Walking on a line: A motor paradigm using rotation and reflection symmetry to study mental body transformations

Brain and Cognition 70 (2009) 191–200

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Brain and Cognition

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Walking on a line: A motor paradigm using rotation and reflection symmetryto study mental body transformations

Bérangère Thirioux a,b,*, Gérard Jorland b, Michel Bret c, Marie-Hélène Tramus c, Alain Berthoz a

a Laboratoire de Physiologie de la Perception et de l’Action, Collège de France, Franceb Ecole des Hautes Etudes en Sciences Sociales, Paris, Francec Arts et Technologies de l’Image, Université Paris 8, Saint-Denis, France

a r t i c l e i n f o a b s t r a c t

Article history:Accepted 2 February 2009Available online 18 March 2009

Keywords:Reflection–rotation symmetryEmbodimentSelf-locationMental body transformationSpontaneous perspective-taking

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* Corresponding author. Address: Laboratoire de Phde l’Action, Collège de France, 11, place Marcelin BerFrance. Fax: +33 (0) 1 44 27 13 82.

E-mail address: berangere.thirioux@college-de-fra

Researchers have recently reintroduced the own-body in the center of the social interaction theory. Fromthe discovery of the mirror neurons in the ventral premotor cortex of the monkey’s brain, a humanembodied model of interindividual relationship based on simulation processes has been advanced,according to which we tend to embody spontaneously the other individuals’ behavior when interacting.Although the neurocognitive mechanisms of the embodiment process have started being described, themechanisms of self-location during embodiment are still less known. Here, we designed a motor para-digm which allows investigating in ecologically more valid conditions whether we embody another per-son’s intransitive action with an embodied or disembodied self-location. Accordingly, we propose aphenomenological model of self–other interaction showing how perspective-taking mechanisms mayrelate on mental body transformation and offering a promising way to investigate the different sortsof intersubjectivity.

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1. Introduction

Since the last decade, cognitive neuroscientists (Blanke & Metz-inger, 2009; Gallagher, 2000, 2005; Metzinger, 2003, 2008; Ruby &Decety, 2001) have restored the own-body in the center of the the-ory of the self, revealing its importance for the development of acomprehensive selfhood theory. While providing us with the feel-ing of being positioned at a specific location in space, the own-bodyis immediately experienced as the spatial location of the self (Arzy,Thut, Mohr, Michel, & Blanke, 2006; Blanke et al., 2005), leading tothe coherent and normal experience of the spatial unity betweenthe self and the body – or ‘‘embodied self-location” (Blanke & Metz-inger, 2009). In the same time, the own-body has also been reintro-duced in the theory of social interactions, as phenomenologists atthe beginning of the 20th had already assumed (Lipps, 1897,1903, 1906, 1913; Vischer, 1927; Husserl, Hua XIII-XVI). First, themechanisms of self-location have been hypothesized to be involvedin the mechanisms of self–other distinction (Decety & Sommerville,2003; Ruby & Decety, 2001, 2004). Indeed, the own-body as origi-nate center of orientation mediates our own egocentered visuo-spatial perspective (Zahavi, 1994) that we can experience only ina direct way (Husserl, Hua XIII-XVI; Berthoz, 2004; Vogeley & Fink,

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ysiologie de la Perception etthelot, 75231 Paris Cedex 05,

nce.fr (B. Thirioux).

2003; Zahavi, 1994) whereas taking the other’s visuo-spatial per-spective has been hypothesized to rely on a mental translocationof the egocentric viewpoint (Vogeley & Fink, 2003), i.e., on a ratherindirect process (Berthoz, 2004; Jorland, 2004; Jorland & Thirioux,2008). Second, inspired by the discovery of the mirror neurons inthe ventral premotor cortex of the monkey’s brain (di Pellegrino,Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Rizzolatti, Fogassi, &Gallese, 2001), current theories of social interactions have proposeda human embodied model of interindividual relationship based onsimulation processes (Carr, Iacoboni, Dubeau, Mazziotta, & Lenzi,2003; Fadiga, Craighero, & Olivier, 2005; Gallese, 2007; Rizzolatti,Fadiga, Fogassi, & Gallese, 1999). Accordingly, understanding an-other individual’s actions (Buccino et al., 2001), emotions (Carret al., 2003) or intentions (Iacoboni et al., 2005) may require theobservation of his (her) behavior in a specific context and then addi-tional cognitive elements (Gallagher, 2008) such as the activation ofinternal models by which we simulate the other’s behavior (see alsoZahavi, 2008; for the rehabilitation of the direct perception in socialcognition, see Gallagher, 2008). For instance, it has been shown thatindividuals change their breathing when observing other individu-als performing effortful actions (Blakemore & Frith, 2005; Paccalin& Jeannerod, 2000) and a recent fMRI study on sense of touch re-vealed that touch observation activates the secondary somatosen-sory cortex, suggesting visuo-tactile mirroring mechanisms in thisbrain area (Ebisch et al., 2008). In the aggregate, the own-bodymay thus be involved in several phenomenological aspects of theself, such as self-location, self–other distinction, and self–other

192 B. Thirioux et al. / Brain and Cognition 70 (2009) 191–200

interaction, suggesting that embodied self-location may be some-how crucial for social cognitive abilities.

Therefore, the sole role of embodied self-location within self–other interactions has been recently discussed on the basis of datafrom neurological patients with experiences of disruption of thespatial unity between the self and the body, the so-called out-of-body experiences (OBEs; Blanke, Landis, Spinelli, & Seeck, 2004;Blanke & Mohr, 2005; Blanke, Ortigue, Landis, & Seeck, 2002; Brug-ger, Regard, & Landis, 1997; Devinsky, Feldmann, Burrowes, &Bromfield, 1989; Irwin, 1985; Kahane, Hoffmann, Minotti, & Ber-thoz, 2003; Lhermitte, 1939, 1951) and ‘‘Heautoscopic hallucina-tions” (HAS; Blanke & Mohr, 2005; Blanke et al., 2004; Brugger,2002; Brugger, Agosti, Regard, Wieser, & Landis, 1994; Hécaen &Green, 1957; Lance, Cooper, & Misbach, 1974; Lhermitte, 1939,1951; Menninger-Lerchenthal, 1935). During OBEs, neurologicalpatients with lesions or epileptic discharges in a posterior brain re-gion – i.e. predominantly in the right temporo-parietal junction(TPJ; Blanke et al., 2002; Kahane et al., 2003) – experience thattheir ‘‘self” is localized outside their bodily boundaries and seethe world and their own-body from this perspective – or ‘‘disem-bodied self-location”1 (Arzy et al., 2006; Blanke et al., 2002, 2004,2005; Brugger et al., 1997). In HAS, patients with damages predom-inantly to the left TPJ see in front of them the reduplication of theirown-body, (Doppelgänger; Brugger, 2002) with the preservation oflateral asymmetries (if the patient moves his own left hand, hisreduplicated body moves his own left hand also, i.e., inversely tothe left/right reversal as in a mirror; for remarks concerning this ter-minology, see Brugger (2002)), and alternatively experience of see-ing their ‘‘body” from their own visuo-spatial perspective or fromtheir double’s perspective (Blanke & Mohr, 2005; Blanke et al.,2004; Brugger, 2002). Interestingly enough, in addition to thehypothesis of disturbed vestibular processing, (Kahane et al., 2003;Schwabe & Blanke, 2008), OBEs and HAS have also been hypothe-sized to relate on deviant perspective-taking abilities which occurunder normal conditions when individuals are interacting (Brugger,2002). Suggesting that visuo-spatial perspective-taking requires spa-tial cognitive abilities relying on bodily processing such as mentalbody transformations (Zacks, Rypma, Gabrieli, Tversky, & Glover,1999) and notably mental imagery with disembodied self-location,this hypothesis was reinforced by data from electrical neuroimagingstudies with healthy subjects showing that imagining oneself in thebody position of another individual (own-body transformation task)activates also the TPJ (Arzy et al., 2006; Blanke et al., 2005), whereasembodied self-location activates the lateral occipito-temporal cor-tex, including the extrastriate body area (EBA; Arzy et al., 2006).

Therefore, this hypothesis led us here to address two observa-tions. First, the human embodied model of social interactions re-mains unclear how the mechanisms of self-location (i.e.,embodied or disembodied) are involved when individuals areinteracting. To the best of our knowledge, it is still unknownwhether individuals when embodying the behavior of anotherindividual, embody it with an embodied or disembodied self-loca-tion. The only fact that the other’s behavior is simulated or repro-duced is per se not enough to infer which mechanisms of self-location are involved. We rather propose that focusing on howthe other’s behavior is simulated/reproduced may be useful inunderstanding the mechanisms of self-location within self–otherinteractions. Second, most studies on self-location do not investi-gate self-location when individuals are interacting spontaneouslywith each other and are often conducted in highly constant andconstrained contexts rendering the experimental situations lessecologically valid. Typically, behavior and brain activity are tested

1 The terms ‘‘embodied and disembodied self-location” have been proposed byArzy et al. (2006).

in a prone/seated position, although most interactions with hu-mans occur in the standing or walking position (Bavelas, Black,Chovil, Lemery, & Mullett, 1988; Bavelas, Black, Lemery, MacInnis,& Mullett, 1986; Bavelas, Black, Lemery, & Mullett, 1986; Bavelas,Black, Lemery, & Mullett, 1987; Parsons, 1987; Reed & McGoldrick,2007; Scheflen, 1964). Moreover, participants are generally in-structed explicitly on how to perform mental transformation ofperspective (Ruby & Decety, 2001; Ruby & Decety, 2004; Vogeleyet al., 2004) or to perform left–right judgments on static and sche-matic figures either by imagining themselves in the figure’s bodyposition (Own-Body-Transformation-task) or by imagining the fig-ure as the reflection of their own-body (Mirroring-task; Arzy et al.,2006; Blanke et al., 2005; Zacks et al., 1999). Hence, the experi-mental investigation of spontaneous mental body transformationswithin self–other interactions is still missing.

Here, we applied the causal principle according to which ‘‘formfollows function” (Bavelas et al., 1988) and asked whether the formof the motor behavior exhibited by the own-body may provide anempirical criterion to infer the multisensorial and namely the vi-suo-spatial function at work within social interactions. We proposea simple motor paradigm which, adapted from the rotation andreflection symmetry model by Bavelas et al. (1988), enables toinvestigate whether individuals automatically embody, withoutexplicit task, another person’s behavior and, if so either by keepingtheir embodied self-location or locating mentally themselves in theother’s body position (disembodied self-location). Inspired by theEinfühlung Theory of Lipps (1906), who claimed that a typical caseof embodied simulation processes occurs while observing a danc-ing tightrope walker, we designed a behavioral study in which par-ticipants interacted spontaneously with a life-sized virtualtightrope walker walking forward, backward and leaning to her leftor right on a rope.

Here, we report results showing that participants automaticallyembodied the avatar’s leaning movements. Moreover, the form ofthe participants’ motor behavior (i.e., automatic leaning move-ments to their right and left when the tightrope walker was lean-ing to her own right and left, respectively) revealed thatparticipants, using mental imagery, located spontaneously them-selves in the avatar body position, suggesting that they embodiedthe avatar’s visuo-spatial perspective. Our motor paradigm maythus be useful in understanding how spontaneous perspective-tak-ing mechanisms may rely on mental body transformations.

2. Methods

2.1. Paradigm

To investigate whether individuals, under spontaneous condi-tions and without explicit instruction, embody another person’sbehavior and, if so, with either an embodied or disembodied self-location (how), we designed a motor paradigm which allows par-ticipants to employ spontaneously their own transformation strat-egy, comparable to strategies used in daily life. For this, wepursued the traditional psychological approach, an approach focus-ing on elementary mimicry that is used to investigate, from thebody posture, how individuals act together without explicit taskinstructions (Bavelas, Black, Lemery, MacInnis et al., 1986; Bavelas,Black, Lemery, & Mullet, 1986; Bavelas et al., 1987, 1988; Char-trand & Bargh, 1999; O’Toole & Dubin, 1968; Scheflen, 1964; Stot-land, 1969; Tessari, Rumiati, & Haggard, 2002). Notably, weadapted the rotation and reflection symmetry paradigm by Bavelaset al. (1988). According to Bavelas et al. (1988), if individuals A andB are facing each other and B is leaning to his right, A can copy B’stilt by leaning either to his left or right (Fig. 1a and b). In the firstcase, A reacts by mirroring B’s tilts. We hypothesized that such

Fig. 1. (a,b) Reflection and rotation symmetry. Two individuals, A and B, are facing each other. (a) In reflection symmetry, A is leaning to his left when B is leaning to his right(yellow arrows) and vice-versa (red arrows) as if A were watching himself in a mirror. (b) In rotation symmetry, A is leaning to his left when B is leaning to his left (yellowarrows) and vice-versa (red arrows), as if A were taking B’s perspective. (c,d) Experimental setup. (c) Participants, standing in the Romberg position on a red line onthe ground, held a metal bar horizontally in front of them. We presented a life-sized virtual female tightrope walker on the screen in front of the participants. The line on theground prolonged the rope in the movie. (d) Goniometer. To record the participants’ tilts, a goniometer was fixed in the middle of the metal bar. (e, f) Movie samples. Thetightrope walker, shown in front orientation, leaned either to her right (e) or left (f) while walking forward and backward. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

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‘‘reflection symmetry” (as labelled by Bavelas et al., 1987, 1988)indicates that A is keeping his own visuo-spatial perspective(first-person perspective; 1PP), by imagining his own-body at hisactual body position (physical or ‘‘embodied” position; see alsoArzy et al., 2006), but that A is further performing a mental trans-formation by imagining that B’s tilts are his own-body movementsas reflected in a mirror (Fig. 1a). By contrast, we hypothesized thatthe second case or ‘‘rotation symmetry” (Bavelas et al., 1987, 1988)reflects that A is performing a mental own-body-transformation(OBT; Arzy et al., 2006; Blanke et al., 2005; Fig. 1b) and imagininghimself in the B’s body position to take the B’s visuo-spatial per-spective (third-person perspective; 3PP). Hence, the type of sym-metry (either reflection or rotation) may provide an empiricalcriterion to infer which sort of mental body transformations isbeing performed within self–other interaction (i.e., either imagin-ing the other’s body as one’s own-body as reflected in the mirroror imagining one’s own-body in the other’s body position). Reflec-tion and rotation symmetry thus indicates that the ‘‘self” is inter-acting with the ‘‘other” from its own visuo-spatial perspective viaan embodied self-location and from the other’s visuo-spatial per-spective, via a disembodied self-location, respectively.

2.2. Participants

Ten healthy volunteers took part in this experiment (six fe-males) with a mean age of 33 years (SD ± 9.32). All participants

were right-handed according to the Edinburgh handedness inven-tory (Oldfield, 1971) and had normal or corrected-to-normal vi-sion. None reported neurological or psychiatric disorders. Allwere naive to the purpose of the experiment and gave written in-formed consent. The experimental protocol was approved by thelocal ethics research committee and has been performed in accor-dance with the ethical standards laid down in the declaration ofHelsinki.

2.3. Experimental setup

We presented a colour movie of a computer generated femaletightrope walker (Fig. 1e and f). The movie lasted 3.5 min with aframe rate of 25 Hz. It was controlled by a PC with a WindowsXP operating system. The animated tightrope walker was designedand programmed using the AnyFlo System (Bret, 1988), which gen-erates virtual avatars with a neural network model taking into ac-count the rules of natural movements and biomechanicalconstraints of the body (2/3 power law; Berthoz, 1997; Viviani &Flash, 1995; Viviani & Terzuolo, 1982).

Within the movie, the female tightrope walker was shown infront-facing orientation. During the first eight seconds, she stoodwith her right foot in front of the left on the rope. Then, she startedwalking forward and backward on the rope with the arms horizon-tally outstretched. While walking, she executed lateral tilts in ran-dom order to the right (Fig. 1e) or left (Fig. 1f). In either direction,

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the tightrope walker executed 23 tilts with an amplitude between20� and 30� and a duration of either 1 s (8�), 2 s (10�), or 3 s (5�).To reinforce both the interaction with the tightrope walker and theecological features of the task, the tightrope walker’s forward andbackward movements lasted irregularly and appeared randomly,preventing that participants infer regularity or routine cues whichwould have facilitated the task. Without such indications, partici-pants were obliged to pars the anatomical features of the tightropewalker’s movements in order to anticipate her forward/backwardmovements. We presented the same movie to all participants.Moreover, the animated tightrope walker was displayed life-sizedby a rear-projector onto a large screen (2 m � 2 m). To mimiceveryday social encounters and to reinforce interactions givingparticipants the impression to act in the same spatial environmentas the tightrope walker, participants stood on a red line (3 m/10 cm; length/width) which prolonged on the ground the avatar’srope on the screen (Fig. 1c). Before the movie started, we askedparticipants to stand in the so-called ‘‘Romberg position” (Rom-berg, 1846) with one foot in front of the other on the red line,i.e., in a standing position approximating the unbalanced positionas the tightrope walker, and to choose the most comfortable posi-tion (i.e., either with the right or left foot in front of the other;Fig. 1c). Six participants chose standing with their left foot in frontof the right. Hence, when the tightrope walker appeared, 6 partic-ipants were positioned in reflection symmetry to the tightropewalker’s position and 4 in rotation symmetry. This procedureavoided a bias towards either rotation or reflection symmetrydue to the initial foot position. Participants held a metal bar(length: 1 m) horizontally in front of them. Their hands wereplaced at equal distance from a goniometer (Intertrax) whichwas fixed in the middle of the bar (Fig. 1d). The output of the goni-ometer fed into the analogue input of a PC (Windows XP operatingsystem) recording continuously the participants’ movements.

2.4. Task and procedure

The experiment included one baseline condition and one testcondition. All participants started with the baseline conditionand continued, after a 10 min pause, with the test condition. Inthe baseline condition, we tested the participants’ stability andposture. They were asked to stand for 5 min, motionless in theRomberg position, while holding the metal bar horizontally in frontof them and fixing a red point in the middle of a large black rect-angle displayed on the screen. We determined the maximal ampli-tude of the metal bar with respect to the horizontal. On average,the maximal amplitude of the tilts occurring during 5 min was4.30� (±1.03). Accordingly, we set the threshold for lateral move-ments to 5� in the test condition.

In the test condition, to investigate whether participant wouldautomatically (i.e., without conscious action planning) embodythe tilts of the tightrope walker and, if so with either an embodied(tilts in reflection symmetry) or disembodied self-location (tilts inrotation symmetry), we did not mention the avatar’s leaningmovements in the briefing. Participants were only instructed tokeep their feet on the red line on the ground in front of themand to walk forward and backward along the line when the tight-rope walker was walking forward and backward. The exact instruc-tion was: ‘‘You are going to see a tightrope walker. She will moveforward and backward. Please, walk forward if she is walking for-ward and backward if she is walking backward.”

2.5. Data analysis

We determined the number of left and right tilts from the par-ticipants’ movements recorded by the goniometer (Fig. 2). Accord-ing to the baseline condition, we analyzed only the lateral

movements whose amplitude was greater than 5� (as a control,we manually noted when the participants leaned during theexperiment). To determine whether the participants’ tilts wereperformed in rotation (left–left or right–right) or reflection (left–right or right–left) symmetry, we synchronized the onset of themovie with the onset of the recording. For each participant, we cal-culated the number (and percentages) of tilts in reflection androtation symmetry, respectively.

Moreover, we defined the time point when the tilt goes beyondthe threshold as the onset of the movements (t1 for the partici-pant; t1’ for the tightrope walker; Fig. 2b). We calculated reactiontimes (RT) by the difference between t1 and t10. For each partici-pant, we calculated the mean RT for tilts in reflection and rotationsymmetry, respectively. Finally, we defined the point when the tiltgoes below the threshold as the offset of the movements (t2, t20;see Fig. 2b). We calculated the duration of the tilt by the differencebetween t2 (t20) and t1 (t10).

3. Results

3.1. Spontaneous embodiment

All 10 participants automatically leaned when the tightropewalker was leaning. On average, participants performed 14.1 ± 1.4tilts (mean ± SEM) corresponding to about one third of the tightropewalker’s leaning movements (Fig. 3a). In the aggregate, most of thetilts (133) occurred within 2 s after the onset of an avatar’s leaningmovement (frequency: 0.117 ± 0.016 tilts/s). The remaining tilts (8)occurred in a period from 2 s after one avatar’s leaning movementonset to the next one (frequency: 0.007 ± 0.008 tilts/s). The differ-ence in frequency was significant (two-tailed, paired t-test:p < 0.001), showing that most of the participants’ tilts were inducedby the interaction with the avatar and not by the participants’ for-ward/backward movements (or random unbalances). This suggeststhat the participants’ tilts were due to a spontaneous embodimentprocess. We found no gender difference as leaning movements ofmales (14.8 ± 1.4 tilts, n = 4) and females (13.7 ± 2.1 tilts, n = 6) oc-curred similarly often (two-tailed, unpaired t-test: p = 0.444).

To test for a possible learning effect in our data, we investigatedwhether the tilt frequency changed during the experiment. Forthat, we divided the movie in three time periods ranging fromthe avatar’s leaning movement no. 1 to no. 15, from no. 16 to no.30, and from no. 31 to no. 46, respectively. This tilt repartitionshowed that participants leaned more often in the first two thirds(first third: 5.4 ± 1.3 tilts; second third: 5.5 ± 1.3 tilts) of the exper-iment than in the last third (3.2 ± 1.3 tilts; Fig. 3b). The tilts oc-curred thus immediately from the beginning of the experimentand did not increase during the experiment, suggesting no learningeffect and rather a spontaneous embodiment process.

The analysis of the leaning movement duration shows that, forthe avatar’s leaning movement with a duration of 1, 2 and 3 s, themean duration of the participants’ tilts was 1.2 s (±0.1), 1.9 s (±0.1)and 2.5 s (±0.2), respectively (Fig. 3c). A linear regression analysisrevealed a high correlation between the duration of the partici-pants’ tilts and that of the avatar’s leaning movements (meanslope: 0.66; R2 = 0.99), confirming that when the duration of theavatar’ tilts increased, the duration of the participants’ tilts in-creased as well, but slightly less. This duration adaptation suggestsalso a phenomenon of motor resonance.

3.2. Form of embodiment

The analysis of tilt symmetry showed that more tilts (9.7 ± 1.3)were performed in rotation symmetry than in reflection symmetry(4.4 ± 0.7; two-tailed, paired t-test: p < 0.001; see Fig. 3a). This

Fig. 3. Percentage and duration of the tilts. (a) Percentage of tilts. Participants embodied about 1/3 of the avatar’s leaning movements (small disc). In these cases, participantsleaned mostly in rotation symmetry and much less in reflection symmetry (large disc). (b) Time course of tilts. Participants leaned more often in the first two thirds of theexperiment than in the last third. The ratio rotation/reflection symmetry sees to be rather constant. Mean (and SEM) of 10 participants. (c) Duration of the tilts. When theduration of the tightrope walker’s tilts increased, the duration of the participants’ tilts increased as well. The dotted line indicates equal duration for tightrope walker’s andparticipants’ tilts. Mean and SEM of 10 participants.

Fig. 2. Data analysis. (a) Sample’s data of the participant no. 3. We synchronized the movie onset with the recording onset and determined whether participants leaned inrotation or reflection symmetry when the tightrope walker was leaning. In this example, the participant leaned four times in rotation symmetry (+) and once in reflectionsymmetry (*). The other five tilts of the tightrope walker did not evoke a tilt. (b) Details of the synchronization between one tilt in rotation symmetry (in red) and thecorresponding tilt of the tightrope walker (in blue). We analyzed only participants’ movements for which the amplitude of the lateral tilt was larger than 5� (threshold). Wedefined the point in time when the tilts goes beyond the threshold as the onset of the movements (t1 for the participant; t10 for the tightrope walker). Consequently, thereactions time (RT) is given by the difference between t1 and t10 . Finally, we defined the point in time when the tilt returns below the threshold as the offset of the movements(t2, t20). Thus, the duration of the movement is given by the difference between t2 (t20) and t1 (t10). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

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predominance of rotation symmetry (about 2/3) seemed to be con-stant along the experiment (Fig. 3b). Moreover, the sort of symme-try seemed to affect reaction times as participants tended toperform longer in rotation (824 ± 86 ms) than in reflection symme-try (613 ± 91 ms; two-tailed, paired t-test: p = 0.078).

Leaning movements of males and females seemed to occur sim-ilarly often in rotation (males: 10.8 ± 0.5; females: 9.0 ± 2.0; two-tailed, unpaired t-test: p = 0.177) and reflection symmetry (males:4.0 ± 0.9; females: 4.7 ± 0.9; two-tailed, unpaired t-test: p = 0.312),respectively. Thus, as with tilts in general, we found no genderdifference.

There was no significant difference between tilts to the left(7.6 ± 1.2) and tilts to the right (6.5 ± 1.2; p = 0.168). Thus, the factthat all participants were right-handed did not affect the leaningmovements.

4. Discussion

4.1. A motor paradigm to investigate self–other interactions

Here, we investigated the mechanisms of spontaneous embodi-ment and self-location when healthy participants interacted with a

virtual tightrope walker. To render the experimental conditionsecologically more valid, we designed a motor paradigm controllingthree phenomenological key aspects of interindividual relation-ships: i.e., interactive, naturalistic and spontaneous, as in the fol-lowing. First (interactive aspect), both participants and avatarfaced each other in standing position. We used the same body po-sition as most social interactions occur in the same body positions(seated/seated or standing/standing), this ‘‘similarity” having beenshown to favor exchange or share of actions, information, ideas,opinions and feelings (Scheflen, 1964). We further used the stand-ing position as most interactions have also been shown to occur instanding or walking position (Bavelas, Black, Lemery, MacInniset al., 1986; Bavelas, Black, Lemery, & Mullet, 1986; Bavelaset al., 1987, 1988; Parsons, 1987; Reed & McGoldrick, 2007; Schef-len, 1964) and brain mechanisms to differ depending on the bodyposition (Arzy et al., 2006; Gaunet & Berthoz, 2000; Lobmaier &Mast, 2007). To reinforce interaction, participants further acted ina comparable spatial environment as the tightrope walker and per-formed the same intransitive action: i.e., walking forward andbackward on a line designed on the ground when the avatar waswalking forward and backward on a rope. Second (naturalistic as-pect), the tightrope walker was displayed life-sized on the screen

196 B. Thirioux et al. / Brain and Cognition 70 (2009) 191–200

and generated by a neural network model (Bret, 1988) taking intoaccount the rules of natural movements and biomechanical con-straints of the body (2/3 power law; Berthoz, 1997; Viviani & Flash,1995; Viviani & Terzuolo, 1982). Third (spontaneous aspect), par-ticipants were blind-tested as, the tightrope walker in addition toher forward/backward movements performed irregularly and ran-domly leaning movements that we did not mention in the briefing.To test for the participants’ spontaneous behavior, we further useda goniometer which was fixed in the middle of the metal bar holdby the participants and recorded continuously the parameters ofthe movement independently of explicitly given responses (suchas pressing a response button). In addition, our motor paradigm al-lowed us to investigate whether and how participants would spon-taneously embody the avatar’s tilts. Hence, according to theobserved leaning direction (i.e., rotation/reflection symmetryaccording to the direction of the avatar’s leaning movement), wehypothesized whether participants embodied the avatar’s move-ments from their own visuo-spatial perspective via an embodiedself-location or from the avatar’s visuo-spatial perspective, via adisembodied self-location.

4.2. Spontaneous embodiment

Our data showed that all 10 participants reproduced automati-cally about one third of the avatar’s leaning movements. One mayobject that the participants’ tilts are due to the forward/backwardmovements per se, inducing possible random unbalances. If so, thetilts should have occurred independently of the avatar’s leaningmovements. Only a few tilts occurred in a time period broadly ex-tended according to the stimulus onset (i.e., 2 s after the onset ofan avatar’s leaning movement), suggesting that most of the tiltswere rather motor effects induced by the interaction with thetightrope walker. Moreover, if tilts were random unbalances, weshould not have found a significant difference between the per-centage of tilts in rotation and reflection symmetry, which wasthe case. It is also very unlikely that participants, misunderstand-ing the task, reproduced the avatar’s leaning movements in addi-tion to its forward/backward movements. First, we did not noticecomprehension difficulties by the participants during the briefingand, second, it is highly improbable that the 10 participants‘‘mis-transformed” the tasks in a similar way. We rather believein accordance with the current human embodied model of socialrelationships (Carr et al., 2003; Fadiga et al., 2005; Gallese, 2007;Rizzolatti et al., 2001) that participants here spontaneouslyembodied the avatar’s leaning movements. Our finding replicatesprevious works on ‘‘non deliberate imitation” (Hull, 1933) reveal-ing that individuals tend to imitate, independently of the consciouswill, the nonverbal motor behaviors of another person being ob-served (Bavelas, Black, Lemery, MacInnis et al., 1986; Bavelas,Black, Lemery, Mullet, 1986; Bavelas et al., 1987, 1988; Chartrand& Bargh, 1999; Darwin, 1872/1965; Dromard, 1905; Lakin & Char-trand, 2003; O’Toole & Dubin, 1968; Stotland, 1969), for instanceby ‘‘wincing at” [someone else’s] ‘‘injury”, ‘‘smiling at” [someoneelse’s] ‘‘joy” or ‘‘leaning with” [someone else’s] ‘‘effort” (Bavelaset al., 1988, from which the quotations are taken; Bavelas, Black,Lemery, MacInnis et al., 1986; Bavelas, Black, Lemery, Mullet,1986; Bavelas et al., 1987; O’Toole & Dubin, 1968). Pleading in fa-vor of an automatic imitation (i.e., without conscious action plan-ning), the participants’ tilts here concord with the hypothesis thatperceiving an action activates automatically the observer’s corre-sponding motor programs (Bertenthal, Longo, & Kosobud, 2006)by the use of a direct perceptual-motor mapping process in whichperception and action are coupled by a common representationalcoding (Wohlschläger, Gattis, & Bekkering, 2003; see also Berten-thal et al., 2006; Butterworth, 1990; Grafton, Arbib, Fadiga, & Riz-zolatti, 1996; Gray, Neisser, Shapiro, & Kouns, 1991; Iacoboni et al.,

1999; Iacoboni et al., 2005). Hence, it is likely that participantshave automatically translated a dynamic complex visual input pat-tern into motor commands ‘‘in such a way that the resulting move-ment visually matches the model movement” (Wohlschläger et al.,2003). The duration data concord also with this hypothesis as theduration of the participants’ tilts increased and decreased whenthe avatar’s tilts increased and decreased, respectively. This dura-tion concordance may thus reflect a phenomenon of motor reso-nance, as if participants adapted their movements’ duration tothat of the avatar. This corroborates previous studies on passiveobservation of actions, showing that individuals change theirbreathing when merely observing other individuals performingeffortful actions (Blakemore & Frith, 2005; Paccalin & Jeannerod,2000). According to these authors, this motor resonance responsereveals that individuals prepare to perform such actions them-selves. Here, our results further show that this duration adaptationoccurs in active self–other interaction, suggesting that embodi-ment may consist of spatial (reproduction of the leaning move-ment) and temporal (adaptation of the movement duration)components, at least in the context of our experiment.

Moreover, inspired by the discovery of mirror neurons in theventral premotor cortex of the monkey’s brain (di Pellegrinoet al., 1992; Rizzolatti et al., 2001), the human embodied modelof social interactions based upon simulation processes (Carret al., 2003; Gallese, 2007) has recently stressed that both the mo-tor resonance and the direct mapping system are facilitated andmediated by the activation of the so-called human mirror neuronsystem (hMNS; Bertenthal et al., 2006). Numerous studies haveshown (for a review, see Iacoboni & Dapretto, 2006) that observa-tion and imitation of another’s person actions (Buccino et al.,2001), emotions (Carr et al., 2003) and intentions (Iacoboni et al.,2005) enhances neural activity in the premotor cortex, intraparie-tal operculum, and inferior frontal gyrus (Grèzes, Armony, Rowe, &Passingham, 2003; Iacoboni et al., 1999; Koski, Iacoboni, Dubeau,Woods, & Mazziotta, 2003; Schulte-Rüther, Markowitsch, Fink, &Piefke, 2007), comparable to activity changes when the observersthemselves execute these actions, have these intentions, or feelthese emotions. This suggests that the hMNS underlies the embodi-ment mechanisms as well as the mechanisms of self-attribution ofanother person’s actions, intentions or emotions. Hence, the mirrorsystem would provide ‘‘neural precursor mechanisms for the hu-man ability to imitate” (Koski et al., 2003) by allowing the directmapping of the representation of observed and executed actions(Bertenthal et al., 2006; Hommel, Müsseler, Aschersleben, & Prinz,2001; Prinz, 1990, 1997), promoting a self–other overlap (Lozano,Hard, & Tversky, 2006; see also Davis, Conklin, Smith, & Luce,1996; Galinsky & Moskowitz, 2000; Vorauer & Cameron, 2002).Moreover, according to the mirror hypothesis, mirroring physicallyanother person’s body posture or action would increase the activa-tion of this self-attribution process (Carr et al., 2003; Iacoboni et al.,2005; Schulte-Rüther et al., 2007). Here, we further assume thatself-attributed the other individuals’ actions, emotions and inten-tions are rather experienced from one’s own visuo-spatial perspec-tive as one’s own actions, emotions and intentions than from theother individual’s visuo-spatial perspective (Jorland & Thirioux,2008). This hypothesis concords with recent studies, revealingthe dominance of the right frontal cortex in self-referential pro-cessing and first-person perspective (Devinsky, 2000; Fossatiet al., 2003; Northoff & Bermpohl, 2004; Vogeley & Fink, 2003),as well as in self-recognition tested in front-facing images of one’sown face (Keenan, Wheeler, Gallup, & Pascual-Leone, 2000). Hence,even if the embodiment process observed in the present study maybe explained, to a certain extent, by the activation of visuo-motormirroring mechanisms, the most prominent outcome of our exper-iment concerning the sort of symmetry being performed (i.e., rota-tion symmetry) may hardly be explained by the sole hypothesis of

B. Thirioux et al. / Brain and Cognition 70 (2009) 191–200 197

mirroring mechanisms activation. If so, we rather believe that par-ticipants would have physically mirrored the avatar’s leaningmovement, i.e., performed reflection symmetry, which was notthe case.

4.3. Embodiment with disembodied self-location

Participants here embodied the avatar’s tilts by leaning mostlyin rotation symmetry (i.e., with the preservation of lateral asym-metries) and much less in reflection symmetry (i.e., left/rightreversal). Confirming that an embodiment process does not neces-sarily be exhibited by the sole physically mirroring body posture,our finding concords with the proposal by Wohlschläger et al.(2003) to use strictly the term ‘‘direct mirroring” (or ‘‘mirroringmechanisms”) for the description of the specific behavior whenindividuals physically mirror another person’s behavior and notfor the systematic description of related embodiment processes,such as adaptation of the movement’s duration or breathingchange. We cannot exclude that the tilts in rotation symmetry inour study may have been induced by the mere activation of bodyrepresentations probably involved in imitation processes, as pro-posed by the ‘‘active intermodal matching model” (AIM; Meltzoff& Moore, 1997, 2002). According to the AIM, imitation dependsupon a supramodal system in which one’s action and another indi-vidual’s action are represented as non-decomposed units (Chiava-rino, Apperly, & Humphreys, 2007): when imitating, individualswould match their own movements with the other individuals’movements, via a representation of both actions ‘‘within a singlerepresentational framework of organ relations” (Chiavarino et al.,2007). Hence, analyzing the other individuals’ movement fromthe anatomical parts that they are using to produce their move-ments, we would reproduce the same movement with the sameanatomical part (Chiavarino et al., 2007). Accordingly, rotationsymmetry here would possibly have been induced by the natureof our stimulus as the avatar’s forward/backward movements weredesigned to last irregularly and to appear randomly, rendering dif-ficult to infer the avatar’s motor intention. Hence, participants mayhave parsed the anatomical features of the tightrope walker’smovements to deduce whether she was about to walk forward orbackward, leading them to rather refer to the structural represen-tation of the body (i.e., corresponding to the body in general;Sirigu, Grafman, Bressler, & Sunderland, 1991).

Therefore, we rather believe that rotation symmetry here re-flects the features of perspective-taking mechanisms (Jackson,Meltzoff, & Decety, 2006; Lozano et al., 2006), suggesting thatparticipants have reproduced spontaneously the avatar’s leaningmovement by referring to its visuo-spatial perspective – orembodiment with a disembodied self-location. According to ourmodel, rotation symmetry is the motor effect of a specific sortof mental rotation (i.e., own-body transformation; Blanke et al.,2005) by which individuals mentally rotate their body by 180�to align it with another individual’s body, which is not requiredfor reflection symmetry (Franz, Ford, & Werner, 2007). Hence,considering that rotation symmetry is a more cognitivelydemanding process than reflection symmetry (Mohr, Brugger, &Blanke, 2006), RTs should be longer in the former than in the lat-ter. Our response speed data concord with this hypothesis, show-ing that RTs tend to be faster in reflection (613 ms) than rotationsymmetry (824 ms). This corroborates previous studies on explicitmental body transformations tasks, revealing that participantsperform faster left–right judgments while imagining the body ofa schematic figure as their own-body as reflected in a mirror(embodied self-location; Arzy et al., 2006) than imagining them-selves in the figure’s body position (disembodied self-location;Arzy et al., 2006; see also Blanke et al., 2005; Parsons, 1987; Zackset al., 1999). Hence, our findings suggest that the leaning direc-

tion (rotation/reflection symmetry) associated to specific RTsmay provide empirical criteria to investigate whether individualsembody another person’s behavior with an embodied or disem-bodied self-location. Considering its cognitive demand, one maytherefore object that rotation symmetry is a counter intuitivebehavior. Therefore, previous studies have proposed that perspec-tive-taking may not be reduced to its cognitive or time demandbut rather understood as what improves action understanding(Lozano et al., 2006). For instance, previous works on verbaldescription of actions have shown that individuals spontaneouslytend to adopt a third-person perspective when describing a goal-directed action performed by another person (by using expres-sions such as ‘‘she putts the block on the left” instead of ‘‘sheputts the bloc on my right”, from Hard, Lozano, & Tversky,2006; see also Hard, Tversky, & Lang, 2006; Lozano, Hard, & Tver-sky, 2008; Lozano et al., 2006). Moreover, perspective-taking hasbeen shown to promote action understanding and learning (Loz-ano et al., 2006) as it allows a spatial coding centered on the otherindividual body, leading to understand his/her action as his/herown one. Concordant with these previous works, our findings fur-ther revealed that spontaneous perspective-taking mechanismsmay also be used and tested during motor and active interactionand observation of intransitive action. Nevertheless, the rotationsymmetry dominance in our study contrasts with the findingsby Bavelas et al. (1988). In their experiment, seated participantsreacted mostly by ducking to their left when a seated experi-menter facing them and telling them a story about ‘‘the dangersof being a short person in a party” (Bavelas et al., 1988), illus-trated her words by ducking to her right (i.e., reflection symme-try), avoiding her of ‘‘being hit in the head by the elbow of amuch taller person” (as the experimenter reported; Bavelaset al., 1988). First, this behavioral difference may be explainedby the participants’ different body positions: seated (previouswork) or standing (present work). Second, the visual stimulationwas either a female human being (previous work) or a virtual fe-male tightrope walker (present work). Third, participants were inaddition verbally stimulated in the Bavelas et al. study (1988),which was not the case in ours. More importantly (fourth), Bav-elas et al., (1988), starting from the hypothesis that rapport corre-lates with mirroring behavior (LaFrance & Broadbent, 1976),ensured rapport in that the experimenter told each participant a‘‘light and humorous story” (Bavelas et al., 1988) while holdingeye-contact. The stimulus per se regarding the ‘‘dangers of beinga short person” (Bavelas et al., 1988) may also have increasedemotional rapports. We suggest that this threefold priming effectled participants to attribute to themselves the experimenter’semotional situation and to mirror her movement (see also Lakin& Chartrand, 2003). Hence, it seems that according to the context(i.e., cognitive, emotional, or neutral. . .), participants may embodyanother individual’s behavior either with an embodied or disem-bodied self-location, extending the current debate on imitationaccording to which mirror and anatomical imitations coexist (Ber-tenthal et al., 2006; Wohlschläger et al., 2003). This also pointsout the necessity to discriminate between different sorts of inter-subjectivity and thus to understand how the mechanisms ofembodiment, self-location and mirroring/perspective-taking mayrelate.

4.4. A phenomenological model of self–other interaction

We propose a model which may be useful in understanding thedifferent neurocognitive mechanisms underlying the self–otherinteractions (Fig. 4). In line with the ancient (Husserl, Hua XIII-XVI; Merleau-Ponty, 1945) and current hypotheses of the phenom-enology (Berthoz, 2004; Blanke & Metzinger, 2009; Jorland, 2004;Jorland & Thirioux, 2008; Zahavi, 1994, 2008), we hypothesize that

Fig. 4. Self–other interaction schema. One key aspects of the self is self-location, i.e., the experience that the self is localized at a specific position in space within one’s bodilyborders, or embodied self-location mediating our own egocentered visuo-spatial perspective (perceptive mechanisms). Depending on the context, self–other interactions maybe carried out either from our own visuo-spatial perspective (1PP) or from the other’s visuo-spatial perspective (3PP). In the first case, we imagine that the other’s body is ourown-body, as reflected in a mirror, reducing the other’s visuo-spatial perspective to our own one (mirroring process). Therefore, we still perceive the world from our imposedand physical visuo-spatial perspective but perform an additional process to imagine the other’s body as our own-body as reflected in a mirror. In the second case, we take theother’s visuo-spatial perspective by performing another sort of mental body transformation – own-body-transformation – by which we imagine ourselves in the other’s bodyposition (or disembodied self-location). This perspective-taking is a mental imagery process, additional to the basal perceptive process by which we continuously perceive theworld from our physical 1PP. During self–other interaction, the perspective reduction and change (visuo-spatial function level) are respectively mediated by mental imagerywith embodied and disembodied self-location (self-location mechanisms level) and may be reflected at a motor level either by reflection or rotation symmetry.

198 B. Thirioux et al. / Brain and Cognition 70 (2009) 191–200

the self experiences under normal conditions an embodied self-location (Blanke & Metzinger, 2009) which mediates its own egoc-entered visuo-spatial perspective (Berthoz, 2004; Jorland & Thir-ioux, 2008; Zahavi, 1994). We further consider it as referring to a‘‘perceptive level” as both embodied self-location and 1PP enableour direct perception of the world (including objects and other indi-viduals). Indeed, the imposed first-person and ‘‘invariable perspec-tive” (Merleau-Ponty, 1945, p. 107) of our body is a ‘‘physicalnecessity” (ibid), giving rise to a spatio-temporally determined per-ception. Hence, the sole visuo-spatial perspective that we can expe-rience perceptively (i.e., physically, spatio-temporally and in proper;Husserl, Hua XIII-XVI) is our own one. The certitude of our percep-tive determination is maintained when interacting with someoneelse, as this certitude enables us to distinguish continuously our-selves from the other individuals (anyhow we are interacting, weare always convinced to be physically where we are and we still per-ceive the other individual from our own perspective). However,within self–other interaction, mechanisms of mental imagery mayoccur in addition to this basal perceptive mechanism. We here pro-pose that depending on the context, we may perform additionalmental perspective transformations (or a-perceptive process), lead-ing to interact with other individuals either from our own visuo-spatial perspective or from the other’s visuo-spatial perspective.In the first case, we would perform a specific sort of a mental bodytransformation (i.e., a mirroring process) and imagine that theother’s body is our own-body as reflected in a mirror (Arzy et al.,2006), reducing his/her perspective to our own one (Fig. 4). Thismay occur for instance when we interact with individuals experi-

encing emotional situations that we have already experienced orwhen we are affectively linked to these individuals, what rathertriggers the trend to attribute to ourselves their emotions. In the sec-ond case, we take the other’s visuo-spatial perspective by perform-ing another sort of mental body transformation (i.e., own-body-transformation; Blanke et al., 2005) by which we imagine ourselvesin the other’s body position (or disembodied self-location; Arzyet al., 2006; Blanke et al., 2005). This perspective-taking is also amental imagery process, additional to the basal perceptive processby which we still perceive the world from our physical and imposed1PP. This may occur for instance when we engage in understandingor learning another person’s action. We propose that during self–other interaction, these mechanisms of mental perspective transfor-mation (i.e., reducing mentally another individual’s visuo-spatialperspective to our own one or taking another individual’s perspec-tive) are respectively mediated by mental imagery with embodiedand disembodied self-location (Fig. 4) and may be reflected at a mo-tor level either by reflection or rotation symmetry. Our modelshould now be further developed and tested in various contextualfeatures of self–other interaction (for instance we consider thatself–other interaction may also occur via the mere mechanism of1PP maintenance without neither mirroring nor perspective-takingprocess) and in neuroimaging studies to investigate whether spon-taneous reflection and rotation symmetry activates the hMNS andthe right temporo-parietal junction, respectively (Arzy et al.,2006; Blanke et al., 2005). We believe that this model associatedto our motor paradigm may offer a promising way to investigateself–other interactions in general.

B. Thirioux et al. / Brain and Cognition 70 (2009) 191–200 199

5. Conclusion

In the present study, we investigated the mechanisms of self-location during spontaneous embodiment processes. For that, wedesigned a new paradigm allowing, in ecologically more valid con-ditions, the investigation of the motor and physical features of theembodiment process, i.e., whether and how another person’sbehavior is spontaneously embodied when being observed. Weshow that when interacting with a virtual tightrope walker with-out explicit given instruction, participants spontaneously locatedmentally themselves in the avatar’s body position to take its vi-suo-spatial perspective, suggesting that embodiment process isnot necessarily exhibited by a physically mirroring body posture.We further propose a model of self–other interaction showinghow perspective-taking mechanisms may relate on mental bodytransformation and enabling to deepen the description of the dif-ferent sorts of intersubjectivity.

Acknowledgments

Bérangère Thirioux was supported by the WAYFINDINGEuropean Project ‘‘Finding your way in the world: on the neuro-cognitive basis of spatial memory and orientation in humans”,according to the convention established between the Centre Na-tional de la Recherche Scientifique and WAYFINDING. Theauthors thank France Maloumian (Laboratoire de Physiologie dela Perception et de l’Action, Paris, France) for illustrations pre-paring. Bérangère Thirioux thanks sincerely Thomas U. Otto fordiscussion.

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