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:

The reaction to bitter, which requires twice as long in the above case as that of any other quality, was considerably shorter when the stimulus was applied to the root of the tongue. This suggests that the differences found by Kiesow may in part, at least, depend on the accessibility and perhaps also on the number of such various types of taste buds as there may be.

Taking these reactions times as they stand, the average time for the four taste qualities may be said to be about one-half a second. As compared with the reaction times of other sense modes, taste is more sluggish than any other sense, with the possible exception of smell. Averaging the results of numerous observers for the range of stimulus qualities and intensities that have been employed, the comparative times are somewhat as follows:

It should, of course, be borne in mind that these are but averages of figures which vary considerably with a large number of factors, although it is true that the influence of these factors can itself be subjected to precise measurement.

Determinants of Reaction Time to Taste

By way of illustration of the numerous incidental and extraneous factors that influence reaction time to the taste qualities we may instance the temperature of the solution. In what seems to have been a carefully conducted series of observations Chinaglea has recently shown the nature of this influence. As Kiesow had already found, the temperature of the solution (within the pain limits) does not influence the intensity of stimulus required to produce the weakest sensation, and hence does not modify the threshold or limen. But such changes Chinaglea showed to have a measurable influence on the reaction time to taste stimuli. Lowering the temperature of the solution below that of the mouth does not affect reaction time to salt, but it lengthens the time for the other qualities. Raising the temperature of the solution above that of the mouth quickens the reaction to sweet, but lengthens the reaction to bitter and sour.