Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, PC104
Regional variation in glutamate taste recognition thresholds on the human tongue.
L. F. Donaldson1, L. Bennett1, L. Rooshenas1, B. Feakins1, E. K. Richardson1, N. Jones1, C. Kenyon1, V. Smith1, S. Raichura1, J. K. Melichar1
1. Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom.
Many textbooks continue to publish the human tongue map, in which specific regions of the tongue are denoted as being the area in which the four basic tastes, salt, sour, sweet and bitter, are detected, despite the fact that this map was shown to be incorrect 30 years ago (1). Although there are regional differences in sensitivity for detection of different taste modalities on the tongue, in general the four well-known taste modalities can be detected in all areas of the tongue, in addition to the soft palate and oropharynx. Taste recognition is also related to age in that in the elderly population, taste generally becomes blunted with increasing age. Much less is known about the fifth taste, umami, the taste of glutamate. In this study we have investigated possible regional variations in sensitivity for glutamate taste detection across the human tongue, and the relationships between glutamate recognition threshold and 1) age, and 2) perception of taste intensity. Monosodium glutamate (MSG) recognition thresholds were determined in 34 healthy volunteers (age range 19-71; 6 male 28 female) at the tip (n=34), the sides (n=7) and the back (n=19) of the tongue. Solutions of MSG ranging in concentration from ~180mM to ~300µM were applied to each region of the tongue, each concentration a minimum of 5 times. Subjects indicated whether or not they could recognise the taste stimulus at each concentration. MSG thresholds were calculated for each region using psychophysical taste function curves. Taste intensity of a supra-threshold stimulus (1M) was measured using a 150mm generalised labelled magnitude scale anchored at “barely detectable” and “strongest imaginable sensation of any kind”. Data shown are mean ± SEM. The back and sides of the human tongue had similar MSG recognition thresholds, with only slight differences between the regions (back, 14±3mM; left, 25±9mM; right 26±10mM; ns; Kruskal Wallis + Dunn’s post-hoc). The MSG recognition threshold at the tip of the tongue was significantly greater than that at the back of the tongue (tip, 60±13mM, p<0.01, Dunn’s) than the tip of the tongue. There was no relationship between age and MSG threshold, but there was a significant negative correlation between MSG threshold and intensity ratings (p=0.03, r=-0.4, n=30, Spearman rank correlation). There is no significant drop in MSG sensitivity with increasing age in young/middle aged adults. The perceived intensity of MSG taste is related to the recognition threshold - the lower the threshold, the more intense the taste. There are regional differences in MSG taste threshold across the human tongue, but MSG could be tasted in all regions tested. The greatest difference in MSG recognition thresholds exists between the tip and the back of the tongue.
Where applicable, experiments conform with Society ethical requirements