Contralateral and ipsilateral responses in primary somatosensory cortex following electrical median nerve stimulation—an fMRI study

Objective Ten healthy adult subjects were examined using functional magnetic resonance imaging (fMRI) to investigate responses in the contralateral and ipsilateral primary somatosensory cortex (SI) following electrical stimulation of the median nerve. Methods The right and left median nerves were stimulated alternately at the wrist in the different sessions. First, the location of the response in contralateral SI was identified following median nerve stimulation, and then, a spherical search volume with a 10 mm radius centered on the region of the contralateral response was determined. Whether or not fMRI activation occurred within this sphere following ipsilateral stimulation was examined using a 3 T MR imager. Results A response in contralateral SI was observed in 8 of the 10 subjects in right and left hemisphere. Responses in ipsilateral SI were observed in 6 of 8 subjects in right hemisphere, and the region of the response tended to be posterior to the contralateral region. On the other hand, in left hemisphere, the ipsilateral responses were found in three. Conclusions In the present study, not only contralateral SI but also ipsilateral SI was activated following median nerve. The location of the ipsilateral activation was significantly more posterior than the contralateral one in right hemisphere. Significance The region of activation in ipsilateral SI was located in the posterior portion of post central gyrus, corresponding to around BA2 and 5 in human.