I know how you feel coding others' somatosensory experience in the observer 's somatosensory cortex

My doctoral thesis aims at exploring the role of the somatosensory cortices in the visual coding of others’ tactile experiences. Several studies posit the existence in the human brain of a system which match the somatosensory and visual experience of touch (for a review see, Keysers et al., 2010); in particular, the neural network involved in first-hand tactile stimulation is also responsible for understanding others’ somatic sensations. Firstly, by using online high-frequency repetitive transcranial magnetic stimulation (rTMS) I assessed whether the first (SI) and the second (SII) somatosensory cortices play a functional role in the visual processing of tactile events (Experiment 1). Healthy participants performed a discrimination task of visual stimuli depicting touch (a finger touching a hand) and a control task, with the visual discrimination of movements, not comprising a tactile component (the movement of a finger). rTMS over SI selectively impairs subject’s ability to discriminate visual stimuli depicting a tactile event, suggesting that SI, a cortical area traditionally viewed as modality‐specific, is implicated in the visual processing of touch. Instead, SII is not involved in the visual discrimination of touch. Then, I assessed whether the visual processing of touch in SI is specific for the view of human to human contact, or it applies to the sight of ‘any’ touch (Experiments 2 and 3). Using the same rTMS paradigm, I show that in healthy subjects interfering with SI activity specifically impairs the visual detection of the human touch, without affecting the visual perception of contact between objects, nor between human body‐parts and objects. Experiment 4 investigated whether SI is also involved in understanding others' sensations conveyed by tactile events, and whether this mechanism shows hemispheric specialization. Healthy subjects underwent a picture‐based affective go/no‐go task while receiving offline low‐frequency rTMS to the right or left SI, or the right or left dorsolateral prefrontal cortex (DLPF); DLPF was chosen as active control site as it was shown to be involved in encoding the affective valance of emotional pictures (Bermpohl et al., 2005). Disruption of the right, but not left, SI activity by rTMS selectively reduces participants' performance, but only when the affective state is conveyed by touch; intriguingly, this interfering effect is associated with individual empathic ability to adopt others’ subjective perspective. Then, the same task was given to a group of brain-damaged patients, to determine if specific brain lesions were associated with impaired recognition of the emotional valance of a visually presented somatic experience (Experiment 5). The main finding is that lesions affecting the right hemisphere are associated with a poorer performance in the affective go/no-go task, regardless of the visual tactile component. Finally, I explored the neural underpinnings of mirror-touch synaesthesia (Experiment 6). When subjects with mirror‐touch synaesthesia view a tactile stimulation on others they also feel the same somatic sensation on their own body, even in absence of a real touch (Blakemore et al., 2005). By using a facilitatory paired‐pulse transcranial magnetic stimulation (ppTMS) protocol, I show that mirror‐touch responses and synaesthesia‐like sensations can be induced even in non‐synaesthetes by increasing the excitability of SI, or by boosting its activity via ipsilateral posterior parietal cortex (PPC). Functionally connectivity between ipsilateral premotor cortex and SI is not involved in mirror-touch synaesthesia. Again, synaesthetic–like responses in non-synaesthetes are associated with different in emphatic abilities. Overall, this series of studies demonstrates that: I) SI is causally involved in processing the sight of human‐to-human contacts (Experiments 1-3); II) besides being involved in low-level visual processing of touch, SI of the right hemisphere participates in higher-level functions related to the encoding the affective valance of others’ touch (Experiments 4,5); III) a right-hemisphere lesion may impair patient’s ability of understanding others’ somatosensation (Experiment 6); IV) the vicarious activation of SI by the sight of touch is associated to individual differences in affective and cognitive empathy (Experiments 4-6); V) synaesthesia‐like mirror‐touch sensations can be induced through the enhancement of SI activity, or by boosting its activity via PPC (Experiment 6).