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'''Classical conditioning''' (also '''Pavlovian''' or '''respondent conditioning''', '''Pavlovian reinforcement''') is a form of [[associative learning]] that was first demonstrated by [[Ivan Pavlov]] (1927).<ref name="Conditional Reflexes">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/).</ref> The typical procedure for inducing classical conditioning involves presentations of a neutral [[stimulus (physiology)|stimulus]] along with a stimulus of some significance, the "unconditioned stimulus". The neutral stimulus could be any event that does not result in an overt behavioral response from the organism under investigation. 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 neutral stimulus presented along with the unconditioned stimulus, it would become a ''conditioned stimulus'' (CS). 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]]''.

The original and most famous example of classical conditioning involved the [[saliva]]ry 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.

==Types==  
* '''Forward conditioning''': Learning is fastest in 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]].
* '''[[Extinction (psychology)|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.

===Generalization in classical conditioning===
In classical conditioning, after conditioning, subjects that show the conditioned response not just to the original conditioned stimulus but also to new stimuli that resemble that stimulus are displaying what is called ''generalization''. In addition, the magnitude of the response to the new stimulus also depends on the degree of similarity between the new stimulus and the original conditioned stimulus. For example, in an experiment conducted by Ivan Petrovich Pavlov (1927/1960), a dog conditioned to salivate to a 1,000-hertz (cycles-per-second) tone also salivated to tones of other frequencies, but the further the tone was in frequency from the original conditioned stimulus, the less the dog salivated to it.

===Extinction and spontaneous recovery classical conditioning===
Pavlov was also looking at the permanence of a conditioned reflex. Once the dog has learned to salivate to the sound of the bell, will this behavior continue to occur even if the bell sounded for many trials without any food stimuli? It was found that that without the US (food in this case), the bell elicited less and less salivation on subsequent trials eventually becoming extinct as a behavior. Even with extinction, there was no clear evidence that the animal returned to its fully unconditioned state. The conditioned reflex can be partially renewed after a certain time period has passed, a phenomenon known as spontaneous recovery. Furthermore, just a single pairing can recover the conditioned reflex (which can also go on and be extinguished after another series of trials without US). Pavlov concluded that the conditioned reflex is not entirely lost during extinction, but possibly inhibited. In this state, the behavior can be recovered after the passage of time or the recurrence of the unconditioned stimuli. These results are further validated in Bouton's 1994 study.

===Classical ISI discrimination conditioning===
This is a discrimination procedure in which two different CSs are used to signal two different [[interstimulus interval]]s. 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. dogss leared dis

===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+/CS− trials). Typically, organisms show CRs on CS+/US trials, but suppress responding on CS+/CS− trials.

;Phase 3
:In this retention test, the previous CS− is paired with the US. If conditioned inhibition has occurred, the rate of acquisition to the previous CS− should be impaired relative to organisms that did not experience Phase 2.

===Blocking===
{{main|Blocking effect}}
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.

;Test
: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.

==Applications==
===Little Albert===
{{main|Little Albert experiment}}
[[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 experimenters 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 object—even 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 (psychology)|flooding]] and [[systematic desensitization]]. 

Classical conditioning is short-term, usually requiring less time with therapists and less effort from patients, unlike [[humanistic psychology|humanistic]] therapies.{{Fact|date=January 2008}} The therapies mentioned are designed to cause either aversive feelings toward something, or to reduce unwanted fear and aversion.

===Conditioned drug response===
Certain drug reactions can also be partly viewed in terms of classical conditioning. Conditioned drug reactions can occur if a drug is repeatedly paired with a stimulus after a time which the stimulus along can evoke in the drug user the same effects as the drug itself. This is sometimes the case with caffeine; habitual coffee drinkers find that simply the smell of coffee gives them a feeling of alertness (Psychology 102). In other cases, repeated use of a drug can cause the body to develop a compensatory reaction to the drug in which the body enters a state that will counterbalance the effects of the drug. For example, if a drug causes the body to become less sensitive to pain, the compensatory reaction will be one that makes the user more sensitive to pain, to counteract the drug's pain-relieving effects. Thus, in the context of drug use this compensatory reaction is used to restore the body to homeostasis. Upon the repeated pairing of a drug and a stimulus, over time the stimulus, which has come to be associated with the effects of drug use, can trigger compensatory reactions in the body that counteract the effects of the drugs and thereby contributing to drug tolerance. Additionally, in the cases where the stimulus is absent at the time of drug use, the user is likely to overdose since there is no stimulus to elicit the compensatory reaction to counteract the effects of the drug (Psychology 103).

===Conditioned hunger===
Signals that consistently precede food intake can become conditioned stimuli for a set of bodily responses that prepares the body for food and digestion. These reflexive responses include the secretion of digestive juices into the stomach and the secretion of certain hormones into the blood stream, and they induce a state of hunger (Psychology 104). An example of conditioned hunger is the "appetizer effect". Any signal that consistently precedes a meal, such as a clock indicating that it is dinnertime, can cause us to feel hungrier than before the signal. The lateral hypothalamus (LH) is involved in the initiation of eating. The nigrostriatal pathway, which includes the substantia nigra, the lateral hypothalamus, and the basal ganglia have been shown to be involved in hunger motivation.

===Conditioned sexual response===
From an evolutionary viewpoint, classical conditioning involves an individual preparing itself for important biological events in the individual's life, particularly sexual activity. For example, a stimulus that is conditioned to occur before sexual interaction prepares animals both mentally (increased sex drive) and physically (sexually-aroused body responses). Sexual arousal can actually be conditioned in human subjects by pairing a conditioned stimulus like a picture of a jar of pennies with an unconditioned stimulus like an erotic film clip. Similar experiments involving blue gourami fish and domesticated quail have shown that these conditioning techniques often increase the number of offspring, compared to unconditioned males. These findings could possibly be beneficial in terms of conditioning techniques aimed to increase fertility rates in infertile individuals and endangered species (Psychology, Gray).

==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 A. Rescorla|Robert Rescorla]] undertook the following experiment.<ref>Rescorla, R (1973) Effect of US habituation following conditioning. ''Journal of Comparative and Physiological Psychology'', 82 137–143</ref> 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 [[habituate]]d 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.<ref>Psychology, Peter Gray ''Third Edition'' p. 121</ref> His theory still continues and is applied in everyday life.<ref name="Conditional Reflexes"/>

==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<ref>Laurence Sterne: The Life and Opinions of Tristram Shandy, Gentleman; Vol. 1, Chapter 1. IV</ref> 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,—on the first Sunday-night of every month throughout the whole year,—as certain as ever the Sunday-night came,—to wind up a large house-clock, which we had standing on the back-stairs head, with his own hands:—And being somewhere between fifty and sixty years of age at the time I have been speaking of,—he 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,—but the thoughts of some other things unavoidably popped into her head—& vice versa:—Which 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.


==References==

<references/>

==Further reading==
* Dayan, P., Kakade, S., & Montague, P.R. (2000). Learning and selective attention. ''Nature Neuroscience'', 3, 1218–1223. [http://www.nature.com/neuro/journal/v3/n11s/pdf/nn1100_1218.pdf Full text]
* Jami, S.A., Wright, W.G. & Glanzman, D.L. (2007). [http://www.jneurosci.org/cgi/reprint/27/12/3064 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). ''[http://psychclassics.yorku.ca/Pavlov/ Conditioned Reflexes]: An Investigation of the Physiological Activity of the Cerebral Cortex'' (translated by [[Gleb Vassilievitch von Anrep|G.V. Anrep]]). London: Oxford University Press. [http://books.google.com/books?id=cknrYDqAClkC&printsec=frontcover&dq=%22conditioned+reflexes%22&hl=en&ei=SsMCTfCwF4GUvAOwyLXNBg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCcQ6AEwAA#v=onepage&q&f=false Google preview of Dover 2003 reprint]
* 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.
* [http://en.wikibooks.org/wiki/Animal_Behavior/Learning#Associative_Learning wiki book on Animal behavior]

==External links==
* [http://www.scholarpedia.org/article/Classical_Conditioning Scholarpedia Classical conditioning]
* [http://www.scholarpedia.org/article/Computational_Models_of_Classical_Conditioning Scholarpedia Computational models of classical conditioning]
* [http://www.scholarpedia.org/article/Hermissenda Scholarpedia Hermissenda]