The representation of single features

Within this first version of the multi-component model, information to be maintained was represented mainly within the phonological loop or the visuo-spatial sketchpad, the two maintenance stores. Nevertheless, as stated above, in case a buffer was completely occupied, the central executive could take over some of the maintenance activities (Baddeley & Hitch, 1974). Over the years however, the maintenance function of the central executive has been abandoned (Baddeley, 1986; Baddeley & Logie, 1999). In his book in 1986, Baddeley described the central executive as an attentional controller, with a main function of

supervising. The maintenance function of the central executive was largely ignored. In 1999, Baddeley and Logie rendered the abandon of the maintenance function of the central

executive explicit and explained its rationale. First of all, theoretically they were unhappy about giving too much power and flexibility to the central executive and a strict separation

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between processing and storage activities would allow making more progress in our understanding of working memory processes. A second reason to abandon the storage

function of the central executive was the growing evidence that working memory could make use of other sources of storage, like for example long term memory. The implication of long term memory in working memory had meanwhile become a benchmark of some working memory theories (e.g., Ericsson & Kintsch, 1995), and several researchers even claimed working memory to be the activated part of long term memory (e.g., Anderson, 1993; Cowan, 1988; Engle, Kane, & Tuholski, 1999; Lovett, Reder, & Lebiere, 1999). It was certain that additional storage space was needed next to the maintenance buffers. However, as this storage space might for example be situated within long term memory, there was no longer need to attribute any storage space to the central executive. Thirdly, a series of experiments (Logie &

Duff, 1996, cited in Baddeley & Logie, 1999) combining arithmetical verification with verbal storage resulted in only small detrimental effects from the one on the other. Logie and Duff suggested hence a functional separation between the maintenance and processing activities. It has however to be noticed that the initially observed trade-off between maintenance and processing, used as an argument to attribute some maintenance capacity to the central executive, was not that devastating as expected either (Baddeley & Hitch, 1974). Not surprisingly, subsequent research continued thus to dispute the magnitude of the effect of maintenance on processing and vice versa (e.g., Baddeley, 2000; Cocchini, Logie, Della Sala, MacPherson, & Baddeley, 2002; Logie, 1986). This result is important as it has strong

implications for one’s vision on working memory. Accepting this trade-off between maintenance and processing implies both processes to fundamentally rely on the central executive. Neglecting the trade-off implies a strict functional separation between processing and maintenance. Despite the continuous debate, the multi-component model has clearly opted for a strict functional separation between processing and maintenance. Several other theories favor however the concept of a common resource for maintenance and processing activities (e.g., Anderson, 1993; Barrouillet et al., 2004; Case, 1985; Cowan, 1988; Daneman

& Carpenter, 1980; Just & Carpenter, 1992).

Within the later versions of the multi-component model, maintenance of single

features is thus accomplished within the domain-specific maintenance stores. Verbal material is to be maintained within the phonological loop while visuo-spatial material is to be

maintained in the visuo-spatial sketchpad. The existence of the phonological loop within working memory and its reliance on phonemic coding was first validated by the negative

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effects articulatory suppression and phonemic similarity had on verbal reasoning and learning, as we have described above (Baddeley & Hitch, 1974). Additional evidence was supplied by the word length effect. Baddeley, Thomson, and Buchanan (1975) showed that participants were able to recall a greater amount of short words like “sum” or “hate”, then longer words like “aluminium” or “university”. This word length effect did not seem to be the result of the number of syllables or phonemes, but instead the result of the time it took to pronounce words. Baddeley et al. (1975) observed a high correlation between speech rate and memory span and concluded that participants could maintain an amount of words equal to the amount they could pronounce in approximately two seconds. The word length effect added thus evidence to the phonemic nature of the phonological loop and supported the hypothesis of temporal decay within working memory. The activation of memory traces that had not been rehearsed within these two seconds fell beneath the threshold for active maintenance.

However, in the meantime it has become obvious that different accounts for the word length effect were possible, without having recourse to the articulatory component (e.g., Cowan et al., 1992; Neath & Nairne, 1995). We will elaborate on these proposals in chapter two on the working memory resources.

According to the multi-component model at that time, the effects of articulatory suppression, phonemic similarity and word length all seemed to point to a phonological store that is dependent on articulatory rehearsal to refresh the memory traces, preserving them from temporal decay (Baddeley, 1986). Further investigation of these phenomena imposed a

fractionation of the phonological loop. It had been shown that articulatory suppression

neutralized the effect of phonemic similarity and the word length effect when the material was presented visually. However, this appeared not to be the case when it was presented auditorily (Baddeley, 1986). In order to account for this result, the phonological loop was divided in a passive and an active system. Maintenance of verbal material is accomplished in a passive phonological store, on which active articulatory rehearsal can act to impede forgetting. An auditory presentation gives thus direct access to the phonological store, while a visual presentation of the material requires an articulatory coding before it can enter this store. As articulatory suppression hinders this articulatory coding, the effects of phonemic similarity and word length are abolished. The irrelevant speech effect relies on this fractionation of the phonological loop as well. The effect refers to a decrease in memory performance caused by the auditory presentation of irrelevant verbal material (Colle & Welsh, 1976). Due to its

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auditory nature, unattended speech enters thus directly into the phonological store, causing as such corruption of the information to be maintained (Salame & Baddeley, 1982).

The phonological loop is probably the module of the multi-component that has been investigated the most. Its existence and functioning have thus largely been documented within the literature. This is unfortunately not the case for the visuo-spatial sketchpad, whose

existence remains an issue of debate up until today (Barrouillet & Camos, 2015; Morey, 2009;

Phillips & Christie, 1977; Vergauwe, Camos, & Barrouillet, 2014) and investigations of its functioning have resulted in less straightforward conclusions. Although neuropsychological research had indeed shown evidence for a separation between verbal and visuo-spatial

processes (e.g., De Renzi & Nichelli, 1975; Milner, 1971), this does not necessarily imply that a visuo-spatial memory store with its own domain-specific maintenance mechanisms does exist. A visuo-spatial memory store was however inserted in the multi-component model in analogy with the phonological loop. Evidence for this kind of store was mainly supplied by studies making use of the selective interference paradigm. This paradigm is based on several assumptions. First of all, it is assumed that a domain-specific verbal resource exists and this resource is responsible for the maintenance of verbal information. This verbal resource can be interfered with by the execution of a concurrent task that equally recruits this same resource, like the constant uttering of verbal material (i.e., articulatory suppression). Secondly, if a domain-specific visuo-spatial resource exists, then it should as well be possible to interfere with this resource by executing a concurrent task relying on these visuo-spatial resources like for example the constant tapping of a spatial pattern (e.g., Farmer, Berman, & Fletcher, 1986;

Salway & Logie, 1995). Finally, a concurrent verbal task should not or only minimally interfere with the maintenance of visuo-spatial material that relies on the visuo-spatial

resource just as a concurrent visuo-spatial task should not or only minimally interfere with the maintenance of verbal material relying on the verbal resource. This pattern of selective

interference has indeed been observed and replicated on many occasions (e.g., Brooks, 1968;

Farmer et al., 1986; Logie, Zucco, & Baddeley, 1990; Shah & Miyake, 1996) suggesting thus the existence of the visuo-spatial sketchpad. It should however be mentioned that several more recent studies failed to show the expected pattern using the selective interference paradigm. Instead, it was observed that a concurrent verbal task interfered well with the maintenance of verbal material and less with the maintenance of visuo-spatial material, while the concurrent visuo-spatial task interfered as much with the verbal and the visuo-spatial maintenance task (e.g., Bayliss, Jarrold, Gunn, & Baddeley, 2003; Vergauwe, Barrouillet, &

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Camos, 2010). This pattern of results supports the existence of a domain-specific verbal resource, while it puts into question the existence of domain-specific visuo-spatial resources.

Nevertheless, in the meantime this visuo-spatial memory store with its domain-specific resource was accepted within the multi-component model and has resulted in a further investigation of its functioning. From the beginning already, the question about a separation between visual and spatial working memory was pending. Baddeley and

Lieberman (1980) investigated the nature of the visuo-spatial sketchpad and observed that a spatial concurrent task disrupted specifically visuo-spatial memory but not verbal memory, while a visual concurrent task affected especially the verbal memory task and not the visuo-spatial memory task. This pattern indicated first of all a distinction between visual and visuo-spatial processes, and secondly led to the conclusion of a prioritized implication of spatial coding within the visuo-spatial sketchpad. Logie (1995) continued to investigate the functioning of the visual and spatial components within the visuo-spatial sketchpad and proposed a structural equivalence with the phonological loop. He assumed memory traces within the visuo-spatial sketchpad to be formed by processes closely linked to perception, on which active rehearsal programs then have to act. Concretely, this was accounted for by proposing a visual and a spatial maintenance component within the visuo-spatial sketchpad. The visual maintenance store was termed the visual cache and was supposed to be the equivalent of the phonological store. Several of the characteristics of the phonological store could indeed be transposed to the visual cache. For example, an equivalent to the phenomenon of phonemic similarity, called visual similarity, was observed (Hitch, Halliday, Schaafstal, & Schraagen, 1988; Hue

& Erickson, 1988; Walker, Hitch, & Duroe, 1993). Like the phonological store, Logie assumed the visual cache to be an essentially passive store maintaining in this case visual information with visually presented information entering automatically in this store. The spatial maintenance store, termed the inner scribe, served then to maintain spatial information as well as to actively rehearse visuo-spatial information, maintained otherwise passively. The mechanism by which this rehearsal is achieved was supposed to be linked closely to the planning of movements.

Although the visuo-spatial sketchpad was thus suggested as mirroring the phonological loop, and indeed some resemblances do exist, the same level of symbiosis observed between the phonological store and articulatory rehearsal could not be demonstrated for visual and spatial maintenance. Instead, growing evidence was shown for a separation between visual and spatial maintenance. Initially, Baddeley and Lieberman (1980) had shown

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a primordial role for the spatial component within the maintenance of visual and spatial information. However, they had used of the Brooks matrix (Brooks, 1967, 1968) to evaluate visuo-spatial memory. Within this task, participants have to imagine a 4*4 matrix. They are then read sentences describing where they have to place a specific digit. First, a starting square is indicated where they have to imagine the number one, next a sentence like “place digit two in the square to the left” is read and so on. Participants create hence a spatial sequence of the digits one to eight within this imagined matrix. It has to be noticed that this task is essentially spatial in nature and it is thus not surprising that Baddeley and Lieberman observed interference from a secondary spatial but not from a secondary visual task. Della Sala, Gray, Baddeley, Allamano, and Wilson (1999) used a more complete design to study the functioning of visual and spatial working memory. They combined both visual and spatial memory with both a visual and a spatial processing task. This allowed them to observe the typical pattern of selective interference between the visual and the spatial domain anyway.

This separation was further confirmed by Klauer and Zhao (2004) and Darling, Della Sala, and Logie (2007) as well. They also made use of the selective interference paradigm but optimized the specific tasks used. Their results led them to comply with the vision of visuo-spatial working memory being dependent on either domain-specific visual or domain-specific spatial mechanisms for their maintenance. The nature of these mechanisms is not yet entirely clear, but visual and spatial maintenance have been suggested to rely on visual imagery and the representation and planning of movement respectively (Baddeley & Logie, 1999; Logie, 1995). Within the multi-component model, visuo-spatial resources were thus integrated as a pillar to the framework. In chapter two on the working memory resources, we will present some alternative views on the role of visuo-spatial resources within working memory performance.

To summarize, within the multi-component model, information is thus supposed to be maintained within domain-specific buffers, with domain-specific rehearsal mechanisms acting upon these representations. One might however wonder how the multi-component model incorporates representations of information belonging to different domains, for example a street name with its location on a map. It is clear that domain-specific maintenance stores and domain-specific rehearsal mechanisms do not suffice to accomplish the maintenance of cross-domain feature associations. The multi-component model’s vision on the maintenance of associated features will be discussed in the next section.

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