The involvement of domain-specific verbal resources within working memory
maintenance has been supported by the observation of several phenomena. We have described in chapter one the phenomena that had led Baddeley (1986) to conclude on the verbal nature of his firstly implemented maintenance buffer, the phonological loop. These concerned first of all the effect of articulatory suppression and phonemic similarity, and later on the word length effect. The verbal nature of the phonological loop was accounted for by supposing a
separation between a passive maintenance buffer and an active rehearsal mechanism.
Articulatory suppression impedes the functioning of the verbal rehearsal mechanism while phonologically similar items lead to confusion between the memory traces. In both cases, memory performance is thus negatively affected. The word length effect further specified the functioning of the phonological loop, but this elaboration has been subject to a range of criticisms. To recapitulate, the word length effect consists in the better memory for shorter
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words then for longer words. Baddeley et al. (1975) had shown a direct link between speech rate and memory: one could maintain as much verbal information as one could pronounce in about two seconds. Their explanation for the word length effect was based on two
assumptions: memory traces decay over time and active verbal rehearsal compensates for this decay by recovering the memory traces. Longer words take longer to pronounce and the number of longer words that can be repeated within these two second is thus smaller than the number of shorter words. If a word is not rehearsed within this two seconds interval, the probability that its trace is recovered diminishes, leading to lower memory performance.
Cowan et al. (1992) specified that the observed word length effect might in fact rather be an output effect than an actual constraint of the verbal rehearsal mechanism on memory. He observed that it was the pronunciation time at recall that played a major effect. If one had to start by recalling longer words, this led to an increased difficulty for the recall of subsequent words, in comparison with starting by recall of the shorter words. This had been implemented in a clever paradigm (see Figure 2.1), leaving unchanged the order of presentation of the words and as such the assumed order of verbal rehearsal, but varying the output order.
Figure 2.1: Example of the trial design in the study by (Cowan et al., 1992).
For example, the presentation of a sequence of words could start by longer words and end by shorter words (or vice versa). At recall, participants could be asked to recall in the presented order (forward recall), or recall the list backwards. In both conditions, this concerned thus a same amount of words and the same time to verbally rehearse the list of words. Nevertheless, the condition starting with longer words to recall, systematically led to lower recall performance. Cowan argued hence that this result was caused by the fact that during the verbal output, rehearsal of the other words could not take place, leading to a decay of their traces. Starting the recall by the longer words led hence to longer periods during which rehearsal could not take place, and thus more decay of the other words. This account still supports both assumptions made by Baddeley on the word length effect, namely decay and verbal rehearsal to counteract decay. However, other accounts for the word length effect have remained less faithful to these principles. As a result, the exclusive reliance on verbal
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rehearsal as the maintenance strategy for verbal material has been undermined. For example, in another paper in 1992, Cowan continued to investigate the output effects within verbal memory recall. An in depth investigation of the temporal course of speech-related parameters during recall suggested an important role for the inter word pauses at recall. The shortness of these pauses (about 320 ms) suggested that these could not have been used to verbally rehearse the material. Instead Cowan suggested a rapid scanning mechanism (Sternberg, Monsell, Knoll, & Wright, 1978; Sternberg, Wright, Knoll, & Monsell, 1980) that could act as a source of reactivation of the memory traces, taking place within these inter word pauses.
Decay was still a factor to be taken into account, but the reliance on verbal rehearsal to counteract decay was not a necessary mean. In the meantime, several other explanations for the word length effect have been proposed without even having recourse to the notion of decay (e.g., Hendry & Tehan, 2005; Lewandowsky & Oberauer, 2008; Neath & Nairne, 1995). Without decay, there is no need for a verbal resource to counter this decay.
Lewandowsky and Oberauer (2008) questioned the nature of the word length effect in light of the increasing evidence for an attentional mechanism to refresh memory traces that could act concurrently with articulation. If refreshing can take place during the articulation of the words, we should not even observe a word length effect. The cumulative nature of
attentional refreshing and verbal rehearsal, as shown by Camos et al. (2009, see chapter one) could however account for its occurrence. Additionally, Mora and Camos (2013) showed clear evidence for the word length effect to occur, whether participants could make use of attentional refreshing or not. The more important factor appeared to be the possibility to make use of verbal rehearsal. Blocking the verbal rehearsal mechanism by making use of
articulatory suppression did abolish the word length effect. This study confirms thus the reliance on a verbal mechanism to explain the word length effect.
Initially, the phenomena leading Baddeley to suppose a verbal maintenance resource (articulatory suppression, phonemic similarity and the word length effect) had especially been shown within the context of short term memory. The study by Mora and Camos (2013) on the word length effect as well as other studies of Camos on articulatory suppression and the phonological similarity effect (Camos et al., 2009; Camos, Mora, & Barrouillet, 2013) had all made use of complex span tasks. These phenomena have thus clearly been shown to apply within a working memory context as well.
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To summarize, a number of phenomena strongly support the idea of a verbal resource contributing to working memory performance. The widespread use of articulatory suppression within working memory tasks, blocking this verbal resource, reflects well this general
acceptance. Despite this overall agreement, some theories continue to deliberately neglect its existence (e.g., Oberauer, Lewandowsky, Farrell, Jarrold, & Greaves, 2012). The involvement of a domain-specific verbal resource has been shown to be compatible with the involvement of a domain-general attentional resource. However, the distribution of the roles for both of these resources differs strongly form one model to another. We will explicit this relationship in the sections on the resources for the maintenance of single features and feature
associations.