Word-length effects in immediate memory: Overwriting trace decay theory

IAN NEATH ) 0 1 JAMES S. NAIRNE 0 1 0 We thank Nelson Cowan, Denny C. LeCompte, Randi C. Martin, Rich ard Schweickert, and Aimee M. Surprenant for comments on an earlier Sciences Building, Purdue University , West Lafayette, IN 47907-1364 ( 1 Purdue University , West Lafayette, Indiana Memory is worse for items that take longer to pronounce, even when the items are equated for frequency, number of syllables, and number of phonemes. Current explanations of the word-length effect rely on a time-based decay process within the articulatory loop structure in working memory. Using an extension of Nairne's (1990) feature model, we demonstrate that the approximately linear relationship between span and pronunciation rate can be observed in a model that does not use the concept of decay. Moreover, the feature model also correctly predicts the effects of modality, phonological similarity, articulatory suppression, and serial position on memory for items of different lengths. We argue that word-length effects do not offer sufficient justification for including time-based decay components in theories of memory. - People can remember items that take less time to pro nounce better than items that take longer to pronounce. Mackworth (1963) first reported the high correlation be tween reading rate and memory span, but because she was primarily interested in identifying limits in iconic memory (or the visual image, as she termed it), she did not measure word length precisely. Nonetheless, over five experiments with a variety of stimuli, including pictures, letters, digits, and colors, she found that "the amount reported was pro portional to the speed of reporting the individual items" (Mackworth, 1963, p. 81). M. 1. Watkins (1972) and M. 1. Watkins and O. C. Watkins (1973) reported effects of word length as a function of modality and serial position in both free and serial recall. In all cases, recall was worse for words offour syllables than for words of one syllable. Baddeley, Thomson, and Buchanan (1975) systemati cally explored the effect of word length in terms of pronun ciation time. They demonstrated that even when items are equated for number of syllables and word frequency, if one set of words takes less time to pronounce than another set, memory will be better for the shorter items. Ellis and Hen nelly (1980) showed that apparent differences in memory span between Welsh- and English-speaking subjects might be ascribed to the relatively longer time needed to say the Welsh digits than the English digits. Cowan et al. (1992) demonstrated a word-length effect even when the phone mic components of the long and short items were closely matched. The predominant explanation of these findings involves Baddeley's (1986, 1992) concept of working memory. Ac cording to this account, short-term retention of verbal in formation depends on the articulatory loop. As part of this structure, there is a passive, phonological store that is sus ceptible to time-based decay. A covert rehearsal process can refresh or reactivate the traces in the store to counter, tem porarily, the effects of decay. If rehearsal of a particular item does not occur within a certain length of time, the memory trace for that item will have decayed too far to be usable. The amount of verbal information that can be re tained is therefore a tradeoff between the decay rate (which is assumed to be fixed) and the covert rehearsal rate, which can vary. Specifically, the relationship between memory span, s, for verbal items oftype i, is a linear function ofpro nunciation rate, r, and the duration of the verbal trace, r: Perhaps the most compelling evidence supporting this view is the relative consistency of the presumed rate of decay, r, As Schweickert and Boruff (1986, p. 420) put it, 'The mean of these trace duration estimates is 1.6 sec. Considering the variety of methods for presenting the stim uli and measuring the spans and pronunciation rates, one is struck more by the agreement than by the differences." The goal of the present paper is to demonstrate that the re lationship expressed in Equation 1 need not arise from decay. Rather, we show that an interference-based model, Nairne's (1988, 1990) feature model, easily handles the relevant patterns of data. THE FEATURE MODEL The feature model (Nairne, 1988, 1990) was designed to account for the major effects observed in immediate memory settings, including the recency effect, the modality effect, the terminal and preterminal suffix effects, and the effects ofarticulatory suppression, temporal grouping, and phonological similarity. Nairne distinguished between two types offeatures that comprise a memory trace: Modality dependent features represent the conditions of presenta tion, including presentation modality, whereas modalityindependent features represent the nature ofthe item itself and are generated through internal processes such as cat egorization and identification. Modality-dependent infor mation can interfere only with modality-dependent fea tures, and modality-independent information can interfere only with modality-independent features. Rather than placing the locus of echoic memory in a separate struc ture, such as precategorical acoustic store, or PAS (Crow der & Morton, 1969), Nairne follows O. C. Watkins and M. 1. Watkins (1980) in that echoic and nonechoic repre sentations of an item are viewed as different aspects (or features) of a common memory trace. Nairne (1990) assumed that recall would depend on the match between a degraded trace in primary memory and a particular undegraded trace in secondary memory. Thus, a major function of primary memory was to serve as a means of constructing and maintaining cues that might in dicate which secondary-memory trace was present on a particular list (see also Raaijmakers & Shiffrin, 1981). Memory traces are represented by vectors of features, and each individual feature typically takes as a value +1, - 1, or 0; the actual values used for each feature are generated randomly for each trial. Primary memory traces do not exist in a vacuum; rather, they exist as part of a stream of ongoing mental activity (see Johnson & Raye, 1981; Nairne & McNabb, 1985). When interpreting a primary memory trace, the subject needs to discriminate the trace not only from other list traces but also from traces that are generated internally. By definition, traces that are generated by purely internal ac tivity contain only modality-independent features. Exter nally produced list traces contain modality-dependent fea tures because there is always a presentation modality. But it seems likely that there will be more modality-dependent features for auditory presentation than for visual presen tation. The justification for this assumption rests on two lines of evidence. First, there is an extensive literature on the speech-like encoding of information presented visu ally and also for the speech-like nature ofsubvocal rehearsal (for rev (...truncated)