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Abstract The working memory of people with intellectual disability has been found to generally lag behind their mental age. However, studies concerning the structure of working memory or its connections to other cognitive functions are rare. The present study employs a versatile battery of tests for the evaluation of working memory structure in adults with intellectual disability of unknown aetiology. In addition, connections between working memory and cognitive skills valid for everyday functioning are evaluated. Working memory performance in the study participants was found to stem from two distinct components which could be regarded to represent phonological and general working memory. General working memory was closely related to intelligence, whereas phonological working memory was not. The subjects in the study group differed in their working memory profiles. These distinct profiles were significantly related to academic skills (e.g. reading, writing and mathematics) and sentence comprehension because the profile of the working memory predicted these abilities even when the intelligence and educational background of the participants was taken into consideration. Keywords everyday cognition, working memory Introduction Studies of the capacity of immediate memory in intellectual disability (ID) have used the terms working memory (WM; Mackenzie & Hulme ; Hulme & Mackenzie ; Wang & Bellugi ; Vicari et al. ; Vicari et al. ; Grant et al. ) and short-term memory (STM; e.g. Marcell & Weeks ; Simon et al. ) alternately. Despite the inconsistency in terminology, the general finding has been that people with ID perform more poorly in digit span forwards and backward tasks, as well as in some other span tasks (e.g. the Corsi blocks task) compared to normally developing children of similar mental age, regardless of level of intelligence or aetiology of ID (e.g. Hulme & Mackenzie ; for reviews, see Pulsifer ; Pennington & Bennetto ). In contrast, some studies have not found such a lag compared to mental age in forward span tasks (Vicari et al. ). The structure of the immediate memory process has rarely been studied in ID. Dissociations between verbal and visual WM components (e.g. Wang & Bellugi ), as well as complex and more basic WM processes (Vicari et al. ), have Journal of Intellectual Disability Research pp 579 © Blackwell Science Ltd Working memory structure and intellectual disability H. Numminen, 1 E. Service, 2 T.Ahonen, 1 T. Korhonen, 1 A.Tolvanen, 1 K. Patja 3 & I. Ruoppila 1 1 Department of Psychology, University of Jyväskylä, Jyväskylä, Finland 2 Department of General Psychology, University of Helsinki, Helsinki, Finland 3 Unit of Child Neurology, University of Helsinki, Helsinki, Finland Correspondence: Heli Numminen, Department of Psychology, University of Jyväskylä, PO Box , Jyväskylä, Finland.

Working memory structure and intellectual disability

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Page 1: Working memory structure and intellectual disability

Abstract

The working memory of people with intellectualdisability has been found to generally lag behindtheir mental age. However, studies concerning thestructure of working memory or its connections toother cognitive functions are rare. The present studyemploys a versatile battery of tests for the evaluationof working memory structure in adults withintellectual disability of unknown aetiology. Inaddition, connections between working memory andcognitive skills valid for everyday functioning areevaluated. Working memory performance in thestudy participants was found to stem from twodistinct components which could be regarded torepresent phonological and general working memory.General working memory was closely related tointelligence, whereas phonological working memorywas not. The subjects in the study group differed intheir working memory profiles. These distinct profileswere significantly related to academic skills (e.g.reading, writing and mathematics) and sentencecomprehension because the profile of the workingmemory predicted these abilities even when theintelligence and educational background of theparticipants was taken into consideration.

Keywords everyday cognition, working memory

Introduction

Studies of the capacity of immediate memory inintellectual disability (ID) have used the termsworking memory (WM; Mackenzie & Hulme ;Hulme & Mackenzie ; Wang & Bellugi ;Vicari et al. ; Vicari et al. ; Grant et al.) and short-term memory (STM; e.g. Marcell& Weeks ; Simon et al. ) alternately.Despite the inconsistency in terminology, thegeneral finding has been that people with IDperform more poorly in digit span forwards andbackward tasks, as well as in some other span tasks(e.g. the Corsi blocks task) compared to normallydeveloping children of similar mental age,regardless of level of intelligence or aetiology of ID(e.g. Hulme & Mackenzie ; for reviews, seePulsifer ; Pennington & Bennetto ). Incontrast, some studies have not found such a lagcompared to mental age in forward span tasks(Vicari et al. ).

The structure of the immediate memory processhas rarely been studied in ID. Dissociationsbetween verbal and visual WM components (e.g.Wang & Bellugi ), as well as complex andmore basic WM processes (Vicari et al. ), have

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© Blackwell Science Ltd

Working memory structure and intellectual disability

H. Numminen,1 E. Service,2 T.Ahonen,1 T. Korhonen,1 A.Tolvanen,1 K. Patja3 & I. Ruoppila1

1 Department of Psychology, University of Jyväskylä, Jyväskylä, Finland2 Department of General Psychology, University of Helsinki, Helsinki, Finland3 Unit of Child Neurology, University of Helsinki, Helsinki, Finland

Correspondence: Heli Numminen, Department of Psychology,

University of Jyväskylä, PO Box , Jyväskylä, Finland.

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been suggested. Structural evaluations haveconcentrated on single syndromes: a verbal WMcomponent (Wang & Bellugi ) or complex WMprocesses (Vicari et al. ) have been found to bedeficient in Down’s syndrome, whereas visuo-spatialWM has been found to be deficient in Williamssyndrome (e.g. Wang & Bellugi ; Grant et al.).

One weakness of the studies of immediatememory in ID is the lack of theoretical analysisconsidering differences between STM and WM.Recently, it has been suggested that STM should beincluded in WM, which, in addition to short-termstorage, is responsible for controlled attention(Engle et al. ). It has also been suggested thatdetermination of memory system activation inmemory tasks depends on a person’s age,intelligence and developmental level, as well as thetask requirements (Engle et al. ). Engle et al.() suggested that controlled attention may berequired more often and in simpler memory tasksat lower developmental levels. As developmentproceeds, many memory tasks can possibly beexecuted by more rudimentary STM processes andneed for controlled attention is lessened. For peoplewith ID, it is possible that some tasks reported asmeasuring immediate memory require theinvolvement of both controlled attention andrudimentary information processing, i.e. WM.

An influential theory by Baddeley & Hitch ()described WM as a multicomponent system whichis responsible for the temporary storage and activeprocessing of information. According to theframework, WM consists of at least threecomponents: the central executive; and twosubordinate slave components, the phonologicalloop and the visuo-spatial sketchpad (Baddeley &Hitch ; Baddeley ). It has been suggestedthat the phonological loop and the visuo-spatialsketchpad represent rudimentary STM processes,while the central executive reflects controlledattention (Engle et al. ). In many ways, thecentral executive handles similar functions asattention (cf. Shallice ). It controls, supervisesand regulates information flow between the WMcomponents (Engle et al. ). In addition, it isresponsible for the retrieval of information fromother memory systems, such as long-term memory(Baddeley ). The phonological loop maintains

and stores verbally coded information. It consists ofa phonological store for auditory images of speechand an articulatory rehearsal process which canrefresh the images through a kind of abstract innerspeech process. The visuo-spatial sketchpad isinvolved in the processing and maintenance ofmaterial with a strong visual or spatial component,or of verbal material which has subsequently beenencoded in the form of visual imagery (see Logie).

Central executive functioning can be measuredby complex tasks which require dual processing(the reading span task; Daneman & Carpenter), or simultaneous active manipulation andstorage of information (digit span backwards).Assessment of the phonological loop hastraditionally been based on serial memory for digitsor words. More recently, repetition of and memoryfor so called non-words (words which resemble reallanguage but have no meaning) has been employed(see Gathercole & Baddeley ; Grant et al.). Replacing digits with non-words accentuatesthe phonological character of the task because thestimuli have no meaning and are not familiar lexicalunits. Visuo-spatial sketchpad functioning can bemeasured by tasks which rely on memory for visualobjects and/or their visuo-spatial configuration(Wilson et al. ), or memory for seriallypresented locations; for instance, the Corsi blockstask (Milner ). In ID, central executivefunctioning has been studied by the digit spanbackwards task (Vicari et al. ), phonologicalloop functioning by the digit span forwards task(e.g. Mackenzie & Hulme ; Hulme &Mackenzie ; Wang & Bellugi ; Vicari et al.) or non-word repetition task (Grant et al.), and visuo-spatial sketchpad functioning bythe Corsi blocks task (Wang & Bellugi ; Vicariet al. ).

Working memory capacity has been found toaffect cognitive functions supporting everydayactivities including reasoning, languagecomprehension, long-term learning and mentalarithmetic (e.g. Baddeley & Hitch ; Gathercole& Baddeley ; Baddeley et al. ). It has beensuggested that the central executive has closeconnections with action planning (Baddeley ;see Shallice ), general WM capacity (Just &Carpenter ) and fluid intelligence (Engle et al.

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). The phonological loop appears to play acrucial role in vocabulary acquisition, speechproduction, reading development, skilled readingand language comprehension (for reviews, seeGathercole & Baddeley ; Baddeley et al. ).In addition, the visuo-spatial sketchpad is importantfor geographical orientation and for planning spatial tasks, for example. To date, there have beenfew studies concerning the relationship betweenWM and cognitive functions affecting everyday lifein ID. Language learning has been studied inrelation to phonological WM skills in Williamssyndrome (Grant et al. ), and vocabularyacquisition as well as language learning and usehave been studied in relation to the phonologicalWM skills of a single subject with Down’ssyndrome (Vallar & Papagano ). An assessmentof the relationship between other WM componentsand everyday cognition, as well as connectionsbetween WM and academic skills (e.g. reading skilland everyday memory functioning in ID) remainsto be carried out.

The present study is the first to use versatilemethodology designed for the assessment ofdifferent WM components in people with ID ofmixed aetiology. The primary aim was to evaluatethe structure of WM in ID. Furthermore, thepresent authors explore the connections betweenWM, estimated intelligence, vocabulary, academicmeasures of reading, writing and basicmathematical skills, and everyday memory, asreflected in the Rivermead Behavioural MemoryTest (RBMT; Wilson et al. ). The secondaryaim of the present study was to evaluate theconsequences of individual differences in WMcapacity to cognitive skills valid for everydayfunctioning in ID.

Subjects and methods

Subjects

The present study is part of the psychologicalfollow-up test phase of a nation-wide, populationbased, cross-sectional, multidisciplinary studybegun in Finland in . In , all people from to years of age suspected of having ID whowere living in a selection of municipalitiesrepresenting Finland with regard to socio-

economical structure (n = ) were examinedusing psychological tests to assess if they met thecriteria of ID (IQ < ). Out of those suspected, individuals were diagnosed as having ID in. In , of these subjects could beidentified by their social security code and ofthem were found to be still alive. In , ofthese people were re-tested by using severalassessment methods for intellectual functioning andeveryday cognition.

For the WM examination, a subsample of

women and men with ID of mixed aetiology wasselected. Mixed aetiology was defined as a diagnosisof unknown cause of ID and absence of severeneurological signs. A separate group of participantswith Down’s syndrome was also tested, but theresults are reported elsewhere. The criteria forselection were a re-tested IQ between and , anage under years, an ability to follow testinstructions, an ability to communicate verbally, anabsence of acute psychiatric disorders, and a highlevel of everyday functioning operationalized asnon-institutional living and attendance of shelterworking or other activities.

The participants and/or their guardiansconsented to take part in the present study. Theages of the subjects ranged from to years(mean = . years, SD = . years). The agerange was determined by participation in the firstwave of the follow-up study and the upper limit wasrestricted to years to decrease the possibleeffects of the ageing on WM functions. All studyparticipants had good motor abilities, and theirvision and hearing were comparable to those of thesame age-group in the general population. Out ofthe studied sample, eight people were found to beundergoing medication which could possibly affectmemory (e.g. antidepressants, sedative painkillers,antiepileptic drugs, muscle relaxants andantipsychotic drugs): all drugs interfering with thedopamine and cholinergic systems, or disordersaffecting these systems, potentially produce memoryproblems (e.g. Staubli et al. ). The medicationof the individual participants was highly variableand no specific effects were expected. Therefore, theparticipants were not excluded from the study inorder to avoid a further reduction in the size of thegroup. In addition, the possible effects wereexplored in the analysis of the data.

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Working memory and cognitive measures

The data discussed in the present study werederived from the individual testing of theparticipants. The tests were presented in quiet andconfidential situations, which lasted for an averageof . h. The presentation order of the tests wasvaried from person to person in a semi-randomfashion to control for the possible systematic effectof fatigue and the possible reciprocal effects of thetests.

Working memory tasks were chosen to reflect thetriarchic WM framework described by Baddeley &Hitch (; cf. Table ). The estimation of WM,intelligence, academic skills and everyday memorywas partly done by using standardized methods, forwhich the prescribed procedure was followed. Allnon-standardised tests are described in detail. Ineach WM test, the length of the longest sequencerecalled (span) was scored as well as the totalpoints received from the test (number of successfultrials). All tests and tasks which were performed arepresented in Table .

Academic skills

The academic skills assessed included theevaluation of reading, writing and basicmathematics. The reading test included readingaloud of single words and meaningful text. Thewriting test included a word dictation task as wellas spontaneous writing elicited by a picture.Mathematical skills assessed included theknowledge of numbers, basic counting operations,as well as concepts of age, time and money.Reading, writing and mathematics were used ascompound variables in the analyses.

Finnish non-word repetition

Finnish non-word repetition was constructed from non-word pairs with nine syllables (four or fivesyllables in each non-word), which corresponded toFinnish language words regarding phonotacticstructure and pronunciation. The non-words werepresented as pairs on a tape, which was pausedafter each pair to let the participant immediately

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Table 1 Assessment methods used

Cognitive process Method

Working memoryPhonological loop Finnish non-word repetition

Finnish non-word span (modified from Service 1998)Digit span forwards (Wechsler 1981)*

Visuo-spatial sketchpad Corsi blocks (Milner 1971)Visuo-spatial test (modified from Wilson et al. 1987)

Central executive Digit span backwards (Wechsler 1981)*Complex span (modified from Daneman & Carpenter 1980)

Intelligence Raven’s coloured matrices (Raven 1956)*WAIS-R subtest, similarities (Wechsler 1981)*

Language skills Finnish passive vocabulary (Ruoppila 1964)*Sentence comprehension (part of the complex span task, modified from Daneman & Carpenter 1980)

Academic skillsReading Ability to read single words and full textWriting Ability to write single words and dictated textMathematics Knowledge of numbers, basic counting operations, and comprehension of age, time and money

Everyday memoryStory recall, RBMT test battery (Wilson et al. 1985)*Total Score, RBMT (Wilson et al. 1985)*

*Standardized procedure.

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repeat the non-words. The responses were taperecorded for later scoring. One point was given foreach perfectly correctly repeated non-word (allphonemes right). Recurring problems inarticulation or the order of recall had no effect onscoring.

Finnish non-word span

Finnish non-word span (modified from Service) consisted of combinations of simple non-words (consonant–vowel–consonant–vowel) whichwere phonotactically legal in Finnish. Theparticipants were allowed to read or hear the non-words included in the test (in alphabetical order)before the experiment began. The non-words werepresented by a tape recording at a rate of one persecond in sequences starting from the length of twonon-words. The participants were told toimmediately repeat the non-words after eachsequence. The sequence length increased by oneafter every five trials. Testing was finished when theparticipant failed to repeat three or more out of thefive trials of a certain sequence length. One pointwas given for every correctly repeated sequence (allphonemes correct, non-words in correct order).Span length was estimated as one less than the lastsequence length tested and was increased by half apoint for two correct repetitions of the lastsequence length. Recurring problems in articulationhad no effect on scoring.

Digit span forwards

In the digit span forwards task (Wechsler ),additional two-digit sequences were added and thetask was begun from the sequence length of twoinstead of the three recommended in order toprevent a floor effect.

Corsi blocks

A -block version of the Corsi blocks task (Milner) was employed. The test administrator pointedat one block or a sequence of blocks which theparticipant was told to point to in the same order.Sequences were determined in advance so thatthese appeared to be random. The finishing andscoring criteria were the same as thoserecommended for the digit span forwards test(Wechsler ).

Visuo-spatial test

The visuo-spatial test (modified from Wilson et al.) consisted of matrices of squares, half ofwhich were black and half white. A model matrixwas shown to the participants for s, after whichthey had to mark the squares which had been blackin the model on a empty matrix on their answersheet. The test begun from a matrix size of four.Matrix size was increased by two squares afterevery four trials. Testing was finished after all fourattempts at a certain size of the matrix had beenunsuccessful. One point was given for every totallycorrect reproduction.

Complex span

Complex span (originally based on Daneman &Carpenter ; modified from Service et al.Submitted) was presented on a MacintoshPowerBook computer. The test consisted ofsentences which were presented throughheadphones and were related to simultaneouspictures on the computer screen. The participanthad to indicate whether the heard sentence wastrue or false in relation to the picture by pressingone of two keys (green for true and red for false).Simultaneously, the subject also had to memorizethe last word(s) of the heard sentence(s). Thesentences were presented in growing sets, startingwith one sentence. After each set (all) last word(s)of the sentence(s) in the set had to be orallyrecalled. There were four trials of each of the setsizes from one to six sentence–picture pairs. Thefour trials of the sentence–picture pairs, testing thefirst set size of one, were presented to theparticipant slowly, the instruction was explainedthoroughly and incorrect answers were corrected.The test began with the same sentence–picturepairs which had been presented with theinstructions. Testing was stopped when theparticipant failed on all of the four trials at acertain sequence length, i.e. set size. One point wasgiven for each trial on which all last words (–) ofthe sentence set were remembered. Order of therecall had no effect on the scoring.

Sentence comprehension

True/false decisions in the complex span task werealso scored and were used as a measure of sentence

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comprehension. Order of the recall had no effect onthe scoring.

Statistical analysis

In general, inter-item reliability was estimated bycalculating Cronbach’s alpha. Because most WMmeasures had cut-off points and the tests were notperformed entirely, the present authors assessed thereliabilities of these tests by correlations calculatedbetween the length of the longest sequence recalled(span) and the total score (number of successfultrials) attained during the test. Relationshipsbetween the variables were evaluated bycorrelations. Pearson’s correlation coefficients werecomputed for those variables which were normallydistributed. For the remaining variables, thereported correlation coefficients are Spearman’scorrelations. The structure and profiles of WMperformance in the study group were analysed byprincipal components analysis and hierarchicalcluster analysis with Ward’s method using a squaredEuclidean measure. A principal componentsanalysis was performed to evaluate the correlativestructure of the whole range of WM tests.Hierarchical cluster analysis was performed toevaluate the individual profiles of WM performancein the study group. The analysis was based on

valid cases. The analysis was based on a selection of the WM tasks with the complex span taskexcluded because of many missing values (the testwas not presented to eight participants).Correlations, regression analysis and analysis ofvariance () were used to estimate theconnections between WM performance,intelligence, academic abilities and everydaymemory measures, as well as age, sex, educationand medication of the participants. Therelationships between WM performance andacademic abilities and everyday memoryperformance were further analysed using analysis ofcovariance (). The results of the intelligencetests and the educational background of theparticipants were added as covariants using anenter method. Measures of reading and writingwere divided into three classes for the toreach the normal distribution because thedistributions of the raw scores were not normal. Onother measures, the raw scores were used. All

analyses were performed with the SPSS computersoftware (SPSS ). The results were consideredto be significant at P < ..

Results

Based on Raven’s coloured matrices (Raven ),the mean mental age of the subjects was . years(SD = . years), and estimated on the basis oftheir scores in the similarities subtest of WechslerAdult Intelligence Scale – Revised (WAIS-R;Wechsler ), their mean verbal IQ was .

(SD = .). All measures were highly reliable(inter-item estimation) and most measures werenormally distributed (see Table ). The number ofmissing values was small.

Working memory performance

Intercorrelations between the different WM tasks, aswell as correlations between WM tasks andeveryday cognition measures are presented inTable . Contrary to the triarchic division in WMtheory (phonological loop, central executive andvisuo-spatial sketchpad), the test performance ofsubjects with ID loaded on only two distinctcomponents which could be regarded to represent different structures of WM (see theprincipal components analysis in Table ). Thecomponents explained .% of the variation inWM performance. On the basis of the loadings of individual tests component was named the general component and component wasnamed the phonological component. Thephonological component reflected the tasksdesigned for phonological loop assessment, whiletasks designed for the central executive and thevisuo-spatial sketchpad evaluation loaded on thegeneral component. The components weredifferently related to the two measures ofintelligence. When entered into the model, bothintelligence measures accounted for a significantportion of variation in the general component(multiple regression analysis: F(2, 43) = .,P < ., R2 = .; b (Raven) = ., t = .,P < .; b (similarities) = ., t = .,P < .), explaining equal amounts of variation in the general component (the b-values were equal). However, intelligence measures were

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Table 2 Descriptive data on working memory tasks, intelligence measures, measures of language skills and academic skills, as well as mea-

sures of everyday memory

Theoretical Reliability/Cognitive process Task Mean SD range normality* Number

Working memoryPhonological loop Non-word repetition 15.18 8.85 0–40 0.90 (N) 45

Non-word span 2.20 0.81 0–8 0.88 (A)† 46Digit span forwards 3.65 0.85 2–9 0.87 (N)† 45

Visuo-spatial Sketchpad Corsi Blocks 3.20 1.11 0–9 0.95 (N)† 46Visuo-spatial test 3.65 1.23 1–10 0.93 (A)† 46

Central Executive Digit span backwards 1.72 1.19 0–8 0.94 (A)† 46Complex span 1.45 0.72 0–6 0.89 (A)† 38

Intelligence Raven 16.83 5.61 0–36 0.84 (N) 45Similarities (WAIS-R) 8.67 7.10 0–32 – (A) 46

Language skills Passive vocabulary 51.76 8.06 0–68 0.86 (N) 45Sentence comprehension 12.68 6.11 0–16 – (N) 38

Academic skillsReading 27.73 18.51 0–43 0.94 (A) 44Writing 7.31 4.77 0–13 0.88 (A) 43Mathematics 26.54 8.26 0–39 0.80 (N) 44

Everyday memoryStory recall (RBMT) 15.48 10.68 0–84 0.74 (N) 46Total score (RBMT) 14.20 4.47 0–22 0.83 (N) 46

*Distribution: (N) normal; and (A) abnormal.†Reliability assessed by correlation.

Table 3 Correlations between working memory tests, and measures of intelligence, academic skills and everyday memory

Visuo-Non-word Non-word Digit span Corsi spatial Digit span Complex Similaritiesrepetition span forwards blocks test backwards span Raven (WAIS-R)

Working memoryNon-word repetition 0.67** 0.32* 0.09 –0.01 0.28 0.36* –0.17 0.27Non-word span 0.67** 0.13 0.02 –0.04 –0.01 –0.03 –0.04 0.02Digit span forwards 0.32* 0.13 0.07 0.08 0.28 0.20 –0.13 0.10Corsi blocks 0.09 0.02 0.07 0.49** 0.47** 0.37* 0.55** 0.51**Visuo-spatial test –0.01 –0.04 0.08 0.49** 0.33* 0.41* 0.53** 0.34*Digit span backward 0.28 –0.01 0.28 0.47** 0.33* 0.40** 0.32* 0.31*Complex span 0.36* –0.03 0.20 0.37* 0.41* 0.40* 0.10 0.36*Language skillsVocabulary 0.17 0.03 –0.02 0.22 0.44** 0.32* 0.30 0.17 0.55**Sentence 0.39* 0.17 0.35* 0.37* 0.41* 0.46** 0.85** 0.11 0.39*

comprehension

Academic skillsReading 0.46** 0.26 0.36* 0.29 0.10 0.48** 0.48** –0.07 0.27Writing 0.39* 0.07 0.47** 0.36* 0.13 0.41** 0.38* –0.09 0.44**Mathematics 0.24 0.06 0.27 0.35* 0.36* 0.52** 0.41* 0.35* 0.47**

Everyday memoryStory recall (RBMT) –0.04 –0.06 0.04 0.35* 0.41** 0.22 0.44** 0.42** 0.49**Total score (RBMT) 0.10 –0.06 –0.06 0.41** 0.28 0.28 0.31 0.24 0.54**

*P < .; **P < ..

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not significantly related to the phonologicalcomponent.

The WM performance of the subjects with IDhad distinctive features which could be clustered

into four characteristic WM profiles based onhierarchical cluster analysis (see Fig. ). The fourgroups were considered to reflect differences inWM capacities.

Relationship between working memoryperformance and everyday cognition

From the tests loading on the phonologicalcomponent, the non-word repetition and the digitspan forwards test correlated significantly withreading and writing as well as sentencecomprehension, but not with other everydaycognition measures.The non-word span task did notcorrelate with any everyday cognition measures(Table ).Working memory tests which loaded on thegeneral component all correlated significantly withmathematical skill and sentence comprehension. Inaddition, tasks intended for visuo-spatial sketchpadassessment were differently related to everydaycognition measures because the Corsi blocks test wassignificantly related to writing and the measures of

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Table 4 Principal components analysis result (varimax-rotated

model) for the working memory measures

Component

Measure General Phonological

Eigenvalue 2.52 1.81Percentage of variation 36.0 25.8

Working memoryNon-word repetition 0.13 0.91Non-word span –0.14 0.87Digit span forwards 0.18 0.52Corsi blocks 0.76 0.02Visuo-spatial test 0.76 –0.19Digit span backwards 0.76 0.25Complex span 0.70 0.25

Figure 1 Different working memory profiles of the subjects.

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everyday memory, and the visuo-spatial test was significantly related to vocabulary and storyrecall.Tasks for central executive assessmentcorrelated significantly with all academic measuresand the complex span task correlated with storyrecall.

Compound variables representing thephonological and general components werecorrelated with everyday cognition measures. Testsloading on the phonological component correlatedsignificantly with reading (Spearman’s correlationcoefficient: r = ., P < .), writing (Spearman’scorrelation coefficient: r = ., P < .) andsentence comprehension (Pearson’s correlationcoefficient: r = ., P < .), but not with othereveryday cognition measures. Working memory testswhich loaded on the general component correlatedsignificantly with all academic measures(Spearman’s correlation coefficients: readingr = ., P < .; writing r = ., P < .;Pearson’s correlation coefficient: mathematicsr = ., P < .), language skills (Pearson’scorrelation coefficients: vocabulary r = .**,P < .; sentence comprehension r = .**,P < .) and everyday memory measures

(Pearson’s correlation coefficients: story recall(RBMT) r = ., P < .; total score (RBMT)r = ., P < .).

The Raven’s coloured matrices test wassignificantly related to mathematical skills and storyrecall, whereas the similarities test was significantlyrelated to all academic and everyday cognitionmeasures except reading. Age had a significanteffect only on performance in the visuo-spatial test,younger persons performing better than older ones(Spearman’s correlation coefficient: –.). Neitherthe sex of the participants nor their use ofmedication had a significant effect on the results.

The groups with characteristic WM profilesdiffered significantly from each other on all othermeasures except the RBMT total score, where thedifference only approached significance (Table ).When the effects of the two intelligence measuresand education were controlled, the groupingaccording to WM performance still significantlyaccounted for variability () in sentencecomprehension (F(3, 30) = ., P < .), reading(F(3, 36) = ., P < .), writing (F(3, 35) = .,P < .) and mathematics (F(3, 36) = .,P < .).

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Table 5 Differences between working memory profile groups in cognitive measures

Significant differencesMeasure F-value Degrees of freedom Significance between single profiles

IntelligenceRaven 7.15 3, 41 0.001 2 > 1Similarities (WAIS-R) 4.29 3, 41 0.01 2 > 1Language skillsVocabulary 3.00 3, 40 0.04 NS*Sentence comprehension 10.07 3, 33 0.001 2 > 1

3 > 1

Academic skillsReading 8.53 3, 39 0.001 2 > 1

4 > 14 > 3

Writing 7.21 3, 38 0.001 2 > 12 > 3

Mathematics 8.91 3, 39 0.001 2 > 12 > 3

Everyday memoryStory recall (RBMT) 3.06 3, 41 0.04 NSTotal score (RBMT) 2.42 3, 41 0.08 NS

*NS: no significant differences between groups.

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Discussion

The present study explored WM structure, as wellas the connections between WM and everydaycognition measures in individuals with ID. Workingmemory performance in the tested group wasfound to stem from two distinct components: thephonological component and the generalcomponent (the latter had loadings from bothcentral executive and visuo-spatial sketchpad tasks).These components corresponded to the theoreticalcomponents of the WM framework only with regardto the phonological component which reflected thetasks selected for phonological loop assessment(Baddeley & Hitch ; Baddeley ). Thegeneral WM component more or less reflected morestrategic WM functions and controlled attention,and was highly connected to general intelligence(cf. Engle et al. ). The present results showthat at least one component of WM, probablycorresponding the phonological loop, can bedissociated from the general cognitive impairmentrelated to low IQ and general WM deficiency in ID.

Recent studies with general population samplesdo not support simple pairings of tests and memorycomponents (cf. Engle et al. ). A finding byGathercole & Pickering (in press) suggested thatthe sensitivity of WM tasks is different inchildhood. The above authors studied WMstructure in children from to years of age andfound that, at this developmental stage, visuo-spatial tasks require strong involvement of thecentral executive, i.e. controlled attention. Becausethe above results are seemingly similar to those ofthe present study, it could be hypothesized thatdevelopmental changes in memory processinvolvement in distinct WM tasks also occur in ID.In addition, it is probable that these changes followthe pace of mental age increase. One small group ofsubjects, i.e. profile (n = ), with a dissociationbetween visuo-spatial sketchpad and centralexecutive tasks, showed that a separate visuo-spatial sketchpad component could also be found in the WM structure of the subjects with IDin the present study. More studies are needed inorder to evaluate which tasks are more sensitivethan the present ones to the fragmentation of thevisuo-spatial sketchpad from the central executivein ID.

The participants in the present study differed intheir performance on the WM tasks. Based on thesedifferences, they could be divided into groupswhich represented distinctions in the quantitativeand qualitative nature of their WM. Thesedistinctions significantly predicted sentencecomprehension and academic abilities, even whenthe two intelligence measures and educationalbackground were taken into consideration. Thepresent findings support the view that WM hasstrong validity in the evaluation of cognitivecapacities of people with ID, and that it also affectslearning at lower levels of general intelligence,although it partially reflects the general capacity aswell.

Earlier studies concerning the connectionbetween WM and cognitive abilities have suggestedthat problems in WM subprocesses can lead tospecific deficits of cognition. For instance,impairment of the phonological loop would lead toproblems in language development (Bishop et al.), reading (Taylor et al. ) and vocabularyacquisition (Gathercole & Baddeley ).Phonological WM tasks predicted reading andwriting skills better than mathematical skills in thegroup with ID. Similar results to those of thepresent study on the relationship betweenphonological WM and reading and writing, but notmathematics, have been reported for children withlearning disabilities but normal IQs (Taylor et al.). Interestingly, the correlations between thephonological WM tests and vocabulary in thepresent study were not significant. The currentfindings are similar to those found for children withspecific language impairment (Bishop et al. ).It is possible that these results are related to thequalitative differences in WM functions in ID andlearning disabilities in comparison with the generalpopulation. General WM skills and generalintelligence may have a more prominent effect onvocabulary acquisition in ID than in the generalpopulation. It is also possible that the learningprocess may be based on massive repetitions inenvironmental input since low IQ is generallyrelated to a small vocabulary which consists offrequent words, and the phonological skills of theperson may play a less prominent role than in thosewho learn from less exposure. However, it seemsthat good phonological WM could partly

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compensate for a deficiency of general WM inreading and writing at least.

The estimation of everyday memory was addedto the present study in order to evaluate memoryfunctions which are more closely related toeveryday life. Performance in everyday memorymeasures (i.e. RBMT story recall and total score)correlated with the WM tests loading on the generalcomponent, but only story recall was significantlydifferent for distinct WM profile groups. Everydaymemory measures seem to be closely connected togeneral ability because differences in WMperformance in the study group did not reachsignificance in the explanation of the RBMT totalscore when the effects of intelligence and educationwere taken into consideration.

Acknowledgements

This study was financed by The Academy ofFinland. We wish to thank the psychologists whohelped in data collection, Jaana Saastamoinen,Tuula Saarimäki, Raija Hiljanen, and PäiviHäggblom, as well as Dr Juhani E. Lehto forcomments on the manuscript. We also want tothank all the study participants and their relativeswho made the study possible.

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