Mirror touch : electrophysiological and behavioural evidence on the effects of observing others' tactile sensations on somatosensory processing in the observer and possible links to trait empathy

Recent neuroimaging evidence suggests that the sight of somebody else being touched vicariously triggers activity in the secondary (SII) and possibly also the primary (SI) somatosensory cortex in the absence of any actual tactile stimulation on the onlooker’s own body. The present PhD thesis aimed to investigate electrophysiological and behavioural correlates of such shared neural representations for actually experienced and merely observed touch, importantly, not only in the context of observing somebody else being passively touched on their body but also in the context of witnessing somebody else perform actions with a tactile component (such as actively touching an object). In addition, the present thesis intended to explore possible links between variations in the strength of touch observation-related modulations in somatosensory processing and interindividual differences in dispositional empathy. The obtained electrophysiological data indicated that, first of all, the sight of others’ passive tactile sensations modulated somatosensory activity relatively consistently at both early and late processing stages within the first 200 ms after tactile stimulus onset. These modulations occurred independently of whether the touch target was actually a human body or merely an inanimate object (Exp. 2.1). The perspective from which a body part was observed to be touched did differentially affect touch observation-related ERP modulations, but only during later-stage somatosensory processing (Exp. 2.2). The electrophysiological evidence further suggested that while the somatotopic organisation of vicarious somatosensory activations might not be fine-grained enough to represent which location within a given body part was seen to be touched (Exp. 2.3), it might nevertheless be sufficiently detailed (at least in SI) to code the touched location at the level of different body parts (Exp. 2.4). The sight of others’ action-embedded tactile sensations was, too, found to alter ongoing somatosensory activity but the pattern of modulations was rather complex and fragile (Exp.s 3.1-3.6), possibly in the context of movement observation-related vicarious somatosensory activity which might sometimes have obscured much subtler touch observation-related resonance responses, especially if participants were not sufficiently aware of the tactile component in the observed actions. Behaviourally, the sight of other’s (active) touch sensations was nevertheless associated with systematic shifts in tactile perception (Exp.s 4.1.3 and 4.1.5), even though the measurability of such changes appeared somewhat task-sensitive (Exp. 4.1.4). Finally, a highly complex pattern of correlations between the strength of touch observation-related ERP modulations and interindividual differences in trait empathy associated the automatic sharing of others’ bodily states with complex emotional and cognitive empathy phenomena. How we respond to others’ somatic sensations thus appears to be fundamentally linked to how readily we respond emotionally to others’ mental and emotional states and how easily we can infer those states by intentionally putting ourselves into somebody else’s shoes. More research will be needed to shed more light on the intricate interplay between low-level resonance mechanisms and higher-order affective and cognitive processes in mediating the empathic understanding of others’ and the occurrence of appropriate other-related emotional responses.