Somatotopic representation of the tongue in human secondary somatosensory cortex

Objective To clarify the somatotopic representation of the tongue secondary somatosensory cortex (SII) in humans. Methods Somatosensory evoked magnetic fields (SEFs) were recorded from nine subjects after stimulating four body sites, left antero (LA) and postero (LP) lateral margins of the tongue, left median nerve at the wrist (Hand), and left tibial nerve at the ankle (Foot). Results Clear neural activities were recorded from the bilateral SII in both hemispheres after the four sites were stimulated. The tongue SII for LA and LP was located close to the hand SII and significantly more anterior than the Foot SII. There was no significant difference in the location of dipoles between the LA and LP areas of the tongue SII. The mean peak latencies of the tongue SII for LA and LP were significantly shorter in the hemisphere contralateral to the stimulation than the ipsilateral hemisphere. Conclusions The tongue areas are considered to occupy a small region in SII with insufficient spatial separation to differentiate anterior from posterior areas even using magnetoencephalography which has a higher spatial resolution than electroencephalography (EEG). Significance This is the first systematical study to clarify the activated regions in SII following stimulation of the tongue.