Theoretical implications for vicarious perception of touch and pain

The experimental work reported in this thesis contributes to understanding the role of self-other distinction in vicarious touch and pain. Support is provided for the suggestion that vicarious pain is associated with broader mechanisms beyond somatosensory mirroring, as proposed by the Self-Other Theory of MTS (see Ward & Banissy, 2015). In particular, processes relevant to maintaining the sense of bodily self, discussed in Chapter 5, may play an important role in modulating vicarious pain perception.

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Less support is provided for the involvement of other self-other distinction processes in modulating vicarious pain perception. Stimulus variability factors, specifically animacy (human vs. dummy) and visual perspective (1st vs. 3rd person) were not found to modulate performance on an objective measure of conscious vicarious tactile perception in a bottom-up way, in Chapter 3. However, this lack of modulation may be explained by the non-synaesthete participant sample who took part in the study. Since these individuals were not expected to experience conscious vicarious sensations of touch under sham stimulation conditions, any modulating effect of animacy or perspective may not be detected on this particular task. Indeed, previous research which has recorded neural responses to stimuli varying in animacy (Deschrijver et al., 2015) and visual perspective (Canizales et al., 2013) has indicated a modulating effect on unconscious vicarious perception of touch and pain. Future research should continue to vary self-other distinction factors such as animacy in both a bottom-up (e.g., using visibly inanimate body parts) and a top-down way (e.g., using a gloved hand and instructing participants that the hand belongs to either a human or a robot, see Liepelt & Brass, 2010), and also incorporate individual variability factors relevant to animacy perception to effectively assess the modulating effect on vicarious perception.

Furthermore, in Chapter 4 self-other control ability was comparable across conscious vicarious pain responders and controls, on a task requiring the inhibition of other-relevant and promotion of self-relevant representations. Investigating the functional role of rTPJ in modulating vicarious tactile perception also provided little support for the involvement of self-other control processes. Increasing excitability in this region with tDCS has previously been linked with improved self-other control ability (Santiesteban et al., 2012; Hogeveen et al., 2015), but did not have a significant effect on conscious vicarious tactile perception in Chapter 3. These results contrast with

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predictions based on Self-Other Theory, and with prior evidence indicating impairments in self-other control associated with MTS (Santiesteban et al., 2015b) and conscious vicarious pain (Derbyshire et al., 2013).

Evidence for Threshold and Task Control theories was lacking in the present studies. While vicarious pain responder groups did not show differences in self-other control ability, these groups also performed comparably on a domain-general inhibitory control task. The question of whether previously observed impairments in self-other control in conscious vicarious responder groups (Derbyshire et al., 2013; Santiesteban et al., 2015b) reflect domain-general deficits in inhibitory control, as proposed by Task Control Theory (Heyes & Catmur, 2015), therefore remains unclear.

Additionally, attempts to modulate vicarious perception using transcranial current stimulation in Chapter 3 did not provide strong support for a Threshold Theory of conscious vicarious experience (see Ward & Banissy, 2015). Conscious vicarious perception of touch did not significantly increase (after correction for multiple comparisons) following tDCS or tRNS aimed at increasing cortical excitability in primary somatosensory cortex. However, this result does not necessarily conflict with Threshold Theory. Factors such as individual variability in responsiveness to transcranial current stimulation (see Krause & Kadosh, 2014) may have influenced the effectiveness of stimulation in this case. Additionally, even if hyper-excitability in somatosensory cortex is involved in MTS, this may not be sufficient to induce the condition if other neural mechanisms (such as self-other distinction) are involved.

The lack of significant effects reported in the experiments of Chapters 3 and 4 (as discussed above) must be considered in light of a potential lack of statistical power with which to identify these effects. In Chapter 3, for instance, data from 22 participants

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was analysed for Experiment 1 and 23 for Experiment 2. While in each case this is fewer than participated in the study being replicated (N = 32; Bolognini et al., 2013), in this previous study participants were divided into two groups in terms of the hemisphere targeted with tDCS (left or right) whereas in the current experiments all participants received stimulation on the same hemisphere. Further, the number of experimental trials was increased in the current experiments (180 per task compared with 144), further increasing statistical power. Nevertheless, achieved power for the crucial t-test comparison between SI and sham RTs in Experiment 2 was only 0.51, insufficient to detect significance. Power calculations indicate that a sample size of 44 would be required to achieve a 0.80 level of power. With this in mind, it should be noted that although the effects reported by Bolognini and colleagues were not replicated here, the current results alone do not provide strong evidence for the null hypothesis.

In Chapter 4 similar problems arise. Power calculations indicate that sufficient power (0.96) was achieved for the interaction effect between pain responder group and congruency on the imitation-inhibition task where between-group differences were predicted, despite a null result being found. While this indicated a sufficient overall sample size, the analysis does not account for the unequal groups in the study (Controls N = 24, Sensory-Localised N = 10, Affective-Generalised N = 3). Certainly, more than 3 participants would be required to detect significant differences in the Affective- Generalised group versus controls. With the obtained ratio of Sensory-Localised to control participants, estimated sample sizes of 38 Controls and 16 Sensory-Localised responders would be required to achieve 0.8 power for paired comparisons, based on effect sizes obtained with mirror-touch synaesthetes in prior work (Santiesteban et al., 2015b). Again, we can conclude that despite the null results obtained in this Chapter, strong evidence for the null hypothesis is not provided. Across both areas of

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investigation (Chapters 3 and 4) further work with larger sample sizes is warranted in order to clarify discrepancies with prior work.

Based on the evidence presented in this thesis and reviewed here, mechanisms associated with the sense of bodily self-awareness appear to be of relevance to conscious vicarious perception (at least of painful stimuli). Chapter 5 of this thesis presents only a preliminary investigation of bodily self-awareness in relation to depersonalisation and interoception, but provides a promising avenue for future research. Both MTS and conscious vicarious pain have previously been linked to greater plasticity of body representations (Aimola Davies & White, 2013; Cioffi et al., 2016; Maister et al., 2013; Osborn & Derbyshire, 2010), with these individuals more likely to incorporate others into their own bodily self-concept. In line with Self-Other Theory, it has been suggested that this broader plasticity of the bodily self may underlie conscious vicarious experiences of touch and pain (see Banissy & Ward, 2013; Ward & Banissy, 2015). Plasticity of the bodily self in MTS and conscious vicarious pain could be further studied using existing paradigms from the body representations literature. For instance, visual adaptation to a distorted version of one’s own arm (e.g, larger or smaller) alters perceived tactile distances on the arm, indicating a rescaling of the implicit body model (Taylor-Clarke, Jacobsen & Haggard, 2004; see also Longo, Azañón & Haggard, 2010). For individuals with MTS or conscious vicarious pain, an extended plasticity of bodily representations to incorporate others may mean that similar effects are observed following visual adaptation to another person’s arm. Methods such as these provide interesting future directions for research and may provide further insight into bodily self-awareness and vicarious perception.

The theories discussed above, while originally developed to explain vicarious tactile perception, have also been adopted to explain vicarious perception of pain. While