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Issue Editor Foreword: The Challenge of Understanding, Assessing, and Providing Intervention for Linguistic and Working Memory Impairments

Section Editor(s): Archibald, Lisa M. D. Issue Editor

doi: 10.1097/TLD.0000000000000000
Issue Editor Foreword

Researchers focus on several different cognitive systems to understand how the brain learns. This endeavor must be acknowledged as an imperfect approach that is often frustrated by the apparent rich integration between the systems under study and other proposed neural mechanisms. The question arises over and over again: Are these distinct cognitive mechanisms or different sides of the same processes? In these two special issues of Topics in Language Disorders (TLD; Volume 33:3–4), we have considered this question with regard to linguistic knowledge, working memory, and other cognitive factors.

The integration between memory and existing language is obvious: The ability to hold information in memory facilitates a long-term representation in the brain (Baddeley, Gathercole, & Papagno, 1998) and, correspondingly, activation of existing (linguistic) knowledge facilitates the acquisition of new representations/knowledge (Gathercole, 2006). So, when we encounter an unfamiliar word like “ghid,” we rely on immediate memory systems to encode and retain the new form. Conversely, when we encounter new information in a familiar context such as the classic “point to the chromium one” (Wilkinson, Dube, & McIlvane, 1996), we can use our existing linguistic knowledge to associate a meaning and connect the unfamiliar word within our existing knowledge network. In fact, as our existing language knowledge develops, we humans rely more and more on our linguistic system to facilitate all kinds of cognitive tasks including memory, reasoning, problem solving, and so forth. As a result, the line between language and memory blurs, making these systems hard to study and understand.

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Over the two issues in this series, a number of cognitive processes are considered. Immediate memory refers to both short-term memory and working memory. Short-term memory is the ability to hold information in mind briefly. Working memory subsumes short-term memory and additionally involves some cognitive manipulation of the information being held in a person's current focus of attention. Working memory is considered in all of the articles because it has been associated with both language learning (Gathercole & Baddeley, 1993) and language impairment (Montgomery, 2002).

Working memory has been proposed as a domain-general system supported by two domain-specific short-term memory stores (Baddeley & Hitch, 1974). The domain-general central executive constrains performance on working memory tasks; that is, tasks involving some active cognitive processing of information. This domain-general central executive resource is of particular interest because of findings that complex cognitive performance is predicted better by working memory tasks than by short-term memory tasks (Engle, Tuholski, Laughlin, & Conway, 1999). Importantly, working memory and short-term memory tasks both require the immediate recall of material, but working memory tasks also involve active manipulation of the information, placing demands on the central executive. Typically, working memory tasks involve either verbal or visuospatial information. Kidd (2013) discussed the overlap between verbal working memory and language in detail. Archibald (2013) investigated the distinction between working memory and language using a different approach. On the basis of the premise that working memory relies on a domain-general resource to support learning, Archibald examined both verbal and visuospatial working memory measures and their relationship to language performance. In this issue, Schuchardt, Bockmann, Bornemann, and Maehler (2013) consider the working memory and language profiles of children with academic learning disabilities with or without specific language impairment (SLI).

The domain specificity within short-term memory is recognized either explicitly in working memory models (Baddeley & Hitch, 1974) or acknowledged as sources of interference (Cowan, 2001). There is considerable evidence for distinct neural mechanisms both for processing phonological versus visuospatial information (Paivio, 1971) and for corresponding stores in short-term memory (Pickering, Gathercole, & Peaker, 1998). A deficit in the ability to process and store phonological information (Archibald & Gathercole, 2006a) has been implicated in SLI, the unexpected developmental difficulty in acquiring language.

Findings for children with SLI regarding immediate memory for visuospatial information have lacked consistency, with some studies reporting group differences (Bavin, Wilson, Maruff, & Sleeman, 2005) and others not (Archibald & Gathercole, 2006b). In this issue, Alt (2013) further investigates visuospatial processing in children with SLI using a fast mapping paradigm. As well, given evidence that verbal associations and visual imagery support learning (Reed, 2010), Washington and Warr-Leeper (2013) consider the role of visual supports in SLI intervention. It must be acknowledged that humans will use their powerful verbal strategies to support learning and retention whenever possible, even in tasks presumed to be largely nonverbal (Jones & LeBaron, 2002). The challenge of assessing the verbal and nonverbal influences on performance is addressed in three articles in this two-issue series in TLD (Botting, Psarlou, Caplin, & Nevin, 2013; Leclercq, Maillart, & Majerus, 2013; Stokes, Moran, & George, 2013).

Long-term memory refers to a person's ability to encode, store, and retrieve information by establishing neural patterns that are retained over long periods of time and, perhaps, indefinitely in some cases. Knowledge in long-term memory is stored in a preconscious or unconscious state and may be activated through associations. According to Cowan (2001), activated long-term memories are held in the current focus of attention, which essentially defines working memory. Typically, long-term memory is divided into the procedural system, supporting rule-based learning, and the declarative system, supporting the development of semantic and episodic knowledge. These systems are discussed in detail by Lum and Conti-Ramsden (2013) and considered by Kidd (2013) and Archibald (2013). The extent to which these memory systems are separable from linguistic knowledge is a theme recurring across several of the articles is this TLD series (Archibald, 2013; Kidd, 2013; Leclerq et al., 2013; Lum & Conti-Ramsden, 2013; Stokes et al., 2013).

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Briefly, the theoretical suggestions addressed in these two TLD issues considered the separability of working memory and language. Considerable debate has focused on whether verbal working memory and linguistic tasks tap a unitary language processing factor (MacDonald & Christiansen, 2002) or reflect distinct cognitive processes (Archibald, 2013). Kidd (2013) reviewed evidence that verbal working memory is an emergent property of long-term linguistic knowledge. Consistent with this suggestion, Archibald (2013) reported loadings on a language processing factor for listening recall, a commonly used verbal working memory task involving judging the veracity of sentences and remembering sentence-final words in sequence. Similarly, Stokes et al. (2013) and Leclercq et al. (2013) reported converging findings that nonword repetition is constrained by the ability to access phonological representations from long-term memory in both late talkers and children with SLI, respectively.

Nevertheless, language and working memory were found to load on separable factors by Archibald (2013). In this case, a domain-general working memory factor emerged with loadings from both verbal and visuospatial working memory tasks. These findings suggest that cross-domain measures of working memory are needed to assess working memory independent of language abilities. Results reported by Schuchardt et al. (2013) in this issue provide some indirect evidence of the separability of language and working memory impairments. In particular, Schuchardt et al. (2013) reported that children with learning disorders with or without SLI had similar impairments in both phonological short-term memory and the central executive of working memory. Thus, the presence or absence of SLI was not associated with a working memory impairment. Indeed, a domain-general working memory impairment characterized all groups with a learning disorder regardless of language status. On the contrary, children with SLI did have a markedly severe phonological short-term memory impairment compared with all other groups. In agreement with many previous reports of phonological deficits across a variety of measures (Bortolini & Leonard, 2000; Claessen, Leitão, Kane, & Williams, 2013), these findings suggest a unique domain-specific phonological impairment in SLI.

It is well recognized, however, that SLI groups commonly score more poorly than age-matched peers on nonverbal tasks as well (e.g., Bavin et al., 2005). The study reported by Alt (2013) was aimed at investigating potential domain-general working memory deficits in SLI by using a visual fast mapping paradigm. Although there was some evidence that children with SLI were less efficient at fast mapping and retrieving visual information, the effect was small and largely relied on differences in a feature with potential for verbal coding (i.e., color). Alt (2013) concluded that although visual working memory deficits were not ruled out in her study, such deficits are considerably less severe than verbal working memory deficits for children with SLI.

A systematic review by Lum and Conti-Ramsden (2013) provided further insight into the interactions between immediate and long-term memory and language abilities. Although SLI deficits on verbally based learning and retrieval measures of declarative memory have been reported, they have not been found for corresponding nonverbal tasks. As well, group differences on the verbal declarative memory measures were not reliable when controlling for verbal working memory differences. On the basis of these findings, Lum and Conti-Ramsden proposed the preliminary conclusion that problems with learning and retrieval in declarative memory are secondary to working memory and/or language difficulties. Few studies were available for review that specifically addressed procedural memory. However, preliminary results indicated consistent SLI deficits to learn information implicitly regardless of whether the material to be learned is verbal or visual. In particular, children with SLI appear to require more exposures to learn information implicitly.

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It is clear from the preceding review of the cognitive processes and theoretical discussions addressed in these special issues that devising assessments of working memory and language abilities is a challenging one. Children with SLI will score more poorly on any task allowing for the use of verbal strategies regardless of whether or not that task is meant to be a test of visual/nonverbal abilities. Two problems in interpretation arise from the use of verbal strategies. First, testers are not always aware of the opportunities for verbal coding on tasks they intend to assess nonverbal skills. As a result, deficits may be attributed to nonverbal skills that really arise because children with SLI cannot use verbal strategies as efficiently as their language-typical peers. Second, testers may intend to assess a related cognitive skill such as working memory using a verbally based task. In this case, SLI deficits may be attributed to working memory that are really due to language impairment.

Botting et al. (2013) investigated the nature of the verbal/nonverbal boundaries in short-term memory tasks directly. Four tasks were devised ranging from entirely verbal to entirely nonverbal. The verbal task involved both auditory-verbal input and output, whereas the nonverbal task involved both visuospatial input and output. The remaining two tasks involved either visual input and auditory output or visual input and output. Importantly, the stimuli in these two combined verbal-visual tasks were pictures of familiar objects, a feature designed to encourage the use of verbal strategies. Lower scores were found for the SLI group than for the control group on all but the purely nonverbal task. Additional analyses suggested that children with SLI were less efficient at using verbal encoding strategies and relied on visual encoding to a greater extent than the control group. On the basis of this pattern of findings, Botting et al. suggested that the SLI deficit might be in verbal encoding rather than a (domain-general) limitation in working memory capacity.

It is clear from the preceding discussion that clinicians who wish to assess particular linguistic or cognitive skills must select appropriate tests carefully. A number of tests include verbal measures of short-term memory and working memory. For example, a common measure of phonological short-term memory is forward digit span, a task requiring the serial recall of digits. Typically, backward digit span is considered to be a working memory measure because the task additionally requires a reordering of the presented elements. Some findings indicate that forward and backward digit spans tap the same underlying mechanism, possibly because the processing load of reordering digits is fairly low (Rosen & Engle, 1997). Other verbal working memory measures include the previously mentioned listening recall task, counting recall in which the child counts target shapes on each plate and then remembers the tallies from each plate in sequence, and serial recall tasks such as number letter recall in which the child hears a list of numbers and letters and must reorder the items on recall by category and sequence. Many standardized tests have working memory subtests including the Automated Working Memory Assessment (AWMA; Alloway, 2007), Wide Range Assessment of Memory and Language (WRAML; Sheslow & Adams, 2003), Wechsler Intelligence Scale for Children, Fourth Edition (WISC-4; Wechsler, 2004), Test of Auditory Processing Skills, Third Edition (TAPS-3; Martin & Brownell, 2005), and the Woodcock Johnson Test of Achievement, Third Edition (Woodcock, McGrew, & Mather, 2007).

Corresponding visuospatial short-term and working memory measures are more difficult to find. The most common visuospatial short-term memory tests involve presentations of sequences of locations (e.g., blocks on a board; dots on a grid), and the child must point to the indicated locations in sequence at recall. The AWMA (Alloway, 2007) and the WRAML (Sheslow & Adams, 2003) include tasks of this nature. Visuospatial working memory tasks involve some visuospatial processing, usually mental rotation, and then recall of locations. Subtests of this type are included in the AWMA. Tasks tapping short-term and working memory that combine verbal and symbolic stimuli are included in the WRAML.

Beyond the stimuli presented in these tasks, the test design is an important consideration. Memory measures use a span procedure aimed at assessing an individual's memory capacity by determining the number of items that can be held in mind accurately. Tasks begin at an easy level with the child required to recall lists of just a couple of items and progress by adding one item to the list until the child fails to reliably recall the items. One consideration here is how to determine whether the child's responses are reliable. Some tests present only two lists at each length (e.g., WISC-4; Wechsler, 2004), which may create opportunities for spurious errors. In the AWMA (Alloway, 2007), four accurate responses in up to six attempts are required before list length is increased. In this case, more reliable behavior is demonstrated but test time and opportunities for fatigue effects may be increased.

Another consideration is whether the test provides an interpretation of targeted constructs. For example, a single composite score is calculated for performance across the forward and backward digit spans in the WISC-4 (Wechsler, 2004). Corresponding scores for short-term memory (forward digit span) and working memory (backward digit span) are not available. Other tests may not organize themselves according to the constructs of short-term and working memory but do provide scaled scores for each particular subtest (e.g., WRAML, Sheslow & Adams, 2003; TAPS-3, Martin & Brownell, 2005). Domain-specific short-term and working memory subtest and composite scores are calculated in the AWMA (Alloway, 2007).

Finally, it is important to distinguish verbally based immediate memory measures from those targeting linguistic knowledge. Archibald (2013) provided an analysis of commonly used standardized tests of verbal abilities. Results demonstrated that the linguistic tasks included in the Clinical Evaluation of Language Fundamentals, Fourth Edition (Semel, Wiig, & Secord, 2003), loaded on a language processing factor. The language processing factor was distinct from a domain-general working memory factor with loadings from both verbal and visuospatial working memory measures. It is important to note that some tasks did show secondary loadings across factors and different loading patterns with age. It is clear that the demands of these verbal tasks are not static over development or within individuals.

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There was considerable consistency in the findings across the studies included in these special issues on language and working memory. Children with SLI have a particular vulnerability to learning, retaining, and processing verbal information. Visuospatial processing, on the contrary, is a relative strength in SLI. The question of how this relative strength may be incorporated in intervention was addressed by Washington and Warr-Leeper (2013). Greater learning efficiency and syntactic growth were found for children with SLI who received intervention incorporating visual supports for grammatical elements compared with those whose intervention did not include such supports. The findings suggest that specifically targeted visual supports can provide an effective support to learning for children with SLI.

It must be noted that the use of visual supports should be approached with some caution. On the basis of findings of the verbal demands of visual tasks in their study, Botting et al. (2013) suggest that verbal demands may be “hidden” within visual tasks and strategies. If such is the case, the introduction of visual supports may place additional verbal demands on the child with SLI (see also Ebbels, 2008). Thus, it is important to carefully evaluate task demands in nonsimplistic ways and the effectiveness of visual support strategies for children with SLI.

The aim of these two special issues of TLD has been to evaluate the cognitive processes involved in language learning and impairment. The concepts of working memory and existing linguistic knowledge have been highlighted specifically. It is clear that within the realm of verbal tasks, these constructs are tightly integrated. Nevertheless, the relative deficits in verbal compared with nonverbal visuospatial processing in children with SLI are clear. Assessments and interventions may capitalize on this domain specificity to better understand core deficits and achieve therapeutic benefits.

—Lisa M. D. Archibald

Issue Editor

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