neutral stimulus

Meaning: In classical conditioning, a neutral stimulus (NS) is a stimulus that does not produce the response being studied before any learning has happened. It is repeatedly paired with an unconditioned stimulus (US), which triggers an unconditioned response (UR) without learning. After enough pairings, the NS alone triggers a learned response, the conditioned response (CR), and it is then called the conditioned stimulus (CS). The pairing phase is called acquisition.

On the MCAT: A passage describes an experiment, such as a tone sounded just before food is given, or a light flashed before a puff of air to the eye. You are asked to label each element as NS, US, UR, CS or CR. To find the NS, ask which stimulus caused no response at first and was later paired with the US. Timing matters. Conditioning usually works best when the NS comes shortly before the US. Questions may also ask what happens in extinction, when the CS is presented repeatedly without the US and the CR weakens.

Common confusion: The NS and the CS are the same physical stimulus at different stages. It is neutral before conditioning and conditioned after. Neutral does not mean unnoticed. The stimulus may get attention, but it does not cause the target response, such as salivation. The US is never the neutral stimulus, because it triggers the response without learning.