Acquisition, Extinction, and Spontaneous Recovery In classical conditioning, the CS is originally neutral and produces no specific response. After several trials pairing the CS with the US, the CS alone comes to elicit the salivary response (the CR). Learning tends to take place fairly rapidly and then levels off as stable responding develops. In extinction, the CR diminishes quickly until it no longer occurs. A rest period, however, is typically followed by spontaneous recovery of the CR. In fact, a well-learned CR may show spontaneous recovery after more than one rest period even though there have been no additional learning trials.