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What Is a Conditioned Stimulus in Psychology?

A conditioned stimulus is a stimulus that evokes a conditioned response. A previously neutral stimulus is associated with an unconditioned stimulus through the classical conditioning process. Eventually, this neutral stimulus starts to evoke the same response, at which point it becomes known as the conditioned stimulus.

For example, imagine that you hear the sound of an alarm (a neutral stimulus) every day before you each lunch. The sight of your lunch is the unconditioned stimulus that automatically triggers an unconditioned response. After this unconditioned stimulus is repeatedly paired with the alarm, the alarm becomes a conditioned stimulus that triggers a conditioned response.

The Conditioned Stimulus in Classical Conditioning

Classical conditioning is a type of learning that relies on forming associations with something that naturally triggers a response. By pairing a stimulus with something that already automatically causes a certain reaction, people will learn to display that same reaction when they are presented with a conditioned stimulus.

How this process works:

  • Prior to conditioning, an unconditioned stimulus naturally and automatically triggers a response.
  • Conditioning is used to pair a neutral stimulus with the unconditioned stimulus.
  • After the association is made, the previously neutral stimulus will lead to the same reaction.
  • At this point, it is referred to as a conditioned stimulus, and the ensuing reaction is known as the conditioned response.

Examples of a Conditioned Stimulus

It can be helpful to look at a few examples of how a conditioned stimulus can be used:

  • Sound of a bell: The bell in Pavlov’s classic experiment is a good example of a conditioned stimulus. While it was initially neutral, pairing it with the presentation of food (the unconditioned stimulus) eventually caused it to become a conditioned stimulus.
  • Business signs: The sight of a sign for your favorite restaurant might trigger feelings of hunger because it has become a conditioned stimulus signaling the presentation of food.
  • Perfume or cologne: A particular scent you associate with a particular person or memory can be a conditioned stimulus that triggers emotions linked to that memory.
  • Text notifications: The sound or vibration of a text message notification can become a conditioned stimulus, leading you to check your phone even when you are not expecting a message.
  • Food aversions: If you get sick after eating something, that food can become a conditioned stimulus that causes you to feel queasy whenever you encounter it.
  • Stimuli associated with alcohol or drugs: Conditioned stimuli that are associated with drug or alcohol use can contribute to cravings that make recovery more challenging.

The Conditioned Stimulus in Pavlov’s Experiments

Ivan Pavlov was a famous physiologist known for the discovery of classical conditioning. In his experiments, he presented dogs with food (the unconditioned stimulus) which naturally caused the dogs to salivate (the unconditioned response). 

He noticed that the dogs would also salivate to other stimuli associated with the presentation of food, such as the sound of a bell (the conditioned stimulus). Once the association was formed, the dogs would salivate when they heard the bell, even though the food was not present.

The Conditioned Stimulus in the Little Albert Experiment

The “Little Albert” experiment was a classic psychology experiment conducted by the behaviorists John B. Watson and Rosalie Rayner. The experiment paired the sound of a loud noise (the unconditioned stimulus) with the sight of a white rat (a neutral stimulus), which led to a fear response (the unconditioned response).

After repeated pairings, the sight of the white rat alone could trigger the fear response. At this point, the white rat was the conditioned stimulus, and the feelings of fear were the conditioned response.

The Unconditioned Stimulus vs. the Conditioned Stimulus

Sometimes, it can be a little challenging to figure out which is the unconditioned stimulus and which is the conditioned stimulus. To tell them apart, look at the stimulus itself. 

Is it something that automatically and naturally leads to a response? If yes, then it is an unconditioned stimulus. 

Is it something that needs to be learned by pairing it with something else in order to form a response? If yes, then it is a conditioned response.

Higher-Order vs. Second-Order Conditioning

Sometimes, a conditioned stimulus can be paired with another stimulus which can then also evoke the conditioned response. In this case, the second conditioned stimulus will cause the behavior without ever being directly paired with the unconditioned stimulus.

For example, an animal might first learn to associate the sound of a bell with the presentation of food, which then leads to salivation.

In second-order conditioning, a light might then also be paired with the sound of the bell. After conditioning, the light will also come to evoke the conditioned response.

The first conditioned stimulus can be used to establish the first step of the learning process and then serve as a basis for learning subsequent information about another stimulus.

Factors That Affect Responses to the Conditioned Stimulus

There are a few important factors that can affect how an organism responds to a conditioned stimulus: stimulus generalization, stimulus discrimination, and extinction.

Researchers have also found that the nature of the conditioned stimulus can also impact the motivational properties it acquires.

Stimulus Generalization

Stimulus generalization happens when an organism generalizes a response to stimuli that are similar to the conditioned stimulus. In Watson’s experiments, Little Albert generalized the fear response to white objects similar to the white rat, including stuffed toys. 

Stimulus Discrimination

Stimulus discrimination involves the ability to distinguish between a specific conditioned stimulus and other things that are similar. This can be important because it means that the organism will only respond to the specific conditioned stimulus and not other similar signals.


If an unconditioned stimulus no longer follows the conditioned stimulus, extinction may eventually occur. By not pairing the two together, the relationship will weaken over time, and the response to the conditioned stimulus will eventually disappear.

Why the Conditioned Stimulus Is Important

The conditioned stimulus can play an important part in the learning process. It is important to understand how it works so that people can know how certain behaviors are learned and how they can also be unlearned.

For example, post-traumatic stress disorder (PTSD) is a mental health condition in which trauma (the unconditioned stimulus) leads to feelings of distress, fear, and anxiety (the unconditioned response).

After a trauma has passed, certain environmental, physiological, cognitive, and emotional cues can become conditioned stimuli that trigger fear responses. This can lead to problems with intrusive thoughts, flashbacks, and fear responses that are similar to what happened immediately after the traumatic event.

Treatments for PTSD may focus on helping people unlearn these associations by changing how people think about and appraise these PTSD triggers.

Key Points to Remember

  • A conditioned stimulus (CS) is initially neutral but becomes associated with an unconditioned stimulus (UCS) through repeated pairing.
  • After association with the UCS, the CS alone can evoke a conditioned response (CR) similar to the response produced by the UCS.
  • The process of classical conditioning involves the gradual formation of this association between the CS and the UCS.
  • CS can vary widely, including auditory, visual, olfactory, or even abstract stimuli, and they play a crucial role in learning and behavioral adaptation.


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Lee J. C. (2021). Second-order conditioning in humans. Frontiers in Behavioral Neuroscience, 15, 672628.

Meyer, P. J., Cogan, E. S., & Robinson, T. E. (2014). The form of a conditioned stimulus can influence the degree to which it acquires incentive motivational properties. PloS one, 9(6), e98163.

Rehman, I., Mahabadi, N., Sanvictores, T., & Rehman, C. I. (2023). Classical Conditioning. In StatPearls. StatPearls Publishing.

VanElzakker, M. B., Dahlgren, M. K., Davis, F. C., Dubois, S., & Shin, L. M. (2014). From Pavlov to PTSD: the extinction of conditioned fear in rodents, humans, and anxiety disorders. Neurobiology of Learning and Memory, 113, 3–18.