word-length effect in verbal short-term memory in individuals with down's syndrome

613

Abstract

Background Many studies have indicated that indi-viduals with Down’s syndrome (DS) show a specificdeficit in short-term memory for verbal informa-tion. The aim of the present study was to investigatethe influence of the length of words on verbalshort-term memory in individuals with DS.Methods Twenty-eight children with DS and

control participants matched for memory span weretested on verbal serial recall and speech rate, whichare thought to involve rehearsal and output speed.Results Although a significant word-length effectwas observed in both groups for the recall of alarger number of items with a shorter spoken dura-tion than for those with a longer spoken duration,the number of correct recalls in the group with DSwas reduced compared to the control subjects. Theresults demonstrating poor short-term memory inchildren with DS were irrelevant to speech rate. Inaddition, the proportion of repetition-gained errorsin serial recall was higher in children with DS thanin control subjects.Conclusions The present findings suggest that pooraccess to long-term lexical knowledge, rather than

Correspondence: Kazue Kanno, Doctoral Program of Disability

Sciences, University of Tsukuba, Tsukuba-shi, Ibaraki -,

Japan.

overt articulation speed, constrains verbal short-term memory functions in individuals with DS.

Keywords Down’s syndrome, serial recall, short-term memory, word-length effect

Introduction

The impairments to short-term memory in individ-uals with Down’s syndrome (DS) are considered tobe one of the causes of their inhibited cognitive andlanguage development. Much research has beendone on short-term memory, and some studies haveshown that individuals with DS have poor verbalshort-term memory relative to mental-age-matchedcontrols (Bilvosky & Share ; Rohr & Burr; Pueschel et al. ). It has also been sug-gested that the performance of short-term memoryfor verbal information in these individuals is infe-rior to that for visuospatial information (Marcell &Armstrong ; Kay-Raining Bird & Chapman; Wang & Bellugi ; Jarrold & Baddeley). However, the mechanisms for these impair-ments remain almost completely unknown.

One of the models of short-term memory is that of the phonological loop, a subsystem in theworking memory model developed by Baddeley(). This phonological loop is composed of the

Journal of Intellectual Disability Research

pp –

© Blackwell Science Ltd

Word-length effect in verbal short-term memory in individuals with Down’s syndrome

K. Kanno1 & Y. Ikeda2

1 Doctoral Program of Disability Sciences, University of Tsukuba,Tsukuba-shi, Ibaraki, Japan2 Institute of Disability Sciences, University of Tsukuba,Tsukuba-shi, Ibaraki, Japan


K. Kanno & Y. Ikeda • Word-length effect and short-term memory614

© Blackwell Science Ltd, Journal of Intellectual Disability Research , –

two separate subsystems – namely, a storage com-ponent (the phonological store) and a rehearsalcomponent (the articulatory subvocal rehearsalprocess) – and is considered to be the system sup-porting verbal short-term memory. Speech-basedinformation is time-based and automaticallyaccessed in a phonological store. This informationdecays over time and the memory trace in the storeis continuously maintained by active rehearsal in anarticulatory subvocal rehearsal process.

The findings of several authors point to a word-length effect in verbal short-term recall. Baddeleyet al. () demonstrated that a greater number of items could be recalled when list items had ashorter spoken duration than when they werelonger. This word-length effect was thought toreflect the temporal constraints of the articulatorysubvocal rehearsal process. Because short wordscould be spoken more rapidly, more of them couldbe rehearsed in a given time period (Baddeley et al.). Thus, by investigating the word-length effectin serial recall, it would be possible to determinewhether memory depends on the temporal lengthof the memory items, and whether or not rehearsaloccurs. Overt articulation speed is also consideredto be an important factor in verbal short-termmemory. Many studies have found that speech rateis related to memory span (e.g. Hulme et al. ;Hitch et al. ; Cowan et al. ). Speech rate isthought to be the speed at which stored memoryitems are rehearsed (Hulme et al. ; Hitch et al.) or the output speed of memory items(Cowan et al. ), and it is considered to con-strain the ability of the short-term memory to storeverbal information.

No consensus has been reached on the word-length effect in verbal short-term serial recall byindividuals with DS. Hulme & Mackenzie ()examined the word-length effect in the serial recallof words in participants matched for mental agewith children with DS, children with intellectualdisability (ID) and typically developing children.The above authors found no word-length effect inDS children and indicated that the verbal short-term memory deficit in individuals with DSreflected the absence of rehearsal. On the otherhand, Vallar & Papagno () found a word-lengtheffect in a young DS woman with good languagedevelopment.

One of the factors underlying these disparateconclusions may be a difference in experimentalprocedures. Hulme & Mackenzie () presentedfour-word lists prepared with two different condi-tions: () a long-word condition using words with along pronunciation duration; and () a short-wordcondition using words with a short pronunciationduration. In contrast, Vallar & Papagno ()investigated the word-length effect using lists con-sisting of two to six words, and compared thememory span for each list under both the long- andshort-word conditions. With the procedure used byHulme & Mackenzie (), the lists consisting offour words were used for all participants, and there-fore, a floor effect would be produced in any par-ticipant whose memory span size was smaller thanfour items. If the recall of participants was poor inboth the short- and long-word conditions, then noword-length effect would be seen. Gathercole &Baddeley () investigated the word-length effectin language-disordered children, and showed thatan effect appeared with lists containing up to fiveitems, but this effect disappeared when lists of morethan six items were used. Hall et al. () investi-gated this effect in children who were poor readersand reported the same results, i.e. no effect wasseen with a five-item list. Thus, it may be a pre-sumed that the word-length effect is not preservedwith list lengths which greatly exceed span, andtherefore, it is still too early to conclude from theresults that no word-length effect is seen in childrenwith DS and rehearsal does not occur.

In the present study, a procedure using lists withnumbers of items equal to the memory spans of thestudy participants was adopted. The word-lengtheffect in serial recall of words by individuals withDS was then investigated and compared with typi-cally developing children who were matched formemory span.

Subjects and methods

Participants

The study participants were children with DS( boys and girls) drawn from a special schoolfor children with ID. All participants with DS had akaryotype with a standard trisomy . The partici-pants were chosen to exclude children with hearing




impairments. Furthermore, the word-hearing testfor DS children established by Ishida () wasemployed, and only children with a % word-hearing rate participated in the study. A digit-spantest (Wechsler ) was also administered, and theparticipants were taken from those whose span sizewas three items. The control group consisted of typically developing children of kindergarten age(five boys and five girls). As with the DS group, adigit-span test (Wechsler ) was administeredand only children with a span size of three wereincluded in the control group. Table shows thechronological and mental ages of the participants.Mental age was measured by the Tanaka-BinetIntelligence Scale (TIER ).

Materials and procedure

All children were measured individually duringschool hours in a quiet room in the school. Individ-uals were tested in a single session, first with theserial recall task and then the speech-rate task.

Serial recall of words

The serial recall of words task was done under both the long- and short-word conditions. Memoryitems are usually chosen from a pool of one-syllablewords for the short-word condition and three-syllable words for the long-word condition inEnglish-speaking countries. However, because ofthe difficulty of selecting single-syllable words inthe Japanese language, the words were drawn froma pool of nine, two-syllable words [zou (elephant),tora (tiger), basu (bus), fune (ship), shatsu (shirt),bara (rose), isu (chair), kutsu (shoes) and nashi(pear)], and nine four- to five-syllable words[hikouki (plane), enpitsu (pencil), himawari (sun-

flower), reizouko (refrigerator), nagagutsu (rubberboots), ninjin (carrot), daikon (radish), niwatori(chicken) and kabutomushi (beetle)]. These memoryitems were selected for each condition from thewords to which - and -year-old children respondwhich are included on the Tables of Vocabularyobtained from Japanese Children by Association Method(NLRI ), and matched for word frequency.Three lists consisting of three words were preparedfor each condition. The examiner read aloud a listof words at a rate of one word per second. The par-ticipants were required to repeat two words in prac-tice trials, and this was repeated until they couldrepeat all the presented words in the same order.Three trials were conducted under each condition.

Speech rate

Speech rate was measured for two- and four-syllable words not used in the serial recall task,which came from same Tables of Vocabulary obtainedfrom Japanese Children by Association Method (NLRI). In speech-rate tasks, word pairs are oftenused as the stimulus materials (Hulme & Tordoff), but following indications that this task isvery difficult in individuals with ID (Hulme &Mackenzie ), the present authors did not useword pairs for tests of individuals with DS in thepresent study. For each set of words (two or foursyllables), five single words were prepared. Partici-pants were asked to repeat one two-syllable wordand one four-syllable word times as quickly aspossible. The participants were given a practice trialusing words not included in the memory set beforebeing required to repeat the five sets of single two-and four-syllable words.

Analysis

In the serial recall of words, the number of correctrecalls and types of response were analysed. Thenumber of correct recalls was calculated as themean number of words recalled in the same orderas the lists presented for each condition. Partici-pants responses were classified into five categoriesfor each list, and the proportion of each was calcu-lated. These categories are defined in Table . Inthe speech-rate task, the time for the participants to repeat the words times was recorded, and

Table 1 Mean (± SD) chronological and mental ages of the

participants

Age (years)

Group Chronological Mental

Down’s syndrome (n = 28) 16.10 ± 1.64 6.02 ± 0.98Controls (n = 10) 5.42 ± 0.50 –




the number of words uttered per second was calculated.

Results

Figure compares the number of correct recallsunder the short- and long-word conditions in thegroup with DS and the control subjects. A two-factor analysis of variance (ANOVA) with factors ofGroup (children with DS and typically developingchildren) and Conditions (short and long) showedthat the main effects of Group and Conditions werestatistically significant at the % level (Group,F1,36 = .; Conditions, F(1,36) = .). TheGroup ¥ Conditions interactions were not signifi-cant (F1,36 = ., i.e. short words were recalledmore successfully than long ones by both groups,indicating a word-length effect; the group with DSalso showed a lower number of recalled words thanthe control subjects.

The type of response of the group with DS and control subjects are compared in Fig. . A chi-square test with factors for Group (childrenwith DS and typically developing children) andResponses (correct, order error, repetition-gainederror, one-word omission error and two-word omis-sion error) revealed a bias in Responses that wassignificant at the % level [c2

(4) = .]. A residualanalysis found a higher proportion of correct

responses in the control subjects [residual = .

(> .), P < .]. In repetition-gained error, theproportion in the group with DS was higher [resid-ual = . (> .), P < .]. The control subjectsmade more one-word omission errors, while thegroup with DS made a greater number of repeti-tion-gained errors in the error responses.

Table shows the mean speech rate and standarddeviations of the group with DS and control sub-jects. A two-factor ANOVA with factors for Group(children with DS and typically developing chil-dren) and Word Length (short and long) revealed asignificant main effect of Word Length at a level of% (F1,36 = .). No significant difference wasseen in Group main effect or the Group ¥ WordLength interactions (Group, F1,36 = .; interac-tions, F(1,36) = .). It was shown that the speechrate was slower for the four-syllable words than the

Table 2 Definitions of the five categories of response

Category Definition

Correct response All items in their correct serialorder

Erroneous responses:order Items switch places either adjacently,

or with one or more interveningitems between them

repetitions gained Repetition of an item at recall, anditems which are not in the input listare recalled

one-word omission One word missing from the recallprotocol

two-word omission Two words missing from the recallprotocol

Figure 1 Number of correct recalls by the group with Down’s syn-

drome (�) and the controls (�).

Two-wordomissions

One-wordomissions

Repetitionsgained

Order

Correct

1

0.8

0.6

0.4

0.2

0

Pro

port

ions

of r

espo

nse

type

s

Down’ssyndrome

Controls

Figure 2 Proportions of response types used by the group with

Down’s syndrome and the controls.




two-syllable words in both groups, and that therewas no difference in speech rate between the groupwith DS and control subjects.

Discussion

The purpose of the present study was to investigatethe word-length effect in serial recall by individualswith DS, and to determine whether the verbalshort-term memory of children with DS dependson the time duration of memory items. Both groupswere sensitive to the word length of memory items;recall of lists of long words was poorer than that oflists of short ones. This differs from the results ofHulme & Mackenzie (). The present studyadopted lists containing a number of items equal to the participants’ memory spans. As result, theauthors have shown that a similar word-lengtheffect is seen in both children with DS and typicallydeveloping children matched for memory span.

Then, based on results of the present study, whatshould be considered the deficit of verbal short-term memory in children with DS? The fact that aword-length effect was exhibited in children withDS demonstrates that memory in these individualsdepends on the temporal duration of the memoryitem. However, the children with DS had loweroverall recall accuracy than the control group. It hasbeen pointed out that speech rate, an indicator ofthe rehearsal and output speed of items, is relatedto verbal short-term memory capacity (Hulme et al.; Hitch et al. ; Cowan et al. ). Becauseof the fact that the speech rate of patients withapraxia of speech was much slower than that of acontrol group, Waters et al. () suggested thatthe short-term memory impairment of these

patients was caused by the absence of rehearsal.However, there was no difference in speech ratebetween children with DS and typically developingchildren in the present study, and therefore, itwould seem that the smaller number of correctrecalls by children with DS could not have arisenfrom their slow rehearsal and output speed.

An analysis of the types of responses revealedthat the most common type of error in the controlsubjects was a one-word omission error, while itwas repetition-gained error in the group with DS.Thus, in the typically developing children, thedecay of stored memory items led to the errorresponses, but in the children with DS, errorsoccurred because of repetitions in the presentedmemory items or because they recalled items other than those presented. The type of mistakenresponses in the two groups was clearly different.Moreover, in the group with DS, word replace-ments also occurred in which a presented memoryitem was recalled as a semantically associated word;for example, neko (cat) was recalled as nezumi(mouse), or kabutomushi (beetle) as kuwagata (stagbeetle). These results suggest that their memorydeficit reflects an impairment in the process ofreferring the memory item to lexical knowledge inlong-term memory. Hulme et al. () found thatmemory span is larger with the use of non-wordsthan with use of real words, and indicated thatlong-term memory contributes to memory span.The above authors also emphasized that the repre-sentation of phonological information within long-term memory is crucial in this process. The resultsof the present study do not clearly show whethersemantic coding in long-term lexical knowledge hasan important role in serial recall by children withDS or whether the representation of phonologicalinformation is essential. In the future, it will be nec-essary to investigate verbal short-term memory inchildren with DS using novel words or non-wordswhich do not mediate in their long-term memory.

In conclusion, the present study shows that theserial recall in both children with DS and typicallydeveloping children is constrained by the durationof the memory item. In addition, it seems likelythat the poor short-term memory performance ofthe group with DS did not reflect a slow speechrate, but rather, restricted access to lexical knowl-edge in long-term memory. Future studies on the

Table 3 Mean (± SD) speech rates of the participants

Speech rate

Two-syllable Four-syllableGroup words words

Down’s syndrome (n = 28) 2.09 ± 0.56 1.35 ± 0.36Controls (n = 10) 2.26 ± 0.48 1.49 ± 0.19




effect of long-term lexical knowledge on serial recallin children with DS will be important.

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Accepted May

word-length effect in verbal short-term memory in individuals with down's syndrome

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