What happens when the conditioned stimulus no longer foretells the presentation?

Abstract

Classical conditioning was first discovered by Ivan P. Pavlov in the early 1900s. It can be conceptualized as learning about event sequences that occur independently of one's actions in one's environment. Specifically, one learns that a preceding event (stimulus) becomes a signal for a subsequent event. This article touches on a variety of issues. It provides examples of classical conditioning with humans, describes different forms of classical conditioning, elaborates on one of many models of classical conditioning, presents a few of the key phenomena, and finally illustrates application of classical conditioning to some treatments of clinical conditions such as phobias and drug addiction.

V.B.1.b.iv. Fear Classical Conditioning

Classical conditioning has been increasingly used to study the learning of fear. This paradigm can be considered a hybrid of autonomic and somatic classical conditioning because fear causes numerous autonomic changes, which could be measured as the CR. However, in the rat, the most common subject for studies of this type, fear can also be measured with the somatic response of freezing. In the typical paradigm, a tone CS is paired with a shock US. The shock US is delivered to the rat through an electrified floor grid. With pairing of the CS and the US, a fear CR develops in response to the CS. In this case, the fear CR is freezing (the rat holds completely still).

VI. Summary

Classical conditioning theories have become considerably more complex since the early formulations by O. H. Mowrer and others. Modern conditioning models emphasize the role of cognitive factors such as memory processes and expectancies in the etiology and maintenance of conditioned responses. As theories of fear have developed, other pathways to fear acquisition have been added, although classical conditioning continues to be seen as important. Classical conditioning theories have led to a number of important treatments, with the most widely used being the exposure therapies for reducing fear. According to contemporary views, extinction of the CR can be regarded as a process of exposure to corrective information. Exposure involves having the person repeatedly exposure himself or herself to a feared stimulus until fear abates. Patients play an active role in choosing what they will be exposed to, and when the exposure will occur. Exposure therapies can successfully reduce conditioned fears and fears arising from other forms of learning.

Of the exposure therapies, graded in vivo exposure and flooding are among the most effective treatments of phobias, and play an important role in treating disorders in which fear plays a prominent role (e.g., social phobia, agoraphobia). For patients who are extremely phobic, the least demanding form of exposure (systematic de-sensitization) is typically the exposure intervention to be used first. Graded in vivo exposure is particularly important because it involves teaching patients skills for overcoming their fears. Patients can continue to apply these skills on their own, without the aid of a therapist. Exposure therapies can be combined with other psychological interventions, such as relaxation training and cognitive restructuring. For the average phobic patient, combination treatments tend to be no more effective than exposure alone. However, there are likely to be exceptions to this rule, and some patients may benefit most from a combination of psychotherapeutic procedures. Combining exposure with antianxiety drugs does not improve outcome, and may actually impair the effects of exposure. The benefits of exposure therapy tend to be long lasting, with no evidence of symptom substitution. Patients sometimes relapse, although their reemergent fears can usually be successfully treated with a further course of exposure therapy.

Publisher Summary

This chapter provides an overview of classical conditioning concept in learning behavior. While classical conditioning is often thought of as a simpler form of learning than operant conditioning; in fact, the complexity of classical conditioning from a procedural viewpoint rivals that of operant conditioning. It is generally agreed that classical conditioning, along with operant conditioning, constitutes the majority, if not all, of the learned behaviors. In general, classical conditioning involves the pairing of two stimulus events, typically a neutral conditioned stimulus (CS), and an unconditioned stimulus (US). That an association between these two events is learned is reflected in the acquisition of a conditioned response (CR) to the CS. The CR is usually topographically similar in the form to the unconditioned response (UR) to the US, although this is not universally the case. However, for classical conditioning the CR in no way changes the CS–US stimulus arrangements. One area where classical conditioning has been used extensively is in the study of drug effects on learning or acquisition. Classical conditioning procedures hold a number of advantages over operant conditioning in the study of learning.

2 Basic Conditioning Procedure

The procedure of classical conditioning consists of the repeated presentation of two stimuli in temporal contiguity. First, a neutral stimulus (NS) is presented—that is, a stimulus that does not elicit regular responses or responses similar to the unconditioned response (UR). Immediately after that, the US is presented. Because of this pairing, the NS will become a CS and, therefore, will be capable of provoking a conditioned response (CR) similar to the UR that, initially, only the US could elicit (Fig. 1).

On the initial trials, only the US will elicit the salivation response. However, as the conditioning trials continue, the dog will begin to salivate as soon as the CS is presented. In salivary conditioning, the CR and the UR are both salivation. However, in many other conditioning situations, the CR is very different from the UR. According to Pavlov, the animals learn the connection between stimulus and response (CS–UR). Currently, it is understood that animals learn the connection between stimuli.

B. The Rabbit Model System of Nictitating Membrane/Eyeblink Classical Conditioning

Much of the general literature on classical conditioning is based on data collected with the human eyeblink conditioning paradigm and in the rabbit nictitating membrane or “third eyelid” paradigm first introduced by Isadore Gormezano (Gormezano et al., 1962; Schneiderman et al., 1962). Evidence has converged from a number of sources to suggest that the cerebellum ipsilateral to the conditioned eye is essential for eyeblink classical conditioning in rabbits and humans.

The most extensive body of literature linking the cerebellum and eyeblink classical conditioning comes from research with animals. This research is eloquently described by its primary instigator and motivator, Richard F. Thompson et al., this volume, and will be mentioned only briefly here. A variety of techniques including electrophysiological recording of multiple and single units, electrolytic and chemical lesions, physical and chemical reversible lesions, neural stimulation, genetic mutations, and pharmacological manipulation have been used to demonstrate that the dorsolateral interpositus nucleus ipsilateral to the conditioned eye is the essential site for acquisition and retention (Berthier and Moore, 1986, 1990; Chen et al., 1996; Clark et al., 1992; Gould and Steinmetz, 1994; Krupa et al., 1993; Lavond et al., 1985; Lincoln et al., 1982; McCormick et al., 1981; McCormick and Thompson, 1984a,b; Steinmetz et al., 1992; Thompson, 1986, 1990; Yeo et al., 1985). Involvement of the cerebellar cortex has also been demonstrated during normal acquisition, although it may not be essential (Chen et al., 1996; Lavond and Steinmetz, 1989).

Classical or Pavlovian conditioning, the simplest form of associative learning demonstrated in a variety of animals (ranging from mollusks to humans), is hypothesized to be an elemental unit for complex learning. Classical conditioning ensues when an initially neutral conditional stimulus (CS) is paired in close temporal proximity with a biologically significant unconditional stimulus (US) that elicits a reflexive unconditional response (UR). Through the formation of a CS–US association, the CS comes to evoke a conditional response that typically mimics the UR and has an adaptive value. Classical conditioning is an ideal model system for investigating the neurobiology of learning and memory because (a) the stimuli are discrete and well defined; (b) only two stimuli are involved, and thus the learning or association of CS and US must occur at brain site(s) where the two pieces of information converge; (c) the onset and offset of the stimuli can be controlled precisely by the experimenter; and (d) this form of learning is ubiquitous across the phylogenetic scale. This article will discuss those types of classical conditioning whose neural basis is relatively well understood.

C. Pavlovian Conditioning: the Establishment of Drug “Cues”

Pavlovian conditioning refers to the behavioral and physiological changes brought about by experiencing a predictive relationship between a neutral stimulus and a consequent biologically significant event (Pavlov, 1927). In a Pavlovian conditioning experiment, a contingency is arranged between the presentation of the neutral stimulus, and the delivery of a biologically significant outcome, so that the animal learns that a specific stimulus predicts the impending delivery of the unconditioned stimulus. Both stimuli are presented independently of the animal's behavior. The acquisition of Pavlovian conditioning has been well established for both appetitive and aversive reinforcers.

Pavlovian conditioning may manifest itself behaviorally in procedures that measure “Pavlovian approach” (or “autoshaping”), Pavlovian-to-instrumental transfer (or PIT) and conditioned reinforcement. Thus, Pavlovian conditioned stimuli not only become attractants that grab attention and subserve approach behavior, they can energize or motivate instrumental behavior (as in the PIT paradigm), and they can also act as conditioned reinforcers of instrumental behavior. All three processes may contribute significantly to the development of compulsive responding. We will discuss them separately in the following sections.

Classical conditioning occurs when neutral stimuli become associated with a psychologically significant event. The main result is that the ‘neutral’ stimuli come to evoke responses or emotions that can contribute to many clinical disorders. Recent research emphasizes the fact that conditioned stimuli evoke whole systems of physiological, emotional, and behavioral responses that help the organism prepare for the significant event. Basic research on classical conditioning has many other implications for understanding the development of clinical disorders, including (but not limited to) anxiety disorders and drug dependence, as well as their therapy.

B Classical vs. Instrumental Conditioning

Classical conditioning or Pavlovian conditioning is best described as a procedure for presenting stimuli. In the classical conditioning paradigm, an unconditioned stimulus and a conditioned stimulus are presented. An unconditioned stimulus, such as shock, is one which reliably produces an unconditioned response, such as leg flexion. A conditioned stimulus is one which has been shown not to produce the unconditioned response. The conditioned stimulus and the unconditioned stimulus are then presented to the organism in a specified order and temporal spacing, and the experimenter is interested in the development of a response to the conditioned stimulus (the conditioned response) which is similar to the unconditioned response. Some theorists consider that classical conditioning is an essential part of instrumental conditioning. Instrumental conditioning paradigms will be the major concern here. In such paradigms the organism's behavior determines what stimuli, positive or negative, will be received.

Regarding the use of aversive stimuli, three general types of instrumental situations are usually distinguished: punishment training, escape training, and avoidance training (for a fourth type called the preservation procedure see Church, 1963). In punishment training the organism makes some response which produces the aversive stimulus. In escape training the organism receives the aversive stimulus and by making some response terminates it. In avoidance training, generally speaking, the aversive stimulus is absent, and the organism by making some response prolongs its absence.