Commentary on: Ruchkin D.S., Grafman J., Cameron K., Berndt R.S. (2003). Working Memory Retention Systems: A State of Activated Long-Term Memory. Brain and Behavioral Sciences, vol. 26, p. 250. Developmental Evidence for Working Memory as Activated Long-Term Memory Sergio Morra Università di Genova morra@nous.unige.it ABSTRACT There is remarkable agreement between the authors' psycho-physiological views and my own model, based on developmental-experimental evidence, of working memory as activated long-term memory (LTM). I construe subvocal rehearsal as an operative scheme that maintains order information and demands attentional resources. Encoding and retrieving operations also demand attention. Another share of resources is used for keeping activated specific LTM representations. I find a remarkable congruency between the authors' views, based on psycho-physiological evidence, and my own views, based on developmental-experimental evidence. These were expressed in a formal model of verbal short-term memory tasks (Morra, 2000). I summarize here some assumptions of that model, and highlight their parallels with the target article: First, words are represented in LTM as cognitive units (figurative schemes). When stimulus items are presented, encoding operations enable activation of those units. The encoding is automatic in case of auditory presentation, but effortful for visual items. This is congruent with the general thesis of activation of LTM representations, defended in the target article, and with its particular statements on processing streams and phonological recoding (section 3.1). In turn, the idea of activated LTM representations is also congruent with other similar models of working memory (e.g., Cowan, 1988, 1999; Pascual-Leone, 1987; Engle, Cantor & Carullo, 1992; Shiffrin, 1976). Second, subvocal rehearsal is an optional strategy that yields the benefit of encoding order information in a simple way. In itself, the rehearsal procedure is a LTM representation (an operative scheme for repeating speech materials); its activation also demands a share of the individual s limited attentional resources. Most often, rehearsal is useful because its benefit is larger than its attentional cost. However, young children may not rehearse (although they are obviously able to repeat speech) because this strategy demands too large a fraction of their very limited attentional resources. An alternative strategy could be just to try to keep activated the single
relevant LTM units, without specific encoding of order information. These assumptions on rehearsal seem consistent with the authors claims on rehearsal operations that involve attentional control, and storage (i.e., LTM representation) of attentional control operations in the frontal cortex (sections 3.1 and 5). Third, still another operative scheme is involved in retrieval and, in order to be activated, it also consumes attentional resources. This assumption does not have a direct parallel in the target article, that does not address overt recall, but is broadly consistent with its claims on control operations. Fourth, because the operative schemes currently used for encoding, rehearsal, or retrieval consume attentional resources, it follows that only a part of the individual s limited attentional capacity remains available for activating LTM representations of the stimulus words. This is consistent with (and perhaps more specific than) the authors suggestion that the amount of information in the focus of attention is limited (section 5). Fifth, phonological encoding often prevails for various reasons, such as automatic recoding of an auditory input, or usefulness of phonological rehearsal. However, phonological encoding is not the only possibility. The cognitive units used by a participant to represent stimulus items could also be semantic codes, number codes in the case of digit span tasks, or any appropriate representation of lexical items. The nature of the particular codes that participants use may affect the rate of decay or the amount of interference among those representations that do not remain fully activated because they cannot be kept within the focus of attention. This seems consistent with sections from 3.2 to 3.4 in the target article. A model with only one free parameter was tested successfully for goodness of fit in a series of experiments with primary school children, also reported by Morra (2000). Therefore, we have behavioral data from experimental developmental research that supports a model, based on the general view of working memory as activated LTM. What about the alternative view, that working memory is accounted for by specific buffer stores, perhaps co-ordinated or supervised by an executive control system? If one assumes that there are specific short-term stores, then one should also specify their limitations. This task proved awfully difficult; for instance, some valuable reviews (Cornoldi, 1995; Logie, 1995) noted that it is problematic to define the capacity limitation or even the appropriate measurement unit for visual
and spatial short-term storage. However, one important exception is that the limited capacity of the articulatory (or phonological) loop seemed to be well-established, i.e., people, both adults and children, can remember as much as they can rapidly utter in 1.5 or 2 seconds (e.g., Baddeley, 1986). Such an estimate was based on the word-length effect, and particularly the ratio of recall to articulation rate, or the slope of the regression equation of recall on articulation rate (given a nearzero intercept in the equation). This seemed to be the only precise statement on capacity limitations generally agreed upon by supporters of the buffer stores view of working memory. Our results were in contrast with this claim; more important, different and inconsistent estimates of the capacity of the hypothesized articulatory loop were obtained across experiments, conditions, and techniques of estimation (Morra, 2000). In our experiments the finding of a word-length was replicated, as well as a correlation between rate of articulation and recall, which suggests that rehearsal skill actually contributes to memory performance. However, it seems that an appropriate account of the role of rehearsal has to be different from that proposed within storage frameworks. Moreover, previous experiments from our laboratory (e.g., Morra, 1989, 1990; Morra, Mazzoni & Sava, 1993) suggested the conclusion that the quantitative predictions of the articulatory loop model were not either supported in experiments with adult participants. It seemed to us that the classical findings of a near-zero intercept in the regression equation and a constant capacity of the articulatory loop (measured in units of time) were due, at least in part, to particular features of the experiments from which those results were found, such as the use of supra-span memory lists and English language. Publishing those results proved extremely difficult (see Morra, 1998, 2001; see also Anderson & Matessa, 1997); however, also from other studies (e.g., Cheung & Kemper, 1993; Hulme, Maugham & Brown, 1991; Nicolson & Fawcett, 1991) now we know that language and span versus supra-span procedure do actually affect the relationship of short-term recall to rate of articulation. Therefore, studies with adult participants had already suggested that the time-limited capacity of a short-term phonological storage system is questionable. Our experiments with children (Morra, 2000) only strengthened this conclusion. Also the target article reaches the conclusion that the existence of separate buffer stores for short-term memory is questionable. It is interesting to note, also at this point, how well our conclusions agree, even though they are based on so different lines of research.
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