Adaptation and Fatigue
Curious phenomena in the case of all the senses are those known as adaptation and fatigue. It is a familiar experience that the illumination of a room which seems upon entrance to be yellowish quickly comes to appear merely light. After a few minutes wearing colored goggles the tinge they give to objects seems to disappear and we say we have become “adapted” to the color. “We become rapidly adapted to a constant stimulus so that we fail to notice the weight of our hats, the temperature of the room we are in, the odors of the subway.” Searching for spectacles which meanwhile perch upon the nose is the result of adaptation. A related phenomenon is the fact that a darkened room which on entrance seems perfectly black comes in time to show its contents as more or less clearly marked off from one another.
Some adjustment—in the sense organs, perhaps, or perhaps in the brain centers—takes place in the presence of a constant stimulus. The general result of this adjustment is that the particular sense quality involved fades away: colors tend toward gray, pressures tend to disappear, temperatures tend toward a neutral point, and sounds become indifferent. This adjustment is not fatigue in the usual sense of the word. The area that has become adapted to a given pressure is still sensitive and will feel even a lighter weight if this be substituted for the one to which it has become adapted. It is adapted to a continuous stimulus, but it is sensitive to any change in the stimulus.
In the case of smell it is notorious that odors constantly present soon cease to be observed or even to be observable. Even the most disagreeable and insistent odors fade away in time. In a few minutes stale cheese comes to have no discernible odor, while the odors of tobacco smoke and various perfumes disappear equally quickly. In this sense adaptation seems to be much more like exhaustion or fatigue than in the cases of sound, sight, and pressure, and it may require a considerable interval of freedom from the stimulus before the quality returns.
The laws and effects of adaptation are by no means the same for all the senses. Thus, in the case of smell, adaptation to certain odors seems to increase our sensitivity to other odors. In the case of taste the effects are by no means clear nor consistently reported by different observers. In general it seems to be true that the effect of adaptation to a given taste quality has no demonstrable effect on the remaining qualities, and that this effect, as in the case of smell, is of the general character of exhaustion. Taste, along with smell, seems to have not yet developed any peripheral or central mechanism whereby adaptation may take place without actual loss of sensibility.
Acquired Tastes
In a very different sense the word “adaptation” is often employed to express the phenomenon of habituation in the case of “acquired tastes.” Here the habituation is not to the taste quality, in a sensory sense, but represents a change in the feeling or affective tone which characterizes or accompanies this quality.
The easier case to understand is that in which the continued indulgence in a substance, such as ice cream, candy, tobacco, sets up organic effects which have their unpleasant accompaniment. Here it happens that a taste originally very pleasant becomes indifferent or even disgusting. The unpleasantness in such a case is rather easily seen to arise, not from a taste quality alone, but from the total state of the moment. On a later occasion the first appearance of the taste quality may, by well-recognized associative mechanisms, arouse the organic revulsions or memories of them, with the attendant disagreeable effect. The originally agreeable taste then appears to have become disagreeable.
Cases of the reverse order are equally familiar, in which a taste originally unpleasant comes, with repetition, to lose its disagreeable character, or even to become distinctly pleasing. Indeed, in many such cases habituation results in the establishment of a craving for the quality which was originally repulsive. Here the repetition of the taste quality seems to set up defensive adjustments and adaptations of a profound organic kind rather than the earlier protective reactions of refusal and rejection. Once this adjustment or adaptation takes place the presence of the original stimulus is called for as part of the new condition of balance, and the craving, or appetite, results. In this account, it must be confessed, we speak in terms of vague generalities, since it is not easy to state the precise nature of these biological adaptations. But their existence in the case of many users of such things as olives, garlic, tobacco, liquors, and various drugs is a matter of common experience.