SHORT TERM MEMORY Dissertation submitted for the Degree of Doctor of Philosophy in the University of Stirling by Neil Thomson University of York November, 1979 ii 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 character­ istic 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 assump­ tion 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 iii from both stores in order to test this hypothesis. The results support the hypothesis in showing that variables known to affect ST~ 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. iv PREFACE The experiments reported here are original and were carried out by me and unless it is stated otherwise the ideas expressed are original. I would like to thank Dr. Alan Baddeley, who supervised this work while he was at Stirling University, for his support and encouragement. I am grateful to Dr. Peter Hamilton and Dr. Philip Smith for taking over the supervision of this work when Dr. Baddeley left Stirling. I should also like to thank Mrs. Marilyn Glicker who typed this thesis. Neither this dissertation nor any part of it has been submitted to any other university. v CONTENTS Chapter I Introduction 1 Research Review 4 II The Chunking HlEothesis 28 Experiment 1 32 Experiment 2 40 Experiment 3 45 Experiment 4 52 III The Word Length Effect 68 Experiment 5 69 Experiment 6 85 Experiment 7 96 Experiment 8 111 IV General Discussion 123 Summary of Experimental Results 123 Inferences drawn from the Results 124 Some Theoretical Implications 125 Some General Implications 1 ItS Conclusions 1",1 V References 14 t VI Appendices 15' - 1 - INTRODUCTION The variety of meaning in human communication is mainly achieved by combining simple elements into different sequences. This ability to use serial order has long been recognized as a question of central importance in Psychology (Lashley, 1951). It is still, however, not well understood despite the abundance of research into serial recall and the mechan­ isms underlying it. The latter have been most commonly conceptualized as short- and long-term memory (STM and LTM) and STM in particular has conventionally been regarded as an input system which receives and holds information prior to its encoding in LTM (Waugh & Norman, 1965; Atkinson and Shiffrin, 1968). An alternative proposal has been that STM is an output store involved in the production of speech (Morton, 1970; Ellis, 1976). Although placing STM at opposite ends of the information processing system, both proposals require that the store be capable of dealing with serially ordered information. Speech as a form of human communication is serial by nature while the permanent storage of information cannot be. ThUS, given that people decode serial information into a permanent message and vice-versa, the constraints of serial ordering will be felt most strongly at the points of perception and production. At input, the store would be responsible for holding serially organized information until it could be encoded into a permanent message, whereas at output, the store would hold material translated from a permanent message while it was being produced serially. In neither case would STM be - 2 - holding material in the code used for permanent storage and consequently the meaning of the message should not interact with the properties of the store. Further, it might be expected that the store would have a relatively small capacity as the "perceptual units" of speech are fairly short whether taken as syllables, words, tone groups or phrases. By this account, then, STM is a peripheral store with a limited capacity, employing some form of speech code and insensitive to the meaning of a message. This description of STM is very much in agreement with those derived from experimental work as will become apparent from the research review below. However, there is one very important experi­ mental result which would seem to contradict directly the assumption that STM is insensitive to the meaning of a message and it is this contradiction that provides the starting point for the thesis. Miller (1956) showed that the amount people could remember in a serial recall task was 7±2 chunks where a chunk was defined as a subjective meaningful unit, e.g. HPD might be three chunks while PHD would be one. In so far as chunks are "meaningful units" they are units of permanent storage rather than units of some peripheral code and consequently, Miller's result would not be predicted by the view that STM is a peripheral store. Thus, there is a contradiction in the literature between the experimentally supported idea that STM is a peripheral store employing a speech code and the fact that the capacity of STM is constant when measured in units of permanent storage. The general aim of this thesis is to resolve this contradiction by - 3 - testing the chunking hypothesis. This is not as straight­ forward as it might seem, because the chunk is defined subjectively and hence the hypothesis is essentially irrefutable. Typically this problem has been avoided by making the simplifying assumption that such experimenter defined units as words, digits and letters constitute chunks to the subject. Where lists are composed of sequences of these items chosen to be unrelated one to another this assumption seems plausible and allows a weaker version of Miller's hypothesis to be tested: that memory span is a constant number of chunks where chunks are defined experimental units. It will be shown that for tasks which rely heavily on STM this hypothesis fails for a variety of definitions of a chunk and Miller's results will be explained by arguing that he used tasks in which there was a large LTM contribution to recall. It will be assumed that, because LTM is responsible for the storage of a permanent message, it employs a semantic code and consequently is responsible for chunking. The experimental work is based on two main assumptions: first, thatat least two stores mediate performance on memory span and serial recall tasks, viz. STM and LTM, and secondly, that STM is a peripheral store responsible for holding the units of perception or production in a prescribed order. Before introducing the experiments, these two assumptions will be evaluated in the context of previous research. - 4 - RESEARCH REVIEW The idea that STM is a peripheral store and particularly one involved in speech perception or production implies that it should employ a speech code; acoustic if it is an input store and articulatory if it is an output store. Conrad (1964) showed that errors produced in immediate serial recall were similar to those produced when attempting to identify verbal material in noise and, in particular, items that sounded similar tended to be confused. He called this the acoustic similarity effect (AS) and it implies that STM is sensitive to the physical properties of a message as would be expected of a peripheral store. Further, he showed that the AS effect was not dependent on the material being presented auditonally, but also occurred with visually presented material. This latter result is particularly important as it suggests that the store mediating performance is not modality specific. Thus, if it is to be argued that STM is an input store, it must occur after the material has been encoded in a form that is independent of modality. This would mean that the store is not subject to the same constraints of serial ordering as a truly peripheral store responsible for receiving serially organized messages. However, the experimental evidence reviewed below suggests that STM is a peripheral output store responsible for holding serially organized articulatory commands while they are being produced and consequently would not be modality specific. - 5 - Baddeley (1966a; 1966b) extended Conrad's results and showed that, while AS was detrimental to immediate serial recall, semantic similarity had little effect on performance. However, when list length was increased and a rehearsal preventing task interpolated between presentation and recall, AS had little effect while semantic similarity consistently impaired performance.