Tactile information from the human hand reaches the ipsilateral primary somatosensory cortex

Neuromagnetic responses to median nerve stimulation were studied in six healthy right-handed subjects. In the rest condition, only the right and left median nerves were alternately stimulated at the wrists. In two other conditions, continuous superficial tactile stimulation was concurrently applied to either the left or right hand. Tactile stimulation of palm and fingers of one hand enhanced, in the ipsilateral primary somatosensory cortex (SI), responses to median nerve stimulation of the other hand. This effect was stronger in the left than the right SI. Our data provide evidence in humans for the access of cutaneous information from the hands to ipsilateral SI, probably via excitatory transcallosal pathways. This interhemispheric information transfer may represent a neurophysiological substrate of somatosensory fusion between the hands.