Seeing and feeling touch: subjective experiences of vicarious touch and the underlying neural mechanisms

<p dir="ltr">The sense of touch plays a fundamental role in social interactions, shaping our perception of ourselves and others. Although touch is usually experienced directly through physical contact, we can also experience tactile sensations vicariously, that is, by seeing someone else being touched. This phenomenon, known as ‘vicarious touch’ or ‘mirror touch’, is thought to rely on a simulation process where one’s own tactile system is similarly co-activated. The research in this thesis aims to increase our understanding of the subjective experience and underlying mechanisms of vicarious tactile perception in both typical adults and individuals with mirror-touch synaesthesia.</p><p dir="ltr">For my initial study, I developed the Validated Touch-Video Database (VTD), comprising 90 videos that depicted tactile interactions with a hand. I collected validation data on various parameters, including the hedonic tone of the viewed touch, as well as assessments of arousal and perceived threat. In my second study, I used these videos to examine the prevalence and characteristics of reported vicarious touch experiences in a large undergraduate sample. I then used a subset of these videos to design a videobased questionnaire. This was done to categorise participants based on their vicarious touch experiences - either experiencing it (<i>vicarious touch group</i>) or not (<i>no vicarious touch group</i>) as part of my third study, which examined neural correlates using electroencephalography (EEG). I conducted a fourth follow-up EEG study focusing on a distinct set of participants who indicated experiences of mirror-touch synaesthesia. Their claims were verified using a modified version of the video questionnaire. To assess the assumptions of simulation theories, I explored the neural overlap between observing and feeling touch in these three groups using EEG with time-resolved multivariate pattern analysis (MVPA). The timecourse of overlap can shed light on the nature of representations activated by viewing touch. For instance, neural overlap with an earlier tactile signal is consistent with a sensory-driven representation, while overlap with a later signal could indicate a more conceptual or abstracted representation. By contrasting the three groups, I gained insights into differences in neural overlap, which could potentially explain variations in the personal experiences of vicarious touch.</p><p dir="ltr">Over three studies, I consistently found that many individuals report localised vicarious sensations, like touch or pain, when they watch the touch videos. I documented different sensation patterns which matched different facets of the observed touch. Through neural decoding techniques, I identified distinct brain patterns related to both observing and directly feeling touch on different body parts (the little finger versus the thumb). Only the vicarious touch and mirror-touch synaesthesia groups demonstrated strong evidence for neural overlap between these experiences. Individuals with mirror-touch synaesthesia exhibited neural overlap earlier in tactile processing compared to the vicarious touch group. This could potentially account for their more regular and pronounced vicarious touch experiences. In the vicarious touch group, the overlap happened between late tactile information and an early stage in the visual trial, as the object neared the hand, hinting at primarily anticipatory processes. On the other hand, in the mirror-touch synaesthesia group, tactile representations coincided with both the beginning and end of the visual trial, suggesting that tactile sensations persist even after the touch event has been observed in the videos. Lastly, the mirror-touch synaesthesia group had higher emotional empathy scores than the vicarious touch group, hinting at a potential link between shared touch perceptions and increased empathic tendencies.</p><p dir="ltr">Together, these findings provide insight into vicarious sensory processing in typical adults and mirror-touch synaesthetes. By integrating validated touch videos, subjective reports, and neural data, this comprehensive approach deepens our understanding of both the subjective experiences and neural mechanisms of vicarious touch.</p>