Unconditioned Stimulus in Classical Conditioning: Definition and Examples

What is Unconditioned Stimulus (US)?

An unconditioned stimulus (US or UCS) is one of the key terms used in classical conditioning, others include unconditioned response, neutral stimulus, conditioned stimulus, and conditioned response.

In classical conditioning, an unconditioned stimulus (US) is a stimulus that naturally and automatically triggers a specific response without any prior learning. It is something that elicits a natural and instinctive reaction from an organism.

For example, the smell of your favorite food making you feel hungry or a cold breeze causing you to shiver are unconditioned stimuli. These stimuli produce a response without any conditioning or training.

Ivan Pavlov’s experiments on the digestive response of dogs are widely recognized in the study of classical conditioning. His work demonstrated how certain stimuli, like food, can automatically elicit unconditioned responses, such as salivation.

Examples of Unconditioned Stimulus

Let’s look at some examples of the unconditioned stimulus (US).

Pavlov’s Dog Experiment

Pavlov’s groundbreaking experiment involved pairing the sound of a bell with the presentation of food to his dogs. At first, the bell had no special meaning to the dogs; it was just a neutral sound. However, after repeatedly sounding the bell before feeding them, something extraordinary happened. The dogs started salivating in anticipation of the food, even when the food itself was absent.

The sound of the bell, which was initially a neutral stimulus, had become a conditioned stimulus. It triggered a conditioned response—the dogs’ salivation. The unconditioned stimulus, in this case, was the food, which naturally and automatically produced the unconditioned response of salivation.

Related: Conditioned Stimulus in Classical Conditioning: Definition and Examples

Through this experiment, Pavlov brilliantly demonstrated the power of classical conditioning and shed light on the remarkable ways in which organisms learn and respond to their environment.

Unconditioned Stimulus in The Little Albert Experiment

In the famous “Little Albert” experiment conducted by behaviorists John B. Watson and Rosalie Rayner, they demonstrated how emotional reactions could be classically conditioned in humans.

They exposed a 9-month-old boy, Albert, to various stimuli, including a white rat, rabbit, monkey, masks, and burning newspapers.

Initially, Albert showed no fear. However, whenever they presented the white rat, they simultaneously made a loud noise (unconditioned stimulus), causing Albert to cry (unconditioned response). Over time, Albert associated the white rat with the loud noise and developed a fear response.

The white rat, which was once a neutral stimulus, became a conditioned stimulus that elicited fear in Albert. This experiment highlighted the role of the unconditioned stimulus in conditioning emotional responses.

Also Read: What is Psychology? Definition, Types, History, Uses, and Goals

Onion’s Smell Making Eyes Water

The pungent smell of an onion as you cut it serves as an example of an unconditioned stimulus. The natural, inherent properties of the onion’s odor automatically trigger a response in your body, causing your eyes to water. Without any prior conditioning or learning, the onion’s smell elicits an involuntary and instinctive reaction.

This unconditioned stimulus exemplifies how certain stimuli can directly provoke an unconditioned response, demonstrating the innate connection between sensory cues and automatic bodily reactions.

Smell of Your Favorite Food Inducing Hunger

The enticing smell of your favorite food serves as an example of an unconditioned stimulus. When you catch a whiff of that delicious scent, it naturally triggers a physiological response, making you feel hungry. This unconditioned stimulus evokes a primal reaction in your body, without any prior conditioning or learned associations.

The scent of your favorite food directly stimulates your appetite, creating an automatic and unlearned response. The powerful connection between the aroma and your hunger demonstrates how certain stimuli can instinctively elicit a specific, unconditioned response, showcasing the remarkable influence of sensory cues on our physiological state.

What is a Neutral Stimulus?

Neutral Stimulus: A Stimulus That Initially Doesn’t Trigger a Response

A neutral stimulus is like a blank canvas, lacking any inherent ability to elicit a response. It doesn’t cause any noticeable reaction on its own. However, it has the potential to become meaningful when paired with other stimuli to trigger a response through the process of conditioning.

Difference Between Unconditioned Stimulus and Conditioned Stimulus

The difference between an unconditioned stimulus and a conditioned stimulus lies in their innate abilities to evoke a response.

An unconditioned stimulus is naturally capable of triggering a response without any prior learning, while a conditioned stimulus acquires its ability to elicit a response through association with an unconditioned stimulus.

An unconditioned stimulus is like an instinctive button, instantly provoking a reaction, whereas a conditioned stimulus is more like a learned trigger that gains significance over time.

Unconditioned stimuli are innate and unlearned, while conditioned stimuli acquire their power by being repeatedly paired with unconditioned stimuli, forming new connections in our minds.

Next Read: Conditioned Response in Classical Conditioning: Definition and Examples


  • Beck HP, Levinson S, Irons G. Finding Little Albert: A journey to John B. Watson’s infant laboratoryAm Psychol. 2009;64(7):605-14. doi:10.1037/a0017234
  • Clark, R. E. (2004). The classical origins of Pavlov’s conditioning.  Integrative Physiological & Behavioral Science 39 (4), 279-294.
  • Kimmel HD. Inhibition of the unconditioned response in classical conditioning. Psychological Review. Published online 1966:232-240. doi:10.1037/h0023270

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