Various Measures of Sensitiveness

In a general way it is well known that exceedingly weak solutions of many substances are sufficient to provoke sensations of taste. It is also known that weak tastes which some individuals are able to detect or to recognize correctly go quite unobserved by others. The same thing is true of differences between tastes. The connoisseur is sensitive to minute differences in the flavor of wine, tobaccos, and sauces. Through practice the expert taster of these substances acquires a skill which is quite incomprehensible to the inexperienced. In part only is such skill a matter of special sensory activity. It is in large measure a matter of perception rather than one of sensation,—a knowledge of what signs to look for and how to interpret these signs,—rather than an increased sensitiveness to stimuli. In the same way the skilled gardener, hunter, or scout is alert to the significance of particular signs and clues and this alertness and apt interpretation may make him appear to have senses of exceeding acuteness, although this may by no means be borne out by actual measurements.

The psychological problems involved in the measurement of keenness of taste are mainly two in number. One problem concerns itself with the question, What is the faintest stimulus that can be sensed,—the weakest taste that can be appreciated? The other concerns itself with the sensitivity to difference between tastes, and would be expressed by some such question as, How slight a change in the amount or intensity of the stimulus is required for one to be able to perceive a change in the intensity of the taste sensation?

Unfortunately for our knowledge of tastes, both these problems are very difficult to approach experimentally. Whether or not a given weak stimulus will provoke a taste sensation depends on very many things other than the strength of the solution. The amount of solution applied, the extent of surface excited, the duration of the application, the temperature of the solution, the state of rest or movement of the sense organ, and the nature of preceding stimuli, among other things, are important.

The Threshold of Taste Sensation

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:

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:

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.

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.

Adaptation and Fatigue

Curious phenomena in the case of all the senses are those known as adaptation and fatigue. It is a familiar experience that the illumination of a room which seems upon entrance to be yellowish quickly comes to appear merely light. After a few minutes wearing colored goggles the tinge they give to objects seems to disappear and we say we have become “adapted” to the color. “We become rapidly adapted to a constant stimulus so that we fail to notice the weight of our hats, the temperature of the room we are in, the odors of the subway.” Searching for spectacles which meanwhile perch upon the nose is the result of adaptation. A related phenomenon is the fact that a darkened room which on entrance seems perfectly black comes in time to show its contents as more or less clearly marked off from one another.

Some adjustment—in the sense organs, perhaps, or perhaps in the brain centers—takes place in the presence of a constant stimulus. The general result of this adjustment is that the particular sense quality involved fades away: colors tend toward gray, pressures tend to disappear, temperatures tend toward a neutral point, and sounds become indifferent. This adjustment is not fatigue in the usual sense of the word. The area that has become adapted to a given pressure is still sensitive and will feel even a lighter weight if this be substituted for the one to which it has become adapted. It is adapted to a continuous stimulus, but it is sensitive to any change in the stimulus.

In the case of smell it is notorious that odors constantly present soon cease to be observed or even to be observable. Even the most disagreeable and insistent odors fade away in time. In a few minutes stale cheese comes to have no discernible odor, while the odors of tobacco smoke and various perfumes disappear equally quickly. In this sense adaptation seems to be much more like exhaustion or fatigue than in the cases of sound, sight, and pressure, and it may require a considerable interval of freedom from the stimulus before the quality returns.

The laws and effects of adaptation are by no means the same for all the senses. Thus, in the case of smell, adaptation to certain odors seems to increase our sensitivity to other odors. In the case of taste the effects are by no means clear nor consistently reported by different observers. In general it seems to be true that the effect of adaptation to a given taste quality has no demonstrable effect on the remaining qualities, and that this effect, as in the case of smell, is of the general character of exhaustion. Taste, along with smell, seems to have not yet developed any peripheral or central mechanism whereby adaptation may take place without actual loss of sensibility.

Acquired Tastes

In a very different sense the word “adaptation” is often employed to express the phenomenon of habituation in the case of “acquired tastes.” Here the habituation is not to the taste quality, in a sensory sense, but represents a change in the feeling or affective tone which characterizes or accompanies this quality.

The easier case to understand is that in which the continued indulgence in a substance, such as ice cream, candy, tobacco, sets up organic effects which have their unpleasant accompaniment. Here it happens that a taste originally very pleasant becomes indifferent or even disgusting. The unpleasantness in such a case is rather easily seen to arise, not from a taste quality alone, but from the total state of the moment. On a later occasion the first appearance of the taste quality may, by well-recognized associative mechanisms, arouse the organic revulsions or memories of them, with the attendant disagreeable effect. The originally agreeable taste then appears to have become disagreeable.

Cases of the reverse order are equally familiar, in which a taste originally unpleasant comes, with repetition, to lose its disagreeable character, or even to become distinctly pleasing. Indeed, in many such cases habituation results in the establishment of a craving for the quality which was originally repulsive. Here the repetition of the taste quality seems to set up defensive adjustments and adaptations of a profound organic kind rather than the earlier protective reactions of refusal and rejection. Once this adjustment or adaptation takes place the presence of the original stimulus is called for as part of the new condition of balance, and the craving, or appetite, results. In this account, it must be confessed, we speak in terms of vague generalities, since it is not easy to state the precise nature of these biological adaptations. But their existence in the case of many users of such things as olives, garlic, tobacco, liquors, and various drugs is a matter of common experience.

The Early Development of Taste

In the chapter on “The Evolution of Taste” it will be shown that very early in the development of the forms of animal life there is present a form of sensibility to that type of stimulation which, in our own experience, provokes sensations of taste. The “chemical” sense is thus seen to be a very primitive mode, and adjustments to chemical factors in the environment are present at a very low level of organic development. It is a general rule that capacities which appear thus early in the animal series (phylogeny) also appear relatively early and relatively complete in the development of the individual (ontogeny) of more elaborate forms. So far as we are able to discover, this rule holds for the development of the sense of taste. In a number of cases individual infants have been carefully observed in order to note the order of development of the various senses and the adjustments to stimuli in these different modes. In several cases large numbers of newborn infants have been tested immediately after birth, with the same questions in mind.

These studies show that not only is sensibility to taste present at the time of birth, but that the newborn infant reacts in different ways to the various taste qualities. On the first experimental application of taste stimuli distinguishable reactions, such as quiet sucking, grimacing, nausea movements, facial expressions, and varied mimetic behavior, indicate that at least in a rudimentary way the various taste qualities are responded to in a selective or discriminative manner.

Thus, Kussmaul, in 1859, tested twenty-one children with solutions of sugar and of sulphate of quinine. In general the sweet and the bitter caused “the same mimetic facial movements as are observed in adults.” There seemed, however, to be certain individual differences in sensitivity, and occasionally sweet and bitter provoked facial reactions which were not distinguishable. Guezer, in 1873, studied fifty newborn infants by giving them tastes of sugar, quinine, and weak acetic acid. The sugar, as a rule, produced “pleasurable sucking,” the quinine and acid produced “unpleasant ‘bitter’ expression and even nausea movements.” Kroner, in 1882, recorded studies of the taste reactions of his own children at birth. He observed that they reacted immediately after birth to sweet and bitter with the characteristic facial expression of the adult. He was convinced that the sense of taste was at birth the best developed of all the senses.

The most elaborate study of this kind yet recorded is that of Peterson and Rainey. These observers report tests of 1,060 newborn infants, varying in nationality, color, sex, and period of gestation. The experiments included tests on all the senses. As taste stimuli for salt, sour, sweet, and bitter they employed solutions of salt, acetic acid, simple syrup, and tincture of gentian. The tests of taste showed “with great regularity mimetic reactions to these stimuli characteristic of adults, grimaces of discomfort, or expressions of content and liking.” “The gustatory nerve not only reacts differently to salt, sweet, bitter, and sour at birth, but the same mimetic reactions are observed in premature infants. This nerve is therefore ready to receive taste impressions some time before the normal period of birth.”