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:
Seconds
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Salt .307
Sweet .446
Sour .536
Bitter 1.082
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:
Reaction to Second
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sound .146
touch .149
sight .189 (?)
smell .500 (?)
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.
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