According to Pazzaglia andZantedeschi (2016), a human body representation is an internal portrayal of ourcorporal structure and allows for feelings of ownership over our limbs, withsuch knowledge being essential for any successful interactions.
The ordinaryfamiliarity of our own bodies can make it appear as though the bodyrepresentations we experience are largely infallible. Many normally functioningindividuals take this supposedly fixed experience of the body for granted, withdistortions and malleability often attributed to numerous neurological andpsychiatric conditions (Longo, 2017). Despite this, research has shown thatsome degree of fluidity in body representations of normally functioning individualsmay be an aspect of healthy cognition. This essay shall explore the extent towhich body representations can be manipulated in humans with healthy cognitionand those with apparent disorders of bodily perception, with some suggestingthat a small amount of flexibility in normal cognition is vital for socialinteraction, and others revealing that not all disorders which seemingly affectour bodily perceptions involve distortions in body representations. Ultimately,it will suggest that body representations have some degree of malleabilityacross individuals, however there are many external limitations and exacerbatingfactors that must be accounted for when determining this fluidity. An automatic feeling of ownershipover our limbs is a fundamental aspect of body representation (Ehrsson et al,2004).
Therefore, one of the most common depictions of fluidity in bodyrepresentations in healthy cognition is the Rubber Hand Illusion (RHI); anillusion of limb ownership during which the subject’s real hand is occluded anda rubber hand is placed in front of them, synchronous brush strokes are thenapplied to the real hand and rubber hand, thus creating the subjective sense thatthe rubber hand belongs to them demonstrated through altered proprioception andneural activity in the premotor cortex (Botnicik and Cohen, 1998). This perceptis seen to be the result of visual-tactile correlation; usually a viewed andfelt touch occurring simultaneously must be occurring at one bodily location soproprioception is pulled towards this (Graziano, 1999); when visual andproprioceptive cues are in conflict, the more reliable visual cues are often considered.This is theorised to be the result of bottom-up processing with the onlylimitation being the need for synchronous brush strokes (Armel andRamachandran, 2003); the percept is created without any cognitive input, thussuggesting that the illusion could potentially incorporate any object orinstance, demonstrating exceptional malleability of body representation in normallyfunctioning participants. Previous studies have shown that subjects were ableto perceive touch sensations arising from a table following repeatedsynchronous stimulations (Ramachandran et al, 1998), suggesting that bodyrepresentations are malleable enough to incorporate such objects into thisschema. Such misperception is said to be cognitively impenetrable; participantsfell for this illusion even when they knew they were viewing an experimenter’shand rather than their own (Welch, 1972). However, self-report measures ofintensity of ownership demonstrate that using a table for such an illusionelicits a reduced response that using a far more realistic rubber hand; thereare reported limitations so what we will accept as a part our body (Pavani et al,2000). This effect doesn’t consistently hold when pushing the limits of apre-existing bodily schema, suggesting some fixed body representation despitevisual-tactile coherence due to top-down constraints (Lloyd, 2007).
Accordingto a number of studies, even a rubber hand is only embodied throughproprioceptive drift when of suitable form and orientation (Haans et al, 2008;Tsakiris and Haggard, 2005); subjects were sensitive to inconsistencies inorientation when turning the hand by just 10° (Constantini and Haggard, 2007).Knowledge of the form of our hand is essential for accurate movement in space(Gandevia and Phegan, 1999), therefore the RHI only tends to occur alongsidesynchronous brushstrokes, when correctly aligned with the subject’s real hand,and when the form is largely accurate. This indicates that our bodyrepresentations must be fluid to some degree as we are able to incorporateexternal components into this schema, however, there are limits to the RHIwhich demonstrate some implicit knowledge and stability of representation.
Fixed representations may belargely available in adulthood, however the RHI has displayed developmentalprogression, perhaps indicating a certain degree of malleability early on inlife; body representations may become more fixed over time as we develop aconsistent view of our bodily limits. Children aged 18 – 30 months are morelikely to make scale errors when integrating body-size information into actions(deLoache et al, 2004); suggesting plasticity in representations due bodilychanges throughout childhood which must be incorporated into current schema.When conducting the RHI with children aged 4 – 9 years, Cowie, Tamar, andBremner (2013) found that felt position was captured more for the fake hand inchildren than adults; children’s’ post-illusion pointing drifted significantlyfurther towards the fake hand suggesting that vision of an appropriatelyoriented hand is a powerful cue to own-hand position in children. Thisindicates that children have access to multisensory bodily perception ininfancy, with sensitivity to visual-tactile cues causing them to respond to therubber hand as if it was their own, however there may be a late-maturingvisual-proprioceptive process which underlies self-localisation and reducesmalleability of body representations as you reach adulthood (Ehrsson et al,2012). Children are far more reliant on sight as appose to proprioception earlyin life for localisation of limbs (Graziano, 1999), allowing manipulation offelt hand position to be far easier in children, with previous studies showingthat this effect can be up to three times larger in children compared to adults(Cowie, Sterling, and Bremner, 2016). The optimal combination of multisensorycues appears to show protracted development throughout infancy and into latechildhood and adolescence (Bremner et al, 2013); at which point, the handtypically reaches adult size, so the outcome of the RHI might be expected tobecome less plastic and more stable (Bee, 2000). Pointing responses appear toreach adult levels at 10 – 11 years old, at which point children perceive handlocation using an adult-like balance of sensory cues (Cowie, Sterling, andBremner, 2016).
This suggests that body representations may be relatively fixedin adulthood, however plasticity is required in childhood allowing forincorporation of new information into our body schemas, demonstrating aspectsof fluidity throughout development. Malleability ofbody representations in adult healthy cognition has been shown to be relativelylimited, depending upon held knowledge of bodily capabilities, although it hasbeen suggested that large and systematic distortions of body representationsmay be a key aspect of healthy cognition (Longo, 2017). Tactile size anddistance perception of limbs has long been shown to demonstrate significantdistortions; when moving a compass across various areas of the body, thedistance between the points is perceived to change depending upon the region ofskin that it was moved across (Weber, 1978).
The size of stimuli moving acrosssensitive regions like the palm were viewed as larger than the same sizestimuli moving across the back of the hand. Further studies have suggested thatstimuli moved across the width of the hand are perceived as 40% larger thanthose running across the length (Longo and Haggard, 2011). This results fromsomatosensory maps which show highly disproportionate representation of skinregions depending upon their sensitivity, also known as cortical magnification;knowledge of our bodies is neutrally mediated and reflected through innatetopographic maps in the brain, demonstrating this unique organisation(Pazzaglia and Zantedeschi, 2016). This has been modelled through the PenfieldHomunculus (Penfield and Boldrey, 1937); we are adapted to have high tactilesensitivity on specific skin surfaces, allowing use to perform otherwiseimpossible dexterous behaviours. This has also been found to a larger degreewhen estimating body size through the Body Image Task (BIT), during whichparticipants had to judge the location of their limbs when given an image oftheir head as a landmark; healthy adults overestimated theirshoulder-width-to-height ratio by over 40% (Fuentes, Longo, and Haggard, 2013).Such distortions have also been shown through position sense; even with theabsence of vision, adults are relatively poor in localisation of body parts,seen to also be a result of these somatosensory maps. When participants wereasked to judge the locations of parts of their arm, this localisation was inaccurate,and these parts were perceived as closer together than the actual locations;suggesting a distorted representation of arm length (Gurfinkel and Levick,1991). Similar results have been found when judging location on the back of thehand; hand width is often overestimated, whilst finger length is underestimatedin a radial-gradient formation – increasing progressively from the thumb to thelittle finger (Longo and Haggard, 2012), however the magnitude of thesedistortions is reduced significantly when judgements are made about the palm ofthe hand.
To conclude, body representations may show some degree of fluidity,even in healthy cognition, as shown by these distortions of judgement; however,this also demonstrates that we have a fixed means of processing these inputsthrough the somatosensory cortex.Bodyrepresentations can also appear to be fluid in healthy cognition depending uponseveral external inputs which may manipulate how we view our body. In a studyby Gandevia and Phegan (1999), administration of anaesthesia to the thumbsignificantly increased its perceived size; perceived size increased by 60 – 70%. They proposed this to be the result of the acute removal of afferent inputfrom the thumb resulting in the enlargement of the receptive fields of thecortical cells representing skin areas surrounding this site, thus causing thebrain to accept these adjacent locations as associated with the thumb. Thishighlights the lability of body representations in healthy cognition, withanaesthesia even impacting our cortical representations in the somatosensorycortex, suggesting that are mode of processing these inputs may not be asstable as proposed.
Similar findings demonstrate the role of pain inmanipulations of body representations; when a cold-sensitive class of C fibres,producing a sensation of pain, were increased in input in the thumb, this alsocaused the thumb to feel larger (Calford and Tweedale, 1991). These fibreslimit the receptive fields of the somatosensory neurons at this site, producingfeelings of enlargement. However, such findings have been shown to be limitedto smaller body parts like fingers and lips, perhaps suggesting that ouroverall body size is relatively fixed despite external inputs. Another significantexternal input that should be accounted for is our social experiences, throughwhich distortions of body representations have been shown across much largerfields, often encapsulating the whole body. According to Mead (1934), largerdistortions of viewed body size may reflect a generalisation based upon bodyschema created through social interactions; we construct what we believe ourown body should look like based upon the observed bodies of others. Therefore,external inputs must be assessed when determining fluidity of bodyrepresentations, as it is suggested that they exacerbate manipulation of bodyimage on a large scale, even in healthy cognition.Bodyrepresentations have largely been studied in a clinical population due to theextreme malleability they present; despite pathologies not reflecting normalfunctioning, they still demonstrate how body representations can bemanipulated.
Key pathologies which appear to reflect vast discrepancies injudgement of body size are Eating Disorders, specifically Anorexia Nervosa;classified as disturbance in the way one’s body weight or shape is experiences(DSM-IV, 1994), often estimating body size as larger than objectively true.Body image dysfunction is integral to this syndrome, indicating some degree ofinability to accurately assess your own body size, often attributed tounrealistic social standards (Bruch, 1962); with social input important inassessing body representation, as discussed previously. Despite, severalstudies suggesting a poor ability in gauging your body size to be key inAnorexia, there is debate over the specific nature of disturbances and whetherthe main deficit within this disorder is this perceptual body size distortion,or whether it is more reliant upon cognitive evaluative dissatisfaction, basedlargely upon attitudinal body image (Cash and Deagle, 1997). Previous studieshave equated these variables and mostly assess perceptual distortion due tothis being deemed as a more accessible measure (Hsu and Sobekiewicz, 1991),however others suggest that distortions of body representation may not be asfundamental to Anorexia as previously thought (Cash and Brown, 1987) and thatbody-image disturbances should be abandoned as a diagnostic criterion forEating Disorders (Hsu, 1982). Research comparing perceptual and attitudinalmeasures of Anorexia in women demonstrated that metric estimations of bodysize, through measures requiring estimations of body size as particular sites(e.g.
Askevold, 1975), were significantly poorer than in a control group withmoderate effects of 0.61 – 0.64. However, attitudinal measures such as,adjustment of an image to convey body size preferences, combined withjudgements of actual body size to quantify the extent of discrepancy betweenself and ideal percept (e.
g. Cash and Szymanski, 1995), demonstratedsubstantially larger effect sizes (1.10 – 1.13), suggesting a greater relianceon social input forming exceptionally poor body attitudes in Anorexia, asappose to inaccuracy in body representations. Although, another extremedisorder which reflects large perceptual disturbances is Schizophrenia, withmany patients demonstrating lack of bodily awareness and the knowledge that, inadulthood, these representations are largely fixed (Ehrsson et al, 2004).
Previousfindings have shown that the RHI is intensified and has a rapid onset inSchizophrenia; demonstrating consistency with the notion of a weaker and moreflexible sense of self (Peled et al, 2003). Following this, the RHI wasconducted on a group of Schizophrenia patients using skin-conductanceresponses, proprioceptive drift, and temperature measurements to determine thestrength in these patients compared to normally functioning individuals(Thakkar et al, 2011). The RHI was consistently stronger in patients, with morevivid self-reported feelings of ownership, and with proprioceptive drift beingthree times in the synchronous condition larger than the healthy controls.Increased vividness was also associated with schizotypty symptoms in healthyparticipants; suggesting that malleability in body representations has anassociation with psychosis. Skin cooling in the stimulated hand was alsopresent following this illusion, relating to a sense of disownership of thestimulated hand (Moseley et al, 2008). Schizophrenia demonstrates extrememalleability in body representations, largely associated with psychoticsymptoms, with some patients experiencing the illusion even before tactilestimulation began. To conclude, disorders appearing to impact body representationmay not always involve perceptual discrepancies, however certain pathologiesdemonstrate huge flexibility in these representations, suggesting that theyaren’t fixed across the population.Anotherpathology which has been relatively ignored regarding stability of bodyrepresentations is Autism Spectrum Disorder (ASD), despite the idea that theseindividuals demonstrate poor empathy and imitation, both of which have beenlinked to malleability of body representations (Cascio et al, 2012).
When carrying out the RHI among children withASD, in comparison to typically developing (TD) children, it was shown thatthose with ASD took longer to fall for this illusion, with a more fixedrepresentation than normally-functioning adults, whilst TD children have previouslybeen shown to be much more malleable in this respect (Haswell et al, 2009). Itwas theorised that children with ASD depend more heavily on proprioceptive thanvisual input when the two are incongruent, largely dissimilar to TD childrenwho are far more efficient at combing visual-tactile cues. This perhapsdemonstrates malleability of body representation being compromised in thosewith ASD, analogous with a modified cortical representation of the bodily self;giving rise to a diminished capability for perspective-taking and empathy(Lombardo et al, 2009). Falling for the RHI and this slight fluidity in bodyrepresentation has been related to the ability to empathise with others, thisbeing a complex social process requiring the integration of multiple affectivecomponents (Decety, 2010). This suggests that, perhaps some small degree offluidity in body representations is important in normally-functioningindividuals, and completely fixed body representations are indicative of socialdisorders such as ASD.Overall,research suggests that some level of plasticity in body representations isnormal, even in healthy cognition; with extreme stability sometimes indicatingdisordered cognition, suggesting that body representations aren’t completelyfixed. This flexibility can be exacerbated alongside familiar external factors;like experiencing pain and social comparison, and is especially pronounced throughoutearly to mid-childhood. Large distortions in body representations are asignificant aspect of certain psychopathologies, especially Schizophrenia,however Anorexia which is also said to be a disorder of body representation, isbased on attitudinal dysfunction as appose to perceptual deficits.
Despite cleardemonstrations of lability, it needs to be understood that there are limitationsto this, especially in healthy cognition, which restrict any extrememalleability. These involve held knowledge of bodily capabilities and the fixedareas of the brain in which sensory inputs are processed; the somatosensorycortex, despite demonstrating distortions of body representation, is arelatively fixed means of processing these inputs and provides limitations toflexibility.