Numerous investigations have concerned themselves with the task of discovering the weakest solution of various substances that will provoke their respective sensory qualities. The main results of these studies have been the demonstration of two facts, namely, that exceedingly weak stimuli may arouse sensations of taste and that this minimum solution varies greatly in amount according to the substance which is in question.
Valentin, in 1842, measured the lower threshold for solutions of sugar, salt, quinine, and sulphuric acid and found the following proportions to represent the least amounts able to arouse the corresponding sensation:
| Sugar | 1.200 | parts to 100 parts of water |
| Salt | 0.300 | |
| Acid | 0.001 | |
| Quinine | 0.003 |
Numerous other investigators have reported figures of this character. Thus, Nichols and Bailey give the following as averages of the lower thresholds in the case of measurements on forty-six women:
| Sugar | 1 part to | 204 parts water |
| Salt | 1 part to | 2,000 parts water |
| Acid | 1 part to | 3,300 parts water |
| Bitter | 1 part to | 456,000 parts water |
It is neither profitable nor interesting to draw close comparisons between the various sets of measures, since they vary considerably with the substance used, and since, after all, as one writer remarks, “the experiments are chiefly valuable as gratifying our curiosity.” Various students of individual differences have sought to determine the presence of sex differences, age differences, race and group differences, in these minimal taste stimuli. But the incidental factors are so numerous and so beyond experimental control that, if such differences exist, it has never been possible consistently to demonstrate their nature or amount.
Relative Sensitivity of Taste and Smell
A recent investigation by Parker and Stabler was directed toward a question which possesses a certain interest. Reasoning that, since taste and smell are both “chemical” senses, it might be possible to compare their respective sensitivities in terms of the strength of solution required to affect them both, they attempted to make such comparison in the case of one substance. Pure ethyl alcohol is a substance which has both a distinctive taste and a distinctive odor. These investigators found that the minimum amount of this substance that could be sensed by taste was 24,000 times as great as the least amount that could be detected by odor.
The Discrimination of Tastes
In the case of our second general question, that concerning the amount of change in the strength of solution required to produce a felt difference in the intensity of the taste sensation, various difficulties are involved. In the first place, there is no known way of measuring the intensity of that mode of stimulation which may be responsible for the excitation of the taste bud. In such cases as the sensibility to weight and light it is easily possible to measure the intensity of the stimulus in terms of pounds, candle power, or similar physical units. In the cases of temperature and sound the problem is much more difficult, since we do not know precisely what aspect of the stimulus should be used as indicating the intensity of the process at the point of stimulation. Taste and smell offer still greater difficulties, inasmuch as we do not know even with moderate certainty the real nature of the stimulation,—whether, for example, it be mechanical or chemical, or both. Consequently, although it is possible to state that in general a change in the stimulus intensity required to produce a sensation change which will be correctly reported in a certain percentage of the trials is 1% for light, 33% for sound, 5% for lifted weights, no such coefficients of change can even be suggested for taste and smell. Only the general statement can be made that, within limits, increased strength of solution means increased intensity of sensation.