conditioned stimulus

Meaning: In classical conditioning, a neutral stimulus (NS) produces no reflex at first. When it is repeatedly paired with an unconditioned stimulus (US), which naturally triggers an unconditioned response (UR), the NS becomes a conditioned stimulus (CS). The CS then elicits a conditioned response (CR), which is usually similar to the UR. The learning is an association: the CS signals that the US is coming. In Pavlov's experiments, food was the US, salivation to food was the UR, the bell became the CS, and salivation to the bell alone was the CR.

On the MCAT: Passages describe a pairing procedure and ask you to label each element. A common question is to identify the CS in a new scenario, such as a tone paired with an air puff to the eye or a hospital smell paired with nausea from chemotherapy. Expect questions on timing: acquisition is usually strongest when the CS comes shortly before the US. Also expect questions on what happens when the CS is presented repeatedly without the US (extinction), the reappearance of the CR after a rest period (spontaneous recovery), and responses to similar stimuli (generalization) or only to the exact one (discrimination).

Common confusion: Students swap the CS and the US. The US works without any training. The CS works only after learning. A cue to separate them is to ask which stimulus would trigger the response in a subject who has never been trained. That one is the US.