In the case of some of the sensory modes it is possible to arrange the various elementary qualities in a schema or graph, representing in a diagrammatic way their relations to each other, the results of their combination, their influence on each other, etc.
Thus, in the case of vision the conventional “color pyramid” expresses the various relations between the different elementary colors and the different degrees of brightness. Red, yellow, green, and blue occupy the corners of the base of a double pyramid. The upper apex represents white and the lower apex black. On the side between red and yellow are found the various oranges which result from mixing red and yellow light in varying proportions. On the remaining sides are represented the combinations of yellow and green, green and blue, blue and red. Along the vertical axis range the different grays. Cross sections of the pyramid indicate, at different levels, the result of mixing the different colors with these grays, thus yielding the tints and shades of the colors. Along the base, the colors which are at the extreme ends of any diagonal passing through the center are complementary,—they neutralize each other when mixed and under other circumstances each tends to induce the other by contrast. The visual manifold may thus be adequately schematized on a three-dimensional figure.
In a similar way the various tones, in the case of hearing, may be arranged along a one-dimensional line, which represents the tonal scale. Is it possible to arrange in any such systematic way the elementary taste qualities so as to indicate their relationship to each other? Before suggesting such a diagram it will be well to have in mind just what relationships the various taste qualities do as a matter of fact display.
The testimony of daily experience would probably be at once that the various elementary tastes may combine to produce new tastes of a more complex or even of a unitary character. Thus, the taste of lemonade is distinctive enough. Yet even casual observation suffices to show that the sweet and the sour components have by no means lost their identity, since each can be singled out in attention and recognized as the familiar elementary quality. Red and blue may fuse to produce a violet or a purple from which the original elements can by no means be singled out and identified through direct inspection. But it seems to be the rule that tastes do not behave in this way, although the demands of daily experience do not readily lead us to discover the fact. “Think, for instance,” writes Titchener, “of the flavor of a ripe peach. The ethereal odor may be ruled out by holding the nose. The taste components,—sweet, bitter, sour,—may be identified by special direction of the attention upon them. The touch components—the softness and stringiness of the pulp, the puckery feel of the sour—may be singled out in the same way. Nevertheless, all these factors blend together so intimately that it Is hard to give up one’s belief in a peculiar and unanalyzable peach flavor. Indeed, some psychologists assert that this resultant flavor exists; that in all such cases the concurrence of the taste qualities gives rise to a new basic or fundamental taste, which serves, so to say, as background to the separate components. There is, however, no need to make any such assumption. It is a universal rule in psychology that when sense qualities combine to form what is called a perception, the result of their combination is not a sum but a system, not a patchwork but a pattern.... Hence, just as it would be absurd to say that the plan of the locomotive is a new bit of steel or the pattern of the carpet a new bit of colored stuff, so is it wrong to say that the peach character of a certain taste blend is a new taste quality.”
The mixture of stimuli provoking two taste qualities does not, then, produce intermediate qualities such as the orange which results from the mixture of red and yellow. Instead, in this case, the two qualities do one of these three things: (a) they may remain separate and distinct; (b) they may fluctuate individually and alternate with each other in their appearance; (c) they may tend to neutralize each other. If the stimuli are very intense, oscillation is the common result. If the stimuli are weak, some degree of neutralization is reported to be the rule. Only in one case, namely, the mixture of sweet and salt, does a new taste seem to emerge, which does not resemble either of the original qualities. Kiesow finds that such a mixture, in the case of weak solutions, gives rise to a quality described as “flat,” “vapid,” or “insipid,”—the alkaline taste which we have already considered.
In the case of color, there may be found for every quality or mixture an opposite quality or mixture which when combined with the former either completely neutralizes it or at least reduces its intensity. Thus blue and yellow, of the proper tones and proportion, cancel each other, leaving only an experience of gray. So do a certain olive color and a particular violet, a certain orange and a particular bluish-green, a certain red and a particular green.
We have already suggested that in case of weak taste qualities a similar effect is present. “With the low intensities there is in most cases a partial compensation, which is least for sweet and sour, better for salt and bitter, better still for sour and bitter, sour and salt, sweet and bitter.” These facts are utilized in daily life in the countless combinations of dressings, sauces, seasonings and condiments used in the preparation of food. We take sugar with our tea, our coffee, our chocolate, our strawberries, our grapefruit, and our lemon juice, and realize that it to some degree counteracts or neutralizes the bitter or the sour taste of these foods in their original form. “Salt corrects the sweetness of an over-ripe melon.” In our salad dressings, sauces, gravies, relishes, and bitters we find the means of reënforcing or toning down the taste qualities to suit our own particular fancy.
In part, of course, these effects are not achieved through the mere process of neutralization. The addition of touch qualities, such as the pucker of vinegar, the sting of pepper, the bite of mustard, and the burn of onion, plays its own part in the constitution of a flavor, regardless of their compensating influence on the pure taste qualities.
In line with the fact that taste and odor are easily confused, and contributing perhaps to this confusion, is the fact that tastes and odors are related to each other through their antagonism, almost if not quite as definitely as are the qualities within each of the separate modes. Thus, the sickening odor of many medicines is somewhat palliated if they are taken in fermented juices or with the sour acids of fresh fruits. “Quinine, which tastes bitter and has no smell, is corrected by essence of orange peel, which has an aromatic smell and no taste.” Titchener pertinently remarks that these results may in part arise from the simple process of distracting attention from an unpleasant item to a more agreeable part of the experience. On the other hand, the special effectiveness of the introduction of odors into the complex rather than pleasant sights and sounds suggests that the results in the case of taste and smell are not solely a matter of attention, but are in part, at least, dependent on the essential relationships between the qualities of these two modes of sensation. In the chapter on “The Evolution of Taste” certain light is thrown on the closeness of these relationships by our knowledge of the intimate biological connection between taste and smell. In certain lower forms of animal life it is indeed quite impossible to draw any clear line between these two features of “the chemical sense.”
In general, then, although the facts of compensation, antagonism, and complementariness are to be observed within the field of the taste qualities, the relations disclosed are by no means as definite nor as systematic as they are in the case of vision. For a given primary color quality there exists only one other elementary quality which stands to it in the relation of antagonist. But we have seen that in the cases of both sour and bitter there is at least some degree of antagonism with all three of the other qualities, while both sweet and salt antagonize in some degree both sour and bitter. Moreover, at least the sour, the bitter, and the sweet appear to show antagonistic relations to certain qualities of smell.
In none of these cases has there been presented clear evidence showing the ability of one quality to totally efface another, so that no taste whatever is present. In the case of colors, however, the result of such combinations in the right proportions may easily be a total absence of color quality. It is true that occasional instances of such effects in taste have been reported, but the general rule seems to be that the extreme degree of neutralization leaves an experience which is recognized as a taste, but which is described as “flat” or as “insipid.” It is possible, of course, that this “insipid” taste quality is the tactile and kinæsthetic residue of the total experience, much as the “gray” which results from the combination of complementary colors may be described as the brightness residue of the total momentary effect. But in the latter case the residue would be distinctly “visual” although not “color.” In the case of taste nothing corresponding to the “brightness” of vision is recognized, and the residue as we have described it would consequently belong to a different mode of sensation.
The phenomena known as contrast are very familiar sense experiences. Not only is it true that in the fields of perception and feeling the tall, the good, the wholesome, the fast, the daring, and the pleasant have their qualities enhanced when they accompany or follow upon the diminutive, the wicked, the foul, the slow, the cowardly, and the disagreeable; in the case of more simple sense experiences also contrast effects are often both immediate and striking. The apparent temperature of the air or water varies with the conditions from which we emerge into them. The sudden calm after a thunderstorm seems even more empty than the same conditions in Indian summer. The palest complexion assumes a moderate rosiness if green ribbons and fabrics are suitably arranged about or near it. Even a pure gray strip of paper becomes a rich pink line or a yellowish band when placed across a background of saturated green or blue.
Daily experience entails many such instances of contrast in the case of the taste qualities as well. A ripe apple may surprise us by its unexpected sourness if we come to it direct from a box of bonbons. Experiments designed to investigate the presence and character of taste contrasts are especially interesting and their results are in many ways curious. If, under proper experimental precautions, a salt solution is applied to one side of the tongue and a drop of tasteless distilled water is simultaneously applied to the other side, the tasteless water is reported as sweetish. If, instead of the distilled water, one apply a sugar solution of such weakness that its taste could not under ordinary circumstances be recognized, the sweetness becomes clearly apparent. Under the same circumstances a solution otherwise producing a weak sensation of sweetness is reported as being “very sweet.” The salt solution, that is to say, induces by contrast the quality of sweetness in tasteless substances and enhances the degree of an otherwise weak quality aroused at another region of the tongue.
In much the same way a sugar solution induces saltiness, or sourness, or perhaps bitterness, according to the individual, the occasion, and the circumstances. Sometimes the salt induces a sour instead of the sweet. The bitter, however, seems unable to induce other qualities by contrast, and is at least seldom induced by the other qualities.
In this as in other respects the bitter quality seems to show idiosyncrasies. Thus, it is generally accepted that no papillæ are ever sensitive only to bitter stimuli. Many primitive languages are said to contain in their vocabulary no word for bitter: it is not uncommon in daily experience to find bitter confused with sour; bitter seems to be especially easily antagonized by certain odors; it does not display striking contrast phenomena; and its reaction time is exceptionally slow.
The type of contrast which we have thus described in the case of the tastes is known as simultaneous contrast. Both stimuli are applied at the same time to different parts of the sense organ. What is known as successive contrast can also be experimentally produced. Here one of the stimuli follows the other after an interval in which nothing is applied or, still better, in which the mouth is carefully rinsed with water. This is the type of taste contrast with which we are most familiar in daily life. The same contrasts may be induced experimentally by this method as result from the simultaneous method. But the inducing stimuli in this case must be rather more intense than is necessary for the production of simultaneous contrasts. In much the same way in perception as in sensation the contrast between two extremes or opposites is better realized when both are present together than when one follows the other after an interval.
The general facts of taste contrast are succinctly summarized by Titchener in the following way:
(1) Salt and sour contrast: the sour induced by salt being clearer and stronger than the salt induced by sour.
(2) Sweet and sour contrast: the sweet induced by sour being clearer and stronger than the sour induced by sweet.
(3) Salt and sweet contrast: the sweet induced by salt being clearer and stronger than the salt induced by sweet.
(4) Bitter shows no contrast at all.
(5) The order of qualities, as regards ease of induction, is sweet, sour, salt, bitter.
Suggested by the phenomena of contrast are the somewhat related facts of after sensations or after images, as they are sometimes called. When one looks for a moment at a candle or other source of light and then quickly extinguishes it or looks away from it, one still continues to see, for a time, a luminous form, which may persist for a considerable time after the removal of the stimulus. In such a case the color and brightness of this after image may be the same as those of the original object, and the after image is hence said to be positive. Under certain conditions the colors of the after image are complementary to those of the original and the brightness relations of the various parts are reversed. The after image is then said to be negative. Or if after looking at a colored object one transfers his gaze to a gray expanse there appears upon this gray field an outline of the original object, with colors which are complementary or antagonistic to those of the original. After sensations of pressure arising under special conditions have been described, and positive after effects of warm and cold stimuli seem also to be demonstrable. Even after sensations of sound, somewhat weak, transitory, and by no means easily detected, have been described. In all these cases except vision the after sensations are of the positive type only.
In the case of taste, and of smell also, it is difficult to investigate the presence of such after sensations, inasmuch as it is by no means easy to be sure that some trace of the stimulus does not remain in or near the sense organ. An experience reported as a positive after sensation might easily enough represent only the effect of persistent stimulation by these traces of the substance. At least one investigator is convinced that in his observations of taste experiences “the sensation continued after the tongue was so carefully dried off that no particles of the tastable substance were left.” Similarly, experiences of tastes being “left in the mouth” are very common. But our inadequate control over the disposition of the sapid substance and the complicated chemical relation which exists between various substances and between some substances and the natural juices secreted in the vicinity of the taste organ makes it impossible to assert with certainty either the presence or the absence of after sensations of taste.
The foregoing facts concerning the phenomena of mixture, fusion, antagonism, contrast, and after sensation show at once the impossibility, in our present state of knowledge, of arranging the taste qualities in any such systematic scheme as is represented by the color pyramid and the tonal scale in the cases of vision and hearing. It by no means follows, however, that such orderly arrangements have not been attempted.
Fig. 1.
Kiesow, one of the most famous students of the sense of taste, proposed that a circle with a vertical and a horizontal diameter indicated would best represent the various relations between the taste qualities. At top and bottom would stand salt and sweet; to left and right, bitter and sour. Along the horizontal diameter would be placed the mixtures of bitter and sour, and along the vertical diameter would range the various results of mixing salt and sweet. The mixtures of salt-sour, sweet-sour, bitter-sweet, and bitter-salt would stand in their appropriate places about the circumference or periphery of the circle.
Wundt tentatively adopts a similar scheme when he says: “The system of taste sensations is, accordingly, in all probability to be regarded as a two-dimensional continuity, which may be geometrically represented by a rectangular surface at the angles of which the four primary qualities are placed, the various mixed qualities being placed along the side and on the inner surface.”
To such suggestions, however, Kuelpe objects that: “There is no indication of a continuous transition between the four qualities which tastes appear to present, as there is between the qualities of tone sensations. They form, not a one-dimensional manifold, but a discrete system of unknown relations.”
Titchener, one of the most careful students of sense experience, is less emphatic, but he “doubts whether, in the present state of our knowledge, this idea (that of Wundt) can be accepted.” He doubts “whether the sweet-sour of lemonade stands to its originals as blue-green stands to blue and green, or as orange to red and yellow; and also whether bitter should lie in the same plane with the other three taste qualities. We must suspend judgment; in the meantime, Kiesow’s figure provides us with a working hypothesis.”
Ladd and Woodworth align themselves with Kuelpe and conclude that, “there is no clear indication that the tastes can be arranged in a linear scale, as the primary colors are, nor that any taste stands to any other definitely in the relation of opposite or complementary. On the whole it appears as if the four tastes were rather isolated from each other, each representing almost an independent sense.”