A stimulus that automatically elicits a response without prior learning (Chance 2009).
The unconditioned stimulus (UCS) is a term described in the learning literature to describe a stimulus that automatically elicits a response (Chance 2009). The UCS is innate; no prior learning has to occur in order for the UCS to elicit a response. Unconditioned stimuli have “survival value” or are pertinent for survival (Domjan 2015) and examples can include smell, food, water, pain, temperature, and sexual stimulation. Conversely, the conditioned stimulus (CS) is a stimulus that elicits a response after it is associated with the UCS. Unlike the UCS, the CS requires prior learning in order to elicit a response, which becomes the conditioned response (CR; Krause and Corts 2014).
Standard Paradigm of the Unconditioned Stimulus
Ivan Pavlov, a famous nineteenth century physiologist, first described classical conditioning and the UCS when he began research on the physiology of dogs’ digestion. He found when he presented a dog with meat powder, the dog would immediately begin to salivate. The dog did not have to have any prior training to salivate following the presentation of the food stimulus. Thus, food served as an UCS to elicit salivation, the unconditioned response (UCR; Chance 2009). Upon several presentations of the meat powder to the dog, Pavlov noticed that the dog would begin to salivate even before being presented with the meat powder. Stimuli in the dog’s environment that once did not elicit a response, such as the sight of the food dish (neutral stimulus; NS), began to elicit salivation. So through association with the UCS, the food dish (CS) began to elicit a CR (Chance 2009).
Time Presentation of the Unconditioned Stimulus
The time between the presentation of the UCS and CS is known as contiguity. Contiguity may have an impact on how effective the UCS is at forming an association with the CS. Generally, the shorter the interval between the presentation of the CS following the UCS, the more effectively a CR will be formed (Chance 2009). There are four basic presentations of pairing the UCS with the CS: trace, delayed, simultaneous, and backward (Mazur 2016). In trace conditioning, the CS is presented and removed followed by the UCS. In delayed conditioning, the onset of the US occurs while the CS is still present. Delayed conditioning is sometimes differentiated between short-delay (e.g., few seconds in between pairing the UCS with the CS) and long-delay (e.g., several seconds to minutes or longer between pairing the UCS with the CS). If the UCS and the CS are presented at the same time, it is known as simultaneous conditioning. Finally, in backward conditioning, the onset and offset of the UCS precede the presentation of the CS. Relative to the trace, delayed, and simultaneous conditioning, backward conditioning is the least effective for learning the association between the UCS and CS. Furthermore, in order for the CS to continue to have an effect or elicit the CR, it must occasionally be paired with the UCS. Otherwise, extinction, or the reduction or removal of a response may occur (Mazur 2016).
Conclusion: Examples of Unconditioned Stimuli in Humans
The UCS is a stimulus that elicits a response automatically or involuntarily. Applied examples of unconditioned stimuli can include the following: (1) A puff of air (UCS) elicits a blink (UCR), (2) hot temperature (UCS) elicits perspiration (UCR), and (3) loud noise (UCS) elicits increased heart rate (UCR).
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