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.”
CHAPTER IV
Time Relations of Taste Qualities
The Inertia of the Taste Organs
Many experiments have been undertaken in the effort to measure the inertia or sluggishness of the various senses. Inasmuch as the nervous structure is a physical system set in operation by the incidence upon it of external agencies, in each of its parts it requires a certain time in order to be set going; and, once set in operation, acquires a certain momentum which necessitates that a certain time elapse before it is again in a state of equilibrium. At least the sense organs all show such inertia, so that, in a given case, only a limited number of distinct sensations can be produced in a given time by successive stimulation. A measure that has often been used for expressing such facts is the maximum number of separate excitations to which the sense organ responds in a unit of time, as one second. This measure, to be sure, varies considerably with numerous conditions and circumstances, such as the nature of the stimulus, the part of the sense organ affected, its previous condition, the intensity of the stimulation, etc. This measure, which may be said roughly to indicate the duration of a sensation (including its positive after image), is very short for touch, somewhat longer for sound, and still longer for vision. Because of the nature of the stimuli in taste and smell and the difficulty of accurately controlling their application and removal, satisfactory measures of the inertia of these sense organs have never been secured.
Reaction Time to Taste Stimuli
One fact, however, seems to be fairly clear from experiment, although it would by no means be suspected from casual observation, namely, that the various taste qualities are not equally prompt in the time required for them to appear after the application of the stimulus. Salt and sweet come rather quickly as compared with sour and bitter, the order of speed being salt, sweet, sour, bitter. How much this may depend merely on such differences as may exist in the structure and location of the various taste buds it is impossible to say. Kiesow points out that taste sensations are tardy and gradual in their appearance. If the person being stimulated be required to indicate by a signal the instant at which the taste quality appears, it is possible to measure, in very small units of time, the interval between the superficial application of the solution and the appearance of the sensation. This is called the “reaction time” to the taste stimulus. When the stimuli were applied to the tip of the tongue Kiesow found the following figures to represent average reaction times to his different solutions: