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17.5: Pavlov's Stimulus Substitution Model Of Classical Conditioning

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    For most of the twentieth century, Pavlov’s originally pro- posed stimulus substitution model of classical conditioning was widely accepted. Pavlov viewed conditioning as a mechanistic (automatic) result of pairing neutral and biologically significant events in time. He believed that the established conditioned stimulus became a substitute for the original unconditioned stimulus. There were four assumptions under- lying this stimulus substitution model:

    • Classical conditioning requires a biologically significant stimulus (i.e., US).
    • Temporal contiguity between a neutral stimulus and an unconditioned stimulus is necessary for the neutral stimulus to become a conditioned stimulus.
    • Temporal contiguity between a neutral stimulus and an unconditioned stimulus is sufficient for the neutral stimulus to become a conditioned stimulus.
    • The conditioned response will always resemble if not be identical to the unconditioned response.

    Does Classical Conditioning Require a Biologically Significant Stimulus?

    Higher-order conditioning is a procedure or process whereby a previously neutral stimulus is presented in a predictive relationship with a second, previously established, predictive stimulus. Learning is inferred from the occurrence of a new response in the presence of this previously neutral stimulus. For example, after pairing the tone with food, it is possible to place the tone in the position of the US by presenting a light immediately before it occurs. Research indicates that a conditioned response (salivation, in this case) will occur to the light even though it was not paired with a biologically significant stimulus (food).


    Watch this video for a demonstration of higher-order conditioning.

    is Temporal Contiguity Necessary for Conditioning?

    Human beings have speculated about the learning process since at least the time of the early Greek philosophers. Aristotle, in the fourth century bce, proposed three laws of association that he believed applied to human thought and memory. The law of contiguity stated that objects or events occurring close in time (temporal contiguity) or space (spatial contiguity) become associated. The law of similarity stated that we tend to associate objects or events having features in common such that observing one event will prompt recall of similar events. The law of frequency stated that the more often we experience objects or events, the more likely we will be to remember them. In a sense, Pavlov created a methodology permitting empirical testing of Aristotle’s laws. The law that applies in this section is the law of temporal contiguity. Timing effects, like many variables studied scientifically, lend themselves to parametric studies in which the independent variable consists of different values on a dimension. It has been demonstrated that human eyelid conditioning in which a light is followed by a puff of air to the eye is strongest when the puff occurs approximately 500 milliseconds (1⁄2 second) after the light. The strength of conditioning at shorter or longer intervals drops off within tenths of a second. Thus, temporal contiguity appears critical in human eyelid conditioning, consistent with Pavlov’s second assumption.

    An Exception: Acquired Taste Aversion

    Acquired taste aversion is the only apparent exception to the necessity of temporal contiguity in predictive learning (classical conditioning). This exception can be understood as an evolutionary adaptation to protect animals from food poisoning. Imagine if members of the Nukak—an indigenous tribe in Colombia that is said to be at imminent risk of extinction—got sick after eating a particular food and continued to eat the same substance. There is a good chance the tribe members (and tribe!) would not survive for long. It would be advantageous to avoid foods one ate prior to becoming ill, even if the symptoms did not appear for several minutes or even hours. The phenomenon of acquired taste aversion has been studied extensively. The time intervals used sometimes differ by hours rather than seconds or tenths of seconds. For example, rats were made sick by being exposed to X-rays after drinking sweet water (Smith & Roll, 1967). Rats have a strong preference for sweet water, drinking it approximately 80% of the time when given a choice with ordinary tap water. If the rat became sick within 1⁄2 hour, sweet-water drinking was totally eliminated. With intervals of 1 to 6 hours, it was reduced from 80% to 10%. There was even evidence of an effect after a 24-hour delay! Pavlov’s dogs would not associate a tone with presentation of food an hour later, let alone 24 hours. The acquired aversion to sweet water can be interpreted as either an exception to the law of temporal contiguity or contiguity must be considered on a time scale of different orders of magnitude (hours rather than seconds).

    is Temporal Contiguity Sufficient for Conditioning?

    Pavlov believed not only that temporal contiguity between CS and US was necessary for conditioning to occur but also that it was all that is necessary (i.e., that it was sufficient). Rescorla (1966, 1968, 1988) has demonstrated that the correlation between CS and US (i.e., the extent to which the CS predicted the US) was more important then temporal contiguity. For example, if the only time one gets shocked is in the presence of the tone, then the tone correlates with shock (i.e., is predictive of the shock). If one is shocked the same amount whether the tone is present or not, the tone does not correlate with shock (i.e., provides no predictive information). Rescorla demonstrated that despite temporal contiguity between tone and shock in both instances, classical conditioning would be strong in the first case and not occur in the second.

    Another example of the lack of predictive learning despite temporal contiguity between two events is provided in a study by Leon Kamin (1969). A blocking group received a tone (CS 1) followed by shock (US) in the first phase, while a control group was simply placed in the chamber. The groups were identical from then on. During the second phase, a com- pound stimulus consisting of the light and a tone (CS 2) was followed by shock. During a test phase, each component was presented by itself to determine the extent of conditioning.

    In the blocking group, conditioning occurred to the tone and not to the light. Conditioning occurred to both elements of the compound in the control group. It is as though the previous experience with the tone resulted in the blocking-group subjects not paying attention to the light in the second phase. The light was redundant. It did not provide additional information.

    A novel and fun demonstration of blocking in college students involved a computerized video game (Arcediano et al., 1997). Subjects used a laser gun (the space bar) to try to protect the earth from invasion by Martians. Unfortunately, the enterprising Martians had developed an anti-laser shield. If the subject fired when the shield was in place, their laser-gun would be ineffective, permitting a bunch of Martians to land and do their mischief. A flashing light preceded implementation of the laser-shield for subjects in the blocking group. A control group did not experience a predictive stimulus for the laser-shield. Subsequently, both groups experienced a com- pound stimulus consisting of the flashing light and a complex tone. The control group associated the tone with activation of the laser-shield, whereas, due to their previous history with the light, the blocking group did not. For them, the tone was redundant.


    Watch this video for a demonstration of blocking.

    The blocking procedure demonstrates that temporal contiguity between events, even in a predictive relationship, is not sufficient for learning to occur. In the second phase of the blocking procedure, the compound stimulus precedes the US. According to Pavlov, since both components are contiguous with the US, both should become associated with it and eventually elicit CRs. The combination of Rescorla’s (1966) and Kamin’s (1969) findings lead to the conclusion that learn- ing occurs when individuals obtain new information enabling them to predict events they were unable to previously predict. Kamin suggested that this occurs only when we are surprised. That is, as long as events are proceeding as expected, we do not learn. Once something unexpected occurs, individuals search for relevant information. Many of our activities may be described as “habitual” (Kirsch et al., 2004) or “automatic” (Aarts & Dijksterhuis, 2000). We have all had the experience of riding a bike or driving as though we are on “autopilot.” We are not consciously engaged in steering as long as events are proceeding normally. Once something unexpected occurs we snap to attention and focus on the immediate environmental circumstances. This provides the opportunity to acquire new information. This is a much more active and adaptive under- standing of predictive learning than that provided by Pavlov’s stimulus substitution model (see Rescorla, 1988).

    Must the Conditioned Response Resemble the Unconditioned Response?

    We will now examine the fourth assumption of Pavlov’s model, that the conditioned response always resembles the unconditioned response. Meat powder reflexively elicits salivation, and Pavlov observed the same reaction to a conditioned stimulus predictive of meat powder. Puffs of air reflexively elicit eye blinks, and taps on the knee elicit knee jerks. The conditioned responses are similar to the unconditioned responses in research involving puffs of air and knee taps as unconditioned stimuli. It is understandable that Pavlov and others believed for so long that the conditioned response must resemble if not be identical to the unconditioned response. However, Zener (1937, p. 393) took movies of dogs undergoing salivary conditioning and disagreed with this conclusion. He observed, “Despite Pavlov’s assertions, the dog does not appear to be eating an imaginary food. . . . It is a different response, anthropomorphically describable as a looking for, expecting, the fall of food with a readiness to perform the eating behavior which will occur when the food falls.”

    Kimble (1961, p. 54) offered the possible interpretation that “the function of the conditioned response is to prepare the organism for the occurrence of the unconditioned stimulus.” Research by Shepard Siegel (1975, 1977, 1984, 2005) has swung the pendulum toward widespread acceptance of this interpretation of the nature of the conditioned response. Siegel’s research involved administration of a drug as the unconditioned stimulus. For example, rats were injected with insulin in the presence of a novel stimulus (Siegel, 1975). Insulin is a drug that lowers blood sugar level and is often used to treat diabetics. Eventually, a conditioned response was developed to the novel stimulus (now a CS). However, rather than lowering blood sugar level, the blood sugar level increased to the CS. Siegel described this increase as a compensatory response in preparation for the effect of insulin. He argued that it was similar to other homeostatic mechanisms designed to maintain optimal levels of biological processes (e.g., temperature, white blood cell count, fluid levels, etc.). Similar compensatory responses have been demonstrated with morphine, a drug having analgesic properties (Siegel, 1977) and with caffeine (Siegel, 2005). Siegel (2008) has gone so far as to suggest that “the learning researcher is a homeostasis researcher.”

    Siegel has developed a fascinating and influential model of drug tolerance and overdose effects based upon his findings concerning the acquisition of compensatory responses (Siegel, 1983). He suggested that many so-called heroin over- doses are actually the result of the same dosage being consumed differently or in a different environment. Such an effect has actually been demonstrated experimentally with rats. Whereas 34% of rats administered a higher than usual dosage of heroin in the same cage died, 64% administered the same dosage in a different cage died (Siegel et al., 1982). As an experiment, this study has high internal validity but obviously could not be replicated with human subjects. In a study with high external validity, Siegel (1984) interviewed survivors of suspected heroin overdoses. Most insisted they had taken the usual quantity but indicated that they had used a different technique or consumed the drug in a different environment. This combination of high external validity and high internal validity results makes a compelling case for Siegel’s learning model of drug tolerance and overdose effects.

    Drug-induced compensatory responses are consistent with the interpretation that the conditioned response constitutes preparation for the unconditioned stimulus. Combining this interpretation with the conclusions reached regarding the necessity of predictiveness for classical conditioning to occur leads to the following alternative to Pavlov’s stimulus substitution model: Classical conditioning is an adaptive process whereby individuals acquire the ability to predict future events and prepare for their occurrence.


    1. Describe the basis for concluding that extinction is an inhibitory as opposed to an unlearning process.
    2. Define and give examples of the following classical conditioning phenomena: acquisition, extinction, spontaneous recovery, stimulus generalization, and stimulus discrimination.
    3. Explain how the fact that spontaneous recovery occurs indicates that the connection between a conditioned stimulus and conditioned response is not broken during the extinction process.
    4. State the assumptions underlying Pavlov’s stimulus substitution model of classical conditioning. Describe the research findings addressing each of the assumptions. Show how the research findings are consistent with the description of classical conditioning as an adaptive learning process.


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    This page titled 17.5: Pavlov's Stimulus Substitution Model Of Classical Conditioning is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Kate Votaw.

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