As you experience the world, one event often signals the occurrence of another. A flashing light in your rearview mirror signals that a police officer may pull you over. A ringing phone in the middle of the night signals that there may be a family emergency. Once you learn the connection between the first event and the second, your reaction to the first one changes. You may react anxiously to the flashing light and the ringing phone—even though the first time you ever saw a light flash or heard a phone ring, you were perfectly calm. The change in responding to one stimulus, after you learn its connection to a second one, is called learning through classical conditioning.

You’re already familiar with classical conditioning from everyday life. You’ve seen people’s reaction to a stimulus change once they learn that it predicts a second stimulus. Here are some examples.

Suppose a child hears a parent address her by her full name rather than a nickname—for instance, “Elizabeth” rather than “Lizzy”—and, immediately afterward, the parent scolds the child. If this happens a few times, the child learns that when she hears her full name, scolding will follow. She learns, in other words, that one stimulus (full name) predicts another (scolding). Once she learns this, her emotional response to the first stimulus will change. Through classical conditioning, she’ll become fearful when hearing her full name.

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Classical conditioning also can produce positive emotions. Imagine that you hear a musical jingle in the distance outside your window. Soon afterward, an ice cream truck, the source of the jingle, pulls up in front of your house. Once you learn to associate the distant jingle with the subsequent arrival of ice cream, you will (if you’re an ice cream lover) come to like the jingle.

Classical conditioning can change your reactions not only to specific sights and sounds (e.g., the jingle), but also to types of people and places. Suppose you once experienced the breakup of a relationship while visiting an amusement park. In the future, you might find amusement parks less amusing than you used to because you associate them with the breakup. Your emotional reaction to a type of situation, amusement parks, changes as a result of learning through classical conditioning. In this chapter’s opening story, you saw how a boy’s reactions to a type of object, buttons, changed after a frightening experience. That change was a case of classical conditioning.

How, exactly, does classical conditioning work? As you’ve seen elsewhere in this book, the question can be answered at different levels of analysis. Let’s first look at psychological processes in classical conditioning. Then we’ll move to a biological level of analysis, examining brain mechanisms that underlie this form of learning.

The Russian biologist Ivan Pavlov (1849–1936) initiated research on psychological processes in classical conditioning. He did so only after an earlier line of research that was purely biological. In studies of the digestive system, Pavlov placed different stimuli in a dog’s mouth and recorded the amounts of salivation produced. He found, for example, that food triggers salivation, which contributes to the chemical processing of food, but that marbles do not because the dog can spit out marbles without needing extra saliva (Pavlov, 1928). (Pavlov’s research on the digestive system earned him a Nobel Prize in 1904.) This connection between food and salivation is a biologically determined reflex, that is, an involuntary reaction that occurs automatically, without learning.

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PHYSIOLOGICAL REACTIONS TO PSYCHOLOGICAL STIMULI. While studying this biological process, Pavlov noticed something of psychological interest. Sometimes dogs salivated when no physical stimuli were in their mouths. After a few days in the lab, a dog might salivate when it heard the footsteps of a laboratory assistant who previously had brought it food.

Pavlov knew that no natural, inherited connection existed between the sound of footsteps and salivation. There was, however, a learned connection. The dogs learned that the sound of footsteps predicted the arrival of food. Once they did so, they began responding reflexively to the sound, which triggered the physiological response, salivation.

If you’re a pet owner, you probably have noticed this sort of learning yourself. Your dog may react excitedly not only when it sees dog food, but when it sees you open the cabinet door where you store her food. If so, your dog has learned that the open door signals that soon she’ll be dining.

Pavlov devised a research procedure to discover the exact processes through which animals learn to associate different types of stimuli. Let’s examine his classic procedures in detail.

STIMULUS–RESPONSE CONNECTIONS. Pavlov recognized that the essence of classical conditioning is a learned connection between two types of stimuli: (1) an unconditioned stimulus and (2) a conditioned stimulus.

An unconditioned stimulus (US) is one that elicits a reaction in an organism prior to any learning. The reaction it elicits is called an unconditioned response (UR). For example, if a sudden and very loud noise occurs in front of you, you will startle: Your head will jerk back and your hands will move up in the air. You do not have to learn to startle; the startle response occurs the first time you ever encounter the loud noise (Quevedo et al., 2010). The noise, then, is an unconditioned stimulus (US) and the startle response is an unconditioned response (UR). In his main experiments, the US Pavlov used was food and the UR was salivation.

The second type of stimulus, the conditioned stimulus (CS), is one that elicits a response from an organism only after the organism learns to associate it with an unconditioned stimulus. At first, the conditioned stimulus is neutral; it does not elicit any response. But once an organism (person or animal) learns that the neutral stimulus predicts an unconditioned stimulus, its reaction changes: The previously neutral stimulus becomes a CS that elicits a response, usually of the same general type as was elicited by the US. For instance, if, on a number of occasions, you experienced a flashing light and then a loud startling noise, you would learn that such a light predicts the noise and the light would elicit a startle response. Returning to an earlier example, once Lizzy learned that “Elizabeth” predicts scolding, the name “Elizabeth” elicited a fear response. In the terminology of classical conditioning, the response that is triggered by a conditioned stimulus is called a conditioned response (CR).

Let’s use the terminology you just learned—US, UR, CS, CR—to reexamine what Pavlov originally observed in his lab. Food was a US that elicited a UR, salivation. The sound of footsteps originally was a neutral stimulus. However, after the dogs learned that footsteps predict food, footsteps became a CS that produced the CR of salivation.

Pavlov developed formal research procedures (Figure 7.1) in which he substituted the sound of a bell for the experimenter’s footsteps. Pavlov rang the bell (the CS), gave the dog food (the US), and collected saliva to measure the dog’s response. He later determined whether the dog salivated upon hearing the bell. Inevitably, it did. All dogs learn through classical conditioning.

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It’s not just dogs who learn through classical conditioning. All members of the animal kingdom do it. In principle, any organism might learn any association between any conditioned stimulus and any unconditioned stimulus. Let’s look at classical conditioning in simple organisms.

CLASSICAL CONDITIONING IN SIMPLE ORGANISMS. Even fruit flies can learn through classical conditioning. In research, fruit flies have been placed in containers with two different odors. In one of the containers, the flies receive a mild electric shock, whereas in the other container, they receive no shock. Later, researchers put the flies in a third container that has one of the odors on one end, and the other odor on the other end. What happens? They fly away from the odor previously paired with shock (Dudai, 1988). This means that this simple organism, a fruit fly, learned to associate the CS, odor, with the US, shock.

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Another simple organism that learns through conditioning is the honeybee (Bitterman, 2006). Researchers placed honeybees in an area containing dishes of two different colors. Initially, the colors were neutral; honeybees didn’t prefer one to the other. The researchers then put a neutral stimulus (plain water) on one dish and an attractive US (a solution of water and sugar) on the other, and allowed the honeybees to fly to both dishes to learn what they contained. Later, researchers placed the honeybees in an area with empty dishes of the same two colors. Which color dish did the bees fly to first? They consistently flew to the color of dish that previously had contained the sugar solution. In other words, they learned that color, the CS, predicts sugar, the US.

What about more complex organisms? Let’s look at classical conditioning in organisms with the most complex brains of all: human beings.

CLASSICAL CONDITIONING IN HUMANS: EMOTION. Early in the twentieth century, an American psychologist, John Watson, read about Pavlov’s experiments and became intrigued. Could Pavlov’s research with dogs be extended to humans? To find out, he and a colleague, Rosalie Rayner, ran an experiment on the classical conditioning of human emotions. The emotion they studied was fear, and the human was an 11-month-old named Little Albert. (The guidelines for ethical treatment of research participants that you read about in Chapter 2 were not yet in place when they ran the study in 1920.)

Watson and Rayner’s experiment featured two key items, in addition to Little Albert himself: a white rat, which served as a conditioned stimulus, and a steel bar that made a loud noise—the unconditioned stimulus—when struck with a hammer. At the outset of the study, Little Albert seemed to like the rat, watching it closely and displaying no fear. However, he was quite afraid of the noise. Whenever Watson struck the bar with the hammer, the noise caused Little Albert to startle and cry (an unconditioned response).

After observing Little Albert’s reactions to the two stimuli, the rat and the noise, Watson and Rayner combined the two. They took the rat out of a basket and banged on the steel bar just as they showed the rat to Albert. Albert startled and fell over (Watson & Rayner, 1920). Watson and Rayner then repeated the pairing of the CS and US (the rat and the noise) six more times. Each time, Little Albert startled, fell over, or cried.

Next came the critical trial. Watson and Rayner showed the rat to Little Albert without banging on the bar. What do you think happened? Now Albert was afraid of the rat. When he saw it, he cried, fell over, and crawled away from it as fast as he could (Watson & Rayner, 1920). Through classical conditioning, Little Albert had learned a new emotional reaction—fear of the rat. Further tests indicated that Little Albert eventually developed fears of many objects that were in some way similar to the rat: a rabbit, a fur coat, and even a Santa Claus mask (Figure 7.2).

Note that Little Albert’s fear did not result from anything the rat did. Its behavior didn’t change. What changed is that Albert learned a CS–US association: He learned that the rat was a CS that predicted the noise, a US. Upon learning this, Little Albert became afraid of the rat. Watson and Rayner’s finding demonstrated that human emotions could be shaped by classical conditioning.

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CLASSICAL CONDITIONING IN HUMANS: SELF-ESTEEM AND SEXUAL RESPONSE. The effects of classical conditioning are not limited to simple responses, such as fear, or to young children, such as Little Albert. Conditioning can influence the complex responses of adults. Conditioning, for example, can affect people’s overall feelings about themselves, or their self-esteem.

In one self-esteem study, experimenters first asked participants to provide information about themselves (e.g., their first name and birthday). Next, classical conditioning trials paired this personal information with different types of facial expressions shown on a computer screen. In one experimental condition, all the faces were smiling. In another, they were mixed: smiling, frowning, and neutral. Afterward, the experimenters measured participants’ self-esteem. Conditioning influenced people’s feelings about themselves. The trials in which personal information consistently was paired with smiling faces distinctively increased self-esteem (Baccus, Baldwin, & Packer, 2004).

Another adult response that can be shaped by classical conditioning is sexual arousal. Conditioning can convert neutral stimuli into sexually arousing ones. In a study conducted with heterosexual men and women, the CS was a non-arousing photo of a person of the opposite gender, and the US was an arousing 30-second film of heterosexual activity. In the conditioning trials, the experimenters displayed the photo and then, immediately afterward, the film clip. They then measured participants’ sexual response to the photo, using physiological equipment that records arousal in the genitals. Classical conditioning altered sexual response. After conditioning, the photo alone aroused sexual response (Hoffmann, Janssen, & Turner, 2004).

Here’s the key point about these studies of flies, honeybees, Little Albert, self-esteem, and sexual arousal: Classical conditioning can be found everywhere. All complex organisms can learn when a CS predicts a US; afterward, their response to the CS changes.

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You’ve learned the basic fact of classical conditioning: Once organisms associate a conditioned stimulus with an unconditioned stimulus, the previously neutral CS evokes the same type of response as did the US. Yet there’s more to learn. Pavlov and later researchers investigated five key aspects of the learning process: acquisition, generalization, extinction, habituation, and compensatory responses.

ACQUISITION. In the study of learning, acquisition is attaining the ability to perform a new response; after learning, the organism is said to have “acquired” the response. When Pavlov’s dogs first salivated (the CR) upon hearing a bell (the CS), or Lizzy first responded fearfully (the CR) when hearing the name “Elizabeth” (the CS), the CR had been acquired.

Research has answered three basic questions about the acquisition of conditioned responses. The first is, if you want an organism to quickly acquire a CR, how long should you wait after presenting the CS, before presenting the US?

Try to answer this for yourself. Imagine Pavlov’s lab: To get an animal to learn, you ring the bell (the CS) and then wait—how long?—before presenting the food (the US).

If you answered, “not long at all,” you’re right! You should present the food almost immediately after ringing the bell. If you wait longer—that is, ring the bell but don’t present the food until a few minutes later—the dog might not recognize the connection between the bell and food. Research confirms that the amount of time elapsing between the presentation of the CS and US strongly affects learning. Animals learn most quickly when the gap between the CS and US is only a fraction of a second (Figure 7.3).

A second finding in the study of acquisition shows that animals have a keen sense of time. They learn not only that the CS predicts the US, but also the amount of time that transpires between the two (Savastano & Miller, 1998). Animals learn to respond to the CS at exactly the point in time in which the US is most likely to appear. For example, when the CS is a tone, the US is a puff of air to the eye, the UR is an eyeblink (caused by the air puff), and the duration between the CS and US is half a second, animals will learn to close their eyes precisely half a second after they hear the tone (Gluck, Mercado, & Myers, 2008).

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A third question is whether animals learn to associate the CS and US gradually or suddenly. If an animal learned gradually, it might respond slightly to a CS after 10–20 pairings of the CS and US, moderately after another 10–20 pairings, and fully after 50 or more CS–US pairings. If it acquires the CS–US connection suddenly, the animal might abruptly “get it” and quickly transition to a state at which it responds strongly to the CS.

Research shows that animals suddenly “get it.” Within just a few trials, they change abruptly from a state in which they do not respond to the CS to one in which they respond to it strongly and consistently (Gallistel, Fairhurst, & Balsam, 2004). Although psychologists have been studying learning for more than a century, only relatively recently have they come to appreciate the sudden manner in which organisms acquire a new response.

In summary, research on acquisition shows that (1) organisms will best acquire the ability to respond to a CS if the duration between CS and US is brief; (2) they will learn to respond precisely when the US is most likely to occur; and (3) they will learn all this quickly, rapidly transitioning from no response to a full response to the CS.

GENERALIZATION. In Pavlov’s experiments, dogs learned to salivate when a bell rang. What do you think would have happened if Pavlov changed bells—that is, switched to a bigger bell with a deeper tone or a smaller one with a higher pitch?

It turns out that when you change bells—or vary any other aspect of the conditioned stimulus—organisms still respond, but not quite as strongly. Psychologists call this generalization. Generalization is a learning process in which conditioned responses are elicited by stimuli that vary from the conditioned stimulus that originally was paired with the unconditioned stimulus.

You’ve already seen two cases of generalization. Little Albert’s fear generalized: He became afraid not only of the white rat, but also of other white or furry objects—stimuli that varied from, yet in some ways were similar to, the rat. The button fear described in our opening story generalized, too. The 9-year-old boy was afraid of not only the buttons at school, but also buttons in general.

Generalization occurs systematically. Stimuli that vary more and more from the original CS elicit less and less of a CR (Bower & Hilgard, 1981; Figure 7.4). You saw this, too, in the opening story; the boy was less fearful of metal buttons than plastic ones. Laboratory experiments document generalization. In one study (Lissek et al., 2008), conditioning trials paired pictures of geometric rings (neutral stimuli) with uncomfortable electric shocks (USs). Participants later viewed pictures of other rings whose sizes varied systematically from the rings that were shown originally. Classically conditioned fear generalized to rings of other sizes. And as the depicted rings increasingly departed in size from the original ones, participants’ physiological responses became weaker.

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Generalization explains experiences that otherwise would be puzzling. Consider the case of Pearson Brack, a bombardier in World War II. After a series of successful flights, Brack developed a strange symptom. When he saw from the plane’s altimeter that he had ascended to 9000 feet, he started to become uncontrollably anxious: He trembled, turned pale, and started breathing rapidly. Once the plane descended a few hundred feet, he was fine. But if it ascended again, he became anxious again, with the anxiety increasing as it reached 10,000 feet, at which point Brack would faint. What was the cause? It couldn’t be the altitude itself; other crew members felt fine and Brack previously had flown at these altitudes.

A psychological analysis suggested that the cause was classical conditioning (Mischel, 1968). The reading of 10,000 feet had become a conditioned stimulus that triggered panic. Lower altitudes triggered lower levels of anxiety, due to generalization. Key evidence for this conclusion was that, on the mission before his symptoms began, enemy fighters attacked Brack’s plane. It rolled, dove uncontrollably, and almost crashed. Brack sustained an injury in the turmoil. The attack likely occurred while Brack’s plane was flying at exactly 10,000 feet. That altitude, then, was paired with a US (the attack) and thus became a powerful CS, with effects that generalized to nearby altitudes.

Responses do not generalize to all stimuli. Organisms discriminate between stimuli. Discrimination in classical conditioning is a learning process in which organisms respond to one stimulus but not another. If, for example, Pavlov had trained a dog using a bell as a CS and then had switched stimuli—say, from Pavlov ringing a bell to Pavlov singing a song—the dog likely would not respond to the singing in the same way that it responded to the ringing. It would discriminate between the two stimuli.

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EXTINCTION. What do you think would happen, after conditioning, if a researcher kept ringing the bell, but there was never another trial in which the dog received food? Would the dog salivate to the bell, trial after trial, forever? No, it wouldn’t. What actually happens is extinction. In classical conditioning, extinction is a gradual lessening of a conditioned response when a CS is presented repeatedly without any presentations of the US. When a CS occurs repeatedly but the US does not follow, organisms gradually display less and less of the conditioned response. Eventually, they stop responding altogether; the CR becomes extinct. Your dog eventually would stop salivating when you rang the bell if there was no more food. Little Albert’s fear would eventually have extinguished if Watson and Rayner had repeatedly presented the rat without ever again banging on the steel bar.

Interestingly, when extinction occurs, it does not mean that an organism has completely forgotten the CS–US connection. If, after extinction in a traditional Pavlovian experiment, you wait a day and then start ringing the bell again, the dog will salivate again the first few times it is rung. The dog will exhibit spontaneous recovery, the reappearance of an extinguished CR after a period of delay following extinction (Rescorla, 2004; Figure 7.5).

Humans, too, exhibit spontaneous recovery. A study of fear conditioning among college students shows how this works (Huff et al., 2009). The study had three main steps:

HABITUATION. The fourth conditioning process, habituation, differs from the previous three (acquisition, generalization, and extinction) in that it does not involve pairing a CS and US. Instead, habituation is a change in behavior that occurs when one stimulus, which normally evokes a response in an organism, merely is presented repeatedly. The organism’s response to the stimulus gradually lessens; in colloquial terms, it “gets used to” the stimulus.

Habituation occurs all the time. On a cold winter day when you first put on a coat, it may feel heavy. Not long afterward, you barely notice that you’re wearing it; you habituate to the coat’s weight. If you’re in a room with a noisy air conditioning or heating system, you might notice the noise at first, but eventually you become unaware of it. You habituate to the sound. If you move from a rural area to a city, you may at first be bothered by the around-the-clock lights and sounds, but soon you habituate to them.

In summary, research on acquisition, generalization, extinction, and habituation yielded consistent results. Pavlov’s research findings meant that a wide range of responses (in a wide range of organisms, and in both the laboratory and everyday life) could be understood in terms of a small set of learning principles.

COMPENSATORY RESPONSES AND DRUG TOLERANCE. Besides learning that a CS predicts a US, animals also do something else. They learn to prepare for the anticipated effects of the US. Once they learn that a CS signals an imminent US that will affect them physiologically, their bodies automatically respond in a manner that prepares them for this effect. This preparation is called a compensatory response. Compensatory responses are biological reactions that are the opposite of the effects of a stimulus and thereby attempt to counterbalance those effects (Siegel, 2005; Siegel & Ramos, 2002).

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In their research, Pavlov’s colleagues gave dogs injections of adrenalin, a drug that increases heart rate. When they did this repeatedly, they found that the drug gradually had less and less of an effect on heart rates. The dogs, it appeared, had developed physiological reactions that counteracted the effects of the drug. To test this idea, researchers placed the dogs in an area of the lab where they usually received the adrenalin injections, but gave them an injection of a neutral substance rather than adrenalin. In response, the dogs’ heart rates decreased. When the dogs experienced the conditioned stimuli (the area of the lab, the injection needle) that normally signaled the presentation of a drug that increased heart rate, they automatically reduced their heart rate to compensate for the anticipated effects of the drug (Siegel & Ramos, 2002).

This conditioning research on dogs is directly relevant to humans. People exhibit drug tolerance, reacting less and less to a fixed amount of a drug. The compensatory response discovered by researchers directly contributes to this reduction in drug effects (Siegel, 2005). When people take a drug repeatedly, their bodies automatically produce biological responses that counteract the drug’s effects. As a result, they need higher amounts of the drug to obtain a given level of drug effectiveness.

Compensatory responses are situation-specific. They occur in situations in which an organism (nonhuman animal or person) has experienced a drug previously. This pairing of situation and drug turns the situation into a conditioned stimulus that triggers the compensatory response. Other situations, in which a drug has not been used previously, do not trigger compensatory responses. This means that the same person will respond differently to the same drug in different situations (Siegel, 2005).

The situation-specificity of drug tolerance explains tragic cases of overdose in the use of illegal drugs. Heroin users sometimes suffer fatal overdoses even when postmortem exams reveal that the level of the drug in their bloodstream was not excessively high (Siegel, 2001). Such overdoses usually occur when users take the drug in a new situation. In familiar situations, their bodies produce a compensatory response. But in the unfamiliar situations, they don’t. Users’ tolerance of the drug thus is lower, and overdose is more likely.

Such overdoses can even occur with prescription drugs. Siegel and Ellsworth (1986) report the case of a cancer patient taking morphine for pain relief. He usually received the drug in the same situation: his dimly lit bedroom. One day, he happened to take the same amount of the drug in a different setting—a brightly lit living room—and died of a drug overdose.

In summary, organisms can tolerate different amounts of a drug in different situations. A drug may produce modest effects in one situation, but large effects in another.

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CULTURAL OPPORTUNITIES

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Pavlov’s work did have limitations. Later researchers sometimes made discoveries that were inconsistent with Pavlov’s original findings. Let’s look at three of them—blocking, learned helplessness, and the Garcia effect.

ACQUIRING NEW INFORMATION. Pavlov believed that if a CS is paired with a US, animals always will learn to associate the two. It turns out, however, that sometimes they don’t. They only learn to associate a CS with a US if that CS provides new information about a US, that is, information the animal did not have previously.

We know this from research on blocking. In blocking, animals fail to learn that a CS predicts a US if they already can predict the US based on other cues (Rescorla, 1988). Their prior knowledge blocks the learning of new CS–US associations.

In blocking research (Kamin, 1968), researchers first exposed animals to a series of trials in which one CS, a sound, predicted the occurrence of the US, a shock. Next, animals experienced a second set of trials in which two stimuli, the sound plus a light, predicted the same US. Researchers then examined whether animals would respond when only the second stimulus, the light, was presented. Pavlov thought they would—but they didn’t! Although the light had been paired repeatedly with the shock, animals never learned to associate the two. Their prior knowledge of the sound–shock association link blocked their learning of the light–shock association (Kamin, 1968).

Blocking research shows that classical conditioning is more complex than Pavlov thought. Animals don’t merely learn simple associations between two stimuli. They acquire information about their environment as a whole (Rescorla, 1988). When they already have enough information—that is, when they already can predict the occurrence of a US—they don’t bother to pick up more information that is redundant. This finding prompted psychologists to develop new theories of learning that emphasize animals’ overall knowledge of environmental cues that predict biologically significant events (Rescorla & Wagner, 1972).

LEARNED HELPLESSNESS. Pavlov showed that animals learn when two events (the occurrence of a CS and a US) are related. Later research showed that animals also can learn when two events are not related. When they learn that (1) aversive outcomes are not related to (2) their own behavior—in other words, that their behavior cannot control the aversive occurrence—they experience a severe reduction in motivation known as learned helplessness.

In classic learned helplessness research (Maier & Seligman, 1976; Figure 7.6), dogs were placed in a box with two sides separated by a barrier that they could easily jump over. Experimenters placed a dog in one side of the box and then administered an electric shock to that side. Dogs quickly escaped the shock by jumping over the barrier. In so doing, they learned that their behavior (jumping over the barrier) could control the outcome (avoid the shock).

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Then, with a different group of dogs, the experimenters introduced an additional procedure at the outset of the experiment. Before entering the box, these dogs experienced a series of electric shocks that they could not avoid. Next, they entered the box and received the shocks that were easily avoidable by jumping over the barrier. Unlike the first group of dogs, this second group never figured out how to avoid the shocks. Instead of running around and trying to jump over the barrier when the shocks were administered, as the first group of dogs did, this second group just gave up. They lay down passively on the floor while the shocks were administered.

The second group of dogs experienced learned helplessness. The researchers concluded that the dogs’ experience with uncontrollable shocks caused them to expect that shocks and their own behavior were unrelated. This, in turn, caused them to respond helplessly: to not even try to escape the later shocks, which they easily could have avoided (Maier & Seligman, 1976).

Though conducted with dogs, this research has profound implications for humans. Life often seems like it can’t be controlled. You try to lose weight, but fail. You ask people out on a date, and they say no. You apply for jobs, and don’t get them. When this happens, there’s a risk: You might wrongly conclude that nothing you can do will work—that all attempts will always fail. If you draw this conclusion, you might experience learned helplessness, give up, and thus fail to try out some new strategies that could succeed.

Psychologists have developed interventions to combat this risk. One was designed for first-year college students (Wilson & Linville, 1985), a group at risk of concluding, falsely, that good grades at college are beyond their control. In the intervention, people were informed that many students get lower grades than they expect when they first start college, but that in future semesters these same students earn much higher grades. The intervention emphasized that the transition from low to higher grades was normal. Therefore, low grades at first did not mean that academic success was beyond one’s control.

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This simple intervention worked. Compared with students in a control group who did not receive it, students who received the anti-learned-helplessness intervention earned higher grades the next semester (Wilson & Linville, 1985).

BIOLOGICALLY RELEVANT STIMULI AND CONDITIONED TASTE AVERSION. Another finding that moves beyond Pavlov’s work involves conditioned taste aversion, the rapid learning of a connection between food (specifically, the taste of a food) and illness that occurs after consuming that food (Garcia & Koelling, 1966). Organisms learn very speedily that the food is a CS that signals the UR, illness. Conditioned taste aversion is also called the Garcia effect, in honor of the researcher who discovered it, John Garcia.

Have you ever eaten some unusual dish, gotten sick, and then decided never to eat it again? (This happened to your author once in college; I haven’t eaten moussaka, a Greek casserole dish, ever since.) It sounds like a simple case of classical conditioning: Sickness is the UR, the unusual food is the CS, and you learn to associate the two. In some ways, conditioned taste aversion is similar to other cases of classical conditioning; for example, as in other forms of conditioning, the CR generalizes (from the food flavor with which conditioning occurred to similar flavors; Richardson, Williams, & Riccio, 1984). Yet the example of conditioned taste aversion does not fit the traditional Pavlovian conditioning paradigm in three respects:

What’s different about the association of food and sickness? Garcia recognized that the difference lies in organisms’ evolution. Across the course of evolution, some CS–US connections have been biologically relevant, whereas others have not. The connection between food and illness has been biologically relevant; throughout evolutionary history, organisms that failed to learn this connection would repeatedly become ill and thus would be less likely to survive and reproduce. Compare this to the CS used by Pavlov: a bell. It is biologically irrelevant; over the course of their evolution, dogs rarely if ever encountered ringing bells that signaled the presence of food. The Garcia effect, then, shows that organisms learn biologically relevant associations much more rapidly than Pavlovian theory would have anticipated.

Garcia’s findings opened the door to research on how a species’ evolutionary past affects its ability to learn in the present (Domjan & Galef, 1983). Seligman (1970) has suggested that the link between the evolutionary past and contemporary learning can be understood with the concept of preparedness. Preparedness refers to the ease with which an organism can learn to associate a stimulus and response. Thanks to evolutionary pressures, some stimulus–response connections (e.g., food and illness) are easy to learn; they are highly prepared. Others, which were not important during the course of evolution (e.g., bell ringing and food), are low in preparedness and therefore more difficult to learn.

In sum, contemporary research has moved beyond Pavlov in a number of ways. Researchers now understand that organisms acquire information about the overall nature of their environment, that this information includes cases in which stimuli and responses are unconnected, and that some links between stimuli and responses are easier to learn than others, thanks to predispositions that organisms acquired over the course of evolution.

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These advances are important, but they do not overturn basic foundations laid by Pavlov more than a century ago. Through the psychological processes of classical conditioning, a wide spectrum of organisms learns new responses to the environment when encountering stimuli that predict one another. These psychological findings prompt contemporary researchers to seek the biological bases of conditioning processes, as you’ll see now.

The human brain contains billions of neurons (see Chapter 2). Each one of them communicates to a large number of other neurons through neurotransmitters—chemical substances that travel from one neuron to another. Because the brain is the biological basis of behavior, whenever your emotions or behavior change as a result of classical conditioning, a change must have occurred somewhere in this soupy sea of nerve cells and biochemicals. But where? With so many neurons to search through, how could a researcher ever locate the changes that constitute the biological basis of learning?

One way is to study a simpler nervous system possessed by an organism with far fewer neurons than a human being. The neuroscientist Eric Kandel (2006) has identified biological bases of learning by studying a very simple animal, Aplysia.

Aplysia are sea slugs—big ones, commonly more than a foot long. To scientists, these creatures have three valuable characteristics: (1) Their nervous system has only about 20,000 neurons; (2) the individual neurons are quite large, and thus easy to study; (3) Aplysia can learn through conditioning, despite being such simple organisms. Kandel’s insight, then, was that he could identify the biological bases of classical conditioning by studying these simple creatures.

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In one line of research (reviewed in Kandel, Schwartz, & Jessell, 2000), Kandel and colleagues identified the nervous-system changes that occur in habituation. They did this by studying a simple reflex: When touched by an object, an Aplysia reflexively retracts its gill (the organ it uses to breathe). If the Aplysia is touched repeatedly, it habituates to the stimulus and gradually stops withdrawing the gill. Kandel discovered the exact biological change responsible for habituation. It involves communication between one neuron, which detects the stimulus, and another neuron, which controls gill movement. When the stimulus strikes the Aplysia repeatedly, the neuron that detects this stimulus gradually sends lesser amounts of neurotransmitters to the neuron that controls motor movement. When this latter neuron receives less neurotransmitters, it is less likely to fire and thus to cause the gill to retract (Figure 7.7). The changes that occur when Aplysia learn to associate a CS and US are similar, but they involve activity in a combination of neurons that converge on the neuron that controls gill movement (Kandel, 1991).

Kandel’s research fulfills the promise formulated by Pavlov. The great Russian scientist anticipated that the psychology of learning could be explained in terms of the biology of the nervous system. By discovering biological mechanisms of classical conditioning in the Aplysia, Kandel has taken a critical step toward achieving this goal. More research is needed; it is a large step from the nervous system of sea slugs to the brains of humans. Nonetheless, Kandel’s research begins to provide psychology with a complete, three-level—person, mind, and brain—picture of classical conditioning (Figure 7.8).

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