Unconditioned Stimulus in Classical Conditioning

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In the learning process known as classical conditioning, the unconditioned stimulus (UCS) is one that unconditionally, naturally, and automatically triggers a response. In other words, the response takes place without any prior learning.

For example, when you smell one of your favorite foods, you may immediately feel hungry. In this example, the smell of the food is the unconditioned stimulus.

Examples of the Unconditioned Stimulus

In Ivan Pavlov's classic experiment with dogs, the smell of food was the unconditioned stimulus. The dogs in his experiment would smell the food and then naturally begin to salivate in response. This response requires no learning, and it simply happens automatically.

Some more examples of the unconditioned stimulus include:

  • A feather tickling your nose causes you to sneeze. The feather tickling your nose is the unconditioned stimulus.
  • Cutting up an onion makes your eyes water. The onion is the unconditioned stimulus.
  • Pollen from grass and flowers cause you to sneeze. The pollen is the unconditioned stimulus.
  • Your cat running to its bowl whenever it smells food. The scent of food is the unconditioned stimulus.
  • A loud bang causes you to flinch away from the sound. The unexpected loud noise is the unconditioned stimulus because it automatically triggers a response with no prior learning.

In each of these examples, the unconditioned stimulus naturally triggers an unconditioned response or reflex. You don't have to learn to respond to the unconditioned stimulus - it simply occurs automatically.

The Role of the Neutral Stimulus

For the purposes of classical conditioning or learning, you need a neutral stimulus as well as an unconditioned stimulus. In other words, for conditioning to take place, you must first start by pairing a previously neutral stimulus with an unconditioned stimulus.

A neutral stimulus doesn't trigger any particular response at first, but when used together with an unconditioned stimulus, it can effectively stimulate learning. A good example of a neutral stimulus is a sound or a song.

When it is initially presented, the neutral stimulus has no effect on behavior. As it is repeatedly paired with an unconditioned stimulus, it will begin to cause the same response as the UCS.

For example, the sound of a squeaky door opening may initially be a neutral stimulus. If that sound is repeatedly paired with an unconditioned stimulus, such as you feeding your cat, that sound will eventually come to trigger a change in your cat's behavior. Once an association has been formed, your cat may react as if it is being fed every time it hears the squeaky door open.

Timing of Learned Behavior

Throughout the classical conditioning process, there are a number of different factors that can influence how quickly associations are learned. How much time that passes between presenting the initially neutral stimulus and the unconditioned stimulus is one of the most important factors in whether or not learning will actually occur.

The timing of how the neutral stimulus and the unconditioned stimulus are presented is what influences whether or not an association will be formed, a principle that is known as the theory of contiguity.

UCS and Classical Conditioning

In Ivan Pavlov's famous experiment, for example, the tone of the buzzer was initially a neutral stimulus, while the smell of food was the unconditioned stimulus. Presenting the tone close to presenting the smell of food resulted in a stronger association. Ringing the buzzer, the neutral stimulus, long before the unconditioned stimulus led to a much weaker or even nonexistent association.

Different types of conditioning may use different timing or order between the neutral stimulus and the UCS.

  • In simultaneous conditioning, the neutral stimulus is presented at the exact time as the unconditioned stimulus. This type of conditioning leads to weak learning.
  • In backward conditioning, the unconditioned stimulus is given first, and the neutral stimulus is presented afterward. This type of conditioning also tends to result in weak learning.
  • In trace conditioning, the neutral stimulus is presented briefly and then stopped, then the unconditioned stimulus is presented. This type of conditioning produces good results.
  • In delayed conditioning, the neutral stimulus is presented and continues while the unconditioned stimulus is offered. This type of conditioning produces the best results.
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2 Sources
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  1. Boakes RA, Costa DSJ. Temporal contiguity in associative learning: Interference and decay from an historical perspective. Journal of Experimental Psychology: Animal Learning and Cognition. 2014;40(4):381-400. doi:10.1037/xan0000040

  2. Prével A, Rivière V, Darcheville J, Urcelay GP. Conditioned reinforcement and backward association. Learning and Motivation. 2016;56:38-47. doi:10.1016/j.lmot.2016.09.004

Additional Reading
  • Pavlov I. Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex. London: Oxford University Press; 1927.