Acid and NaCl self-adaptation with micro-drop stimulation of fungiform papillae.

A small drop of either citric acid, HCl, or NaCl, judged to be approximately equal in magnitude, was applied to two papillae for a fixed duration (15 sec). The stimulus was then reapplied to the same papillae and judged for magnitude of sensation ("self-adaptation" design). NaCl was about 50% less intense after 15 sec of prior exposure whereas citric acid showed no significant decline and HCl showed either no decline or a significant increase in magnitude. When citric acid was tested with a longer exposure time (55 sec), significant decline in citric acid magnitude occurred for all subjects. These outcomes are discussed with regard to the problem of cutaneous irritation caused by certain stimuli and the difficulty of extracting cutaneous sensations from the sensory experience of strong concentrations of acids and salts.