In Chapter 2 the rubber hand illusion paradigm (Botvinck & Cohen, 1998) was used to investigate the role played by affective touch in the sense of body ownership. Specifically, the velocity of touch during the illusion was manipulated in order to observe the effect of affective/slow vs. neutral/fast touch on the multisensory integration process taking place during the illusion. The results confirmed previous findings that slow velocity, light touch on hairy skin is perceived as more pleasant than fast touch (Löken et al., 2009). Importantly, the experiment reported in Chapter 2 demonstrated for the first time that using slow, pleasant touch during the rubber hand illusion produces higher levels of subjective embodiment compared to fast, neutral touch. The results further showed that slow, synchronous stroking did not affect the perceived location of the participants’ own hand during the illusion (i.e. proprioceptive drift). This study seemed to suggest that affective touch had a greater effect on subjective than behavioural measures of body ownership.
To better understand the role of affective touch in bodily self-consciousness, the relation between affective touch and another interoceptive modality (i.e. cardiac awareness) was examined in Chapter 3. The heartbeat counting task (Schandry, 1981) and the affective touch task were combined in the context of the rubber hand illusion, in order to explore the relationship between interoception and exteroception in the sense of body ownership. The aim of this study was to investigate for the first time the interplay between different interoceptive modalities, namely cardiac awareness and affective touch, in body representation. In particular, this Chapter explored whether interoceptive sensitivity would modulate the extent to which affective touch influences the multisensory process taking place during the rubber hand illusion, as measured using 1) subjective self-reports (i.e. an embodiment questionnaire), 2) an objective, behavioural measure (i.e. proprioceptive drift), and 3) physiological changes in the body (i.e. the temperature drop previously observed as a consequence of acquiring ownership of the rubber hand). In line with the findings of Chapter 2, the results confirmed that the subjective (i.e. embodiment questionnaire) but not the objective (i.e. proprioceptive
drift) component of the illusion can be enhanced by slow, affective touch (see also Chapter 2; Lloyd et al., 2013; but see van Stralen et al., 2014 for an opposing result), resulting in a stronger conscious experience of acquiring ownership over the rubber hand. Given the recent evidence arguing for a role of affective touch as an interoceptive modality for the sense of body ownership, a potential relationship between the bodily pleasure raised by affective touch and the extent to which participants were aware of their body from within (i.e. interoceptive sensitivity) was hypothesised. However, as tested in the present study, this prediction could not be confirmed. Correlational analysis showed a non-significant relation between the performance on the heartbeat detection task and on the affective touch task. Therefore, these two interoceptive modalities might underlie different aspects of interoception. These results are discussed in more details below.
Chapter 4 presented an investigation of the neurobiology of affective touch.
This double-blind, placebo-controlled, randomised, cross-over study aimed to explore the effect of intranasal oxytocin on the perception of affective touch and body awareness, as measured by means of the heartbeat detection task and the rubber hand illusion. The importance of tactile social interactions on physical and psychological wellbeing, and in particular slow touch (i.e. massage), is supported by clinical and experimental studies (e.g. Field, 2010 for a review). However, the neurobiological mechanisms of touch, and in particular affective touch, are still under investigation. Animal studies have associated the perception of affective touch with the release of oxytocin (Uvnäs-Moberg et al., 2015 for a review); more recently these findings have been partially extended to humans too (e.g. Morhenn et al, 2012). The study described in Chapter 4 attempted to further investigate the neurobiology of affective touch by exploring the effect of intranasal oxytocin and placebo on the perception of slow (CT optimal) vs. fast (non-CT optimal) strokes delivered on participants’ forearms. The findings highlighted a trend for oxytocin compared to placebo to improve the discrimination between slow and fast touch; however the results were not statistically significant and future studies should investigate this further on a larger sample. Additionally, the performance on the heartbeat detection task following the self-
administration of oxytocin and placebo were compared in order to investigate any effect of the active compound on interoceptive awareness. No differences in performance on the heartbeat counting task were found following the self-administration of oxytocin or placebo. This finding supports the existing idea of interoceptive sensitivity as a rather fixed trait (Schandry, 1981; Tsakiris et al., 2011, but see Ring, Brener, Knapp & Mailloux, 2015). Finally, the sense of body ownership was manipulated and measured by means of the classical rubber hand illusion. No differences were found in the occurrence of the illusion following the self-administration of oxytocin vs. placebo, suggesting that this multisensory integration process is resistant to the effect of intranasal oxytocin. However, given the small sample size and limited statistical power observed in this study, the results and their interpretation should be taken with caution and further investigated in future research.
Chapter 5 applied some of the above methodologies to investigate affective
touch in a clinical population characterised by body image distortions, lack of awareness and social difficulties, i.e. anorexia nervosa. Participants with anorexia nervosa and healthy controls were asked to rate the pleasantness of the touch that they perceived on their forearms, while simultaneously looking at faces displaying different facial expressions (accepting, neutral and rejecting faces), which are known to differently engage the attentional resources in anorexia nervosa compared to healthy controls. Results showed that participants with a diagnosis of anorexia nervosa perceived affective touch as less pleasant compared to healthy controls. No differences between the two groups were found in the perception of fast, emotionally neutral touch. Therefore, this finding seemed to suggest a specific impairment in the perception of touch mediated by the CT afferents system. Hence, these data might indicate a potential involvement of a distorted affective touch system in the disordered body image and social anhedonia which characterise this clinical population. Contrary to the prediction, no differences were found between anorexia nervosa participants and healthy controls in the social modulation of facial expressions, in the sense that both groups showed an increase in pleasantness ratings when touch was presented together with accepting faces compared to rejecting faces. These data might suggest an intact top-down modulation of
interpersonal touch in anorexia nervosa, and therefore open novel avenues to the development of different therapeutic approach to anorexia nervosa by means of this unimpaired channel.
The relationship between affective touch and body awareness in the context of social cognition was further investigated and discussed on Chapter 6 and Chapter 7. In
Chapter 6, the focus was on the distinction between self and other and the effect on
bodily pleasure and cardiac awareness. Participants completed a heartbeat counting task and an affective touch task while they were looking at themselves or someone else in the mirror. The participants were split in two groups (high vs. low perceivers) based on whether their ability to count their own heartbeat while looking at a blank screen was above or below the median of the group (see Ainley et al., 2012). The findings showed a significant difference in interoceptive sensitivity between high and low interoceptive sensitivity groups also in the self- and other-mirror observation conditions, respectively. Previous studies showed that observing the self in a mirror can improve interoceptive sensitivity (measured using the heartbeat counting task) compared to a blank screen condition, but only in participants with low interoceptive sensitivity at baseline (Ainley et al., 2012). The study reported in Chapter 6 failed to replicate the findings of Ainley and colleagues, suggesting that visual feedback does not affect performance on the heartbeat counting task in either of the two groups. It should be noted that the sample of the study presented in Chapter 6 is much smaller than the sample tested in the study of Ainley and colleagues (64 vs. 129 participants); hence this could explain the discrepant findings, which are discussed in more depth below. Performance on the affective touch task indicated that perceiving touch whilst looking at someone else in a mirror (and being looked back at) reduced the perceived pleasantness, compared to looking at a blank screen or at one’s own face in the mirror. To a certain extent, this finding was in line with recent evidence showing that exteroception, but not interoception, can be affected by the awareness of being watched (Durlik et al., 2014). In fact, the performance on the heartbeat detection task did not seem to be affected by a condition in which a recording camera was on (‘being watched condition’); in contrast the tactile perception was enhanced when there was social self-focus. Similarly, the present study
showed that being observed by someone else reduced the perceived pleasantness of tactile stimulation, whereas it did not affect the performance on the heartbeat counting task. This could suggest that being observed by someone else (not the person performing the touch) does affect the perception of touch, since the nature of tactile stimulation itself is interpersonal, and therefore probably more sensitive to social manipulation.
In contrast, in Chapter 7, the effect of the togetherness of self and other on bodily pleasure was investigated. Namely, participants were asked to rate the pleasantness of touch while perceiving it together (synchronously or asynchronously) with someone else. The results showed that visual feedback of synchronous tactile stimulation on another person’s arm increased the perceived pleasantness of the touch more than asynchronous touch. Moreover, this effect seemed to apply only when the touch was delivered at CT optimal velocities, with no effect of synchronicity found when touch was applied at fast, CT-non optimal velocities. Finally, no effect of visual feedback was found, in the sense that there were no differences in pleasantness ratings reported after looking at the participant’s own arm or the confederate’s arm being stroked in synchrony, for both CT- and CT-non-optimal velocities. This seemed to suggest that the mere knowledge of someone else being touched in a pleasant way in synchrony to the participant was enough to increase the pleasantness compared to the asynchronous condition.