Quantitative evaluation of the sensory disturbance of the tongue is important clinically. However, because the conventional electrophysiological approach to the peripheral nerve cannot be used in the mandible owing to the deep route of the lingual nerve, we applied evoked potentials in the central nervous system. Somatosensory evoked magnetic fields (SEFs) following electric stimulation were recorded in 10 healthy subjects by means of pin electrodes placed on the tongue mucosa. Three or four components (P25m, P40m, P60m, and P80m) were identified over the contralateral hemisphere with unilateral stimulation. Because none of the components were consistently detected in all subjects, we evaluated the root mean square (RMS) of 18 channels over the contralateral hemisphere. To estimate the activated cortical response, we calculated the difference in mean RMS amplitude between 10 and 150 ms and that of the baseline period (aRMS=RMS[10, 150]-RMS[-50, -5]). The aRMS values for right-sided and left-sided stimulation were 10.18+/-7.92 and 10.99+/-8.98 fT/cm, respectively, and the mean laterality index, expressed by [(left-right)/(left+right)] was 0.025+/-0.104. This parameter can be useful for evaluating patients with unilateral sensory abnormality of the tongue.

No abstract

Sophisticated tongue movements contribute to speech and mastication. These movements are regulated by communication between the bilateral cortex and each tongue side. The functional connection between the cortex and tongue was investigated using oscillatory interactions between whole-head magnetoencephalographic (MEG) signals and electromyographic (EMG) signals from both tongue sides during human tongue protrusion compared to thumb data. MEG-EMG coherence was observed at 14-36 Hz and 2-10 Hz over both hemispheres for each tongue side. EMG-EMG coherence between tongue sides was also detected at the same frequency bands. Thumb coherence was detected at 15-33 Hz over the contralateral hemisphere. Tongue coherence at 14-36Hz was larger over the contralateral vs. ipsilateral hemisphere for both tongue sides.Tongue cortical sources were located in the lower part of the central sulcus and were anterior and inferior to the thumb areas, agreeing with the classical homunculus.Cross-correlogram analysis showed the MEG signal preceded the EMG signal. The cortex-tongue time lag was shorter than the cortex-thumb time lag. The cortex-muscle time lag decreased systematically with distance. These results suggest that during tongue protrusions, descending motor commands are modulated by bilateral cortical oscillations, and each tongue side is dominated by the contralateral hemisphere.

Tongue movements contribute to oral functions including swallowing, vocalizing, and breathing. Fine tongue movements are regulated through efferent and afferent connections between the cortex and tongue. It has been demonstrated that cortico-muscular coherence (CMC) is reflected at two frequency bands during isometric tongue protrusions: the beta (β) band at 15-35 Hz and the low-frequency band at 2-10 Hz. The CMC at the β band (β-CMC) reflects motor commands from the primary motor cortex (M1) to the tongue muscles through hypoglossal motoneuron pools. However, the generator mechanism of the CMC at the low-frequency band (low-CMC) remains unknown. Here, we evaluated the mechanism of low-CMC during isometric tongue protrusion using magnetoencephalography (MEG). Somatosensory evoked fields (SEFs) were also recorded following electrical tongue stimulation. Significant low-CMC and β-CMC were observed over both hemispheres for each side of the tongue.Time-domain analysis showed that the MEG signal followed the electromyography signal for low-CMC, which was contrary to the finding that the MEG signal preceded the electromyography signal for β-CMC. The mean conduction time from the tongue to the cortex was not significantly different between the low-CMC (mean, 80.9 ms) and SEFs (mean, 71.1 ms). The cortical sources of low-CMC were located significantly posterior (mean, 10.1 mm) to the sources of β-CMC in M1, but were in the same area as tongue SEFs in the primary somatosensory cortex (S1). These results reveal that the low-CMC may be driven by proprioceptive afferents from the tongue muscles to S1, and that the oscillatory interaction was derived from each side of the tongue to both hemispheres. Oscillatory proprioceptive feedback from the tongue muscles may aid in the coordination of sophisticated tongue movements in humans.

No abstract