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Unconditioned Stimulus in Classical Conditioning – Psychology

In classical conditioning, unconditioned stimulus (US or UCS) is defined as any stimulus that can naturally and automatically trigger a response without prior learning or practice. It is also called the primary reinforcer. The involuntary response is a reflex triggered whenever the UCS is present.

Examples of an Unconditioned Stimulus Triggering An Unconditioned Response

Russian physiologist, Ivan Pavlov, first discovered classical conditioning when he was feeding his dogs. The dogs would smell the food and automatically salivate. This natural response did not require any prior learning. So the food’s smell was the UCS​1​.

So, how to find the unconditioned stimulus?

An UCS can trigger a response naturally. This response is a biological reaction. A person or animal usually does not have control over this behavior​2​.

A dog begs for food with a bowl of dog food in front of it, is an example of unconditioned stimulus

Here are some examples of unconditioned stimulus.

  1. Touching a hot iron
    Touching a hot iron makes you withdraw your hand right away.
    The hot iron is the UCS.
  2. Eating
    Putting food into your mouth causes your mouth to water.
    The food is the UCS.
  3. Hurting your foot
    Dropping a rock on your foot makes you scream in pain.
    The rock is the UCS.
  4. Inhaling dust
    Dust entering your nose causes you to sneeze.
    Dust is the UCS.

The Difference Between Unconditioned Stimulus and Neutral Stimulus

An unconditioned stimulus elicits a natural, reflexive response, called the unconditioned response (UCR).

A stimulus that doesn’t naturally elicit a response is a neutral response. For example, food is a UCS for dogs and can cause salivation. But ringing a bell by itself doesn’t trigger the same response. The bell’s sound is hence a neutral stimulus.

dust enters nose causing woman to sneeze

The difference between Unconditioned Stimulus and Conditioned Stimulus

A neutral stimulus initially doesn’t trigger any particular response. However, when a neutral stimulus is presented together with an UCS, an association can form.

Classical conditioning happens when a neutral stimulus is paired with an UCS repeatedly to create associated learning. The previously neutral stimulus becomes a conditioned stimulus (CS) and can trigger the same response as the UCS.

So initially, the neutral stimulus does not affect a specific behavior. But after repeatedly presented together with the UCS, it becomes a CS, and the person or animal subconsciously learns to react with the same response when it’s present. This response is then called a conditioned response (CR).

For example, in Pavlov’s experiments, he sounded a bell whenever he brought food to his dogs. After multiple repetitions, the dogs learned to expect food and naturally salivated when they heard the bell’s sound even when they did not see the food. When UCS (food) was paired with a previously neutral stimulus (the sound of a bell), the neutral stimulus became a CS.


  1. 1.
    Kimmel HD. Inhibition of the unconditioned response in classical conditioning. Psychological Review. Published online 1966:232-240. doi:10.1037/h0023270
  2. 2.
    Clark RE. Classical Conditioning and Brain Systems: The Role of Awareness. Science. Published online April 3, 1998:77-81. doi:10.1126/science.280.5360.77

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