Before dismissing the question of adequate stimuli one other set of phenomena ought to be mentioned. There are cases in which sapid substances dissolved in the blood produce taste sensations. In cases of diabetes, where sugar is present in the blood, a sweet taste is experienced in the absence of stimuli upon the tongue surface. Also, in cases of jaundice, where there is an excess of bile, a bitter sensation may be experienced. Here the stimuli affect either the taste nerves or the taste buds directly. There is no reason to think that the stimulation process differs in any other respect from the normal, except in the mode of access to the taste buds.
Inadequate Taste Stimuli
Our second question concerns the possibility of other stimuli to taste, or of inadequate stimuli. Among these forms of stimulation three will be considered, namely, mechanical, thermal, and electrical. Are taste sensations produced by mechanical stimulation of the sense organ? Some of the older experimenters reported that tapping the tongue lightly or putting it under slight pressure aroused taste sensations of sour and bitter. But there does not seem to be good ground for such a conclusion. No doubt under poorly controlled conditions such mechanical stimuli might serve to force sapid substances already upon the tongue into the taste pores and thus stimulate the taste nerves. This, however, would be only an indirect effect of mechanical stimulation and not at all analogous to the visual sensations produced by mechanical stimulation of the eyeball.
It is rather interesting to note in this connection that mechanical stimulation of the chorda tympani nerve, which carries the impulses from the anterior two-thirds of the tongue, is said to produce taste sensations. In cases in which the eardrum has been removed, stimulation of the nerve in the middle ear is said by some authorities to produce sweet and bitter sensations, and by others nothing but pricking sensations.
Thermal stimuli when applied to the tongue do not cause taste sensations, but only sensations of warmth and cold. Thermal stimuli, however, are recognized to have considerable influence upon the effect of taste stimuli. Two cases are to be noted, that in which the sapid substance itself is either warm or cold and that in which the mouth has been subjected to warm or cold stimuli before the sapid substance is introduced into the mouth. The first condition has received the more attention on account of the importance of temperature in the supposed chemical reaction in the taste buds and the consequent possibility of interpreting the facts in favor of a chemical theory of taste. Both problems have been investigated by Kiesow and others, and the conclusions are conflicting. Some find that there is a certain optimum temperature for sapid bodies at which the taste sensations are best obtained, although there is no perfect agreement as to what this optimum temperature is. It varies from 55 to 120 degrees F. for different authors. At the optimum temperature the least quantity of the sapid substance is necessary to arouse a taste sensation, while a deviation from this temperature in either direction requires a stronger stimulus to arouse the same sort of a sensation. Kiesow, on the other hand, believes that the temperature of the sapid substance makes no difference, but only the temperature of the mouth previous to receiving the sapid substance. For instance, he holds that the sensitivity is just as acute when the temperature of the sapid substance is 32 degrees F. as when it is 100 degrees F. But if the tongue is placed in water at 32 degrees F. for a few minutes it then becomes insensitive to sapid substances,—a kind of anesthesia results. He believes that the conflicting results obtained by the other investigators are merely the results of distraction of the attention. Whenever the temperature rises or falls beyond a certain point, then this temperature sensation becomes more impressive than the weaker taste sensation, and the taste sensation drops out of consciousness. It is a well-established fact, at any rate, that thermal stimuli cannot directly arouse taste sensations.
Taste sensations aroused by passing an electric current through the tongue were reported as early as the middle of the eighteenth century by Sulzer. Since that time a great number of experimental researches have accumulated in the attempt to answer the question whether an electric stimulus can directly arouse taste sensations, as it will produce sensations of light when it stimulates the eye. In all of this work there has been rather general agreement as to the character of the taste sensations present during electrical stimulation. Thus, when the anode (positive pole) comes into contact with the tongue the taste is said to be sour, while at the point where the cathode (negative pole) touches the tongue a bitter taste is reported. There is less general agreement upon the character of this cathode taste, however, than upon that of the anode taste, some describing it as burning, some as bitter, sweet, or alkaline.
The great differences of opinion do not concern the character of the taste aroused by the electric current so much as the real source of the stimulation of the taste buds. It was suggested by Humboldt about 1800 that the taste sensations were not caused directly by the electrical stimulation of the taste buds, but rather by certain sapid substances which were set free within the mouth by the action of the current on the saliva. It is known that fluid salts can be broken down by electrolysis in such a way that at the anode an acid reaction, and at the cathode an alkaline reaction, can be detected. These facts have lent support to the assumption that the saliva thus acted upon by the electric current produces tastable substances. An interesting experiment attributed to Volta about 1800 seemed for a time to refute the whole electrolysis theory. He used an alkaline fluid for the anode, into which the tip of the tongue was dipped. Even in this case the characteristic sour taste was still experienced. But this experiment, like all of the others which seem to refute the electrolysis theory of electric stimulation, neglects the fact that the electrolysis may occur within the taste bud itself, and hence could not be detected by any test of the tongue surface, nor could the taste sensations thus produced be prevented by immersing the tongue in any kind of a solution, since this need not displace the fluids within the taste bud. The experience of both bitter and sweet tastes at the cathode, if these experiences are genuine, offers some difficulty for the electrolysis theory. The most that can be said is that it is quite likely that electrolysis takes place within the taste bud, but that, in addition to this, there may possibly be a certain direct action of the electrical stimulus upon the taste buds or their nerve endings.
From this review of the different ways in which the taste organ may be affected it appears that the evidence is not conclusive that mechanical, thermal, or electrical stimuli may arouse taste sensations. Chemical stimulation, which is usually included among the inadequate stimuli for the other senses, is the adequate stimulus for the taste mechanism.