Learning is the alteration of behavior as a result of experience. When an organism is observed to change its behavior, it is said to learn. Many theories have been formulated by psychologists to explain the process of learning. Early in the twentieth century, learning was primarily described through behaviorist principles that included associative, or conditioned response. Associative learning is the ability of an animal to connect a previously irrelevant stimulus with a particular response. One form of associative learning--classical conditioning--is based on the pairing of two stimuli. Through an association with an unconditioned stimulus, a conditioned stimulus eventually elicits a conditioned response, even when the unconditioned stimulus is absent. The earliest and most well-known documentation of associative learning was demonstrated by Ivan Pavlov, who conditioned dogs to salivate at the sound of a bell. In operant conditioning, a response is learned because it leads to a particular consequence...