The acquisition phase of classical conditioning consists of the develpment of the CR as trials proceed. On each trial the CS is paired with the US. For early trials the animal does not make CRs. Then the first CRs appear, but they are small. As the trials proceed, the CRs rapidly become strong. Finally, the CRs reach their maximum strength, called the asymptote.
In the extinction phase of classical conditioning, the CS is presented on each trial as during acquisiton, but the US is not presented. As the extinction trials proceed, the CRs weaken and eventually disappear.
In reacquisiton the CS is again paired with the US. The CRs quickly return to asymptote. Thus this phase is sometmes called rapid reacquisition.
Once a CR is learned to a CS, the subject will generalize the CR to similar CSs. In Pavlov's work he found that if a particular tone was used as CS, the dog would make CRs to similar tones. Generalization is condidered to be a fundamental behavioural process. That is, organisms have a tendency to generalize tresponses form situation to situation.
Although generalization is a fundamental process, organisms can learn to make a discrimiation. Pavlov performed research with tones that differed in frequency. If one tone served as the CS paired with a US, whereas the other tone was presented without the US, the dog learned to make CRs only to the tone paired with the US. This is an example of the formation of a discrimination. (During the acquisiton process, once CRs appeared they were made to both CSs , demonstrating generalization. The discrimination only appeared later in training).
Research demonstraitng sponstaneous recovery begins with pairing a CS with a US many times(acquisition). Next, the CS is presented without the US many times(extinction). The subject is then given a waiting period, perhaps as short as one day. When presented the CS (without the US) the subject will make a CR for a number of trials. This defines spontaneous recovery of the CR.