Functional and structural brain differences associated with mirror-touch synaesthesia

Observing touch is known to activate regions of the somatosensory cortex but the interpretation of this finding is controversial (e.g. does it reflect the simulated action of touching or the simulated reception of touch?). For most people, observing touch is not linked to reported experiences of feeling touch but in some people it is (mirror-touch synaesthetes). We conducted an fMRI study in which participants (mirror-touch synaesthetes, controls) watched movies of stimuli (face, dummy, object) being touched or approached. In addition we examined whether mirror touch synaesthesia is associated with local changes of grey and white matter volume in the brain using VBM (voxel-based morphometry). Both synaesthetes and controls activated the somatosensory system (primary and secondary somatosensory cortices, SI and SII) when viewing touch, and the same regions were activated (by a separate localiser) when feeling touch — i.e. there is a mirror system for touch. However, when comparing the two groups, we found evidence that SII seems to play a particular important role in mirror-touch synaesthesia: in synaesthetes, but not in controls, posterior SII was active for watching touch to a face (in addition to SI and posterior temporal lobe); activity in SII correlated with subjective intensity measures of mirror-touch synaesthesia (taken outside the scanner), and we observed an increase in grey matter volume within the SII of the synaesthetes' brains. In addition, the synaesthetes showed hypo-activity when watching touch to a dummy in posterior SII. We conclude that the secondary somatosensory cortex has a key role in this form of synaesthesia

Transcranial electric stimulation and cognitive training improves face perception

Recently, there has been much interest the effectiveness of cognitive training programmes across a variety of cognitive and perceptual domains. Some evidence suggests that combining training programmes with noninvasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance training gains, but to date this has only been examined in numerosity and arithmetic tasks. In this study, we examined whether tRNS modulated the effects of a face recognition training programme. Participants completed a face discrimination training task for an hour per day over five days. Each day, training was preceded by twenty minutes of active high frequency tRNS or sham stimulation, targeted at the posterior temporal cortices or the inferior frontal gyri (IFG). Participants who received active stimulation to the posterior temporal cortices showed significant improvement on a facial identity discrimination task (the Cambridge Face Perception Test) after training, whereas those receiving sham or IFG stimulation showed no performance change. There was no evidence of an effect of stimulation on a face memory task (the Cambridge Face Memory Test). These results suggest that tRNS can enhance the effectiveness of cognitive training programmes, but further work is needed to establish whether perceptual gains can be generalised to face memory

Suppressing sensorimotor activity modulates the discrimination of auditory emotions but not speaker identity

Our ability to recognize the emotions of others is a crucial feature of human social cognition. Functional neuroimaging studies indicate that activity in sensorimotor cortices is evoked during the perception of emotion. In the visual domain, right somatosensory cortex activity has been shown to be critical for facial emotion recognition. However, the importance of sensorimotor representations in modalities outside of vision remains unknown. Here we use continuous theta-burst transcranial magnetic stimulation (cTBS) to investigate whether neural activity in the right postcentral gyrus (rPoG) and right lateral premotor cortex (rPM) is involved in nonverbal auditory emotion recognition. Three groups of participants completed same-different tasks on auditory stimuli, discriminating between the emotion expressed and the speakers' identities, before and following cTBS targeted at rPoG, rPM, or the vertex (control site). A task-selective deficit in auditory emotion discrimination was observed. Stimulation to rPoG and rPM resulted in a disruption of participants' abilities to discriminate emotion, but not identity, from vocal signals. These findings suggest that sensorimotor activity may be a modality-independent mechanism which aids emotion discrimination. Copyright © 2010 the authors

The relationship between mirror-touch synaesthesia and empathy: new evidence and a new screening tool

People with mirror-touch synaesthesia (MTS) report experiencing tactile sensations on their own body when seeing another person being touched. Although this has been associated with heightened empathy and emotion perception, this finding has recently been disputed. Here, we conduct two experiments to explore this relationship further. In Study 1, we develop a new screening measure for MTS. We show that MTS is related to vicarious experiences more generally (including to itch and pain), but is not a simple exaggerated version of normality. For example, people with MTS report videos of scratching as ‘touch’ rather than ‘itchiness’ and have localized sensations when watching others in pain. In Study 2, we show that MTS is related to increased emotional empathy to others and better ability to read facial expressions of emotion, but other measures of empathy are normal-to-low. In terms of theoretical models, we propose that this is more consistent with a qualitative difference in the ability to selectively inhibit the other and attending to the self, which leads to heightened activity in shared self-other representations (including a mirror system for touch, but also includes other kinds of vicarious experience)

Superior Facial Expression, But Not Identity Recognition, in Mirror-Touch Synesthesia

Simulation models of expression recognition contend that to understand another's facial expressions, individuals map the perceived expression onto the same sensorimotor representations that are active during the experience of the perceived emotion. To investigate this view, the present study examines facial expression and identity recognition abilities in a rare group of participants who show facilitated sensorimotor simulation (mirror-touch synesthetes). Mirror-touch synesthetes experience touch on their own body when observing touch to another person. These experiences have been linked to heightened sensorimotor simulation in the shared-touch network (brain regions active during the passive observation and experience of touch). Mirror-touch synesthetes outperformed nonsynesthetic participants on measures of facial expression recognition, but not on control measures of face memory or facial identity perception. These findings imply a role for sensorimotor simulation processes in the recognition of facial affect, but not facial identity

Consciously feeling the pain of others reflects atypical functional connectivity between the pain matrix and frontal-parietal regions

Around a quarter of the population report ‘mirror pain’ experiences in which bodily sensations of pain are elicited in response to viewing another person in pain. We have shown that this population of responders further fractionates into two distinct subsets (Sensory/localised and Affective/General), which presents an important opportunity to investigate the neural underpinnings of individual differences in empathic responses. Our study uses fMRI to determine how regions involved in the perception of pain interact with regions implicated in empathic regulation in these two groups, relative to controls. When observing pain in others (minor injuries to the hands and feet), the two responder groups show activation in both the sensory/discriminative and affective/motivational components of the pain matrix. The control group only showed activation in the latter. The two responder groups showed clear differences in functional connectivity. Notably, Sensory/Localised responders manifest significant coupling between the right temporo-parietal junction (rTPJ) and bilateral anterior insula. We conclude that conscious experiences of vicarious pain is supported by specific patterns of functional connectivity between pain-related and regulatory regions, and not merely increased activity within the pain matrix itself

Transcranial random noise stimulation and cognitive training improves face perception

Several studies have found that cognitive training can improve face recognition. However, the effects tend to be relatively small and short-lived. Recent research has found that non-invasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance and extend the effects of cognitive training in other domains, but this has not been examined for face recognition. In this study, we examined whether tRNS modulated the effects of a face recognition training program in people with typical face recognition abilities. Participants completed a face discrimination training task for one hour per day over fi ve days. Training was preceded by twenty minutes of active high frequency tRNS or sham stimulation to lateral occipitotemporal cortices. Participants completed a battery of face processing tasks assessing face memory (the Cambridge Face Memory Test, CFMT), face perception (the Cambridge Face Perception Test, CFPT), and patterns of eye-movements to faces (free-viewing of faces and social scenes); these took place before training, after training, and at a one-week follow-up session. Participants who received active stimulation showed signifi cant improvement on the CFPT following training, whereas those who received sham stimulation did not show any training gains. There was no improvement for inverted faces, and neither the active or sham stimulation group showed an improvement on the CFMT, or any change in eye-movement patterns. These results suggest that tRNS can enhance the effectiveness of face recognition training programmes, but further work is needed to establish whether perceptual gains can be generalised to memory

Mirror-touch synaesthesia: the role of shared representations in social cognition

Synaesthesia is a condition in which one property of a stimulus results in conscious experiences of an additional attribute. In mirror-touch synaesthesia, the synaesthete experiences a tactile sensation on their own body simply when observing touch to another person. This thesis investigates the prevalence, neurocognitive mechanisms, and consequences of mirror-touch synaesthesia. Firstly, the prevalence and neurocognitive mechanisms of synaesthesia were assessed. This revealed that mirrortouch synaesthesia has a prevalence rate of 1.6%, a finding which places mirror-touch synaesthesia as one of the most common variants of synaesthesia. It also indicated a number of characteristics of the condition, which led to the generation of a neurocognitive model of mirror-touch synaesthesia. An investigation into the perceptual consequences of synaesthesia revealed that the presence of synaesthesia is linked with heightened sensory perception - mirror-touch synaesthetes showed heightened tactile perception and grapheme-colour synaesthetes showed heightened colour perception. Given that mirror-touch synaesthesia has been shown to be linked to heightened sensorimotor simulation mechanisms, the impact of facilitated sensorimotor activity on social cognition was then examined. This revealed that mirror-touch synaesthetes show heightened emotional sensitivity compared with control participants. To compliment this, two transcranial magnetic stimulation (TMS) studies were then conducted to assess the impact of suppressing sensorimotor activity on the expression recognition abilities of healthy adults. Consistent with the findings of superior emotion sensitivity in mirror-touch synaesthesia (where there is facilitated sensorimotor activity), suppressing sensorimotor resources resulted in impaired expression recognition across modalities. The findings of the thesis are discussed in relation to neurocognitive models of synaesthesia and of social cognition

The brain's response to pleasant touch: an EEG investigation of tactile caressing

Somatosensation as a proximal sense can have a strong impact on our attitude toward physical objects and other human beings. However, relatively little is known about how hedonic valence of touch is processed at the cortical level. Here we investigated the electrophysiological correlates of affective tactile sensation during caressing of the right forearm with pleasant and unpleasant textile fabrics. We show dissociation between more physically driven differential brain responses to the different fabrics in early somatosensory cortex - the well-known mu-suppression (10-20 Hz) - and a beta-band response (25-30 Hz) in presumably higher-order somatosensory areas in the right hemisphere that correlated well with the subjective valence of tactile caressing. Importantly, when using single trial classification techniques, beta-power significantly distinguished between pleasant and unpleasant stimulation on a single trial basis with high accuracy. Our results therefore suggest a dissociation of the sensory and affective aspects of touch in the somatosensory system and may provide features that may be used for single trial decoding of affective mental states from simple electroencephalographic measurements

Individual differences in vicarious pain perception linked to heightened socially elicited emotional states

For some people (vicarious pain responders), seeing others in pain is experienced as pain felt on their own body and this has been linked to differences in the neurocognitive mechanisms that support empathy. Given that empathy is not a unitary construct, the aim of this study was to establish which empathic traits are more pronounced in vicarious pain responders. The Vicarious Pain Questionnaire (VPQ) was used to divide participants into three groups: (1) non-responders (people who report no pain when seeing someone else experiencing physical pain), (2) sensory-localized responders (report sensory qualities and a localized feeling of pain) and (3) affective-general responders (report a generalized and emotional feeling of pain). Participants completed a series of questionnaires including the Interpersonal Reactivity Index (IRI), the Empathy Quotient (EQ), the Helping Attitudes Scale (HAS), and the Emotional Contagion Scale (ECS) as well as The Individualism - Collectivism Interpersonal Assessment Inventory (ICIAI) and a self-other association task. Both groups of vicarious pain responders showed significantly greater emotional contagion and reactivity, but there was no evidence for differences in other empathic traits or self-other associations. Subsequently, the variables were grouped by a factor analysis and three main latent variables were identified. Vicarious pain responders showed greater socially elicited emotional states which included the ECS, the Emotional Reactivity Subscale of EQ and the HAS. These results show that consciously feeling the physical pain of another is mainly linked to heightened emotional contagion and reactivity which together with the HAS loaded on the socially elicited emotional states factor indicating that, in our population, these differences lead to a more helpful rather than avoidant behavior