Elucidation of a lingual detection mechanism for high-viscosity solutions in humans

While perception of high-viscosity solutions (η > 1000 cP) is speculated to be linked to filiform papillae deformation, this has not been demonstrated psychophysically. Presently, just-noticeable-viscosity-difference thresholds were determined using the forced-choice staircase method and high-viscosity solutions (η = 4798-12260 cP) with the hypotheses that the tongue would be chiefly responsible for viscosity perception in the oral cavity, and that individuals with more, longer, narrower filiform papillae would show a greater acuity for viscosity perception. Subjects (n = 59) evaluated solutions in a normal, "unblocked" condition as well as in a "palate blocked" condition which isolated the tongue so that only perceptual mechanisms on the lingual tissue were engaged. Optical profiling was used to characterize papillary length, diameter, and density in tongue biopsies of a subset (n = 45) of participants. Finally, psychophysical and anatomical data were used to generate a novel model of the tongue surface as porous media to predict papillary deformation as a strain-detector for viscosity perception. Results suggest that viscosity thresholds are governed by filiform papillae features. Indeed, anatomical characterization of filiform papillae suggests sensitivity to high-viscosity solutions is associated with filiform papillae length and density (r = 0.68, p < 0.00001), but not with diameter. Modelling indicated this is likely due to a reciprocal interaction between papillae diameter and fluid shear stress. Papillae with larger diameters would result in higher viscous shear stress due to a narrower gap and stronger fluid-structure interaction, but a larger-diameter papilla would also deform less easily.