Classical (Pavlovian) Conditioning

Classical conditioning encompasses the association between two stimuli, an unconditioned stimulus that elicits an automatic response (e.g. blink response to an air puff in the eye), and a conditioned stimulus that does not. When these two stimuli are presented together repeatedly, they become associated and the organism blinks on presentation of the sound only.

Defining Characteristics of Classical Conditioning

Ivan Pavlov is credited with discovering classical conditioning and first officially describing the process to an English-speaking audience with the publication of ‘Conditioned reflexes’ in 1927.

The most well-known form involves pairing a neutral conditioned stimulus (CS) with a biologically relevant unconditioned stimulus (US) that automatically elicits an unconditioned response (UR), leading the conditioned stimulus (CS) to elicit a conditioned response (CR) qualitatively similar to the unconditioned response (UR).

Accordingly, the defining characteristics of classical conditioning includes

1. the presentation of an unconditioned stimulus (US) that reliably elicits an unconditioned response (UR);
2. the use of a conditioned stimulus (CS) that has been shown by test to produce a response that initially does not resemble the UR;
3. the repeated presentation of the CS and US with a specified order and temporal interval; and
4. the emergence of a new response to the CS, the conditioned response (CR), which is similar to the UR.

Pavlov’s first reported example used dogs as subjects, a metronome conditioned stimulus (CS), food as the unconditioned stimulus (US), and salivation as the unconditioned response (UR) and conditioned response (CR) (Pavlov, 1927, p. 26).

Overview

1. Basic Excitatory Phenomena

Acquisition is the primary phenomenon of Pavlovian conditioning and refers to the growth in conditioned responding resulting from pairing a conditoned stimulus (CS) and unconditioned stimulus (US) over time (Pavlov, 1927, p. 26).

A conditioned stimulus (CS) that produces conditioned responding is sometimes referred to as an excitor. An example of an excitor is a brief tone that, due to repeated pairings with an air puff to the eye, elicits anticipatory eyeblinking.

An animal that has learned to blink to a tone may also blink when presented with a novel auditory stimulus. The magnitude of this generalized responding is a function of the similarity of the new stimulus to the originally trained conditioned stimulus (CS) (Pavlov, 1927, p. 111).

Generalization functions can be modified by discrimination learning, in which some stimuli are reinforced while others are nonreinforced.

For example, the generalized responding from a tone to a burst of white noise can be reduced by interspersing presentations of the noise alone.

A tone paired with an air puff to the eye gains more than the ability to elicit anticipatory blinking. It also develops the ability to serve as a conditioned reinforcer for another, second-order, conditione stimulus (CS) (Pavlov, 1927, p. 33).

Second-order conditioning tends to lead to lower levels of responding than does first-order conditioning. Thus, a light paired with a tone excitor will likely elicit less conditioned eyeblinking than will the tone itself.

Conditional reinforcers are contrasted with primary reinforcers that do not need prior training to be able to establish conditioned responding.

Unlike in instrumental conditioning, Pavlovian reinforcers refer to stimuli that may be appetitive or aversive.

2. Basic Inhibitory Phenomena

Inhibitory phenomena are those that manifest in opposition to conditioned responding. When stimuli both reduce responding to simultaneously presented excitors and are slow to become excitors themselves, as compared to neutral stimuli, they are called inhibitors (Rescorla 1969a).

This type of inhibition is sometimes referred to as operational inhibition. It is contrasted with the theoretical inhibition that is used as an explanation for transient decreases in excitatory responding.

Extinction refers to the loss in conditioned responding that occurs when an excitor is subsequently presented in a manner that breaks the conditioned stimulus (CS)-unconditioned stimulus (US) relationship (Pavlov, 1927, p. 49).

For example, our tone excitor will stop eliciting conditioned eye-blinks if it is repeatedly presented alone. Although extinction is explained by appeal to inhibitory mechanisms, extinguished stimuli do not typically become operational inhibitors (Rescorla, 1969a).

Suppression of responding may also be observed when a nonreinforced stimulus is interspersed among the reinforced trials of another conditioned stimulus (CS) and an unconditioned stimulus (US).

For example, if noise alone presentations are interspersed among tone air puff pairings, generalized responding to the noise will be reduced.

The suppression of responding observed in a discrimination procedure is referred to as differential inhibition. Unlike extinguished stimuli, differentially conditioned stimuli may become operational inhibitors (Pavlov, 1927, p. 120).

A particularly powerful form of differential inhibition results from a conditioned inhibition procedure in which a stimulus is nonreinforced in the presence of an excitor that is reinforced when presented alone.

For example, if our tone signals an air puff except when it is presented together with a light, the light will become a conditioned inhibitor capable of passing the operational tests for inhibition (Pavlov, 1927, p. 68).

Perhaps it is not surprising that with extended training, organisms may come to withhold responding to a CS until closer to the time of the US. The decrease in responding to early parts of a CS that may result from extended training is referred to as inhibition of delay (Pavlov, 1927, p. 88).