Classical Conditioning

• Nestor A. Schmajuk, Duke University

During classical (or Pavlovian) conditioning, human and animal subjects change the magnitude and timing of their conditioned response (CR), as a result of the contingency between the conditioned stimulus (CS) and the unconditioned stimulus (US). Whereas in classical conditioning, the US and the CS determine the form of the CR, in operant conditioning the strength and frequency, but not the form, of the response is independent of the US.

This article briefly enumerates some of the results obtained by creating different CS-US contingencies and mentions a few scientists who, among others, participated in those discoveries.

Introduction

• US: Unconditioned stimulus
• UR: Unconditioned response
• NS: Neutral stimulus
• CS: Conditioned stimulus
  • CS1: Conditioned stimulus 1
  • CS2: Conditioned stimulus 2
• CR: Conditioned response

Classical Conditioning was first described by Pavlov (1927), who placed hungry dogs on a stand and delivered food powder (the US) following a tone (the CS) presentation. At the beginning, he observed that only the food presentation elicited salivation (the UR). After repeating the tone-food (CS-US) pairings a number of times, he observed that the dogs began to salivate (the CR) before the food was delivered.

Excitatory conditioning

Pavlov (1927) was the first to describe acquisition of classical conditioning.

• Acquisition. After a number of CS-US pairings, the CS elicits a conditioned response (CR) that increases in magnitude and frequency.
• Partial Reinforcement. The US follows the CS only on some trials and might lead to a lower level of conditioning.
• Generalization. A CS2 elicits a CR when it shares some characteristics with a CS1 that has been paired with the US.
• US- and CS-specific CR. The nature of the CR is determined not only by the US but also by the CS.
• Interstimulus Interval (ISI) effects. Conditioning is maximal at an optimal ISI and gradually decreases with increasing ISIs.
• Intertrial Interval (ITI) effects. Conditioning to the CS increases with longer ITIs.

Inhibitory conditioning

Rescorla (1969) showed that a CS becomes inhibitory when the probability that the US will occur in the presence of the CS, p(US/CS), is smaller than the probability that the US will occur in the absence of the CS (p(US/noCS).

• Conditioned Inhibition. Stimulus CS2 acquires inhibitory conditioning with CS1 reinforced trials interspersed with CS1-CS2 nonreinforced trials.
• Extinction of Conditioned Inhibition. Inhibitory conditioning is extinguished by CS2-US presentations, but not by presentations of CS2 alone.
• Differential conditioning. Stimulus CS2 acquires inhibitory conditioning with CS1 reinforced trials interspersed with CS2 nonreinforced trials.

Extinction

Pavlov (1927) was also the first to describe extinction, external disinhibition, spontaneous recovery, and reinstatement.

• Extinction. When CS-US pairings are followed by presentations of the CS alone or by unpaired CS and US presentations, the CR decreases.
• External Disinhibition. Presenting a novel stimulus immediately before a previously extinguished CS might produce renewed responding.
• Spontaneous recovery. Presentation of the CS after some time after the subject stopped responding might yield renewed responding.
• Renewal. Presentation of the CS in a novel context might yield renewed responding.
• Reinstatement. Presentation of the US in the context of extinction and testing might yield renewed responding.
• Reacquisition. CS-US presentations following extinction might result in faster or slower reacquisition.

Preexposure Effects

Lubow and Moore (1959) found that conditioning was retarded if the CS had been presented by itself previous to the reinforced trials.

• Latent inhibition. Preexposure to a CS followed by CS-US pairings retard the generation of the CR.
• Context preexposure. Preexposure to a context facilitates the acquisition of fear conditioning.
• US–Preexposure effect. Presentation of the US in a training context prior to CS-US pairings retards production of the CR.
• Learned irrelevance. Random exposure to the CS and the US retards conditioning even more than combined latent inhibition and US preexposure.

Combination of multiple conditioning events

Brogden (1939) demonstrated sensory preconditioning.

• Sensory preconditioning. When CS1-CS2 pairings are followed by CS1-US pairings, presentation of CS2 generates a CR.
• Second order conditioning. When CS1-US pairings are followed by CS1-CS2 pairings, presentation of CS2 generates a CR.

Conditioning of multiple stimuli

Wagner et al. (1968) showed that conditioning to a CS depended, not on the number of CS-US presentations, but on its quality as a predictor of the US compared to other CS present at the time of conditioning. Kamin (1968) discovered that conditioning of one CS would block conditioning to a second CS trained simultaneously with the first CS.

• Relative validity. Conditioning to X is weaker when training consists of XA trials alternated with XB trials, each type reinforced half of the time; than when training consists of reinforced XA trials alternated with XB nonreinforced trials.
• Blocking. Conditioning to CS1-CS2 following conditioning to CS1 results in a weaker conditioning to CS2 than that attained with CS2-US pairings.
• Unblocking by increasing the US. Increasing the US increases responding to the blocked CS2.
• Unblocking by decreasing the US. Responding to CS2 can be increased by decreasing the US.
• Overshadowing. Conditioning to CS1-CS2 results in a weaker conditioning to CS2 than that attained with CS2-US pairings.
• Backward Blocking. Conditioning to CS1 following conditioning to CS1-CS2 results in a weaker conditioning to CS2 than that attained with CS2-US pairings.
• Overexpectation. Reinforced CS1-CS2 presentations following independent reinforced CS1 and CS2 presentations, result in a decrement in their initial associative strength.
• Superconditioning. Reinforced CS1-CS2 presentations following inhibitory conditioning of CS1, increase CS2 excitatory strength compared with the case when it is trained in the absence of CS1.

Recovery from conditioning of multiple stimuli

Kaufman and Bolles (1981) and Matzel, Schachtman and Miller (1985) described cases of recovered responding to a CS that initially did not elicit a CR.

• Recovery from latent inhibition. Presentation of the US in the context of preexposure and conditioning results in renewed responding to the preexposed CS.
• Recovery from overshadowing. Extinction of the CS1 results in increased responding to the overshadowed CS2.
• Recovery from forward blocking. Extinction of the blocker CS1 results in increased responding to the blocked CS2.
• Recovery from backward blocking. Extinction of the blocker CS1 results in increased responding to the blocked CS2.

Non-Linear combinations of multiple stimuli

Bellingham, Gillette-Bellingham, and Kehoe (1985) demonstrated positive and negative patterning, as well as the CR strength as a function of the proportion of simple and compound acquisition trials.

• Positive Patterning. Reinforced CS1-CS2 presentations intermixed with nonreinforced CS1 and CS2 presentations result in stronger responding to CS1-CS2 than to the sum of the individual responses to CS1 and CS2.
• Negative Patterning. Nonreinforced CS1-CS2 presentations intermixed with reinforced CS1 and CS2 presentations result weaker responding to CS1-CS2 than to the sum of the individual responses to CS1 and CS2.

Occasion setting

Holland (1983) suggested that, in addition to acting as a simple CS (by predicting the occurrence of a US), a CS could also act as an occasion setter (by modulating the responding generated by another CS). More informally, rather than signaling the delivery of the US, an occasion setter indicates whether another cue is to be reinforced (or not reinforced), setting the occasion for its reinforcement (or nonreinforcement).

• Simultaneous Feature-positive Discrimination. Reinforced simultaneous CS1-CS2 presentations, alternated with nonreinforced presentations of CS2, result in stronger responding to CS1-CS2 than to CS2 alone. In this case, CS1 gains a strong excitatory association with the US.
• Serial Feature-positive Discrimination. Reinforced successive CS1-CS2 presentations, alternated with nonreinforced presentations of CS2, result in stronger responding to CS1-CS2 than to CS2 alone. In this case, CS1 acts as an occasion setter.
• Simultaneous Feature-negative Discrimination. Non-reinforced simultaneous CS1-CS2 presentations, alternated with reinforced presentations of CS2, result in weaker responding to CS1-CS2 than to CS2 alone. In this case, CS1 gains a strong inhibitory association with the US.
• Serial Feature-negative Discrimination. Non-reinforced successive CS1-CS2 presentations, alternated with reinforced presentations of CS2, result in weaker responding to CS1-CS2 than to CS2 alone. In this case, CS1 acts as an occasion setter.

Temporal properties

Pavlov (1927) observed that the peak CR occurred at the end of a long CS, a phenomenon he called inhibition of delay. Smith (1968) showed that the peak CR is approximately located at the time of the US presentation, independently of the US intensity.

• ISI effects. Conditioning is negligible with short ISIs, increases dramatically at an optimal ISI, and gradually decreases with increasing ISIs.
• ITI effects. Conditioning to the CS increases with longer ITIs.
• Timing of the CR. CR peak tends to be located around the end of the ISI.
• Temporal specificity of blocking. Blocking is observed when the blocked CS, is paired in the same temporal relationship with the US as the blocking CS.
• Temporal specificity of occasion setting. A serial feature-positive discrimination is best when the feature-target interval during testing matches the training interval.

The article on Computational Models of Classical Conditioning explains how different models address this large number of properties.

References

Bellingham, W.P., Gillette-Bellingham, F., and Kehoe, E.J. (1985) Summation and configuration in patterning schedules with the rat and rabbit. Animal Learning and behavior, 152-164.

Brogden, W.J. (1939). Sensory pre-conditioning. Journal of Experimental Psychology, 25, 223-232.

Holland, P. C. (1983). Occasion setting in Pavlovian feature positive discriminations. In M. L. Commons, R. J. Herrnstein, & A. R. Wagner (Eds.), Quantitative analyses of behavior: Discrimination processes (Vol. 4), pp. 183-206. New York: Ballinger.

Kasprow, W., Catterson, D., Schatchman, T. & Miller, R. (1984). Attenuation of latent inhibition by postacquisition reminder. Quarterly Journal of Experimental Psychology, 36B, 53-63.

Kamin, L.J. (1968). "Attention‑like" processes in classical conditioning . In M.R.Jones (eds.), Miami Symposium on the Prediction of Behavior: Aversive Stimulation (pp 9‑33). Miami: University of Miami Press.

Kaufman, M.A., and Bolles, R.C. (1981). A nonassociative aspect of overshadowing. Bulletin of the Psychonomic Society, 18, 318-320.

Kehoe, E.J., & Graham, P. (1988) Summation and configuration: Stimulus compounding and negative patterning in the rabbit. Journal of Experimental Psychology: Animal Behavior Processes, 14, 320‑333.

Lubow and Moore (1959) Lubow, R.E., & Moore, A.U. (1959). Latent inhibition: The effect of non‑reinforced preexposure to the conditional stimulus. Journal of Comparative and Physiological Psychology, 52, 415‑419.

Matzel, L., Schachtman, T. & Miller R. (1985). Recovery of an overshadowed association achieved by extinction of the overshadowing stimulus. Learning and Motivation, 16, 398-412.

Pavlov, I. (1927). Conditioned reflexes. London: Oxford University Press.

Rescorla, R.A. (1969) Conditioned inhibition of fear resulting from negative CS-US contingencies. Journal of Comparative and Physiological Psychology, 67, 504-509.

Smith (1968) CS‑US interval and US intensity in classical conditioning of the rabbit's nictitating membrane response. Journal of Comparative and Physiological Psychology, 66, 679‑687.

Wagner, A.R., Logan, F.A., Haberlandt, K., and Price, T. (1968) Stimulus selection in animal discrimination learning. Journal of Experimental Psychology, 76, 171‑180.

Internal references

• Nestor A. Schmajuk (2008) Computational models of classical conditioning. Scholarpedia, 3(3):1664.

• Peter Jonas and Gyorgy Buzsaki (2007) Neural inhibition. Scholarpedia, 2(9):3286.

• Florentin Woergoetter and Bernd Porr (2008) Reinforcement learning. Scholarpedia, 3(3):1448.

• Wolfram Schultz (2007) Reward. Scholarpedia, 2(3):1652.

See Also

Computational Models of Classical Conditioning, Motivation, Operant Conditioning, Reinforcement, Reinforcement Learning, Rescorla-Wagner Learning Rule, Reward

Sponsored by: Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia

Reviewed by: Anonymous

Accepted on: 2007-02-13 06:00:05 GMT