The purpose of the present paper is to reconsider the problem of stimulus equivalence (SE). Primary emphasis is placed on the related areas of stimulus generalization, transposition, and discrimination learning as examples of SE. Recent evidence has indicated that the only comprehensive approach to SE, the Hull-Spence system, is inadequate to account for many phenomena in these areas. A theory of SE based on a combination of information and adaptation-level concepts is proposed as an alternative to the Hull-Spence theory. A number of investigations were conducted in order to test certain assumptions of the proposed theory. The results of these studies seem to provide rather striking support for the approach presented in this paper. A main advantage of the proposed theory is that the use of adaptation-level (AL) concepts permits quantitative predictions when an appropriate mathematical model for the calculation of AL is available. It is felt, however, that both in lieu of and in addition to quantitative predictions, that the present system has considerable heuristic value that may stimulate further research.
According to Hull's theory of extinction (3), the amount of reactive inhibition (In) is a function of the effort required to make a response, while conditioned inhibition (sis) is a habit of not responding, reinforced by the reduction of //? during rest.Primary evidence cited by Hull (3, p. 279) as supporting the IK concept is a study by Mowrer and Jones (6), which indicated that rats required to depress a bar requiring an SO-gm. effort, made fewer responses during extinction than rats pressing 42.5 or 5 gm. This study has been criticized by Applezweig (1) for lack of control of habit strength during the acquisition procedure. Though ^Ss received the same number of reinforced trials on the same bar loadings, more acquisition trials were given all Ss at the S-gm. effort level than at the other levels. Therefore, the group extinguished on the 5-gm. effort might be expected to make more responses because of greater habit strength. The appositeness of the criticism is shown by Applczweig's results indicating that if rats were selected and specifically trained on heavier weights, responses to extinction did not vary as a function of bar loading. Applezweig's criticism is supported by Maatsch, Adelman, and Denny (4). In their experiment rats pressing bars requiring efforts of 5, 40, and 80 gin. showed no differences in meeting extinction criteria.However, in all three of the above studies the extinction results seem to be confounded by the acquisition procedure. That is, all 6s did not receive equal training at each effort level. Since the extinction results in the above studies may have been a function of either the effort involved in making the response or the acquisition procedure, and in view of the conflicting results, it would seem that the relationship between effort and extinction remains questionable.Basic to an evaluation of the fn concept is the establishment of the relationship between
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