Ivan Pavlov

Meaning: Pavlov was studying digestion in dogs when he noticed they salivated before food arrived. He then paired a neutral stimulus (a bell or metronome) with food. Food is the unconditioned stimulus (US), and salivating to food is the unconditioned response (UR), which is reflexive and unlearned. After repeated pairings, the bell becomes a conditioned stimulus (CS), and salivating to the bell alone is the conditioned response (CR). Acquisition is this learning phase. Extinction is the weakening of the CR when the CS is presented repeatedly without the US. Spontaneous recovery is the reappearance of a weakened CR after a rest period. Generalization is responding to stimuli similar to the CS, and discrimination is responding only to the CS.

On the MCAT: A passage describes an everyday case, such as nausea at the smell of a food linked to past illness, or a person who tenses at a dentist's drill sound. You label the NS, US, UR, CS and CR, or predict what happens when the CS is presented alone or the timing of the pairing changes. Conditioning is generally strongest when the CS comes just before the US.

Common confusion: The UR and CR are often the same behavior, such as salivation, but they differ in the trigger. The US triggers the UR, and the CS triggers the CR. Pavlov's work is also not operant conditioning (Skinner), which links voluntary behavior to its consequences.