A tool to induce cross-modal Hebbian-like plasticity within the primary somatosensory cortex

Background In recent years neuroscientific evidence highlighted the cross-modal, mirror-like, properties of the primary somatosensory cortex (S1): this area is activated not only by one’s own somatosensation, but also by the observation of tactile stimulation of another person’s body. We explored whether the activity of S1, and the associated tactile processing, could be enhanced by activating Hebbian plasticity-induction mechanisms through their visual, mirror-based, recruitment. To this aim, we developed a novel, cross-modal, Paired Associative Stimulation protocol (cm-PAS). Methods Cm-PAS consists in the repetitive, time-locked pairing of a visual stimulus depicting a hand being touched and a Transcranial Magnetic Stimulation (TMS) pulse over S1. In four experiments we tested the i) timing ii) cortical area iii) visual stimulation selectivity of the cm-PAS, and iv) the role of predictive coding mechanisms. Cm-PAS effects were assessed by measuring tactile acuity (behavioural index) and Somatosensory Evoked Potentials (SEPs, neurophysiological index in experiment 3). Results Results show an enhancement of tactile acuity only with strict delay between the TMS pulse and the visual touch stimulus; an increase of SEPs following only S1 stimulation paired with a visuo-tactile stimulus. Moreover, we found that preventing the anticipation of the visual touch changes the time-course of cortical-visual interactions.