Preferred-chewing-side-dependent Two-point Discrimination and Cortical Activation of Tactile Tongue Sensation

Objectives: The purpose of this study was to investigate 1) whether the sensory threshold of the tongue differed according to the side, and 2) whether the pattern of hemispheric cortical activation by tactile tongue stimulation differed, with special attention to the preferred chewing side (PCS). Methods: Ten healthy adults participated in the study. The PCS was determined with the use of a mandibular kinesiograph. Experiment 1 (behavioral study): The mean thresholds of the two-point discrimination (TPD) of the anterior, canine and posterior regions on both sides of the tongue, and those between PCS and non-PCS in each region were statistically compared. Experiment 2 [functional magnetic resonance imaging (fMRI) study]: Tactile stimulation was delivered to the either side of the tongue with acrylic balls via a mandibular splint. The runs were measured with the use of a T2*-weighted gradient echo-type echo planar imaging sequence in a 1.5T scanner. Activated voxel numbers in the bilateral S1 were statistically compared. Results: Experiment 1: The threshold of TPD increased in order of the anterior, canine and posterior regions. Moreover, it was significantly smaller on the PCS than the non-PCS in both the canine and posterior regions. Experiment 2: There was a marked difference in the activated pattern localized in SI contralateral to the stimulated side; a broader activation area with greater fMRI signal intensity in S1 contralateral to the PCS was found. The activated voxel numbers in S1 contralateral to the PCS was significantly greater than that in S1 contralateral to the non-PCS. Conclusion: The present study shows that the PCS is associated with the asymmetric tactile sensation and cortical activation of the tongue. The sensory acuity of the tongue on the PCS may play an important role in a functional coupling between the jaw and tongue to maximize the efficiency of chewing.