NaCl thresholds in man: Thresholds for water taste or NaCl taste?

Water is not tasteless to man. Water can produce any of the 4 basic taste qualities if it is preceded by adaptation to an appropriate substance. After adaptation to the NaCl in saliva, water tastes predominantly bitter. In detection threshold experiments, this bitter water taste can be confused with solute tastes so that some detection thresholds for NaCl may actually represent water thresholds instead. Water also appears to have a taste after adaptation to saliva in the rat. Just as with man, some apparently low thresholds for NaCl detection in rat may be water-taste thresholds. The apparently high preference threshold for NaCl in rat may be a true NaCl threshold based on adaptation to saliva. The scaling of suprathreshol d NaCl in man provides insights into NaCl thresholds. Briefly, the taste of moderate NaCl concentrations disappears after constant stimulation for about 30 sec. (Abrahams, Krakauer, & Dallenbach, 1937; Bartoshuk, 1968; Bartoshuk, McBurney, & Pfaffmann, 1964; Hahn, 1934; McBurney & Pfaffmann, 1963). After such adaptation, concentrations higher than the adapting concentration still taste salty; however, concentrations lower than the adapting concentration are altered in quality and taste bitter-sour (Bartoshuk, 1968; Bartoshuk et al., 1964; McBurney, 1969a; McBurney & Shick, 1971). The bitter-sour taste is maximal when the test solution is water. This is one example of a general phenomenon that occurs across species: The taste of water changes as a function of the nature of the substance that precedes it on the tongue (Bartoshuk, 1968, 1972; Bartoshuk, Harned, & Parks, 1971; Bartoshuk, Lee, & Scarpellino, 1972; McBurney, 1969a; McBurney & Bartoshuk, 1972). These observations are important for behavior because saliva contains appreciable concentrations of Na and Cl ions: That is, saliva can act as an adapting solution and thereby alter