Oral Structure Representation in Human Somatosensory Cortex –A magnetencephalographic study

Objectives: To determine the precise mapping of the functional topographic organization of the representation areas in the whole intraoral structures, and to elucidate the bilateral neuronal projection feature on the S1 cortex, we recorded somatosensory-evoked magnetic fields (SEFs) reflecting cortical earliest responses induced by pure tactile stimulation using magnetoencephalography and a newly developed piezo-driven tactile stimulation device.

Methods: SEFs were recorded by whole-head magnetoencephalography in 10 right-handed healthy adults on stimulation of each of 6 sites on the oral mucosa: superior/inferior buccal, anterior/posterior tongue, and upper/lower lip mucosa, with the index finger as a control.

Results: Following tactile stimulation of six sites of oral mucosa, SEFs with peak latency of 15 msec (1M) reflecting initial cortical responses were identified in bilateral hemispheres. In contrast, SEF with peak latency of 30 msec following right index finger tactile stimulation was identified only in the contralateral hemisphere. Equivalent current dipoles (ECDs) generating 15 msec components were found along the posterior wall of the central sulcus bilaterally. These ECD locations for oral mucosa representing areas located inferiorly to those for the index finger, with the following pattern of organization from top to bottom along central sulcus: index finger, upper or lower lip, anterior or posterior tongue and superior or inferior buccal mucosa, with wide distribution having 30% occupancy in the S1 cortex. The values of source strength for 1M in the ipsilateral hemisphere were smaller than those in the contralateral hemisphere.

Conclusions: The result of this study clearly indicated that sensory afferents innervating the intraoral region project to both contralateral and ipsilateral S1 cortexes via the trigeminothalamic tract where contralateral projection is predominant. In addition, the present study provides detailed topographic organization of the representing areas of whole intraoral structures in the S1 cortex by adding these areas to the classical “sensory homunculus”.