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 o verall taste percept, much as do quality, intensity, and the temporal chara cteristics of tastes. The difficulty in eliminating the confounding factor of a tactile sensation may partially account for the paucity of such studie s, since a gustatory stimulus cannot be presented as a liquid without a tac tile component. As a step toward understanding the localizability of gustat ory sensations, we designed a yoked stimulator and an experimental procedur e to control for tactile cues. Lateral discrimination was evaluated at the tip of the tongue with four taste stimuli (sodium saccharin, sodium chlorid e, citric acid! and quinine hydrochloride) by presenting a taste and a blan k solution simultaneously at two locations on the tongue. We found that sub jects could lateralize all four taste stimuli in the absence of any discrim inative tactile cues. Subjects' ability to lateralize varied as a psychomet ric function of the stimulus concentration. Detection thresholds, measured in a forced-choice two-interval staircase procedure with the same yoked sti mulator that was used in the lateralization task, were always lower than la teralization thresholds, and both lateralization and detection thresholds w ere correlated within subjects. Subjects were unable to lateralize taste cu es on a nongustatory surface under the upper lip at the highest tested conc entrations, at which performance was 100% on a gustatory surface (dorsal an terior tongue). These results show that (1) taste compounds can be laterali zed in the absence of any discriminative mechanical cue (but only on the gu statory epithelium) and (2) although the localization of a compound does no t logically require conscious detection of the taste (cf. blind sight), sub jects always detected a taste when they were able to lateralize.