THE SENSITIVENESS OF TASTE
_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:
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.
_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.”
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