What best describes a conditioned stimulus (CS)?

• A. A neutral stimulus presented before the unconditioned stimulus, that after pairing elicits a conditioned response.
• B. A stimulus that directly triggers a response without conditioning.
• C. A stimulus used only for punishment.
• D. A reward given to reinforce behavior.

Answer: (A)

In classical conditioning, a conditioned stimulus is a neutral cue that, after being paired with an unconditioned stimulus, begins to evoke a learned response. The key idea is that the CS starts off neutral and only gains predictive power after association with the unconditioned stimulus, so the organism learns to respond to the CS itself. This fits the description of a neutral stimulus presented before the unconditioned stimulus, which, after pairing, elicits a conditioned response.

The other scenarios describe things outside this learned signaling process: something that directly triggers a reflex without conditioning is the unconditioned stimulus, and aspects involving punishment or reinforcement pertain to operant conditioning rather than the CS-US pairing mechanism.