Habituation is a nonassociative learning phenomenon that is often confused with extinction. Extinction results when the CS fails to predict the occurrence of the associated US, that is, the CS no longer elicits the CR. In contrast, habituation occurs when the US is repeatedly presented until the associated UR is no longer elicited. For instance, the occurrence of a strange loud noise will evoke a vig orous orienting response from most dogs. However, if the noise is repeated many times, dogs may learn to ignore it. In effect, they have learned that their original reaction is no longer appropriate, determining that the noise is irrelevant to them and that it can be safely ignored. If subsequently exposed to the same noise a day or two later, the dogs' reaction will have probably returned to nearly the same strength as it was prior to habituation. This effect is known as spontaneous recovery. Spontaneous recovery affects both habituated URs and extinguished CRs.
Sensitization produces the opposite effect of habituation. The sensitization effect is produced by exposing dogs to an intense sample of the US sufficient to elicit a startle or surprise reaction. Subsequent exposures to the US at lower intensities (perhaps previously ignored) will produce a noticeable increase in UR magnitude. Another method for sensitizing dogs to a US of low salience is to pair it with a different US of stronger intensity. Such US-US pairings are very useful in dog training. For instance, the vocal reprimand, while possessing some surprise/startle properties, is easily fatigued through repeated use but may be potentiated by being presented simultaneously (compound conditioning) with a startling US like the toss of a shaker can. Sensitization techniques are especially useful in training situations involving avoidance conditioning and aversive countercondi-tioning.
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