Learning is the process by which new knowledge, behaviors, attitudes, and ideas are acquired. Learning can occur through both unconscious and conscious pathways. Classical conditioning is one of those unconscious learning methods and is the most straightforward way in which humans can learn. Classical conditioning is the process in which an automatic, conditioned response is paired with specific stimuli. Although Edwin Twitmyer published findings pertaining to classical conditioning one year earlier, the best-known and most thorough work on classical conditioning is accredited to Ivan Pavlov, a Russian physiologist born in the mid-1800s. Pavlov had such a great impact on the study of classical conditioning that it is often referred to as Pavlovian conditioning.[1][2][3]

Classical conditioning was stumbled upon by accident. Pavlov was conducting research on the digestion of dogs when he noticed that the dogs’ physical reactions to food subtly changed over time. At first, the dogs would only salivate when the food was placed in front of them. However, later they salivated slightly before their food arrived. Pavlov realized that they were salivating at the noises that were consistently present before the food arrived; for example, the sound of a food cart is approaching.

To test his theory, Pavlov set up an experiment in which he rang a bell shortly before presenting food to the dogs. At first, the dogs elicited no response to the bells. However, eventually, the dogs began to salivate at the sound of the bell alone.

To understand classical conditioning, it is essential to be familiar with the following terms. A neutral stimulus is a stimulus that at first elicits no response. Pavlov introduced the ringing of the bell as a neutral stimulus. An unconditioned stimulus is a stimulus that leads to an automatic response. In Pavlov’s experiment, the food was the unconditioned stimulus. An unconditioned response is an automatic response to a stimulus. The dogs salivating for food is the unconditioned response in Pavlov’s experiment. A conditioned stimulus is a stimulus that can eventually trigger a conditioned response. In the described experiment, the conditioned stimulus was the ringing of the bell, and the conditioned response was salivation.

It is important to note that the neutral stimulus becomes the conditioned stimulus. Furthermore, it is important to realize that the unconditioned response and the conditioned response are the same except for which stimulus they are elicited by. In this case, salivation was the response, but the unconditioned response was triggered by food, whereas the conditioned response was triggered by the bell which indicated the coming of food.

Pavlov recorded several phenomena associated with classical conditioning. He found that the rate of acquisition, the initial stages of learning, depended on the noticeability of the stimulus and the time in between the introduction of the neutral stimulus and the unconditioned stimulus. In Pavlov’s experiment, this would translate to the time in between the bell ringing and the presentation of food. Second, Pavlov observed that the conditioned response was vulnerable to extinction. If the conditioned stimulus is continuously supplied in the absence of the unconditioned stimulus, then the conditioned response becomes weaker and weaker until it disappears. In Pavlov’s experiment, this would translate to Pavlov ringing the bell without giving food to the dogs. Eventually, the dogs would stop salivating to the sound of the bell. However, spontaneous recovery was also observed. Even if a substantial amount of time had passed, the conditioned response would easily recover if the neutral stimulus and the unconditioned stimulus were paired again. Lastly, he found that stimulus generalization and stimulus discrimination can occur. Stimulus generalization occurs when the dog can respond to stimuli that are similar to the conditioned stimulus. For example, if Pavlov’s dogs salivated at the sound of another ringing sound such as a cell phone ringing, that would be stimulus generalization. Stimulus discrimination, on the other hand, is being able to differentiate between similar stimuli and respond only to the correct stimuli.[4][5][6][7]

People can use classical conditioning to exploit others for their gain. A prominent example of this occurs in advertising. The advertiser generally attempts to get consumers to associate their product with a particular response or feeling so they are more likely to buy the product. Advertisers can use music or mouth-watering food in their ads to create an association with their product. These types of associations can lead to increased spending as well as poor eating habits especially if the product is unhealthy food. 

Most psychologists now agree that classical conditioning is a basic form of learning. Furthermore, it is well-known that Pavlovian principles can influence human health, emotion, motivation, and therapy of psychological disorders. There are many clinically related uses of classical conditioning. For example, former drug users often have a craving when they are in a drug-related environment or around people that they associate with previous highs. Drug counselors often advise these people to stay away from settings that could trigger a desire to take drugs again. Also, it has been proven that classical conditioning can even affect the human immune system. When a particular taste accompanies a drug that influences an immune response, sometimes the taste itself can induce the immune response at a later time. Another example can be found in the overcoming of phobias. One patient, who had feared to get into an elevator for 30 years, forced herself to enter 20 elevators a day. After 10 days, her fear had almost completely vanished.

O.H Mowrer discovered that certain behaviors can be reconditioned when he successfully developed a therapy for bed-wetters. In his therapy, the child would sleep on a liquid-sensitive pad connected to an alarm. Once moisture was detected, the alarm would go off. After repetition, bladder relaxation became associated with waking up and 75% of the time, frequent bed-wetters were healed and longer wet the bed while they slept.

Another example of an effective therapy that is used to cure phobias is counterconditioning, which pairs the trigger stimulus with a response that is contrary to fear. Two counterconditioning techniques that have been proven to be effective are exposure therapy and aversive therapy. In general, exposure therapies are therapies that expose people to what scares them. Two types of exposure therapies are systematic desensitization and virtual reality exposure therapy. In systematic desensitization, a pleasant, relaxed state is associated with increasing anxiety-triggering stimuli. This therapy is common in the treatment of phobias. Virtual reality exposure follows the same concept as general exposure therapy but uses virtual reality to do instead of real-life situations. Aversive conditioning has the goal of substituting a negative response for a positive response to a harmful stimulus. This is essentially the reverse of systematic desensitization in which a positive response is replaced with a negative response to a harmless stimulus. One common example of aversive conditioning is mixing alcohol with an extremely bitter taste or lacing fingernails with something that causes severe nausea. The problem with this therapy is that patients can differentiate between situations inside and outside of the psychiatrist’s office. An alcoholic understands that if he drinks alcohol, it will normally not have that bitter taste. For this reason, a combination of conditioning therapies is the best approach to treat certain issues.[8][9]

A combination of both behavior modification therapy with medications can lead to better clinical outcomes than if either option is used alone. Some studies show enhanced outcomes when certain drugs are used in the psychological treatment of anxiety disorders and even post-traumatic stress disorders. Despite the clinical efficacy of these studies, it should be noted that each individual can have a different treatment plan that is optimal to only them. Some people might respond better to only therapy or medications rather than a combination of both. Furthermore, combination treatments can be more of a "mix and match" treatment where two patients can have similar amounts of treatment success but use different medications and therapy options.[10][11][12]