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.