Primary somatosensory cortex and Hebbian associative learning: a novel cross-modal Paired Associative Stimulation (PAS) protocol

An increasing number of evidence suggests the existence of a Tactile Mirror System in the human brain: the same cortical network implicated in tactile perception, which comprises the primary somatosensory cortex (S1), also responds to the mere observation of tactile events. It has been 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 introducing a novel cross-modal Paired Associative Stimulation (PAS) protocol. In the cross-modal PAS, a visual stimulus depicting a hand being touched is repeatedly presented, paired with a Transcranial Magnetic Stimulation (TMS) pulse over S1. In experiment 1, we tested both the temporal and the cortical specificity of the plasticity effects induced by the protocol. Results showed that cross-modal PAS was effective in eliciting Hebbian associative plasticity, modulating subjects’ tactile acuity (measured with a 2-Point Discrimination Task) selectively when the Inter-Stimulus Interval (ISI) between the two paired stimulations was 20 ms and the TMS pulse is delivered over S1. In experiment 2, we looked for evidence for a selectivity of the visual responsiveness of S1 and the role of expectancy during the cross-modal PAS. We also aimed to uncover possible neurophysiological changes in S1 by pairing the measurement of subjects’ tactile acuity with the recording of Somatosensory-Evoked Potentials (SEPs). To sum up, our study provides novel insight of the visual activity of S1, showing that our cross-modal PAS can induce plastic changes in S1, in line with an Hebbian associative learning rule. This evidence also 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 (low-level) stages of sensory processing