Classical Conditioning (also Pavlovian or Respondent Conditioning) is a form of associative learning that was first demonstrated by Ivan Pavlov. 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). Classical conditioning has been demonstrated in numerous species using a variety of methodologies. 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 Classical Conditioning of Aplysia gill and siphon withdrawal reflex.
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 was 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 bells to call the dogs to their food and, after a few repetitions, the dogs started to salivate in response to the bell. Thus, a neutral stimulus (bell) became a conditioned stimulus (CS) as a result of consistent pairing with the unconditioned stimulus (US - meat powder in this example). Pavlov referred to this learned relationship as a conditional reflex (now called Conditioned Response).
Types of Classical Conditioning
Types and variations of classical conditioning are all derived from the same source. 
The onset of the CS precedes the onset of the US. Three common forms of Forward Conditioning are: Short-delay, Long-delay, and Trace.
- Short-delay Conditioning
- The onset of the US is delayed relative to the onset of the CS. In this procedure, the CS may completely overlap with the US, or the CS may terminate at some point before the US offset. The term "short" refers to the Interstimulus interval (ISI), and is determined by the type of classical conditioning. For example, in some forms of classical conditioning, such as Eyeblink conditioning, ISIs in the range of 100 to 750 msec are typically considered short. In other forms of classical conditioning, such as in Taste aversion, ISIs in the range of minutes to 1 or 2 hours are considered short.
- Long-delay Conditioning
- In this procedure, the onset of the US is still delayed relative to the onset of the CS, but ISIs are longer than in the Short-delay Procedure. While the difference between Short and Long may appear trivial, the distinction is important because some forms of conditioning are best learned with a long delay, while others are best learned with a short delay.
- 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.
The CS and US are presented at the same time.
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The onset of the US precedes the onset of the CS. Rather than being a reliable predictor of an impending US (such as in Forward Conditioning), the CS actually serves as a signal that the US has ended. As a result, the CR is said to be inhibitory.
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.
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.
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.
Variations of Classical Conditioning Procedures
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 CS- and its presentation is never followed by the US. After a number of trials, the organism learns to discriminate CS+ trials and CS- trials such that CRs are only observed on CS- trials.
During Reversal Training, the CS+ and CS- are reversed and subjects learn to suppress responding to the previous CS+ and show CRs to the previous CS-.
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 the 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+/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.
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This form of classical conditioning also involves three phases.
- Phase 1:
- A CS (CS1) is paired with a US.
- Phase 2:
- CS1 is presented in compound with a new CS (CS2), and the compound is paired with the US.
- Phase 3:
- CS2 is paired with the US. Blocking is measured as impairment in the rate of learning to CS2 relative to organisms that did not experience Phase 2. Essentially, acquisition to CS2 is blocked during compound training because CRs had already formed to CS1.
Classical Conditioning Applied
John B. Watson's Little Albert
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John B. Watson proposed that emotions (such as fear) can be conditioned in humans. To study this, Watson conditioned an eleven month old child, Albert, to fear various neutral objects (including a rabbit, a dog and some wool) by repetitively pairing their presence with a loud noise. Watson's experiment was successful and resulted in little Albert displaying severe fear responses to the previously unfeared objects. The goal of Watson's experiment had been to prove that behaviour is learned, in contrast to the then-prevalent, Freudian belief that behaviour came from the unconscious.
While informative, Watson's study clearly violates modern society's ethical ideals. Currently, fear conditioning is studied in animals, usually for the purpose of learning about phobias or anxiety disorders such as post traumatic stress disorder.
Behavioral Therapies Based on Classical Conditioning
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In human psychology, implications for therapies and treatments using classical conditioning differ from operant conditioning. Therapies associated with classical conditioning are aversion therapy, flooding, systematic desensitization, and implosion therapy. Implosion therapy and "flooding" involve forcing the individual to face an object/situation giving rise to anxiety; both of these techniques have been criticized for being unethical since they have the potential to cause trauma.
Classical conditioning is short-term, usually requiring less time with therapists and less effort from patients, unlike humanistic therapies. The therapies mentioned in the last paragraph are intended to cause either aversive feelings toward something, or to reduce the aversion altogether. Classical conditioning is based on a repetitive behaviour system.
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This is a form of psychological therapy that is designed to eliminate, for example, sexual behaviour by associating an aversive stimulus such as nausea with sex. Because the aversive stimulus performs as a US and produces a UR, the association between the stimulus and behaviour leads to the same consequences each time. If the treatment has worked, the patient will not have a compulsion to engage in such behaviours again. This sort of treatment has been used to treat alcoholism as well as drug addiction.
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Patients might learn that the object of their phobias or fears are not so fearful if they can safely relive the feared stimulus. However, anxiety often obstructs such recovery. This obstruction is overcome by reintroducing the fear-producing object gradually by a process known as reciprocal inhibitions. A person constructs a hierarchy of events leading to the feared situation. This hierarchy is approached step by step and anxiety is relieved at every level. The fear is eventually removed if the therapy is performed correctly.
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 theory 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 (UCS), and can think, feel or respond to the CS as if it were actually the UCS.
The opposing theory, put forward by cognitive behaviorists, is stimulus-stimulus theory (S-S theory). Stimulus-stimulus theory, referred to as S-S theory, is a theoretical model of classical conditioning that suggests a cognitive component is required to understand classical conditioning and that stimulus-response 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. Stimulus-stimulus 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 UCS? S-R theory would suggest that the rats would continue to respond to the UCS, 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. 
Neural structures involved in classical conditioning
Dopamine neurons in the pars compacta of substantia nigra and the medially adjoining ventral tegmental area show short, phasic activations after presentation of appetitive US. These phasic dopamine responses transfer to the onset of conditioned stimuli. It has been suggested that the ventral striatum corresponds to the critic and responds during both pavlovian and instrumental conditioning and the dorsal striatum corresponds to the actor which mainly responds during operant conditioning. Amygdala has long been associated with pavlovian fear conditioning, but recent views suggest that amygdala also responds to appetitive stimuli. Neurons within the orbitofrontal cortex discriminate between visual stimuli that predict appetitive and aversive reinforcers  The cerebellum also appears to be involved in classical conditioning. Researchers demonstrated that lesions to pathways from the cerebellum stop the conditioned response, but do not stop the unconditioned response.
- Backward conditioning
- Blocking effect
- Conditioned taste aversion
- Eyeblink conditioning
- Fear conditioning
- Latent inhibition
- Learned helplessness
- Operant conditioning
- Placebo (origins of technical term)
- Rescorla-Wagner model of conditioning
- Reward system
- Preparedness (learning)
- Second-order conditioning
- Taste aversion
- Edwin B. Twitmyer
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