Taste Thresholds and Intensity

Perceptual scientists often study sensory responses at two levels: responses to weak and to strong stimuli. The simplest form of weak stimulus study is the measurement of absolute detection threshold. In taste research, this is the lowest level of a sapid chemical that can be distinguished from pure water. Threshold concentrations for taste stimuli range from micromolar (μmol) levels for [Page 982]many toxic compounds, such as quinine (a commonly experienced bitter drug and beverage fla-vorant) to low millimolar (mmol) ranges for NaCl (a salty stimulus), citric acid (a sour stimulus), and glutamate (a savory stimulus), and even to higher millimolar range for some sugars (sweet stimuli). The simplest form of a strong stimulus study is the measurement of sensation magnitude with questions like, “How strong is the salty taste?” This entry discusses the relation between taste thresholds and perceived intensity of taste stimuli.

Usually these two types of measures correlate because they are encoded by the same peripheral physiological systems. But what do lay people appreciate of the differences between these two sensory responses? Someone with a high fever or a migraine headache might declare that their sensitivity to stimuli, such as lights, sound, touches, and odors, is high. By this they could mean that they are either bothered by barely detectable stimuli that would otherwise not be noticed or that they are pained by intermediately intense stimuli that would otherwise seem only weak to moderate in strength, or most likely both. The general habit of linking together our sensitivities to threshold-level stimuli and to suprathreshold stimuli is born of a common understanding that absolute threshold sensitivity and the magnitude of suprathreshold responses are related. Our life experiences confirm the relationship between these two.

But these two measures are often not correlated for taste stimuli. For example, someone who can detect extremely low levels of salt in water, lower than most others can, may find very high concentrations of salt in water to be only weakly salty. Or conversely, someone who requires relatively high concentrations of quinine to notice it might report that moderately stronger concentrations of quinine are overwhelmingly bitter. Observations such as this, which indicate a lack of correlation between threshold and suprathreshold measures, suggest that these measures may involve different cellular and molecular processes.

The popular understanding of the relationship between our perceptual responses to weak and strong stimuli contains two assumptions. First, we implicitly assume that detection and suprathreshold ratings of stimuli are based upon the same systems, a correct assumption for organs such as the eye, the ear, and the skin. Second, we assume that the sensory systems that detect a weak stimulus grow in their response with stimulus strength similarly for different people. However, at the absolute threshold for taste stimuli, detection level sensations are not usually accompanied by a quality of taste. Rather, it is merely the detection that something rather than nothing is present in the water. At this low level of stimulation in the human mouth, we cannot be certain of the identity of the receptor system that detects the stimulus. It is possible that detection at these low levels is not based upon activation of the taste system, but rather a somatosensory or osmotic input provides the signal.

...