Fig. 3.

An oral texture-sensing mechanism mediates the influence of fat on choices. (A) Rheology and tribology measurements. Rheology examines liquid flow to characterize stimulus viscosity; tribology quantifies lubrication and friction between moving surfaces as the CSF. We measured viscosity by testing the liquids in a rotational rheometer (Left). We measured CSF using a custom-designed tribometer (Right) with biological tissue (fresh pig tongues) mimicking oral surfaces. We loaded 30 mL of testing liquid between pig tongues pulled by a slider controlled by an Instron Testing System from tongue base (posterior) toward tongue tip (anterior) at constant velocity (v = 16 mm/s). (B) Sliding friction measurements. Curves show CSFs for liquids with increasing fat content, measured from a single tongue (three repetitions per stimulus). CSF was normalized to the coefficients measured with water. (C) Viscosity and CSF as a function of fat content in our stimulus set for the two flavors (orange: peach and purple: blackcurrant; linear regressions). Measurements are shown for 13 stimuli: the four factorial stimuli with each of the two flavors, diluted cream with each flavor, diluted fruit juices, and water. (D) Mediation analysis. The aggregated influence of nutrient content on choices (total effects, path c) was decomposed into indirect effects mediated by texture (path a, path b) and direct effects (path c’). Mediation effects were significant if texture parameters replaced regression coefficients of the total effects ($c^{\text{'}}=c-b$). (E) Mediation effects of viscosity and CSF on the influences of fat and sugar on choices. Logistic regressions included fat and sugar regressors (“nutrient”) or additional viscosity (“+ Viscosity,” brown) or CSF (“+ Friction,” orange) regressors. (F) Path diagrams describing correlational relationships between nutrient content, texture parameters, and choices. Because the effect of fat on choices was fully accounted for by CSF (all animals) and viscosity (animal Ya), we included only the direct sugar effect. Significance of path coefficients derived from bootstrap (1,000 iterations).