Homogeneity of thermal tasters and implications for mechanisms and classification

Thermal tasting, an important type of individual variation in orosensation, is a phenomenon by which some individuals perceive thermally-induced taste sensations simply by having the tip of their tongue warmed or cooled. These individuals, known as thermal tasters, report a variety of thermally-elicited tastes (typically sweet, sour, salty, bitter, metallic) and the tastes reported can vary with the temperature regime used (warming or cooling) and location on the tongue tested. Importantly, when compared to thermal non-tasters, thermal tasters are more responsive to aqueous solutions of basic tastants and to beverages. The mechanism(s) underlying thermal tasting are not well understood and it is unknown if the increased orosensory responsiveness of thermal tasters is universal or if it is driven by a subgroup of thermal tasters. Thermal taste data from 12 previous studies was combined to obtain a large sample of thermal tasters (n = 254) who were divided into subgroups based on the type of thermally-elicited taste reported and the temperature regime/location at which it was experienced. Sweet thermal tasters (n = 77) were 9 times more likely than non-sweet thermal tasters (n = 177) to experience thermally-elicited sensations during lingual warming (p < 0.0001). Similarly, sour thermal tasters (n = 89) were 8 times more likely than non-sour thermal tasters (n = 165) to report thermally induced tastes during cooling (p<0.0001). However, no differences in orosensory responsiveness based on these or other subgroups were identified, suggesting that the heightened orosensory responsiveness of thermal tasters may be centrally-mediated. We discuss how these findings inform our understanding of the mechanism(s) underlying thermal taste and the identification of thermal taste subgroups, along with practical implications of methodological differences in determining thermal taste status.