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Prior research suggests that different types of touch can affect sleep, but whether there is a consistent association between tactile intimacy and sleep quality is unclear. Here, we report a pre-registered systematic review (PROSPERO - CRD42020158683) of studies examining the association between tactile intimacy and sleep quality in healthy adults. The databases PsycINFO, PubMed, Web of Science, the Cochrane Central Register of Controlled Trials and EMBASE were searched on August 7th, 2020. 13 studies met the inclusion criteria and were synthesised qualitatively. Most commonly, articles researched sexual intimacy in relation to sleep quality, but some studies also investigated nonsexual affective touch and emotionally neutral touch. Some evidence for a connection between sexual function, sexual satisfaction and masturbation with sleep quality was found, however, no evidence for an association between sexual frequency or sexual positions and sleep was found. Interestingly, studies employing more subjective approaches were more likely to report an association between touch and sleep, potentially highlighting a discrepancy between self-reported and the objectively measurable association between touch and sleep.

Vicarious perception describes our ability to co-represent the experiences of others, by matching observed states onto representations of our own experience. For instance, seeing another person feel touch or pain elicits activity in regions associated with first-hand touch and pain sensation, including somatosensory cortices. Vicarious touch and pain perception is thought to facilitate complex social processes such as empathy, and also shows substantial inter-individual variability. For a minority of people, a physical sensation of touch (mirror-touch synaesthesia) or pain (conscious vicarious pain) is felt on their own body when observing someone experience the same sensation. Current theory suggests increased excitability in somatosensory cortices may underlie conscious vicarious experience. Recently, broader impairments in self-other distinction have also been implicated. This thesis first attempted to modulate vicarious tactile perception with transcranial current stimulation targeted at somatosensory cortices or the right temporo-parietal junction (linked to self-other control). A lack of modulation provided minimal support for either somatosensory excitability or self-other distinction accounts. Behaviourally, conscious vicarious pain responders and control participants did not significantly differ in self-other control abilities. Additional self-other distinction processes (beyond self-other control) were next considered. This revealed atypical bodily self-awareness in conscious vicarious pain responders. Lastly, perception of animacy was modulated by stimulus and perceiver variability, but did not significantly differ between mirror-touch synaesthetes and controls, providing implications for vicarious perception from inanimate stimuli. Collectively, this thesis highlights broader impairments involved in conscious vicarious perception, and the importance of the sense of bodily self-awareness for social perception and interaction in typical adults.

Our capacity to share the experiences of others is a critical part of social behaviour. One process thought to be important for this is vicarious perception. Passively viewing touch activates some of the same network of brain regions as the direct experience of touch. This vicarious experience is usually implicit, but for some people, viewing touch evokes conscious tactile sensations (mirror-touch synaesthesia). Recent work has attempted to induce conscious vicarious touch in those that do not normally experience these sensations, using transcranial direct current stimulation (tDCS). Anodal tDCS applied to primary somatosensory cortex (SI) was found to induce behavioural performance akin to mirror-touch synaesthesia on a visuotactile interference task. Here, we conducted two experiments that sought to replicate and extend these findings by examining: a) the effects of tDCS and high frequency transcranial random noise stimulation (tRNS) targeted at SI and temporo-parietal junction (TPJ) on vicarious tactile perception, b) the extent to which any stimulation effects were specific to viewing touch to humans versus inanimate agents, and c) the influence of visual perspective (viewing touch from one’s own versus another’s perspective) on vicarious perception. In Experiment 1, tRNS targeted at SI did not modulate vicarious perception. In Experiment 2, tDCS targeted at SI, but not TPJ, resulted in some modulation of vicarious perception, but there were important caveats to this effect. Implications regarding mechanisms of vicarious perception are discussed. Collectively, the findings do not provide convincing evidence for the potential to modulate vicarious tactile perception with transcranial electrical current stimulation.