It is the fastest method of ingraining a conditioned response. It involves providing the conditioned stimulus before the appearance of the unconditioned stimulus. This develops a connection that helps in implying that the CS will be followed by the US. It is of two types.

• Delay Conditioning: The CS and US are presented at the same time, such that their psychological presentation overlaps.
• Trace Conditioning: The CS and US are separated by a short period of time, called the trace interval or the conditioning interval. The CS begins and ends before the US is introduced.

The CS and US are introduced and then withheld together simultaneously. It takes place in two steps, and two CS are used (CS1 and CS2). The CS1 is introduced with the US to produce an initial forward conditioning response. Then, the CS2 is provided along with the CS1 in absence of the US, in order to connect the CS2 and the US. It acts like an equality function, similar to that used in math, i.e. If a = b and b = c, then a = c. The US is immediately followed by a CS. It differs from forward conditioning due to the fact that the CS serves as an inhibitory function. In forward conditioning, the CS signals the onset of the US, whereas in backward conditioning the CS marks the end of the US. The US is presented at regular intervals of time, such that the time intervals act as a CS, thereby conditioning the subject to produce a CR at the same time intervals shortly before the US is provided. This method utilizes the organism's internal biological clock (CS). The CS and US are presented together, but the US is presented at other times as well without the presence of the CS. In this case, conditioning does not occur as the CS fails to elicit a CR. Instead, a CR is elicited by the ability of the organism to predict the occurrence of the US regardless of the presence or absence of the CS. This method is a type of conditioning reversal. A pre-established CS is used and repeatedly presented without the US till the subject stops producing a CR. However, various experiments have proved that extinction does not fully reverse conditioning.

It refers to the psychological pairing of a CS and a CR, as a result of successful conditioning. In other words, it signifies a strong CR in the presence of a CS but in the absence of a US. The more number of times a CS-US pairing is repeated, a stronger CR is generated. The strength of the CR also depends on the nature of the CS and US. It refers to the process of negating or removing the learned behavior. It occurs by employing the method of the same name till the CR is "extinguished". It occurs when a strong and unfamiliar stimulus is registered by the subject at around the same time as a CS. This leads to a distracted state of mind, in turn reducing the functional effect of the CS on the psyche of the subject. It is often employed to break or reduce the subject's association between the CS and CR. It refers to regaining a CR that was extinguished as a result of extinction. It may occur via five methods.

• Reacquisition: If the relevant CS is introduced along with the appropriate US, a CR may be elicited. Reacquisition occurs faster than acquisition.
• Spontaneous Recovery: If the CS of an extinguished CR is presented after a significant period of time, the CR may be presented weakly due to the extinction.
• External Inhibition: If an externally inhibited CS is introduced, without the occurrence of the external stimulus, the CR may be recovered temporarily.
• Reinstatement: If a US is provided to the subject within the same experimental parameters and environment as that during the response conditioning and extinction, the CR is elicited based on the memory of the CS. The CS generates the CR in any conditions after, the conditioning has been reinstated.
• Renewal: It is a natural process in which a subject recalls the conditioning, after undergoing extinction and then being placed in the same surroundings where it acquired the CR initially.

It occurs when two or more CS elicit the same CR. Similarities between the various CS lead to the development of a stronger CR in response to its specific CS. In case the two CS are very different in nature, the CR elicited by them will also show a few differences. This occurs when two CS elicit two very different CR. Where CS1 elicits a particular CR, the CS2 may induce a different CR2 or may not elicit any CR at all. This can be observed by presenting a CS1 with a US, and a CS2 without any US (extinction). It involves the use of a familiar or mild CS for conditioning purposes. This process is not conducive to quick and strong acquisition of learned behavior. It implies that associations between familiar objects take longer to develop than those between unfamiliar objects. It involves two steps. First, the organism is taught a mechanical function, like pressing a button. Once this function is learned, it is conditioned using a CS and a negative US (shocks or loud sounds). While the organism is carrying out the learned function, the CS is introduced, followed shortly by the negative US. This causes the development of an association between the function, CS, and the US. Due to the negative nature of the US, the organism slows or stops carrying out the mechanical function in order to escape the US. This is an example of an emotion-based conditioned response, fear being the emotion in this case. If paired with a positive US, the frequency of the function would increase. This phenomenon is used to test the strength of conditioning. This occurs in three steps. Initially, a normal conditioning is carried out with a positive CS (CS+), till the desired CR is obtained. Next the organism is exposed to two sets of signals - positive CS with a US (CS+/US) and positive CS with negative CS (CS+/CS-) without a US. This results in the production of CR in CS+/US cases and not in the CS+/CS- cases. Further, these organisms are subjected to two types of tests.

• Summation test: Again, the CS- is presented along with CS+ to the organism. If the organism's response to this pairing is lower than that to CS+ alone, it is said to be conditioned inhibition.
• Retardation test: When the CS- is presented together with the US, if the organism has been subject to conditioned inhibition, it will show a very slow acquisition of the conditioning as compared to if the organism had been conditioned without the second step of conditioning.

It occurs in two phases. First, a CS1 is presented with a US till a CR is obtained. Then, another CS2 is added, i.e. CS1 and CS2 are presented with the US. Then the organism is tested by being subjected to both CS individually without the US. The organism shows a CR in response to CS1 but fails to do so in case of CS2, thereby exhibiting a blocking effect created by the first conditioning over the second.

This theory has been put forward by Pavlov. It claims that conditioning does not imply the acquisition of new behavior, but is merely the organism's tendency to respond in pre-learned ways to new stimuli. According to Pavlov, the CS merely substitutes the US as a response-eliciting stimulus, and this theory is known as the stimulus-substitution theory. However, this theory faces various limitations, one of which is the fact that the CR is weaker than the UR in most cases, and in some cases the CR may even be the opposite of the UR. CR is opposite a UR in cases where a neutral US is paired with a negative CS. Many researchers have carried out such experiments and propose that this contrary nature is seen only when the evocation of the UR does not involve the central nervous system. It was created by Robert A. Rescorla (University of Pennsylvania) and Allan R. Wagner (Yale University) in 1972, as a model of conditioning that theorizes that an organism learns from the difference between expected events and actual events. It is based on the following assumptions.

• The expected response to a US is based on the summation of all related stimulus exposed to the organism during the course of a trial.
• Excitation and inhibition are opposite in nature, and cannot be elicited by the same stimulus.
• The strength of a stimulus is exhibited by the strength of the response it elicits.
• The CS is a salient constant.
• This model successfully explains the phenomenon of blocking and hence is accepted and regarded as one of the most influential models of learning.

According to this theory, when an organism is conditioned using a US and CS, it acquires a CS/US association as well as a context/US association. The context refers to the environmental conditions present during conditioning. The theory tests the difference in the comparative response to both associations. A CS is successful in eliciting a CR only if the CS/US relation is stronger than the context-US relation. Repeated CS/US presentations, leads to it becoming gradually stronger than the context-US pairing, till a strong CR is elicited by the CS. This theory claims that a response to a stimulus is determined by the discrepancy of the onsets and offsets of the CS and US. This discrepancy is recorded and used by the organism to predict the probability of a US occurring after a CS. Many experiments have been successful in proving that humans and animals are capable of learning to time events. However, this theory cannot account for some associative models and other research findings.

Classical conditioning is a useful tool to gain an insight towards the neural structures and processes. Its various forms are used to study and understand the neural structures and functions that are involved in learning and memory. They include the induction of conditioned reflex responses by way of fear conditioning, eye-blink conditioning, etc. Unconditioned reflexes are more stable than conditioned reflexes, that can be modified and extinguished. The differences between these two types of reflexes can be seen by observing the neural processes involved. While an unconditioned, natural reflex is generated by the lower divisions of the higher nervous system, the sub-cortical nuclei, brain stem and spinal cord, the conditioned reflexes are a function of the cerebral cortex. Conditioning is used to modify behavior patterns to help people with behavioral and cognitive disorders. They include therapies such as aversion therapy, desensitization, flooding, etc. Aversion therapy involves helping the patient relinquish an undesirable habit, by inducing an aversive reaction to the habit, in the subject via the help of classical conditioning. Desensitization, on the other hand is used to relieve patients of their phobias. Conditioning is used to help them relax and be untroubled by the stimulus that evokes phobias or anxiety in them. Another form of desensitization is the use of flooding therapy. It involves the exposure of the patient to the anxiety-causing stimulus in a repeated matter, till the patient is desensitized and the stimulus ceases to evoke an anxious response (extinction). It involves the administration of a stimulus along with a drug, such that an association is formed between the two, eventually leading the stimulus to evoke a physiological response similar to that produced by the drug, e.g, coffee drinkers feeling alert by the mere smell of coffee. However, it has been seen in some cases that CR acts as a compensatory mechanism produced to offset the effects of the drug. For example, repeated use of anti-pain medication causes the user to develop drug tolerance towards the doses, causing increase in dosage. If in such a case the CR is absent, the increased drug dose would lead to an overdose reaction. Stimuli received by the organism prior to food intake, can condition the organism to stimulate all digestive processes in anticipation of food. These processes may include salivation, secretion of digestive enzymes, and the release of hunger-related hormones. The anticipatory CS cause the organism to feel hungrier than they actually are. It involves the evocation of an emotional response such as phobia, fear, disgust, nausea, anger, sexual arousal, and joy in response to a CS. This type of conditioning is believed to be an adaptive mechanism in organisms, designed to protect it from harm or to prepare it for biological activity. Common examples of this type of conditioning include, the feeling of nausea when a certain food is smelled which caused an upset stomach in the past; the fear of an object that has caused pain in the past; the feeling of disgust when someone is seen behaving in an unsavory manner.

♦ Techniques used to train dogs and other such animals, to respond in a particular way to a specific word or phrase, are typical examples of conditioning. ♦ Rewarding good behavior and punishing bad behavior in a person helps in developing a better general conduct in that person. ♦ The use of taste aversion therapy is used to wean a person off a certain substance or food. ♦ The association of certain places or people with a distinct smell can lead to them being remembered, when the relevant scent is smelled. ♦ Sprinkling chilli powder on the fingertips of toddlers prevents them from biting their nails. ♦ A celebrity you hate endorses a particular product or brand. This makes you hate the product as well. ♦ When chewing gum is chewed on for too long, the jaw starts paining. Repeated occurrence of this causes you to refuse when someone offers you a chewing gum. ♦ When sitting on a creaky chair, every time you move, the chair makes a loud noise. This causes you to sit as still as possible in that chair. ♦ Every time you speak to a particular person, you are made fun of. Eventually you start avoiding that person. ♦ The smell of your date's cologne evokes memories of the good times together, leading you to believe that a similar experience could possibly reoccur. ♦ You accidentally bang your foot against a particular piece of furniture on multiple occasions. Later, you start being cautious around that furniture. ♦ In the novel, Brave New World by Aldous Huxley, classical conditioning is used to maintain social peace among the different castes in society.