Working memory structure in 10- and 15-year old children with mild toborderline intellectual, disabilities

Mariet J. van der Molen *

University of Amsterdam, Department of Psychology, Roetersstraat 15, A7.26, 1018 WB Amsterdam, The Netherlands

1. Introduction

Working memory and short-term memory deficits are known to exist in children with mild to borderline intellectualdisabilities (ID; IQ score 55–85; Henry, 2001; Maehler & Schuchardt, 2009; Pickering & Gathercole, 2004; Van der Molen, VanLuit, Jongmans, & Van der Molen, 2007, 2009). Working memory (WM), the ability to maintain and process informationsimultaneously during the performance of a cognitive task, is considered a central construct in cognitive psychology (e.g.,Cowan, 1999; Engle, Kane, & Tuholski, 1999) and plays an important role in scholastic activities like language comprehension(e.g., Daneman & Merikle, 1996) and arithmetic (e.g., Bull & Scerif, 2001). Most WM theories indicate that WM can bedifferentiated from short-term memory in the typically developing population (e.g. Engle, Tuholski, Laughlin, & Conway,1999). To be able to adequately interpret the findings that WM and short-term memory (STM) are weak in children with mildto borderline ID, it is important to test the validity of the two constructs in this population.

This study was guided by Baddeley’s WM model (1986, adapted and extended in 2000, originally Baddeley & Hitch,1974) as it is probably the most influential WM model (Engle, Kane, & Tuholski, 1999; Garon, Bryson, & Smith, 2008).Furthermore, it is a model which is frequently used for studying STM and WM in children. In this model, WM isconceptualized by four components. The visuo-spatial sketchpad is responsible for the temporarily storage of static, visualinformation and dynamic, spatial information, whereas the phonological loop is responsible for the temporary storage of

Research in Developmental Disabilities 31 (2010) 1258–1263

A R T I C L E I N F O

Article history:

Received 13 July 2010

Accepted 20 July 2010

Keywords:

Working memory

Short-term memory

Mild intellectual disability

Memory structure

A B S T R A C T

The validity of Baddeley’s working memory model within the typically developing

population, was tested. However, it is not clear if this model also holds in children and

adolescents with mild to, borderline intellectual disabilities (ID; IQ score 55–85). The main

purpose of this study was therefore, to explore the model’s validity in this population.

Several verbal and visuo-spatial STM and WM tasks, were administered to 115 children

with mild to borderline ID (mean age 10 years) and to 98, adolescents with mild to

borderline ID (mean age 15). Structural equation modeling (LISREL) shows, that Baddeley’s

working memory model does not fit the data of the 10-year and 15-year old, participants.

Principal components analyses on the other hand show a hazy pattern with on the one,

side an indication for a ‘general’ component with loadings of visuo-spatial short-term

memory and, working memory tasks and a separate verbal short-term memory

component. On the other hand there, is also an indication of a modality specific memory

structure; a visuo-spatial- versus a verbal, component. A straight-forward dichotomy

between STM and WM indicates apparently an, oversimplification, at least it is for children

and adolescents with mild to borderline ID.

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* Tel.: +31 20 5256857; fax: +31 20 6390279.

E-mail address: m.j.vandermolen@uva.nl.

Contents lists available at ScienceDirect

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doi:10.1016/j.ridd.2010.07.019

verbal information. Both short-term memory stores are co-ordinated by the central executive, an attentional controlsystem. Assessing the central executive is commonly done by using complex memory tasks requiring simultaneous storageand processing of information (Alloway, Gathercole, Willis, & Adams, 2004). These concepts of differentiated STM and WMsystems are also seen in other WM models (e.g., Cowan, 1999; Engle, Kane, & Tuholski, 1999). Furthermore, in 2000,Baddeley added the fourth component, the episodic buffer, which stores information in a multi-dimensional code andserves as a temporary interface between the two short-term memory stores and long-term memory and is controlled by thecentral executive as well. As the episodic buffer is theoretically still very much in development and as yet hardly measured,it is not considered in the current study.

Researchers have shown justification for Baddeley’s WM model for use in typically developing young adults (seeOberauer, 2005, for an overview). Furthermore, there is some indication that the model holds for typically developingchildren from 4 years old on (Alloway, Gathercole, & Pickering, 2006), although the results are not straightforward.Numminen et al. (2000; see also Numminen, Service, & Ruoppila, 2002) did not find support for Baddeley’s model in adultswith ID (mean age 49 years; mean IQ score 63). In their study, a battery of six WM and STM tasks were administered. In factoranalysis, the two verbal STM tasks loaded on one factor, hence representing verbal STM. The other four assessed visuo-spatialSTM and WM tasks loaded on a second factor, which the authors named the ‘general component’ (Numminen et al., 2000).The results are explained in terms of the visuo-spatial tasks being depended on WM processes in these adults. This is in linewith studies by Gathercole and colleagues where a similar visuo-spatial STM–WM factor was found with explorative factoranalyses in 4- and 5-year-old typically developing children (Alloway et al., 2006) and in 6- and 7-year-old typicallydeveloping children (Gathercole & Pickering, 2000). It is argued that at a young (mental) age, the central executive is requiredin visuo-spatial tasks more than at older age. Furthermore, in general, visual STM tasks are said to place significant demandson the central executive (Gathercole, Pickering, Ambridge, & Wearing, 2004). However, the findings in the Numminen et al.study show that verbal STM can be differentiated from WM in adults with ID.

In conclusion, some restraint in accepting the validity of STM stores versus WM in children is justified. It is suggested thatSTM and WM are different but related constructs and its relationship depends on a person’s age, intelligence anddevelopmental levels (Engle, Tuholski, Laughlin, & Conway, 1999). Possibly WM and STM are rather equivalent in childrenbecause their STM performances depend more on the central executive than in adults (Hutton & Towse, 2001). As childrenwith mild to borderline ID have a young age and because their intelligence is lower than average, it might well be that STMand WM cannot be distinguished within this group. This raises the question to which extent Baddeley’s account of WMprovides an adequate framework to understand STM and WM performances and weaknesses in children with mild toborderline ID. The main purpose of this study is therefore to explore the validity of Baddeley’s WM model in this population.To see if the relationship between STM and WM differs between ages, two age groups are included in the current study;children with a mean age of 10 years and adolescents with a mean age of 15 years.

2. Method

2.1. Participants

A total of 213 young people with MID attending special schools for mild intellectually disabled pupils were available for thisstudy. A criterion for entrance in this type of school is an IQ score in the range 55–85. One group consisted of 115 children (61boys) ranging in age from 9 to 12 years. The other group consisted of 98 adolescents (55 boys) ranging in age from 13 to 16 years.Participants diagnosed by psychiatrists as having attention deficit/hyperactive disorder, pervasive developmental disorder-nototherwise specified, or other specific etiologies were excluded because these psychiatric problems are associated with specificworking memory strengths and weaknesses (Gathercole & Alloway, 2006), which might influence the results.

Informed consent was obtained for every participant. All children and adolescents had normal or corrected vision andwere reported to be healthy; none of them were taking psychotropic medication. Ethnicity and social economical status werecomparable across the two groups. All participants were born in The Netherlands.

2.2. Measures

To assess intelligence, we administered a non-verbal IQ test, the Standard Progressive Matrices (SPM; Raven, Court, &Raven, 1992). Behavioral problems were assessed with the Teacher Report Form (TRF; Achenbach, 1991). See Table 1 for theparticipants’ details. Analysis revealed that gender did affect one of the seven measures of the test battery, that is the VisualPatterns test (F(1, 197) = 4.72, p< .05). Boys scored higher than girls on this task.

2.2.1. Short-term memory tasks

Two verbal and two visual STM tests were used. Digit Recall and Nonword Recall (Pickering & Gathercole, 2001) bothmeasure verbal STM. Both tests require repeating digits or nonwords in the same order as presented. Digit Recall starts withtwo digits up to eight, while Nonword Recall starts with one nonword up to six. For these, and all of the following spanmeasures (except the Visual Patterns test), there are six trials per list length. List lengths increase incrementally, provided atleast four of the six trials are completely correct. The omitted trials are awarded one point each. Memory scores represent thenumber of trials that were completely correct. Scores vary from 0 to 42 (digits) or from 0 to 36 (nonwords).

M.J. van der Molen / Research in Developmental Disabilities 31 (2010) 1258–1263 1259

Visual STM was assessed by using Block Recall and the Visual Patterns test. Block Recall is identical to the Corsi test (seeLezak, 1995), but in this study we used the instructions from Pickering and Gathercole (2001). The experimenter taps asequence of three-dimensional blocks that the child has to repeat in the same order. The task starts with one block up tosequences of nine blocks. Scores vary from 0 to 54. In the Visual Patterns test (Della Sala, Gray, Baddeley, & Wilson, 1997) thechild is shown a matrix depicted on a stimulus card, varying from 2� 2 to 5� 6 squares with half of the squares beingmarked. After inspecting a stimulus card for three seconds, the child has to indicate the marked squares using a blank grid onthe response sheet. Three stimulus cards are available for each of the 14 difficulty levels. List length increases incrementally,provided at least two of the three trials are completely correct. Scores vary from 0 to 42.

2.2.2. Working memory tasks

Two verbal and one visual WM tests were used. The two verbal WM tests were Backward Digit Recall and Listening Recall.Backward Digit Recall (Pickering & Gathercole, 2001) requires repeating spoken lists of digits, but in the reverse order. Thereis some controversy about what type of test this is; it seems to measure STM in adults but WM in children (St Clair-Thompson, 2010). Listening Recall (Pickering & Gathercole, 2001) requires listening to simple statements to determinewhether they are true or false, while at the same time remembering the last word of each statement. Following each trial,these last words are to be repeated in the same order as presented. Trials in Backward Digit Recall start with two digits up toseven, while Listening Recall starts with one sentence, up to a maximum of six. Scores vary from 0 to 36 for each of thesetests.

Visual WM was examined using a manual version of the Spatial Span (Alloway, 2007). A card is shown with two shapes ofwhich the right one has a red dot on top. The right shape can be exactly the same (p� p) or opposite (p� q) to the left shapeand it can be rotated in three different ways (08, 1208 and 2408). The child has to decide whether the shape at the right is thesame to the left shape or opposite. At the same time, the position of the red dot on the right shape has to be remembered,which can be at three different locations according to the three rotation possibilities. After each trial, the child has to point toone of three dots (at 08, 1208 or 2408) to indicate which dots were on the stimuli cards and in which sequence. The trials startwith one card up to a sequence of six. Scores can vary from 0 to 36.

2.3. Data analyses

First the data were screened for outliers. Therefore, all scores were converted to Z scores. Of the 1603 subtest scores, 22had a Z score higher than 2.58 above or below the mean. All those scores were from different subjects except two differentscores which were from the same subject. These data were normalized by changing them with values corresponding to 2.59above or below the mean as appropriate (Field, 2005).

Correlational analyses were carried out to see which STM and WM scores relate most with each other. To evaluatewhether the factor structure of the data was consistent with Baddeley’s model of WM, a confirmatory factor analysis wasperformed using structural equation modelling (LISREL). This method provides means of testing the relationships betweenmeasures, with each model specified in terms of paths between observed variables and latent constructs and between theseconstructs (Alloway et al., 2004). Goodness of fit is tested by means of the x2 statistic, where non-significant x2 valuesindicate a good fit. An additional indication of the extend to which the specified model describes the data adequately is givenby the root mean square error of approximation (RMSEA). A small and significant RMSEA indicates good fit. To furtherexplore the factor structure of the data, principal component analysis with Varimax rotation and Kaiser normalization wasperformed.

3. Results

3.1. Correlational analyses

Correlations among all STM and WM variables were conducted for the 10-year old and 15-year old separately and aredisplayed in Table 2. The two verbal STM tasks shared moderately high correlations for both the 10- and 15-year old children.Both visuo-spatial STM tasks correlated moderate in both age groups. Correlations between verbal STM and visuo-spatialSTM measures were low, although in the 10-year old age group, Digit Recall correlated moderate with Block recall. For the10-year old children, verbal STM and visuo-spatial STM measures correlated moderate with all three WM tasks. For the 15-year old group the two verbal STM measures correlated moderate with both verbal WM tasks, but not with the visual WM

Table 1

Participants characteristics by group.

Children Adolescents

M SD M SD

Age 10.42 .63 15.25 .85

Fluid intelligence 25.76 8.48 33.91 6.64

TRF total score 57.62 7.52 57.02 8.44

M.J. van der Molen / Research in Developmental Disabilities 31 (2010) 1258–12631260

task while visuo-spatial STM tasks correlate moderately low with all WM tasks. Finally, the three verbal and visual WMmeasures correlated moderately high for the 10-year old children and rather low for the 15-year old.

3.2. Structural equation modeling

Model 1 tested Baddely’s account of WM and consists of three separable factors which represent the phonological loop(PhL), the central executive (CE) and the visuo-spatial sketchpad (VSS). Model 1 does not adequately fit the data,x2(11) = 65.72, P =<.001, RMSEA = .148.

To make sure the lack of fit was not caused by differences in performance of the 10- and 15-year old children the modelwas fitted to both groups separately as well. The model does not adequately fit the data of the 10-year olds, x2(11) = 50.204,P =<.001, RMSEA = .149. Neither does it adequately fit the data of the 15-year olds, x2(11) = 24.91, P = .009, RMSEA = .114.

3.3. Principal component analyses

Factor loadings of .3 and higher from the principal component analyses are presented in Table 3.In the total dataset two factors with an Eigenvalue above 1 were found which respectively explained 40.0% and 20.5% of

the variance. The first factor consists of visual STM tasks, a verbal WM task (Backward Digit Recall) and the visual WM task.The second factor consists of both verbal STM tasks and one of the two verbal WM tasks (Listening Recall), although this taskalso loads moderately on the first factor.

The findings in both the 10 and the 15-years-old groups separately were mostly similar to the findings in the combinedgroup. In the 10-years-old group, two factors with an Eigenvalue above 1 were found which respectively explained 41.0% and19.5% of the variance. In the group of 15-years-olds, two factors with an Eigenvalue above 1 were found which respectivelyexplained 34.0% and 19.4% of the variance. The only notable difference is that for the 15-year-olds both verbal WM tasksloaded on the second factor together with both verbal STM tasks.

4. Discussion

The present study indicates that Baddeley’s WM model (1986) does not adequately describe WM performances in 10-year and 15-year old children with mild to borderline ID. Although both verbal and visuo-spatial STM can be separated fromeach other, indicated by correlational and principal components analyses, WM could not be differentiated from both STMstores.

Table 2

Correlations between STM and WM scores per age group.

Variable 1 2 3 4 5 6

1. Digit Recall –

2. Nonword Recall 10-year old .58 –

15-year old .543. Block Recall 10-year old .41 .19 –

15-year old .16 .16

4. Visual Patterns 10-year old .29 .07 .48 –

15-year old .22 .02 .335. Backward Digit 10-year old .52 .18 .41 .33 –

15-year old .32 .24 .31 .21

6. Listening Recall 10-year old .58 .39 .27 .26 .48 –

15-year old .34 .36 .26 .05 .27

7. Spatial Span 10-year old .40 .24 .38 .47 .39 .5015-year old .04 .00 .35 .16 .13 .12

Correlations> .30 and P< .01 are in bold.

Table 3

Factor loadings >.30 from principal component analyses.

Test Total subjects 10-year old 15-year old

1 2 1 2 1 2

Verbal STM Digit Recall .81 .83 .81

Nonword Recall .84 .82 .82

Visual STM Block Recall .76 .69 .77

Visual Patterns .79 .80 .64

Verbal WM Backward Digit .62 .36 .56 .45 .40 .50

Listening Recall .43 .62 .42 .68 .65

Visual WM Spatial Span .68 .67 .70

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The results are not clear in understanding the relationship between STM and WM in this population. The results ofexplorative factor analyses indicate rather clearly that in the total group of 10- and 15-year old children with mild toborderline ID, but especially in the 10-year old, visuo-spatial STM is strongly related to the central executive and the scorescan be combined to one ‘general factor’. This could mean that memory is structured in a verbal STM store and a ‘general’factor accountable for visuo-spatial and WM performances. It is consistent with Numminen et al. (2000), who suggest theexistence of a distinctive ‘verbal STM’ and a ‘general’ factor in adults with ID. Also, the same two factors were found withexplorative factor analyses in studies with young typically developing children (e.g. Gathercole & Pickering, 2000). In fact,there is growing evidence to suggest that executive functioning, or the attentional control system (Engle, Tuholski, Laughlin,& Conway, 1999), support the more dynamic aspects of visuo-spatial tasks even in the normal population (e.g. Alloway et al.,2006; Holmes, Gathercole, & Dunning, 2009; Vandierendonck, Kemps, Fastame, & Szmalec, 2004). But for example Hambrick,Kane, and Engle (2005) consider both visual and spatial STM as being more dependent on the attentional control system thanverbal STM, regardless of the task used (see also Miyake, Friedman, Shah, Rettinger, & Hegarty, 2001). This might be thereason for the finding of one factor consisting of both visuo-spatial STM and WM tasks.

However, for the 15-year old, the visuo-spatial STM and WM tasks loaded together on one factor, while tasks for verbalSTM and verbal WM loaded on a second factor. This might indicate a modality specific STM and WM which is in line with thefindings of Shah and Miyake (1996) in undergraduate students. Again, this might also be a question of task purity. One of thetwo administered verbal WM tasks, Backward Digit Recall is said to be rather a verbal STM task instead of a (verbal) WM task(Engle, Tuholski, Laughlin, & Conway, 1999). St Clair-Thompson (2010) specifies this by arguing that Backward Digit Recall isa test for WM in children and a test for STM in adults. If this is true, then the task has functioned as a WM task in thispopulation of children with mild to borderline ID. Furthermore, studies have shown verbal WM weaknesses in children andadolescents with ID (e.g. Henry, 2001; Van der Molen et al., 2007), while visual WM seems to be relatively preserved (Van derMolen et al., 2007). Above that, after a visual WM training, adolescents with mild to borderline ID performed better on (non-trained) visual WM tasks, but not on verbal WM task (Van der Molen, Van Luit, Van der Molen, Klugkist, & Jongmans, 2010).These results support a modality specific memory structure.

Overall, a straight-forward dichotomy between STM and WM indicates apparently an oversimplification (Hutton &Towse, 2001), at least it is for children and adolescents with mild to borderline ID. Furthermore, when STM- and WM tasksare not clearly tapping different underlying constructs, the tasks seem to be organized along modality, visual versus verbaltasks. As it is known that WM is important for the development of scholastic abilities like reading (e.g. Hitch, Towse, &Hutton, 2001) and mathematics (e.g. Bull & Scerif, 2001) and for memory performances in everyday life (Van der Molen, VanLuit, Van der Molen, & Jongmans, 2010), it is important to further explore WM and STM in the population with mild toborderline ID and also examining the validity of the assessment of both constructs in this group.

Acknowledgements

The author would like to thank all children and staff of the schools who participated in this study. Furthermore thanks toBonnie Krausz, Djenie Menig, Bonnie Noordegraaf, Sander de Vries and Hugo van der Weide for assistance with datacollection and to Iris Smits for her statistical support.

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