Pavlov (1849–1936) was one of the first scientists to demonstrate how animals learn through classical conditioning. Pavlov was a Russian physiologist, not a psychologist, who was studying the digestive system in dogs (Hunt, 2007). He accidentally discovered how animals learn . Pavlov was measuring the amount of saliva that dogs produced in response to various foods. However, over time, he also noticed that the dogs began to salivate not only at the taste of the food, but also at the sight of food or the food bowl, and even at the sound of the laboratory assistants’ footsteps (Pavlov, 1927). Salivating to food in the mouth is a reflex, so no learning is involved. However, dogs do not naturally salivate at the sight of an empty bowl or the sound of footsteps—they had learned to make the associations.

These unusual responses intrigued Pavlov, and he wondered what accounted for the behavior (Pavlov, 1927). He designed a series of carefully controlled experiments to see what other stimuli could cause the dogs to salivate. He was able to train the dogs to associate food with other unrelated (neutral) stimuli, such as the sound of a bell, a light, and a touch on the leg. Pavlov proposed that the dogs had two types of responses to their environment: (1) unconditioned (unlearned) responses, or reflexes, and (2) conditioned (learned) responses, which they learned through experience.

In Pavlov’s experiments, the dogs always salivated (response) when given some meat (stimulus). Thus, the meat is an example of an unconditioned stimulus (UCS) and the dogs’ salivation is an unconditioned response (UCR). In his classical conditioning experiments, to teach the dog a new association, Pavlov presented a neutral stimulus (NS) immediately before an unconditioned stimulus (UCS), and repeated the pairings many times. For example, Pavlov would ring a bell and then give the dogs the meat (Figure 6.3). The bell is a neutral stimulus (NS), because it does not naturally elicit a salivation response. Prior to conditioning, the dogs did not salivate when they just heard the bell.

By pairing the bell sound with giving the dog some meat many times, eventually, the previously neutral stimulus (the bell ringing) also began to elicit salivation. Thus, the neutral stimulus had become the conditioned stimulus (CS). The dogs began to salivate to the bell alone. The behavior caused by the conditioned stimulus is called the conditioned response (CR). Pavlov’s dogs had learned to associate the bell (CS) with being fed, and they began to salivate (CR) in anticipation of food.

Acquisition, Extinction, and Spontaneous Recovery

Pavlov conducted many experiments to try better understand the constraints under which the dogs could be conditioned to associate a neutral stimulus with food. For example, how long did the learning last for? He found that during the acquisition (learning) phase of the studies, when he always paired the neutral stimulus with food—the dogs always salivated. But, what if he played the bell many times but never followed up with any food? As you might have guessed, over time, the dogs began to produce less and less saliva, until the conditioned response disappeared—this is called extinction. On the following day, Pavlov was quite surprised to find that when he rang the bell, but did not give any meat—the dog started to salivate again. The CR had returned—this is called spontaneous recovery. Acquisition and extinction involve the strengthening and weakening, respectively, of a learned association. Pavlov found that the CR seems to persist for a long time. He also discovered that the CR could be quickly returned to previous levels simply by reminding the dog of the association by presenting a few bell—meat pairings.

Little Albert, Fear Conditioning, and Phobias

You might be wondering whether people also learn through classical conditioning? The answer is yes, they do. Before we go into several real-life applications of classical conditioning in people, we will describe a historical study with a human infant. The study was conducted by John B. Watson and his research assistant, Rosalie Rayner, at Johns Hopkins University in Baltimore at the beginning of the 20^th century. Watson was one of a group of psychologists in the United States who felt that the methods of introspection and psychoanalysis that were popular in Europe, were too subjective to be considered scientific. Watson believed that psychology should focus on behaviorism—that is, studies that depended on measuring outward observable behaviors (Hunt, 2007). Building on the work of Pavlov, Watson and Rayner conducted a series of studies designed to try and evoke learned fears in an 9-month-old boy, whom they referred to as Albert B. This study has been cited many times—especially as an example of an unethical study in Psychology. There has also been much speculation about the fate of “Little Albert” as the study participant came to be known.

Initially, Little Albert was shown and touched various stimuli, including a rabbit, a dog, a monkey, some masks, some cotton, and a white rat. He was not afraid of any of them, and so they can be considered neutral stimuli. In one study, Watson and Rayner conditioned Little Albert to associate a white rat with feeling scared. Every time they handed Albert the rat, Watson would make a loud clanging sound by hitting a hammer on a metal bar behind the little boy. The sound was very loud and scary and made Little Albert cry. Watson repeated the rat-clanging sound pairings seven times until Albert became frightened of the sight of the rat, even before the noise began. Can you figure out the UCS, CS, UCR, and CR in this study? This highly distressing study (which would not be permitted today) showed that emotions could become conditioned responses. According to Watson, Albert exhibited another phenomenon of classical conditioning—stimulus generalization. He was also scared by a somewhat similar stimulus—a Santa Claus mask with a white beard, even though he was not directly conditioned to fear it. Stimulus generalization is quite common in classical conditioning. For example, Pavlov found that once trained with the bell—meat pairings, dogs would also salivate to other similar sounds, such as a musical note on a piano. However, Pavlov then taught the dogs to recognize the difference between the two stimuli, by playing the piano note—without any meat repeatedly, and consistently ringing the bell before the meat all the other times. Thus, over time the dogs learned that the piano note was not associated with meat, but the bell was. This is called stimulus discrimination.

According to Watson, Little Albert was also conditioned to fear a rabbit (Figure 6.5) and a dog. We do not know whether these fears persisted through his life or how strongly he feared the animals. Albert B apparently left the hospital soon after the study and was not traceable. However, there have been many inaccurate portrayals of these studies in the literature, including embellishments by Watson himself (Harris, 1979).

Some clinical psychologists have suggested that phobias—a strong and irrational fear of a specific object, activity, or situation can arise though classical conditioning. For example, a child who is bitten or extremely frightened by a barking dog may learn to fear other dogs too (Oar et al., 2019). Somewhat relatedly, classical conditioning has also been used to help explain the experiences of people with posttraumatic stress disorder (PTSD). PTSD is a severe trauma and stressor-related disorder that can develop after exposure to a serious, often life-threatening traumatic event, (American Psychiatric Association, 2013). PTSD occurs when the individual develops a strong association between the situational factors surrounding the traumatic event (e.g., sounds or smells associated with guns, bombs, etc.) and the trauma itself. The trauma is a UCS because it automatically produces a fear response (UCR). Loud noises, flashing lights, or thinking about the situation in which the trauma occurred can become conditioned stimuli (CS) and so will produce a conditioned response (CR) of feeling extremely scared (Keane, Zimering, & Caddell, 1985). Thus some of the symptoms of PTSD may be explained by classical conditioning.

Taste Aversion

In general, learning often takes a lot of practice. For example, we saw that Pavlov’s dogs needed many repetitions of the CS-UCS pairings to learn the association. However, conditioned fears can develop after a single CS-UCS pairing, if the response that is evoked is especially intense. Taste-aversion, is another example of classical conditioning that can occur after one single CS-UCS pairing. You probably have had the experience of being ill after eating something and never wanting to eat it again. Even looking at the food or thinking about it makes you feel nauseous. Typically, we only learn to associate events that are close together in time. However, with taste aversion, several hours can pass between the CS (something eaten) and the UCR (nausea, vomiting etc.) caused by something like bacteria in the food (UCS). Garcia and Koelling (1966) showed that there are some biological constraints when learning to avoid specific foods. The conditioned stimulus has to be food-related, e.g., taste or smell; other non-food stimuli (like flashing lights) will not work. They discovered this by conducting a study investigating how rats learn to avoid poisons. They put a bottle of water containing a chemical that made rats sick, in each test cage. For one group of rats, they had sweetened the water so it had a distinctive taste, and for the other group, they flashed lights and played sounds whenever the rats drank from the water bottle. They found that rats that drank the sweet water learned to avoid the flavor, but none of the rats exposed to lights and sounds learned to avoid the bottle when the lights or sounds were presented.

Taste aversion and fear conditioning are two ways in which classical conditioning can contribute to a species’ survival by helping organisms learn to avoid stimuli that pose real dangers to health and welfare (Garcia & Rusiniak, 1980; Garcia & Koelling, 1966). However, cancer patients who are treated with chemotherapy also often develop taste aversions, but these are not helpful. These aversions can occur when a healthy food is eaten just prior to a chemotherapy session, and then the patient gets sick to their stomach after the session (Holmes, 1993; Jacobsen et al., 1993; Hutton et al., 2007; Skolin et al., 2006). Chemotherapy drugs often make people sick, but one way to avoid developing taste aversions to healthy foods during treatment is to ensure that the patient eats something with relatively low nutritional value just before a session. Broberg and Bernstein (1987) used this approach by having children undergoing chemotherapy eat a strongly flavored candy just before a session. The children developed an aversion to the candy flavor, but not to the nutrition-rich food they ate before the candy.