Learning may be defined broadly as a stable change within an organism due to an experience or set of experiences — a change that is reflected in changed behaviors, thoughts, and/or emotions. We are members of a species, Homo sapiens[∂], that has evolved in ways that allow us to be very aware of and very good at learning about our environments (see Section 8 for more on evolution and evolutionary psychology). It seems obvious that humans learn much more about more diverse aspects of the environment, and do so more easily, than any other species that has ever existed. Because learning is fundamental to human life, early experimental psychologists focused much of their research on understanding how we learn. The doctrine of associationism gave them a way to conceptualize the learning process: the most important kinds of human learning involve the formation of associations between events — between mental events in structuralism and between environmental events in behaviorism. This type of learning is known as associative learning[∂].

Some psychologists and biologists study nonassociative forms of learning — learning that does not involve the formation of associations. One type of nonassociative learning is called habituation, which is decreased responsiveness to a frequently repeated stimulus. For example, when a fan turns on in a ventilation system while you are sitting in a quiet room, you may be startled by it at first but you quickly stop responding to the noise and, eventually, stop noticing it. Habituation has been very important in memory research. Eric Kandel, a neuroscientist who was awarded the Nobel Prize in Physiology or Medicine in 2000, has studied habituation in the sea slug, Aplysia californica, for decades. This research led to a detailed understanding of biochemical factors important for learning and memory (you will learn more about memory in Section 6). Although studies of nonassociative learning in relatively simple forms of animal life have led to important findings, most experimental psychologists have focused on the study of associative learning in humans and other mammals, such as mice, rats, monkeys, and apes. Some, like Skinner, have studied associative learning in classes of species other than Mammalia, such as Aves (birds). For example, Skinner, 1948) performed a study of what he called "superstitious behavior" in pigeons.

An example should make clear what is meant by the term associative learning. Let’s say that a friend invites you to dinner at an expensive restaurant. After eating a very large and calorie-rich meal, you drink too much of a sweet-tasting liqueur. Although you feel happy and content for an hour or so, you eventually become nauseous and begin to vomit. A few weeks later, you smell something similar to the smell of the liqueur, instantly feel an emotion of disgust, and begin gagging. This example is outlined in Figure 1. The experience in which the gustatory (taste) and olfactory (smell) sensations of the liqueur were paired with the somatic (bodily), gustatory, and olfactory sensations of nausea and vomiting has caused changes in your cognitions, emotions, and behaviors: you now are digusted and become physically ill by sensations similar to those you experienced when you ate and drank too much several weeks before. These cognitive, emotional, and behavioral changes are caused by a type of associative learning called classical conditioning.

Figure 1. An outline of the learning of disgust and associated behaviors

Reflexes and Classical Conditioning

You learned about reflexes in previous sections. A reflex consists of a particular stimulus and a particular response. When an individual experiences a "reflexive stimulus," such as the taste of candy placed in the mouth, a "reflexive response," salivation in this case, occurs automatically (involuntarily). We can't help but salivate when candy is placed in our mouths, and we don't have to consciously think about responding: the salivation is independent of conscious thought. This is because all reflexes depend on activity in the spinal cord or brain stem, and conscious thought is due primarily to activity in the cerebral cortex.

Classical conditioning takes advantage of reflexive responding to stimuli. Classical conditioning may be defined as a procedure in which a neutral stimulus — a stimulus that initially does not elicit a particular reflexive response — is followed repeatedly by a reflexive stimulus — a stimulus that elicits the reflexive response. Figure 2 contains an illustration of the classical conditioning procedure:

Figure 2. An outline of the classical conditioning procedure

For example, the following would represent a classical conditioning procedure: a neutral stimulus of a ringing bell is followed immediately by a reflexive stimulus of the taste of candy, which involuntarily causes the reflexive response of salivation.

Early behaviorists assumed that the repeated pairing of a neutral stimulus with a reflexive stimulus caused an unknown change in the central nervous systems of organisms (Lashley, 1930; Pavlov, 1927). This change may be conceived of in the following way:

(a) The presentation of a neutral stimulus activates a particular sensory area of the CNS. The presentation of a reflexive stimulus activates a second sensory area of the CNS. The activation of the reflexive-stimulus area causes the activation of a third area that produces a reflexive response (see Figure 3).

Figure 3. Activation of CNS areas associated with the neutral and reflexive stimuli, and the reflexive response

(b) The repeated presentation of the neutral stimulus followed immediately by the reflexive stimulus causes a new neural connection to develop between the two sensory areas activated by these stimuli (see Figure 4).

Figure 4. The development of a new neural connection during classical conditioning

(c) Finally, when the sensory area linked to the neutral stimulus is activated, the sensory area linked to the reflexive stimulus becomes active, even when the reflexive stimulus is not presented. This occurs because of the development of a new neural connection between the two areas, which causes the activation of the reflexive-response area.

Because of the development of the new neural connection — a connection that developed because of the repeated pairing of the neutral and reflexive stimuli — presenting the (formerly) neutral stimulus alone now causes the reflexive response to occur. Although there was no evidence for decades that something similar to these hypothesized CNS changes actually do occur during learning, behaviorists assumed, based on what then was known about the brain, that they must be occurring.

Associationists claimed that organisms (including humans) have no active role in what they learn: conscious thinking and reasoning is not necessary for the learning of new ways of responding to environmental events. Thus, the learning of an association between the neutral stimulus and the reflexive stimulus illustrated in Figures 2 and 3 is passive and machine-like: the close pairing in time of the two stimuli changes the CNS automatically. In essence, classical conditioning causes the development of a learned reflex: the formerly neutral stimulus now reflexively elicits a response that (usually) is very similar to the reflexive response elicited by the reflexive stimulus.

Terminology in Classical Conditioning

In the terminology of classical conditioning, the neutral stimulus is called the conditioned stimulus. We can think of the word conditioned as meaning learned, so that the conditioned stimulus (abbreviated CS) is the stimulus that the organism learns to respond to reflexivly (involuntarily) when it is followed repeatedly by a reflexive stimulus that already elicits a reflexive response (see Figure 5). The reflexive stimulus is referred to as the unconditioned stimulus. We can think of the word unconditioned as meaning unlearned so that the unconditioned stimulus (abbreviated UCS) is the stimulus that the organism already responds to automatically because it is part of a reflex.

Figure 5. An outline of the classical conditioning procedure using the terminology of behaviorism

In defining conditioned as learned, we see that the conditioned response (abbreviated CR) is the response that the organism learns to produce reflexively when presented with a conditioned stimulus (but only after the conditioned stimulus has been paired repeatedly with an unconditioned stimulus). Lastly, in defining unconditioned as unlearned, it follows that the unconditioned response (abbreviated UCR) is the response produced reflexively by the unconditioned stimulus.

As an example of classical conditioning, let's consider the reflex where newborn infants salivate reflexively to the taste of their mother’s milk. In classical conditioning terminology, the taste of mother's milk is the UCS that elicits the inborn reflexive response of salivation, which is the UCR. The UCS is preceded each time by the sight of an approaching nipple. Since the baby does not initially salivate to the sight of the nipple, this stimulus is the CS (the initially neutral stimulus). The repeated pairing of the CS and the UCS eventually leads to the development of an association between the two stimuli. We know that a CS-UCS association has formed when the baby salivatates to the CS (the sight of the nipple). The learned salivation is the CR. Figure 6 illustrates this example.

Figure 6. The classical conditioning of salivation to the sight of the mother's nipple

Classical conditioning has been used to explain the development of mental disorders characterized primarily by anxiety and depression — the kinds of mental disorders referred to by psychodynamic[∂] theorists as neuroses[∂] (see Dollard & Miller, 1950). One common type of neurosis is phobic disorder, which will be described in the next section.

Study Questions for Section 5-7

1. How would you define learning in your own words?
2. What are some examples of learning not mentioned above?
3. What is the most important kind of human learning?
4. What is the major distinction between associative and nonassociative learning?
5. What is habituation?
6. How is the reflex concept important for classical conditioning?
7. How would you define classical conditioning in your own words?
8. What is being associated in classical  conditioning?
9. How do you know when an association has formed between the CS and UCS?
10. According to early behaviorists, what is the physical basis of the association between the CS and UCS?
11. What does it mean to say that classical conditioning leads to the development of a learned reflex?
12. According to the early behaviorists, what role does the conscious mind have in the formation of a conditioned response?
13. What are two examples of classical conditioning from your everyday life?
14. In each of your examples, what is the CS, the UCS, the CR, and the UCR?

Go to Quiz 5-7 questions

Go to Readings Section 5-8