Cognitive Executive Function in Down’s Syndrome

Copyright The British Psychological SocietyReproduction in any form (including the internet) is prohibited without prior permission from the Society

Cognitive executive function in Downs syndrome

John Rowe1*, Anthony Lavender2 and Vicky Turk31City and Hackney Teaching Primary Care NHS Trust and City and East LondonMental Health NHS Trust, London, UK

2Centre for Applied Social and Psychological Development, Salomons, CanterburyChrist Church University, Kent, UK

3Oxleas NHS Trust, Kent, UK

Objectives. The study investigated executive function in adults with Downssyndrome.

Design. Participants with Downs syndrome (N 26) were compared withnon-Down syndrome learning disabled participants (N 26).Method. The two groups performance on a range of tests of executive functionwere compared. Groups were matched on age and a measure of vocabulary.

Results. The Downs syndrome group performed at a significantly lower level on anumber of tests of executive function.

Conclusion. It is suggested that impaired executive function in Downs syndrome isdue to abnormal development of the prefrontal cortex in Downs syndrome. Tests ofexecutive function may be sensitive to cognitive changes with the onset of dementia inpeople with Downs syndrome. Longitudinal studies examining changes in executivefunction in people with Downs syndrome are recommended.

In Downs syndrome (DS) there is a significantly increased risk for dementia of theAlzheimer type (DAT) with the onset being at an earlier age. However, because of

pre-existing cognitive impairment, people with DS do not generally reach the level of

cognitive ability against which decline due to dementia can easily be measured. There is

a recognized need for methods for detecting the early cognitive changes with DAT onset

in DS (Das & Mishra, 1995; Devenny et al., 1996; Haxby, 1989; Holland & Oliver, 1995;

Schapiro, Haxby & Grady 1992). The pre-morbid assessment of people with DS order to

establish individual baselines of cognitive and adaptive functioning, against which

future decline due to possible DAT onset can be measured, is recommended (Burt &Aylward, 1998; Burt & Aylward, 2000; NHS Executive and Social Service Inspectorate,

2001; Oliver, 1998; Turk, Dodd, & Christmas, 2000). Early detection enables timely care

planning, and consideration of interventions such as cholinergic therapy.

* Correspondence should be addressed to John Rowe, Learning Difficulties Service, St Leonards Primary Care Centre, Nuttall St,London N1 5LZ, UK (e-mail: [email protected]).

TheBritishPsychologicalSociety

5

British Journal of Clinical Psychology (2006), 45, 517

q 2006 The British Psychological Society

www.bpsjournals.co.uk

DOI:10.1348/014466505X29594


Early diagnosis, via cognitive assessment, may be aided by the identification of

cognitive domains of relative weakness in DS. Holland, Hon, Huppert, and Stevens

(2000) and Holland, Hon, Huppert, Stevens, and Watson (1998) hypothesize that

functions served by neural substrates affected by early DS neuropathology, either

directly or through being the target of projections from affected areas, will be sensitive

to further insult due to lack of reserve capacity. That is, they will be unable to sustainfurther decrement without the effects being manifest.

A survey by Holland et al. (2000) of prevalence rates of dementia in people with DS

in a UK health district found that a number of cases, particularly in younger age bands,

met diagnostic criteria for frontal-type dementia (Gregory & Hodges, 1996). They argue

that the presentation and course of dementia in DS reflects, or is affected by, pre-existing

abnormalities in brain development, specifically hypoplasia of the frontal cortex. In DS,

the frontal cortex, limbic system, and brain stem, generally show reduction in volume

(De la Monte & Hedley-Whyte, 1990; Jernigan, Bellugi, Sowell, Doherty, & Hesselink,1993; Raz et al., 1995; Wisniewski, 1990) and decreased cellular density, particularly in

those layers from which callosal projections arise, and the region of the corpus callosum

serving frontal neural projections has been found to be underdeveloped (Coyle,

Oster-Granite, & Gearhart, 1986; Wang, Doherty, Hesselink, & Bellugi, 1992).

Holland et al. (2000) and Holland et al. (1998) point out that the frontal lobe

abnormalities found in DS may result in the early manifestation of dementia taking the

form of frontal-type dementia. They do not propose that younger people with DS and

dementia have frontal dementia but that early stage DAT can manifest in a similar form tofrontal dementia. They hypothesized that early behavioural changes at onset of DAT are

the consequence of early, unrecognized, cognitive decline in functions served by the

frontal lobes. Functions served by the prefrontal cortex and its projection areas include

executive function, summarized as goal formation, planning, and carrying out goal-

directed plans effectively (Lezak, 1995, pp. 650651).

Some authors have suggested the presence of executive dysfunction in DS

(e.g. Jernigan et al., 1993; Wisniewski, 1990) and some studies have found impairment

on some tests of executive function (e.g. Bellugi, Wang, & Jernigan, 1994; Burack,Benedetto, & Frye, 1996; Frith & Frith, 1974; Gibson, 1991). It has, however, been noted

that executive function in DS has rarely been the subject of specific study (Pennington &

Bennetto, 1998; Crnic & Pennington, 2000).

This study examines aspects of executive function in people with DS and free from a

diagnosis of DAT in comparison to a control group matched on age and a measure of

vocabulary. It was hypothesized that the DS group would perform at a lower level on

tests of executive function.

Method

DesignA between-groups design (participants with DS versus learning disabled participants

without DS) was used.

ParticipantsA sample size of 26 was required to achieve a power level of 0.80 (Bilkman & Rog, 1998),

assuming an effect size of 0.80.

John Rowe et al.6


The DS sample was made up of 18 males and 8 females (N 26), and the age rangewas 23 years to 40, the upper age limit being set to exclude participants likely to have

preclinical DAT. The non-DS control sample was made up of 15 males and 11 females

(N 26), age range 19 years to 55, with learning disabilities of unknown aetiology.All participants lived in community settings and were recruited from 4 day centres for

people with learning disabilities. All were educated in the UK. In both groups

the presence of significant sensory impairment, psychiatric or physical illness, or

challenging behaviour were grounds for exclusion, as were any pre-existing concerns

relating to deterioration in behaviour, skills or cognitive ability. DS participants were

required to have a medical diagnosis of DS confirmed by the appropriate learning

disabilities service. Participants with learning disabilities of known developmental

aetiology or with autistic spectrum disorder were excluded from the control group as

there is evidence for executive deficits in autism and possibly in some specific genetic

disorders (Pennington & Bennetto, 1998).

MeasuresExecutive function is not a unitary function, and there is the possibility that

developmental neuropathology may differentially affect aspects of executive function.

Tests were therefore selected to assess various aspects of executive function.

Pennington and Ozonoff (1996) list areas of executive function open to

neuropsychological assessment: set shifting; planning/problem solving; working

memory; inhibition/perseveration; and fluency. To this list attention was added as it

has been considered to overlap with executive functions and be served by the prefrontal

cortex (Manly & Robertson, 1997; Mirsky, Anthony, Duncan, Ahearn, & Kellam, 1991;

Posner and Peterson, 1990). A measure of vocabulary was included to match

participants. A measure of motor speed was included to control for the speed element in

the timed tests that had a motor speed component.

Tests were selected to cover Pennington and Ozonoffs areas. The criteria for test

selection were previous use with learning disabled participants, acceptable reliability

and validity, portability, minimal reliance on complex verbal instructions, brevity, and

availability in the UK.

Set shifting

Weigl Colour-Form Sort TestThe Weigl Colour-Form Sort Test (Goldstein & Scheerer, 1953) assesses ability to

categorize across two dimensions, involving ignoring a salient dimension (colour) to

categorize by a less salient dimension (form). Instructions for administration and scoring

were taken from Byrne, Bucks, and Cuerden (1998). Test material consisted of nine

tokens. These were three circles, three triangles, and three squares, each shape

coloured blue, red, or yellow on one side and white on the reverse. The participant was

asked to sort the tokens so that they went together and then asked to sort them in a

different way. The scoring system takes into account the established finding that sorting

by form is more difficult than sorting by colour for people with organic impairment, and

therefore awards a higher score to those who sort by form without prompting.

Responses were scored as follows.

Cognitive executive function 7


Planning/problem solving

Tower of LondonThe Tower of London (Shallice, 1982) assesses problem solving and spatial planning.

Instructions for administration and scoring were taken from Anderson, Anderson, and

Lajoies (1996) standardized administration and scoring procedures for a paediatric

population, chosen as it gives credit for longer solution times than Shallices (1982)

scoring system. Scores were allocated as follows.

Test material consisted of a board, 4 inches by 12 inches, with three pegs of

increasing height and three rings coloured red, blue, and green. The rings were wooden

discs two inches in diameter and half an inch thick with a half-inch diameter central

hole. A5 coloured photographs of the target configurations were provided.

The participant was asked to match the target configuration, shown on a colouredphotograph, from a standard start configuration by moving the rings in accordance with

set rules. The first problem was a training task, following which 12 problems of

increasing complexity were presented, giving a maximum score of 108.

Ravens Coloured Progressive MatricesRavens Coloured Progressive Matrices (RCPM, Raven, Court, and Raven, 1990) is a

widely used and standardized test of non-verbal reasoning ability based on figural test

stimuli. Although this test is primarily used as a measure of fluid intelligence, Carpenter,

Just, and Schell (1990), Duncan (1995), and Duncan, Burgess, and Emslie (1995) report

specific impairment on this task in people with frontal lobe lesions compared with

controls. Pennington and Ozonoff (1996) suggest this is because the frontal lobes are

important for fluid intelligence but less so for crystallized intelligence. It was thereforedecided to include this as a test of executive function, rather than a measure of

intellectual ability on which to match participants.

Instructions for administration and scoring were taken from Raven et al. (1990) with

a maximum score of 36.

Response ScoreParticipant sorts by form or colour without prompting and shifts set when told. 5Participant sorts to form and shifts to colour when given a cue. 4Participant sorts to colour and shifts to form when given a cue. 3Participant sorts to form only and does not shift to colour. 2Participant sorts to colour only and does not shift to form. 1Participant is unable to sort by form or colour. 0

Solution Time ScoreLess than 5 seconds 9610 seconds 81120 seconds 72140 seconds 64160 seconds 5Greater than 60 seconds 4

John Rowe et al.8


Working memoryNo suitable tests of the central executive component of working memory

were identified. Tasks tapping this function listed by Pennington and Ozonoff (1996)

were considered too complex for participants. Backwards digit span and spatial span,

which involve a processing element, were piloted but found to show floor effects with

many participants. Forward digit span and spatial span were used. These tap the verbaland visual short term memory (STM) components of working memory in terms of

Baddeleys (1986) model of working memory but not the central executive processing

component.

Digit spanDigit span assesses verbal STM via immediate recall of a spoken digit string, although

Lezak (1995, p. 359) considers this more a test of attention than STM. Stimulus material

and instructions for administration and discontinuation criteria were taken from the Digit

Span subtest of the Wechsler memory scale third edition (WMS-III, The Psychological

Corporation, 1997). One point was given for each digit string correctly repeated, giving amaximum score of 16.

Spatial spanSpatial span assesses visual STM. People with frontal lobe lesions have been found to

perform less well on this task than controls (De Renzi, Faglioni, and Previdi, 1977).

The participant observes the experimenter tapping blocks in sequence and then is asked

to reproduce that sequence. Stimulus material and instructions for administration and

discontinuation criteria were taken from the Spatial Span subtest of the WMS-III. One

point was given for each correctly reproduced sequence, giving a maximum of 16.

Inhibition/perseveration

Motor perseveration (finger tapping)This motor perseveration test was used originally by Luria (1980). Instructions for

administration and scoring were taken from the Middlesex Elderly Assessment of

Mental State (MEAMS) manual (Golding, 1989). The participant was asked to tap the

table once if the examiner tapped twice, and tap the table twice if the examiner tapped

once. Ten trials were given. Practice trials were given until the participant gave both

correct responses. The score was the total number of correct trials (maximum 10).

Fluency

Verbal (semantic) fluencyVerbal Fluency, in which the participants produce as many words belonging to the same

category (e.g. same initial letter or same semantic category) as they can within a timed

period, is an established test of executive function. The semantic category of animalnames was used as this form of the test was considered easier for learning disabled

clients than generation of members of an abstract category, such as common initial

letter, and less dependent on literacy skills. Harrison, Buxton, Hussain, and Wise (2000)

report that the animal names version of this test correlates well with the initial letter

(FAS) version. The participant was asked to say as many animal names as they could



within one minute. The score was the total number of unique animal names generated

in 60 seconds.

Attention

Attention SustainedLeiter International Performance Scale revisedAttention Sustained is a visual search and cancellation task from the Leiter InternationalPerformance Scale-Revised (LIPS-R) attention and memory battery (Roid & Miller, 1997).

Instructions for administration were taken from the LIPS-R manual (Roid and Miller,

1997), except that all booklets (A, B, and C) were administered. On each trial

participants were required to strike out with a pencil line target items within a given

time period (30 seconds or 60 seconds). The score was not the LIPS-R scaled score but

the total number of target items cancelled minus the total number of errors (non-target

items cancelled), giving a maximum of 355 points.

Verbal ability

British Picture Vocabulary Scaleshort formParticipants were not matched on the common tests of intellectual function, WAIS-III,

LIPS-R, and RCPM, as subtests from the WAIS-III and LIPS-R and the entire RCPM were

used as tests of executive function. To match groups on intellectual ability, a vocabularymeasure (British Picture Vocabulary Scale short form, BPVS, Dunn, Dunn, Whetton,

and Pintile, 1982) was used. Though not a measure of general intelligence, vocabulary is

an important contributor to general intelligence (Kaufman & Lictenberger, 1999; Lezak,

1995, p. 539; Satler & Ryan, 1998) and has been used to match learning disabled

participants in other studies (Jarrold & Baddeley, 1997; Jarrold, Baddeley, & Hewes,

1999; Jarrold, Baddeley, & Hewes, 2000) and can readily be assessed. Administration was

modified from the BPVS manual (Dunn et al., 1982). The score was the total number of

correct answers, discontinuation criteria were four incorrect responses out of sixsuccessive items. Testing began at the first trial, (not at the BPVS manual age-indicated

trial) giving a maximum score of 32.

Motor speed

Crayton Oliver Card Sorting TestAs the Attention Sustained and Tower of London tests have a motor speed component,

this was separately assessed by a motor task that had minimal attentional or problem

solving demands. The Crayton Oliver Card-sorting task (Crayton, Oliver, Holland,

Bradbury, & Hall, 1998) was used. Test material consisted of twenty-six black and twenty-

six white tokens measuring 7 mm by 10 mm, made locally from 3 mm Perspex.

Instructions for administration were taken from Oliver and Crayton (1993). Twenty-

five black and twenty-five white tokens were placed in a randomly sorted pile in front ofthe participant. One extra black and one extra white card were placed to the participants

left and right respectively as models. The participant was asked to sort the tokens into

piles by colour, black to the left and white to the right. The score was the time taken from

touching the first card to completion. Note: no reliability or validity data were located for

this task.

John Rowe et al.10


Data managementData were analysed via SPSS v. 9.0 for Windows. As the hypothesis predicts that the DS

group would be relatively impaired on tests of executive function, one-tailed levels of

significance were used except where stated. Normality of distribution was assessed via

the Kolmogor-Smirnov test. Homogeneity of variance was assessed via the F-test.

Comparison of means was via the unrelated t test if requirements for parametric testswere met, otherwise the Mann-Whitney test was used. The level of significance was set

at p , :05 unless corrected for multiple comparisons. Effect size was calculated bydividing the difference between group means by their mean standard deviation.

Results

Group characteristicsCharacteristics of the DS and non-DS control groups are shown in Table 1. There was no

significant between-group difference for age or vocabulary (BPVS score). A significantbetween-group difference for Crayton Oliver Card Sorting Test time was found with the

DS group showing a longer (slower) sorting time.

Executive functionBefore correction for multiple comparisons significant between-group differences were

found for scores on: Verbal Fluency, Weigl Colour-Form Sort Test, Digit Span, AttentionSustained, Motor Perseveration, Tower of London, and RCPM. The Spatial Span group

effect was not significant. Results are shown in Table 2. As multiple comparisons

(eight) were made on tests of executive function Bonferroni correction was carried out

and the level of significance set to p , :006. After Bonferroni correction a significantgroup effect remained for the Weigl Colour-Form Sort Test, Attention Sustained, and

RCPM.

Differences in short term memoryTwo-way ANOVA was carried out to assess the effects of modality of presentation

(verbal versus visual) in short term memory in the two groups by comparing scores on

the Digit Span with scores on the Spatial Span. The between-group interaction effect,

group by modality, was significant, (F1; 49 16:81, p :027). The DS group showedlowered Digit Span performance than the non-DS group, whilst both groups showedsimilar levels of performance on the Spatial Span.

Effects of motor speed on timed tests of executive functionThere was a significant between-groups difference on motor speed scores as measured

by the Crayton Oliver Card Sorting Test. The executive function tests included two witha timed motor speed component, the Attention Sustained and Tower of London tests.

ANCOVA was carried out on Attention Sustained scores, but not on Tower of London

scores as they did not reach significance after correction and the data were not suitable

for parametric analysis. After controlling for motor speed the group effect on Attention

Sustained remained significant; F1; 49 5:244, p :027.



Table1.

Gro

up

char

acte

rist

ics

DS

groupN

26

Non-D

Sgr

oupN

26

Mea

sure

Mea

nSD

Ran

ge(m

ax

min

)M

ean

SDR

ange

(max

m

in)

t=Z

Sig.

pva

lue

Effec

tsi

zed

BPV

Ssc

ore

14.5

03.8

07

22

15.6

44.1

27

25

0.9

30.3

6ns

0.2

9C

OC

STsc

ore

102.9

635.2

242

186

78.5

827.1

038

149

2.7

30.0

09

0.7

8A

ge33.2

55.3

023.8

40

33.4

59.6

519

55

0.2

70.8

0ns

0.0

3

CO

CST

C

rayt

on

Oliv

erC

ard

Sort

ing

Test

.

John Rowe et al.12


Table2.

Gro

up

com

par

isons

on

test

sofex

ecutive

funct

ion

DS

groupN

26

Non-D

Sgr

oupN

26

Mea

sure

Mea

nSD

Ran

ge(m

ax

min

)M

ean

SDR

ange

(max

m

in)

t=Z

Sig.

pVal

ue

Effec

tsi

zed

Ver

bal

fluen

cy10.2

73.8

04

20

12.6

94.1

75

20

2.1

90.0

33

0.6

1W

CFS

T2.5

01.3

01

5

4.0

40.9

92

5

4.7

8,

.001

1.3

4D

igit

span

3.8

82.0

50

9

5.4

02.4

52

12

2.3

8.0

21

0.6

7Sp

atia

lsp

an4.1

91.7

40

9

4.0

81.9

80

10

0.2

1.4

15ns

0.0

6A

tten

tion

sust

ained

115

57

10

199

178

46

20

259

5.1

2,

.001

1.2

2M

oto

rper

seve

ration

4.4

63.9

20

10

7.6

42.9

60

10

5.7

4.0

20

0.9

2To

wer

ofLo

ndon

26.5

11.4

77

46

33.5

13.7

621

78

2.4

8.0

13

0.5

5R

CPM

13.5

13.4

68

18

16

4.8

312

28

4.4

0,

.001

0.2

7

WC

FST

W

eigl

Colo

ur-

Form

Sort

Test

.



Discussion

The two groups did not differ significantly in age or verbal ability as measured by the

BPVS. The DS group were significantly slower on the Crayton Oliver Card Sorting Test.

On all tests of executive function, except for Spatial Span, the DS group scored at a

lower level than the control group. After correction for multiple comparisons significant

between-group differences were found for performance on the Weigl Colour-Form Sort

Test, Attention Sustained, and RCPM. Performance differences on the Tower of London

only narrowly missed significance.

Effect size gives a measure of the degree of overlap between experimental and

control groups, and hence the magnitude of deficit. Whilst statistically significant

deficits were found on measures of executive function in DS, these were not necessarily

clinically significant, for which an effect size of d . 3:00 would be required

(Zakanis, Leach, and Kaplan, 1999). Standard deviations and hence variance on Verbal

Fluency, Digit Span, Attention Sustained, and Motor Perseveration were high and would

argue for the importance of comparison against individual baseline scores as opposed to

comparison with group norms.On measures of visual STM (Spatial Span) and verbal STM (Digit Span) a significant

between-group interaction for modality was found. The two groups showed similar

performance on the visual STM measure but the DS group showed significantly lower

performance on the verbal STM measure. The results support previous findings

(Baddeley, 1993; Bower & Hayes, 1994; Jarrold & Baddeley, 1997; Jarrold, et al., 1999;

Wang & Bellugi, 1994) of lowered verbal STM in DS with relatively unimpaired visual STM

and provide evidence for the hypothesis that verbal and visual STM are dissociated.

Tests of executive function could usefully be included in baseline neuropsychological

batteries, though a longitudinal study would need to be carried out to determine their

sensitivity and specificity. A number of tests used in this study were shown to be

potentially suitable for administration to a population with high-moderate to mild

learning disabilities and with a UK educational background. These are the: Weigl Colour-

Form Sort Test, Verbal (semantic) Fluency, WMS-III Digit Span and Spatial Span, Tower of

London, RCPM, MEAMS Motor Perseveration, LIPS-R Attention sustained. Some tests

(Digit Span, Spatial Span, and Motor Perseveration) showed floor effects with some

participants, rendering them unsuitable for use with more impaired clients.

The sensitivity and specificity of tests of executive function to onset of DAT in DS

would need to be determined by a longitudinal study.

People with DS without DAT are not generally reported to show social and adaptive

impairments relative to people with learning disabilities of other aetiologies.

Lezak (1982) points out that executive functions are at the heart of all socially useful,

personally enhancing, constructive, and creative activities. It might be expected that

executive function impairment would adversely affect these domains. This is an area for

future research.

This study found that impairment of executive function, relative to intellectual ability

(as estimated by lexical knowledge), is a feature of DS, possibly as a consequence of

impaired development of the prefrontal cortex. Deterioration of executive function as

determined by neuropsychological testing may be an indicator of DAT onset in DS.

Future research on this topic could usefully consider matching participants on a wider

range of measures including adaptive and social functioning.

John Rowe et al.14


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Received 20 November 2002; revised version received 4 August 2004


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Cognitive Executive Function in Down’s Syndrome