What Is an Unconditioned Stimulus?

Unconditioned Stimulus
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In classical conditioning, the unconditioned stimulus (UCS) is one that unconditionally, naturally, and automatically triggers a response. For example, when you smell one of your favorite foods, you may immediately feel very hungry. In this example, the smell of the food is 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, 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.
  • Pollen from grass and flowers causes you to sneeze. The pollen from the grass and flowers is the unconditioned stimulus.

In each of these examples, the UCS naturally triggers an unconditioned response. You don't have to learn to respond to the UCS.

How Timing Impacts Acquisition of a Behavior

The amount of time that passes between presenting the initially neutral stimulus and the unconditioned stimulus is one of the most important determinants of whether learning will actually occur. The timing of how the neutral stimulus and the UCS are presented is what influences whether or not an association will be formed, a principle that is known as the congruity hypothesis.

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

Different types of conditioned may utilize different temporal relationships 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 tends to result in weak results.
  • In trace conditioning, the neutral stimulus is presented briefly and then halted, then the unconditioned stimulus is presented. This type of conditioned produces good results.
  • In delayed conditioning, the neutral stimulus is presented and continues while the unconditioned stimulus is offered. This type of conditioned produces the best results.

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Reference

Nicholas, L. (2008). Introduction to Psychology. Cape Town: Juta and Company.

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