Tactile acuity as an index of plasticity induced by a novel cross-modal PAS protocol

In the past few years, an increasing number of studies have shown the existence of a Tactile Mirror System in the human brain. This system relies on a shared network of brain areas responding to both the experience of a tactile sensation and to the observation of the same tactile sensations experienced by others. One crucial area of this system is the primary somatosensory cortex (S1). A hypothesis put forward is that the cross-modal responses of the Tactile Mirror System could be a by-product of Hebbian associative learning: the contingency between a seen touch and a felt touch on one’s own body may reinforce the synapses between somatosensory and visual neurons. We developed a novel cross-modal paired associative stimulation (cmPAS) protocol to test the hypothesis that the mirror-like responses of S1 may arise from Hebbian associative plasticity mechanisms. In this new protocol, a Transcranial Magnetic Stimulation (TMS) pulse over S1 is repeatedly paired with the view of a hand being touched. The successful modulation of S1 reactivity by the cmPAS was assessed at a behavioral level, measuring changes in the observer’s tactile acuity as tested with a 2-point discrimination task (2PDT). In the first experiment, we investigated the efficacy of the cmPAS and the temporal specificity of the Hebbian plasticity mechanisms induced by it by varying the inter-stimulus intervals (ISIs) between the TMS pulse and the visual stimuli. In the second experiment, we examined the cortical specificity of our protocol applying TMS also over the primary visual cortex (V1). We found that cmPAS was effective in decreasing participants’ sensory threshold, specifically when the ISI between the visual stimulus and the TMS pulse was of 20 ms, and only when TMS was applied over S1. In the third experiment, we investigated the specificity of the visual stimulus in eliciting changes in the sensory threshold: we assessed the effect of a control condition in which TMS pulses were paired with a picture depicting a static hand, that was not touched. Crucially, results showed that the cmPAS protocol induced tactile enhancement only when the visual stimulus paired with the TMS pulse depicted a touch. These results highlight the central role of mirror-touch mechanisms in the success of the cmPAS. Taken together these findings prove the effectiveness of the cmPAS in modulating participants’ tactile acuity as a consequence of the effective mirror-like recruitment of S1.