Aplasic phantoms and the mirror neuron system: An enactive, developmental perspective

Aplasic phantoms and the mirror neuron system:An enactive, developmental perspective

Rachel Wood & Susan A. J. Stuart

Published online: 22 July 2009# Springer Science + Business Media B.V. 2009

Abstract Phantom limb experiences demonstrate an unexpected degree of fragilityinherent in our self-perceptions. This is perhaps most extreme when congenitally absentlimbs are experienced as phantoms. Aplasic phantoms highlight fundamental questionsabout the physiological bases of self-experience and the ontogeny of a physical, embodiedsense of the self. Some of the most intriguing of these questions concern the role of mirrorneurons in supporting the development of self–other mappings and hence the emergenceof phantom experiences of congenitally absent limbs. In this paper, we will examine thehypothesis that aplasic phantom limb experience is the result of an ontogenetic interplaybetween body schemas and mirror neuron activity and that this interplay is founded onembedding in a social context. Phantom limb experience has been associated with thepersistence of subjective experience of a part of the body after deafferentation throughsurgical or traumatic removal.Wemaintain that limited association is inconsistent with theextent to which phantom limb experience is reported by aplasic individuals.

Keywords Aplasia . Phantom limb experience . Body schema . Enactivism .

Mirror neuron system . Embodiment

Introduction

Phantom limb experiences among individuals who suffer the loss of a limb throughtrauma or disease provide an intriguing illustration of an inherent uncertainty at the

Phenom Cogn Sci (2009) 8:487–504DOI 10.1007/s11097-009-9138-2

S. A. J. Stuart (*)University of Glasgow, 11 University Gardens, Glasgow G12 8QH, UKe-mail: [email protected]

R. WoodDipartimento di Neuroscienze - Sezione di Fisiologia, Universita’ di Parma, Via Volturno 39,I-43100 Parma, Italye-mail: [email protected]: [email protected]

R. WoodCentre for Computational Neuroscience and Robotics, University of Sussex, Brighton BN1 9QG, UK

center of our apparently veridical self-experience. We know that human perceptualsystems can be easily fooled by visual illusions and other “tricks” that subtlymisrepresent a state of affairs in the world. Yet still, we hold on to the idea that ourinherent notions of self are immune to perceptual legerdemain. Thus, I may bevulnerable to an assortment of visual, auditory, and proprioceptive illusions but stillfeel that no such chicanery can trick me about my own physical form. Evenmanipulations such as the infamous “rubber hand illusion” (RHI; Botvinick andCohen 1998) can only make me feel as if the fake hand on the table might belong tome if what I see happening to the rubber hand matches what I feel happening to myown hand. As soon as these conditions are broken, my normal self-perceptionreasserts itself. In fact, quite stringent conditions must be met in order to bring aboutthe RHI, the most important of which is temporal synchronicity of sensory inputs.Second only to temporal synchronicity is the need for coherence with existing visualand proprioceptive information about the body (Tsakiris and Haggard 2005). Therubber hand can be, in some limited sense, incorporated into the subject’s ongoingsense of itself only so long as the seen stimulation of the fake hand and the feltstimulation of the real hand are synchronized and the rubber hand is also in aposition which is congruent with rest of the subject’s body.

The RHI helps to demonstrate the importance of coherence in the subject’sexperience of her physical self, an alien object can be incorporated into the bodyschema if the object is in the right place and sensory feedback comes at the righttime. I can be tricked into temporarily owning a fake hand but only if the claim doesnot violate the coherence of my lived body schema. So the RHI which might, at firstsight, seem to be evidence for some instability in our embodied notions of selfactually attests to their fidelity, showing as it does that a carefully orchestrated set ofcircumstances is required for the illusion to be sustained. By contrast, however,phantom limb experiences demonstrate that our self-perceptions might be rathermore fragile than we like to think. Here, it is not manipulation or subterfuge thatcreates an illusion, but rather, the body itself generating sensations and perceptsapparently pertaining to missing parts. This is illustrated in its most extreme form inthe case of aplasia when congenitally absent limbs are experienced as phantoms.Much of the data on phantoms of congenitally absent limbs has been obtainedthrough self-reports from aplasic individuals [the experiments conducted by Bruggeret al. (2000) and Gazzola et al. (2007) being notable exceptions]. According to suchreports, upper and lower limb phantoms are experienced and, in the extremely rarecase of tetra-amelia in which neither upper nor lower limbs develop fully, both armand leg phantoms may be experienced by the same person (Price 2006).

Phantoms may be extremely vivid and present almost all of the time or be quitefleeting experiences which occur every few months. The phantom may represent theentire missing limb (a complete forearm for instance) with all phantom parts beingequally vivid or the phantom may, for example, be limited to the vague sensation ofa hand with undifferentiated, “finger-like” projections (Price 2006). Phantoms arereported both by users and nonusers of prosthetic limbs (Saadah and Melzack 1994;Melzack et al. 1997; Price 2006). In some instances, the emergence of a phantomlimb appears to be linked to an injury or medical procedure affecting the limb stump,and yet in others, the first phantom experience appears not to be linked to anyobvious physiological event. Melzack et al. (1997) document the phantom

488 R. Wood, S.A.J. Stuart

experience of two aplasic adolescents, both missing an arm below the elbow andwho both report bad weather or rain as being among the triggers for their phantomexperiences [p. 1610]. The role of attention in eliciting phantoms appears to beambiguous. Some subjects report that their phantom disappears when they attend totheir limb stump or think about the phantom itself, but others say that the phantomemerges when they think about or bump their stump against some object [ibid.]. Theexistence of aplasic phantoms and the strong heterogeneity of aplasic phantomexperience highlight fundamental questions about the physiological bases of self-experience and the ontogeny of a physical, embodied sense of being a self. How canwe understand the role played by coherence in the generation of aplasic phantomsand what does this tell us about embodiment and self-experience?

Self as perspective

If we consider phantom limbs as a form of distortion or disturbance of the sense ofself, then it is useful to spend a little time considering notions of selfhood and howthese might contribute to our understanding of how aplasic phantoms come about.There is in fact no generalized, theoretical consensus about what the notion of “self”means and there exist a great many competing definitions of the term (Zahavi 2005).Some authors go so far as to deny that such things as selves exist in any substantialor forensic sense (Stuart 2006), others even preferring to posit representations toaccount for the sense of perspectival ownership or phenomenological “mineness” ofexperience (Metzinger 2003a, b). On this view, complex representational processesgenerate the experience of being a self from the top down, the self is thus a by-product of cognition or, rather, there is no self, only representational content. This isa more extreme position even than the standard materialist account in which theexperience of being a self amounts to no more than the net product of brain activity.The materialist position is possibly best exemplified in Churchland’s analysiswherein I feel like a self because my brain makes it so; the activity of my brain iscoherent and continuous, thus so is my sense of myself (Churchland 1981). Theaccount offered by eliminative materialism, being rooted in a neuromechanistic levelof analysis has plenty to contribute to the question of where the feeling of selfhoodcomes from but, by definition, says less about that what it is comprised of. Bycontrast, phenomenological approaches attempt to couple investigation of thecontent of experience with exploration of what it means to be a self.

Merleau-Ponty describes two components of selfhood: ipseity realized byembodied interaction with the world and temporality referring to the continuity ofexperience in time. The coupling of “aboutness” and temporal coherence provides amodel of self-experience, which goes beyond the notion that a concatenation of brainprocesses is responsible for the feeling of perspectival ownership at the heart of self-experience. The prereflective bodily self is extended spatially through preattentional,real-time representations that are generated by the interplay of proprioceptive,somatosensory, and vestibular inputs. Since spatial extension necessarily impliestemporal extension and the organism’s spatiality is, for the most part, transparent,which is to say that it possesses a nonself-conscious immediacy, so, too, in theprereflective mode, is the agent’s temporality. Embodied sensory immersion in a

Aplasic phantoms: An enactive, developmental perspective 489

temporally continuous experiential world implies a point of view, a locus on whichthe flow of experience is centered.

Nevertheless this ceaseless welling up of time is not a simple fact to which Iam passively subjected, for I can find a remedy against it in itself, as happensin a decision which binds me or in the act of establishing a concept. Itwithholds me from what I was about to become, and at the same time providesme with the means of grasping myself at a distance and establishing my ownreality as myself (Merleau-Ponty 1962, p. 380).

The phenomenological conception of self exemplified by Merleau-Ponty and,more recently, Zahavi and others (see, for example, Gallagher 2005 and Legrand2006) might be characterized as emanating from the inside and projecting out. Theseapproaches to selfhood focus on the phenomenal content of experience from a locusof perspective, that is, the self as a phenomenological point of view. Conversely, thisfocus can also be inverted to informative effect; thus, the experience of self can alsobe understood as shaped by the outside projecting in. On this view, self is broughtinto relief by the medium in which it is immersed, and phenomenological experiencearises at the interface between the experiencing individual and the world of things asthey are encountered.1 Put differently, self is made manifest in the context of itsrelationship with other.2

Merleau-Ponty defines ipseity in terms of embodiment and “being-in-the-world”(Merleau-Ponty 1962, p. 408) concepts that serve to emphasize the necessarycoupling between the self as a locus of experience and the world of phenomena to beexperienced.

[T]his view makes it clear that self-awareness is not to be understood as anawareness of an isolated, worldless self, nor is the self located and hidden inthe head. To be self-aware is not to interrupt the experiential interaction withthe world in order to turn one’s gaze inwards; on the contrary, self-awareness isalways the self-awareness of a world-immersed self. The self is present to itselfprecisely and indeed only when it is engaged in the world (Zahavi 2005, p. 10).

Merleau-Ponty talks of the world of objects as bringing forth the body toconscious experience; the body is experienced through interaction with things,

1 We might understand this by employing the metaphor of the cell membrane whose tensegrity, ortensional integrity, is maintained by compression and pressure from each side of the membrane’s surface.Its embodied immersion has the membrane in sharp relief at the interface. Though here we might beaccused of moving towards a philosophy of immanence, of the necessary balance of the constituted/constitutive forces, rather than remaining strictly within the phenomenological conception.2 We refer here only to the physical world of objects, but would argue that the capacity to think reflexivelyof oneself as a self is only possible in socially embedded agents, and these are agents which naturallypossess an endogenous intersubjectivity (Gallagher 2007), that is, the practical knowledge of oneself thatguides from the inside out. It is through the combination of a socially embedded self-awareness andprereflective bodily self-awareness—derived through active felt agency—that a subjective conception ofourselves by ourselves (first-person) and an intersubjective conception by others of us as persons (second-person) is entailed. It is this notion of personhood which extends the self temporally into a world ofmorally culpable agents and morally vulnerable patients. Thus, it is that agency is ontically andepistemically prior to self, and it is agency and reflective awareness which entails personhood as judgedfrom a first-person (subjective), second-person (empathetic/intersubjective), and third-person (objective)perspective.


which, in turn, are perceived in terms of the individual’s intentions towards them. Onthis view, immersion in a world of manipulable objects calls forth “habitualintentions” or ways of being-in-the-world by which the self experiences both bodyand world. Thus, a world of graspable things is experienced by the individual interms of affordance. At the same time, the individual experiences her embodied selfvia her capacity to act on such things. In the case of someone who has lost a limb,the world evokes those same habitual intentions but now there is a discontinuitybetween the available affordances and the capacity to act. The world remains full ofmanipulable things but the subject lacks an effector and somehow this mismatchmust be reconciled. Thus, phantom limb experience might be understood as “calledforth” by the world of objects, a relic of the habitual mode of being-in-the-world of aself or agent that has yet to remap the relationship between embodiment andaffordance [ibid.].

This body–environment circuit of motor intentionality is constitutive ofwhat Merleau-Ponty calls the “intentional arc” subtending the life ofconsciousness, which integrates sensibility and motility, perception andaction (Merleau-Ponty 1962, p. 136). The intentional arc and being-in-the-world overall are neither purely first-person (subjective) nor purely third-person (objective), neither mental nor physical. They are existential structuresprior to and more fundamental than these abstractions (Thompson 2005,p. 410).

Self and other

At first sight, this analysis has little to contribute to the understanding of aplasicphantom experience; in the case of congenital limb absence, there is no history ofembodied interaction with the world to account for the phantom, that is, there is nohistory for the absent body part. The aplasic infant develops her capacities to actwith whatever effectors are available. Her mapping of action to object is alteredthough her embodied self is still founded in the coupling between the givenness ofher first-person phenomenological experience and her immersion in a world ofinteractive possibilities. Yet, in trying to understand the roots of aplasic phantomexperience, it may perhaps be useful to remember that the external world of thingsincludes other people and that these are entities that are, more or less, like ourselves.Recent discoveries in neuroscience have helped to illuminate some of the processesby which we recognize and interpret the actions of other conspecific agents orselves, and these findings may have some bearing on how the embodied self is, inturn, shaped through its interaction with others.

Mirror neurons are hypothesized to play a fundamental role in intersubjectivity,allowing an individual to understand the intentions of others by observing theiractions (Rizzolatti et al. 2001). Mirror neurons, networks of which have beenidentified in the inferior frontal gyrus (region F5), the inferior parietal lobule (DiPellegrino et al. 1992; Gallese et al. 1996; Rizzolatti et al. 1996), and the cortex ofthe superior temporal sulcus (Perrett et al. 1989, 1990) in macaques and humansubjects, are active when we engage in certain goal-directed actions and when we


perceive similar actions in others (usually conspecifics). Mirror neuron networks canthus be said to link perception and action, providing a mechanism by which theobserved behavior of others is “mirrored” at the neuronal level. It is proposed thatmirror mechanisms underpin intention understanding (and thus social3 behavior) bymeans of motor “resonance” whereby perceived actions provoke concomitantactivity in the motor cortex of the observer, as if the observer herself were acting.

“In order to be triggered by visual stimuli, mirror neurons require an interactionbetween a biological effector (hand or mouth) and an object” (Rizzolatti andCraighero 2004, p. 170). In the rostral part of the inferior parietal lobule, area PF ofVon Economo (1929), a third of the neurons are somatosensory, just over one tenthare visual, and the other 56% are bimodal (somatosensory and visual) neurons.“Virtually all mirror neurons show congruence between the visual actions theyrespond to and the motor responses they code” (Fadiga and Craighero 2007, p. 102).

It is claimed that:

Each time an individual sees an action done by another individual, neurons thatrepresent that action are activated in the observer’s premotor cortex. Thisautomatically induced, motor representation of the observed action corre-sponds to that which is spontaneously generated during active action andwhose outcome is known to the acting individual. Thus, the mirror systemtransforms visual information into knowledge (Rizzolatti and Craighero 2004,p. 172).

Mirroring, thus, can only occur for actions that are in the behavioral repertoire ofthe observer and so we might expect impaired intention understanding andconcomitant social deficits in individuals born with a reduced or altered behavioralrepertoire. Developmental aplasias such as tetra-amelia (characterized by the absenceof both upper and lower limbs) would be good candidate syndromes in which toobserve such deficits induced by dysfunctional mirror systems. However, theliterature does not support the notion of impaired action understanding (orsystematic social deficits) in cases of congenital limb absence. In addition, studiesof phantom limb in aplasic limb absence indicate the presence of neural structures tomap nonexistent body parts and motor responses (Brugger et al. 2000) providingfurther reason to think carefully about how we conceptualize the role of mirrormechanisms in aplasic phantoms.

Rizzolatti et al. (2001) ask “What are the neural mechanisms that underlie actionunderstanding?” where, by action understanding, they mean “the capacity to achievethe internal description of an action and to use it to organise appropriate futurebehaviour” [p. 661]. They propose a “direct-matching hypothesis” which states thatanother’s action is understood when the visual representation of the observed actionis mapped on to our own motor representation of the same action.

3 For the sake of clarity, let us state simply that our definition of “social” is broad and not the one favoredby, for example, some linguists and sociologists as being specific to human beings and including a set ofsocial structures, norms, institutions, culture, language, and so on. “Social” in our sense is more broadlybiological, applying to living organisms that interact in a collective coexistence. See De Jaegher and DiPaolo (2007) for an operational, enactive definition of the social which is compatible with our use of theword.


Gallagher (2006) mentions the many people, including de Vignemont (2004),Gallese (2005), Hurley (2005), and Jeannerod and Pacherie (2004), who haveclaimed that “representations in those brain areas that are activated when I performan action and when I perceive another person perform the action, are neutral inregard to determining agency” [p. 2], and those who are now concerned with findingthe “Who system” (Georgieff and Jeannerod 1998) which makes agent determinationpossible. But their quest is misguided. There is a givenness of myself that isimmediately manifested in my experience (Henry 1963) without which consciousexperience would be meaningless (Merleau-Ponty 1962). This givenness mightcorrespond with what Sheets-Johnstone (2003) describes as the “energic qualities”that are present in my goal-directed agency, and which make it clear to me, withoutthe need for any secondary system, that it is me who is the agent. Without the“energic qualities,” one could imagine the feeling of mineness or agency beingabsent and, subsequently, the issue of agent determination not arising. At a reflectivelevel, one can imagine concluding, it is not me who is acting, so it must be you, butat a prereflective level, this is simply unnecessary; it is happening at a “completelyneurological and sub-personal” level, “intentions in almost all cases come alreadyclothed in agency” (Gallagher 2006, p. 3).

The given body

The self-givenness4 of action is, at least in part, enabled by the possession of a bodyschema, that is, the system of motor capacities, abilities, and habits that enablemovement and the maintenance of posture and which function without the necessityof perceptual monitoring. The body schema “is not a perception, a belief, or anattitude … [but] … a system of motor functions that operates below the level of self-referential intentionality, although it can enter into and support intentional activity. Itinvolves a set of tacit performances, preconscious, subpersonal processes that play adynamic role in governing posture and movement” (Gallagher and Meltzoff 1996,p. 6). Gallagher and Meltzoff frame their analysis around the supposition that thebody schema, in some form at least, is innately specified. Furthermore, they suggestthat the evidence from developmental psychology supports the notion of this innateschema as providing a mechanism to account for both early imitation and aplasicphantoms “the evidence raises the possibility that the basic framework of a bodyschema is innate” (Gallagher and Meltzoff 1996, p. 7).

The body schema as described by Gallagher and Meltzoff (they are careful todifferentiate between body schema and body image, a distinction that they point outis not maintained in many discussions of aplasic phantoms) is very much predicatedon action, it is not a perception of the body but rather an experience of it; so, in fact,here it seems that Gallagher and Meltzoff’s own characterization of the conceptmitigates against their assumption of innateness. The infant (aplasic or otherwise) is

4 Perhaps we might understand self-givenness in terms of Husserl’s concept of “eidetic intuition”: thedirect givenness which “refers to the acts in which ‘objects show up in person’” (Depraz et al. 2003, p. 45)and which primarily reveals itself as a perceptual and imaginative act concerned with disclosing anessence [ibid., p.55]. Self-givenness is concerned with the revelation of the tight experiential couplingbetween body and ownership of the experience.


born with a system of motor functions and, crucially, with a history of prenatal, inutero motor behavior. The uterine medium is sufficiently dissimilar from the postbirth environment as to require significant relearning of proprioceptive contingencieson the part of the infant, processes which can be seen as further shaping anddirecting the ontogeny of a lived body schema.

Either the body schema depicted by Gallagher and Meltzoff is meant to refer tothis history of uterine movement in which case the schema is clearly not innate but aproduct of fetal experience or it simply comprises a set of motor capacities orpossibilities for action within the infant’s nascent behavioral domain. It is not clearin what sense this set of morphological affordances can be usefully distinguishedfrom the body itself. It is obviously quite possible to draw a clear ontologicaldistinction between notions of what the body is and what it can do but then how isthe capacity for action represented as a schema. One answer might be to argue thatthe body schema resides in the neural implementation of the motor system, that thecapacity to act is thus represented in neural networks. This level of descriptionintuitively seems not to do justice to Gallagher and Meltzoff’s original notion of thebody schema as a structure involving “performances” underlying intentional activity.Thus, we come back to a characterization that links motor capabilities (possibilitiesfor action) with a history of being in a world (abilities, habits). This richer conceptseems more plausible as a means of sustaining the notion of body schema as thelocus of linkages between mere movement and intentional action.

Merleau-Ponty also distinguishes body image and body schema, arguing that theattempt to account for phantoms by reference to body image generates no newinsight while the body image is held to be constitutive of the physical experience ofself rather than “the residue of habitual cenesthesis” (Merleau-Ponty 1962, p. 86).Going still further, he rejects the reduction of body schema to the sum total of“associations established during experience,” arguing that it is better characterized“as an attitude directed towards a certain existing or possible task” [ibid.], that is, away of being-in-the-world. For Merleau-Ponty:

The body schema functions as if it were an “innate complex” (p. 84), that is, asstrongly and pervasively as if it were innate, but, as an acquired habit with adevelopmental history, it is not innate. It follows that the existence of aphantom limb is based on a history of sensory inputs, and the continuation ofsensory inputs at the stump. Sensory impulses “establish and maintain itsplace, prevent it from being abolished, and cause it still to count in theorganism.” They are the sine qua non by which we “build up the phantom”(1962, p. 86) (Gallagher and Meltzoff 1996, p. 6).

Gallagher and Meltzoff reject this view of the acquired body schema, citingevidence that, in one study of 30 aplasics, 17% experienced phantoms (Weinsteinand Sersen 1961)5. Their argument centers on the idea that, if the body schemadevelops, then it should be true to the physical form of the individual and aplasicsshould not experience phantoms. Thus, in the nonaplasic infant, there is a

5 Melzack et al. (1997) found that 20% of congenital aplasics in their questionnaire study to haveexperienced phantoms (n=76). Saadah and Melzack (1994) report a phantom incidence of around 10% inanother questionnaire study of 100 aplasics.


rudimentary, innate body schema6 ready to be elaborated through action, actioninterpretation, and imitation. In the aplasic case, the infant has the same rudimentarybody schema and this innate structure is sufficient to overcome the lack of visual,tactile, and proprioceptive awareness for the missing limb(s) and to produce thephantom.

But surely, we might ask exactly the opposite question: if phantom experienceproves the innateness of the body schema, then why are aplasic phantoms not morecommon? One answer to this question might be that the phantom represents thefailure of an innate schema to adapt to the actual form of the body, and thus, a failureto incorporate the absence of missing limbs. Thus, the majority of aplasics havebody schemas which accurately reflect the individual’s particular physical form, butin some cases, the missing limbs are still “present” in some rudimentaryconfiguration in the body schema. But this solution seems to beg the question ofwhat develops and when. If the account is now that the body schema is innate andthat it is ontogeny which fits it to the infant, then it is not clear what special meritthis explanation for the aplasic phantom has over and above an alternative view inwhich the body schema develops from the outset.

In order to make a case for the ontogeny of the body schema as a system of motorcapabilities, it is crucial to take account of the evidence pertaining to the infant’shistory of embodied action prior to birth. In an ultrasound study of fetal movementkinematics, Zoia et al. (2007) found an unexpected degree of action planning in themovements of 22-week-old fetuses. Kinematic patterns for bringing the hand to themouth (for thumb-sucking) and for bringing the hand to the head (for touching) werefound to be quite different both in terms of peak velocity and movement duration.By 22 weeks, hand reaches become straighter and more accurately aimed withacceleration and deceleration phases of the movement predicated on the size andsensitivity of the target. These findings provide strong evidence that the neonatecomes into the world with a nontrivial history of embodied action and, while it istrue that the medium in which it is moving has quite different characteristics fromthe environment in which it will soon find itself, the noteworthy aspect is that theinfant already has an experiential history with its own body [ibid.].

We can see further evidence that the neonates’ experience of motion comes fromstudies of biological motion perception in newborn infants. Two-day-old infantsshow robust, orientation-dependent sensitivity to biological motion when exposed topoint-light displays of dots moving in a coherent pattern predicated on walking. In astudy by Simion et al., infants consistently preferred patterns based on a hen walkingabove random displays. Such data could be explained by recourse to an innate“biological motion detector module” designed (perhaps by evolution) to differentiatebetween animate and inanimate objects in motion (Simion et al. 2008). Alternatively,we might also propose that it is fetal experience with vestibular and proprioceptivemotion cues which shapes the preference for coherent motion patterns and that thisvery early experience helps to calibrate shared visuomotor circuits involved in mirror

6 This notion of an innate body schema challenges the earlier acquired postural model favored by, amongstothers, Piaget (1962), Merleau-Ponty (1962), Wallon (1947, 1965), and Simmel (1966). In their model, theinfant’s experience is entirely interoceptive with the external perceptual abilities, which mark theexteroceptive domain, only developing after a matter of months.


neuron systems, a point also raised by Simion et al. These findings contribute to anunderstanding of the newborn infant as a fully embodied agent with a ready historyof coupling to an environment. On this view, the body schema develops with theinfant and the aplasic phantom cannot be explained as the result of an “innate”structure that fails to map the body accurately.

Caution is required here; there is a risk that, in seeking to question one perspective onthe causes of aplasic phantoms, we may inadvertently perpetuate a much more insidiousset of assumptions about developmental causation in general. Aplasic phantoms can beinterpreted as evidence to support the notion of an “innate” body schema, though here wequestion that view by examining evidence for the ontogeny of the body schema and, byextension, the phantom. This process inevitably entails discussion of “acquired”behaviors and characteristics. However, that should not be taken as tacit endorsementof the conventional innate–acquired dualism in developmental theory. Oyama provides aconstructivist understanding of ontogeny which is highly compatible with enactiveapproaches to cognition (Varela et al. 1991). The concept of enaction wherebyknowledge is indelibly tied to the praxis of action is valuable here as we are seeking tobuild an account in which the aplasic phantom develops through a history of embodiedinteraction in the (social) world. Oyama argues strenuously against the “dichotomi-zation” of development and the prevalence of the “gene as information” metaphor(1985) [these sentiments are memorably echoed by Thelan and Smith in their rejectionof the structure versus process “Balkanization of developmental phenomena” (1994,p. 38)]. In ontogeny, there are multiple sources of what might be termed “information”(a.k.a. causes) both within and without the organism; no one of these sources can beusefully accorded causal privilege over another. Development occurs as the unfoldingof a process; otherwise understood as the dynamics of a self-organizing system. Any“cause” exists as such only in the wider context of a process enacted in time. Thenotion of developmental “information” thus becomes obsolete regardless of whether itis considered to be located internally in genetic material or externally in some aspect ofan environment. By extension, internal–external, innate–acquired dichotomies aresimilarly rendered redundant in the context of a dynamic process of self-organizationby which a system constructs itself through time [ibid.]. Our own position with regardto the innate–acquired dichotomy accords most closely with Oyama’s.

Phantom ontogeny

Aplasic phantoms seem most often to appear in early childhood or even later:

[I]n the majority of cases of aplasic phantoms the onset of the phantom takesplace relatively late. In cases where specific ages are provided to indicate theonset of the aplasic phantom, the age of onset ranges from 4 to 30 years, withthe majority of subjects experiencing the onset of the phantom between theages of 5 and 8 years (see Poeck 1964; Saadah and Melzack 1994; Weinsteinand Sersen 1961, and Weinstein et al. 1964) (Gallagher and Meltzoff 1996,p. 8).

In a review of 39 cases of aplasic phantoms, Price (2006) reports 11 in which thephantom has been experienced for as long as the subject could remember; however,


it is clearly not possible to establish from this description how early in life thephantom became apparent. The most obvious reasons for the lack of data early onsetphantoms are either (1) that younger children are simply incapable of accuratelyrecognizing and distinguishing a phantom from a nonphantom experience or (2) thatthey are incapable of reporting on any phantom experiences they might have. Yet, itis significant that there are no reports in the literature of infants with congenitalaplasia trying to use absent limbs or in any other way indicating that they experiencesomatosensory feedback from their missing limbs. Interestingly, Melzack et al.(1997) report that phantoms emerge later in congenital aplasics than in amputees;according to their data, the average age of phantom onset in congenital aplasia was9 years (n=15) compared to 2.5 years following amputation (n=26). They alsofound that, the older a child was at amputation, the shorter the time to the emergenceof a phantom. There are clear methodological problems associated with attemptingto resolve these questions; indeed, it seems unlikely that such issues could ever beresolved with absolute certainty. However, if we accept the extant literature on theage of onset for phantoms, it again appears likely that some developmentalinteraction underlies their eventual emergence.

We can add to this phenomenological picture of phantom ontogeny with newphysiological data about the neural correlates of phantom experience. In a case studyof a woman with tetra-amelia, Brugger et al. (2000) used transcranial magneticstimulation (TMS) of the sensorimotor cortex to generate contralateral phantom handand finger sensations. Movements of the subject’s existing upper arm structuresproduced activations in areas of cortex deprived of afferences or efferences (silentareas) and these activations mapped to phantom hand sensations. In addition,phantom sensations were obtained in the absence of concomitant motor-evokedpotentials (MEPs), a result which contrasts with data obtained from traumaticamputees. The subject of this study A.Z., born with neither forearms nor legs,reported having had vivid phantom limb experiences for “as long as she couldremember” [ibid., p. 6168]. Her phantoms comprised forearms with hands andfingers and legs with feet plus first and fifth toes. Interestingly, A.Z., who uses awheelchair but not prosthetics, reported that when manipulating objects with an armstump she would feel the phantom fingers but lose awareness of the forearm,whereas when contact with the object stopped, the fingers would return to their distalposition.

TMS was used to map A.Z.’s sensorimotor cortex with MEPs obtained from thedeltoid muscles. In 13 out of 18 stimulation sites where a response was elicited, acoincidence of deltoid MEPs and phantom sensations were reported; four stimulationsites elicited phantom movement sensations without corresponding MEPs. Phantommovement sensations were experienced exclusively in the limb contralateral to thestimulated hemisphere. These were described as slow movements of one or severalfingers or of the whole hand. In contrast with TMS-elicited contralateral phantommovements, functional magnetic resonance imaging (fMRI) data showed volitionalphantom movements to produce bilateral activations, indicating a difference betweenexternally triggered and self-generated phantoms. Phantom finger and hand move-ments failed to produce activation of primary motor cortex; however, bilateralactivations of the dorsal premotor cortex and superior posterior parietal cortex wereobserved.


Brugger et al. conclude: “These data indicate that body parts that have never beenphysically developed can be represented in sensory and motor cortical areas” [ibid.,p. 6167]. While they acknowledge that these findings could be taken as evidence insupport of an innate body schema, they also invoke shared neural networks foraction preparation and observation (i.e., mirror neurons) to account for the findingthat absent limbs are mapped in the brain.

In the absence of a physical substrate for the execution of an action, habitualperception of conspecifics moving extremities could still activate networksmediating a visuomotor limb representation. This activation may give rise tophantom sensations in at least a minority of individuals with limb aplasia[ibid., p. 6172].

The aplasic individual inhabits a world of graspable, manipulable objects and ofconspecifics who execute their intentional actions though the means available tothem (including the “exercise” of effectors the aplasic lacks). The aplasic person isable to perceive others moving their limbs and, possibly, mirror their actionneuronally which would also provide some low-level somatosensory experience.Significantly, the aplasic individual is also able to attend to the goals of others’actions, that is, not just the “how” of action but also to the “why.” By these means, itis possible to postulate that mirror mechanism activity might reach a threshold suchthat it signals the presence of a limb even where no actual limb exists. It is importantto emphasize the strongly embodied sense in which mirror neurons are postulated tocontribute to this aspect of phantom generation. Action observation–executionmapping takes the form of motor resonance. Observation of an action evokes neuralactivity concomitant with execution of the action, the neural structures whichproduce this response are located in the brain of the observer but the correspondencethey enact is entirely founded on the body and its capacity to act.

By extension, the obvious difficulty with this account arises from the necessity formirrored actions to be in the behavioral repertoire of the observer. In other words,there is no mirror mechanism response for actions which the observer has nopersonal experience of performing. But a solution to this might be found in thehistory of observing the species-typical action primitives of conspecifics which hasan ontogenetic priming effect on mirror neurons, such that, for instance, handactions, are remapped onto the effector systems available to the aplasic person. Thisanalysis suggests that mirror mechanisms should be relatively flexible with regard tothe mapping of body parts to actions with the means of execution being ofconsiderably less importance than the goal. Thus, the correspondence betweenactions should lie not in the effector systems employed but in their goals. So, the useof a hand to bring a cup to the mouth and drink can be (goal-)equivalent to the use ofa foot to lift the cup to the mouth in order to drink.

This notion is, to at least some extent, supported by recent experimental work byCatmur et al. (2008) who found that, in morphologically typical adults, mirrorresponses could be obtained for hand to foot mappings. In this fMRI study, twogroups of participants were trained to execute movements in response to the actionsof a model. The control group was trained to respond with a congruent effector(hand to hand and foot to foot) while the other group responded with an incongruenteffector (foot to hand). Catmur et al. found countermirror activity in the incongruent


condition. Thus, brain areas that responded most strongly to observation of handactions in the control group responded more to foot actions in the incongruent group.

This far from trivial finding demonstrates that motor resonance is inherentlyplastic and it further indicates that the means of execution does not predicate theresponse of the mirror mechanism.

The results of this study confirm the predictions of experience-basedtheories that postulate that the mirror system consists of links betweenneural populations coding for sensory and motor action representations,and that these are forged through correlated experience of seeing anddoing actions (Heyes 2001; Keysers and Perrett 2004; Brass and Heyes2005). These theories further suggest that the processes that create these linksare stimulus-general associative learning mechanisms (Catmur et al. 2008,p. 1213).

Thus, if observational input were sufficient to stimulate the aplasic mirror neuronsand to establish rudimentary mappings between disparate effector systems, then ahistory of interaction with morphologically typical conspecifics might well besufficient to develop the mirror mechanism to such an extent that it produces analtered body schema and even eventually generates phantom limb experiences. Thisis not to neglect the essential correspondence, that is, the “mirror” in the system.This correspondence between self and other cannot be established simply throughobservation. In that case, the motor system would play no part in the process of self–other mapping or in any consequent action understanding. It is necessary forobservation and action to be coupled through a history of interaction and this, wepropose, is the social component in the ontogeny of the body schema. By extension,this is also the level at which the body schema interacts with the mirror mechanismand, therefore, the level from which phantom experience arises.

Evidence in support of this view comes from an fMRI study by Gazzola et al. (2007)in which they examine aplasics’ mirror system responses to hand actions. Two aplasicand 16 typically developed participants were scanned while they watched video-recordings of hands manipulating objects while they themselves simultaneouslymanipulated objects using various effectors, including mouth, toes, and for typicallydeveloped subjects, hands. The aplasic participants activated regions associated withmouth and foot execution in response to observation of hand actions, that is, theymapped feet to hands, thus generating a goal-level correspondence between actionsperformed with different effectors. These results highlight once again the question ofhow the correspondence between observed and executed actions is realized: is theassociation between an observed action and an equivalent goal or is it based simply onthe corresponding means of achieving the goal?

Our stated position is that the association occurs at the level of goalequivalence. This notion of correspondence as founded on the goal rather thanthe means of an action should, however, not be taken to mean that neuronalactivation is “in itself” a “representation of” or indeed “thought” of a goal orintended outcome. The claim of goal rather than means equivalence is borneout by work carried out on the mirror systems of monkeys. The monkey mirrorsystem comprises “broadly congruent” (bcMN) and “strictly congruent” (scMN)mirror neurons where scMNs respond for observed actions with the same goal


and the same effector as the executed action and bcMNs also respond when theaction is performed using a different effector. Thus:

[T]he idea that the observation of an action also recruits motor programs of actionswith corresponding goals but differing means endows the observer with theflexibility of mapping the observed action onto the behavioural alternative that ismost suited under his present circumstances (Gazzola et al. 2007, p. 1239).

This flexibility also helps to account for the lack of any deficit in actionunderstanding that we might expect to see if the altered morphology of aplasiaprecluded the use of motor resonance-based mirror mechanisms for acts performed bytypically developed conspecifics. In fact, even more impressively, it helps to account forany systematic deficit in perspective taking or other social cognitive function.

Price (2006) argues for a developmental account of body image formation andaplasic phantom experience. Price does not distinguish body image from bodyschema but uses image to refer to the putative innate structure by which the body ismapped. In this paper and elsewhere, this would be referred to as the body schema.One of the fundamental claims Price makes is that, prior to birth, an in utero“primitive bilateral body image [schema]” is formed on the basis of spontaneousfetal movement. Post birth, the infant’s visual and sensory systems are broughttogether through the mirror neuron system which can also facilitate the incorporationof prostheses into the body schema, thus contributing to the development ofexperiential phantoms. “This hypothesis is capable of explaining all cases of aplasicphantoms without recourse to an independent genetically determined neuralrepresentation of self” [ibid., p. 320]. Price proposes that, to evaluate thedevelopmental account of aplasic phantom experience, it would be necessary toexclude opportunities for the individual to acquire visuomotor mappings for theabsent limbs and suggests studies of phantom limb experiences among congenitallyblind subjects as a test of the validity of the ontogenetic perspective.

Studies of phantom limb incidence amongst the congenitally blind could provide avaluable test of the validity of the hypothesis presented in this paper. Drawings andmodels of human figures made by blind children indicate that they hold a mentalrepresentation of human form in which body parts critical to their exploration ofthe world—such as hands and arms—are exaggerated in size (Critchley 1953;Kinsbourne & Lempert 1980). This phenomenon alone suggests that theirrepresentations of the body are built upon somatic sensory experience. Aphantom limb, or limbs, experienced by a congenitally blind subject withbilateral aplasia and no history of prosthesis usage would be the strongestpossible evidence for the existence of an autonomous hard-wired body image.On a lower explanatory level, unilateral aplasic phantom in a congenitally blindsubject would indicate that a process other than visual input is involved, be it dueto a bilateral representation of body image, or genetic hard-wiring [ibid.].

However, as both aplasia and congenital blindness are individually rare andtogether even rarer, it is unlikely that enough people with both conditions could befound to make this a viable experimental approach. It would also be unlikely, if theywere to occur together, that the individual would be of an age and ability to reportthe experience of phantoms before they were to have a prosthetic limb fitted.


Conclusions

In this examination of aplasic phantom limb experience, we have sought to offer analternative to the view that an innate body schema is necessary to account forphantoms of congenitally absent limbs. We have presented an account in which ahistory of embodied action (both pre-birth and post birth) plays a constructive role inthe ontogeny of the body schema. The developmental aspects of this account,particularly with respect to the role of embedding in a social world, fit well withinthe enactive approach (Varela et al. 1991; Thompson 2007). On this view, the aplasicphantom does not emerge as a vestigial fragment of a rudimentary, innate structurethat has failed to adapt to the lack of certain limbs, rather, and much more positively,the aplasic phantom can be viewed almost as a developmental acquisition, a socialconstruction which reflects the ontogenetic interaction between the body schema, asan element of the prereflective, basic self, and the role of the mirror neurons inmediating the bidirectional relationship of self and other.

Recent experimental evidence shows that action observation–execution mappingsare inherently plastic and that it is the goal rather than the means of attainment that iscentral to mirror mechanism activity (Catmur et al. 2008). In addition, aplasicsubjects have been shown to map observed hand actions to executed foot actions,demonstrating that effector mapping is flexible (Gazzola et al. 2007). Theseexperiments help to explain how it is that aplasic subjects show no deficit in actionunderstanding despite their inability to replicate the means of attainment of the goalin an observed action. By extension, they also provide some insight into evidence toshow neural mappings for congenitally absent limbs (Brugger et al. 2000).Previously, such evidence has been taken to indicate that the neural mapping ofbody parts, that is, the formation of the body schema, is innate, hard-wired, and atleast, at a rudimentary level developmentally isolated from the growth or failure togrow of limbs and other elements of morphological structure. A view which byextension entails that such neural structures must, therefore, not be experiencedependent; the fetus’ history of spontaneous movement in utero being irrelevant tothe neural structures mapping its body.

In contrast, here we have presented an account that places a strong emphasis on aconstructive continuity of experience between prenatal and postnatal life and whichendeavors to take account of the developmental trajectory traversed by the aplasicinfant. On this view, the fact that a neural mapping can be demonstrated when itstarget, a particular limb, is absent does not mean that we need to couch our questionsabout the neural system which underpin that mapping in terms of innateness versusacquisition. There are a number of other possibilities and it seems most useful tostart from a position that can take into account the ontogeny of a system, thus we canask questions such as, at what point in fetal development did the aplasia occur? Ifneural structures are in any way affected by afferent feedback from the target sites,then such inputs will be available from the earliest stages of embryogenesis and willcontinue until development is diverted in aplasia. This interpretation of an “altered”trajectory is, at least, one which can be contrasted with the view of aplasia as acomplete absence of development.

The claim that phantoms of congenitally absent limbs do not require explanationin terms of an innate body schema is complementary with the notion that the aplasic


phantom can be understood ontogenetically, as a fundamentally social acquisition.Thus, just as nonaplasic phantom limb experience might be understood as “calledforth” by the world of graspable, manipulable objects (Merleau-Ponty 1962), so theaplasic phantom could be enacted through the medium of intersubjectivity and thevisual and somatosensory experience of others’ actions in a world of graspable,manipulable objects. The experience of interacting with conspecifics, of joint actionoften featuring the coupling of hands and other effectors in the service of sharedgoals, could provide the stimulus for the aplasic individual’s incorporation of absentlimbs into her own body schema. Thus, the aplasic individual might be said toacquire a somatosensory mapping of self to other that is richer and more complexthan that of her typically developed counterpart. On this view, her body schemaexplicitly accounts for the other within the most basic structures of herself; hermirror system could provide the substrate for this flexible negotiation of dual modesof action. The aplasic phantom might, therefore, be seen to exemplify the notion of“differently abled” in its most positive sense, signifying the capacity to extend one’sown embodiment to take experiential account of the other.

Acknowledgements The authors would like to thank Vittorio Gallese, Alessandra Umilta and EzequielDi Paolo for helpful discussions on the themes of this paper. RW’s work on this paper was supported bythe EU Marie Curie - Research Training Network 035975 “DISCOS - Disorders and coherence of theembodied self”.

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Aplasic phantoms and the mirror neuron system: An enactive, developmental perspective