Classical conditioning (also Pavlovian or respondent conditioning, Pavlovian reinforcement) is a form of associative learning that was first demonstrated by Ivan Pavlov (1927). The typical procedure for inducing classical conditioning involves presentations of a neutral stimulus along with a stimulus of some significance. The neutral stimulus could be any event that does not result in an overt behavioral response from the organism under investigation. Pavlov referred to this as a conditioned stimulus (CS). Conversely, presentation of the significant stimulus necessarily evokes an innate, often reflexive, response. Pavlov called these the unconditioned stimulus (US) and unconditioned response (UR), respectively. If the CS and the US are repeatedly paired, eventually the two stimuli become associated and the organism begins to produce a behavioral response to the CS. Pavlov called this the conditioned response (CR).
Popular forms of classical conditioning that are used to study neural structures and functions that underlie learning and memory include fear conditioning, eyeblink conditioning, and the foot contraction conditioning of Hermissenda crassicornis.
One of Pavlov's dogs with a surgically implanted cannula
to measure salivation
, Pavlov Museum, 2005
The original and most famous example of classical conditioning involved the salivary conditioning of Pavlov's dogs. During his research on the physiology of digestion in dogs, Pavlov noticed that, rather than simply salivating in the presence of meat powder (an innate response to food that he called the unconditioned response), the dogs began to salivate in the presence of the lab technician who normally fed them. Pavlov called these psychic secretions. From this observation he predicted that, if a particular stimulus in the dog's surroundings were present when the dog was presented with meat powder, then this stimulus would become associated with food and cause salivation on its own. In his initial experiment, Pavlov used a bell to call the dogs to their food and, after a few repetitions, the dogs started to salivate in response to the bell.
Diagram representing forward conditioning. The time interval increases from left to right.
- Forward conditioning: During forward conditioning the onset of the CS precedes the onset of the US. Two common forms of forward conditioning are delay and trace conditioning.
- Delay conditioning: In delay conditioning the CS is presented and is overlapped by the presentation of the US.
- Trace conditioning: During trace conditioning the CS and US do not overlap. Instead, the CS is presented, a period of time is allowed to elapse during which no stimuli are presented, and then the US is presented. The stimulus free period is called the trace interval. It may also be called the conditioning interval.
- Simultaneous conditioning: During simultaneous conditioning, the CS and US are presented and terminated at the same time.
- Backward conditioning: Backward conditioning occurs when a conditioned stimulus immediately follows an unconditioned stimulus. Unlike traditional conditioning models, in which the conditioned stimulus precedes the unconditioned stimulus, the conditioned response tends to be inhibitory. This is because the conditioned stimulus serves as a signal that the unconditioned stimulus has ended, rather than a reliable method of predicting the future occurrence of the unconditioned stimulus.
- Temporal conditioning: The US is presented at regularly-timed intervals, and CR acquisition is dependent upon correct timing of the interval between US presentations. The background, or context, can serve as the CS in this example.
- Unpaired conditioning: The CS and US are not presented together. Usually they are presented as independent trials that are separated by a variable, or pseudo-random, interval. This procedure is used to study non-associative behavioral responses, such as sensitization.
- CS-alone extinction: The CS is presented in the absence of the US. This procedure is usually done after the CR has been acquired through "Forward conditioning" training. Eventually, the CR frequency is reduced to pre-training levels.
 Procedure variations
In addition to the simple procedures described above, some classical conditioning studies are designed to tap into more complex learning processes. Some common variations are discussed below.
 Classical discrimination/reversal conditioning
In this procedure, two CSs and one US are typically used. The CSs may be the same modality (such as lights of different intensity), or they may be different modalities (such as auditory CS and visual CS). In this procedure, one of the CSs is designated CS+ and its presentation is always followed by the US. The other CS is designated CSâ and its presentation is never followed by the US. After a number of trials, the organism learns to discriminate CS+ trials and CSâ trials such that CRs are only observed on CS+ trials.
During Reversal Training, the CS+ and CSâ are reversed and subjects learn to suppress responding to the previous CS+ and show CRs to the previous CSâ.
 Classical ISI discrimination conditioning
This is a discrimination procedure in which two different CSs are used to signal two different interstimulus intervals. For example, a dim light may be presented 30 seconds before a US, while a very bright light is presented 2 minutes before the US. Using this technique, organisms can learn to perform CRs that are appropriately timed for the two distinct CSs.
 Latent inhibition conditioning
In this procedure, a CS is presented several times before paired CS-US training commences. The pre-exposure of the subject to the CS before paired training slows the rate of CR acquisition relative to organisms that are not CS pre-exposed. Also see Latent inhibition for applications.
 Conditioned inhibition conditioning
Three phases of conditioning are typically used:
- Phase 1:
- A CS (CS+) is paired with a US until asymptotic CR levels are reached.
- Phase 2:
- CS+/US trials are continued, but interspersed with trials on which the CS+ in compound with a second CS, but not with the US (i.e., CS+/CSâ trials). Typically, organisms show CRs on CS+/US trials, but suppress responding on CS+/CSâ trials.
- Phase 3:
- In this retention test, the previous CSâ is paired with the US. If conditioned inhibition has occurred, the rate of acquisition to the previous CSâ should be impaired relative to organisms that did not experience Phase 2.
This form of classical conditioning involves two phases.
- Phase 1:
- A CS (CS1) is paired with a US.
- Phase 2:
- A compound CS (CS1+CS2) is paired with a US.
- A separate test for each CS (CS1 and CS2) is performed. The blocking effect is observed in a lack of conditioned response to CS2, suggesting that the first phase of training blocked the acquisition of the second CS.
 Little Albert
John B. Watson, founder of behaviorism, demonstrated classical conditioning empirically through experimentation using the Little Albert experiment in which a child ("Albert") was presented with a white rat (CS). After a control period in which the child reacted normally to the presence of the rat, the experimentors paired the presence of the rat with a loud, jarring noise caused by clanging two pipes together behind the child's head (US). As the trials progressed, the child began showing signs of distress at the sight of the rat, even when unaccompanied by the frightening noise. Furthermore, the child demonstrated generalization of stimulus associations, and showed distress when presented with any white, furry objectâeven such things as a rabbit, dog, a fur coat, and a Santa Claus mask with hair.
 Behavioral therapies
In human psychology, implications for therapies and treatments using classical conditioning differ from operant conditioning. Therapies associated with classical conditioning are aversion therapy, flooding and systematic desensitization.
Classical conditioning is short-term, usually requiring less time with therapists and less effort from patients, unlike humanistic therapies. The therapies mentioned are designed to cause either aversive feelings toward something, or to reduce unwanted fear and aversion.
 Theories of classical conditioning
There are two competing theories of how classical conditioning works. The first, stimulus-response theory, suggests that an association to the unconditioned stimulus is made with the conditioned stimulus within the brain, but without involving conscious thought. The second, stimulus-stimulus theory involves cognitive activity, in which the conditioned stimulus is associated to the concept of the unconditioned stimulus, a subtle but important distinction.
Stimulus-response theory, referred to as S-R theory, is a theoretical model of behavioral psychology that suggests humans and other animals can learn to associate a new stimulus, the conditioned stimulus (CS), with a pre-existing stimulus, the unconditioned stimulus (US), and can think, feel or respond to the CS as if it were actually the US.
The opposing theory, put forward by cognitive behaviorists, is stimulus-stimulus theory (S-S theory). S-S theory is a theoretical model of classical conditioning that suggests a cognitive component is required to understand classical conditioning and that S-R theory is an inadequate model. It proposes that a cognitive component is at play. S-R theory suggests that an animal can learn to associate a conditioned stimulus (CS) such as a bell, with the impending arrival of food termed the unconditioned stimulus, resulting in an observable behavior such as salivation. S-S theory suggests that instead the animal salivates to the bell because it is associated with the concept of food, which is a very fine but important distinction.
To test this theory, psychologist Robert Rescorla undertook the following experiment. Rats learned to associate a loud noise as the unconditioned stimulus, and a light as the conditioned stimulus. The response of the rats was to freeze and cease movement. What would happen then if the rats were habituated to the US? S-R theory would suggest that the rats would continue to respond to the CS, but if S-S theory is correct, they would be habituated to the concept of a loud sound (danger), and so would not freeze to the CS. The experimental results suggest that S-S was correct, as the rats no longer froze when exposed to the signal light. His theory still continues and is applied in everyday life.
 In popular culture
One of the earliest literary references to classical conditioning can be found in the comic novel The Life and Opinions of Tristram Shandy, Gentleman (1759) by Laurence Sterne. The narrator Tristram Shandy explains how his mother was conditioned by his father's habit of winding up a clock before having sex with his wife:
My father, [...], was, I believe, one of the most regular men in every thing he did [...] [H]e had made it a rule for many years of his life,âon the first Sunday-night of every month throughout the whole year,âas certain as ever the Sunday-night came,âto wind up a large house-clock, which we had standing on the back-stairs head, with his own hands:âAnd being somewhere between fifty and sixty years of age at the time I have been speaking of,âhe had likewise gradually brought some other little family concernments to the same period, in order, as he would often say to my uncle Toby, to get them all out of the way at one time, and be no more plagued and pestered with them the rest of the month. [...]
[F]rom an unhappy association of ideas, which have no connection in nature, it so fell out at length, that my poor mother could never hear the said clock wound up,âbut the thoughts of some other things unavoidably popped into her headâ& vice versa:âWhich strange combination of ideas, the sagacious Locke, who certainly understood the nature of these things better than most men, affirms to have produced more wry actions than all other sources of prejudice whatsoever.
Another example is in the dystopian novel, A Clockwork Orange in which the film's anti-hero and protagonist, Alex, is given a solution to cause severe nausea, and is forced to watch violent acts. This renders him unable to perform any violent acts without inducing similar nausea.
The song "Dinner Bell" by the group "They Might Be Giants" talks about "salivating dogs".
In the song "Cenoir Studies 02" by rapper "Canibus" (Germaine Williams), he says "Sometimes I wonder who's listening, the auditory Pavlovian conditioning's so sickening".
 See also
- ^ a b Pavlov, I.P. (1927/1960). Conditional Reflexes. New York: Dover Publications (the 1960 edition is an unaltered republication of the 1927 translation by Oxford University Press http://psychclassics.yorku.ca/Pavlov/).
- ^ Rescorla, R (1973) Effect of US habituation following conditioning. Journal of Comparative and Physiological Psychology, 82 17â143
- ^ Psychology, Peter Gray Third Edition p. 121
- ^ Laurence Sterne: The Life and Opinions of Tristram Shandy, Gentleman; Vol. 1, Chapter 1.IV
 Further reading
- Dayan, P., Kakade, S., & Montague, P.R. (2000). Learning and selective attention. Nature Neuroscience 3, 1218 - 1223. Full text
- Jami, S.A., Wright, W.G. & Glanzman, D.L. (2007). Differential Classical Conditioning of the Gill-Withdrawal Reflex in Aplysia Recruits Both NMDA Receptor-Dependent Enhancement and NMDA Receptor-Dependent Depression of the Reflex. The Journal of Neuroscience, 27, 3064-3068.
- Kirsch, I., Lynn, S.J., Vigorito, M. & Miller, R.R. (2004). The role of cognition in classical and operant conditioning. Journal of Clinical Psychology, 60, 369 - 392.
- Pavlov, I.P. (1927). Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex (translated by G.V. Anrep). London: Oxford University Press.
- Rescorla, R.A., & Wagner, A.R. (1972). A theory of Pavlovian conditioning. Variations in effectiveness of reinforcement and non-reinforcement. In A. Black & W.F. Prokasky, Jr. (eds.), Classical Conditioning II New York: Appleton-Century-Crofts.
- wiki book on Animal behavior
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