In this paper we examine the role played by working memory demands in determining problem difficulty during the solution of Tower of Hanoi Problem isomorphs. W e do so by describing a production system model that accounts for subjects' performance on these problems via a dynamic analysis of the memory k>ad imposed by the problem and of changes in that load during the problem solving episode. W e also present the results of detailed testing of the model against human subject data. The model uses a highly constrained working memory to account for a number of features of the problem solving behavior, including the dichotomous (exploratory and final path) nature of the problem solving, the relative difficulty of the problems, the particular moves made in each state of the problem space, and the temporal patterning of the final path moves.

A problem solving experiment is described where the difficulty Ss experienced in solving a panicular puzzle is manipulated using a dual task paradigm. Although Ss show impaired performance solving the puzzle the first time, performance improves considerably on a second trial and Ss are not impaired by a second trial memory load. In spite of the improvement in performance, Ss are unable to report virtually any information about the problem or their solution strategies. A model is presented that describes the pattern of performance across the levels of memory load and across the two trials. The theoretical implications of this model are discussed.