Using the terminology of classical conditioning, the conditioned stimulus (CS) is a learned stimulus that triggers a conditioned response. In Pavlov’s experiment, the sound of a bell was the conditioned stimulus, and the dogs’ salivation was the conditioned response.
A conditioned stimulus is “a previously neutral stimulus that, after being repeatedly associated with an unconditioned stimulus, evokes a conditioned response.” It gains this power only through learning, never through any built-in property of its own.
Before conditioning, psychologists call this same stimulus a neutral stimulus (NS): one with no built-in power to cause the response. Pairing it with an unconditioned stimulus (US) changes that.
A US already triggers an automatic response with no learning needed, called the unconditioned response (UR). Once conditioning succeeds, the stimulus becomes a CS. Its own learned reaction is the conditioned response (CR), similar to the UR but not identical.
The best-known work on classical conditioning came from Ivan Pavlov in the early 1900s, during his studies of digestion in dogs. His impact was so large that classical conditioning is often called Pavlovian conditioning. The name stuck.
For example, imagine you get horrible food poisoning the first time you eat sushi. Sushi starts out as a neutral stimulus.
Through its association with getting sick (the unconditioned stimulus), sushi becomes a conditioned stimulus. You might then feel sick from just the smell of sushi or the sight of raw fish.
Key Takeaways
- Definition: A conditioned stimulus is a previously neutral stimulus that, after being repeatedly paired with an unconditioned stimulus, comes to trigger a learned response on its own.
- Four Terms: The unconditioned stimulus (US) automatically produces the unconditioned response (UR). Once a neutral stimulus is paired with the US, it becomes the CS, which then produces the conditioned response (CR).
- Acquisition: Pairing the CS just before the US, again and again, builds the strongest and most reliable conditioned response.
- Extinction: Presenting the CS repeatedly with no US weakens and eventually stops the conditioned response, though the original learning is not fully erased.
- Not Just Pairing: A stimulus only becomes a strong CS when it genuinely predicts the US. One that occurs just as often without the US gains little conditioned power, however many times it is paired.
- Anxiety Link: A 2022 meta-analysis of dozens of studies found that fear generalising too far beyond the original conditioned stimulus is a modest but reliable marker of clinical anxiety (Fraunfelter, Gerdes, & Alpers, 2022).
The Four Terms of Classical Conditioning
What Makes a Stimulus “Conditioned”
A conditioned stimulus is defined by its history, not by any feature built into it. A bell, a taste, a room or a single word can all become one, provided it has been paired with something meaningful.
It works by signalling. Once learned, it becomes a cue that something important is about to happen.
A stimulus only becomes a strong CS, though, when it carries real information about what follows. Repetition alone is not enough. One paired with an outcome just as often as that outcome happens on its own gains little conditioned power, however many times the pairing occurs.
This is the modern, information-based view of learning: contingency, not mere contiguity. It replaced an older idea that mere repeated pairing was, by itself, enough.
CS, US, UR and CR: How They Relate
Classical conditioning connects two stimuli so that one comes to stand in for the other. An unconditioned stimulus (US), such as food or a loud noise, reliably and automatically produces an unconditioned response (UR), such as salivation or a startle.
A neutral stimulus produces no such response at first.
Once it is paired consistently with the US, it becomes the conditioned stimulus (CS). Presented alone, the CS then produces the conditioned response (CR).
The same stimulus carries two different names across this timeline. Before pairing, it is simply a neutral stimulus (NS); once conditioning succeeds, that identical stimulus is renamed the CS. Nothing about the stimulus itself changes, only its acquired power to produce a response.
The CS–CR pairing is always learned. The US–UR pairing is automatic and unlearned.
What is an example of a conditioned stimulus?
Pavlovs Dogs Experiment
Ivan Pavlov noticed that his dogs began salivating before food even arrived, in response to the footsteps and sounds that reliably came first. He turned this observation into a controlled experiment.
Aim: To test whether a signal with no natural connection to food could be trained to make dogs salivate.
Method: Pavlov rang a bell just before presenting food to the dogs, repeating the pairing across many trials while measuring how much they salivated.
Results: At first, the bell alone produced no response. Eventually, the dogs began to salivate at the sound of the bell, even with no food present.
Conclusion: The bell had become a conditioned stimulus, and salivation to the bell alone the conditioned response. A reflex can be transferred to an arbitrary signal purely through repeated pairing.
Little Albert (Watson & Rayner) Experiment
Behaviorist John B. Watson and graduate student Rosalie Rayner were the first to classically condition emotion in a human. Their case became famous as Little Albert.
Aim: To test whether fear could be classically conditioned into a human infant by pairing a neutral stimulus with a naturally frightening one.
Method: Albert was the subject. He was about 11 months old, and was shown a white rat, a rabbit, cotton wool, burning newspaper and masks, with no fear of any.
Watson and Rayner then paired the white rat with a loud clang made by striking a steel bar, across roughly seven trials.
Results: Albert came to cry and show fear at the sight of the rat alone. The fear had transferred completely. It also generalised to a rabbit, a dog, a fur coat and a Santa Claus mask.
Conclusion: A previously neutral object can become a conditioned stimulus for fear once paired with a frightening unconditioned stimulus, supporting a learned-acquisition account of phobias.
The study has serious limits: it used a single child with no control group, and it would be considered unethical today, since Albert’s fear was never removed.
Advertising
Classical conditioning is a powerful tool in advertising. Adverts pair a product with catchy music, bright colours, cute babies or attractive models, features that create an enjoyable response on their own.
When shoppers later see the product in the store, that same positive feeling resurfaces. They become more likely to buy.
Here, the advertisement is the unconditioned stimulus (US), and the enjoyment it produces is the unconditioned response (UR). Because the product is shown alongside the US, it becomes associated with it and turns into the conditioned stimulus (CS).
Gorn (1982) demonstrated this directly. Participants viewed pictures of different-coloured pens, one paired with pleasant music and the other with unpleasant music. When later offered a free pen, most people chose the colour that had been paired with the pleasant music.
Extinction and Spontaneous Recovery
Extinction
A conditioned stimulus is not permanent. Extinction is the gradual weakening of the conditioned response when the CS is presented repeatedly without the US. Pavlov found that sounding the bell again and again with no food caused salivation to fade until it stopped.
Extinction does not erase the original learning, though.
It reflects new learning that competes with the old association, rather than deleting it outright. The organism learns to inhibit the response to the CS, not to unlearn the CS itself.
This is why an “extinguished” conditioned stimulus can regain its power so easily.
This matters clinically. A treatment built only on extinction is fragile: the underlying CS–US link is still there, ready to reassert itself outside the room where treatment happened.
Spontaneous Recovery, Renewal and Reinstatement
Spontaneous recovery is the reappearance of an extinguished response after a rest period, once the CS is presented again. A dog whose salivation to the bell had faded would often salivate again after some hours away, even with no further pairings.
It usually returns weaker. It also fades again faster the second time.
Two related effects matter clinically. Renewal happens when an extinguished response returns in a new context. Reinstatement happens when the original US reappears on its own.
Together, these effects show that a conditioned stimulus keeps its underlying power even when it looks behaviourally silent. Extinction adds a separate, more fragile layer on top, one that is tied to context.
This is why fear can resurface after treatment, and why modern exposure therapy is designed to build learning that holds up across many different settings.
FAQs
1. What is a conditioned stimulus?
In classical conditioning, a conditioned stimulus is a previously neutral stimulus that, after being repeatedly associated with an unconditioned stimulus, evokes a conditioned response.
2. Is fear a conditioned response?
Yes, fear can be a conditioned response. Fear conditioning is a behavioral paradigm where organisms learn to predict aversive events or fear new stimuli.
This can be done by pairing a neutral stimulus (e.g., a tone) with an aversive stimulus (e.g. an electrical shock). Eventually, the neutral stimulus alone would be able to elicit the state of fear.
3. What is the difference between the conditioned stimulus and the unconditioned stimulus?
The conditioned stimulus begins as a neutral stimulus that eventually comes to automatically trigger a conditioned response after becoming associated with an unconditioned stimulus.
While a conditioned stimulus produces a learned response to the previously neutral stimulus, the unconditioned stimulus naturally and automatically triggers a response without any previous learning. There is no necessity to learn to respond to an unconditioned stimulus.
4. What is the difference between the conditioned stimulus and the neutral stimulus?
A neutral stimulus is a stimulus that does not produce an automatic response. But, in classical conditioning, the neutral stimulus will become a conditioned stimulus when consistently paired with an unconditioned stimulus.
For example, in Pavlov’s experiment, the sound of the bell began as a neutral stimulus but eventually became the conditioned stimulus that triggered the conditioned response of salivation.
5. How does a neutral stimulus become a conditioned stimulus?
A neutral stimulus becomes a conditioned stimulus when used together with an unconditioned stimulus.
With repeated exposures to both the neutral stimulus and the unconditioned stimulus at the same time, the neutral stimulus will begin to elicit a response known as a conditioned response.
Once the neutral stimulus elicits this conditioned response, the neutral stimulus becomes known as a conditioned stimulus.
6. What is the difference between the conditioned stimulus and the discriminative stimulus?
A discriminative stimulus refers to something, like a person or an event, that precedes a behavioral response. So, the discriminative stimulus comes first and then, the behavior follows as a direct result of this stimulus.
A conditioned stimulus, on the other hand, produces the response, while the discriminative stimulus signals the opportunity to respond.
For example, if a bell tone were the conditioned stimulus, discrimination would involve being able to tell the difference between the bell sound and other similar sounds. Or, in advertising, the packaging on a certain brand of snack cookies might help buyers discriminate between multiple similar products.
7. Is generalization a conditioned response?
Yes, generalization can be a conditioned response. In classical conditioning, stimulus generalization is the tendency for the conditioned stimulus to evoke similar responses after the response has been conditioned.
Once a person or animal has been trained to respond to a stimulus, very similar stimuli may produce the same response as well.
The Little Albert experiment described above is an example of stimulus generalization. In this experiment, Little Albert was conditioned to fear a white rat. He thus continued to exhibit a fear of objects similar to the white rat, such as a dog, rabbit, fur coat, and a white Santa Claus beard.
References
Clark, R. E. (2004). The classical origins of Pavlov’s conditioning. Integrative Physiological & Behavioral Science, 39 (4), 279-294.
Gorn, G. J. (1982). The effects of music in advertising on choice behavior: A classical conditioning approach. Journal of marketing, 46 (1), 94-101.
Harris, B. (1979). Whatever happened to little Albert?. American psychologist, 34 (2), 151.
Hawkins, D., Best, R., & Coney, K. (1998.) Consumer Behavior: Building Marketing Strategy (7th ed.). Boston, MA: McGraw-Hill.
Pavlov, I. P. (1897/1902). The work of the digestive glands. London: Griffin.
Pavlov, I. P. (1927). Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. Translated and edited by Anrep, GV (Oxford University Press, London, 1927).
Pavlov, I. P. (1928). Lectures on conditioned reflexes. (Translated by W.H. Gantt) London: Allen and Unwin.
Pavlov, I. P. (1955). Selected works. Moscow: Foreign Languages Publishing House.
Watson, J.B. (1913). Psychology as the behaviorist Views It. Psychological Review, 20, 158-177.
Watson, J. B., & Rayner, R. (1920). Conditioned emotional reactions. Journal of Experimental Psychology, 3(1), 1–14. https://doi.org/10.1037/h0069608
