Synesthesia, Mirror Neurons, and Mirror-Touch
Abstract and Keywords
Mirror-touch synaesthesia describes a condition in which individuals experience the sensation of being touched on their own body when observing touch to another person. This chapter reviews studies examining the prevalence and characteristics of developmental and acquired cases of mirror-touch synaesthesia, the neurocognitive mechanisms that contribute to the tactile experiences evoked in mirror-touch, and how mirror-touch synaesthesia may be used to help inform us about mechanisms of social perception in non-synaesthetes.
As noted elsewhere in this volume, synesthesia is a condition where a stimulus in one attribute (the inducer) triggers a conscious experience of another attribute (the concurrent) not typically associated with the inducer. For example, in grapheme-color synesthesia the letter “A” (or “B” or “C” and so on) may trigger synesthetic experiences of colors (Cohen Kadosh and Henik 2007; Rich and Mattingley 2002). A large body of synesthesia research has focused on synesthesia involving color, which is often reported as being the most common concurrent of the condition (Baron-Cohen et al. 1996; Rich, Bradshaw, and Mattingley 2005; Simner et al. 2006). More recently, however, a newly documented form of synesthesia has been described in which individuals experience tactile sensations on their own body when simply observing touch to another’s body (Banissy and Ward 2007; Blakemore et al. 2005; Holle et al. 2011). This variant of synesthesia, known as mirror-touch synesthesia, is the focus of the present chapter and here I will describe the prevalence and characteristics of developmental mirror-touch synesthesia, the neurocognitive mechanisms that contribute to this variant, and the extent to which we can use this interpersonal variant of synesthesia to inform us about mechanisms of social perception more generally. I shall also discuss cases in which mirror-touch synesthesia has been reported to be acquired after injury, and I will consider them in relation to acquired synesthesia more generally. (p. 585)
Developmental Mirror-Touch Synesthesia: Prevalence and Characteristics
Synesthesia has been considered by some as having three defining characteristics: (1) concurrents are conscious perceptual or percept-like experiences; (2) experiences are induced by an attribute not typically associated with that conscious experience; (3) these experiences occur automatically (Ward and Mattingley 2006). This section describes evidence indicating how mirror-touch synesthesia matches onto these three criteria and also considers wider characteristics of this variant of synesthetic experience.
With regard to the automaticity of developmental mirror-touch synesthesia, the question is whether mirror-touch synesthetes experience their synesthetic touch sensations immediately/automatically, or whether they require some type of conscious effort (of a kind not normally associated with “synesthesia” proper). Banissy and Ward (2007) reported a behavioral study of ten developmental mirror-touch synesthetes in which they explored this aspect of mirror-touch synesthesia by developing a “visuo-tactile synesthetic Stroop experiment.” In the task, synesthetes and matched non-synesthetic controls were asked to detect a site touched on their own facial cheeks while observing touch to another person’s facial cheeks or to a corresponding object. Participants were asked to report the site of actual touch (left side of the body, right side of the body, or no touch at all) and to ignore observed touch (which was either on the left, right, or both sides). For synesthetes, but not controls, the synesthetic experience evoked by observed touch could either be congruent or incongruent with the site of actual touch.1 For example, if observing touch to the left cheek evokes synesthetic touch on the right cheek then actual touch to the right cheek would be congruent with their synesthesia, but actual touch to the left cheek would be incongruent with their synesthesia (Figure 30.1). Synesthetes, but not control participants, were faster at detecting the location of actual touch during the congruent condition relative to the incongruent condition. Further, synesthetes produced a higher percentage of errors consistent with their synesthesia (e.g., reporting touch on trials involving no actual touch; hereafter referred to as “mirror-touch errors”). No significant differences were found when the observed stimuli (i.e., observing touch to another person’s cheek) were replaced with a stimulus that did not evoke synesthetic experiences of touch (e.g., a flash of light on the cheek rather than a touch; Banissy et al. 2009). In other words, synesthetes had previously reported that observing a flash of light against another person’s body did not cause touch sensations against their own body, and as a consequence, there were no “mirror-touch errors” found under these conditions. The (p. 586) findings therefore provided evidence supporting the suggestion that mirror-touch synesthetes experience touch on their own body when observing bodily touch and imply that the synesthetic experience in mirror-touch synesthesia is percept-like.
Using the aforementioned “visuo-tactile congruity paradigm” as a measure for the authenticity of mirror-touch synesthesia, my colleagues and I (Banissy et al. 2009) examined the prevalence of developmental mirror-touch synesthesia among university undergraduates at University College London and the University of Sussex. Five hundred and sixty-seven participants were recruited for the study. All were undergraduate students recruited before/after classes being held at each university. Participants were given a description of synesthesia (including examples of what did and did not constitute synesthesia) and were then administered a questionnaire asking about different variants of synesthesia. They were not told that the study was a prevalence study, nor did they receive specific details about mirror-touch synesthesia (i.e., they received a general description of synesthesia rather than specific details about individual subtypes). One question on the questionnaire related to mirror-touch synesthesia, in which participants were asked to indicate the extent to which they agreed with the question “Do you experience touch sensations on your own body when you see them on another person’s body?” All participants who gave positive responses to this question (approximately 10.8% of all (p. 587) subjects) were contacted and interviewed about their experiences. These participants were presented with a series of videos that showed another person, object, or cartoon face being touched, and asked to indicate the location (if any) in which they experienced a tactile stimulus themselves and the type of experience. Typical responses of potential mirror-touch synesthetes (approximately 2.5% of all subjects) included reports that observing touch elicits a tingling somatic sensation in the corresponding location on their own body, and that a more intense and qualitatively different sensation is felt for painful stimuli (e.g., videos of a pin pricking a hand rather than observed touch to the hand). Of these 14 subjects, nine showed a significantly different pattern of performance compared to matched non-synesthetic subjects on the visuo-tactile synesthetic Stroop experiment developed by Banissy and Ward (2007), indicating a prevalence rate of 1.6%. In comparison to previous prevalence estimates of other types of synesthesia this places mirror-touch synesthesia as one of the more common forms of synesthesia, along with grapheme-color synesthesia (1.4% prevalence) and day-color synesthesia (2.8% prevalence; Simner et al. 2006).
A study examining the perceptual characteristics of the inducer in mirror-touch synesthesia indicates a number of common factors that mediate the synesthetic experience in mirror-touch (Holle et al. 2011). In that study, a group of 14 previously verified mirror-touch synesthetes were presented with a series of film clips and asked to report the presence/absence of synesthetic experience, the location of experience, and the intensity of the experience. Films included painful stimulation (e.g., a person being prodded by a knife), thermal stimulation (e.g., a person being touched by a candle), and tactile stimulation (e.g., a person being touched by another person). The target of the stimulation (i.e., the person/thing touched) also differed insofar as whether we showed touch to another person, touch to an object, touch to a dummy body part, or touch shown only within a static photograph. The findings showed that observed touch to another person in a video evoked a significantly more intense synesthetic experience than observing similar touch in static photographs, and was also more intense than observing touch to either dummy body parts or objects in videos. This implies that visual recognition of bodies alone (in the case of dummy body parts) is not driving mirror-touch synesthesia. The intensity of synesthetic sensations did not differ by the body part observed (face, hand, arm, leg, etc.) or by the spatial orientation of the body part when observing touch to a real person (e.g., whether the hands of another person appeared as crossed or uncrossed when touched). Painful stimuli to a real face (e.g., a sharp object prodding the face) did, however, evoke a stronger experience than a non-painful tactile stimulus to the face (e.g., a feather stroking the face).
In addition to commonalities, some important individual differences have also been found across mirror-touch synesthetes (Banissy and Ward 2007; Banissy et al. 2009; White and Aimola Davies 2012). It appears that mirror-touch synesthetes can be divided into at least two subgroups based upon the spatial mapping between observed and felt (synesthetic) touch (Figure 30.2). Some synesthetes report that an observed touch to the left cheek is felt on their right cheek (as if the observed person is a mirror reflection of oneself—and this type of experience is hereafter referred to as the “specular” (p. 588) subtype), whereas others report synesthetic touch on their left cheek when observing touch to another person’s left cheek (as if self and other share the same anatomical body space—hereafter referred to as the “anatomical” subtype). Our own studies indicate that the specular subtype is the more common, with approximately 80% of cases studied to date reporting a specular spatial mapping (see Banissy et al. 2009). This bias towards a specular mapping in synesthetes is consistent with studies on imitation behavior indicating that both adults and children tend to imitate in a specular mode (Franz, Ford, and Werner 2007; Schofield 1976). The possibility that there may be at least two spatial frames of reference that could be adopted when observing another’s tactile experiences is also consistent with neurophysiological findings in primates documenting anatomical and specular spatial frames of reference that mediate bimodal visual-tactile cells in the macaque parietal cortex. These cells respond when the monkey is touched and when the monkey observes touch to the same body part of someone else (Ishida et al. 2009).
In addition to differences in the spatial frame of reference adopted by mirror-touch synesthetes another intriguing characteristic shown by some but not all mirror-touch synesthetes, is the extent to which observing touch to objects can elicit synesthetic interactions. This type of experience has been reported as a consistent experience in approximately 18% of cases studied by our group (e.g., Banissy et al. 2009). For some of these “object-touch” synesthetes, this experience is reported to occur in the synesthete’s fingertip that corresponds to the finger observed touching the objects, but for others, synesthetic touch is linked onto particular body locations that are thought to correspond to the object being touched (e.g., when looking directly at a monitor being touched by (p. 589) another, the synesthetic touch experience maps onto the face, but when standing in front of the monitor the experience maps onto the trunk).
One further feature where developmental mirror-touch synesthesia shares characteristics with more commonly studied variants of synesthesia is how consistent the sensations are over time. In grapheme-color synesthesia, for example, if “A” is red at time 1 then it will be at time 2, several weeks, months, years, or even decades later (Baron-Cohen, Wyke, and Binnie 1987; Simner and Logie 2007). The experiences of mirror-touch synesthetes are also enduring, and an individual’s spatial subtype (i.e., whether they belong to the specular or anatomical category) is consistent both across time (Holle et al. 2011) and across different body parts (Banissy and Ward 2007). There are also additional characteristics that appear common to mirror-touch synesthesia and other variants of the condition. Mirror-touch synesthetes have been found to show an increased tactile sensitivity (Banissy, Walsh, and Ward 2009), which is in line with evidence of heightened perceptual processing of the synesthetic concurrent in other variants of synesthesia (e.g., increased color responsiveness in synesthetes who experience color (Yaro and Ward 2007) and increased color and tactile responsiveness in synesthetes who experience both touch and color (Banissy, Walsh, and Ward 2009)). It is also common for mirror-touch synesthetes to report an additional type of synesthesia (e.g., Banissy, Walsh, and Ward 2009; Banissy et al. 2009) and a similar trend is found in other types of the condition (Simner et al. 2006).
One aspect in which developmental mirror-touch may be considered to differ slightly from other more commonly studied variants of synesthesia is that the mappings in mirror-touch synesthesia appear to be non-arbitrary, insofar as somatotopy is typically preserved between observed and felt touch (e.g., observing touch to face will normally trigger an experience on the synesthetes’ face). Indeed, when mirror-touch synesthesia was first documented there was some resistance (and arguably still is) to the notion that it was a variant of synesthesia because the experience was simply “too literal” to be synesthesia. On closer inspection this may not be such an apparent difference. While it was once believed that synesthetic experiences were consistent arbitrary associations, this view is no longer widely held and there is growing evidence of non-arbitrary associations in other variants of synesthesia: for example, between pitch and lightness in tone-color synesthesia (Ward, Huckstep, and Tsakanikos 2006); number and lightness in number-color synesthesia (Cohen Kadosh, Henik, and Walsh 2007); grapheme frequencies and color in grapheme-color synesthesia (Simner et al. 2005); and phonology and tastes in lexical gustatory synesthesia (Ward and Simner 2003). Direct links have also been reported: for example, in lexical-gustatory synesthesia food-words often taste of the denoted food (e.g., the word “sausage” tends to taste of sausage; Ward, Simner, and Auyeung 2005) and color words sometimes map onto the same colors in linguistic-color synesthesia (e.g., the word “red” is colored red; Gray et al. 2002; Rich, Bradhaw, and Mattingley 2005). In this regard, there is a growing consensus to view mirror-touch synesthesia being part of the “synesthesia family,” as opposed to an unusual experience that is more common in and shares phenomenological similarities with synesthesia (e.g., mitempfindung; Burrack, Knoch, and Brugger 2006).
(p. 590) In sum, mirror-touch synesthesia describes an automatic percept-like tactile experience when simply observing touch to another person (or possibly to an object). Despite only recently being systematically investigated, it appears to be surprisingly common and shares some similarities with other more commonly studied variants of synesthesia (e.g., in terms of consistency over time and in relation to extended perceptual characteristics). There are, however, some important individual differences between mirror-touch synesthetes, including the frame of reference adopted when perceiving touch to another person (specular/anatomical distinction). These will be considered below in relation to potential neurocognitive mechanisms that may mediate the synesthetic experience in mirror-touch synesthesia.
Developmental Mirror-Touch Synesthesia: Neurocognitive Mechanisms
In addition to describing the perceptual characteristics of mirror-touch synesthesia, another important question is what mechanisms evoke synesthetic experiences of touch in this variant of synesthesia. Several biasing principles have been suggested as mechanisms that mediate what forms of synesthesia will or will not be developed (e.g., Bargary and Mitchell 2008; Cohen Kadosh and Walsh 2008; Eagleman 2009; Hubbard and Ramachandran 2005; Ramachandran and Hubbard 2001; Sagiv and Ward 2006; Smilek et al. 2001). One common biasing principle that has been associated with accounts of synesthesia is the role of adjacency between neighboring brain regions (e.g., between adjacent visual grapheme and color processing areas in grapheme-color synesthesia; Ramachandran and Hubbard 2001). The principle of adjacency is less clear in developmental mirror-touch synesthesia because there are no apparent neighboring brain areas that may mediate visuo-tactile experiences. An alternative biasing principle that may be more relevant is the “normal” architecture for multisensory interactions (Sagiv and Ward 2006). Moreover, there is now good evidence for a network of brain regions that are recruited by non-synesthetes when observing touch to others (Blakemore et al. 2005; Ebisch et al. 2008; Keysers et al. 2004; McCabe et al. 2008) and mirror-touch synesthesia may reflect hyperactivity within this network (Blakemore et al. 2005). Here, this possibility is discussed and additional neurocognitive mechanisms that may mediate individual differences between mirror-touch synesthetes are described.
The observed touch network is comprised of the primary and secondary somatosensory cortices, premotor cortex, intraparietal sulcus, and the superior temporal sulcus (Blakemore et al. 2005; Ebisch et al. 2008; Keysers et al. 2004; McCabe et al. 2008). The overlap between brain areas that are involved in passively experiencing touch to oneself and observing touch to another person (i.e., primary somatosensory and secondary somatosensory cortices) has been interpreted as evidence of a mirror-touch system in which observed touch is matched to the observer’s own sensorimotor representation (p. 591) of touch. This interpretation builds upon the findings of mirror neurons within the monkey premotor cortex and inferior parietal lobule (Gallese et al. 1996; Rizzolatti and Craighero 2004), which respond both when a monkey performs an action and when the monkey watches another person perform a similar action. In humans, indirect evidence of brain areas with similar mirroring properties has been found for action (Buccino et al. 2001), pain (Avenani et al. 2005; Singer et al. 2004), disgust (Wicker et al. 2003) and other emotions (Carr et al. 2003; Warren et al. 2006). Therefore, the overlap between the brain areas that become active when observing touch and experiencing touch are consistent with the notion of a mirror system for touch in the human brain.
Blakemore et al. (2005) examined the role of the mirror-touch system in non-synesthetes and a single mirror-touch synesthete named “C.” C reports experiencing touch on her own body when observing another person being touched, but not when observing inanimate objects being touched. Her experiences mirror observed touch to another person, such that observing touch to another person’s left facial cheek leads to a sensation of touch on her own right facial cheek (i.e., she adopts a specular frame of reference). Using functional magnetic resonance imaging Blakemore and colleagues investigated the neural systems underlying C’s synesthetic experience by contrasting brain activity when watching videos of humans relative to objects being touched (the latter did not elicit synesthesia) in the synesthete and in 12 non-synesthetic control subjects. As expected, non-synesthetes activated a network of brain regions during the observation of touch to a human relative to an object (including primary and secondary somatosensory cortex, premotor regions, and the superior temporal sulcus). Similar brain regions were also activated during actual touch, indicating that observing touch to another person activates a similar neural circuit as actual tactile experience—the mirror-touch system. A comparison between synesthete C and non-synesthetic subjects indicated that the synesthete showed hyperactivity within a number of regions within this network (including primary somatosensory cortex) and additional activity in the anterior insula. This suggests that mirror-touch synesthesia is a consequence of increased neural activity in the same mirror-touch network that is evoked in non-synesthetic controls when observing touch to another person (Blakemore et al. 2005) and therefore may be mediated by the “normal” architecture for multisensory interactions.
The fact that additional bilateral anterior insula activation was observed in C, but not in non-synesthetes when observing touch is also of interest. Neural activity in the anterior insula has been related to self-awareness (Critchley et al. 2004) and processing one’s awareness of others (Craig 2004; Lamm and Singer 2010). It is therefore thought to be involved in self-other distinctions (Fink et al. 1996; Kircher et al. 2001; Ruby and Decety 2001) and one possibility is that the additional activation of the insula in mirror-touch synesthesia reflects an error in the neural systems distinguishing between self and other, leading to the source of another person’s tactile experience being mislocated onto the synesthete’s own body (Banissy, Walsh, and Muggleton 2011). Moreover, one possibility is that mirror-touch synesthesia is linked to alterations in perceived body space (i.e., the boundaries of perceived body space between self and other may be more expansive in mirror-touch synesthesia; Aimola Davies and White 2013; (p. 592) Banissy, Walsh, and Muggleton 2011; Banissy et al. 2009) and abnormal activity in the anterior insula is one candidate brain region that may mediate this process (see Banissy and Ward 2013).
In addition to general differences in mechanisms of self-other distinction (i.e., those that distinguish mirror-touch synesthetes from non-synesthetes), it is also likely that there are a number of factors mediating individual differences (e.g., specular/anatomical distinction) between mirror-touch synesthetes. While this has not yet been studied systematically at a neural level, my colleagues and I provided a neurocognitive model to account for these differences and suggested three key mechanisms that are important to mirror-touch synesthesia: (1) identifying the type of visual stimulus touched (“What” mechanism), (2) discriminating between self and other (“Who” mechanism), and (3) locating where on the body and in space observed touch occurs (“Where” mechanism) (Banissy et al. 2009).
The “What” mechanism is considered to be involved in several discriminations related to the nature of the inducer (e.g., “Is this a human or object?”). As noted earlier, one intriguing characteristic shown by some mirror-touch synesthetes is that observing touch to objects can elicit synesthetic interactions (e.g., Banissy and Ward 2007). One brain region of the observed-touch network (Blakemore et al. 2005) that may be central to this is the intraparietal sulcus (IPS). Recent findings indicate that visual object information is processed along the dorsal stream to areas along the medial bank of the intraparietal sulcus (IPS; including IPS1 and IPS2; Konen and Kaster 2008). For mirror-touch synesthetes, this pathway may be particularly important when considered in relationship to visual-tactile body maps within the intraparietal cortex. Single-cell recording in primates has identified bimodal neurons in the intraparietal cortex which fire in response to not only passive somatosensory stimulation, but also to a visual stimulus presented in close proximity to the touched body part (Duhamel, Colby, and Goldberg 1998). Intriguingly, the visual spatial reference frames of such bimodal neurons are dynamic and if the monkey is trained to use a tool the visual receptive field extends to incorporate the tool into the representation of the body (Iriki, Tanaka, and Iwamura 1996). Similar evidence of dynamic multisensory body representations in the parietal cortex has been reported in human subjects (Berlucchi and Aglioti 1997; Bremmer et al. 2001; Colby 1998; Maravita and Iriki 2002). Therefore, one possibility is that the degree to which observing touch to an object is able to elicit visual-tactile synesthetic interactions depends upon the extent to which the object is incorporated into visual-tactile representations of the body, potentially within the intraparietal cortex.
The key process instigated by the “Who” mechanism is to distinguish between the self and other. It has been suggested that mirror-touch synesthesia may reflect a breakdown in the mechanisms that normally distinguish between self and other, leading to altered boundaries of perceived body space and misrepresentations of another’s body onto the synesthete’s own body schema (e.g., Banissy, Walsh, and Muggleton 2011; Banissy et al. 2009; Aimola Davies and White, 2013). Some factors mediating this may include the perspective of the viewed body part and the similarity between the observers and observed. In relation to the later, if similarity is important in mediating activity within the mirror-touch system then one may predict that non-synesthetes should show (p. 593) some level of modulation when observing touch to themselves versus other people. In accordance with this, Serino, Pizzoferrato, and Làdavas (2008) report that, for non-synesthetes, there is an enhancement in tactile sensitivity when observing touch, which is maximized when observing touch to one’s own face rather than another’s face.
The final class of mechanism that also seems important in mediating individual differences between mirror-touch synesthetes involves linking visual representations of body with tactile representations based on spatial frames of reference (Banissy et al. 2009). One way to consider the differences in the spatial frame adopted by mirror-touch synesthetes is through the notion of embodied and disembodied representations of perspective taking (see Brugger 2002; Giummarra et al. 2008). Specular mirror-touch synesthetes appear to process the visual representation of the other body as if looking at their own reflection (i.e., in an embodied manner to oneself), while for the anatomical subtype the spatial mapping between self and other could be considered to be disembodied because the synesthete’s own body appears to share the same bodily template as the others person (i.e., the synesthete is rotating their body into the perspective of the other person). This distinction may then be borne out at the neural level. For example, disembodied experiences have been suggested to arise from functional disintegration of low-level multisensory processing mechanisms (Blanke and Mohr 2005; Bünning and Blanke 2005) and abnormal activity at the temporal parietal junction (TPJ; Arzy et al. 2006; Blanke et al. 2002; Blanke et al. 2004), therefore one may suggest the anatomical, but not specular, subtype will be associated with these neurocognitive mechanisms.
Acquired Mirror-Touch/Mirror-Pain Synesthesia
So far the focus of this chapter has been on developmental cases of mirror-touch synesthesia. Recently, however, a number of studies have begun to describe cases of mirror-touch/mirror-pain synesthesia that have been acquired following sensory loss or brain injury. In this section, these studies are reviewed and considered in relationship to other acquired variants of synesthesia.
The first reported case of an acquired interpersonal synesthesia was related to observed pain rather than observed touch. This anecdotal report, given to clinicians posthumously by the patient’s wife, describes a man who experienced observed pain to others as actual pain on his own body (Bradshaw and Mattingley 2001). The patient was known to have suffered widespread cancer, but as this case was reported post-mortem no information about the functional neural circuitry involved was available. As alluded to earlier, more recently, evidence for the interpersonal sharing of observed pain has been provided (Avenanti et al. 2005; Morrison et al. 2004; Singer et al. 2004). For example, observing pain to another person results in neural activity in similar brain regions (p. 594) as when we experience pain ourselves (Morrison et al. 2004; Singer et al. 2004) and leads to modulation of corticospinal motor representations in a somatotopic manner (e.g., observing pain to the first dorsal interosseous (FDI) muscle modulates the observer’s own FDI muscle activity; Avenanti et al. 2005). These findings provide evidence for a mirror-pain resonance system in all people, in which observed pain is matched to the observer’s own sensorimotor representation of pain, and may be important in both acquired and developmental cases of mirror-pain synesthesia (Fitzgibbon, Giummarra, et al. 2010).
In addition to the discussed case, other cases of acquired mirror-pain and/or mirror-touch synesthesia have been reported. These cases tend to be related to sensory loss following limb amputation. For example, Ramachandran and Brang (2009) report cases of acquired mirror-touch synesthesia following arm amputation. In that study, four patients with upper limb amputations who reported phantom limb sensations, and healthy controls, were shown videos of another person’s hand being touched. Patients reported a consistent tactile experience in their phantom hand when simply observing touch to the intact hand of another person.
In fact, it appears that there might be a particularly high prevalence of mirror-touch/mirror-pain synesthesia in individuals that experience phantom limb sensations following amputation. For example, Fitzgibbon and colleagues (Fitzgibbon, Enticott, et al. 2010) report that 16.2% of self-referred amputees (n = 12 out of 74 amputees) experience sensations of phantom pain (i.e., pain in their phantom limb) when observing pain to others. Furthermore, Goller and colleagues (2013) report that almost a third of amputees (n = 8 out of 28 amputees) report tactile sensations on their phantom limb or stump when observing touch to another person. While these findings are based only on self-reports, and therefore may reflect some false positives, they are high levels when compared to the 10.8% of healthy adults who self-report developmental mirror-touch synesthesia (Banissy et al. 2009).
These acquired cases are in line with reports of developmental mirror-touch synesthesia, but there are some differences. In the case of amputees who report acquired mirror-touch, the most notable difference is the location of synesthetic experience. In developmental mirror-touch synesthesia, somatotopy is typically preserved such that observing touch to another person’s face will evoke a synesthetic sensation on the synesthete’s own face. In phantom limb amputees, synesthetic sensations tend to be evoked on the phantom limb or stump, irrespective of the body part seen (i.e., synesthetic experiences gravitate towards the stump; Goller et al. 2013; Figure 30.3). This difference is consistent with other variants of acquired synesthesia insofar as it is quite common for synesthesia following sensory loss to occur close to the location where stimulation is removed (e.g., Ro et al. 2007). The difference may also be informative about the neural mechanisms that are contributing to mirror-touch/pain in amputees. One possibility is that these experiences reflect a removal of neural signals from the amputated limb that would normally inhibit activity within the mirror-touch system in order to prevent observed touch/pain being experienced when viewing touch to others (Ramachandran and Brang 2009). (p. 595)
Beyond Mirror-Touch Synesthesia: Sensorimotor Simulation and Social Perception
A further reason why mirror-touch synesthesia is of interest is in relation to what this variant of synesthesia can tell us about mechanisms of social perception and cognition in non-synesthetes. As noted earlier, functional brain imaging has linked mirror-touch synesthesia to heightened neural activity in a network of brain regions which are also activated in non-synesthetic control subjects when observing touch to others (the mirror-touch system; Blakemore et al. 2005). Therefore, it is reasonable to consider mirror-touch synesthesia as a consequence of over-activity within the typical system that is activated by us all when observing touch to others and to ask what secondary impact this has on other aspects of perception that the mirror-touch system is thought to be involved in.
(p. 596) One component of perception that the mirror-touch system has been associated with is as a candidate neural mechanism to aid social perception through sensorimotor simulation (Gallese and Goldman 1998; Keysers and Gazzola 2006; Oberman and Ramachandran 2007). Accounts of social perception involving sensorimotor simulation contend that in order to understand another’s emotions and physical states, the perceiver must map the bodily state of the observer onto the same representations involved in experiencing the perceived state oneself (Adolphs 2002, 2003; Gallese and Goldman 1998; Gallese, Keysers, and Rizzolatti 2004; Goldman, and Sripada 2005; Keysers and Gazzola 2006; Oberman and Ramachandran 2007). This view is supported by evidence from electrophysiological, functional brain imaging, and psychophysical studies indicating an involvement of sensorimotor resources in aspects of social perception abilities. For example, responses in expression-relevant facial muscles are increased during subliminal exposure to emotional expressions (Dimberg, Thunberg, and Elmehed 2000) and preventing the activation of expression relevant muscles impairs expression recognition (Oberman, Winkielman, and Ramachandran 2007). Perceiving another’s expressions and producing one’s own also recruits similar cortical sensorimotor regions (e.g., Carr et al. 2003; van der Gaag, Minderaa, and Keysers 2007; Warren et al. 2006; Winston, O’Doherty, and Dolan 2003) and neuropsychological findings indicate that damage to right somatosensory cortices is associated with expression recognition deficits (Adolphs et al. 2000).
A complementary approach to the studies described previously is to consider whether facilitation of sensorimotor mechanisms in mirror-touch synesthesia promotes social perception abilities. For example, Banissy and Ward (2007) examined empathy (the capacity to share the thoughts and feelings of others) in developmental mirror-touch synesthesia in an attempt to determine the relationship between heightened activity in the mirror-touch system and this aspect of social perception. We compared the empathic abilities of mirror-touch synesthetes to non-synesthetic and synesthetic control subjects (i.e., individuals who experience synesthesia but not mirror-touch synesthesia) using the Empathy Quotient (a standardized self-report scale designed to empirically measure empathy; Baron-Cohen and Wheelwright 2004). Mirror-touch synesthetes were found to show significantly higher levels of emotionally reactive empathy compared to controls (e.g., affective components of empathy and instinctive empathic responses to others), but did not differ in their levels of cognitive empathy (e.g., mentalizing and cognitive perspective taking) or in their social skills level. Importantly, synesthetes without mirror-touch synesthesia did not differ from non-synesthetes in their levels of empathy, implying that heightened emotional empathy relates specifically to mirror-touch synesthesia (and the neural system which underpins this condition). In accordance with this, neuroimaging findings indicate that, in healthy adults, emotional empathy (i.e., experiencing an appropriate emotional response as a consequence of another’s state) engages the cortical sensorimotor network (including the premotor cortex, primary somatosensory cortex and motor cortex) more than cognitive empathy (i.e., predicting and understanding another’s mental (p. 597) state by using cognitive processes; Nummenmaa et al. 2008) and grey matter volume in the neural regions linked to sensorimotor resonance correlates with individual differences in emotional empathic abilities for healthy adults (Banissy et al. 2012). Further, neuropsychological findings have demonstrated a functional and anatomical double dissociation between deficits in cognitive empathy and emotional empathy, with emotional empathy being linked to lesions to the human mirror system and cognitive empathy being associated to lesions to the ventromedial prefrontal cortices (Shamay-Tsoory, Aharon-Peretz, and Perry 2009). This functional coupling between emotional and cognitive empathy suggests that emotional empathy may be linked more closely to sensorimotor simulation of another’s state and the evidence that mirror-touch synesthetes only significantly differ from controls on levels of emotional reactivity is consistent with this.
It is not just developmental mirror-touch synesthetes who have been shown to differ in their levels of emotional reactive empathy. A recent study by Goller et al. (2013) indicates that acquired cases of mirror-touch synesthesia are also associated with increases in this aspect of social perception. In their study, the empathic abilities of amputees that reported mirror-touch synesthesia were compared to amputees who do not report mirror-touch synesthesia. As per Banissy and Ward (2007), the authors use the Empathy Quotient to examine empathic abilities and found that amputees who report mirror-touch synesthesia showed higher levels of emotional reactive empathy, but not cognitive empathy or social skills (Goller et al. 2013).
Developmental mirror-touch synesthetes have also been shown to differ in another aspect of social perception that is thought to utilize sensorimotor systems, namely expression processing. My colleagues and I compared mirror-touch synesthetes and non-synesthetic controls on facial expression recognition, identity recognition, and identity perception tasks (Banissy et al. 2011). Based on the hypothesis that mirror-touch synesthetes have heightened sensorimotor simulation mechanisms we predicted that synesthetes would show superior performance on expression recognition tasks but not on the facial identity control tasks that are less dependent on simulation. Consistent with these predictions, mirror-touch synesthetes were superior when recognizing the facial expressions, but not facial identities of others (Banissy et al. 2011). These findings are in accordance with transcranial magnetic stimulation, neuropsychological and functional brain imaging findings indicating the involvement of sensorimotor systems in expression processing but not identity processing (e.g., Banissy et al. 2010; Pitcher et al. 2008). They are therefore consistent with simulation accounts of expression recognition contending that one mechanism involved in expression, but not identity recognition, is an internal sensorimotor re-enactment of the perceived expression (Adolphs 2002; Gallese, Keysers, and Rizzolatti 2004; Goldman, and Sripada 2005; Keysers and Gazzola 2006). When combined with the evidence of heightened affective empathy in developmental and acquired mirror-touch synesthesia, they also help to demonstrate an interesting avenue in which mirror-touch synesthesia may be able to inform us about mechanisms of social perception in non-synesthetes. (p. 598)
In sum, this chapter has described the prevalence and characteristics of mirror-touch synesthesia; the neurocognitive mechanisms that contribute to this experience; and discussed the extent to which mirror-touch synesthesia can be used to inform us about mechanisms of social perception. While much has been learnt already, a number of key questions remain, including the mechanisms that mediate individual differences in mirror-touch synesthesia, the role of mechanisms of self-other distinction in mirror-touch synesthesia, and the extent to which mirror-touch synesthesia shares similarities/differences to other variants of synesthesia. These and other questions will provide interesting avenues for future studies into this variant of synesthetic experience.
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