Short term memory

Abstract

The eight experiments reported in this thesis are designed to investigate the idea that in verbal short-term memory (STM) material decays over time and this decay is prevented by rehearsal. It follows that the capacity of STM when measured in words should be inversely proportional to the time taken to rehearse the words. Consequently, subjects should be able to recall more short duration words than long duration words. In contrast to this hypothesis is the idea that the capacity of STM is a fixed number of chunks, where chunks are a structural characteristic of the material. The first four experiments are designed so that these alternative hypotheses produce conflicting predictions and, in all cases, the hypotheses derived from decay theory are supported. It is shown that serial recall performance is very well predicted by the time taken to say the words and that the relationship between word duration and recall is of the type predicted by decay theory. The second set of experiments are based on the assumption that both STM and long-term memory (LTM) contribute to performance in serial recall tasks. The purpose of the experiments is to determine whether it is the STM or LTM component that is sensitive to word duration. It is predicted, in line with a decay theory of forgetting in STM, that the STM component is sensitive to word duration. The experiments are designed to produce sizable contributions from both stores in order to test this hypothesis. The results support the hypothesis in showing that variables known to affect STM, such as acoustic similarity, interact with word duration, while variables known to affect LTM, such as repeated presentations of the same list, show no such interaction. The results are interpreted in terms of decay theory and the different versions of this theory that have been proposed are considered. It is concluded that while no version of the theory is completely adequate, there is no evidence that invalidates the central assumptions, viz. that in STM items are forgotten by decay and that one of the functions of rehearsal is to prevent this decay.