Hideto Okazaki’s research while affiliated with Johns Hopkins University and other places

Objective: During the mastication of solid food, the tongue pushes the bolus laterally to place it onto occlusal surfaces as the jaw is opened. This movement is referred to as tongue-pushing (TP). TP has an important role in efficient chewing, but its kinematic mechanisms remain unclear. The present study quantified the kinematics of TP and its coordination with masticatory jaw movements. Methods: Videofluorography (VFG) in anteroposterior projection was recorded while 14 healthy young adults ate 6 g each of cookies and meat. Small lead markers were glued to the tongue surface (left, right, and anterior) and buccal tooth surfaces (upper molars and lower canines). The position of the tongue and lower canine markers relative to the upper occlusal plane was quantified with Cartesian coordinates, using the right upper molar as the origin. Jaw motion during chewing was divided into TP and Non-TP cycles, based on the lateral movement of the food and tongue markers. The side of the jaw that compressed food particles was defined as the working side, while the other side was termed the balancing side. Horizontal and vertical displacements of tongue and jaw markers were compared between TP and Non-TP cycles, as well as between food types. Results: The mediolateral displacement of all tongue markers was significantly larger in TP than in Non-TP cycles. Vertical displacement was also significantly greater in TP than in Non-TP cycles for the anterior and working side tongue markers. TP cycles occurred more frequently with meat-chewing than with cookie-chewing. Conclusion: TP is accomplished by rotation and lateral movements of the tongue surface on the working side and the anterior tongue blade, along with medial movement on the balancing side. These movements produce lateral shift and rotation of the tongue surface toward the working side in concert with jaw opening. Designing exercises to improve the strength of the lateral motion and rotation of the tongue body may be useful for individuals with impaired tongue function for eating and swallowing.

Few studies have reported the activation sequence of the swallowing muscles in healthy human participants. We examined temporal characteristics of selected hyoid muscles using fine wire intramuscular electromyography (EMG). Thirteen healthy adults were studied using EMG of the anterior belly of digastric (ABD), geniohyoid (GH), sternohyoid (SH), and masseter (MA, with surface electrodes) while ingesting thin liquid, banana, tofu, and cookie (3 trials each). Onset timing was measured from rectified and integrated EMG. Data were analyzed using repeated-measures ANOVA with Bonferroni correction. When drinking thin liquid, MA, GH, and ABD were activated almost simultaneously, but SH was activated later (using GH onset as 0 s, MA -0.07 (-0.20 to 0.17) second [median (interquartile range)]; ABD 0.00 (-0.10 to 0.07) second; SH 0.17 (0.02 to 0.37) second; P < 0.01). With solid foods, MA contraction preceded GH and ABD; SH was last and delayed relative to liquid swallows (GH 0 s; MA -0.17 (-0.27 to 0.07) second; ABD 0.00 (-0.03 to 0.03) second; SH 0.37 (0.23 to 0.50) second; P < 0.01). The role of the MA differs between solids and liquids so the variation in its timing is expected. The synchronous contraction of GH and ABD was consistent with their role in hyolaryngeal elevation. The SH contracted later with solids, perhaps because if the longer duration of the swallow. The consistent pattern among foods supports the concept of a central pattern generator for pharyngeal swallowing.

When chewing solid food, part of the bolus is propelled into the oropharynx before swallowing; this is named stage II transport (St2Tr). However, the tongue movement patterns that comprise St2Tr remain unclear. We investigated coronal jaw and tongue movements using videofluorography. Fourteen healthy young adults ate 6 g each of banana, cookie, and meat (four trials per foodstuff). Small lead markers were glued to the teeth and tongue surface to track movements by videofluorography in the anteroposterior projection. Recordings were divided into jaw motion cycles of four types: stage I transport (St1Tr), chewing, St2Tr, and swallowing. The range of horizontal tongue motion was significantly larger during St1Tr and chewing than during St2Tr and swallowing, whereas vertical tongue movements were significantly larger during chewing and St2Tr than during swallowing. Tongue movements varied significantly with food consistency. We conclude that the small horizontal tongue marker movements during St2Tr and swallowing were consistent with a "squeeze-back" mechanism of bolus propulsion. The vertical dimension was large in chewing and St2Tr, perhaps because of food particle reduction and transport in chewing and St2Tr.