Types Of Classical Conditioning

Classical conditioning is a generic term that can refer to a variety of different types of classical conditioning procedures and paradigms. Two different parameters can distinguish the type of classical conditioning: (i) the temporal arrangement and spacing of the CS and the US and (ii) the type of response that is measured and conditioned. Some general comments regarding the influence that different types of conditioning have on behavior can be made, but it is impossible to make any specific comments regarding conditioning. The specifics depend on the response system being measured, the nature of the stimuli being used, and the species being conditioned.

A. Temporal Arrangement and Spacing of the CS and the US

The relationship of the CS and the US can be arranged in different ways to produce different conditioning paradigms. These arrangements can greatly influence how the CR develops. Figure 1 illustrates the major classical conditioning paradigms. The upward movement of a line represents the onset of a stimulus, and the downward movement of a line represents the offset of a stimulus.

1. Simultaneous Conditioning

In simultaneous conditioning the CS and the US are presented simultaneously. In this case, because the US always elicits a UR and the CS and US are presented together, it is not possible to determine if CRs are present. In order to determine if a CR has developed, a

Light Frequency Maximum Resolution

Figure 1 Temporal arrangements of the CS and the US (top trace) used in five classical conditioning paradigms. The upward movement of a line represents the onset of a stimulus, and the downward movement of a line represents the offset of a stimulus. A, simultaneous conditioning; B, delay conditioning; C, trace conditioning; D, backward conditioning; E, temporal conditioning. In the case of temporal conditioning, there is no discrete CS. The interval of time between the USs serves as the CS.

Figure 1 Temporal arrangements of the CS and the US (top trace) used in five classical conditioning paradigms. The upward movement of a line represents the onset of a stimulus, and the downward movement of a line represents the offset of a stimulus. A, simultaneous conditioning; B, delay conditioning; C, trace conditioning; D, backward conditioning; E, temporal conditioning. In the case of temporal conditioning, there is no discrete CS. The interval of time between the USs serves as the CS.

CS-alone trial must be presented (i.e., the US is omitted). If the CS elicits a response on the CS-alone trial, this response is a CR. This form of conditioning is not commonly used, and it generally yields only weak conditioning, if any at all.

2. Delay Conditioning

In delay conditioning, the CS onset precedes the US onset. The termination of the CS occurs with the US onset, during the US, at the termination of the US, or at some point after the US. This paradigm is called delay conditioning because the onset of the US is delayed relative to the onset of the CS. Generally, responses that develop to the CS and occur before the onset of the US are CRs. This is the most common conditioning paradigm and generally results in the most robust and rapid conditioning.

3. Trace Conditioning

In trace conditioning, the CS is presented and terminated before the onset of the US. The interval separating the CS offset and the US onset is called the trace interval. This paradigm was named trace conditioning by Pavlov because in order for conditioning to occur, the subject (i.e., the subject's brain) must maintain a memory ''trace'' of the CS. Responses that develop in response to the CS and occur before the onset of the US are CRs. This form of conditioning is also common and yields good conditioning but generally not as readily as delay conditioning.

4. Backward Conditioning

In the backward conditioning paradigm, the US is presented and terminated prior to the CS. A CR is a response that follows the presentation of the CS. This form of conditioning is not commonly used and in most circumstances does not result in conditioning. However, in some situations backward conditioning can occur.

5. Temporal Conditioning

In temporal conditioning there are no discrete CSs. Instead, the US is presented at regular intervals, and over time the CR will be exhibited just prior to the onset of the US. In this case, the CS is the time interval. Temporal conditioning is possible in some experimental paradigms, but in most classical conditioning paradigms it does not result in conditioning.

6. Differential Conditioning

In differential conditioning, two CSs are used. One of the CSs always precedes and predicts the US. This CS is termed the positive CS or the CS+. The other CS is not predictive of the US and occurs alone. This CS is termed the negative CS or the CS. Differential conditioning is indicated by a greater number of CRs in response to the CS+ than to the CS.

7. Controls for Pseudoconditioning

Classical conditioning results in CRs when an association forms between the CS and the US. However, sometimes responses that appear to be CRs, in that they follow the presentation of a CS, result from experience with US only and not because of an association between the CS and the US. This is known as pseudoconditioning. For example, if a very intense US is presented (e.g., a strong shock), the organism might respond to any subsequent stimulus presentation. The response does not occur because of an association between the CS and the US but, rather, because the US sensitizes the subject, making it more likely to respond to any stimulus presentation. To test for the possibility of pseudoconditioning, the CS and the US can be arranged so that the CS does not predict the US. This is known as unpaired training. There are two basic forms of unpaired training. Explicitly unpaired training is used to describe a situation in which the CS and the US never coincide. Random unpaired training is used to describe a situation in which the CS and the US rarely coincide but occasionally (i.e., by chance) do so. Random unpaired training is generally thought to be superior to explicitly unpaired training because during explicitly unpaired training the animal learns that the CS signals a safety period (i.e., the US will not occur).

Because the CS and the US are never paired, an association between them cannot be made. If responses nevertheless follow the presentation ofthe CS, it can be concluded that pseudoconditioning has occurred and not classical conditioning. If unpaired presentations do not result in responses to the CS, then any responses that subsequently develop in response to the CS, after paired training is begun, can be considered true CRs.

B. Types of Classical Conditioning Based on the Measured Response

In addition to the conditioning paradigms discussed previously, different types of classical conditioning can be characterized by the measured response. For example, in Pavlov's experiments, he measured salivation, whereas in Twitmyer's experiments, he measured the knee-jerk response. Although both of these basic experiments are examples of classical conditioning, they involve different response systems.

1. Two Fundamental Response Classes

The nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of all the neural tissue that is encased in bone (i.e., the brain and spinal cord). The CNS is discussed later. Because this section focuses on responses, the PNS will be discussed. In order for a response to occur, the PNS must be engaged (excluding responses of neurons that can be recorded from the CNS). The PNS can be divided into the autonomic nervous system and the somatic nervous system. The autonomic nervous system controls the viscera, and the somatic nervous system controls muscles.

a. Autonomic Classical Conditioning Autonomic classical conditioning refers to any classical conditioning paradigm in which the measured response is under the control of the autonomic nervous system. The autonomic nervous system consists of two divisions, the sympathetic nervous system and the parasympathetic nervous systems. These two systems work together, in opposing directions, to control bodily functions such as heart rate, breathing, dilation and constriction of the pupil, and the control of sweat glands. These systems are not generally under voluntary control but can be modified by classical conditioning. Autonomic conditioning has been used extensively because in addition to involving primarily involuntary responses, autonomic responses can also be used as an index of changes in emotion. Autonomic conditioning is sometimes referred to as conditioned emotional responses because changes in emotion are accompanied by changes in these autonomic measures. For example, many of these measures are also used in polygraph/lie-detection work. The following is a brief overview of the most common types of autonomic conditioning studies.

i. Galvanic Skin Response/Skin Conductance Response Conditioning This response is measured by the change in skin resistance to an electrical current. Research using this response measure dates back to the 1880s. For most of this time, the response was termed the Galvanic skin response (GSR) in honor of Luigi Galvani (1737-1798), an early pioneer in research describing the electrical nature of the body. Today, descriptive terms are commonly used, such as the electrodermal response or the skin conductance response (SCR). This response is measured by passing a small amount of electrical current between two electrodes pasted to the skin. The conductance (the reciprocal of resistance) between the two electrodes is measured. Many different stimuli can induce a SCR. For example, a mild shock, or any new stimulus of sufficient intensity as to attract the attention of the subject, can cause an autonomic response in which sweat glands pump extra sweat into sweat ducts located in the skin. The net effect of this action is an increase in skin conductance (i.e., a skin conductance response). In conditioning experiments in which, for example, a tone CS is paired with a shock or loud noise US, an association is made between the CS and the US and the CS begins to elicit a classically conditioned SCR.

ii. Heart Rate Conditioning Another common autonomic conditioning paradigm is the classically conditioned heart rate response. In this paradigm the change in heart rate (i.e., heartbeats per minute) is measured with electrodes pasted on the chest. For example, a tone CS can be paired with a mild shock US. Initially, the CS does not cause a change in the heart rate, but the shock will increase the heart rate. With continued pairing of the CS and the US, the CS will elicit a CR—a change in heart rate. Interestingly, the CR in this case is change in heart rate to the CS, but the direction of change (i.e., slower or faster) depends on the animal species being testing. For example, the CR with human subjects is an increase in heart rate, which is called conditioned tachycardia (heart rate increasing). However, the CR with, for example, rabbits, is a decrease in heart rate, which is called conditioned bradycardia (heart rate slowing).

iii. Other Examples Another example of auto-nomic classical conditioning is the conditioned pupillary response, which is a conditioned change in the size of the pupil. This response was first conditioned in 1922, but today it is rarely used because it is a difficult response to condition and to measure, and it is subject to a great deal of noise (i.e., spontaneous responses that are unrelated to the CS or the US). As noted previously, salivary conditioning was used by Pavlov in the initial demonstration of classical conditioning and this response was adapted for work with humans by Karl Lashley (1890-1958). Today, this paradigm is almost never used because salivation is a relatively slow response, difficult to measure, and difficult to condition in humans.

b. Somatic Classical Conditioning Somatic classical conditioning refers to any classical conditioning paradigm in which the measured response is under the control of the somatic nervous system. The somatic nervous system as it relates to classical conditioning controls striate or skeletal muscles. Accordingly, any response that requires a motor movement must be controlled by striate muscles and the somatic nervous system. The following is a brief overview of the most common types of somatic conditioning paradigms.

i. Eyeblink Classical Conditioning Eyeblink classical conditioning is by far the most common form of experimental conditioning paradigm with both humans and experimental animals. As early as 1922, eyeblink classical conditioning paradigms were being used. In eyeblink conditioning a CS (typically a tone or light) is paired with a US (e.g., a mild shock or puff of air to the eye). Eyeblink studies have been carried out in a variety of species, including humans, monkeys, rabbits, cats, rats, and mice. The response can be measured with a minitorque potentiometer. A string is attached to the eyelid and any eyelid movement changes the resistance in the potentiometer, which can then be recorded. Electromyographic (EMG) measures can also be used to record the activity of the muscles controlling the eyeblink. Today, the most common method of measuring eyeblink responses in humans is the use of an infrared reflective sensor. In this method, an infrared beam is directed at the eye (usually mounted in goggles) and the amount of light reflected by the eyelid is recorded. Eyeblinks change the amount of light that is reflected and subsequently detected by an infrared sensor.

ii. Nictitating Membrane Classical Conditioning Closely related to eyeblink conditioning, conditioning of the nictitating membrane (NM) is the most frequently used response measure in rabbits, which are the most frequently used experimental subjects for conditioning studies. The NM is often called the "third eyelid'' and it consists of a sheet of cartilage located behind the inner canthus of the eye. When the eyeball is stimulated, the eyeball retracts into the eye socket. This causes the NM to passively sweep across a portion of the eyeball. The NM response is popular because it is simple to measure (usually with a minitorque potentiometer) and because the NM cannot completely cover the eye. Thus, it prevents the subject from avoiding the airpuff US (the eyelids are prevented from closing by the experimenter during NM conditioning).

iii. Leg Flexion Classical Conditioning In the typical leg flexion experiment, an animal is usually restrained so that the legs hang freely, although it has also been used in freely moving subjects. A CS is paired with a mild shock US to the leg, which causes the leg to flex. With pairing of the CS and the US, a CR develops where the leg flexes in response to the CS. The response can be measured by a minitorque potentiometer or EMG.

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).

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