Abstract

Introduction

There is a relationship among working memory, executive functions, and language comprehension and production in children and adults. Working memory capacity measures reflect multiple constructs, domain-specific memory storage and domain-general executive processes (Engle and Kane 2004). Working memory span is constrained by controlled attention (Engle et al. 1999), task switching ability (Towse et al. 1998), and inhibition of irrelevant information (Hasher and Zacks 1988). Individual differences on complex working memory tasks reflect not the number of items stored in memory but the differences in simultaneous processing and executive functions (Engle et al. 1999, Miyake et al. 2001). Better working memory performance is the result of greater ability to control attention, suppress irrelevant information, avoid distraction, focus on task-relevant thoughts, and coordinate simultaneous processing and storage (Engle et al. 1999, Lustig et al. 2001, Miyake 2001).

Executive functions are used when individuals need to develop goals, hold these goals in active memory, and monitor performance to achieve their goals (Stuss 1992). Attentional capacity is responsible for sustaining focus on goal information in the context of distracting stimuli. The ability to inhibit irrelevant information is a fundamental mechanism that underlies cognitive development (Dempster 1992). The present study examines working memory in children with SLI within a theoretical framework that suggests an important role for executive functions, particularly for attentional capacity (e.g. Barrouillet and Camos 2001, Kane and Engle 2002, Cowan et al. 2003).

Children with specific language impairment (SLI) perform more poorly than their typically developing peers in working memory tasks where processing and storage are simultaneously required. Their performance indicated a deficit in processing syntactically simple, short sentences (Ellis Weismer et al. 1999) and sentences of various lengths (Montgomery 2000). Accuracy scores dramatically deteriorated with an increase in syntactic complexity and word length.

Processing differences between children with SLI and the control group were found in the coordination of information storage and retrieval within and across modalities (Hoffmann and Gillam 2004). An increase in the amount of information to be processed resulted in a decrease of resource coordination. The authors concluded that significant constraints on information processing for children with SLI involve limitations in general cognitive capacities and in working memory, particularly in executive functions.

Our previous study on working memory and language comprehension in English-speaking children with SLI revealed a larger effect of syntactic complexity than sentence length on performance accuracy (Marton and Schwartz 2003). Both children with SLI and their age-matched peers showed great difficulty in the Listening Span task (ML), where they listened to sentences with various lengths and syntactic complexity and performed simultaneous processing. They recalled a non-word that was embedded in the sentence and answered a question targeting sentence content. The task required from the participants to pay close attention to sentence content while storing the words to be remembered. Most children with SLI were not able to process simultaneously the non-words and the semantic and syntactic information of the sentences thus, they performed more poorly than their age-matched peers.

The effects of syntactic complexity on language comprehension were consistent with adults’ performance on traditional Listening Span tasks. In adults, increased complexity resulted in increased reaction time and decreased repetition accuracy (MacDonald et al. 1992). The authors interpreted their data within the framework of a processing capacity theory (Just and Carpenter 1992) and concluded that processing more complex sentences requires more working memory capacity. Alternatively, the retrieval of words to be remembered in complex sentence contexts may result in prolonging the recall process for a longer time period than in simple sentential contexts. This prolongation may be long enough for some information to be lost (Cowan et al. 2003). The efficiency of sentence processing is highly related to the demands that sentence structure variations place on memory resources (McElree et al. 2003).

Difficulty with simultaneous processing was also evidenced in the List Recall task (LR), where children listened to sets of sentences and recalled the sentence-final words. In this task, children with SLI made frequent interference errors and showed diminished primacy and recency effects (Marton and Schwartz 2003). These children showed difficulty in switching attentional focus according to test requirements. They were not able to switch their attention from encoding to rehearsal and vice versa.

A storage and processing trade-off was used to explain the performance of children with SLI in tasks that focused on functional working memory capacity (Montgomery 2000). The children recalled as many words as possible under three conditions: no load (simple span), single load (storage and one mental operation) and dual load (storage and two mental operations). The children with SLI had a storage capacity similar to their peers’ when the processing requirements were low. As the processing requirements increased (dual load) children with SLI exhibited a trade-off between storage and processing. In the same study, children with SLI comprehended fewer redundant (long) sentences than the age-matched and the vocabulary-matched younger children. The findings were interpreted as further support for a processing deficit in children with SLI.

The overall objective of the current study was to examine whether the differences in the linguistic structures of English and Hungarian impact verbal working memory performance. Verbal working memory is not a language-independent system (Thorn and Gathercole 1999). Just as SLI may be manifested differently cross-linguistically, reflecting the features of a given language (Leonard 1998, Gosy 2002), the present authors expected that verbal working memory deficits would reflect an interaction of the core deficits associated with SLI and language-specific features. Therefore, working memory performance between groups of Hungarian and English-speaking children with and without language impairments was compared. This issue was addressed in two experiments. The stimuli in this study varied in grammatical (syntactic and morphological) complexity and in length.

Hungarian is a language with relatively free word order and a rich agglutinative morphology (Vinkler and Pleh 1995). The first experiment used an adaptation of our ML and LR tasks (Marton and Schwartz 2003) to compare working memory performance of Hungarian children with SLI with our previous findings in English-speaking children. The effect of syntactic complexity and of sentence length on working memory were examined. Of interest was whether Hungarian children with SLI show a similar deficit to English-speaking children in language comprehension with syntactically complex sentences if the memory load is high. There are no previous studies in Hungarian examining the effects of memory load on working memory by comparing syntactic complexity versus sentence length in children with SLI. A group effect was hypothesized and the effect of syntactic complexity on working memory performance, similar to the English-speaking children’s results, was expected. With regard to the multiple constructs of working memory (Engle and Kane 2004), it was hypothesized that the results on the non-word repetition part of the ML task will reflect storage capacity, whereas the answers to the questions and the results of the LR task will indicate processing capacity. Our previous findings on working memory performance in typically developing children and adults indicated that the two dependent measures of the ML task reflect the contribution of storage and processing efficiency separately (Marton et al. 2005). Whereas processing efficiency was influenced by sentence complexity, non-word repetition performance, reflecting the storage function rather than processing, showed a word-length effect.

The second experiment investigated the impact of specific morphological patterns of Hungarian on working memory performance. Given the agglutinative nature of Hungarian, the authors were able to compare performance on short sentences with simple morphological structures with long sentences with simple morphology, and with long sentences with complex morphological structures. It was hypothesized that sentences with complex morphological structures are more demanding on working memory than sentences with simple morphological structures, even when the sentence length is the same. A group effect was expected with each sentence type.

Experiment 1

Results

Modified Listening Span task

Mixed model analyses of variance (ANOVAs) were used to analyse the differences between and within groups for the ML task. Non-word repetition accuracy was analysed with a group × sentence type × word-length design, whereas the independent variables for the answers to the questions were group and sentence type. Effect sizes (d) were calculated and categorized as small effect size: d=0.2; medium, d=0.5; and large, d=0.8 (Cohen 1988).

The results of the Hungarian children in a baseline measure of single non-word repetition were similar to the results of English-speaking children: children with SLI performed more poorly than their age-matched peers (F(1, 91)=55.75, p<0.001, d=1.51). There was also a word-length effect for both groups (F(2, 91)=16.5, p<0.001), but there was no group × word-length interaction (F(2, 91)=1.58, p=0.19). Children with SLI showed lower scores than their typically developing peers at each word length.

In the Modified Listening Span task, children with TLD repeated significantly more non-words correctly than the children with SLI at each word-length and with each sentence type (F(1, 96)=184.5, p<0.001). The effect sizes were large for each sentence type (simple, d=1.74; complex short, d=1.35; complex long, d=1.66). There was a significant word-length effect for both groups (F(2, 96)=118.23, p<0.001). A significant group × word-length interaction was also found (F(2, 96)=4.88, p<0.01). A post-hoc Tukey analysis (p<0.05) revealed no significant difference in non-word repetition accuracy between the two- and three-syllable non-words for the children with TLD, their performance deteriorated only with the four-syllable non-words. In contrast, the children with SLI showed difficulty beginning with the three-syllable non-words (the same group difference for word length was found for the English-speaking children; Marton and Schwartz 2003).

Syntactic complexity and sentence length did not affect non-word repetition accuracy (F(2, 96)=1. 61, p>0.05). Table 2 shows all means and standard deviations for this task. The data from an error analysis indicate that the increase in word length resulted in more complex errors, particularly in the children with SLI. There were more single errors (only one phoneme substitution or deletion) with the shorter words than with the longer ones. The number of multiple errors (more than one error within a word) increased with the increase of word length (figure 1). Thus, the word-length effect was reflected not only in a larger number of errors, but also in more complex errors, especially in the children with SLI.

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Figure 1

Complexity of errors in non-word repetition in the modified Listening Span task. a: Errors by Hungarian children with SLI. b: Errors by Hungarian children with TLD.

Table 2

Means and standard deviations across tasks in Experiment 1 (percentage correct)

Sentence type/group	Non-word repetition	Question answers	List Recall — free	List Recall — order
Simple (SLI): mean (SD)	46.67 (23.21)	72.92 (18.18)	37.46 (15.43)	10.59 (7.23)
Simple (TLD): mean (SD)	75.18 (17.47)	88.61 (18.38)	52.31 (12.53)	25.3 (15.66)
Complex short (SLI): mean (SD)	40.20 (23.26)	67.51 (19.98)	37.23 (13.64)	9.38 (6.59)
Complex short (TLD): mean (SD)	78.33 (19.97)	81.93 (16.76)	54.54 (16.41)	18.46 (12.92)
Complex long (SLI): mean (SD)	37.06 (24.04)	45.83 (21.25)	28.54 (11.46)	5.91(3.69)
Complex long (TLD): mean (SD)	76.67 (19.71)	54.49 (15.06)	43.23 (9.98)	15.23 (9.38)

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The second dependent variable of the ML task, the answers to the questions, was analysed with a factorial ANOVA (group × sentence type). Both independent variables had an effect on performance accuracy (group, F(1, 64)=7.08, p<0.05, d=0.7; sentence type, F(2, 64)=68.51, p<0.001). There was no group × sentence type interaction (F(2, 64)=0.4, p=0.67). Children with SLI performed more poorly than the typically developing children with each sentence type. To examine the effect of different sentence types on performance accuracy, pair-wise comparisons with one-way ANOVAs were used. The results showed significant differences between the simple short and complex long sentences (F(1, 64)=41.2, p<0.001, d=1.68) and between the complex short and complex long sentences (F(1, 64)=21, p<0.001, d=1.34). The effect sizes were large in both cases. There was no significant difference in performance accuracy between the simple short and complex short sentences (F(1, 64)=2.06, p>0.05, d=0.31). These results indicate a larger effect of sentence length than syntactic complexity on language comprehension. Children’s performance in both groups deteriorated more as the length of the sentences increased (table 2).

There were also some qualitative differences between the groups. Most of the errors made by the children with TLD were omissions and semantic substitutions, whereas the children with SLI produced a high number of interference errors (table 3).

Table 3

Error types for questions in the modified listening (ML) task and for item recall in the list recall (LR) task, Experiment 1 (percentage of all errors)

Task/group	Omissions	Semantic substitutions	Phonological substitutions	Interference	Movement errors	Other
ML-SLI	12	10	3	68	–	7
ML-TLD	39	32	2	22	–	5
RL-SLI	28	–	–	49	19	4
RL-TLD	57	–	–	9	28	6

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List Recall task

First, the results were analysed with the free scoring method (each correctly recalled item received a credit regardless of the order of recall) using a factorial ANOVA (group × sentence type). There was a main effect for both variables (group, F(1, 48)=12.54, p<0.01, d=1.18; sentence type, F(2, 48)=9.77, p<0.001). There was no group × sentence type interaction (for means and standard deviations, see table 2). Pair-wise comparisons of sentence types, using one-way ANOVAs, showed that performance accuracy decreased as the length of the sentences increased. List Recall was significantly better for the simple short sentences than for the complex long sentences (F(1, 48)=5.09, p<0.05, d=0.78), and better for the complex short sentences than for the complex long sentences (F(1, 48)=5.62, p<0.05, d=1.09). There was no significant difference in list recall between the simple short and complex short sentences (F(1, 48)=2.4, p>0.05, d=0.09). Children with SLI performed more poorly than the children with TLD with each sentence type. Their performance pattern across sentence types was similar to that of the typically developing children. Thus, the results are comparable to our findings with the ML task.

Children in both groups showed lower performance scores when the order of recall was also considered. The analysis with the order scoring method showed significant differences between the groups (F(1, 48)=4.27, p<0.05, d=1.12), but not within groups. Children with SLI performed more poorly than their peers. There was no main effect for sentence type (F(2, 48)=0.91, p>0.05).

Similar to the results of the ML task, the error analysis revealed different patterns for the groups. Most of the errors made by the children with SLI were interference errors. In contrast, the main error category for the children with TLD was ‘omissions’ (table 3).

Experiment 2

Results

The results of word repetition were analysed with a factorial ANOVA (group × sentence type) for main effects. There was a significant difference in performance accuracy between the children with TLD and SLI (F(1, 144)=131.65, p<0.001, d=2.78) and across sentence types (F(2, 144)=16.77, p<0.001). (See means and standard deviations in table 5.) The effect sizes were large with each sentence type: d=2.59 for the short sentences with simple morphology, d=3.49 for the long sentences with complex morphemes, and d=2.13 for the long sentences with simple morphemes. There was also a significant interaction for group and sentence types (F(2, 144)=6.31, p<0.01). The post-hoc Tukey test (p<0.05) revealed group differences with each sentence type. There was a significant difference in word repetition accuracy for both groups between the sentences with simple and complex morphology. There was no difference between the morphologically simple short and long sentences. Thus, morphological complexity and not sentence length affected performance.

Table 5

Means and standard deviations for word repetition (WR) and answers (ANS) to the questions (percentage correct) across sentence types in Experiment 2

	WR ST 1	WR ST2	WR ST3	ANS ST 1	ANS ST2	ANS ST3
SLI: mean (SD)	56.48 (18.07)	29.68 (19.53)	65.72 (16.76)	87.47 (13.38)	75.73 (16.98)	85.87 (10.4)
TLD: mean (SD)	92.8 (8.14)	85.04 (11.04)	93.76 (8.2)	97.6 (3.79)	92 (6.9)	95.73 (6.1)

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ST1, short sentences, simple morphology; ST2, long sentences, complex morphology; ST3, long sentences, simple morphology.

Similar to Experiment 1, the main error categories for children with TLD were omissions and substitutions, whereas the children with SLI produced a high number of interference errors (for error type differences, see table 6).

Table 6

Error patterns (per cent of all errors) of word repetition across sentence types in Experiment 2

Sentence type/group	Word omission	Grammatical morpheme omission	Semantic substitution	Phonological substitution	Grammatical morpheme substitution	Interference	Other
ST1-SLI	7	–	0	3	–	90	0
ST1-TLD	68	–	0	0	–	27	5
ST2-SLI	13	17	0	1	15	49	5
ST2-TLD	18	28	0	2	39	13	0
ST3-SLI	18	–	4	2	–	73	3
ST3-TLD	66	–	4	4	–	21	5

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ST1, short sentences, simple morphology; ST2, long sentences, complex morphology; ST3, long sentences, simple morphology.

With a further analysis, the production of the stem (base morpheme) was compared with the whole word (base morpheme plus inflections) in order to see whether children make errors with both the stem and the inflections or they repeat the correct stem but have difficulty with the inflections. There was a significant main effect for group (F(1, 96)=105.86, p<0.001, d=2.07); for morpheme type error (F(1, 96)=20.85, p<0.001, d=1.05), and there was an interaction between group and morpheme type error (F(1, 96)=6.54, p<0.01). A post-hoc Tukey test (p<0.05) showed that the children with SLI performed significantly better in repeating the stem than the whole word with inflections; there were no significant within group differences for the children with TLD (figure 2).

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Figure 2

Complex morpheme repetition in Experiment 2.

One could argue that the differences in word repetition accuracy may not only reflect morphological complexity differences, but a word-length effect because the morphologically complex words were four-syllables long, whereas the simple words were only two-syllables long. Therefore, the effect of morphological complexity on working memory performance was examined by analysing the answers to the questions. The results of a Factorial ANOVA (group × sentence type) showed a main effect for group (F(1, 96)=48.71, p<0.001, d=1.16); for sentence type (F(2, 96)=5.14, p<0.01); and there was a group × sentence type interaction (F(2, 96)=5.66, p<0.01). A post-hoc Tukey test (p<0.05) showed that the children with SLI performed significantly better in answering the questions following the sentences with simple morphology than the ones with complex morphemes. There was no difference in their answers between the short and long sentences with simple morphology. Thus, the pattern was similar to word repetition. The children with TLD answered each question with high accuracy (table 5).

General discussion

The differences between English and Hungarian had an impact on working memory performance in each group (SLI, TLD). The current results clearly support the language-specific effects of verbal working memory functions (Thorn and Gathercole 1999). Experiment 1 revealed better performance accuracy when the sentences were short, either syntactically simple or complex, than when they were long. There was no significant difference between the simple short and complex short sentences. These results differed from our previous findings with English-speaking children. Those children’s verbal working memory performance on similar tasks was more affected by syntactic complexity than sentence length (Marton and Schwartz 2003). Experiment 2 examined the relative influence of sentence length and morphological complexity on working memory performance. It was hypothesized that an increase in morphological complexity caused the decrease in working memory performance accuracy. Previous findings (Engle et al. 1999, Lustig et al. 2001, Miyake 2001) evidenced that working memory performance depends more on the ability to control attention, to suppress irrelevant information and to focus on task-relevant goals than on the ability to store more items. Thus, the simple increase of the number of words in a sentence, without an increase in grammatical complexity, does not influence performance accuracy to the same extent as the increase in morphological complexity. In Experiment 2, children recalled more words and answered more questions correctly following sentences with simple morphological structures than with complex morphology. There was no difference in performance accuracy between the short and long sentences, when morphology was simple. The results of the current study and previous findings (Marton and Schwartz 2003) suggest a larger impact of linguistic complexity than sentence length on performance accuracy. However, complexity affected working memory performance differently across languages. In English, it was the increase in syntactic complexity that resulted in a decrease in working memory performance accuracy. In Hungarian, a language with a rich morphological system, it was the increase in morphology that resulted in decreased performance accuracy, particularly in the children with SLI.

These findings raise issues regarding the nature and role of working memory in language comprehension. Various models of working memory provide different characterizations of the work-space or storage that might be devoted to language processing in general or more specifically to processing words, syntax, and morphosyntax (e.g. Just and Carpenter 1992, Ellis Weismer et al. 1999). The consistency of complexity effects, manifested differently across these languages, suggests that the deficit may best be described by those models that posit a workspace with limited capacity rather than a more traditional capacity-limited storage for material. Tasks that more directly examine memory processes during sentence comprehension may shed additional light on these mechanisms.

The children with TLD produced errors that were predominantly omissions and semantic substitutions comparable to other reported findings (McCormack et al. 2000). As the demands on working memory increased, typically developing children forgot some information or substituted a synonym for the target word. In contrast, the children with SLI produced a high number of interference errors. These errors indicate a deficit in executive functions in children with SLI. Further research is needed to examine whether the errors reflect a problem in suppressing irrelevant information, in attention switching, or in keeping task goals active. There is evidence from an electrophysiological study of lack of localization in brain activity in discourse processing that may be interpreted as poor inhibition (Shafer et al. 2000). Alternatively, these children’s limitations in working memory capacity may have prevented them from keeping task goals active resulting in an apparent deficit in flexibility. The present results are consistent with those of Hoffmann and Gillam (2004), who found that children with SLI show difficulty in coordinating information-processing resources.

The findings of the current study clearly show that children with SLI have difficulty in working memory tasks that are highly demanding on executive functions. It was also evidenced that working memory performance depends on the linguistic characteristics of the language tested. In both English- and Hungarian-speaking children linguistic complexity seems to have a larger effect on verbal working memory performance than the length of the sentences. However, complexity appears to affect working memory performance accuracy differently across languages. Studies examining working memory and language comprehension in children with SLI in languages other than English need to consider the language specific characteristics of that particular language. Furthermore, the results clearly show that sentence complexity has a larger impact on working memory performance than the length of the sentences. Clinicians, who work with children with language processing difficulties, need to consider that even short sentences can be highly demanding if their structure is not simple.

Acknowledgments

Appendix

References