Cross-modal properties of the primary somatosensory cortex: a by-product of Hebbian association learning.

Background: A growing body of evidence suggests the existence of a Tactile Mirror System in the human brain: the observation of tactile events activates the same cortical network implicated in tactile perception, including the primary somatosensory cortex (S1). It has been suggested that such crossmodal, mirror-like, responses of S1 may arise from Hebbian association learning plasticity: the contingency of seeing a touch and the feeling of a tactile sensation may reinforce synapses between visual and somatosensory neurons. We tested this hypothesis with a novel crossmodal Paired Associative Stimulation (PAS). Methods: In the crossmodal PAS, a visual stimulus depicting a hand being touched is repeatedly presented, paired with a Transcranical Magnetic Stimulation (TMS) pulse over S1. We investigated the efficacy of this protocol in inducing timing-dependent crossmodal plasticity, assessed in terms of changes of tactile acuity. Eighteen healthy participants took part in a within-subjects experiment: the inter-stimulus interval (ISI) between the seen touch and the TMS pulse varied across PAS sessions. Results: A significant improvement in tactile acuity was induced by the cross-modal PAS only at a selective (20 ms) ISI. The present result shows that associative plasticity within S1 can be effectively induced through vision, highlighting the existence of crossmodal plastic mechanisms.