Modulating the response of the primary somatosensory cortex with a novel Paired Associative Stimulation protocol

In the last few years, the existence of a Tactile Mirror System in the human brain has been hypothesized: in this system, the same cortical areas implicated in tactile perception, namely the primary somatosensory cortex (S1), also respond to the mere observation of tactile events. In a seminal work, Keysers and Gazzola (2014) suggested that such cross-modal, mirror-like, responses of S1 may arise from Hebbian associative plasticity: the contingency of seeing a touch and the feeling of a tactile sensation on one’s own body may reinforce synapses between visual and somatosensory neurons. We tested this hypothesis by developing a novel cross-modal Paired Associative Stimulation (cmPAS) protocol. In the cmPAS, a visual stimulus (instead of the standard peripheral stimulation) depicting the palm of a left hand being touched is repeatedly presented, paired with a Transcranial Magnetic Stimulation (TMS) pulse over right S1. In Experiment 1, we tested the temporal and the cortical specificity of the plasticity effects induced by the cmPAS. Results show that the cmPAS is effective in eliciting Hebbian Long-Term Potentiation (LTP), modulating tactile acuity (measured by means of a 2-Points Discrimination Task) at the observer’s hand. Importantly, the effect is present only when the Inter-Stimulus Interval (ISI) between the two stimuli is 20 ms and the TMS pulse is delivered over S1. In Experiment 2, we aimed to uncover possible neurophysiological changes in S1 by combining the measurement of subjects’ tactile acuity with the recording of Somatosensory Evoked Potentials (SEPs). We also looked for evidence for a selectivity of the visual responsiveness of S1 by replacing the visual stimulus of a touch with one of a static hand (i.e., no-touch condition). Besides confirming the improvement of tactile acuity, preliminary results from the analysis of SEPs showed an increase of the SEPs’ P40 component peak after the delivery of the cmPAS. Consistently with the literature on the Tactile Mirror System, both these findings were present only when the visual stimulus depicted a touch. To sum up, our study provides novel insight of the cross-modal, visual, responsiveness of S1, showing that our cmPAS can induce plastic changes in S1 that follows the Hebbian associative learning rule. Such changes were detected both at a behavioral and at a neurophysiological level. This evidence offers new insights on the neural substrates of the Tactile Mirror System and, in a broader perspective, of early visuo-tactile interactions in the primary stages of somatosensory processing.