The role of functionality in the body model for self-attribution

Laura Aymerich-Franch; Gowrishankar Ganesh

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ELSEVIER

Review article

The role of functionality in the body model for self-attribution

Laura Aymerich-Franch, Gowrishankar Ganesh*

CNRS-AISTJRL (Joint Robotics Laboratory), Japan

ARTICLE INFO ABSTRACT

Bodily self-attribution, the feeling that a body (or parts of it) is owned by me, is a fundamental component of one's self. Previous studies have suggested that, in addition to a necessary multi-sensory stimulation, the sense of body ownership is determined by the body model, a representation of our body in the brain. It is however unclear what features constitute the body representation. To examine this issue, we first briefly review results on embodiment of artificial limbs, whole bodies and virtual avatars to understand the apparent anatomical, volumetric and spatial constraints associated with the sense of ownership toward external entities. We then discuss how considering limb functionality in the body model can provide an integrated explanation for most of the varied embodiment results in literature. We propose that the self-attribution of an entity may be determined, not just by its physical features, but by whether the entity can afford actions that the brain has associated with the limb which it replaces.

Contents lists available at ScienceDirect

Neuroscience Research

journal homepage www.elsevier.com/locate/neures

Article history:

Received 3 September 2015

Received in revised form 30 October 2015

Accepted 4 November 2015

Available online xxx

Keywords: Body ownership Self-attribution Rubber hand illusion Body model Embodiment Action affordance

Contents

1. Introduction ............................................................................................................................................. 00

2. The rubber hand and beyond ........................................................................................................................... 00

3. The corporeal shape issue .............................................................................................................................. 00

4. Spatial constraints to embodiment.....................................................................................................................00

5. First person perspective................................................................................................................................00

6. A functional body model hypothesis...................................................................................................................00

7. In regard to previous results............................................................................................................................00

8. Coordinate frames and spatial constraints.............................................................................................................00

9. The effects of agency....................................................................................................................................00

10. A visual body image...................................................................................................................................00

11. Hypothesis predictions for verification...............................................................................................................00

12. Neural issues: what and where.......................................................................................................................00

13. Conclusion and future work...........................................................................................................................00

Acknowledgments......................................................................................................................................00

References .............................................................................................................................................. 00

1. Introduction

Who am I? The question of what is our self and how our brain defines self has been a fundamental motivation that has driven philosophy (Kant, 1781; Descartes and Cottingham, 2013), psychology (James, 1890; Jung, 1971) and religion (Rahula, 1959; Sivananda,

* Corresponding author at: CNRS-AIST JRL (Joint Robotics Laboratory), UMI3218/CRT,Tsukuba Central 1,1-1-1 Umezono,Tsukuba,Ibaraki305-8560, Japan. Tel.: +81 9081241047.

E-mail address: gans_gs@hotmail.com (G. Ganesh).

1972) over the course of the human existence. The Oxford dictionary (Oxford English Dictionary, 2010) defines self as "a person's essential being that distinguishes them from others, especially considered as the object of introspection or reflexive action." However, it will be generally agreed that this simple definition is far more complex than perceived. Self can be defined in multiple terms from one's physiology, mental and emotional status to beliefs, social status and spiritual being (closely related to the concept of soul). It can include various facets like self-image, self-perception, ideal-self and self esteem. For instance William James, the well-known 19th century philosopher, divided self into two main categories. The "Me" self, and the "I" self (James, 1890). The "Me" self, which he further

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Fig. 1. The rubber hand illusion (RHI) has been a standard to investigate the sense of ownership over the past decade. (A) The original RHI involves simultaneous brushing of the real hand and a rubber hand in view of the subject. (B) The illusion is not induced if the rubber hand is replaced by a "non-corporeal object" (Guterstam et al., 2013; Tsakiris and Haggard, 2005) but is reportedly possible when (C), the brushing is done on an empty volume of safe. Figures from experiments reconstructed by the authors from (Guterstam et al., 2013).

divided into the material self, the social self, and the spiritual self, refers to the aspects of someone that come from that person's experiences. On the other hand, James saw the "I" self as the thinking self and linked this self to the soul or mind of a person.

In this short review, we will limit ourselves to discussing bodily self-attribution and specifically to what constitutes the body model utilized by the brain for self-attribution. The self we will explore is probably best defined as the bodily self image and by James Williams's definition, part of the "material self".

Bodily self-attribution or body-ownership is a crucial component of the self. Body ownership refers to the special perceptual status of one's own body, which makes bodily sensations seem unique to oneself (Gallagher, 2000; Tsakiris, 2010), that is, the feeling that certain limbs and certain sensed body belongs to me. It is well established that illusory changes in the feeling of body ownership can be generated by correlated stimulations in different combinations of sensory modalities (Botvinick and Cohen, 1998; Armel and Ramachandran, 2003; Ehrsson et al., 2005; Tsakiris et al., 2006; Walsh et al., 2011; Kalckert and Ehrsson, 2012). However, while multi-sensory stimulations are necessary, they are arguably not sufficient to induce the feeling of ownership. Multiple studies have shown that the feeling of ownership toward an artificial limb is additionally modulated by its anatomical (Tsakiris and Haggard, 2005; Haans et al., 2008; Guterstam et al., 2013), volumetric (Pavani and Zampini, 2007), and spatial (Pavani et al., 2000; Austen et al., 2004; Ehrsson et al., 2004; Tsakiris and Haggard, 2005; Costantini and Haggard, 2007; Lloyd, 2007) features. These results support the belief that, in addition to the bottom-up multi-sensory perception, self-attribution is regulated by a top-down perceptual body model, a reference description of our body or/and the space around it in our brain (De Vignemont et al., 2006; Makin et al., 2008; Tsakiris, 2010; Blanke, 2012; Moseley et al., 2012). The specific bodily features that the body model encodes, however, remain unclear.

In this article we will examine what minimal features can explain how our brain represents our body. First, we will briefly review studies on embodiment of artificial limbs, whole bodies and virtual avatars to explore the apparent "top-down" constraints associated with the illusion of body ownership. While the definition of embodiment is varied, at least in the case of artificial limbs and bodies, embodiment is generally agreed to include the sense of ownership. We will thus assume embodiment to represent ownership in this article. Following the review, we will propose limb functionality as the key feature of the body model, and discuss how a body model considering functionality can explain most observations by previous studies.

2. The rubber hand and beyond

Our understanding of body ownership has increased significantly in the last decade after the discovery of the rubber hand

illusion (RHI) (Botvinick and Cohen, 1998) which enables controlled manipulation of limb ownership in the laboratory environment. As is customary with articles dealing with body-ownership, we too will start with a brief description of the RHI. In this illusion, Botvinick and Cohen showed that synchronous touches, applied to a rubber hand in full view of the participant, and the real hand hidden behind a screen, produce the sensation that the touches felt originate from the rubber hand, leading to a feeling of ownership of the artificial rubber hand. In contrast, the illusion of ownership is absent if the touches on the rubber hand and the real hand are not synchronized.

Since the first experiment, multiple versions of the RHI have examined how different physical and spatial features of the rubber hand influence the illusion (Fig. 1). While the similarity of physical features of an embodied artificial limb and the real limb does aid self-attribution, subjects are able to embody limbs with different physical features. It has been shown that color does not determine embodiment of an artificial limb (Holmes et al., 2006; Longo et al., 2009). For instance, Holmes et al. (2006) found that a white rubber hand produced similar levels of embodiment in white and black skin participants. Longo et al. (2009) found that objective similarity (as measured by skin luminance, hand shape, and third-person similarity ratings) did not affect fake limb embodiment. Similarly, a rubber hand with a different skin texture can be embodied, even though the embodiment scores are reportedly lower (Haans et al., 2008). In regard to size, it has been shown that a rubber hand larger than one's real hand (Pavani and Zampini, 2007) and longer arms (Schaefer et al., 2007; Kilteni et al., 2012) can be embodied by subjects, while interestingly, a rubber hand smaller in size than one's real hand is not (Pavani and Zampini, 2007). Smaller size though is no problem when it comes to whole body embodiment as shown by an attractive study by Ehrsson and colleagues (Van der Hoort et al., 2011) where they embodied subjects into dolls ranging in size from 30 cm to 400 cm.

Similar results have been reported for whole body embodiment in virtual reality (VR). Studies that use this technology typically induce embodiment by giving users visual feedback in first person perspective of the virtual environment, which is displayed in accordance to their head movements. VR users are generally able to see the virtual limbs of their avatars in a coincident location with that of their real limbs. Additionally, full-body identification with the digital self-representation (i.e. the avatar) can be achieved by reflecting the avatar's body in mirrors or other reflecting surfaces (González-Franco et al., 2010; Aymerich-Franch et al., 2014), so users gain knowledge of how they look like in the virtual environment. Resemblance of the artificial body to a human body improves embodiment into the avatar (Maselli and Slater, 2013), and the feeling of Presence (discussed also in the next section) in the virtual world (Eastin, 2006; Ratan et al., 2007; Ratan, 2011). Furthermore, customization of avatars increases the extent to which people feel connected to

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their avatars (Lim and Reeves, 2009; Ratan, 2011). However, VR users are able to identify with human-looking avatars even when they present different visual characteristics than their real selves (Kim, 2011; Aymerich-Franch et al., 2012; Maister et al., 2014). For instance, ownership can be induced over a body of a different race, age, or gender (Petkova and Ehrsson, 2008; Maister et al., 2014).

3. The corporeal shape issue

Contradictory results exist on whether a "non-corporeal" entity can be embodied and induce a sense of ownership.

Although similarity of the virtual avatar improves the sense of embodiment in VR (Maselli and Slater, 2013), studies have shown that bodies with extra limbs (Schaefer et al., 2009; Won et al., 2015), a tail (Steptoe et al., 2013), animal bodies (Ahn et al., 2015) and even non-anthropomorphic shapes (Aymerich-Franch, 2010) can be embodied by subjects. Similarly, avatar studies with robots have shown that humans feel identified with very realistic androids (Nishio et al., 2012) as well as non-human looking humanoid robots (Aymerich-Franch et al., 2015) and parts of a robot (results pending publication).

These studies seem to contradict the body-part or full-body illusion studies which highlight a drastic reduction in the illusion when a non-corporeal object, such as when a wooden stick (Tsakiris and Haggard, 2005), a rubber sheet (Haans et al., 2008), a wooden slab (Guterstam et al., 2013) or a cuboid (Lenggenhager et al., 2007), is used instead of a fake hand or body. The RHI is also attenuated when the handedness of the visible rubber hand and the stimulated hand are not congruent (Tsakiris et al., 2006; Petkova and Ehrsson, 2009). But again other studies have claimed that non-corporeal objects such as a box(Hohwy and Paton, 2010) or a table (Armel and Ramachandran, 2003) can be embodied, apparently if followed by the classic RHI with a human-looking rubber hand. It is also relevant to mention here a RHI study (Ehrsson, 2009) that demonstrated that simultaneous brushing of two rubber hands in synchrony with a subject's real right hand induces an experience of having two right hands in the subject. Furthermore, a recent study exhibited that subjects are able to embody a volume of empty space, with no obvious shape, when presented with simultaneous brush strokes from one brush on their hidden hand and another a few centimeters above a table (Guterstam et al., 2013).

4. Spatial constraints to embodiment

There is however, general agreement on the spatial constraints affecting embodiment of artificial limbs and bodies. RHI experiments show that the embodiment of a rubber hand does not occur when the rubber hand is located outside the participant's peri-personal space (Lloyd, 2007) or when its posture is spatially incongruent with respect to the real hand (Ehrsson et al., 2004; Tsakiris and Haggard, 2005; Costantini and Haggard, 2007).

On the other hand, spatial congruency between the self and the body can be disrupted in full-body illusions involving one's own body seen from a different perspective (Ehrsson, 2007; Lenggenhager et al., 2007), mannequins (Guterstam and Ehrsson, 2012), virtual body (Lenggenhager et al., 2007) and robots (Nishio et al., 2012; Alimardani et al., 2013; Aymerich-Franch et al., 2015). Studies in virtual reality have also explored a related phenomena of Presence (Heeter, 1992; Steuer, 1992; Lombard and Ditton, 1997; Lee, 2004), which refers to the feeling of "being there", in the virtual world (Lombard and Ditton, 1997), and self-presence, a subdimension of Presence which describes the state in which the virtual self is experienced as the actual self (Lee, 2004; Ratan, 2010). When people experience presence and self-presence, their behavior within a virtual environment in relation to virtual objects and

virtual people is very similar to that in the physical world (Bailenson et al., 2001, 2003; Garau et al., 2005), even if none of them actually exist. In collaborative virtual environments, people physically located in distant places are able to meet in a single virtual space and also experience a feeling of'co-presence' (Bailenson, 2006). If we can consider these behavioral modifications to indicate embodiment, then the observations from all these studies taken together indicate that the spatial coincidence between the real and virtual world, or the real and virtual body, is not a necessary condition to experience whole body embodiment.

5. First person perspective

Finally, first person perspective is a crucial requirement to create the illusion of embodiment over a virtual body (Maselli and Slater, 2013). For instance, it has been shown that a first person perspective of a life-sized virtual human female body that appears to substitute a male subjects' own body is sufficient to generate a body transfer illusion (Slater et al., 2010). Conversely, when third person perspective is used over an artificial body the illusion of embodiment does not occur, even when the set up includes visuo-tactile synchronization between the real and the artificial body (Petkova and Ehrsson, 2008; Slater et al., 2010; Petkova et al., 2011).

6. A functional body model hypothesis

Our review highlighted results that exhibit that, in addition to the multi-sensory integration, embodiment of an artificial entity (limb or body) is also modulated by various physical and spatial features of the entity. These results support the presence of a "body model", a top down reference of body features that our brain uses to identify our self, and which modulates embodiment of external entities. But what features does the body model refer to? Traditionally the model has been suggested to represent visual, anatomical and structural properties of the body (Schwoebel and Coslett, 2005; Costantini and Haggard, 2007) and the space around the body (Makin et al., 2008). But the variance in the embodiment results, as summarized by our review, makes it difficult to identify a single physical body representation that would explain all the observations. It is of course possible that these observations are simply not related, and are a result of an interaction between multiple body representations. However, that said, a synthesis and integrated representation may still be possible.

We suggest that a body model considering not just the physical features but also the functionality of limbs, can provide an integrated explanation for most of the varied embodiment results observed with rubber hands, and surrogate bodies. We propose that the top-down regulation of embodiment of an entity is determined by whether the entity can afford actions that the brain expects from the limb which the entity replaces (in terms of the perceived multi-sensory stimulations). This idea is motivated by the popular concept of "affordance" in sensory perception (Gibson, 1977). There is however a major difference between our proposal and Gibson's concept of affordance. Affordance refers to a representation of external objects in terms of the actions, defined by one's limb characteristics, environmental constraints and motor skill, which can be performed on it by an individual. On the other hand, here we propose that the body model is a representation of the limb characteristics in terms of specific actions it requires to perform. In other words, according to the functional body model hypothesis, our brain attributes a perceived entity as our limb (or as our body) if the physical properties of the entity are sufficient to afford certain actions the brain has associated to that limb (or body).

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7. In regard to previous results

As an example, reaching and grasping are probably the two important actions that a healthy person's brain associates with his/her arm and hand. A body model considering limb functionality would predict that the sensitivity of any physical feature of an artificial hand/arm (like a rubber hand), on how difficult (slow) it is to embody and be attributed as one's own arm, will depend on how much the particular feature impedes or limits reaching and/or grasping in a real hand. Consistent with this prediction, RHI has been shown to be possible with rubber hands of different color (Holmes et al., 2006; Longo et al., 2009), texture (Haans et al., 2008), and gender (anecdotal observations) as these features do not directly affect hand/arm reach or grasp. On the other hand, a wooden stick (Tsakiris and Haggard, 2005), wooden slab (Guterstam et al., 2013) or rubber sheet (Haans et al., 2008) with no fingers would make grasp impossible and hence are not easily attributed as one's arm.

Substituting two hands and arms for one does not significantly deteriorate its functionality (and may be seen to increases it) and hence multiple rubber hands can be embodied at one time (Ehrsson, 2009). Artificial arms and hands, that are longer/larger than the real hand, would arguably constrain actions less than shorter arms or smaller hands; for example smaller limbs would restrict reachability of the extremities of one's usual reachable space. For this reason, embodiment of limbs is less sensitive to longer artificial arms (Schaefer et al., 2007; Kilteni et al., 2012) and larger rubber hands (Pavani and Zampini, 2007) than smaller rubber hands (Pavani and Zampini, 2007). Finally a change from right to left hand or vice versa would severely affect how an arm would perform grip tasks and hence the RHI is affected by the handedness of the rubber hand with respect to the real hand (Tsakiris et al., 2006; Petkova and Ehrsson, 2009).

Most of the whole body illusion and VR results can be similarly explained by considering action affordance. For our brain, our body affords actions like reaching, locomotion and head movement. A cuboid with no limbs will therefore not be attributed easily as one's body (Lenggenhager et al., 2007). On the other hand, it is easier for the brain to embody avatars that can afford these actions in the given environment, even when the avatars are obviously nonhuman (Ahn et al., 2015), or different in size and gender (Petkova and Ehrsson, 2008; Maister et al., 2014)

8. Coordinate frames and spatial constraints

A body model considering action affordance implicitly couples the human limb features to the space around the limb and body. Actions are expected to be defined in (probably multiple) body and limb centric coordinates, the peri-personal space (Rizzolatti et al., 1997) being arguably one of the key spaces in question. It thus follows obviously that artificial limbs outside one's peri-personal space will not easily be attributed as one's own (Lloyd, 2007), and that visual perspective is a critical determinant of embodiment (Maselli and Slater, 2013). Furthermore, a recent framework put forth by Cisek and Kalaska (2010) suggests the presence of constant competition between affordances in the brain for the selection of a particular action at any instance (and posture). Considering that one's current body posture would determine the ease with which different actions are afforded, it can be expected that the embodiment of artificial limbs is dependent on its postural congru-ency to the real limb (Ehrsson et al., 2004; Tsakiris and Haggard, 2005; Costantini and Haggard, 2007). Though our proposal does not clearly explain the extreme sensitivity of the RHI to the rubber hand orientation as has been reported in (Costantini and Haggard, 2007).

Note that spatial constraints do not limit embodiment of whole bodies as the spatial constraints are relative to the body (in body and limb centric coordinates) that would shift with the body. Whole body illusions have thus been observed to be insensitive to the spatial location of the fake or virtual bodies (Lenggenhageret al., 2007; Blanke, 2012). On the other hand, our model would predict whole body embodiment to be sensitive to posture; it should be difficult to embody avatars with a significantly different posture than the real body.

The coordinate frames of action affordance can also explain why, even though embodiment of a smaller rubber hand is difficult, embodiment of a smaller body (with multiple small limbs) can be achieved with ease (Van der Hoort et al., 2011). In the presence of a first person perspective, a scaling down of the body size would lead to the proportional shrinkage of the functional workspace of the limbs (and the peri-personal space). Embodiment into the smaller body with its smaller limbs is thus possible as the brain (mis)perceives the shrinking of the whole body as a change in the size of the environment (Van der Hoort et al., 2011) and not its limbs.

9. The effects of agency

The role of action affordance in the body model highlights the importance of agency on the embodiment of limbs. The sense of agency, induced by movement synchronization between the real and the artificial limb or body, has been utilized to induce the illusion of ownership of artificial limbs (Tsakiris et al., 2006; Raz et al., 2008; Dummer et al., 2009; Kalckert and Ehrsson, 2012; Nishio et al., 2012; Kalckert and Ehrsson, 2014). It is believed that the key feature enabling embodiment in these movement studies are the dynamic proprioceptive and haptic sensations induced by the motion. These can substitute the tactile input, for example from brush strokes in a regular RHI experiment, and enable multi-sensory integration.

In addition to aiding the bottom-up multi-sensory integration, we propose that the sense of agency also performs a crucial function in regard to the body model; sense of agency can change our body model by modulating the actions our brain associates to a particular limb. Due to this bi-directional effect (bottom-up and top-down), agency is a much stronger modus for inducing embodiment than multi-sensory stimulations. This is probably the reason why subjects can even identify with non-anthropomorphic 3-D shapes in VR when their movements were synchronized with the object movements (Aymerich-Franch, 2012). The importance of the sense of agency in embodiment would also explain the observed increase in the RHI in the presence of agency (Kalckert and Ehrsson, 2012) and the success agency has had in the induction of embodiment in virtual environments (Slater et al., 2009; Sanchez-Vives et al., 2010; Spanlang et al., 2014).

Furthermore, if action affordance plays a key role in embodiment, then it can be concluded that artificial limbs that are successfully embodied, are perceived by the brain as able to afford the required actions. This view is consistent with the reports of heightened sense of agency toward embodied rubber hands after RHI (Longo et al., 2008), even when induction of the illusion does not involve any movement.

10. A visual body image

Similar to affordance in perception literature, we postulate that one's limbs are represented in the brain as a mapping, between the visual limbs and the visual environment, through the actions the limb affords. We believe that a visual body image plays a fundamental role in the self- attribution process, as has also been previously

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discussed by other studies (Botvinick and Cohen, 1998; Costantini and Haggard, 2007; Kammers et al., 2009). Vision is not required in regard to the bottom-up multisensory stimulation (Ehrsson et al., 2005; Petkova et al., 2012), but a subject's visual image (visualization) of her body determines the top-down regulation of body ownership.

We propose that visualization of a limb, aided by multisensory representations is (at least one of the key reason) why subjects can embody an empty volume of space (Guterstam et al., 2013). The visualization is impeded when the space is not empty and is replaced by a non-corporeal object, hence leading to a reduction in the embodiment (Tsakiris and Haggard, 2005; Guterstam et al., 2013). We would also suggest that the RHI induced by self touch and without vision (Ehrsson et al., 2005; Petkova et al., 2012) is possible because, in addition to the multi-sensory tactile and proprioceptive stimulation, the subjects are able to visualize the limb that they feel they are touching. Lack of which, as would be expected from blind subjects who have lost or distorted their visual image of their body, prevents self-attribution of self-touched limbs even though the same multi-sensory stimulations are available (Petkova et al., 2012).

11. Hypothesis predictions for verification

A body model considering limb functionality thus seems to provide an integrated explanation for various observations by previous self-attribution studies. But an obvious question to ask is whether there are any predictions from the functional body model hypothesis that can be used to validate it against previous nonfunctional model beliefs? We discuss three verifiable predictions here.

First, the critical prediction of the functional body model hypothesis is that the brain does not identify a limb or body just by how it looks, but also by how it is used. Thus the model should be verified by examining whether functional constraints on a real limb alter what physical features in an artificial limb (that would replace it) impede its self-attribution. In their elegant monkey study, Costantini et al. (2010) utilized such a functional constraint to examine whether object affordances are affected by environmental constraints on a subject's action. They demonstrated that visual observation of an object, which evokes motor neurons when placed in the peri-personal space of monkey, fails to evoke the motor neurons when a transparent barrier is placed between the monkey and the object. However, while the constraint on object affordances (object being distinct from self) can be extrinsic, as the body model is the representation of one's self, we believe the constraint on the limb functionality would have to be intrinsic, from within one's body and on the real hand (and not the rubber hand). One possibility for an intrinsic constraint is the state of paralysis. If the functional body model hypothesis is true then individuals with loss of motor function (and not sensory function such that they can feel the multi-sensory stimulation) in an arm (and hand) should more easily embody a wooden stick or slab than healthy individuals (Tsakiris and Haggard, 2005; Guterstam et al., 2013). Acute motor axonal neuropathy, acute inflammatory demyeli-nating polyneuropathy and pure motor stroke/hemiparesis are examples of diseases that present predominantly motor, and not sensory, paralysis. This prediction however remains to be tested.

Second, though we concentrated on 'action' functions in this article as these are the most relevant for human limbs, functionality can also be unrelated to action. Taking an example cited before, RHI is not affected by the color and texture of the rubber hand (Holmes et al., 2006; Haans et al., 2008; Longo et al., 2009) because actions, like reaching and grasping, that constitute the major functions of a human hand, are not affected by color or texture. On the other hand,

the functional body model hypothesis would predict that animals that rely on camouflage should be more sensitive to the color of the artificial limb during embodiment. RHI (or equivalent) studies with animals would be required to confirm this prediction.

Third, if our brain defines a limb by its action affordance, then it would be expected that artificial limbs that are successfully embodied, are perceived by the brain to afford the required actions. Embodiment of artificial limbs can thus be expected to influence not just the sensory but also the motor-sensory representations of the limb, as these are known to be essential for performing and controlling actions (Wolpert et al., 2011; Ganesh et al., 2013). Consistent with this prediction, our recent studies (pending publication) show modifications in the motor-sensory predictions, or forward models, after the embodiment of, both a corporeal rubber hand, as well as a relatively non-corporeal robot hand.

12. Neural issues: what and where

Affordance represents a mapping between an object's spatial features, constraints of the environment it is in, and an individual's own limb features and motor skill. This mapping is captured by the actions the individual can perform on the object. Perception of an object, according to Gibson, is not performed by individually comparing its features with those stored in the brain. Object perception represents a "pick up" of its affordance, the mapping as a whole, by the brain. This concept is elucidated by a nice example from a recent book, where the author states ".. .for a person who has never encountered stairs before, there might be some question as to why climbing up the incline would be desirable, but the perceiver's body would pick up the fact that it could use them to go upward either way" (Hinton, 2014). Similarly, we believe the body model represents a mapping between the features that represent one's limb/body and the actions associated with the limb. Self-attribution is achieved by the brain by "picking up" and comparing affordances of an observed (artificial) limb to that stored for the real limb in question.

The inferior parietal region has been associated with the visual distinction of self from another (Ruby and Decety, 2001). Somatosensory regions have also been isolated in regard to the distinction of the self (Ruby and Decety, 2001), body awareness (Hari et al., 1998; Schwartz et al., 2005) and internal representation of one's body (Tsakiris et al., 2007; Tsakiris, 2010). These regions are thus arguably important for the body model.

Interestingly, the inferior parietal region is adjacent to the anterior intraparietal salcus (alPS) which is known for its role in action affordance in humans (Tunik et al., 2005). This view is backed by monkey electrophysiological studies of the anterior intraparietal area (AIP), which is suggested as being the functional equivalent to a human alPS (Culham et al., 2003, 2006; Frey et al., 2005). AIP has connections to the premotor (F5) and has been suggested to play a crucial role in the multi-sensory integration of object size and shape (Murata et al., 1997; Raos et al., 2006; Maranesi et al., 2014) and modulating corresponding motor action (Gallese et al., 1994; Fogassi et al., 2001; Maranesi et al., 2014).

Furthermore, the inferior parietal has reciprocal connections with the superior occipital gyrus, which has been previously observed to be activated when the handedness of the rubber hand is congruent to the real hand (Tsakiris et al., 2007). This region is in the junction of the dorsal and ventral streams and is believed to assimilate information related to visual object shapes and object motions (James et al., 2003). Overall, these results suggest that if our functional body model hypothesis is true, then the body model would probably be represented by the interactions between the inferior parietal with occipital regions, and with the premotor and somatosensory areas through the intraparietal salcus.

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13. Conclusion and future work

In this article we reviewed the results from RHI, whole body illusions, and virtual reality studies to investigate the nature of the body model, the central representation of our limbs and body in our brain. We proposed that the body model for self-attribution includes a mapping between the limb features and actions that the brain expects it to afford. Limb specific actions are likely learnt by the brain through the course of development, by exploring one's physiology and environment. These explorations are possible through both, observed and self-generated actions and can enable the brain to understand not just the movement but also the spatial constraints related to the limb actions. We discussed how considering functionality allows for an integrated explanation for the observed physical, spatial, and agency related constraints on embodiment. However, though some evidence supporting the proposal were discussed, the validation of the functional body model hypothesis requires a dedicated investigation of interactions between functionality and the sense of ownership, in line with the suggestions in the Hypothesis predictions for verification section.

We conclude by highlighting the fact that, in addition to limb and whole body embodiment, the functional body model hypothesis is also consistent with 'tool embodiment' studies. Tool use does not lead to the self-attribution of the tools as part of one's body, but results in modifications in the internal body representations, including the body schema (Cardinali et al., 2009, 2011; De Vignemont, 2010; Sposito et al., 2012; Ganesh et al., 2014), and multi-sensory interactions (Maravita et al., 2001; Holmes et al., 2007), similar to observations after the embodiment of artificial limbs. Perceptual changes due to tools have been shown to present only when the tools lead to specific sensory consequences in relation to a task (Witt et al., 2005). Furthermore, the perceptual changes depend on the shape and functionality of the tool; when a tool is used to estimate the size of an object rather than lift it (Cardinali et al., 2012) and when a stick rather than a light pointer is used to point to a line center in a line bisection task (Berti and Frassinetti, 2000). Therefore similar to artificial limbs, tools are embodied only when they can afford the actions that are relevant to a given task. However, further studies are needed in order to clarify the similarities and differences between tools and artificial limb embodiment in order to evaluate how they can be understood in regard to the self (De Vignemont, 2010).

Acknowledgments

LAF is supported by the Marie Curie IOF Fellowship project 'HumRobCooperation' under grant agreement No. PIOF-CT-622764. GG is partially supported by the Kakenhi 'houga' grant 15616710 from the Japan Society for the Promotion of Science (JSPS).

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