(5) The order of qualities, as regards ease of induction, is sweet, sour, salt, bitter.
After Images of Taste
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 Schema of Taste Relations
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.”