Hebbian associative plasticity in the visuo-tactile domain: a cross-modal paired associative stimulation protocol

We developed and assessed the effects of a novel cross-modal protocol aimed at inducing associative (Hebbian-like) plasticity in the somatosensory cortical system through vision. Associative long-term potentiation can be induced in the primary somatosensory cortex (S1) by means of paired associative stimulation (PAS), in which a peripheral electrical stimulation of the median nerve is repeatedly paired with a transcranial magnetic stimulation (TMS) pulse over S1. Considering the mirror proprieties of S1, the cross-modal PAS (cm-PAS) consists of repetitive observation of bodily tactile stimulations, paired with TMS pulses over the contralateral S1. Through three experiments in healthy participants, we demonstrate that the cm-PAS is able to induce excitatory plastic effects with functional significance in S1, improving somatosensory processing at both behavioral (tactile acuity) and neurophysiological (somatosensory-evoked potentials) levels. The plastic effects induced by cm-PAS depend on the interval (20 m s) between the visual stimulus and the magnetic pulse, the targeted cortical site (S1), and the tactile content of the visual stimulus, which must represent a touch event. Such specificity implies the recruitment of cross-modal, mirror-like, mechanisms in S1, which are able to visually promote associative synaptic plasticity in S1 likely through the recruitment of predictive coding processes