Neuropsychologia 46 (2008) 3014–3018

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The role of the right temporo-parietal junction in maintaining a coherent sense of one’s body Manos Tsakiris a,∗,1 , Marcello Costantini b,1 , Patrick Haggard c a

Department of Psychology, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom Department of Clinical Sciences and Bio-imaging, University “G. d’Annunzio”, Chieti, Italy c Department of Psychology & Institute of Cognitive Neuroscience, University College London, United Kingdom b

a r t i c l e

i n f o

Article history: Received 22 November 2007 Received in revised form 2 June 2008 Accepted 5 June 2008 Available online 14 June 2008 Keywords: Body Body-ownership Multisensory integration Rubber hand illusion Temporo-parietal junction Transcranial magnetic stimulation Self

a b s t r a c t We constantly feel, see and move our body, and have no doubt that it is our own. The brain possesses a distinction between the body and the objects in the outside world. This distinction may be based on a process that monitors whether sensations, events and objects should be attributed to one’s body or not. We controlled whether an external object was represented as part of the body or not, by experimentally inducing a bodily illusion using correlated visual and tactile stimulation. We then studied the role of right temporo-parietal junction (rTPJ) in the processing of multisensory events that may or may not be attributed to one’s body. Disruption of rTPJ using transcranial magnetic stimulation (TMS) made the distinction between what may or may not be part of one’s body on the basis of multisensory evidence more ambiguous, suggesting that the rTPJ is actively involved in maintaining a coherent sense of one’s body, distinct from external, non-corporeal, objects. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction The integration of multiple sensory inputs related to the body produces a sense of self, a sense of body-ownership. Several studies (Botvinick & Cohen, 1998; Ehrsson, Spence, & Passingham, 2004; Tsakiris & Haggard, 2005) suggest that multisensory stimulation drives the experience of body-ownership. For example, in the rubber hand illusion (RHI), participants watch a prosthetic rubber hand being stroked in synchrony with stroking of their own unseen hand. Synchronous, but not asynchronous, multisensory stimulation causes the rubber hand to “feel like it’s my hand” (Botvinick & Cohen, 1998; Tsakiris & Haggard, 2005), to become incorporated in the participant’s representation of her own body. This effect suggests that multisensory evidence can be used to produce a subjective feeling of body-ownership which can extend to external objects such as the rubber hand. Interestingly, replacing the realistic rubber hand with a neutral, non-corporeal, object abolishes the positive effect of synchronous stimulation (Tsakiris & Haggard, 2005, see also Graziano, Cooke, &

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Tsakiris). 1 These authors contributed equally to this work. 0028-3932/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2008.06.004

Taylor, 2000, but cf. Armel & Ramachandran, 2003). Thus, the effect of multisensory stimulation on body-ownership is not simply a passive stimulus-driven process, but rather seems to depend on the modulatory influence of visual, tactile and postural representations of the body (cf. de Vignemont, Tsakiris, & Haggard, 2006; Dinse et al., 1997; Graziano & Botvinik, 2001). Current sensory stimuli are processed and finally tested-for-fit against an abstract body-model that maintains a coherent sense of one’s own body. This abstract body-model would contain a reference description of the visual, anatomical and structural properties of one’s own body (Costantini & Haggard, 2007; Tsakiris & Haggard, 2005). It represents the body as a diachronic physical object that is maintained through time, in contrast to the body-schema model, which is continuously updated as the body moves (Wolpert, Goodbody, & Husain, 1998). Recent studies suggest that the right temporal and parietal lobes and in particular the right temporo-parietal junction (rTPJ) underpins an internal model of the body that could allow the brain to maintain a coherent representation of one’s body. Lesions in this region may result in denial of ownership of the contralateral hand (Bottini, Bisiach, Sterzi, & Vallar, 2002), neglect of the left side of the body (Committeri et al., 2007; Mort et al., 2003) and anosognosia for hemiplegia (Berlucchi & Aglioti, 1997). Direct electrical stimulation of rTPJ in a neurosurgical patient elicited experiences of seeing her body from an external perspective (“out of body experience”), and of illusory spatial transformations of the arms and legs

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(Blanke, Ortigue, Landis, & Seeck, 2002). From this evidence, rTPJ might underpin an internal model of the body that would function as a stored template against which to compare novel stimuli, playing a key role in maintaining a basic sense of embodied self. We focus here on specific brain processes that generate a coherent representation of one’s body on the basis of current multisensory input. This requires assigning sensations either to one’s own body, or to the world beyond the body. We used single-pulse transcranial magnetic stimulation (TMS) during the RHI to investigate the role of rTPJ in the processing of body-related events. We hypothesized that by disrupting activity in the rTPJ, we would impair the test-forfit process that underpins the distinction between corporeal and non-corporeal objects on the basis of visuo-tactile evidence.

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for correct coil positioning, using optical tracking via a frameless stereotactic system (Brainsight, Rogue Research, Montreal, Ontario, Canada). The vertex was localized using the 10-20 EEG system, and used as a control site for non-specific effects of TMS. Transcranial magnetic stimulation was applied using a 70 mm figure-of-eight coil connected to a 2 T Magstim 200 stimulator (Whitland, UK). TMS intensity was chosen for each participant to ensure that no muscle twitches occurred, and that no discomfort was felt during the experiment (mean = 51%; range 38–65% of maximum stimulator output). The coil was polyurethane coated, and it was not covered by a plastic cover, thus allowing the stimulating coil to come into close proximity to the targeted site of stimulation. TMS was delivered 350 ms after the end of visuotactile stimulation. This latency was chosen on the basis of a previous TMS study (Blanke et al., 2005) showing that from 350 to 550 ms TMS over rTPJ interfered with mental rotation of human bodies. Other studies (e.g. Yamaguchi & Knight, 1991) have moreover shown that the involvement of rTPJ in sensory processing is linked to the P300 event-related potential occurring with a latency of 350 ms.

2. Methods 2.2. Experimental procedure 2.1. Experimental design The experimental design was 2 × 2 × 2 factorial. The three factors were (i) the viewed object (rubber hand vs. object), (ii) the site of stimulation (rTPJ vs. vertex), and (iii) the presence of TMS (TMS vs. no-TMS). The first and the second factors were blocked, whereas the third one was pseudo-randomly varied within each block. The experiment consisted of four blocks. In two blocks we stimulated the rTPJ while participants viewed either a rubber hand or a neutral object. In two further blocks we stimulated the vertex. Each block had 40 trials, 20 with TMS and 20 without TMS. In two further blocks participants viewed either the rubber hand or neutral object while receiving asynchronous stroking. No TMS was delivered in these blocks. Each participant performed the blocks in a pseudo-random order. The TMS site for rTPJ was determined on the basis of anatomical landmarks in each participant using high-resolution, T1-weighted magnetic resonance images. The rTPJ site was defined as the junction of the supramarginal, angular, and superior temporal gyrus. The mean location for TMS over rTPJ, transformed into Montreal Neurological Institute (MNI) coordinates, was x = 63.4 ± 0.73, y = −50 ± 1.29, z = 22.7 ± 0.56 (Fig. 1a). MRIs were coregistered with the participant’s head position

At the beginning of each block, the participant’s left hand was placed by the experimenter at a fixed point inside a frame, the top side of which was covered by one-way and two-way mirrors. The two-way mirror was used to make the rubber hand/object appear (during visuo-tactile stimulation) and disappear. The participants were viewing the rubber hand/object in the same depth plane as their own hand. The rubber hand was a prosthetic life-size hand, and the object was a plastic spoon that had the same length as the rubber hand. At the beginning of each block, both the participant’s left hand and the rubber hand/object were out of sight. A pre-test baseline estimate of finger position was obtained prior to visuo-tactile stimulation. Participants saw a ruler reflected on the mirror. The ruler was positioned so as to appear at the same gaze depth as the rubber hand/object. Participants were asked to judge the location of their middle finger, by verbally reporting a number on the ruler. During the judgments, there was no tactile stimulation, and the lights under the two-way mirror were switched off, to make the rubber hand/object invisible, leaving only the ruler visible. After the judgment, the ruler was removed, the lights under the two-way mirror were turned on to make the rubber hand/object appear, and after 700 ms visuo-tactile

Fig. 1. TMS site and experimental protocol. (a) shows the TMS site at the rTPJ rendered over each subject’s cortical surface plot constructed from the individual MRIs. In the individual subjects (S) the MNI coordinates for the TMS site at the rTPJ were the following: S1–S10, (58, −55, 20), (66, −45, 22), (65, −44, 22), (65, −46, 22), (63, −49, 22), (64, −52, 20), (64, −54, 22), (62, −55, 26), (65, −51, 25), (62, −49, 22). (b) shows the temporal sequence of events in an exemplar trial of TMS while looking at a rubber hand.

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stimulation begun. Stimulation was delivered mechanically by two stepper motors to which paintbrushes were attached. The overall amount of stimulation was thus precisely matched across conditions. In order to avoid habituation effects, the speed and the direction of the paintbrushes was unpredictable. The participant’s hand and rubber hand/object always received the same direction of stimulation at the same speed on each stroke. Both the participants’ and the rubber hand were stimulated horizontally on the middle finger for 2300 ms. After the stimulation period, the lights were automatically turned off. In half of the trials, a single TMS pulse was delivered 350 ms after the end of visuo-tactile stimulation. The ruler was then presented again and participants were asked to make a judgment about the felt location of their middle finger. The ruler was always presented with a random offset to ensure that participants judged finger position anew on each trial, and that they could not simply repeat previous responses. Participants were asked “Where is your middle finger?”. After their answer, the ruler was removed, and a new trial started (Fig. 1b). In the asynchronous conditions, the starting position of the paintbrush stimulating the rubber hand/object was offset by 90◦ , resulting in out-of-phase visuo-tactile stimulation. Synchronous and asynchronous conditions differed only in the degree of temporal correlation of visual and tactile stimulation. At the end of each block, participants were asked to have a rest. Following the rest period, their left hand was again passively placed at a pre-determined point. Ten participants (six female, eight right-handed by self-report, mean age 28.6, S.D.: 4) gave their written consent and participated in the experiment that was approved by the UCL/UCLH joint ethics committee.

the viewed object (rubber hand vs. spoon), (ii) the site of stimulation (rTPJ vs. vertex), and (iii) the presence of TMS (TMS vs. no-TMS). The main effect of the viewed object was significant (F(1, 9) = 10.53, p < 0.05). In contrast, the main effects of site of stimulation (F(1, 9) = .57, p > 0.05) and of TMS (F(1, 9) = 4.46, p > 0.05) were not significant. The two-way interaction between the site of stimulation and the presence of TMS was significant (F(1, 9) = 6.39, p < 0.05), because TMS over rTPJ had a pronounced effect on proprioceptive drifts, while TMS over the vertex did not. The two-way interaction between the viewed object and the presence of TMS was significant (F(1, 9) = 9.34, p < 0.05). Importantly, the three-way interaction was significant (F(1, 9) = 19.5, p < 0.05). Follow-up simple effect analysis revealed that TMS over the vertex did not affect proprioceptive drift relative to no-TMS trials (t(9) = 0.23, p > 0.05 for the rubber hand, and t(9) = 0.07, p > 0.05 for the object, two-tailed). In contrast, TMS over the rTPJ significantly reduced drifts when participants viewed the rubber hand (t(9) = 4.67, p < 0.05, two-tailed), but significantly increased drifts when participants viewed the neutral object (t(9) = 2.55, p < 0.05, two-tailed). 4. Discussion

3. Results A baseline pre-test proprioceptive judgment was obtained at the beginning of each block. The mean of the pre-test judgments showed that participants felt their hand to be 17.85 cm (S.D. = 2.34 cm; the real distance was 17.5 cm) lateral to the rubber hand, and no significant differences in the pre-test judgments were observed across conditions. The judgment error from the pretest on each block was subtracted from each post-test judgment in that block. The term “proprioceptive drift” refers to the change in the perceived position of the hand relative to baseline in each condition. A positive drift represents a mislocalization towards the rubber hand/neutral object. In the two control conditions, where the viewed object was stimulated asynchronously with respect to the participant’s own hand, we observed non-significant drifts (rubber hand = 0.1 cm, object = 0.2 cm). Inspection of the mean proprioceptive drifts across conditions (see Table 1) shows that the proprioceptive drifts for the conditions where participants were looking at a rubber hand were overall larger than the proprioceptive drifts while looking at the neutral object. This replicates previous findings on the modulation of the RHI by visual context (Tsakiris & Haggard, 2005). In addition, the no-TMS trials when subjects were looking at a rubber hand showed larger mean drifts in the rTPJ condition than in the vertex condition. Further inspection showed that this difference was driven by a single outlier (subject 7). Importantly, however, the general pattern and significance of results was unaffected by whether this individual was included or excluded. Therefore, the difference between trials with and without TMS was not driven by this unusually high value of proprioceptive drifts observed in the rTPJ condition. Our interest here is in the effect of TMS within each block. We therefore focus on the interaction between TMS site and presence/absence of TMS for each viewing condition. The mean proprioceptive drifts across conditions were analysed by 2 × 2 × 2 repeated-measures ANOVA. The three factors were (i) Table 1 Mean proprioceptive drift in cm and (S.E.) towards the viewed object across conditions

View rubber hand View object

TMS coil over rTPJ

TMS coil over vertex

No-TMS

TMS

No-TMS

TMS

5.31 (1.60) 0.63 (0.74)

4.09 (1.53) 1.04 (0.88)

3.94 (0.72) 0.40 (0.49)

3.99 (0.77) 0.39 (0.46)

We hypothesized that the rTPJ plays a crucial role in maintaining a coherent representation of one’s body. Our results suggest that this area is actively involved in the processing of corporeal and non-corporeal stimuli on the basis of multisensory information. Multisensory integration is essential for the demarcation one’s body as a physical object distinct from external objects and other agents (Lopez, Halje, & Blanke, 2008; Tsakiris, Schütz-Bosbach, & Gallagher, 2007b). When participants received visual and tactile stimulation without TMS, proprioceptive measures suggested that the rubber hand was attributed to the participants’ body, but the neutral object was not. Objects that are visually coherent with a reference model of one’s own body more readily evoke a sense of body-ownership (Tsakiris & Haggard, 2005) and are more strongly incorporated into body representations. Critically, when the rTPJ was disrupted by TMS immediately after multisensory stimulation, the drift in perceived position of the unseen hand towards the viewed object decreased in blocks where participants viewed the rubber hand, but increased when participants viewed the neutral object. The felt location of one’s hand towards or away from the viewed object in the classic RHI manipulations has been shown to correlate with the sense of body-ownership (Botvinick & Cohen, 1998; Longo, Schüür, Kammers, Tsakiris, & Haggard, 2008; Tsakiris, Hesse, Boy, Haggard, & Fink, 2007a), suggesting that proprioceptive drifts towards the viewed object during RHI indicate incorporation and experienced ownership, while proprioceptive drifts away from the viewed object indicate failure of incorporation and disownership. Our design was based on gradual and ongoing accumulation of multisensory evidence during each block. Each brief period of stroking added to that provided earlier in the block. That is, each new multisensory stimulation ‘topped’ up the sense of body-ownership when subjects were looking at a rubber hand, and concomitantly produced greater proprioceptive drift. Previous studies (Costantini & Haggard, 2007; Tsakiris & Haggard, 2005) suggest that this cumulation depends on the visual object that is seen during stroking. The present study shows that TMS over the rTPJ alters this dependence. At first inspection, the fact that TMS over rTPJ decreased the incorporation of the rubber hand while it increased the incorporation of the neutral object (see Fig. 2) may suggest that, paradoxically, the effect of TMS has an opposite direction in the two conditions. In fact, this pattern strengthens our hypothesis that rTPJ is linked to a test-for-fit process used to filter multisensory input to inform and maintain our sense of embodiment. By disrupting activity in

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Fig. 2. The effect of transcranial magnetic stimulation (TMS) as the difference in the proprioceptive drifts in cm between TMS and no-TMS trials for each combination of viewed object and stimulated brain area. Error bars represent standard errors.

the rTPJ, proprioceptive drifts became more comparable between rubber hand and neutral object, suggesting a blurring of the boundary between the effects of viewing a corporeal or a non-corporeal object as a result of TMS. This effect seems to reflect a specific test-for-fit process used to discriminate between visual and tactile events that may be assigned to one’s own body from sensory events that produce a mismatch. Interestingly, Shulman, Astafiev, McAvoy, d’Avossa, and Corbetta (2007) have proposed a similar model of rTPJ activity in an ostensibly different cognitive function such as visual monitoring for targets in the context of distractors. Other studies (Costantini & Haggard, 2007; Lenggenhager, Tadi, Metzinger, & Blanke, 2007; Tsakiris & Haggard, 2005) have suggested that multisensory stimulation during the RHI is modulated by long-term, abstract and canonical representations of the body that relate to visual, anatomical and structural features of body parts. The interference produced by TMS over rTPJ seems to have significantly weakened the modulatory influence of this bodymodel. By definition, this body-model contains substantial prior information about one’s own body. Fit between current stimulation and this model can therefore provide a criterion for distinguishing between ownership and disownership. Our TMS interference in the present study may have altered this criterion and impaired the distinction between corporeal and non-corporeal stimuli. This results in reduced incorporation of the rubber hand, in line with other studies showing altered states of embodiment after TPJ stimulation (Blanke et al., 2005; Le Chapelain, Beis, Paysant, & Andrè, 2001). The results are also consistent with the idea that the rTPJ is involved in the differential processing of corporeal and noncorporeal stimuli. We can postulate at least two mechanisms for the interfering effect of TMS, either of which could explain our results. First, TMS could have added extra noise to the neural signals that provide input to a body/non-body discrimination process. Additional input noise would impair discrimination. Second, TMS could have transiently arrested the test-for-fit process itself, reducing the difference between body and non-body processing. Our results cannot distinguish between these two mechanisms of action. We next consider possible artefactual explanations of our results. Could our results reflect an unspecific effect of TMS over the rTPJ or a purely attentional effect? It seems unlikely that this is the case, because of the opposite pattern of proprioceptive drifts observed in the rubber hand and neutral object conditions after TMS over that area. A second artefactual interpretation would attribute the result to unintentional stimulation of the nearby extrastriate body area (EBA). EBA is an area known to respond selectively to human bodies and body parts (Downing, Jiang, Shuman, & Kanwisher, 2001). A recent study has shown that interfering with

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neural activity in EBA by TMS induces a clear impairment in the visual processing of corporeal stimuli but not of non-corporeal stimuli (Urgesi, Berlucchi, & Aglioti, 2004). Thus, while EBA disruption could explain the altered proprioceptive drifts while looking at a rubber hand, it cannot explain the change in the representation of the neutral object. Our results suggest that rTPJ is actively engaged in a test-for-fit process between current sensory input and a reference bodymodel. Disruption of rTPJ eliminated the differential treatment of multisensory stimuli that are used in the maintenance of a coherent representation of one’s body by blurring the boundary between corporeal and non-corporeal stimuli. An object (i.e. a rubber hand) that would normally have been perceived as part of the participant’s own body came to be treated in a manner more similar to a neutral non-corporeal object. When rTPJ processing was disrupted by TMS, discrimination between the multisensory evidence that may or may not be attributed to one’s body became less definite, rendering the distinction between corporeal and non-corporeal stimuli more ambiguous. Recent studies have highlighted the role of the rTPJ in diverse cognitive functions ranging from low-level processing to high-level social cognitive abilities (Balslev, Nielsen, Paulson, & Law, 2005; Downar, Crawley, Mikulis, & Davis, 2000; Mitchell, 2008; Saxe & Wexler, 2005; Sugiura et al., 2005; Vogeley & Fink, 2003). What can account for the activation of rTPJ in such diverse tasks? To a certain extent, all these cognitive tasks assume a prior representation of a self, that is sufficiently distinct, from other subjects or objects to enter into relations with them. Our results, taken together with recent accounts of the function of rTPJ (for a review see Decety & Lamm, 2007) suggest that this area may underpin a single computational mechanism that is used by multiple cognitive processes. This basic mechanism involves testing sensory events for attribution to the self, and it seems to be lateralized on the right hemisphere (for a review see Keenan, Rubio, Racioppi, Johnson, & Barnacz, 2005). Our results show one specific instance of this general scheme, namely that rTPJ is involved in updating and maintaining this body-model by testing which stimuli are relevant to one’s own body. We suggest that the rTPJ is actively involved in testing the fit between current sensory input and a stored bodymodel. It therefore plays a critical role in maintaining a coherent sense of one’s own body. Several studies suggest that the sense of body-ownership arises from an interaction between bottom-up multisensory regularities, and top-down influences of a cognitive model of the body. This model describes the pre-existing visual, anatomical and structural features of the body. It thus acts as a reference against which current sensory inputs can be compared. Previous neuroimaging studies of the RHI have investigated the neural correlates of the sense of body-ownership that results when current sensory input fits with the stored model of the body (Ehrsson et al., 2004; Tsakiris et al., 2007a). This study, in contrast, has focused on the process of test-for-fit itself. Our results clarify the neural basis of the distinction between what may or may not be part of one’s own body. We suggest that the right temporo-parietal junction contributes to the sense of one’s own body by maintaining a stored reference model of one’s own body, which is used to distinguish between self-related events and other events in the outside world.

Acknowledgements MC was supported by MIUR, and by an RS-ESEP grant to PH. MT was supported by a BBSRC grant to PH. Additional support was provided by Bial Foundation Research Grant 203/08 to PH.

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