The feeling of body ownership — that the various parts of your body are all part of you — is something that we typically take for granted. However, brain damage can disrupt this sensation and leave individuals convinced that an arm or a leg is no longer their own. Even in healthy people, the ‘rubber hand illusion’ can temporarily produce a similar phenomenon. Individuals watch a lifelike rubber hand being touched while their own hand – which is hidden from view – is touched at the same time. This creates the feeling that the artificial hand has become part of their body, while their real hand is left in a ‘disembodied’ state.

How does the brain generate this illusory sense of ownership and accompanying sense of disembodiment? A person’s ability to move their body is thought to contribute to their feeling of body ownership. Therefore, della Gatta, Garbarini et al. asked whether the brain’s ability to move the real hand changes during the rubber hand illusion.

In the experiments, the region of the brain that controls hand movement was artificially stimulated in a number of volunteers. When an individual had been primed by the rubber hand illusion to perceive a fake hand as part of their own body, their brain was temporarily less able to activate the muscles of their real hand. This is as if the brain no longer considered the real hand as part of the body. Thus, the altered sense of body ownership experienced during the rubber hand illusion is not a bizarre fantasy, but corresponds to a physiological reaction that accompanies changes in brain activity.

The next step is to further define and quantify the relationship between the sense of body ownership and the control of body movement. Specifically, how does activity in the brain areas that control movement contribute to the sense of body ownership? And how do these brain regions communicate with one another to generate a sense of self?

The sense of body ownership (i.e. the belief that a specific body part belongs to one’s own body) (Gallagher, 2000) is a fundamental aspect of self-consciousness. Apparently, in normal conditions, the feeling of body ownership does not need any particular explanation; it is immediate and even obvious. However, both pathological cases after brain damage (somatoparaphrenia [Romano et al., 2014] and pathological embodiment [Pia et al., 2016; Fossataro et al., 2016; Garbarini et al., 2013, 2014, 2015; Pia et al., 2013; Garbarini and Pia, 2013]) and experimental manipulations in healthy subjects (e.g. the rubber hand illusion – RHI [Botvinick and Cohen, 1998]) suggest that body ownership, as we normally experience it, is the product of many different and complex operations. It has been suggested that the feeling that our body belongs to us presumably depends on multisensory integration processes arising within a fronto-parietal network, where sensory inputs coming from different modalities are realigned in a unique reference frame (Blanke et al., 2015). Within this network, the ventral premotor cortex seems to play a crucial role, thus establishing, both in monkeys (Graziano, 1999) and in humans (Makin et al., 2008; Ehrsson et al., 2004), an anatomical link between the sense of body ownership and the motor system. Furthermore, it has been proposed that voluntary motor activity of body parts contributes critically to the subjective experience of body ownership (Tsakiris et al., 2010). Within this context, we asked whether the subjective and sometimes illusory sense of body ownership influences objective measures of the sensory-motor system. We took advantage of the RHI paradigm in order to provide a physiological counterpart of the interaction between body awareness and motor control, investigating the relationship between body ownership alterations, such as those occurring during the RHI, and modulation of primary motor cortex excitability.

During the RHI, the subject’s real hand is out of view, while a realistic rubber hand (RH) is positioned in its place. When the experimenter synchronously strokes the index finger of both the real and the fake hand, most subjects, after a few seconds of viewing the fake hand’s finger being touched, attribute their tactile sensation to the RH hand, which they start to perceive as their own. During the illusion, the subject’s hand-centered reference frame shifts towards the RH, and so it has been proposed that, as a consequence, the real hand is subject to a sort of disembodiment (Ehrsson et al., 2004). Accordingly, a feeling of disownership of one’s own hand has been reported as an important behavioural component of the RHI (Longo et al., 2008), while a decrease in the temperature of the real (disembodied) hand has been observed as a neurophysiological correlate of the RHI (Moseley et al., 2008). Moreover, it has been demonstrated that cooling the subject’s hand increases the strength of the RHI, whereas warming the hand decreases it (Kammers et al., 2011). However, another study found that hand-cooling can be present in both the experimental (synchronous) and the control (asynchronous) condition, thus suggesting that it is not a reliable correlate of the subjective feeling of hand disownership in the RHI (Rohde et al., 2013). A further study proposed that somatosensory changes observed in the participants' hand while experiencing the RHI can be explained by cross-modal mismatch between the seen and felt position of the hand, and are not necessarily a signature of disownership (Folegatti et al., 2009).

In the present study, in the main behavioral experiment, we employed a classical RHI procedure to investigate the presence of the illusory experience in our subject sample. In addition, in the control behavioral experiment, the complementary presence of both embodiment of the RH and disembodiment of the real hand was explicitly investigated. Moreover, during the main physiological experiment, we studied the excitability modulation of motor circuits to the real (stimulated) hand during RHI. While subjects received visual-tactile stimulations, either synchronous (to induce the illusion) or asynchronous (control condition), motor evoked potentials (MEPs) were elicited by a single-pulse of transcranial magnetic stimulation (TMS) over the left primary motor cortex (M1) and recorded from the right first dorsal interosseous muscle (FDI). See details in Material and methods and in Figure 1A.

Figure 1

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We hypothesized that, in the motor domain, a disembodiment effect during the RHI might be measurable as a lower excitability of motor pathways to the real hand, i.e. a situation in which a stronger voluntary command is needed to bring enough motor neurons to threshold and thus to elicit movement. Thus, a decrease of FDI MEP amplitude compared to the baseline, specific to the real (disembodied) hand, was expected in the synchronous condition (where the subjects experienced the RHI) and not in the asynchronous (control) condition. Furthermore, according to behavioral studies reporting an increased illusory experience over time (Lewis and Lloyd, 2010; Valenzuela Moguillansky et al., 2013), this inhibitory motor response was expected to increase during exposure to the illusion. Finally, we expected the MEP amplitude decrease to be specific for the stimulated (right) hand, i.e. no amplitude modulation in the non-stimulated (left) hand (see Figure 1B; control physiological experiment).

The main behavioral results showed that both proprioceptive drift towards the RH and embodiment questionnaire rating (emb-q-rating) were significantly higher in the synchronous than in the asynchronous condition (drift=mean ± sd: 4.51 ± 4.2 cm vs. 2.08 ± 2.75 cm; t₍₂₃₎=2.783, p=0.0105, dz=0.58; emb-q-rating=mean ± sd: 2.4 ± 0.64 vs. −2 ± 0.9, Z=4.2857, p=0.000018, dz=3.88; Figure 2A₁ and A₂; see also Figure 2—source data 1). No significant correlation was found between emb-q-rating and proprioceptive drift.

Figure 2

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Figure 2—source data 1

Main experiment behavioral results following asynchronous and synchronous condition.

(A) MAIN EXPERIMENT. For each subject, the proprioceptive drift (estimation of right index finger felt position) mean values, calculated as the difference between pre and post stimulation in synchronous (mean ± sd = 4.51 ± 4.2) and asynchronous (mean ± sd = 2.08 ± 2.75), are reported. (B) MAIN EXPERIMENT. For each subject, the mean rating value of the three ownership statements in synchronous (mean ± sd = 2.4 ± 0.64) and asynchronous (mean ± sd = -2.04 ± 0.9) are reported.

https://doi.org/10.7554/eLife.14972.005

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Figure 2—source data 2

Control experiment behavioral results following asynchronous and synchronous condition.

(A) CONTROL EXPERIMENT. For each subject, the proprioceptive drift (estimation of right index finger felt position) mean values, calculated as the difference between pre and post stimulation in synchronous (mean ± sd = 2.47 ± 2.707) and asynchronous (mean ± sd = 0.075 ± 2.461), are reported. (B) CONTROL EXPERIMENT. For each subject, the mean rating value of the three ownership statements in synchronous (mean ± sd = 2 ± 0.763) and asynchronous (mean ± sd = -0.97 ± 1.387) are reported. (C) CONTROL EXPERIMENT. For each subject, the mean rating value of the three disownership statements in synchronous (mean ± sd = 0.153 ± 1.427) and asynchronous (mean ± sd = -1.15 ± 1.371) are reported.

https://doi.org/10.7554/eLife.14972.006

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In the control behavioral experiment, similar results were found for both proprioceptive drift and emb-q-rating (drift=mean ± sd: 2.47 ± 2.707 cm vs. 0.075 ± 2.461 cm; t₍₁₉₎=5.275, p=0.000043, dz=1.18; emb-q-rating=mean ± sd: 2 ± 0.763 vs. −0.97 ± 1.387; t₍₁₉₎=9.357, p=0.0000001, dz=-2.1; Figure 2,B₁ and B₂; see also Figure 2—source data 2). Furthermore, the disembodiment questionnaire rating (disemb-q-rating) was significantly higher in the synchronous than in the asynchronous condition (disemb-q-rating=mean ± sd: 0.153 ± 1.427 vs. −1.15 ± 1.371; t₍₁₉₎=3.835, p=0.00116, dz==0.86; Figure 2,B₃; see also Figure 2—source data 2). Finally, significant correlations were found between emb-q-rating and disemb-q-rating in both synchronous and asynchronous conditions and in the delta synchronous minus asynchronous (respectively: r-=-0.4911, p=0.0279; r-=-0.7128, p=0.0004; r-=-0.5537, p=0.0113; Figure 2,C_1,C_2,C₃). No significant correlations was found between proprioceptive drift and either emb-q-rating or disemb-q-rating.

For the physiological data, the Friedman non-parametric ANOVA showed a significant effect of condition (χ2 [2, n=24]=9,000,000; p= 0.01111), suggesting a difference between baseline, synchronous and asynchronous conditions, when MEPs were recorded from the stimulated (right) hand. Wilcoxon matched pairs tests, after Bonferroni correction, revealed a significant MEP decrease in the synchronous condition with respect to both the asynchronous (mean ± sd: −0.367 ± 0.362 vs. 0.205 ± 0.395; Z=3.3143, p=0.000919; dz=0.85) and the baseline (mean ± sd: −0.367 ± 0.362 vs. 0.277 ± 0.691; Z=3 .1428, p=0.001673; dz=0.74) conditions (Figure 3A₁; see also Figure 3—source data 1). No significant difference was found between asynchronous and baseline conditions (0.205 ± 0.395 vs. 0.277 ± 0.691; Z=0.1714, p=0.863887; dz=0.08). Examples of MEPs recorded from the FDI muscle of a representative subject are shown in Figure 3. Interestingly, the MEP time-course analysis showed that the inhibitory motor response increases over time, with lower values measured at each time-point. Note that Wilcoxon matched pairs tests, after Bonferroni correction, showed a significant difference between TIME 1 (first five trials) and TIME 4 (last five trials) (mean ± sd: −0.23963 ± 0.425821 vs. −0.5481 ± 0.394248; Z=2.942857; p=0.003252; dz=0.72), Figure 3A₂; see also Figure 3—source data 1.

Figure 3

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Figure 3—source data 1

Main experiment physiological results during baseline, asynchronous and synchronous condition.

(A) MAIN EXPERIMENT. For each subject, the mean MEPs amplitude (row values in µV), recorded during baseline (mean ± sd = 945.204 ± 535.076), asynchronous condition (mean ± sd = 918.075 ± 635.777) and synchronous condition (mean ± sd = 527.328 ± 325.998), are reported. (B) MAIN EXPERIMENT. For each subject, the mean MEPs amplitude (normalized z-scores), recorded during baseline (mean ± sd = 0.277 ± 0.691), asynchronous condition (mean ± sd = 0.205 ± 0.395) and synchronous condition (mean ± sd = -0.367 ± 0.362), are reported. In the z-scores computation, the mean and the sd of the three conditions were used to normalized row data according to the formula x-mean/sd. (C) MAIN EXPERIMENT. For each subject, mean amplitude of 5 MEPs (normalized z-scores) at four time-point recorded during synchronous condition are reported (respectively mean ± sd of TIME 1, 2, 3, 4: -0.239 ± 0.425, -0.284 ± 0.620, -0.389 ± 0.385, -0.548 ± 0.394).

https://doi.org/10.7554/eLife.14972.008

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Figure 3—source data 2

Control experiment physiological results during baseline, asynchronous and synchronous condition.

(A) CONTROL EXPERIMENT. For each subject, the mean MEPs amplitude (row values in µV), recorded during baseline (mean ± sd = 725.665 ± 365.104), asynchronous condition (mean ± sd = 734.398 ± 416.262) and synchronous condition (mean ± sd = 685.368 ± 469.286), are reported. (B) CONTROL EXPERIMENT. For each subject, the mean MEPs amplitude (normalized z-scores), recorded during baseline (mean ± sd = 0.057 ± 0.368), asynchronous condition (mean ± sd = 0.045 ± 0.262) and synchronous condition (mean ± sd = -0.104 ± 0.399), are reported. In the z-scores computation, the mean and the sd of the three conditions were used to normalized row data according to the formula x-mean/sd. (C) CONTROL EXPERIMENT. For each subject, mean amplitude of 5 MEPs (normalized z-scores) at four time-point recorded during synchronous condition are reported (respectively mean ± sd of TIME 1, 2, 3, 4: −0.092 ± 0.483, −0.224 ± 0.435, −0.093 ± 0.656, −0.009 ± 0.587).

https://doi.org/10.7554/eLife.14972.009

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By contrast, in the control physiological experiment, in the three-level one-way ANOVA, no significant effect of condition was detected (F_(2,38)=0,894943; p=0.417068), suggesting that for the non-stimulated (left) hand there is no difference between the synchronous condition and either the asynchronous (mean ± sd: −0.104 ± 0.399 vs. 0.045 ± 0.262; p=0.820737; dz=0.26) or baseline (mean ± sd: −0.104 ± 0.399 vs. 0.0572 ± 0.368; p=0.711483; dz=0.22) conditions (Figure 3B₁; see also Figure 3—source data 2). Moreover, in the four-level one-way ANOVA adopted for analysis of MEP time-course, no significant effect of TIME was found (F_(3,57)=0,842673; p=0.476191), suggesting that in the non-stimulated (left) hand, MEP amplitude is not modulated over time (Figure 3B₂; see also Figure 3—source data 2). Additionally, the Mann-Whitney U-test revealed a significant difference between the delta synchronous minus asynchronous obtained in each experiment, showing a strong physiological effect of the RHI in the main compared to the control experiment (mean ± sd: −390.746 ± 591.662 vs. − 49.0296 ± 238.332; Z=2.074180; p=0.038063).

In the present study, in order to investigate the link between body-ownership and motor system, we took advantage of the well-established RHI paradigm (Botvinick and Cohen, 1998; Ehrsson et al., 2004; Tsakiris et al., 2010; Longo et al., 2008; Moseley et al., 2008; Kammers et al., 2011; Rohde et al., 2013; Folegatti et al., 2009; Lewis and Lloyd, 2010; Valenzuela Moguillansky et al., 2013; Tsakiris and Haggard, 2005), a useful tool to manipulate the sense of body ownership in normal subjects. In the main behavioral experiment, our data show a very strong embodiment effect in the synchronous condition (with a mean rating of 2.4, on a scale of −3 to +3), while none of the subjects reported the illusory experience in the asynchronous condition (with a mean rating of −2, on a scale of −3 to +3). A complementary disembodiment effect (Longo et al., 2008) was investigated and measured in the control behavioral experiment, showing a significant correlation between the reported ownership of the fake hand and disownership of the subject’s own hand. Most importantly, the physiological results provide the first evidence that, during the RHI, the motor excitability of corticospinal hand circuits for the real stimulated hand is greatly reduced. This effect is absent for the real non-stimulated hand. In addition, consistent with behavioral studies reporting an increased illusory experience over time (Lewis and Lloyd, 2010; Valenzuela Moguillansky et al., 2013), the time-course analysis revealed that motor cortex excitability decreases as time of exposure to the illusion increases.

The link between body ownership and motor system activation has been investigated with different behavioral paradigms within the RHI framework. Recently, using a moving version of the RHI, it has been shown that voluntary (but not passive) movement of the real hand decreases the perceptual shift towards the rubber hand, suggesting that the subjective sense of agency strongly contributes to a coherent sense of body ownership (Tsakiris et al., 2010). On the other hand, patients with schizophrenia, who show a specific deficit in predicting the consequences of their voluntary actions (Voss et al., 2010), as well as an altered sense of agency (Garbarini et al., 2016; Daprati et al., 1997; Maeda et al., 2012), are more susceptible to the RHI (Peled et al., 2003; Asai et al., 2011; Thakkar et al., 2011). Other studies on clinical populations with movement disorders suggest that patients with focal hand dystonia (Fiorio et al., 2011) or partial or complete paralysis because of spinal cord injury (Scandola et al., 2014; Tidoni et al., 2014) seem to have some impairment of body ownership, according to their susceptibility to the RHI paradigm. A recent study (Burin et al., 2015), investigating the RHI in movement disorder after brain-damage, shows that hemiplegic patients display a weaker/more flexible sense of body ownership for the affected (paralyzed) hand (where the strength of the RHI is increased) when compared to controls, but an enhanced/more rigid sense of body ownership for the healthy hand (where the strength of the RHI is decreased). In other words, the prolonged absence of movement makes the paralyzed limb more readily disowned, while the healthy limb seems to be more strongly owned. Furthermore, other studies (Kilteni et al., 2016; Schütz-Bosbach et al., 2006) have investigated the modulation of corticospinal excitability during different experimental manipulations related to the sense of body ownership, though none with the specific hypothesis of linking a decrease in subjective ownership with a decrease in motor excitability. Kilteni and colleagues (2016) found that healthy people can experience a pseudo-amputation illusion during a virtual reality procedure, suggesting that this experimental manipulation causes corticospinal excitability changes in muscles associated with the virtually amputated body-part. Schütz-Bosbach and colleagues (2006) studied how an action observation task can induce different changes in corticospinal excitability, depending on the level of ownership experienced with respect to the observed moving hand. Ownership was previously manipulated using the RHI procedure: after asynchronous stimulation, observing others’ actions facilitated the motor system, whereas after synchronous stimulation, identical observed actions, now illusorily attributed to the subject's own body, evoked smaller MEPs. In light of our findings, the absence of facilitatory effect during observation following synchronous stimulation can be interpreted as resulting from the decreased corticospinal excitability induced by the illusion.

Taken together, these results suggest that body ownership and the motor system are mutually interactive and both contribute to the dynamic construction of bodily self-awareness in healthy and pathological brains. In the present study, we found that the excitability of motor pathways in response to stimulation of the real (disembodied) hand is significantly decreased (i.e., MEP amplitude was significantly reduced) when subjects experience the artificial hand as their own. This suggests that an experimental manipulation of the sense of body ownership is accompanied by a coherent modulation of the motor system. However, as the experimental design was not suited to investigate correlations (see details in Material and methods), the present data do not allow us to address the question of whether subjects who experience a larger subjective illusion also show a larger decrease in MEPs. The presence or absence of linear correlations between MEP amplitude and behavioral measures, including both embodiment of the rubber hand and disembodiment of the real hand, will be investigated in future studies, in which physiological parameters of responder and non-responder subjects can be also compared.

It has been suggested that active movements integrate distinct body parts into a unitary body representation (Tsakiris et al., 2010). When this unified representation is altered, as during the visual-tactile conflict induced by the illusion, the excitability of the primary motor cortex is also altered, suggesting that the motor readiness of the real (disembodied) hand could also be reduced. We ascribe the excitability decrease in the primary motor cortex (M1) to cortical inhibitory processes tied to the central processing of hand ownership, possibly reaching M1 via inhibitory input from the premotor cortex (which is known to play a crucial role in the multisensory integration processes that give rise to the sense of body ownership) (Ehrsson et al., 2004).

The present findings, which shed new light on our understanding of the different aspects that contribute to the formation of a coherent self-awareness, suggest that bodily self-consciousness strictly depends on the possibility of movement. The bodily self is primarily and originally construed in terms of motor potentiality for actions (Gallese and Sinigaglia, 2010) . If I believe that the hand is mine, then I must be ready to use it; if not, then the activity of the motor system is accordingly down-regulated.

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Author details

1. Francesco della Gatta

   1. Department of Philosophy, University of Milan, Milan, Italy
   2. Department of Health Sciences, University of Milan Medical School, Milan, Italy

   Contribution

   FdG, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Contributed equally with

   Francesca Garbarini

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0003-3471-6595

2. Francesca Garbarini

   SAMBA Research Group, Department of Psychology, University of Turin, Turin, Italy

   Contribution

   FG, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   Contributed equally with

   Francesco della Gatta

   For correspondence

   francesca.garbarini@unito.it

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0003-1210-0175

3. Guglielmo Puglisi

   Department of Health Sciences, University of Milan Medical School, Milan, Italy

   Contribution

   GP, Conception and design, Acquisition of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

4. Antonella Leonetti

   Department of Health Sciences, University of Milan Medical School, Milan, Italy

   Contribution

   AL, Acquisition of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

5. Annamaria Berti

   Neuroscience Institute of Turin, University of Turin, Turin, Italy

   Contribution

   AB, Conception and design, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0001-9972-5543

6. Paola Borroni

   Department of Health Sciences, University of Milan Medical School, Milan, Italy

   Contribution

   PB, Conception and design, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

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