Tongue-Palate Interaction in Discrete and Sequential Swallowing

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Historically, swallowing motor control was thought to involve a central mechanism that generated patterned responses with little use of sensory input. Although increasing evidence of peripheral modulation has altered this concept, our knowledge about the flexibility in deglutitive motor control and performance is incomplete. This study sought to gain a better understanding by examining lingual motor strategies in light of changing bolus properties (volume, consistency) and task demands (discrete vs. sequential swallowing). Specifically, the timing and patterns of tongue-palate contact and the associated changes in tongue shape and action were examined in five normal adults using simultaneous electropalatography (EPG) and ultrasound. Tasks for discrete swallowing included 5 and 30 cc of water, 5 and 30 cc of gelatin, and saliva. Tasks for sequential swallowing involved drinking 200 cc of water at normal and fast rates. Two analysis schemes were used to make timing and percent-contact measurements: segmentation of the EPG time series into four stages (prepropulsion, propulsion, full contact, withdrawal), and compartmentalization of the pseudopalate into six bins (front, central, back, lateral, medial, midline). Results showed little variation in contact pattern as a function of bolus property or subject, suggesting considerable stereotypy in lingual motor strategies for movement sequencing. However, unlike the conventional description, tongue-palate contact during propulsion was multidimensional with two distinct degrees of freedom in the front-to-back and the lateral-to-midline continua. Significant (Q<. 0 I) timing differences were found in that larger and thinner boluses were propelled faster than smaller and thicker ones, and dry swallows had longer full contact than water. For sequential swallowing during continuous drinking, the tongue used faster movement speed and overlapping gestures to meet the task demands, while propulsive contact pattern remained invariant. Thus, the change was not in motor strategies per se but in the timing coordination of the "drink" and "swallow" sequences. A 3-D model of oral lingual action for swallowing was proposed. Clinical implications were discussed. In sum, results of this study support the theory that swallowing motor control includes a peripheral mechanism capable of modulating centrally generated responses, and that the deglutitive motor program has both invariant and variant parameters.