The procedure used to produce conditioned food aversions resembles classical conditioning. The conditioned stimulus (CS) is usually a novel flavor, and the unconditioned stimulus (US) is often an emetic agent. The unconditioned response (UR) is not well defined and is assumed to be related to the nausea, malaise, or other internal disruptions induced by the US. After one or more pairings with the US, the CS then comes to elicit a conditioned response (CR) that presumably resembles the illness of the UR, and the animal acts as if the CS is aversive and avoids further contact with it. An important control procedure is an unpaired group that receives equivalent experience with both the CS and US but at sufficiently different times that conditioning to the CS does not occur.
Two types of tests commonly used to evaluate the effects of conditioning are the preference test and the taste reactivity test. In the preference test, animals are given access to a choice between the CS and another neutral substance, such as their normal diet or water. After conditioning, intake of the CS is selectively reduced. In the taste reactivity test (Grill & Norgren 1978), the CS is infused directly into the mouth of the subject, and species-typical ingestive or rejection reactions are quantified. After aversion conditioning with emetic agents, subjects increase their rejection reactions and decrease their ingestive reactions.
In sharp contrast to the traditional classical conditioning procedure, the interval from the CS to US in food aversion conditioning can be as long as a few hours and still produce robust conditioning in a single trial (Garcia, Hankins, & Rusiniak, 1974). These features of aversion conditioning help an animal to learn to avoid poisonous foods without multiple experiences with the poison even if the poison does not immediately make the animal sick.
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