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Conditioned taste aversion occurs when an animal associates the taste of a certain food with symptoms caused by a toxic, spoiled, or poisonous substance. Generally, taste aversion is developed after ingestion of food that causes nausea, sickness, or vomiting. The ability to develop a taste aversion is considered an adaptive trait or survival mechanism that trains the body to avoid poisonous substances (e.g., poisonous berries) before they can cause harm. The association reduces the probability of consuming the same substance (or something that tastes similar) in the future, thus avoiding further poisoning. It is an example of classical conditioning or Pavlovian conditioning.
Studies on conditioned taste aversion which involved irradiating rats were conducted in the 1950s by Dr. John Garcia,[1] leading to it sometimes being called the Garcia effect.
Conditioned taste aversion sometimes occurs when sickness was merely coincidental and not related to the substance that caused the sickness. For example, a person who becomes very sick after consuming vodka-and-orange-juice cocktails may then become averse to the taste of orange juice, even though the sickness was caused by the over-consumption of alcohol. Under these circumstances, conditioned taste aversion is sometimes known as the "Sauce-Bearnaise Syndrome", a term coined by Seligman and Hager.[2]
While studying the effects of radiation on various behaviors in the mid to late 1950s, Dr. John Garcia noticed that rats developed an aversion to substances consumed prior to being irradiated. To examine this, Garcia put together a study in which three groups of rats were given sweetened water followed by either no radiation, mild radiation, or strong radiation. When rats were subsequently given a choice between sweetened water and regular tap water, rats who had been exposed to radiation drank much less sweetened water than those who had not. Specifically, the total consumption of sweetened water for the no-radiation, mild radiation and strong radiation rats was 80%, 40% and 10%, respectively.
This finding ran contrary to much of the learning literature of the time in that the aversion could occur after just a single trial and over a long delay. Garcia proposed that the sweetened water became regarded negatively because of the nausea inducing effects of the radiation, and so began the study of conditioned taste aversion.
Many scientists were skeptical of Garcia's findings because it did not follow the basic principles of classical conditioning. However, Garcia replicated his results multiple times. He demonstrated that the particular stimulus used in classical conditioning does matter. An internal stimulus produced an internal response while an external stimulus produced an external response; but an external stimulus would not produce an internal response and vice versa.[3]
Taste aversion does not require cognitive awareness to develop—that is, the subject does not have to consciously recognize a connection between the perceived cause (the taste) and effect (the negative feeling). In fact, the subject may hope to enjoy the substance, but the body handles it reflexively. Conditioned taste aversion illustrates the argument that in classical conditioning, a response is elicited.
Also, taste aversion generally only requires one trial. The experiments of Ivan Pavlov required several pairings of the neutral stimulus (e.g., a ringing bell) with the unconditioned stimulus (i.e., meat powder) before the neutral stimulus elicited a response. With taste aversion, after one association between sickness and a certain food, the food may thereafter elicit the response. In addition, lab experiments generally require very brief (less than a second) intervals between a neutral stimulus and an unconditioned stimulus. With taste aversion, however, the hotdog a person eats at lunch may be associated with the vomiting that person has in the evening.
If the flavor has been encountered before the subject becomes ill, the effect will not be as strong or will not be present. This quality is called latent inhibition. Conditioned taste aversion is often used in laboratories to study gustation and learning in rats.
Aversions can also be developed to odors as well as to tastes.
Taste aversion is fairly common in humans. When humans eat bad food (e.g., spoiled meat) and get sick, they may find that food aversive until extinction occurs, if ever. Also, as in nature, a food does not have to cause the sickness for it to become aversive. A human who eats sushi for the first time and who happens to come down with an unrelated stomach virus may still develop a taste aversion to sushi. Even something as obvious as riding a roller coaster (causing nausea) after eating the sushi will influence the development of taste aversion to sushi. Humans might also develop aversions to certain types of alcohol because of vomiting during intoxication.
Taste aversion is a common problem with chemotherapy patients, who become nauseated because of the drug therapy but associate the nausea with consumption of food.
Taste aversion has been demonstrated in a wide variety of both captive and free-ranging predators. In these studies, animals that consume a bait laced with an undetectable dose of an aversion agent avoid both baits and live prey with the same taste and scent as the baits. When predators detect the aversion agent in the baits, they quickly form aversions to the baits, but discriminate between these and different-tasting live prey.
Stimulus generalization is another learning phenomenon that can be illustrated by conditioned taste aversion. This phenomenon demonstrates that we tend to develop aversions even to types of food that resemble the foods which cause us illness. For example, if one eats an orange and gets sick, one might also avoid eating tangerines and clementines because they look similar to oranges, and might lead one to think that they are also dangerous.
Although the terms "taste avoidance" and "taste aversion" are often used interchangeably, studies with rats indicate they are not necessarily synonymous. "Aversion" is defined as "a strong dislike or disinclination" [4] and "Avoidance" is defined as "stopping oneself from doing".[5] The difference is that in avoidance, the organism is controlling its behavioural responses. Taste avoidance and taste aversion can at times go hand in hand, but they cannot be looked at or be defined the same way.
Studies on rats to determine how they react to different tasting liquids and injections indicate this difference. Scientists measured the facial and somatic reactions of rats after exposure to a flavored solution (sucrose or salt) which do not induce abnormal feelings. However, immediately after the rat ingests the solution, the rat is injected with a drug that induces nausea. The rat subsequently expresses a disgust reaction towards the solution, seen by mouth gaping. This is a Pavlovian conditioned response as the rat is associating the disgust with the solution that it drank immediately before the injection. The rat experiences taste aversion. This is similar to when a human, for example, eats a steak that is perfectly safe and edible and coincidentally contracts a stomach bug and starts vomiting within a few hours of eating the steak. Although the human may know that the vomiting was due to a virus and not from eating the steak, the conditioned response in the brain associates the steak with vomiting due to the timing and the human may avoid steak because he has developed a learned taste aversion to the steak.
When examining taste avoidance, however, the rat may avoid a food yet still enjoy it and choose it over others. In further tests, the rats were tested with another sucrose solution but this time it was paired with a drug that gave positive, euphoric effects, such as amphetamine, cocaine, and morphine. The rats showed positive reactions to the drugs. However, rats react to any change in physiological state as a sign of danger and avoided approaching these solutions. When one of these euphoric solutions was placed next to another solution that had a learned taste aversion, the rat would choose the substance that it had a taste avoidance towards. Scientists theorize that in terms of evolution, because rats are unable to vomit and immediately purge toxins, rats have developed a strong “first line of defense,” which is their sense of taste and smell. This further shows the importance of taste and the correlation between taste and any change in physiological state, whether it be good or bad. Because rats rely upon taste and pairing it with a reaction rather than relying on later responses that involve the gastrointestinal tract, taste avoidance is just as prevalent as taste aversion, though the two don’t necessarily go hand in hand.[6]
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| リンク元 | 「味覚嫌悪」 |
| 関連記事 | 「aversion」「taste」 |