Copyrightc 2013 Wolters Kluwer Health|Lippincott Williams & Wilkins
Working Memory Functioning
in Children With Learning
Disorders and Specific
Language Impairment
Kirsten Schuchardt, Ann-Katrin Bockmann,
Galina Bornemann, and Claudia Maehler
Purpose:On the basis of Baddeley’s working memory model (1986), we examined working
mem-ory functioning in children with learning disorders with and without specific language impairment (SLI). We pursued the question whether children with learning disorders exhibit similar working memory deficits as children with additional SLI.Method:In separate analyses, we compared the following groups of children: (1) 30 children with dyslexia (DYS) and 16 children with DYS re-ceiving special language education and (2) 19 children with combined disorder of scholastic skills (CDSS) and 18 children with CDSS receiving special language education. A control group of 30 typically developing children was included in each comparison. All of the children receiving spe-cial language education met criteria for SLI. To assess the 3 subcomponents of working memory (phonological loop, visual–spatial sketchpad, central executive), the children worked individually on an extensive test battery.Results:We found deficits in the phonological loop and central executive functioning for children with dyslexia (and CDSS) as well as for children with additional SLI. Deficits in phonological functioning were broader and more profound for children with SLI. Deficits in visual–spatial sketchpad could only be found for children with CDSS without SLI.
Con-clusions:Children with isolated learning disorder and children with additional SLI demonstrate
similarities and differences in working memory functioning. These findings support our hypoth-esis that underlying working memory deficits for the different disorders partly overlap but also are distinct and partly distinguish between certain disorders.Key words: combined disorder of scholastic skills,dyslexia,learning disorder,specific language impairment,working memory
L
EARNING DISORDERS REPRESENT one of the most frequent causes for school fail-ure. Children with specific learning disorders such as dyslexia and dyscalculia show generalAuthor Affiliations:Department of Diagnostic and Educational Psychology (Drs Schuchardt,
Bockmann, and Maehler and Ms Bornemann), University of Hildesheim, Hildesheim, Germany.
The authors have indicated that they have no financial and no nonfinancial relationships to disclose.
Corresponding Author: Kirsten Schuchardt, PhD, Department of Diagnostic and Educational Psychol-ogy, University of Hildesheim, Marienburger Platz 22, 31141, Hildesheim, Germany ([email protected]).
DOI: 10.1097/01.TLD.0000437943.41140.36
impairments in acquiring the cultural tech-niques of reading, spelling, and calculating. Dyslexia describes specific deficits in read-ing acquisition (often combined with spellread-ing disorder), and dyscalculia is characterized by deficits in arithmetic skills.
The exact definitions and diagnostic cri-teria of learning disorders differ widely, but some sources are used commonly across the world. International Classification of
Dis-eases, Tenth Revision(ICD-10, 2011) by the
World Health Organization defines interna-tionally accepted diagnostic criteria. Here, learning disorders are described as poor per-formance in reading, spelling, and calculat-ing, respectively, that must be significantly lower than expected with regard to age, in-telligence, and schooling.
It is not unusual that learning disor-ders occur together with specific language impairment (SLI), which is characterized by serious qualitative and quantitative deficits in productive and/or receptive language pro-ficiency. The ICD-10 (Code F80) defines “specific developmental disorders of speech and language” as “disorders in which nor-mal patterns of language acquisition are dis-turbed from the early stages of develop-ment.” It specifies further that the conditions are not directly attributable to neurological or speech mechanism abnormalities, sensory impairments, mental retardation, or environ-mental factors. Specific developenviron-mental dis-orders of speech and language are often followed by associated problems, such as difficulties in reading and spelling.
Approximately 25%–75% of all children with language impairment develop read-ing difficulties (Catts, Adlof, Hogan, & Ellis Weismer, 2005; McArthur, Hogben, Edwards, Heath & Mengler, 2000; Tomblin, Zhang, Buckwalter, & Catts, 2000). On the contrary, according to Catts et al. (2005), every fifth child with dyslexia shows a history of dif-ficulties in language acquisition. If language disorders persist until school age, the rate of children demonstrating specific reading dis-ability climbs to 50% (McArthur et al., 2000). Children with SLI also can experience diffi-culties. These difficulties can emerge prior to formal schooling and persist during the school-age years (Cowan, Donlan, Newton, & Lloyd, 2005; Donlan, Cowan, Newton, & Lloyd, 2007; Eisenmajer, Ross, & Pratt, 2005; Fazio, 1994, 1996, 1999).
Given this high comorbidity rate, recent re-search has started to investigate the common-alities and shared causal factors of learning disorders and SLI. Within recent years, one question has been addressed in particular: Are SLI and learning disabilities distinct disorders with different causal factors or are they var-ious manifestations of the same underlying cognitive factors (Baird, Slonims, Simonoff, & Dworzynski, 2011; Catts et al., 2005; de Bree, Wijnen, & Gerrits, 2010)?
WORKING MEMORY
As possible causal factors underlying SLI and learning disorders, researchers have iden-tified deficits in working memory and diverse aspects of phonological information process-ing, such as phonological awareness (Catts et al., 2005; Eisenmajer, et al., 2005; Nithart et al., 2009). Although various models of working memory have been developed, the model by Baddeley (1986) has proved a par-ticularly useful theoretical tool in numerous studies in this area. The model distinguishes between different components of working memory, with the modality-free central
ex-ecutiveacting as a kind of supervisory system
that serves to control and regulate the cog-nitive processes occurring in its two limited-capacity slave systems, thephonological loop and the visual–spatial sketchpad. Further functions of the central executive that have since been identified by Baddeley (1996) in-clude coordinating the slave systems, focusing and switching attention, and retrieving repre-sentations from long-term memory.
In contrast, Baddeley’s (1986, 1996) two slave systems perform modality-specific op-erations. Verbal and auditory information is stored temporarily and processed in the phonological loop. Whereas verbal or audi-tory information enters the phonological store directly, visual information has to be trans-lated into phonological code before it can do so. Two components of the phonologi-cal loop are distinguished: the phonologiphonologi-cal store and the subvocal rehearsal process. The visual–spatial sketchpad is concerned with re-membering and processing visual and spatial information; it comprises a visual cache for static visual information and an inner scribe for dynamic spatial information (Logie, 1995; Pickering, Gathercole, Hall, & Lloyd, 2001). Later, Baddeley (2000) added a fourth com-ponent to the working memory model, the
episodic buffer, for linking long-term
mem-ory and integrating information from all of the other systems into a unified experience. To date, however, research on working memory
has mostly focused on the three original sub-components, probably because it turned out to be difficult to create valid tasks measuring the functioning of the episodic buffer. WORKING MEMORY AND LEARNING DISORDERS
There is considerable evidence that chil-dren with dyslexia have deficits in phono-logical processing and storage (Schuchardt, Maehler, & Hasselhorn, 2008; Vellutino, Fletcher, Snowling, & Scanlon, 2004). Chil-dren have been found to exhibit a reduced memory span for acoustically presented words, numbers, and nonwords. Numerous studies on dyslexia and accompanying deficits in complex abilities such as text comprehen-sion also detect deficits in central-executive working memory functioning (Landerl, Bevan & Butterworth, 2004; Schuchardt et al., 2008; Siegel & Ryan, 1989). In contrast, hardly any reliable correlations between visual–spatial sketchpad functioning and dyslexia could be found (O’Shaughnessy & Swanson, 1998; Schuchardt et al., 2008).
Researchers studying working memory functioning in children with dyscalculia have reported deficits in the visual–spatial sketch-pad (Schuchardt et al., 2008), whereas mixed results have been reported for the phonolog-ical loop and central executive. Some stud-ies show them not to be impaired (McLean & Hitch, 1999; Schuchardt et al., 2008), whereas others do report deficits in the phonological loop or central executive (McLean & Hitch, 1999; Swanson & Sachse-Lee, 2001). Children with a combined disorder of scholastic skills (CDSS; a term used in Germany on the basis of
theICD-10to describe learning difficulties in
reading and writing as well as mathematics) have been found to have severe deficits in all three working memory components (Maehler & Schuchardt, 2009; Schuchardt et al., 2008). Some of this research (Maehler & Schuchardt, 2009) has shown similar performance pat-terns between children with intelligence below average and children with CDSS, who, by definition, have average intelligence.
Deficits in working memory functioning have been investigated extensively also for children with SLI. Children with SLI display severe deficits in phonological loop function-ing (Archibald & Gathercole, 2006b, 2007; Marton & Schwartz, 2003). Results concern-ing visual workconcern-ing memory are inconsistent. Although some studies did not find any impair-ment of visual spatial sketchpad in children with SLI (e.g., Archibald & Gathercole, 2006a; Riccio, Cash, & Cohen, 2007), others have re-ported significantly lower scores for children with SLI on tasks assessing the visual spatial sketchpad than those for typically develop-ing children (e.g., Hick, Bottdevelop-ing, & Conti-Ramsden, 2005; Hoffman & Gillam, 2004). In addition to these phonological and visual– spatial difficulties exhibited by children with SLI, deficits in central–executive processing are evident (Archibald & Gathercole, 2006b; Marton & Schwartz, 2003; Montgomery & Evans, 2009). These deficits in central execu-tive are not confined to phonological material, they also occur with visual–spatial material. This lends support to the hypothesis that chil-dren with SLI demonstrate a broader impair-ment of central-executive functioning, which is not restricted to phonological information processing.
In summary, it can be stated that specific patterns in working memory functioning have been detected both for learning disorders and SLI and that some similarities in deficit pro-files are evident. Nevertheless, only a few studies have directly compared working mem-ory profiles in children with both specific and comorbid learning deficits although such comparisons are crucial to understanding the potential common or distinct cognitive im-pairments associated with different learning profiles. Catts et al. (2005) examined chil-dren with dyslexia, SLI, and combined SLI and dyslexia. Only children with reading difficul-ties (with or without SLI) performed poorly on phonological awareness and phonologi-cal working memory tasks, whereas the SLI and control groups did not. Catts et al. con-cluded that SLI and dyslexia are distinct dis-orders with overlapping difficulties in basic
cognitive functioning. However, Catts et al. relied on a preschool SLI diagnosis and did not examine whether the children with SLI still met the criteria for SLI at school age. There-fore, it is an open question whether school-aged children with a current diagnosis of SLI, dyslexia, or both SLI and dyslexia will show overlapping or different patterns of working memory functioning.
We also could find no studies on chil-dren with SLI and comorbid severe learning disorder in reading, spelling, andarithmetic skills (CDSS). We, therefore, wanted to inves-tigate whether children with SLI and comor-bid CDSS exhibit working memory impair-ments that are similar in profiles and as severe as those observed for children with CDSS but without SLI.
The purpose of this research was to exam-ine whether children with persisting language impairment and children with learning disor-ders in either reading only or across scholastic domains exhibit the same deficits in working memory. We hypothesized that deficits in the same working memory components would be suggestive of a common underlying cognitive impairment whereas unique deficits would correspond to different underlying impair-ments. In our study, children with learning disorders and children with combined learn-ing disorders and SLI underwent an extensive working memory test battery with tasks on all subcomponents with the exception of the episodic buffer. We then compared the pat-terns of the following groups of children: (1) children with isolated dyslexia and children with comorbid SLI and dyslexia and (2) chil-dren with CDSS (i.e., dyslexia and dyscalculia) and children with SLI and CDSS, and a control group of typically developing children.
METHODS Participants
Five groups of children (second- to fourth-grade students) participated in the study: (1) children with dyslexia but not referred for
special language education (SLE) (DYS; n= 30); (2) children with dyslexia and referred for SLE (SLI + DYS; n = 16); (3) children with CDSS but not referred for SLE (CDSS; n=19); (4) children with CDSS and referred for SLE (SLI+CDSS;n=18); and (5) a con-trol group of typically developing children matched for chronological age (C; n =30). The two groups of children with learning dis-orders (DYS and CDSS) were recruited from the counseling center for children with learn-ing disabilities, which is part of a university program in a city of Germany. All of them at-tended regular primary schools; due to learn-ing problems, they voluntarily attended the diagnostic procedures described later and re-ceived the diagnosis DYS or CDSS. The two groups of children referred for SLE (SLI + DYS and SLI +CDSS) attended a school for SLE. Their SLI-diagnoses were present before they participated in the study. Being referred for SLE and attending a special school re-veals severe problems of language develop-ment in the two SLI groups. The typically de-veloping children in the control group were second to fourth graders from a public ele-mentary school. Only native German-speaking children were included in the study sample. No information on socioeconomic status of the families was available.
Diagnostic classification
The two groups of children with SLI (SLI+ DYS and SLI + CDSS, who all attended a school for SLE) were tested for language de-velopment and intelligence before they were referred to special education and before they took part in the study. Because of time re-strictions, we accepted their intelligence test-ing but we confirmed their language deficits. The two groups with learning disorders (DYS and CDSS) had not been tested before and received their diagnosis through our testing. We did not examine language performance in these groups, because there was no indi-cation for these tests in the context of the counseling center (no language problems re-ported by parents or teachers). Of course, one
might argue that language impairment in chil-dren with learning disorders might be more subtle and harder to identify but yet, observ-able through their lower school achievement. Unfortunately, we cannot rule out this pos-sibility because, due to the routine proce-dure within the counseling center and time restrictions, we did not apply any direct mea-sures for language development to these chil-dren. However, given the severe language impairment of the children receiving SLE, we dare to rely on a significant difference in lan-guage development between the learning dis-order and SLI groups.
All children were screened with standard-ized tests for intellectual ability, spelling, reading, and arithmetic. Different German in-telligence tests had been used for the children with SLI before (CFT-1, Cattell, Weiss, & Osterland, 1980; IDS, Grob, Meyer, & Hagemann-von Arx, 2009; K-ABC, Melchers & Preuss, 2001; SON-R, Tellegen, Winkel, & Laros, 2003), in the counseling center the K-ABC (Melchers & Preuss, 2001) was used. For our study, we relied on the measures for nonverbal intelligence in the different intelligence tests that captured a similar concept of logical reasoning. Mathematical skills were assessed using standardized German mathematical achievement tests for second, third, and fourth graders (DEMAT 2+, Krajewski, Liehm, & Schneider, 2004; DEMAT 3+, Roick, G¨olitz, & Hasselhorn, 2004; DEMAT 4, G¨olitz, Roick, & Hasselhorn, 2006). These multicomponent tests include computation problems, word problems, and geometry problems. Spelling abilities were assessed by the Weingartener spelling tests for second and third graders (WRT 2+, Birkel, 1994a; WRT 3+, Birkel, 1994b) and the West-ermann spelling test for fourth graders (WRT 4/5, Rathenow, 1980). In both of these stan-dardized German achievement tests, children insert dictated words into given sentences. Reading speed abilities were classified on the basis of scores on two subtests of the Salzburg reading test (SLT; Landerl, Wimmer, & Moser, 1997): the word reading subtest “Textlesen” (short or long version, depending
on grade level) and the nonword reading subtest “Wortun¨ahnliche Pseudow¨orter.”
Based on the test battery described previ-ously, the operational criteria for the learn-ing disorder subgroups in this study were as follows: (a) IQ≥80; (b) below-average read-ing, spellread-ing, and/or arithmetic scores (T < 40 [i.e.,Tscores: mean of 50 andSDof 10] or percentile < 16); and (c) a critical dis-crepancy of at least 1.2SDs between IQ and overall performance on the standardized tests of school achievement (DYS groups: discrep-ancy between tests of reading and spelling and intelligence; CDSS groups: discrepancy between tests of reading, spelling,and arith-metic compared with intelligence). The cri-teria for the control group of typically de-veloping children were normal intelligence (IQ > 85) and performance at average in all standardized tests of school achievement (T≥40 in reading, spelling, arithmetic).
Table 1 summarizes the five groups’ de-scriptive statistics. On average, the CDSS and SLI + CDSS groups performed significantly lower on the mathematics abilities test than did the C, DYS, and SLI+DYS groups. At the same time, the DYS, SLI + DYS, CDSS, and SLI+CDSS groups scored significantly lower on spelling and reading tests than the C group did. Inspection of gender distribution patterns across learning disorder groups showed that more (60%) children with CDSS were female, whereas more children with DYS (60%), SLI+ DYS (81%), and SLI+CDSS (61%) were male. Analysis of variance revealed that the five ex-perimental groups did not differ significantly in terms of age,F(4, 107)=1.17,η2=.04, p=.329. The groups differed significantly in terms of intelligence,F(4, 107)=8.11,η2= .23, p =.001. We, therefore, included gen-eral intelligence as covariate in all subsequent analyses.
All of the children receiving SLE completed additional language measures. For receptive and expressive vocabulary, we carried out the computerized German vocabulary and word finding test for 6- to 10-year-old chil-dren (WWT 6–10; Gl¨uck, 2007). For gram-mar, we used the subtests of plural formation
Table 1. Means (SDs) for descriptive characteristics of subgroups: sex, age, IQ, DEMAT math-ematic T scores, WRT spellingTscores, and SLT word and nonword readingTscores
DYS (n=30) SLI+DYS (n=16) CDSS (n=19) SLI+CDSS (n=18) C (n=30) Sex (male/female) 18/12 13/3 8/11 11/7 15/15 Age (months) 108.57 111.56 104.00 108.89 108.77 IQ 99.67 (7.31) 93.87 (9.65) 94.06 (6.93) 89.83 (7.67) 100.53 (6.41) Mathematic 48.00 (8.19) 50.25 (8.60) 31.00 (5.77) 27.72 (5.74) 50.34 (5.51) Spelling 33.13 (4.83) 28.19 (3.83) 32.11 (6.30) 26.06 (3.62) 50.73 (6.42) Word reading 32.62 (7.02) 27.81 (7.96) 31.53 (8.32) 28.17 (8.03) 52.17 (8.42) Nonword reading 33.48 (8.06) 29.81 (10.37) 33.35 (11.42) 30.56 (11.34) 49.04 (7.51)
Note. C=normally performing control children matched for chronological age; CDSS=children with combined disorder of scholastic skills; DYS=children with dyslexia; SLI+CDSS=children with specific language impairment and combined disorder of scholastic skills; SLI+DYS=children with specific language impairment and dyslexia. (Plural-Singular bildung) and imitation of
grammatical structures (Imitation gramma-tischer Strukturformen); and for language comprehension, we used the subtest under-standing of grammatical structures (Verste-hen grammatischer Strukturformen). These measures are all part of the Heidelberger Sprachentwicklungstest (H-S-E-T; Grimm & Sch¨oler, 1991). Table 2 illustrates our find-ings that both the SLI+DYS and SLI+CDSS groups performed below average on language tasks (exception: receptive vocabulary of the SLI+DYS group).
All children in the SLE subgroups met the diagnostic criteria for SLI: (a) IQ ≥ 80 and (b) below-average language scores
(T <40) for vocabulary, grammar, and lan-guage comprehension.
Working memory assessment
Working memory was assessed by a bat-tery of 16 tasks: 7 phonological tasks (mem-ory spans for digits, one-syllable and three-syllable words, one-three-syllable and three-three-syllable nonwords, and images; nonword repetition), 5 visual–spatial tasks (memory span for loca-tions, matrix span, corsi-block), and 4 cen-tral executive tasks (double span, backward spans for one-syllable words and digits, count-ing span). A detailed description of all tasks follows below.
Table 2. Means (SDs) for language characteristics of SLI subgroups: expressive and recep-tive vocabularyTscores, imitation of grammatical structures and plural-formingTscores, and language comprehensionTscores
Vocabulary Grammar
Expressive Receptive
Imitation Of Grammatical
Structures Plural Forming
Language Comprehension SLI+DYS 30.36 (17.27) 46.33 (19.10) 18.13 (4.39) 31.40 (8.22) 35.13 (6.75) SLI+CDSS 21.69 (18.52) 35.29 (10.96) 17.00 (0.00) 29.00 (8.36) 31.06 (4.34)
Note. SLI +CDSS=children with specific language impairment and combined disorder of scholastic skills; SLI+ DYS=children with specific language impairment and dyslexia.
Phonological loop
The digit span task is one of the
con-ventional measures used to assess phono-logical short-term capacity. A series of one to nine digits was presented acoustically at a rate of one digit per second, start-ing with two and continustart-ing up to a max-imum of eight digits. Participants had to repeat the digits immediately in the pre-sented order. The one-syllable and
three-syllable word span tasks and the
one-syllable and three-syllable nonword span
tasks were presented in the same manner as the digit span measure. In the one-syllable and three-syllable word span tasks, familiar German nouns (e.g., Stern = star, Fisch = fish, Erdbeere = strawberry, Briefkasten = letterbox) were used; the one-syllable and three-syllable nonword span tasks are word-like nonwords (e.g., fen, sim, bestrugeln,
reseubelt).
In theimages spantask, participants were presented a series of pictures of easily rec-ognizable objects (e.g.,sun,umbrella,door, car) on a computer screen and were asked to recall them in the order of presentation. This is considered a phonological loop task in which the phonological information is pre-sented visually instead of acoustically because the pictures had to be named internally in or-der to recall them and report them by name.
The Germannonword repetitiontask was developed by Hasselhorn and K¨orner (1997). Children had to repeat 24 word-like nonwords of 2, 3, or 4 syllables immediately after their presentation. Nonwords of different lengths were presented acoustically in random order. The number of correctly repeated nonwords was taken as the score for this task. This task is not a span task, as there was an immediate repetition after each word.
Visual–spatial sketchpad
In the location span task, children were shown a series of green dots at different loca-tions on a 3× 3 matrix and asked to recall these locations in the correct order.
Corsi-block taskswere used to assess the dynamic
spatial component of visual–spatial memory. Nine red blocks were nailed in random po-sitions on a gray board (23×27.5 cm). The experimenter tapped a sequence of blocks at the rate of one per second. The child then attempted to reproduce the sequence of taps in the correct order. We used two variations of the Corsi-block task: simple sequences in-volving short distances between blocks with-out path crossings, and complex sequences involving long distances between blocks with path crossings.
Amatrix spantask was incorporated in the
battery as well to measure the static compo-nent of the visual–spatial sketchpad. This task assesses memory for random visual–spatial patterns of increasing complexity. Patterns of white and black boxes in a 4×4 matrix were presented on the computer, beginning with two black boxes and continuing up to a max-imum of eight black boxes. Immediately after presentation, children were asked to repro-duce the pattern in a blank matrix. Two vari-ations of this task were implemented as well: a simple matrix span with the black boxes arranged in simple patterns and a complex matrix span with the black boxes located at some distance from one another.
Central executive
Measures of the central executive are those that require remembering and processing at the same time. The same items were used for
thebackward digitand word span tasks as
for the forward spans, the only difference be-ing that participants were required to recall the sequences of items in reverse order. In ad-dition, adouble span task was implemented to assess the children’s ability to coordinate the functioning of the phonological loop and the visual–spatial sketchpad. The same pictures as in the images span task were pre-sented but this time in different locations on a 3 × 3 matrix. Children had to recall the pictures simultaneously by verbally recoding the semantic content (phonological demand) and their location (visual–spatial demand) in the order of presentation. Thus, this task is properly viewed as a central executive task
because of its coordinative requirements. It is not a dual task because there is only one reac-tion required: remember the correct pictures in the correct order and location.
The complex counting span task, a mea-sure of storage and processing efficiency, was based on a task designed by Case, Kurland, and Goldberg (1982). A series of yellow cir-cles (target items) and squares (distractor items) was presented in a random, computer-generated pattern. Children were instructed to count the number of circles. Subsequently, another map was presented and children again had to count the number of circles. Finally, the experimenter asked the child to recall the number of circles counted on each map. The number of maps presented per sequence was steadily increased up to a maximum of eight.
Stop criterion
We used the same stop criterion for all span tasks. The length of the sequences presented was increased gradually, beginning with a minimum of two, and increasing to a maxi-mum of eight items. There were four trials at each sequence length. If an error was made, the child was given a second attempt at an item of the same length. If a child succeeded on two successive trials of the same length, the task continued with the next span length. If a child failed on two successive trials of the same length, he or she was not presented with any further sequences of the same length, but with a sequence of one item shorter. The de-pendent measure for all span tasks was the longest sequence of items repeated in correct order. Children were credited an extra one-fourth point (0.25) if they repeated a further sequence of the same length correctly (e.g., a score of 5.25 was awarded if two of four 5-item sequences were recalled correctly, 5.5 if three of four sequences, and 5.75 if all four sequences were recalled correctly).
Procedure
We administered standardized tests for spelling, reading, arithmetic, intelligence, and working memory individually in two separate
sessions with children with SLI and children with learning disabilities. The DEMAT and WRT measures were carried out with control group children in classroom learning groups. All other tests were conducted individually within a period of 3 weeks. Except for the corsi-block task, all working memory tasks were administered by computer. The order of presentation of the working memory tasks was the same for all children (images span, lo-cation span, double span, one-syllable word span, three-syllable word span, corsi-block, nonword repetition, backward word span, backward digit span, counting span, digit span, matrix span, one-syllable nonword span, and three-syllable nonword span) and was carried out in one session.
RESULTS
The first question of our study targeted working memory functioning of children with DYS with and without SLI. To answer this question, we compared children in the DYS, SLI+DYS, and control groups for each work-ing memory subsystem separately.
Table 3 presents means and standard devia-tions for all working memory measures by the three groups. The scores of the seven tasks assessing the phonological loop functioning were entered into a multivariate analysis of variance (MANCOVA). The multivariate main effect proved to be significant,F(14, 136)= 7.53, η2 = .437, p < .001. The univariate
tests also showed significant differences be-tween groups for all phonological tasks (im-ages span,F(2, 73)=12.51,η2=.255,p<
.001; digit span,F(2, 73)=20.26,η2=.357, p<.001; one-syllable word span,F(2, 73)= 14.00,η2=.277,p<.001; three-syllable word span,F(2, 73)=12.85,η2=.260,p<.001; one-syllable nonword span,F(2, 73)=21.43,
η2=.370,p<.001; three-syllable nonword
span,F(2, 73)=12.53,η2=.255,p<.001; nonword repetition, F(2, 73)=82.44,η2=
.693,p<.001).
In the same way, the scores of the five tasks assessing thevisual–spatial sketchpadwere entered into a second MANCOVA. In this case,
Table 3. Means ( SD s) for w orking memory measures of subgroups a nd post hoc tests DYS SLI + DYS C DYS/C Post Hoc T est, p SLI + DYS/C Post H oc Test, p DYS/SLI + DYS Post H oc Test, p Phonological loop Images span 4.22 (0.77) 3.41(0.61) 4.60 (0.84) .151 < .001 .003 Digit span 4.63 (0.64) 3.88 (0.83) 5.19 (0.61) .005 < .001 .001 One-syllable w ord span 4.28 (0.74) 3.50 (0.60) 4.62 (0.66) .149 < .001 .001 Three-syllable w ord span 3.64 (0.47) 3.35 (0.45) 3.86 (0.43) .154 < .001 .002 One-syllable nonword span 3.47 (0.89) 2.92 (0.60) 4.27 (0.50) < .001 < .001 .036 Three-syllable nonword span 1.33 (1.22) 1.06 (1.10) 2.42 (0.74) < .001 < .001 .689 Nonword repetition 17.47 (3.13) 9.56 (3.86) 20.93 (1.78) < .001 < .001 < .001 Visual–spatial sketchpad Location span 5.27 (1.00) 5.50 (1.31) 4.98 (0.80) .480 .215 .749 Corsi-block simple 5.68 (1.34) 6.08 (1.05) 5.77 (1.36) .952 .731 .575 Corsi-block c omplex 4.80 (0.85) 5.17 (0.91) 4.98 (1.08) .742 .803 .429 Matrix span simple 6.72 (1.11) 6.61 (1.62) 6.53 (1.29) .836 .976 .962 Matrix span complex 4.45 (1.63) 4.23 (1.97) 4.72 (1.52) .809 .600 .895 Central e xecutive Backward digit span 3.45 (0.66) 3.31 (0.64) 3.90 (0.68) .027 .015 .781 Backward word span 3.58 (0.56) 3.23 (0.54) 3.82 (0.63) .250 .005 .150 Double span 3.78 (0.76) 3.33 (0.55) 4.04 (0.72) .334 .005 .099 Counting span 3.81 (0.89) 3.08 (0.46) 4.45 (0.97) .013 < .001 .020 Note .C = normally performing c ontrol children matched for chronological a ge; D YS = children with dyslexia; SLI + DYS = children with specific language impairment and d yslexia.
the multivariate group effect was not signifi-cant, F(10, 140)< 1,η2 =.060 (univariate
tests: location span, F (2, 73) =1.56, η2 =
.041; corsi-block simple task, F(2, 73) <1,
η2=.014; corsi-block complex,F(2, 73)<1,
η2=.022; matrix span simple,F(2, 73)<1,
η2=.004; matrix span complex,F(2, 73)<1,
η2=.013).
Third, the scores of the four tasks assess-ing the central executive were entered into a MANCOVA: here, the multivariate group effect, F (8, 142) = 3.69, η2 = .172, p = .001, proved to be significant. Univariate tests showed significant differences between groups on all central executive memory tasks (digit backward span,F(2, 73)=5.37,η2=
.128, p =.007; word backward span, F (2, 73)=5.24,η2=.126,p=.007; double span, F(2, 73)=5.38,η2=.128,p=.007; counting
span,F(2, 73)=13.71,η2=.273,p<.001).
Post hoc tests (Tukey, Table 3) for further analysis of group differences revealed that the DYS and control groups differed in phonologi-cal loop and central executive functioning for most tasks (forward and backward digit span, one- and three-syllable nonword span, non-word repetition, counting span). The SLI + DYS group exhibited deficits in all phono-logical and central executive tasks compared with group C. Here, we presume a broad deficit concerning all tasks. These deficits be-came even more evident in tasks assessing phonological loop and in aspects of central executive compared with children with iso-lated DYS (images span, digit span, one- and three-syllable word span, one-syllable non-word span, nonnon-word repetition, counting span). Just as we expected, no differences ap-peared between the groups on tasks assessing the visual–spatial sketchpad.
In a second step, we compared working memory performance of children who met criteria for learning disorder in reading, writ-ing, and arithmetic only (CDSS) with those who had an additional diagnosis of SLI (SLI+ CDSS) with controls (C). Table 4 presents mean scores on all working memory tasks by subgroup.
The scores of the tasks assessing the
phono-logical loop were entered into a MANCOVA.
The multivariate main effect proved to be significant, F (14, 116) = 11.84, η2 =.588, p < .001. The univariate tests also showed significant differences between groups for all phonological tasks (images span,F(2, 63)= 20.08,η2 =.389,p<.001; digit span,F(2,
63)=41.85,η2=.571,p<.001; one-syllable word span, F (2, 63) = 30.69, η2 = .493, p<.001; three-syllable word span,F(2, 63)
= 28.26, η2 = .473, p < .001; one-syllable nonword span,F(2, 63)=26.63,η2=.458, p <.001; three-syllable nonword span,F(2, 63) =27.70,η2=.468,p <.001; nonword
repetition,F(2, 63)=90.73,η2=.472,p<
.001).
Second, the scores of the five tasks assess-ing the visual–spatial sketchpad were en-tered into a second MANCOVA: the multivari-ate group effect, F (10, 122) = 2.65, η2 =
.179,p=.006, and all univariate tests (loca-tion span, F(2, 64) =3.06,η2 = .087,p= .047; corsi-block simple task,F(2, 64)=3.27,
η2 = .093, p = .045; corsi-block complex, F(2, 64)=7.93,η2=.199,p=.001; matrix span simple,F(2, 64)=6.52,η2=.169,p= .003; matrix span complex,F(2, 64)=6.71,
η2=.137,p=.002) proved to be significant.
In the same way, the scores of the four tasks assessing the central executive were entered into a MANCOVA. Here, the multi-variate group effect proved to be significant, F(8, 124) =6.10,η2 =.282,p<.001.
Uni-variate tests showed significant differences between groups on all central executive mem-ory tasks (digit backward span, F (2, 64) = 8.48, η2 = .209, p = .001; word backward span,F(2, 64)=14.13,η2=.306,p<.001; double span, F (2, 64) = 13.90,η2 =.303, p <.001; counting span,F(2, 64) =28.94,
η2=.475,p<.001).
Post hoc tests (Tukey), illustrated in Table 4, demonstrate that both clinical groups (CDSS and SLI + CDSS) displayed phono-logical working memory deficits compared with controls (all phonological tasks). Deficits were more pronounced for children with
Table 4. Means ( SD s) for w orking memory measures of subgroups a nd post hoc tests CDSS SLI + CDSS C CDSS/C Post H oc Test p SLI + CDSS/C Post H oc Test, p SLI + CDSS/CDSS Post H oc Test, p Phonological loop Images span 3.68 (0.56) 3.34 (0.54) 4.60 (0.84) < .001 < .001 .340 Digit span 4.21 (0.50) 3.65 (0.60) 5.19 (0.61) < .001 < .001 .034 One-syllable w ord span 3.43 (0.60) 3.22 (0.44) 4.62 (0.66) < .001 < .001 .009 Three-syllable w ord span 3.24 (0.47) 2.92 (0.38) 3.86 (0.43) < .001 < .001 .074 One-syllable nonword span 3.21 (0.98) 2.88 (0.55) 4.27 (0.50) < .001 < .001 .329 Three-syllable nonword span 1.22 (0.83) 0.36 (0.83) 2.42 (0.74) < .001 < .001 .024 Nonword repetition 18.16 (4.57) 7.35 (3.93) 20.93 (1.78) .018 < .001 < .001 Visual–spatial sketchpad Location span 4.36 (0.78) 4.90 (1.18) 4.98 (0.80) .048 .960 .155 Corsi-block simple 4.95 (1.13) 5.81 (1.01) 5.77 (1.36) .049 .996 .048 Corsi-block c omplex 3.80 (1.23) 4.97 (0.39) 4.98 (1.08) .001 .999 .005 Matrix span simple 5.26 (1.16) 6.08 (1.04) 6.53 (1.29) .002 .433 .100 Matrix span complex 3.22 (1.03) 4.01 (1.73) 4.77 (1.52) .003 .083 .611 Central e xecutive Backward digit span 3.26 (0.47) 3.07 (1.01) 3.90 (0.68) .012 .001 .705 Backward word span 3.20 (0.32) 3.10 (0.47) 3.82 (0.63) < .001 < .001 .825 Double span 3.34 (0.55) 3.14 (0.54) 4.04 (0.72) .001 < .001 .589 Counting span 3.22 (0.73) 2.82 (0.34) 4.45 (0.97) < .001 < .001 .262 Note .C = normally performing c ontrol children matched for chronological a ge; C DSS = children with combined disorder of scholastic skills; S LI + CDSS = children with specific language impairment a nd combined disorder of scholastic skills.
SLI + CDSS in comparison with children with CDSS only (digit span, one-syllable word span, three-syllable nonword span, nonword repetition).
In contrast, only children with CDSS (with-out SLI) showed deficits associated with visual–spatial sketchpad compared with con-trols (all visual–spatial tasks), whereas chil-dren with SLI +CDSS did not. With regard to central executive functioning, both disabil-ity groups differed from the control group but not from each other (CDSS and SLI+CDSS: deficits in all central executive tasks). DISCUSSION
Researchers currently regard deficits in working memory functioning as one major characteristic for language impairment and learning disorders (Archibald & Gathercole 2006a, 2006b; Schuchardt et al., 2008). Our study investigated the question of whether children with learning disorders and language impairment present with the same working memory deficits. Comparing working mem-ory profiles of children with learning disor-ders (DYS or CDSS) only with profiles for children with additional language impairment (SLI) was intended to help us to discover whether patterns of working memory deficits are disorder-specific. In summary, our results suggested that learning disorders (in reading, spelling, and calculating) and learning disor-ders combined with SLI share some deficits in underlying working memory and are also associated with distinct patterns of working memory difficulties.
Deficits with regard to the phonological loop and central executive were found for learning disorders both with and without SLI; however, the phonological impairment was more severe and broader in children who met criteria for SLI. In fact, all clinical groups displayed significant deficits in tasks involv-ing the phonological loop. Children with combined language and learning impairment (SLI+DYS and SLI+CDSS) performed even worse, however, on the majority of phonolog-ical tasks than children with isolated DYS or
CDSS. Incidentally, we found that the three-syllable nonword span did not differentiate the groups because of significant floor effects. On the contrary, we detected the highest difference in performance on nonword rep-etition tasks for measuring the phonological store.
The finding that children with additional known SLI scored even lower on phonolog-ical loop tasks than children with DYS and CDSS who had not been referred for SLE is partly in line with results of other studies (Archibald & Gathercole, 2006c; Archibald & Gathercole, 2007; Baird et al., 2011; de Bree et al., 2010). Nonword repetition is viewed as a rather pure indicator for the phonolog-ical store, because the task does not imply retrieval from long-term memory. Thus, chil-dren with the additional diagnosis of SLI seem to be more likely to exhibit a particularly pro-nounced storage impairment.
Extending these findings, Hasselhorn and Werner (2000) conducted a study in which they varied acoustic presentation as well as syllable length in nonwords. They highlighted half of the words with white noise to lead to acoustic distortion. In their extended model of the phonological loop, Hasselhorn, Grube and M¨ahler (2000) regarded the acoustic dis-tortion effect as a marker for quality of the phonological store. Children with SLI and language-matched controls performed signifi-cantly worse with increasing syllable length on acoustically distorted tasks. The perfor-mance gap between the groups was reduced in a condition in which children had to repeat distorted three- or four-syllable non-words. Hasselhorn and Werner (2000) con-cluded that children with SLI are disturbed less by white noise (with increasing sylla-ble length) than language-matched controls, because children with SLI benefit less from accurate acoustic presentation. The authors concluded that working memory impairment in children with SLI, therefore, is due to re-duced processing quality in the phonologi-cal store, especially when the soundscapes exceed a certain number of information units.
In addition to the phonological working memory deficits, all four clinical groups dis-played deficits incentral executive function-ing. Therefore, deficits in central executive functioning appear to be characteristic for children with DYS and CDSS (Schuchardt et al., 2008) as well as for children with SLI (Archibald & Gathercole, 2006b; Marton & Schwartz, 2003; Montgomery & Evans, 2009). Nevertheless, it should be considered that most of the central executive tasks used also involve the participation of the phonological loop to some extent because phonological in-formation has to be processed. Therefore, we cannot rule out the possibility that the phono-logical deficit is the predominant impairment in children with learning disorders with or without additional SLI.
When we look at thevisual–spatial sketch-pad, we find different patterns of results. In line with earlier studies, we could observe that children with CDSS exhibit deficits in this domain (Maehler & Schuchardt, 2009; Schuchardt et al., 2008). Interestingly, we could not provide evidence for disadvan-tages in visual-spatial working memory for the children with CDSS + SLI (see Nithart et al., 2009). As only children with combined learning disorder (CDSS) showed deficits in the visual–spatial working memory, it may be that visual–spatial malfunctions influence arithmetic problems of these children. In con-trast, we assume that children with addi-tional language problems (CDSS+SLI) could exhibit arithmetic problems because of lan-guage problems, as their visual–spatial work-ing memory turned out to be unimpaired.
Findings of Donlan et al. (2007) and Fazio (1999) point to the same line of argument. Thus, we would expect mathematical prob-lems in children with SLI especially on tasks that severely depend on language comprehen-sion but not on tasks that can be solved by intact visual–spatial perception and memory. In summary, we would conclude that ana-lyzing working memory functioning in-depth could help us to differentiate between vari-ous disorders according to underlying cogni-tive deficits. Certainly, the assessed patterns of deficits in different clinical groups do not allow for distinct diagnoses of specific dis-orders. Furthermore, other disorders besides the ones included in this study are associ-ated with working memory problems. But at least we could become more precise in rec-ommending intervention measures that rely on different strengths and difficulties of the children.
According to the results of this study, phonological and central executive problems must be expected in children with learn-ing disorders with or without SLI. Therefore, teaching should take these deficits into ac-count and try to reduce the task demands with regard to the amount of information to be processed or integrated. And children with SLI should be supported to profit from their apparently intact visual–spatial memory, for example, by providing training on relevant memory strategies or visual supports to lan-guage intervention activities. Future research is needed to evaluate working memory train-ing programs in order to explore the possibil-ity to overcome working memory deficits.
REFERENCES
Archibald, L. M., & Gathercole, S. E. (2006a). Visuospa-tial immediate memory in language impairment. Jour-nal of Speech, Language, and Hearing Research,49, 265–277.
Archibald, L. M., & Gathercole, S. E. (2006b). Short-term and working memory in specific language impairment. International Journal of Lan-guage & Communication Disorders, 41, 675– 693.
Archibald, L. M., & Gathercole, S. E. (2006c). Nonword repetition in specific language impairment. Psycho-nomic Bulletin & Review,14, 919–924.
Archibald, L. M., & Gathercole, S. E. (2007). Nonword repetition in specific language impairment: More than a phonological short-term memory deficit. Psycho-nomic Bulletin & Review,14, 919–924.
Baddeley, A. D. (1986).Working memory. Oxford: Uni-versity Press.
Baddeley, A. D. (1996). Exploring the central executive. Quarterly Journal of Experimental Psychology,49, 5–28.
Baddeley, A. D. (2000). The episodic buffer: A new com-ponent of working memory?Trends in Cognitive Sci-ences,4, 417–423.
Baird, G., Slonims, V., Simonoff, E., & Dworzynski, K. (2011). Impairment in non-word repetition: A marker for language impairment or reading impairment? De-velopmental Medicine and Child Neurology, 53, 711–716.
Birkel, P. (1994a). Weingartener Grundwortschatz Rechtschreib-Test f¨ur zweite und dritte Klassen (WRT 2+) [Weingarten basic vocabulary spelling test for second and third grades (WRT 2+)]. G¨ottingen, Germany: Hogrefe.
Birkel, P. (1994b). Weingartener Grundwortschatz Rechtschreib-Test f¨ur dritte und vierte Klassen (WRT 3+) [Weingarten basic vocabulary spelling test for third and fourth grades (WRT 3+)]. G¨ottingen, Germany: Hogrefe.
Case, R. D., Kurland, M., & Goldberg, J. (1982). Opera-tional efficiency and the growth of short-term memory span.Journal of Experimental Child Psychology,33, 386–404.
Cattell, R. B., Weiss, R., & Osterland, J. (1980). Grundin-telligenztest CFT1. G¨ottingen, Germany: Hogrefe. Catts, H., Adolf, S., Hogan, T., & Ellis Weismer, S. (2005).
Are specific language impairments and dyslexia dis-tinct disorders?Journal of Speech, Language, and Hearing Research,48, 1378–1396.
Cowan, R., Donlan, C., Newton, E. J., & Lloyd, D. (2005). Number skills and knowledge in children with spe-cific language impairment. Journal of Educational Psychology,97, 732–744.
de Bree, E., Wijnen, F., & Gerrits, E. (2010). Non-word rep-etition and literacy in Dutch children at-risk of dyslexia and children with SLI: Results of the follow-up study. Dyslexia,16, 36–44.
Donlan, C., Cowan, R., Newton, E. J., & Lloyd, D. (2007). The role of language in mathematical development: Evidence from children with specific language impair-ment.Cognition,103, 23–33.
Eisenmajer, N., Ross, N., & Pratt, C. (2005). Specificity and characteristics of learning disabilities.Journal of Child Psychology and Psychiatry and Allied Health Professions,46, 1108–1115.
Fazio, B. B. (1994). The counting abilities of children with specific language impairment: A comparison of oral and gestural tasks.Journal of Speech, Language, and Hearing Research,37, 358–368.
Fazio, B. B. (1996). Mathematical abilities of children with specific language impairment: A 2-year follow-up.Journal of Speech, Language, and Hearing Re-search,39, 839–849.
Fazio, B. B. (1999). Arithmetic calculation, short-term memory, and language performance in children with specific language impairment: A 5-year follow-up.
Journal of Speech, Language, and Hearing Research, 42, 420–431.
Gl¨uck, C. W. (2007).Wortschatz- und Wortfindungstest f¨ur 6- bis 10-J¨ahrige (WWT 6–10). M¨unchen, Ger-many: Elsevier, Urban & Fischer.
G¨olitz, D., Roick, T., & Hasselhorn, M. (2006).Deutscher Mathematiktest f¨ur vierte Klassen (DEMAT 4) [German mathematics test for fourth grades (DE-MAT 4). G¨ottingen, Germany: Hogrefe.
Grimm, H., & Sch¨oler, H. (1991). Heidelberger Sprachentwicklungstest (H-S-E-T). 2. verbesserte Aufl. G¨ottingen, Germany: Hogrefe.
Grob, A., Meyer, C. S., & Hagemann-von Arx, P. (2009). Intelligence and Development Scale (IDS). Intelligenz- und Entwicklungsskalen f¨ur Kinder von 5–10 Jahren. Bern, Switzerland: Huber.
Hasselhorn, M., Grube, D., & M¨ahler, C. (2000). Theoretisches Rahmenmodell f¨ur ein Diagnostikum zur differentiellen Funktionsanalyse des phonologis-chen Arbeitsged¨achtnisses. In M. Hasselhorn, W. Schneider, & H. Marx (Hrsg.), Diagnostik von Lese-Rechtschreibschwierigkeiten(Reihe “Tests und Trends”, Neue Folge, Band 1, S. 167–181). G¨ottingen, Germany: Hogrefe Publishing.
Hasselhorn, M., & K¨orner, K. (1997). Nachsprechen von Kunstw¨ortern: Zum Zusammenhang zwischen Arbeitsged¨achtnis und syntaktischen Sprachleistun-gen bei Sechs- und Achtj¨ahrigen [Repeating non-words: The relationship between working memory and syntactic competence in six- and eight-year-olds]. Zeitschrift f¨ur Entwicklungspsychologie und P¨adagogische Psychologie,29, 212–224.
Hasselhorn, M., & Werner, I. (2000). Zur Bedeu-tung des phonologischen Arbeitsged¨achtnisses f¨ur die Sprachentwicklung. In H. Grimm (Ed.), Enzyk-lop¨adie der Psychologie (Part C, Series III, Vol. 3: Sprachentwicklung, pp. 363–378). G¨ottingen, Germany: Hogrefe.
Hick, R. F., Botting, N., & Conti-Ramsden, G. (2005). Short-term memory and vocabulary development in children with Down syndrome and children with spe-cific language impairment.Developmental Medicine & Child Neurology,47, 532–538.
Hoffman, I. M., & Gillam, R. B. (2004). Verbal and spa-tial information processing in children with specific language impairment.Journal of Speech, Language, and Hearing Research,47, 114–125.
Krajewski, K., Liehm, S., & Schneider, W. (2004). Deutscher Mathematiktest f¨ur zweite Klassen (DE-MAT 2+) [German mathematics test for second grades (DEMAT 2+)]. G¨ottingen, Germany: Hogrefe. Landerl, K., Bevan, A., & Butterworth, B. (2004). De-velopmental dyscalculia and basic numerical capaci-ties: A study of 8–9-year-old students.Cognition,93, 99–125.
Landerl, K., Wimmer, H., & Moser, E. (1997).Salzburger Lese- und Rechtschreibtest (SLT) [Salzburg reading and spelling test (SLT)]. Bern, Switzerland: Huber.
Logie, R. H. (1995).Visual-spatial working memory. Hove, United Kingdom: Erlbaum.
Maehler, C., & Schuchardt, K. (2009) Working memory functioning in children with learning disabilities: Does intelligence make a difference?Journal of Intellectual Disabilities Research,53, 3–10.
Marton, K., & Schwartz, R. G. (2003). Working mem-ory capacity and language processes in children with specific language impairment.Journal of Speech, Lan-guage, and Hearing Research,46, 1138–1153. McArthur, G. M., Hogben, J. H., Edward, V. T., Heath,
S. M., & Mengler, E. D. (2000). On the “specifics” of specific reading disability and specific language im-pairment.Journal of Child Psychological Psychiatry and Allied Health Professions,41, 869–874. McLean, J. F., & Hitch, G. J. (1999). Working memory
im-pairments in children with specific arithmetic learning difficulties.Journal of Experimental Child Psychol-ogy,74, 240–260.
Melchers, P., & Preuss, U. (2001).Assessment battery for children (K-ABC), German version (5th ed.). G¨ottingen, Germany: Hogrefe.
Montgomery, J. W., & Evans, J. L. (2009). Complex sen-tence comprehension and working memory in chil-dren with specific language impairment.Journal of Speech, Language, and Hearing Research,52, 269– 288.
Nithart, C., Demont, E., Majerus, S., Leybaert, J., Poncelet, M., & Metz-Lutz, M. N. (2009). Reading disabilities in SLI and dyslexia result from distinct phonological impairments.Developmental Neuropsychology,34, 296–311.
O’Shaughnessy, T., & Swanson, H. L. (1998). Do im-mediate memory deficits in students with learn-ing disabilities in readlearn-ing reflect a developmental lag or deficit? A selective meta-analysis of the lit-erature. Learning Disability Quarterly, 21, 123– 148.
Pickering, S. J., Gathercole, S. E., Hall, M., & Lloyd, S. A. (2001). Development of memory for pattern and path:
Further evidence for the fractionation of visuo-spatial memory.The Quarterly Journal of Experimental Psy-chology,54, 397–420.
Rathenow, P. (1980).Westermann rechtschreibtest 4/5 (WRT 4/5). Braunschweig, Germany: Westermann. Riccio, C. A., Cash, D. L., & Cohen, M. J. (2007).
Learn-ing and memory performance of children with spe-cific language impairment (SLI).Applied Neuropsy-chology,14, 255–261.
Roick, T., G¨olitz, D., & Hasselhorn, M. (2004).Deutscher Mathematiktest f¨ur dritte Klassen (DEMAT 3+) [German mathematics test for third grades (DEMAT 3+)]. G¨ottingen, Germany: Hogrefe.
Schuchardt, K., M¨ahler, C., & Hasselhorn, M. (2008). Working memory deficits in children with different learning disabilities.Journal of Learning Disabilities, 41, 514–523.
Siegel, L. S., & Ryan, E. B. (1989). The development of working memory in normally achieving and subtypes of learning disabled children.Child Development,60, 973–980.
Swanson, H. L., & Sachse-Lee, C. (2001). A subgroup analysis of working memory in children with reading disabilities: Domain-general or domain-specific defi-ciency?Journal of Learning Disabilities,34, 249– 263.
Tellegen, P. J., Winkel, M., & Laros, J. A. (2003). Non-verbal intelligence-test (SON-R 51/2-17), German ver-sion. G¨ottingen, Germany: Hogrefe.
Tomblin, J. B., Zhang, X., Buckwalter, P., & Catts, H. (2000). The association of reading disability, behav-ioral disorders, and specific language impairment in second grade children.Journal of Child Psychology and Psychiatry and Allied Health Professions,41, 473–482.
Vellutino, F. R., Fletcher, J. M., Snowling, M. J., & Scanlon, D. M. (2004). Specific reading disability (dyslexia): What have we learned in the past four decades. Jour-nal of Child Psychology and Psychiatry and Allied Health Professions,45, 2–40.
