Psychophysics of taste lateralization on anterior tongue.

There have been very few investigations of the spatial properties of taste stimuli localized to specific areas of the oral cavity. This is surprising, since the spatial localization of taste sensations may contribute to the overall taste percept, much as do quality, intensity, and the temporal characteristics of tastes. The difficulty in eliminating the confounding factor of a tactile sensation may partially account for the paucity of such studies, since a gustatory stimulus cannot be presented as a liquid without a tactile component. As a step toward understanding the localizability of gustatory sensations, we designed a yoked stimulator and an experimental procedure to control for tactile cues. Lateral discrimination was evaluated at the tip of the tongue with four taste stimuli (sodium saccharin, sodium chloride, citric acid, and quinine hydrochloride) by presenting a taste and a blank solution simultaneously at two locations on the tongue. We found that subjects could lateralize all four taste stimuli in the absence of any discriminative tactile cues. Subjects' ability to lateralize varied as a psychometric function of the stimulus concentration. Detection thresholds, measured in a forced-choice two-interval staircase procedure with the same yoked stimulator that was used in the lateralization task, were always lower than lateralization thresholds, and both lateralization and detection thresholds were correlated within subjects. Subjects were unable to lateralize taste cues on a nongustatory surface under the upper lip at the highest tested concentrations, at which performance was 100% on a gustatory surface (dorsal anterior tongue). These results show that (1) taste compounds can be lateralized in the absence of any discriminative mechanical cue (but only on the gustatory epithelium) and (2) although the localization of a compound does not logically require conscious detection of the taste (cf. blind sight), subjects always detected a taste when they were able to lateralize.