Embodying Bodies and Worlds (1)

7/27/2019 Embodying Bodies and Worlds (1)

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to three levels of processing body-related signals. The first sense, which we callembodiment1, is the trivial sense of embodiment that applies to somaticsignals. Sensations such as pain, itch, stomach-ache etc, are embodied simplybecause they originate within the body, and signal states of the body: they are

interoceptive, or somatoreceptive, rather than exteroceptive. That is, the orig-inal neural receptors that underlie them transduce states of the body itself,rather than the states of objects outside the body that are transduced byvision, for example. Touch appears as an interesting intermediate case, withboth interoceptive and exteroceptive aspects. Even with touch, however, thisform of embodiment is limited by the spatial distribution of receptors on, orin, the body. Embodiment1 does not, in our view, play a major role in theories ofcognition, except as a basic sensory category to which other forms of embodimentmay be reduced. Embodiment1 is embodiment only in the conceptually-

straightforward sense of originating within the body. We do not discuss it further inthis paper.The second sense of embodiment, which we call embodiment2, involves multi-

sensory integration. This forms the focus of the first part of our paper. By combiningseveral sensory signals, both interoceptive and exteroceptive, the brain generates arepresentation of the body as both a physical object and as a common locus ofsensations. Interestingly, both exteroceptive and interoceptive inputs are combinedin embodiment2. We argue that embodiment2 is the stage at which body ownershiparises. Accordingly, we focus particularly on how viewing ones own body can

modulate somatosensory processing.Embodiment3 represents another stage of cognitive transformation, involving link-ing states across individuals. We use the term embodiment3 to refer to the capacity tounderstand or re-represent the states of others by linking them to states related toones own body, either at the embodiment1 level directly, or via a representation ofones own body at the embodiment2 level.

The paper accordingly has four parts. In the first part, we use the case ofvision of ones own body to explore embodiment2. We show how body repre-sentations have widespread effects on somatomotor processing. Such top-downmodulation provides powerful evidence that somatomotor processing indeed involvesa hierarchy of increasingly abstract and cognitive processing of proximal events. Inthe second part, we emphasise the pervasiveness and persistence of embodiment2, bydiscussing a variety of phenomena where referral to primary sensorimotor experienceremains dominant despite a situation where the body might prima facie be irrelevant.These include clinical and experimental Out of Body Experiences, phantom bodyperception in amputees and patients with spinal cord lesions, apotemnophilia orxenomelia. Third, we consider how high up the cognitive hierarchy the traces ofsensorimotor origins may ascend, taking aesthetic appreciation as a target. Fourth, weconsider the nature of embodiment3, by discussing the concept of somatosensorysimulation, and assessing how tightly it is linked to the primary sensorimotor events(the embodiment1 level), that it is supposed to embody. In a final section, weconsider a case where the project of embodied cognition may seem to fail, byconsidering whether knowledge about others' mental states might sometimes requirecompletely abstract inferential operations, which cannot be reduced to somatomotorsimulation.

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1.1 Evidence for Body Representations in the Brain

Sensorimotor representations abound in the brain (see review in Berlucchi andAglioti 1997; Berlucchi and Aglioti 2010). Any account of the role played by these

different representations in cognition must explain why we have a coherent concept ofself and our own body despite the multiplicity and plasticity of the brains representa-tions of the body. Taxonomies regarding the number and type of body representationshave been developed, following neuropsychological dissociations or other principles.Several brain areas may contribute: insular, cerebellar and subcortical brain regions havebeen included in the list of brain regions that implement body representations.

However, even the lowest-level spinal processes seem to presuppose existence ofappropriate body templates. Giszter and colleagues data suggest that the spinal frogcannot wipe the correct part of its body following a tactile stimulus unless itknows

the length of its own legs (Giszter et al. 1989). This knowledge implies a represen-tation of the body, since limb length is not directly signaled by any single afferentpathway. Using the terminology we developed in the introduction, the embodied1afferent input of a tactile stimulus is functionally useless for controlling behaviourunless the motor system also has access to an embodiment2 level representation,specifying the physical properties of the body, in this case limb length.

Beside these and others somatoreceptive body representations, the presence of aspecific area of the visual brain dedicated to processing bodies provides strongevidence for the importance of embodiment2, since it implies representations of the

body that are exteroceptive in origin.Several fMRI studies demonstrate that the visual perception of full bodies or bodyparts selectively activates lateral (Extrastriate Body Area, EBA) and medial occipito-temporal visual areas (Fusiform Body Area, FBA) (Downing et al. 2001; Peelen andDowning 2005). Further, the viewing perspective of the body (allocentric vs egocentric)(Chan et al. 2004; Saxe et al. 2006), identity recognition (Hodzic et al. 2009), facepresence (Morris et al. 2006) and static posture (Peelen et al. 2006) may modulateEBA activity.

Interestingly, the same circuits may also help to link embodiment2 and embodi-ment3. Any process for matching observed events involving others bodies withproprioceptive, tactile and nociceptive information coming from ones own bodywould clearly benefit from the contribution of a system specifically dedicated toprocessing the physical form of the body. Therefore, a visual area selective for bodyprocessing may allow a direct matching between locations on anothers body andones own. While embodiment1 somatosensations are inherently personal, telerecep-tive senses like vision that underpin embodiment2 can facilitate processing of funda-mental features of others' bodies. Linking the two representations could provide aneural basis for social equivalence between individuals. fMRI has shown that theneural basis for this linkage indeed exists.

Despite its position in the occipital cortex, several studies suggest that this visualarea may already make use of intermodal transformation, i.e., its embodied2 func-tions, to understand somatic events of others, i.e., to perform embodied3 functions. Inparticular, several recent studies suggest that EBA is not really a visual area, but amultimodal area. The evidence comes from tasks involving haptic exploration ofbody shapes (Kitada et al. 2009; Costantini et al. 2011), and even active motor control

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(Astafiev et al. 2004). These results suggest that EBA may not be purely visual, butmay provide a supramodal representation of body form (Urgesi et al. 2007a; Moro etal. 2008), making it well suited for both visual perception of the body and forsimulation of bodily events.

A potent example of the importance of intermodal transformation in embodiment2comes from the fact that simply viewing the body profoundly modulates somatosen-sory processes such as touch and pain. In particular, non-informative vision of onesown body enhances touch perception but down-regulates pain perception (Kennett etal. 2001; Taylor-Clarke et al. 2002; Longo et al. 2009). Interestingly also, the visual-somatosensory core of this system was included in the original proposal of theneuromatrix orpain matrix (Melzack1990). In general, such results are consistentwith the concept of a strong link between visual, exteroceptive and somatosensory,internal representation of ones own body. A strong link of this kind could be used

also to link visual events on others bodies to ones own somatic sensations.

2 Persistence and Pervasiveness of Embodiment2

We argued above that events related to the bodies of others may be referred back toones own body. However, this referral is only useful if the brain maintains a clearrepresentation of ones own body. In fact, several cases can occur in which the linkbetween brain and body is degraded. Embodiment1 is therefore compromised. How-

ever, the brain

s body representations turn out to track the body rather conservatively,and embodiment2 may persist despite changes in embodiment1. This point hasimportant implications forembodied cognition theories, that seek to ground abstractcognition in basic bodily states. Specifically, embodiment2 may be a more solidfoundation for these theories than embodiment1, the brain-body links of embodiment1seem very fragile. And indeed, evidence from patients seems to show that patientswith body disorders (due to physical or neurologic causes) still represent their body ina way that is fundamentally sensorimotor, or that at least refers to somatomotorproperties of the body.

2.1 Amputation and Spinal cord injury

The extreme case of altered brain-body links is the case of missing body parts that arestill perceived as existent. The feeling of receiving sensory inputs from an amputatedlimb, or the sensation to be able to move it, is very common after amputation. Thesepositive hallucinations are called phantom limb sensations and are often associatedwith sensations of pain in the phantom limb. Interestingly these sensations have beenreported both in cases of accidental amputation, and also in aplasic individuals bornwithout arms (Brugger et al. 2000; Melzack et al. 1997). The sensation that some-thing is missing after amputation or at early stages of development in aplasics is takenas evidence for innate body representations (Melzack1990; Melzack et al. 1997). Thereplacement of the missing body part with a projection of what we might normallyexpect our body to be like following amputation has been interpreted in the sameway. Such an innate and immutable body schema representation (Melzack 1990)corresponds to an alternative view of our embodiment2. Whereas we argued above for



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an important role of multisensory interaction in establishing embodiment2, Melzacksposition amounts to genetic specification of embodiment2.

However, the concept of innate body representation cannot readily explain whypeople sometimes experience supernumerary limbs, either as a result of experimental

stimulation (Ehrsson 2009) or following brain damage (Halligan and Marshall 1995).Violation of implicit expectancies about body structure may occur due to temporarymanipulations of multisensory integration processes (somatic illusions), or due tomore lasting neural changes caused by brain damage.

Another condition where embodiment2 seems to be resilient to traumatic neuralchanges is Spinal Cord Injury (SCI). In SCI, the brain-body links that constituteembodiment1 are partly or totally severed. Not only are phantom limb sensationsreported in some SCI patients, but there is also the possibility, at least in principle, forthese patients to recover a normal feeling of embodiment by remapping inputs to the

affected body parts onto those body parts with preserved sensory and motor function.Accordingly, illusory duplication of phantom limbs has been described also in a smallnumber of patients with SCI (Curt et al. 2011; Drysdale et al. 2009).

Amputation and spinal damage induce two conditions of altered embodiment2 thatcontrast in important ways. While amputation changes the form of the body, SCIdamage induces conditions in which the form of the physical body remains unalteredbut the possibility to move or to feel specific parts of the body is impaired. Interest-ingly, in both cases the missing or plegic body part can be masked by a phantomreduplication. This shows that sensorimotor traffic to a body part, rather than its

objective existence, determines how the brain represents that body part. Phantomreduplication may be interpreted as showing the strong persistence of body repre-sentations following interrupted sensorimotor traffic.

The failure to integrate the altered information, or lack of information, comingfrom a disconnected body may represent a failure of specific integrative brain regionsthat support embodiment2. The parietal cortex of the right hemisphere may play a keyrole in this respect. In particular, the Temporo-Parietal Junction (TPJ) is thought toplay an essential role in integrating signals coming from the body, and building-up acoherent sense of the body as a physical object embodiment2. This possibility issupported by studies showing a role for TPJ in the illusory integration of a rubberhand into ones own normal body representation (Tsakiris et al. 2008), or in theillusory perception of being mislocalized in space toward a virtual full body inducedby visuo-tactile illusions (Ionta et al. 2011). Crucially, activity in this region may beimportant also in the production of phantom limb sensations in SCI patients. Forexample, Curt and colleagues reported a patient who suffered an incomplete quadri-plegia after SCI at C3 level. The patient experienced a phantom limb while supine,but very rarely in a sitting position or during the induction of the rubber hand illusion(Curt et al. 2011). That the phantoms were reported while supine but not while sittingreveals that vestibular input may have interacted with the integration of somatosen-sory information, and confirms a vestibular contribution to embodiment2. Further-more, 4 months after injury, the induction of a phantom limb was triggered byapplying the classical multisensory conflict underlying the rubber hand illusion(synchronous/asynchronous visuo-tactile rubber hand stimulation triggered the ap-pearance of the phantom limb). Seven months after injury, however, the phantomlimb was no longer activated by the stimulation of the rubber hand and the patient



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showed a normal RHI. Thus, supine posture and multisensory stimulation may haveinduced a bias in the integration of vestibular and multisensory signals concerning thebody, respectively. The failure to correctly integrate sensorimotor, visual and vestibularinformation may have occurred due to changes in the activity of TPJ, which seems to

play the crucial role in determining body-related illusions (Blanke et al. 2002).Disembodiment of one body-part generally involves extended representation of the

remaining body-parts. This phenomenon is supported by neural plasticity in somato-motor brain regions. When no information comes from the affected limb, patients mayremap limb representations to face and neck representations. Even this form ofminimalbody has still great functional meaning: patients can control external devices andcommunicate with others through their spared facial movements and tactile perceptions.Theoretically, this minimal body may also be used to remap virtually any kind ofcognitive representations and become the bodily space needed to use somatosensory

simulation in support of other functions. Thus, phantom limb perceptions in amputeesand SCI suggest that memory traces re-emerge in the somatosensory cortical represen-tations of the missing body part (Vargas et al. 2009; Mercier et al. 2006). Finallylearning to imagine impossible phantom limb postures had an effect on the bodyrepresentation of amputees and modulated their visual perception of movementsassociated to their phantom limb (Moseley and Brugger 2009). Thus, sensorimotorsimulation remained possible even when the physical body is in fact absent.

2.2 Xenomelia

In striking contrast with the above-mentioned syndromes, there is a clinical conditionwhere the representation of the body seems to be impaired despite normal central (brain)and peripheral functioning of the body. The feeling that a given limb is overrepresented,and is intrusively placed into ones own body representation is termed xenomelia.Affected individuals may express the intense desire to have a given body part (morefrequently the left lower limb) surgically amputated. The condition has therefore alsobeen termed apotemnophilia (see Hilti and Brugger 2010; Giummarra et al. 2011).Xenomelia has long been considered as a psychiatric condition, even though the classicalsyndrome requires excluding psychosis and obvious neurological origins. However,recent neuroscientific research is starting to describe this condition in terms of specificdysfunctions of central body representations. Brang and colleagues have shown thattouches delivered distal to the line of desired amputation trigger increased autonomicresponses with respect to above-line touches or to touches to the same distal site on thecontralateral limb (Brang et al. 2008). These authors have proposed that xenomeliainvolves dysfunctional representation of the body in right superior parietal lobule (SPL).

An attempt to find the neural correlate of this condition has been provided in amagnetoencephalographic study. Analysis of somatosensory evoked activity 40140 ms after tactile stimulation (via MEG recording) revealed significantly reducedactivation in right superior parietal lobule for the affected legs when compared witheither the unaffected leg or legs of healthy controls (McGeoch et al. 2011).

Thus, studies on patients who desire to be amputated converge in identifying theright superior parietal lobule as a candidate for the mis-integration of the felt touchand ones own body representation. This result has been interpreted as if the realsomatic and visual representations of the body did not match a supramodal body



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description, at our embodiment2 level, housed in the right parietal lobe. This descrip-tion would be distorted in these patients. These parietal regions are anatomicallyconnected to somatosensory (S1, S2), motor (premotor and M1), insular and visual(dorsal stream) cortices, and may play an important role in integrating messages

coming from all these regions. If the integration of exteroceptive informationcoming from S1 and interoceptive information from right posterior insula involvesa representation of the full body in SPL, we might say that the SPL generatesembodiment2 by integration of embodied1 signals. This may explain why McGeochet al. (2011) argued that this area underlay the feeling of ownership for parts ofones body. Indeed, limb disownership phenomena typically occur following brainlesions centered upon the right parietal lobe (Berlucchi and Aglioti 2010).

2.3 Out-of-body Experiences and Temporo-parietal Junction (TPJ)

In Out of Body Experiences (OBEs), self and body are perceived as spatiallyseparated. The self is often experienced as floating in the air above the physical bodyand looking down on it. In some cases, two bodies are experienced. The self may thenbe linked to the real body, to the virtual body or to both of them (heautoscopy, cf.Blanke and Metzinger2009; Blanke et al. 2004). In other cases, the self remains inthe real body, but it is seen as if from outside (autoscopy). These conditions have beenlinked to TPJ activity by functional neuroimaging and electroenchephalographicstudies, showing that TPJ is involved in tasks requiring multisensory representation

of one

s own body (Ionta et al. 2011; Blanke et al. 2005). Furthermore, directelectrical stimulation of this region in epileptic patients induces the feeling of beingoutside of their body (Blanke et al. 2002) or the feeling of a presence (Arzy et al.2006). Moreover, transcranial magnetic stimulation over right TPJ affects the abilityto mentally rotate oneself in space (Blanke et al. 2005).

Interestingly, the functions of these areas seem also to go beyond simply generat-ing an embodied2 representation of ones own body. They appear also to contribute tomore abstract cognitive processes. For example, surgical removal of posterior brainregions encompassing the TPJ induces an increase in self-transcendence (ST), apersonality trait supposedly stable over very long periods of time (Urgesi et al.2010). High ST indexes detachment from current actions and body perceptions, weakself-other boundaries, and feelings of a strong connection between the self and theuniverse as a whole (Cahn and Polich 2006; Lutz et al. 2008; Newberg and Iversen2003). Increased ST following TPJ lesion may suggest that TPJ plays an active rolenot only in integrating multisensory signals to generate a representation of the body,but also in excluding other signals, so as to keep the self within the body, andmaintain an orderly separation between the self and the external world. Suchhigher-order cognitive functions are not independent from lower-level somatosensoryprocessing. This link will be highlighted further in the following section.

3 Traces of Sensorimotor Origins Ascend High Up the Cognitive Hierarchy

Extensive experience of using specific objects induces the tendency to incorporate theminto ones own body schema. The case of incorporation of tools has perhaps been the



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best-studied example of this phenomenon (Iriki et al. 1996; Serino et al. 2007).Importantly a similar phenomenon of embodiment2 may apply also to objects devoidof any obvious sensorimotor functional role, but nevertheless associated with ones ownbody at both cognitive and sensorimotor levels. These incidental incorporations suggest

that non-body objects can come to be treated as body parts, if they are appropriatelyassociated with the body. For example, denial of sense of ownership toward a body partafter right hemisphere damage inducing personal neglect can also extend to personalbelongings (Aglioti et al. 1996). A patient with right-hemisphere damage who firstdenied and then accepted ownership of her left hand, also attributed the ring on her lefthand as being either someone elses property, or fundamentally linked to her lifehistory. This example suggests that the extension of body representation can involvesemantic properties of incorporated objects, in this case attribution of ownership.Interestingly, ownership and disownership of the ring were accompanied by affective

as well as semantic changes. Specifically, the emotional valence of the ring changedfrom negative to positive when ownership of ring (and hand) was re-established.If an object may become part of ones own body not only via functional but also

incidental affective associations, the question arises as to whether other emotionally-significant stimuli are processed with reference to body and self. Aesthetic appreci-ation of art objects may provide a challenging and interesting example. In addition tobeing reported as an ineffable cognitive and emotional experience, the appreciation ofan art object may rely upon a sensorimotor simulation of the object as well as of themovements of the artist who created it (Freedberg and Gallese 2007). The viewer may

feel

the art object or event as directly related to her/his own body. The art object, forexample, induces feelings that are fundamentally somatosensory. Art objects maythus play the same functional role in mental life as ones own sensations. Artisticresponses, however indirect, may be embodied2 because they strongly resemblesomatomotor responses, and are strongly linked to ones own bodily experience.

This view predicts that exposure to art should profoundly influence the bodily stateof the observer. A clear example of this is the Stendhal syndrome" characterized byaccelerating heartbeat, dizziness, fainting, confusion and even hallucinations asconsequence of exposure to art works (Magherini 2003). This syndrome is epony-mously associated with the French novelist: Upon leaving Santa Croce, my heartwas beating irregularly (), life was ebbing out of me and I went onwards in fear ofswooning (Stendhal 1812, pp. 271273).

The syndrome has both mental and psychosomatic manifestations. The mentalaspect takes the form of disturbances of the sense of reality, described as feelings ofstrangeness or alienation, and altered perception of sounds and colors. The psycho-somatic symptoms include tachycardia, chest pains, weakness, sweating and some-times stomach pains, each generally accompanied by anxiety and confusion(Magherini 2003). Thus, the ineffable properties of art objects may lead to changesin bodily feelings, and in ones own sense of embodiment.

4 Simulation and Embodiment

So far we have reviewed evidence for mental representations of the body not only atthe somatosensory level (embodiment1), but also visual and multimodal



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(embodiment2). Body representations are pervasive and are maintained even after theloss of peripheral inputs coming from the body. In the previous section, we have seenthat the role of those body representations may not be restricted to the body per se, butcan extend beyond for tools that we manipulate and for art that induce aesthetic

feelings. However, one may want to go even further and propose that body repre-sentations, or what we shall call embodied simulation, can play a role for higherfunctions. We shall now consider the evidence in favour of this view as well as thelimits.

Following Goldmans (2006) theory of simulation, we use the term simulation torefer to purely internal re-enactment of mental events. Simulation can be of twokinds: intra-personal or extra-personal. Intra-personal simulation involves the re-enactment of ones own mental events. For example, in motor control, the notionof somatomotor simulation is often linked to the function of sensorimotor prediction.

The ability of the brains motor systems to simulate sensory outcomes of an impend-ing movement, and thereby adjust ongoing motor control, is described by theories ofinternal forward models (Kawato and Wolpert 1998). The capacity to simulate anaction just before making it, or without making it at all, is a kind of freedom fromimmediacy (Gold and Shadlen 2003) that may allow development of other high-levelcognitive functions relevant to an individuals cognitive capacity, such as imaginationand long-range planning (Pezzulo & Castelfranchi 2007).

Intra-personal simulation can also be extended to inter-personal simulation, thusproviding a starting point for social cognition (Gallese 2003). Inter-personal simula-

tion involves re-enactment of other people

s mental events. This process enables theinterindividual sharing of experiences that originate in the minds of others, andsupposedly the individuation of others states. Hence simulation processes maysupport a variety of functions ranging from lower-level somatomotor control throughto higher-order social cognition.

When simulation happens to invoke somatomotor and affective events, and usesthe neural resources of the brains somatomotor and affective systems, it has beencalled embodied simulation (Goldman and Vignemont2009; Gallese and Sinigaglia2011). In essence, an individual can grasp the mental, sensori-motor and emotionalstates of another if they can link them back to embodiment1 states of their own body.Interestingly, when a person uses this capacity to simulate the somatomotor states ofanother individual, he/she must detach, at least partially, from his/her own currentsensory states (Grush 2004). The term embodiment can thus refer both to bodilyself-awareness, but also to the ability to recreate mental states, including the mentalstates of others, via simulation endowed with sensorimotor and affective format. Weuse the term embodiment3 for this process, since it involves an additional cognitiveprocess of self-other equivalence, over and above embodiment2.

Behavioural, neuroimaging, psychophysiological and neuropsychological evi-dence shows that the brain performs a sensorimotor simulation during perception ofothers actions. The observed action of another may indeed be related back to asimilar action that one makes oneself by classical mirror mechanisms. The firstcomputational problem for such mechanisms is intermodal transformation. Theactions of another individual constitute visual, or perhaps auditory, events as far asthe perceiver is concerned. However, the embodiment2 stage provides a readymechanism to solve this problem, since it already involves an integration of body-



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specific visual information and embodiment1 level somatic signals. More specifically,bimodal visuo-motor neurons in premotor and parietal brain regions of monkeys (diPellegrino et al. 1992; Fogassi et al. 2005) and supplementary motor regions ofhumans (Mukamel et al. 2010) play an important role in this intermodal transfor-

mation, and may thus support socially-resonant neural activities. The strong linkbetween cross-modal transformation and inter-indvidual social resonance has beenshown for perception of others actions (Urgesi et al. 2007a,b; Candidi et al.2008; Moro et al. 2008), discrimination of speech sounds (DAusilio et al. 2009)and discrimination of action sounds (Pazzaglia et al. 2008; Aglioti and Pazzaglia2010, 2011).

One might ask whether these visuo-motor mappings are innate, or shaped byexperience. Typically, this question has been asked by investigating whether visuo-motor transformations that underlie imitative behaviours are fixed, or can be shaped

by preceding exposures in an experimental setting. Observation of an index fingermovement during the execution of a little finger movement, for example, changes thepattern of cortico-spinal facilitation by reversing the association between the ob-served and the simulated movement (Catmur et al. 2007). This evidence has beenused to claim that the mapping of observed actions through the putative activity ofmirror visual-to-motor neurons is experience-based, and may be changed viaHebbian mechanisms (Catmur et al. 2007). On a strong version of this view,sensorimotor simulation is no more than associative learning.

One important issue is how direct perceptuo-motor associations may be

extended to cognitive representation of actions. Some quite abstract associationsof an item to a motor response may be reflected in the activity of the sensorimotorsystem. For example, viewing pictures of athletes, people who could themselvesperform the observed actions showed slower reaction times in a recognition taskwhen the response was made with the limb associated with the motor expertise(i.e. hand for tennis and foot for soccer; Bach and Tipper 2006), relative toanother limb. This effect was also confirmed physiologically: cortico-spinal excit-ability was reduced in the limb associated with athlete whose image was presented(Candidi et al. 2010b).

Action observation classically produces a motor resonance (Fadiga et al. 1995).Interestingly, purely linguistic reference to sensorimotor actions is also reflected inactivations demonstrated by behavioural, neuroimaging and neurophysiological stud-ies (Pulvermuller 2005). In particular the excitability of the cortico-spinal tract isaffected when hearing or reading action-related verbs (Buccino et al. 2005; Candidi etal. 2010a). It is not clear whether the role of motor activations during these tasks isepiphenomenal or causal, but such data at least show that language understandingmay refer back to the motor system.

4.1 Remapping somatomotor simulations

A classic neural signature of embodied simulation, i.e., our embodiment3 level, isthus the recruitment of sensorimotor systems in resonance with sensory and motorevents that are seen to happen to other individuals, along the same lines as theirrecruitment in ones own case. However, overlapping neural activity during first-handpain perception and observation of pain in others has been reported even in



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individuals with congenital insensitivity to pain (Danziger et al. 2009). Crucially, inpatients with congenital insensitivity, empathic pain mapping activated a neuralstructure (the ventromedial prefrontal cortex), which was not activated in healthysubjects. The re-located activity of this structure for responding to the observed pain,

correlated with their measures of empathic personality traits. Therefore, even whenno direct sensorimotor mapping is possible, other neuro-cognitive strategies maysupport non-embodied simulation. The plasticity of the neural substrate shows theimportance of the cognitive function of simulation.

Embodied simulation may thus be important, but not necessary. Non-direct, non-somatomotor simulation is also possible. Although direct somatomotor simulationmay be the preferred and most obvious choice for phylogenetic and ontogeneticreasons, alterations in brain and peripheral sensory structures may impose that thesefunctions are remapped to different neural substrates. The plastic re-allocation of

simulation functions to novel brain areas only serves to emphasise their functionalimportance. Thus, somatomotor simulation may be a special case of a generaladaptive mechanism. In particular, sensorimotor systems have three crucial proper-ties, which must be borne in mind when considering simulation. First, they plasticallyadapt to a number of intrinsic and environmental pressures. Second, they are notcompletely genetically predetermined, but respond to experience. Third, they maychange their organization after damage. Therefore, it seems unlikely that they wouldbe a unique neural substrate for simulation, or that they would provide a single,unique type of simulation. Simulation, indeed, may be regarded as a general mech-

anism that could be implemented in virtually any neural substrate, not only somato-motor and affective systems. Individuals may plastically adapt and use whateverneural resource is available in order to represent the mental states of others, and themeaning of abstract concepts. This notion is based on the plastic nature of neuralcircuits, and on what has been called multiple realizability (Putnam 1975).

The idea of linking back to embodiment1 has been invoked in several areas ofcognition, including memory, imagery, and language understanding (see Barsalou2008 for a comprehensive review on the topic). Exactly how intra- and inter-personalembodied simulation supports cognitive functions is still a matter of debate. Thestrong view that proximal sensorimotor functions are necessary for cognition facesseveral challenges. In particular, individuals with altered sensorimotor interactions,following brain damage, amputation or aplasia may have preserved cognition, con-trary to the strong view. To resolve this impasse, Mahon and Caramazza haveproposed a cascade-like bidirectional flow of information between sensory andintegrative brain regions which would be sufficient to account for most of theavailable data on the relation between cognition and sensorimotor neural activations(Mahon and Caramazza 2008). However, bidirectional flow theories of this kindmake rather few testable predictions.

Although individuals use similar brain regions, neural networks and temporaldynamics to perceive and represent specific objects and concepts, these activationsare not identical between individuals, nor do they demonstrate that a given part of thebrain is dedicated to the representation of specific experiences. The same argumentholds true for an individual who repeatedly re-represents the same object. The neuralsubstrate of the representation may change during the lifespan in the face of thestability of the conscious representation itself. Thus, although theories of situated and



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grounded cognition (Barsalou 2008) propose that representations have stable links totheir perceptual origins (Barsalou 1999), perceptual and somatomotor systems showhigh plastic modifications which are at odds with the stability of conceptual repre-sentations. Sensorimotor systems are strongly affected during development via

experience-based plasticity and might be disturbed by external traumatic events(brain damaged, amputation) that would lead to changes in their structure. Thestability and the tendency of the brain to represent and organize perceptual andconceptual categories in its structure may overcome these phenomena but does notin itself say anything about the functional role of the different brain regions wherethese functions are instantiated.

The main feature of a mental representation is, of course, that it needs to co-varywith the external object it refers to (Clark and Grush 1999; Pezzulo and Castelfranchi2007). However, the high degree of plasticity implies that the neural substrate might be

anywhere or everywhere in the brain. Embodied simulation has been studied because itis readily accessible scientifically, and has reductive scientific appeal. However, theconcept of simulation cannot be tied only to somatomotor and affective mechanisms.

5 Conclusion

Somatosensory and motor simulation is a natural extension of embodiment2 toembodiment3. Instances of 'disembodied bodies' indicate the brain uses past memo-

ries and inferential cognition to drive representation of one's own and others' bodilystates. Embodiment through somatomotor simulation may extend beyond the phys-ical body and the activity of sensorimotor brain regions might allow the embodimentof objects and abstract concepts. Circumstances of altered perceptions such as thoseoccurring in the Stendhal syndrome, which have been left outside scientific consid-eration for a long time, might show that the nervous system can develop non-somaticresources to represent objects and abstract entities as well, but always with a link torepresentation of the body. In conclusion, while we broadly support the concept ofembodied simulation, we think that little is gained by insisting on its directness, i.e.,the link back to embodiment1. Both intra-personal simulation and inter-personalsimulation are pervasive in cognition. To our minds, the steps forward from embodi-ment1 to embodiment2 and embodiment3 is a remarkable development of cognition.Projects of embodied cognition need to remember the magnitude of these stepswhen linking back to embodiment1.

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Embodying Bodies and Worlds (1)