggests that ones own body or, atleast, ones own hand plays a special role in calibrating sizeperception. However, theseresultscanlargelystill beex-plainedbythevisual cueapproach. First, for theeffect ofapparent hand size to occur, the hand needs to be visible andlocated close to the target object, which suggests that the handis used as a familiar-size cue for direct visual comparison withtarget objects. In addition, one is more familiar with the sightof ones own hand, which makes it a stronger familiar-size cuethan does someone elses hand. In addition to the body as animportant familiar-size cue, we recently showed that the verysensation of owning a certain-sized body (body ownership) iscrucial for using the body as a reference in perceiving the sizesof objects (van der Hoort et al., 2011). We used a variation ofthe full-body illusion (Petkova & Ehrsson, 2008) to manipu-late the feeling of ownership of different-sized artificial bodies(van der Hoort et al., 2011). The full-body illusion is inducedbysynchronouslytouchingtheparticipantsbodyandtheartificial body while participants see the artificial body froma first-person perspective through a set of head-mounted dis-plays(HMDs)connectedtoapairofcamerasfacingdowntowardtheartificial body. Underthecontrol condition, thefull-body illusion is significantly diminished by asynchronousvisuotactilestimulation. Usingthismethod, wefoundthatidentical objects at identical distances were perceived as largerandfarther awaywhenparticipantsownedasmall body(80 cm) and as smaller and closer when they owned a largebody(400cm).Thiseffectwassignificantlyreducedwhenbody ownershipwasdisruptedbytheasynchronouscontrolcondition. Importantly, the size of the object on the retina, thebinocular disparitywhenfocusingontheobject, andthedemandcharacteristicswereidentical for boththeillusoryownershipconditionandthecontrol condition. Thus, thealtered visual perception of size and distance is a consequenceofchangedbodysizeandisenhancedbybodyownership.Therefore, this is referred to as the own-body-size effect. Wefound this own-body-size effect using a variety of measures:verbal estimation of object size and distance, indication of theperceived size with two hands (bimanual size estimation), andestimation of the perceived distance by walking that distanceblindfolded.Theaimofthepresent studywastofurtherexplorethepossiblemechanismsandcharacteristicsof thiseffect. Intheory, there are two possible, nonmutually exclusive mecha-nisms that could contribute to the own-body-size effect. First,body ownership could enhance the effect that results from theartificial body serving as a familiar-size cue for direct visualcomparison(i.e., adirect familiar-sizecue). Disruptionofbody ownership could decrease the bodys relevance in thisrespect because a body that is not owned could potentially beof anysizeandis, thus, less informative. Inthis case,preventing the possibility of visual comparison between targetobjects and the artificial body would decrease the own-body-size effect.The second possible mechanism, which is the hypothesiswe propose here, is that ownership of a different-sized bodycausesarescalingofexternalspace, which, inturn, causestargetobjectsinthisspacetoberescaledaccordingly. Thishypothesis predicts that, after the initial rescaling of externalspace,thealteredsizeperceptionoftargetobjectsdoesnotdependonvisual comparisonbetweenthebodyandthoseobjects.Our first aim was to disentangle the relative contribution ofeach mechanisminhowbody ownershipaffectsvisual per-ception.Tothisend, wefirstexcludedtheinfluenceofthebody as a direct familiar-size cue by introducing a new exper-imentalsetupthatallowedustooccludetheartificialbodyafter induction of the full-body illusion (or a period of asyn-chronous stimulation in the control condition) but before thepresentation of target objects and subsequent size estimationsof those objects (Experiment 1). Thus, the crucial differencebetween this and all previous studies (Haggard & Jundi, 2009;Linkenauger et al., 2010; Marino et al., 2010; van der Hoortetal.,2011)isthatnobody(orbodypart)isvisibletotheparticipant when the test object is presented.In Experiment 2, the participants were instructed to look ata fixation point during the period of visuotactile stimulation inorder to control for fixation differences. In Experiment 1 (andinourpreviousstudy), theparticipantsreceivednoinstruc-tions about where to look before the target object presentation.Atten Percept Psychophys (2014) 76:14141428 1415Therefore, the participants could have looked at the artificialbodymoreduringthevisuotactilestimulationwhenit wasdone synchronously, because the visuotactile stimulation mayhaveattractedmoreattention. Thismayhavecausedtheartificial body to be a stronger remembered familiar-size cueinthebodyownershipconditionand, hence, maypartiallyexplain a possible own-body-size effect in Experiment 1. Thefixationpoint inExperiment 2waslocatedjust abovetheartificial bodyat afixeddistancefromthecameras. Thus,theparticipantsneverlookedat theartificial bodydirectly,which controlled for the amount of time looking directly at theartificial body. Furthermore, because the distance of the fixa-tion point from the cameras is identical across the body sizeconditions, anadditional effect ofthismanipulationisthatchanges in eye vergencethat is, oculomotor informationupon the target object presentation are now identical for thevarious body size conditions. Thus, by controlling for fixation,Experiment 2 controls both for how much participants attendto the body and for oculomotor information from eye-vergingmuscles.Comparing the results of these two experiments with thoseof the previous study (van der Hoort et al., 2011) through ameta-analysis allows us to assess the relative contribution ofbody visibility and fixation in the own-body-size effect and,thus, disentanglesthefamiliar-sizecuehypothesisfromtherescaling hypothesis.The second aim of this study was to characterize the own-body-size effect even further by studying its role in visual sizeperceptionatvariousdistances. Itispossiblethattheown-body-sizeeffectappliesonlytothespaceimmediatelysur-rounding the body (i.e., peri-personal space) because the brainrepresentsthisspaceinaspecialmanner. Forexample, thespace near the hands is represented in hand-centered referenceframes (Brozzoli, Gentile, Petkova, & Ehrsson, 2011; Farn,Dematt, &Ldavas, 2005; Graziano, Hu, &Gross, 1997;Spence, Pavani, & Driver, 2004). Furthermore, the mere sightof anobject canprepotentiateactions, but onlywhentheobject is located close to the body (Cardellicchio, Sinigaglia,&Costantini, 2011). Thus, giventhat theperceptionofanobjectssize in relationto one'sownbody is especially im-portant when one is planning to interact with that object, it isreasonabletothinkthat theown-body-sizeeffect onobjectsize perception is stronger in peri-personal space. In contrast,if the own body were to serve as a more fundamental ruler forvisual space perception, which is the hypothesis we proposehere, one would expect its effect to be equal for near space andfar space. Such a result would exclude the possibility that thiseffect is restricted to space near the body or handsa possi-bilitythatcannotbeexcluded,onthebasisofmanyofthepreviouslypublishedreports (Haggard&Jundi, 2009;Linkenauger et al., 2010; Marino et al., 2010). By presentingtarget objects either near the body, in the peri-personal space(1.0mfromthecameras), orfarfromthebody, indistantextrapersonal space (6.0 m from the cameras) (Experiment 3),wecoulddirectlycomparethebodysizeeffectindifferentparts of space and test our hypothesis.General methodEthics statementAll participants gave their written informed consent prior toparticipatingintheexperiment. All experimentswereap-proved by the Regional Ethical Review Board of Stockholm.ParticipantsWe recruited a total of 65 nave, healthy adult participants forthefourexperiments, withthefollowingnumbersforeachexperiment: Experiment 1a, 16 (8 females, 8 males, 25.4 years1.1[ageSE]);Experiment1b, 15(7females,8males,23.7years0.8); Experiment 2, 16(9females, 7males,30.8years2.4); Experiment 3, 18(10females, 8males,24.4 years 1.0). The participants were recruited by adver-tisements placed on the campus of Stockholm University, andtheyreceiveda cinema voucher inexchange for theirparticipation.We chose to recruit nave participants for each individualexperiment toprevent theparticipantsfromadjustingtheirsize estimates according to previous experimental experiences(e.g., estimatesof distancebetweenthehandsafter verbalestimates or vice versa). The use of such a serial design withmultiple experiments, instead of a single factorial design withaverylargenumberofconditions, promisedtogivemorereliable results.Video technology and artificial bodiesThe participants wore a set of HMDs (CybermindVisette ProPAL, CybermindInteractive, Maastricht, theNetherlands;displayresolution=640480,fieldofview=71.5)thatwereconnectedtotwosynchronizedcolor CCTVcameras(ProtosIV, Vista, Workingham, Berkshire, U.K.). Thedis-tancebetweenthecameras(85mm) wasfixedfor all theparticipants. The image from the cameras was directly trans-mitted to the HMDs without any software conversion, and sothere was no noticeable delay. At the start of the experiment,theparticipantssawahomogeneousgrayscreenthat wasplaced a few centimeters in front of the cameras. The first trialbegan when the experimenter lowered this gray screen, whichenabledtheparticipantstoseeareal-time3-Dimageofanartificial body. Twodifferent artificial bodieswereused: asmall body (80 cm) and a large body (400 cm). Both bodiesworecustom-madeclothestomatchtheappearancesacrossthe bodies (blue jeans and white socks). The participants could1416 Atten Percept Psychophys (2014) 76:14141428see the legs and lower abdomen of the artificial body and apart of thetestingroomfromafirst-personperspective(Fig. 1). Questionnaire data from van der Hoort et al. (2011)showedthat theparticipantsnoticedthesmall bodytobeshorterthanthelargebody, but participantsaveragebodyheight estimations(160and190cm, respectively)deviatedstrongly from the actual height of these artificial bodies.Visuotactile stimulationIn this study, we used two visuotactile stimulation conditions.In the synchronous condition, the participants and the artificialbodyweretouchedsimultaneouslyat correspondingloca-tions. In the asynchronous condition, we alternately touchedthe participants body and the artificial body on different bodyparts. The tactile stimulations were random combinations oftaps and strokes (of approximately 1020 cm) applied to thelower legs and feet using a small white ball attached to a rod.Therodwaslongenoughtokeeptheexperimentershandoutside the field of view of the cameras. The size of the rod(whiteball andstick) andthelengthof thestrokeswereproportionaltothesizeofthebodybeingtouchedinorderto optimally match the visual impression of the touches withthe tactile sensation (van der Hoort et al., 2011). This proce-dure ensured that there were no subjectively perceived incon-gruences between the seen and felt strokes that might other-wise reduce or eliminate the illusion.ProcedureUpon arrival, theparticipantsfirstreceived instructionsin aseparate waitingroom. Inthat room, theywere thenblindfolded and led to the testing room by the experimenter.This procedure prevented themfromseeing the true size of theartificial bodies and the target objects in the test room (furtherinformation is provided below). The participants lay down ona bed and were fitted with the HMDs. The participants wereinstructed to keep theireyesclosed when theblindfoldwasremoved and the HMDs were fitted; thus, they never saw theroom without the HMDs. They were instructed to fold theirhands on their chest and keep their head tilted slightly forward(approximately30). Agrayscreeninitiallyoccludedtheentirefieldof viewof theparticipant, andtheexperimentstartedassoonasthisscreeninfront of thecameraswasmoved down entirely (by the experimenter). Each trial startedwith 45 s of synchronous or asynchronous visuotactile stim-ulation (see Fig. 1). Subsequently, the gray screen in front ofthe cameras was moved upward again until it covered half ofthecamerasvisualfields. Theartificialbodywasnowoc-cluded behind the screen, but the upper half of the visual field,which included part of the testing room, remained visible. Thetarget object wasthenshownat afixeddistancefromthecameras (1.2 m) in the half of the visual field that remainedvisiblefor3s. Intotal, weused12homogeneouscoloredcubes as target objects, including four 10-cm, four 20-cm, andfour30-cmobjects. Varioussizeswereusedtoprevent theparticipantsfromrecognizingobjectsizeswithinthecondi-tionsandyieldamorereliablemeasureforsize perception.Within each group of four, each cube had a different color topreventtheparticipantsfromrecognizingobjectsacrossthedifferent conditions. The time needed to present a target objectafter occludingtheupperhalf of thecamerasfieldof viewwas approximately 4 s. The participants then indicated the sizeof the object verbally or with their hands (see below). Afterthis object size estimation procedure, the screen that occludedpart of the visual field was lowered again to start a new trial(see Fig. 1).Experiment 1aTheparticipantswereinstructedtoindicatethesizeofthecube with their hands (i.e., a bimanual size estimation) as soonasthecubewasvisible.Theparticipantswereinstructedtokeeptheirhandsclosetotheirchest toprevent themfromexpecting to see their hands in their visual field (of which thelower part was occluded), with the palms turned inward andthefingersfullyextended. Thedistancebetweenthepalmswas used as a measure of the perceived object size and wasmanuallymeasuredbytheexperimenter witharuler. Theexperiment consisted of four blocks of three trials. Each blockcontainedoneconditionderivedfromafullfactorial22design: body size (small and large) visuotactile stimulation(synchronous vs. asynchronous). We used one block for eachcondition to prevent interference effects across the conditionsand to produce the most robust effect possible by maintainingtheillusionsforaprolongedperiod of time.Three differentobject sizes (10, 20, and 30 cm) were used within each block.Theorder of thethreedifferent-sizedtarget objectswithineach condition and the order of the conditions were random-ized across the participants.Experiment 1bWe instructed the participants to verbally report the estimatedsizeof thetarget objectsincentimeters. Eachobject waspresentedtwicepercondition,becauseweexpectedgreatervariabilityin thismeasureon thebasis of previous findings(van der Hoort et al., 2011); thus, this experiment consisted of24 trials. The other aspects of the procedure were identical tothose in Experiment 1a.Experiment 2The participants were instructed to look at and maintain fixa-tiononasmall redball (i.e., thefixationpoint) that washangingonafishingline1.5mfromthecameras. ThisAtten Percept Psychophys (2014) 76:14141428 1417fixation ball was visible both during the period of visuotactilestimulation and when the artificial body was occluded fromviewbythescreenduringtheobjectsizeestimationproce-dures. The subsequent presentation of a target object (at 1.2 mfrom the cameras) occluded the fixation ball entirely, and theparticipants wereinstructedtofixateonthetarget objectinstead. Theconditionsandtarget objectswereidentical tothose in Experiment 1a.The procedure used in Experiment 2 differs from those inpreviousstudiesofthefull-bodyillusion, inwhichpartici-pants looked at the strokes being applied to the artificial body(Petkova & Ehrsson, 2008; Petkova, Khoshnevis, & Ehrsson,2011; Schmalzl & Ehrsson, 2011; van der Hoort et al., 2011).Thus, toensurethatsuchachangetotheoriginalprotocolwould not reduce the illusion, we administered a questionnaireto quantify the subjective strength of the illusion. At the end ofeach condition (i.e., after three trials), the screen in front of thecameraswasmovedup so thatit occluded theentire visualfield. Theparticipantswerereadtwostatements(test state-ments T1 and T2) that were designed to capture the subjectiveexperienceofthebodyownershipillusionandtwocontrolstatements(C3andC4) that weredesignedtocontrol forexpectancyeffectsandtaskcompliance(seeTable1; thestatements were taken from van der Hoort et al., 2011). Theorder of the questions was randomized between the conditionsand between the participants. The significance of the averagedifferences between the control and test statements was testedfor each condition separately.Experiment 3Theparticipants wereinformed that objectscouldappearatvarious distances and were instructed to estimate the size ofeach target object with their hands according to the proceduresdescribeabovefor Experiment 1a. This experiment usedanotherfullfactorial 22design(bodysizeobject dis-tance). We used the same small and large artificial bodies thatwere employed in the previous experiments, but thevisuotactilestimulationwasalwayssynchronousinthisex-periment. Here, theexperimentalmanipulationwasthedis-tance at which the target objects were shown from the camera(either 1.0minthenear conditionor 6.0minthefarcondition) for both body sizes. These distances were chosenbecause1.0mfromthecameraiswithintheperi-personalspace of both artificial bodies (20 cm beyond the feet of thesmall body and right above the chest of the large body), and6.0 m is considered outside the peri-personal space for bothbodies. The trials in which the large body was used and thetrials in which the small body was used were divided into twoseparate blocks to prevent inference effects between the small-and large-body conditions, but the six trials within each blockwererandomizedintermsofdistanceandobject size. TheFig. 1 Experimental design. Timeline of a single trial (top), the perspec-tive of the participants through the head-mounted displays (middle), andan overview (bottom) are depicted. There were three periods within eachtrial: (1) visuotactilestimulation(synchronousor asynchronous), (2)occlusionof thebody,and(3)objectpresentationandestimation.Thered fixation ball in the middle panel was visible only in Experiment 2.Only the small-body condition is shown1418 Atten Percept Psychophys (2014) 76:14141428order of thetwomainblockswasrandomizedacrosstheparticipants.Statistical analysisFor all experiments, wetooktheweightedaverageof theresponsesineachcondition. Inorder toaveragethesizeestimationsofdifferent object sizes, wemultipliedthesizeestimations of 10 cm objects by 2 and the size estimations of30 cm objects by 2/3. Collapsing the data from all trials intoone condition yields a more precise size estimation per par-ticipant and is valid because there is typically no interactioneffect of object sizeontheown-body-sizeeffect (seetheResults section).Wewerenot interestedinintersubject differencesbut,instead, inthechangedperceptionwithinparticipants; thatis, a size estimation increase from 10 to 20 cm in participantA is an identical effect, as compared with a 4080 cmincreasein participant B. Therefore, we standardized the data in such away that intersubject variability was reduced whileintrasubject variabilitywasmaintained. Foreachcondition,the difference between a participant's weighted score on thatcondition and the participant's overall average was divided bythis participants overall average; (Xi,A A)/A, where Xi,Aisthe weighted score of participant A on condition i and A isthe average of participant A on all trials.Thesedatapointswereusedforfurtheranalysis.Alltheinferential statistics were calculated using SPSS for Windows,release19.0(IBMCorp.,Armonk,NY).EachdatasetwasfirsttestedfornormalitywithaKolmogorovSmirnovtest.Experiment 1b and Experiment 3 had at least one conditionwith significant deviation from normality ( = .05) and were,therefore, statistically analyzed using nonparametric tests. WeusedtheWilcoxonsigned-ranktest totest for significantdifferencesbetweenindividual conditionsandtocomparethese differences (i.e., a nonparametric interaction test). Theeffect sizes of the nonparametric tests were calculated by r2=Z/n.For the normally distributed data sets of Experiment 1a andExperiment 2, we used repeated measure ANOVAs to test thesignificanceof themaineffectsandinteractioneffects. Inaddition, we performed paired t-tests to compare the averagesbetween specific conditions and to compare these differences(this is logically equivalent to examining the interaction termin an ANOVA). The effect sizes of the parametric tests werecalculated by r2= t2/(t2+ df).Finally, we performed a combined analysis of Experiment1a, Experiment 2, andapreviouslypublishedexperiment(Experiment 6invanderHoort et al., 2011)toisolatetheeffectsof bodysize, bodyownership, bodyvisibility, andfixation across different experiments. We used a general linearmodel in which body ownership and body size were definedaswithin-subjectsfactorsandbodyvisibilityandfixationwere defined as between-subjects factors. The p-values werecorrected(Bonferroni) for multiple comparisons whenrequired.ResultsExperiment 1: Body occlusionExperiment 1 was designed to test for an own-body-size effectin the absence of a visible body, which excluded the possibil-ityofdirectvisualcomparisonsbetweentheobjectandthebody. Thecrucial findingwasthat theeffect ofbodysizesurvivedthismanipulationintheownershipcondition. Wefound a significant interaction effect between body size andbody ownership both for bimanual size estimations [Experi-ment 1a: F(1, 15) = 36.185, p < .001, r2= .70] and for verballyreported sizes [Experiment 1b: Z = 1.988, n = 15, p < .05, r2=.51], which was based on a significant difference between thesmall body and the large body in the body ownership condi-tion [Experiment 1a: t(15) = 6.408, p < .001, r2= .72; Exper-iment 1b: Z = 3.181, n = 15, p < .001, r2= .82] and the absenceof a difference in the control condition [Experiment 1a: t(15) =0.286, p = .78; Experiment 1b: Z = .966, n = 15, p = .33] (seeFig. 2b, c). This interaction effect did not depend on the size ofthe object presented at a given trial for Experiment 1a, F(2, 30)= 0.307, p = .74. However, object size had a close to signif-icant effect on the verbal size estimations in Experiment 1b, 2= 5.897, n = 15, p = .052, based on a larger own-body-sizeeffect for the 20-cm objects.Overall, the results of Experiment 1 show that the partici-pantsestimatedobjectstobelargerwhentheyexperiencedhaving a small body and estimated objects to be smaller whenthey experienced having a large body, but not when the senseof body ownership was absent. Furthermore, we found robustTable 1 Questionnaire included in Experiment 2Statements MeasureDuring this part of the experiment T1: it felt as if the touch I felt was caused by the white ball I saw. seven point rating scale (3 to +3)T2: it felt as if the body I saw was my body.C1: it felt as if I had four legs at the same time.C2: it felt as if my own body was turning into plastic.Atten Percept Psychophys (2014) 76:14141428 1419effect sizes for both the bimanual estimations (Experiment 1a)and the verbal reports (Experiment 1b). However, because oflarge interindividual differences in verbal estimations and anassociatedlackofnormalityintheacquireddatasets(alsoseen in van der Hoort et al., 2011), we used the bimanual sizeestimation in subsequent experiments, which tended to yieldnormallydistributedresponses, toallowfor parametricanalyses.Experiment 2: Body occlusion and fixationThesecondexperimentwasdesignedtoexaminetheown-body-size effect in the absence of both direct visual compar-ison and fixation differences. The rationale of Experiment 2was to control for the amount participants fixated directly onthe artificial body while maintaining the crucial experimentalmanipulation of occluding the body from view that was usedin Experiment 1. To this end, the procedure was identical tothat inExperiment 1, withoneexception: Theparticipantswere instructed to focus on a red fixation ball (at 1.5 m fromthe cameras in all conditions) during the visuotactile stimula-tionperiod. Thesubsequent presentationof target objects(always at 1.2 m) occurred in front of the fixation ball. Thepostexperiment reportsrevealedthat 1participant failedtomaintainfixationandwas, therefore,excludedfromfurtheranalyses.Consistent withtheresultsof Experiment 1a, thesizeestimationsshowedasignificant interactioneffect betweenbody size and synchronicity, F(1, 14) = 20.579, p < .001, r2=.58, whichwascausedbyasignificant differencebetweenbody sizes in the body ownership condition, t(14) = 5.246, p .051420 Atten Percept Psychophys (2014) 76:14141428the four conditions of Experiment 2. The rationale for this wastodemonstratethat theillusionwasfullyestablishedeventhough the participants were not directly looking at the artifi-cial body and the strokes applied to it were as in previouslypublishedstudies. Importantly, theparticipants responsesshowedsignificant differencesbetweenthetwoownershipstatements and the two control statements for both the smallbody and the large body, but only in the ownership condition,Zsmall = 3.418, n = 15, p < .001; Zlarge = 3.524, n = 15, p