Taste in the Adult
The most striking characteristics of taste in the adult as compared with that of the child, then, is what appears to be a gradual reduction in the extent of distribution of taste sensitivity and a shifting of the region of the greatest sensitivity. No doubt there are other changes, e.g., a general reduction in delicacy of taste analogous to the sort of change which is noted in the other senses as age advances. In extreme old age such a condition is quite pronounced. In hearing, for example, there is not only a dulling sensitivity but a shortening of the range of audible pitches, especially in the region of the higher pitches. So, on the tip of the tongue, sensitivity may be very much reduced or disappear entirely. These changes in taste are not commonly brought to one’s attention as are those of sight and hearing, because they affect our life less vitally. But there is little doubt that careful tests would reveal them.
Structural and Functional Differences Among Individuals
It has been suggested that individuals differ considerably in the distribution and function of their taste mechanism. In the search for general laws these variations within the limit of normality have been looked upon as troublesome exceptions and not of much interest. And these differences are no doubt responsible for the lack of agreement among investigators on many points. For instance, there are persons whose sensitivity on the tip of the tongue is so poor that sweet tastes can be aroused only with the aid of tongue movements. This and other similar cases have given cause for the belief in the necessity of tongue movements for arousing all taste sensations. Some investigators have found the tonsils and the uvula sensitive, while others have found them insensitive. When one finds such conflicting statements from men like Nagel and Kiesow, individual differences seem a plausible explanation. No special studies have been made of these individual differences. Consequently, about all that one can say is that the distribution of the taste buds in different people is subject to considerable variation and that on this account one cannot definitely mark out the limits of their distribution which shall hold for all persons. The same is true of the distribution of sensitivity to specific taste stimuli, sweet, sour, bitter, and salt substances. Such vague statements as are usually made, namely, that sweet is best tasted upon the tip of the tongue, sour upon the sides, bitter upon the back, and salt over nearly the whole tongue, are true, but when one attempts to assign definite limits to these regions then great individual differences appear.
Individual Differences Due to Pathological Changes
More extreme variations in taste are frequent as an accompaniment of disease or congenital malformation of the cerebrum. These variations may be in the nature of absence of sensitiveness, dull (hypo) sensitivity, or very high (hyper) sensitivity. Only one case, so far as we have found, has been reported of congenital taste blindness, either for all taste sensations or for one or two of them, such as would correspond to hereditary color blindness of the various sorts. The apparent absence of taste in certain idiots is not a form of taste blindness to be compared with color blindness. Such persons will eat sulphate of quinine with as much enjoyment as sweet food, but so will they try to eat wood or stones or paper. Nor are the cases, which are more or less common, of dulled taste sensitiveness as a result of cerebral malformation to be compared with real color blindness, which, as far as is known, at least, depends upon no such cerebral abnormality.
There are cases of acquired taste blindness, either general or for special qualities, which resemble in general character acquired color blindness. In both senses the blindness results from pathological changes in the sensory mechanism, either in the sense organ, its centers within the brain, or its connecting nerve trunks. The nature of the sensory defect depends upon the extent of the pathological change and its location. Thus, there may be general taste blindness if both of the cerebral centers are involved, a defect on only one side of the tongue if one cerebral hemisphere is involved. And if the disturbance is in the nerve trunks, only the anterior two-thirds of the tongue may be affected, or only its posterior third. Again, if the lesion is in the sense organ itself, one or more specific taste qualities only may be lost, or all qualities for a very small portion of the tongue. These latter cases are of much importance in developing an adequate theory of taste function. Nagel describes a case in which all sorts of taste stimuli produced only a sensation of salt on one side of the tongue, while on the other side taste was normal. This peculiar condition was followed by a total loss of sensitivity on the affected side.
Epileptics are said to show taste abnormalities of varying character after an attack, and lasting for hours in some cases. There is usually a loss or dulling of the sense, most prominent for salt and least noticeable for bitter. Such conditions are not peculiar to the taste sense, for there is usually a disturbance of the other special senses also after an epileptic attack.
All sorts of taste abnormalities are found in hysteria, but little is known of their underlying causes in the taste mechanism. There may be either hyper- or hypo-sensitivity, although the latter is more common; and both sides of the tongue or only one side may be affected, or even only a small portion of the tongue may be involved. Here, too, the taste abnormalities are accompanied by disturbances of the other senses.
Criminals, especially those recognized as degenerate, show taste abnormalities, usually extreme dullness of taste, along with the same sort of defect in the other senses. Attempts have been made to find a positive correlation between keenness of taste and number of stigmata of degeneration, but with no success.