What Is an Unconditioned Stimulus?

The role of the unconditioned stimulus in learning

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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. 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.

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.
  • 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.

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.

The Role of the Neutral Stimulus

For the purposes of classical conditioning or learning, you can't have an unconditioned stimulus without a neutral 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.

How Timing Impacts Acquisition or Learning of a Behavior

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 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. Ringing the bell, the neutral stimulus, long before the unconditioned stimulus leads 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 conditioned produces the best results.

Source:

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

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