P121 The neurophysiological underpinnings of mirror-touch synaesthesia: A TMS-EEG study

Abstract

Introduction Mirror-touch synaesthesia (MTS) is the condition in which the view of a touch delivered to another person’s body (but not to objects) elicits subjective conscious tactile sensations on own body. At a neurophysiological level, MTS has been associated to an unusual hyper-activation of the tactile mirror system (Blakemore et al., 2005; Ward and Banissy, 2015). Recent studies showed that increasing the excitability of primary somatosensory cortex (S1), by mean of non-invasive brain stimulation techniques, induces MTS-like responses even in non-synesthetes in a vision-touch spatial congruity task (Bolognini et al., 2013, 2014). However, it is not clear yet whether synaesthesia-like sensations were induced through direct modulation of S1, or by modulating the functional interplay between SI and other connected brain regions. Objectives This study aims at clarifying this issue by exploring changes in S1 cortical excitability and connectivity by means of TMS-EEG technique. Materials and methods 10 healthy volunteers participated to the study. 4 TMS-EEG recordings were performed; in each 180 TMS single pulses were delivered to right S1 while participants were exposed, in a random order, to three conditions: (i) experiencing a real touch on the left hand (touch), the view of video-clips depicting either (ii) a touch to another person’ hand (mirror-touch) or (iii) a touch to a leaf (control stimulus). Concomitant EEG was recorded by a 60 channels cap. TMS pulses were delivered at two SOAs: 50 ms or 150 ms from stimulus onset. Results In the touch condition, TMS at 50 ms elicited greater TEPs than at 150 ms. Conversely, in the mirror-touch condition, TEPs increased at 150 ms as compared to 50 ms SOA. No difference across SOAs were present instead for TEPs in the control condition. Comparing the three conditions, the greatest mean TEPs on right S1 occurred in the touch condition, followed by the mirror-touch, both eliciting greater TEP than the control condition. Conclusions Preliminary results suggest a rise of S1 cortical excitability at 50 ms SOA in the touch condition, i.e. when the tactile stimulus processing is taking place, and, crucially, at 150 ms SOA in the mirror-touch condition. Download : Download high-res image (185KB) Download : Download full-size image Download : Download high-res image (128KB) Download : Download full-size image