Although both early human memory researchers and behaviorists studied processes that today would be considered Pavlovian, 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. Pavlov's systematic investigation of Pavlovian conditioning uncovered most of the primary phenomena, and his sharp and nuanced discussions are still relevant today.
Gottlieb (2011) defined Pavlovian, or classical, conditioning as the “adjustments organisms make in response to observing the temporal relations among environmental or proprioceptive stimuli.” 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 CS to elicit a conditioned response (CR) qualitatively similar to the UR. Pavlov's first reported example used dogs as subjects, a metronome CS, food as the US, and salivation as the UR and CR (Pavlov, 1927, p. 26).
Pavlovian conditioning is most clearly defined and constrained by its method, which involves maintaining strict control over the presentation of stimuli. There appear to be few clear principles that distinguish Pavlovian conditioning from other forms of associative learning, such as instrumental learning and human associative memory, and we agree with Rescorla and Solomon (1967)'s conclusion the Pavlovian conditioning is most distinct in how determined the form of the learned response is by the choice of US. Pavlovian responding is also characteristically resistant to instrumental contingencies. For example, it is difficult to prevent a pigeon from pecking (CR) at a discrete key light (CS) paired with food (US), even if pecking prevents the delivery of food (Williams & Williams, 1969).
Overview
Basic Excitatory Phenomena
Acquisition is the primary phenomenon of Pavlovian conditioning and refers to the growth in conditioned responding resulting from pairing a CS and US over time (Pavlov, 1927, p. 26). A 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 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, 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. Conditioned 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.
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 CS-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 CS and a 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).
Basic Framework
Early theorists most often adopted a contiguity view of learning, in which the pairing of stimuli in time and space was the necessary and sufficient condition for generating conditioned responding. Although Hull (1943) expanded on this view in arguing that drive reduction was also necessary for learning, it is his other contributions that have made him central to contemporary understandings of Pavlovian processes.
Hull (1943) adopted the view of Pavlovian conditioning as an associative process by which the magnitude of the association determined the magnitude of conditioned responding. He presented a simple mathematical learning rule that specified the change in associative (habit) strength that resulted from the pairing of a CS and a US. The vast majority of subsequent quantitative models developed within the associative framework contain Hull's simple learning algorithm (Mackintosh, 1975; Pearce, 1987; Rescorla & Wagner, 1972; Wagner, 1981).
Hull (1943) helped to cement a view of Pavlovian conditioning as an incremental trial-based process involving changes in the associative strength between stimuli. The parameters of his model are what were originally thought to be the determinants of Pavlovian learning. Primary was the number of trials. Secondary included three durations related to the CS-US interval: the length of the CS, the temporal gap between the CS and US, and the extension of the CS past the US; as well as two salience parameters, one for the CS and one for the US.
