Cross-enhancement of the sour taste on single human taste papillae.

The subjective intensity of one taste quality can be increased by prior exposure of the tongue to a different taste quality stimulus. This phenomenon, called cross-enhancement, may be the result of interactions among the physiological mechanisms that code taste quality. Another possible explanation is that the water solvent of the second stimulus acquires a taste after exposure of the tongue to the first stimulus. This water taste could add to the taste of the solute in the second stimulus and result in an increase of its subjective intensity. A third possibility is that taste receptors on the tongue may be sensitized by exposure to a taste stimulus. Using a small number of highly trained subjects, we have demonstrated that sucrose can enhance the intensity of an acid taste on the single papilla. Neither water taste nor sweet taste system activation played any role in the mediation of this enhancement. Through a series of experimentally derived inferential steps, we conclude that this phenomenon depends on the removal of protons from the acid receptors. In addition, we have demonstrated in the single papilla, that suppression of the acid taste when in mixture with sucrose can occur without sweet system activity. We conclude that sugars, through their capacity to bind protons, act to reduce the availability of protons to the acid receptors.