Subjective assessment of liquid volumes by humans during swallowing

To test the hypothesis that during drinking the appreciation of a volume of fluid in the mouth (VM) is important in determining the volume swallowed (VS), we assessed how well subjects recognized different volumes taken into the mouth and swallowed. Five volumes of tap water (5-25 cm3) were drunk in random order with the eyes closed and the subjects assessed these by selecting from an assortment of cylindrical blocks those with volumes which were thought to correspond visually or manually to the test volumes. Two known reference volumes and cylinders each of 15 cm3 were included in the test. Scores for total error magnitude, correct responses and bias showed that subjective assessment of volumes, though poor, was reproducible and that a student group performed better than children and elderly subjects who consistently underestimated VM. Of the factors studied which might contribute to perception of VM, the time taken for water to enter the mouth, but not the interval before swallowing, seemed most important. Raising the water temperature to 37 degrees C, surface anaesthesia of tongue and palate, and sipping as opposed to gulping, did not alter scores in students. The mean VS in unrestricted drinking was 12.6 cm3 for 44 students; repeat measurements in an individual varied by only about 11 per cent.