Sec2Sec3Sec4Sec5Fig1Sec6Fig2AckBibCR1CR2CR3CR4CR5CR6CR7CR8CR9CR10CR11CR12CR13CR14CR15CR16CR17CR18CR19CR20CR21CR22CR23CR24CR25CR26CR27CR28CR29CR30CR31CR32CR33CR36CR34CR35RESEARCH ARTICLEMarion Funk Æ Maggie Shiffrar Æ Peter BruggerHand movement observation by individuals born without hands:phantom limb experience constrains visual limb perceptionReceived: 14 August 2004 / Accepted: 4 December 2004 / Published online: 8 April 2005Ó Springer-Verlag 2005Abstract Increasing evidence suggests that the visualanalysis of other people’s actions depends upon theobserver’s own body representation or schema. Thisraises the question of how differences in observers’ bodystructure and schema impact their perception of humanmovement. We investigated the visual experiences of twopersons born without arms, one with and the otherwithout phantom sensations. These participants, plus sixnormally-limbed control observers, viewed depictions ofupper limb movement under conditions of apparentmotion. Consistent with previous results (Shiffrar M,Freyd JJ (1990) Psychol Sci 1:257), normally-limbedobservers perceived rate-dependent paths of apparenthuman movement. Specifically, biologically impossiblemotion trajectories were reported at rapid display rateswhile biologically possible trajectories were reported atslow display rates. The aplasic individual with phantomexperiences showed the same perceptual pattern ascontrol participants, while the aplasic individual withoutphantom sensations did not. These preliminary resultssuggest that phantom experiences may constrain thevisual analysis of the human body. These results furthersuggest that it may be time to move beyond the questionof whether aplasic phantoms exist and instead focus onthe question of why some people with limb aplasiaexperience phantom sensations whil e others do not. Inthis light, the current results suggest that somestheticrepresentations are not sufficient to define body schema.Instead, neural systems matching action observation,action execution and motor imagery likely contribute tothe definition of bod y schema in profound ways. Addi-tional research with aplasic individuals, having andlacking phantom sensations, is needed to resolve thisissue.Keywords Body schema Æ Action observation Æ Limbaplasia Æ Congenital phantom limbs Æ Crossm odalprocessingIntroductionStudies of phantom limbs after amputation have pro-vided revolutionary insights into the neural plasticity ofthe human brain (Ramachandran and Hirstein 1998;Halligan 2002 for reviews). These studies have also re-vealed that the ‘‘body in the brain’’ (Be rlucchi andAglioti 1997) or ‘‘body schema’’ is a highly flexiblecentral representation of the human body. More gener-ally, research with amputees has reminded us that ‘‘[w]estand to learn most from phantoms if we attend closelyto patients’ subjective reports’’ (Halligan et al. 1999, p.587). Indeed, the quantity and quality of cortical reor-ganization can be critically related to detailed charac-teristics of an individual’s phant om limb experience(Flor et al. 1998; Knecht et al. 1998).In striking contrast to the rapid pace with which ourunderstanding of the perceptual and neural correlates oflimb amputation unfolds stands the near absence ofexperimental investigations of phantom limb phenom-ena in persons with limb aplasia. This may be due to thefact that many authors still doubt the very existence of‘‘congenital phantoms’’ (Skoyles 1990; Flor et al. 1998) .Indeed, the implicit assumption that phantom sensationsof congenitally absent limbs cannot exist has occasion-ally resulted in an embarrassing failure to ask aplasi csubjects about such sensations (Nico et al. 2004).Nonetheless, in the clinical literature, phantoms ofcongenitally absent limbs have been documented forwell over a century (V alentin 1836; Poeck 1964; Bur-chard 1965; Grouios 1996). Relevant overview articlesindicate that approximately 10% (Boonstra et al. 2000)M. Funk Æ P. Brugger (&)Department of Neurology, University Hospital Zu¨rich,8091 Zu¨rich, SwitzerlandE-mail: [email protected]. ShiffrarDepartment of Psychology, Rutgers University,Newark, NJ 07102, USAExp Brain Res (2005) 164: 341–346DOI 10.1007/s00221-005-2255-4to 20% (Weinstein et al. 1964; Melzack et al. 1997)ofindividuals born without limbs experience phantoms oftheir missing limbs. Apart from the high incidenceof pain in post-amputation phantoms, which contrastswith a virtual absence of painful congenital phantoms(Melzack et al. 1997), the phenomenologies of the twotypes of phantoms are comparable. In both cases,postural and movement sensations predominate (Poeck1969; Melzack et al. 1997) while thermal sensations arerare (Lacroix et al. 1992). Importantly, visual inspectionof other peoples’ bodies can trigger phantom sensationsin amputees (Henderson and Smyth 1948) and peoplewith limb aplasia (Melzack et al. 1997). For example,Melzack and colleagues (1997) reported the case of a 14-year-old boy with a missing right forearm and hand,whose phantom hand percept could be elicited by‘‘playing with his friends and looking at their arms’’ (p.1610).These cross-modal interactions between the visualobservation of other people’s bodies and the ob-server’s own body schema form the topic of thepresent report. Some authors (see especially Melzack1990) have taken the very existence of phantom sen-sations of limbs that have never physically developedas unshakable evidence for innate components of bodyschema. The ultimate utility of such conclusions maydepend upon how one defines the term ‘‘body sche-ma’’. Growing evidence from behavioral (Reed andFarah 1995; Sebanz et al. 200 3) and neuroimaging(Gre`zes and Decety 2001) studies suggests that repre-sentations of one’s own bodily actions share a com-mon neural substrate with visual representations of theactions performed by other people. Such findingssuggest that the ‘‘body schema’’ may be best under-stood as a multimodal representation of one’s ownbody that contains input from somatosensory, pro-prioceptive, and vestibular system s as well as visualinformation about human body dynamics.Here we investigate the perceptual, specifically visual,experiences of two people with bilateral congenital ab-sence of arms duri ng the presentation of other peoples’upper limb movements. The rareness of bila teral armaplasia, especially when accomp anied with phantomsensations, precluded