Similarity and Discrimination: A Selective Review and a Connectionist Model

The 1st part of this article evaluates the extent to which 2 elemental theories of conditioning, stimulus sampling theory and the Rescorla-Wagner (1972) theory, are able to account for the influence of similarity on discrimination learning. A number of findings are reviewed that are inconsistent with predictions derived from these theories, either in their present form or in various modified forms. The 2nd part of the article is concerned with developing an alternative, configural account for discrimination learning. In contrast to previous configural theories, the present version is set within the framework of a connectionist network. The studies of stimulus generalization by Pavlov (1927) provided the first demonstration of the influence of similarity on conditioning. Once a dog had been trained to salivate to a tone of a given frequency, it was found that tones of other frequencies would also elicit this response but to a lesser degree. The magnitude of this generalization decrement was directly related to the extent of the difference between the training and the test stimuli. Pavlov's findings have been replicated on many occasions with a wide range of species. There have also been a number of attempts to understand these findings.