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'Classical conditioning' (also Pavlovian reinforcement, Pavlovian or respondent conditioning) is a type of learning procedure effective primarily on implicit memory. In classical conditioning two stimuli are presented in close succession repeatedly. Eventually, one stimulus becomes associated with the natural response of the other.

Origins
Classical conditioning was first noticed by Edwin B. Twitmyer. In 1902, he was studying the knee-jerk reflex response of a hammer striking a person's knee. The strike to the knee was delivered shortly after a bell was sounded. He noticed that on occasion the sound of the bell alone was enough to elicit the knee-jerk response.

The most famous example of classical conditioning was demonstrated by Ivan Pavlov (1927). His demonstration involved the salivary conditioning of 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.



Theory
Presentation of a stimulus evokes a natural response. Pavlov called these the unconditioned stimulus (US) and unconditioned response (UR), respectively. If a neutral stimulus is frequently presented along with the unconditioned stimulus, it becomes a conditioned stimulus (CS). Pavlov used the term "conditioned" because he wanted to emphasize that learning required a dependent or conditioned relationship between a CS and US. If the CS and US always occur together and never alone, this pairing causes the two stimuli to become associated. The organism eventually produces the same behavioral response to the CS alone as to the pairing of the US and CS. Pavlov called this the conditioned response (CR).


 * Theories of classical conditioning

Stimulus-stimulus (S–S) theory is a theoretical model of classical conditioning that suggests a cognitive component (conscious thought) is required to understand classical conditioning. The S–S theory suggests a cognitive activity in which the CS is associated to the concept of the stimulus. In the stimulus-response (S–R) theory however, it is proposed that a cognitive component is merely at play, and not a necessity. S-R theory suggests that an organism can learn to associate a CS with the associated US, resulting in an observable behavior. The difference between the concept of the stimulus, as opposed to the actual stimulus itself is an important distinction.

Forward conditioning
Learning is fastest in forward conditioning. During forward conditioning the onset of the CS precedes the onset of the US in order to signal that the US will follow. In the case of Pavlov, the bell (the CS) was presented before the meat powder (the US), making it an example of forward conditioning. Two common forms of forward conditioning are delay and trace conditioning.


 * Short-delay conditioning: During short-delay conditioning the beginning of the presentation of the US is delayed relative to the start of the CS. A delay in eyeblink conditioning is considered short if it ranges from 100 to 720 milliseconds (msec). In other varieties of conditioning, the delay is considered short as long as it is shorter than one minute. This type of conditioning is the most frequently used.


 * Long-delay conditioning: During long-delay conditioning the start of the US is delayed relative to the start of the CS, but the delays are longer than those in short-delay conditioning: ranging from 750 to 3000 msec in eyeblink conditioning or 5-10 minutes in other varieties of conditioning.


 * 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
During backward conditioning the US is presented before the CS. 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 of the ending of the unconditioned stimulus, 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 acquisition of the CR depends on the correct timing of the interval between presentations of the US. The context in which the US is presented, or the stable physical attributes of the organism to which the US is presented can serve as the CS in this example.

Unpaired conditioning
During unpaired conditioning the CS and US are not presented together. The CS and US are presented far enough apart that associations are not able to form between the stimuli. This procedure is used to study non-associative behavioral responses, such as sensitization.

CS-alone extinction
The CS is presented in the absence of an 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 or extinguished. Essentially, the stimulus is presented until habituation occurs.

One-trial Learning
One-trial learning occurs when it only takes one pairing of the US and CS to elicit the CR. Unlike other forms of conditioning, repeated pairings will not have an effect on the strength of the CR.

Taste aversion which was originally demonstrated by Dr. John Garcia, occurs when a subject associates the taste of a certain food with symptoms which would occur after ingesting a toxic substance (such as nausea or vomiting). This one-trial learning is adaptive since it will prevent a subject from ingesting a substance that would make them sick, however, it is possible for a taste aversion to occur coincidentally.


 * Example of a taste aversion: "I once had an acquaintance who told a very vivid story about eating a chicken enchilada while on vacation. Hours after eating the enchilada, she became violently ill. For years after that, she was unable to bring herself to eat a chicken enchilada and even felt queasy when she smelled foods that reminded her of that particular dish. This was despite the fact that she knew that her illness was not connected to eating that particular item. In reality, she had picked up a nasty stomach virus from one of her traveling companions who had been ill just days before the trip."

Taste aversions can be used as a form of Aversion therapy to help people overcome addictions or bad habits (such as nail biting). Aversion therapy has been used to treat alcoholics by pairing an emetic (which induces vomiting) with the alcohol. Following this experience a CR is produced by the smell and taste of alcohol. Alcoholics who are conditioned with a taste aversion to alcohol are more likely to abstain from alcohol than those who are not.

Acquisition
This is also known as the "learning stage" of classical conditioning.

Usually, the first time an US is paired with a CS is not sufficient to develop a CR. Conditioning occurs when this pairing between a CS and US is repeated several times. The time during which a CR first appears and gradually strengthens is known as the Acquisition stage. The increase in the frequency of the CR shows that the conditioning is occurring.

Generalization
After conditioning, subjects that show the conditioned response not only to the original conditioned stimulus but also to similar new stimuli resembling the original conditioned stimulus are displaying what is called generalization. For example, if a child is conditioned to fear a stuffed white rabbit, they will show fear of similar objects. See Little Albert experiment.

Discrimination
In classical conditioning, discrimination is the ability to differentiate between the CS and other stimuli that have not been paired with the US. Therefore, we get the development of a CR when one stimulus is presented, however not when other very similar stimuli are presented. .

Extinction and spontaneous recovery
Extinction occurs when the frequency of CRs decrease or disappear. This happens when the CS is no longer paired with the US. When the conditioned response, after becoming extinguished, is partially renewed it is known as spontaneous recovery. Following his experiments, Pavlov concluded that the conditioned response 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 stimulus.

Latent inhibition
In this procedure, a CS is presented several times before a paired CS–US training commences. The pre-exposure of the subject to the CS before paired training slows the rate of CR acquisition when compared to subjects that have not been pre-exposed to the initial CS. See Latent inhibition for applications.

Conditioned Inhibition
Three phases of conditioning are typically used in conditioned inhibition. In Phase 1 a CS (CS+) is paired with an US until asymptotic CR levels are reached (meaning the CR shows no significant sign of increase). Phase 2 occurs when trials with an association between the CS+ and US (CS+/US trials) are continued, but interspersed with some trials during which the CS+ is presented with a second CS (CS-), and not with the US (CS+/CS− trials). Typically, organisms show the CR on CS+/US trials, but suppress the response on CS+/CS− trials. Finally, in Phase 3 a retention test is conducted, in which only the previous CS− is paired with the US (CS-/US trials). If conditioned inhibition has occurred, the rate of acquisition of a CR on CS-/US trials for an organism that has experienced Phase 2 should be slower relative to organisms that did not experience Phase 2.

Blocking
The blocking effect of classical conditioning involves three phases. Phase 1 involves the pairing of a CS (CS1) with an US. Phase 2 follows with the presentation of a compound CS (CS1+CS2), paired with an US. In the Test phase, a separate test for each CS (CS1 and CS2) is performed. The blocking effect is observed if there is no CR to CS2, suggesting that the first phase of training blocked the acquisition of the CR to the second CS.

Neural Substrates of Classical Conditioning
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 eyeblink test helped to show that the cerebellum plays a role in classical conditioning, undergoing some neuroplasticity along with parts of other regions. The other parts used in the conditioning process often depend partly on the context of the conditioning, and on the nature of the stimuli and associated responses. Particularly, when rabbits were tested, it showed that the cerebellum is the main part of the brain used in conditioning of motor responses and reflexes. It was noticed through fear conditioning, that the use of the basolateral amygdala was critical in consolidating the memory in conditioning. With imapired use of the amygdala, the subjects are less likely to retain a CR from the conditioning. Additionally, in humans it has been shown that the hippocampus and amygdala are involved in conditioning, and are extensively associated in the process. In forward conditioning, subjects with damage to the hippocampus often perform normally in conditioning involving implicit memory, but do not acquire a CR in varieties that are reliant on some explicit memory. This suggests that the amygdala plays a stronger role in attaining the CR itself, while the hippocampus is used primarily in association of the details and conscious thought sometimes involved in the process of conditioning.

Behavioral therapies
In human psychology, implications for therapies and treatments using classical conditioning differ from operant conditioning. Examples of therapies associated with classical conditioning are aversion therapy, flooding and systematic desensitization.

Classical conditioning therapies are short-term, usually requiring less time with therapists and less effort from patients, unlike humanistic therapies. McGee, Donald Loring. Behavior Modification. Wellness.com, Inc. 2006. Retrieved on 2012-2-14. http://www.wellness.com/reference/health-and-wellness/behavior-modification 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, such as a pipe. After a time, the stimulus alone can evoke the same effects in the drug user 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. 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 that the user becomes more sensitive to pain, to counteract the drug's pain-relieving effects. This compensatory reaction is used to restore the body to homeostasis. When the drug is repeatedly paired with a stimulus, over time the stimulus may become a CS, and can trigger compensatory reactions in the body that counteract the effects of the drug, and thereby contribute to drug tolerance. In specific cases where the acquisition of the compensatory reaction as a CR has been conditioned by a consistently presented CS, when 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.

Conditioned hunger
Stimuli that consistently precede food intake can become conditioned stimuli for a set of bodily responses that prepare the body for food intake 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. An example of conditioned hunger is the "appetizer effect." Any signal that consistently precedes a meal, such as a clock indicating that it is time for dinner, can cause people to feel hungrier than before the signal. The lateral hypothalamus (LH) is involved in the initiation of eating, and 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 the conditioning of male blue gourami fish and domesticated quail have shown that these conditioning techniques often increase the number of offspring of the conditioned, compared to unconditioned males. These findings could possibly be beneficial in conditioning aimed to increase fertility rates in infertile individuals and endangered species.

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.

Another example is in the dystopian novel, A Clockwork Orange in which the novel'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.

Another example is from the TV series The Office. In the episode Phyllis' Wedding Jim conditions Dwight to want a breath mint whenever there is a computer chime.