Oral Thresholds and Suprathreshold Intensity Ratings for Free Fatty Acids on 3 Tongue Sites in Humans: Implications for Transduction Mechanisms

Multiple putative free fatty acid (FFA) transduction mechanisms have been identified in the oral cavity. They reportedly differ in their distribution on the tongue and each has a unique range of ligand specificities. This suggests that there should be regional differences in sensory responses to varying FFAs. This was assessed through spatial testing with caproic (C), lauric (L), and stearic (S) FFAs among 35 healthy adults. Stimuli were applied to the fungiform (FU), foliate (FO), and circumvallate (CV) papillae with a cotton-tipped applicator. Oral detection thresholds were measured by an ascending, 3-alternative, forced-choice, sip and spit procedure. Intensity ratings were obtained on the general labeled magnitude scale. Nongustatory cues were minimized by testing with the nares blocked, eyes covered, and by masking tactile cues with the addition of gum acacia and mineral oil to the stimuli vehicle. Thresholds were obtained from nearly all individuals at each site, and the concentration was similar across the 3 FFAs. Absolute intensity ratings differed significantly with C > L > S overall and at the CV and FO papillae. At the FU papillae, the L and S ratings were comparable. Ratings were highest at the FU followed by the CV and then the FO papillae. Slopes of the concentration-intensity rating functions were higher for L compared with C and S at the CV papillae as well as both L and C compared with S at the FO papillae. However, overall, slopes were comparable across sites. These findings strengthen evidence for oral FFA perception in humans by replicating threshold sensitivity findings and documenting monotonic scaling ability for these stimuli. Further, they challenge current views on transduction as sensory responsiveness was observed at tongue sites not predicted to support FFA detection.