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Spatial-simultaneous and spatial-sequential working memory in individuals with Down syndrome: The effect of configuration
Research in Developmental Disabilities 34 (2013) 669–675
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Research in Developmental Disabilities
Spatial-simultaneous and spatial-sequential working memory in individuals with Down syndrome: The effect of configuration Barbara Carretti a,*, Silvia Lanfranchi b, Irene C. Mammarella b a b
Department of General Psychology, University of Padova, Padova, Italy Department of Developmental Psychology and Socialization, University of Padova, Padova, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 July 2012 Received in revised form 12 September 2012 Accepted 14 September 2012 Available online 1 November 2012
Earlier research showed that visuospatial working memory (VSWM) is better preserved in Down syndrome (DS) than verbal WM. Some differences emerged, however, when VSWM performance was broken down into its various components, and more recent studies revealed that the spatial-simultaneous component of VSWM is more impaired than the spatial-sequential one. The difficulty of managing more than one item at a time is also evident when the information to be recalled is structured. To further analyze this issue, we investigated the advantage of material being structured in spatial-simultaneous and spatial-sequential tasks by comparing the performance of a group of individuals with DS and a group of typically-developing children matched for mental age. Both groups were presented with VSWM tasks in which both the presentation format (simultaneous vs. sequential) and the type of configuration (pattern vs. random) were manipulated. Findings indicated that individuals with DS took less advantage of the pattern configuration in the spatial-simultaneous task than TD children; in contrast, the two groups’ performance did not differ in the pattern configuration of the spatial-sequential task. Taken together, these results confirmed difficulties relating to the spatial-simultaneous component of VSWM in individuals with DS, supporting the importance of distinguishing between different components within this system. The findings are discussed in terms of factors influencing this specific deficit. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: Down syndrome Visuospatial working memory Configuration Spatial-simultaneous Spatial-sequential
1. Introduction Down syndrome (DS) is due to abnormalities on chromosome 21 and it is the most common cause of intellectual disability (Kittler, Krinsky-McHale, & Devenny, 2008). The IQ generally ranges between 25 and 70, with few individuals reaching a mental age beyond 7–8 years. The cognitive functioning of individuals with DS is characterized by speech and language impairments (Chapman & Hesketh, 2000) and their difficulties are often greater in expressive language than in language comprehension. Their non-verbal skills are usually less impaired than their verbal abilities and much the same pattern is observable in their performance in working memory (WM) tasks. In particular, several studies found that children (Jarrold & Baddeley, 1997), adolescents (Hulme & Mackenzie, 1992; Marcell & Weeks, 1988), and adults (Kittler, KrinskyMcHale, & Devenny, 2004; Kittler et al., 2008; Numminen, Service, Ahonen, & Ruoppila, 2001) with DS had a weaker verbal
* Corresponding author. E-mail address: [email protected] (B. Carretti). 0891-4222/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ridd.2012.09.011
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WM than individuals of the same mental age with severe intellectual disabilities or typical development (TD). On the other hand, most of the research on visuospatial working memory (VSWM) suggested that individuals with DS obtain the same scores as TD children matched for mental age (Jarrold & Baddeley, 1997; Laws, 2002; Numminen et al., 2001). It is important to remember, however, that VSWM is not conceived as a unitary system. According to the Logie (1995) model, it consists of a visual store (known as the visual cache) and a rehearsal mechanism (known as the inner scribe). Consistent with this distinction, there is a large body of evidence to support a dissociation between visual and spatial memory, based on studies using the paradigm of selective interference (Della Sala, Gray, Baddeley, Allamano, & Wilson, 1999; Klauer & Zhao, 2004; Quinn & McConnell, 1996) and on neuropsychological (Carlesimo, Perri, Turriziani, Tomaiuolo, & Caltagirone, 2001; Farah, Hammond, Levine, & Calvanio, 1988; Luzzatti, Vecchi, Agazzi, Cesa-Bianchi, & Vergani, 1998), and developmental studies (Gathercole & Pickering, 2000; Hamilton, Coates, & Heffernan, 2003; Logie & Pearson, 1997; Pickering, Gathercole, Hall, & Lloyd, 2001; Pickering, Gathercole, & Peaker, 1998). Lecerf and de Ribaupierre (2005) considered three VSWM components rather than two, including an extra-figural encoding responsible for anchoring objects in relation to an external frame of reference, and an intra-figural encoding based on the relations of each item within a pattern, further broken down into pattern encoding (leading to a global visual image), and path encoding (leading to spatial-sequential positions). Pazzaglia and Cornoldi (1999) and Mammarella, Pazzaglia, and Cornoldi (2008) likewise suggested distinguishing between three components: a visual component in charge of processing shapes and colors, and two kinds of spatial component, both involved in memorizing patterns of spatial locations but presenting them in a different format and consequently using different spatial processes, simultaneous in one case, sequential in the other. Evidence collected with various groups of children supported the distinction between visual and spatial-simultaneous processes (Mammarella, Cornoldi, & Donadello, 2003) and between spatial-simultaneous and spatialsequential processes (Mammarella et al., 2006). The possibility of the pattern of performance differing depending on the VSWM components considered also emerges from the literature on individuals with DS. For instance, an early study by Ellis, Woodley Zanthos, and Dulaney (1989) found that individuals with DS have an unimpaired spatial memory, but an impaired visual memory. A subsequent study by Laws (2002) found that individuals with DS performed significantly better than TD controls matched for receptive vocabulary in the Corsi blocks task (testing spatial working memory), while the two groups’ performance did not differ significantly in a memory for color task (testing visual working memory). Vicari, Bellucci, and Carlesimo (2006) reported individuals with DS having a worse spatial and visual working memory (WM) span than TD individuals, but the difference disappeared after controlling for the role of perceptual and visual abilities. More recently, Lanfranchi, Carretti, Spano`, and Cornoldi (2009) examined the performance of individuals with DS in spatial WM tasks distinguished in terms of presentation format (simultaneous vs. sequential) and demand for controlled attention. The results reported by Lanfranchi et al. (2009) showed that individuals with DS fared worse than TD children of the same mental age on spatial-simultaneous WM tasks, but not on spatial-sequential tasks, thus supporting the distinctions in VSWM drawn by Pazzaglia and Cornoldi (1999). Lanfranchi et al. tentatively interpreted the DS individuals’ worse performance on spatial-simultaneous WM tasks as being due to the difficulty of processing more than one item at a time. This hypothesis finds support in the results of a study by Carretti and Lanfranchi (2010), who showed that individuals with DS were unable to take advantage of structured materials in spatial-simultaneous WM tasks. The task they used contained a confounder, however: the characteristics of the structured pattern were not appropriately controlled because some, but not all, of the pattern could lead to a phonological recoding of the stimuli. This is a point of particular importance because one of the explanations for their results concerned the different use of strategies by DS individuals and TD children. To study these aspects in depth, we analyzed in the present study whether individuals with DS are as able as TD children of the same mental age to take advantage of structured material in spatial-simultaneous and spatial-sequential tasks. Two versions of spatial-sequential and spatial-simultaneous tasks were devised, i.e. pattern configurations (in which the filled cells were grouped to create a pattern), and random configurations (in which the filled cells were separated by spaces and did not create patterns). Unlike the material used by Carretti and Lanfranchi (2010), all the patterns were controlled to avoid the use of phonological recoding strategies. In particular, patterns similar to the shapes of letters were avoided. The main aim of the present study was therefore to analyze whether individuals with DS have a general problem in using structured material (i.e. pattern configurations) to memorize information, or whether this problem applies specifically to spatial-simultaneous WM tasks. Previous findings suggested that structured materials are generally easier to recall than non-structured materials. For example, Attneave (1955) found that memory performance for regular figures was better than for irregular figures. To be more specific, he varied the symmetry in the stimulus, observing memory performance in immediate reproduction, delayed reproduction and recognition paradigms. He demonstrated that symmetrical patterns were always remembered better than asymmetrical patterns containing the same number of cells, and that random patterns were more difficult to remember. More recently, Kemps (2001) examined quantitative and structural complexity using a spatial-sequential task (i.e. the Corsi blocks task) and demonstrated that memory span correlated inversely with the number of blocks in a sequence (quantitative complexity), but it improved when the blocks were put in a matrix rather than a random pattern (structural complexity). Judging from these results, TD children will plausibly take advantage of structured materials in both spatial-simultaneous and spatial-sequential tasks, while we would expect less of an improvement in the performance of individuals with DS, particularly in the spatial-simultaneous tasks (in which they revealed greater difficulties) (Carretti & Lanfranchi, 2010; Lanfranchi et al., 2009).
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Table 1 Participants’ characteristics. Down syndrome
Typical development
M
SD
M
SD
PPVT-R
Raw score Mental age*
56.20 61.90
24.38 22.00
58.00 61.20
18.85 20.00
CPM
Raw score Mental age*
14.05 59.70
4.20 16.00
15.15 64.20
1.98 7.00
Note. PPTV-R, Peabody Picture Vocabulary Test – Revised; CPM, Raven’s colored matrices. * Age is expressed in months.
2. Method 2.1. Participants Twenty individuals with DS took part in the study. Their mean chronological age was 14 years and 2 months (SD = 29 months, range 9 years and 5 months, 17 years and 11 months), and their mean mental age was 5 years and 2 months (SD = 62 months). A control group consisted of 20 TD children with a mean chronological age of 5 years and 5 months (SD = 5 months). The two groups were matched one to one on the raw scores they obtained with the Peabody Picture Vocabulary Test – Revised (PPVT-R, Dunn & Dunn, 1997) (see Table 1). In addition, since individuals with DS show a discrepancy between their verbal and nonverbal abilities, the Raven’s Colored Progressive Matrices (CPM, Raven, Court, & Raven, 1998) were administered to both groups to measure their nonverbal abilities. The two groups’ performance was comparable in both the PPTV-R (F < 1) and the Raven F(1,39) = 1.12, p = .29. 2.2. Materials Visuospatial memory task. The task consisted of a series of matrices presented on a computer screen in which an increasing number of cells were filled. The size of the matrices varied with the number of items to recall (3 4 matrices for levels 1, 2, 3 and 4; 4 4 for levels 5, 6 and 7; and 4 5 for level 8). Each cell in the matrix measured 3 cm 3 cm. Two trials were presented for each level of difficulty. Each task progressed from the shortest lists (containing 1 position to recall) to the longest (maximum 8 positions). To avoid generating frustration, the task was terminated if a participant failed on both lists of the same length, in which case it was assumed that the remaining items would not be recalled correctly. The following conditions were manipulated: (1) the arrangement of the filled cells either in a visual configuration (or pattern), or at random; and (2) the presentation format, which could be simultaneous (requiring the recall of simultaneously presented information) or sequential (involving the recall of items presented one at a time). In the simultaneous condition (see Fig. 1 for an example of the stimuli), each matrix was displayed on a computer screen for 5000 ms, then participants were asked to recall the location of the filled cells. In the sequential condition, each position was displayed for 1000 ms, preceded and followed by an empty matrix of 500 ms. For both conditions, a blue screen appeared for 1000 ms to signal the end of the stimuli’s presentation, then participants were asked to remember the positions without any order constraints, and to point to them on a blank grid. For all conditions, the partial credit unit score method (see Conway et al., 2005) was used to calculate the final score: each position correctly recalled was awarded 1 point, and the mean proportion of the recalled items was calculated on the maximum number of positions presented. 2.3. Procedure During a first session, participants completed the CPM test and PPVT-R. The raw score for the latter test obtained by each individual with DS was used to identify the matching child in the control group, while the CPM scores were used as a measure of the two groups’ non-verbal abilities. Participants subsequently completed the VSWM task. All tasks were administered individually in two sessions with an interval of approximately 1 week between them, each session lasting approximately 25 min. In the second session, the presentation order of the tasks was counterbalanced across participants. 3. Results A 2 2 2 repeated measures ANOVA was performed on the proportion of information recalled, with group (DS vs. TD group) as the between-group variable, and presentation format (spatial-sequential vs. spatial-simultaneous) and configuration (pattern vs. random) as within-group factors (see Fig. 2). The results showed a significant main effect of presentation format F(1,38) = 120.78, MSE = .009, h2p ¼ :76, p < .001, performance being better in the spatial-simultaneous task than in the spatial-sequential one. The main effect of configuration was also significant F(1,38) = 102.72, MSE = .008, h2p ¼ :73, p < .001 with better performances in the pattern than in the random condition. A significant interaction emerged
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Fig. 1. Examples of items in the spatial-simultaneous task. The pattern configuration is shown on the left, the random one on the right.
Fig. 2. Mean proportion of positions recalled by DS and TD individuals as a function of presentation format (simultaneous vs. sequential) and configuration (random vs. pattern). Error bars represent standard errors of the mean.
for the group x presentation format F(1,38) = 7.02, MSE = .009, h2p ¼ :16, p < .05. Post hoc comparisons with the Tukey method, adjusted for multiple comparisons with Bonferroni corrections, indicated that: DS individuals performed marginally less well than the TD group in spatial-simultaneous tasks (Mdiff = .07, p = .065); and both groups did better in the spatial-simultaneous task than in the spatial-sequential one (p < .001 for both groups). The presentation format configuraconfiguration interaction was also significant, F(1, 38) = 45.25, MSE = .005, h2p ¼ :54, p < .001, the pattern condition coinciding with a better performance than the random condition in both spatial-simultaneous and spatial-sequential tasks (for all comparisons, p < .001). The third-order group presentation format configuration interaction F(1,38) = 11.22, MSE = .005, h2p ¼ :23, p < .001 was significant too (see Fig. 2). Post hoc comparisons with the Tukey method adjusted for multiple comparisons with Bonferroni corrections, showed that the DS individuals’ performance was only worse than the TD children’s in the spatialsimultaneous task in the pattern condition (Mdiff = .167, p < .01). Both groups took advantage of the pattern configuration in spatial-sequential and spatial-simultaneous tasks (for all comparisons, p < .001). No other differences were significant. To better understand the results, 2 separate ANOVA were conducted (one for spatial-sequential and one for spatialsimultaneous tasks) 2 (group: DS vs. TD) 2 (configuration: pattern vs. random). The results for the proportion of correctly-recalled information in the simultaneous task showed a main effect of configuration, F(1,38) = 120.59, MSE = .012, h2p ¼ :76, p < .001 (the pattern condition coinciding with a better performance than the random condition), and a significant group configuration interaction, F(1,38) = 8.89, MSE = .012, h2p ¼ :19, p < .01, with the TD group outperforming the group of individuals with DS in the pattern condition (Mdiff = .167 p < .01). Both groups’ performance was better in the pattern than in the random condition, however (TD Mdiff = .347, p < .001; DS Mdiff = .199, p < .001). The effect of group was not significant.
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Fig. 3. Performance of DS and TD individuals as a function of condition (random vs. pattern) and memory load in spatial-simultaneous tasks. Error bars represent standard errors of the mean.
The ANOVA on the spatial-sequential task revealed a main effect of configuration, F(1,38) = 16.06, MSE = .008, h2p ¼ :29, p < .001, with an advantage for the pattern condition. No other effects were significant. 3.1. Spatial-simultaneous task: the effect of configuration as a function of memory load The changes occurring across the different levels of difficulty were analyzed to better understand the results obtained in the spatial-simultaneous task. To do so, a 2 8 repeated measures ANOVA was run on the score obtained at each trial with group (DS vs. TD) as the between-group variable, and memory load (1 vs. 2 vs. 3 vs. 4 vs. 5 vs. 6 vs. 7 vs. 8) and configuration (pattern vs. random) as within-group factors (see Fig. 3). The main effect of configuration F(1,38) = 134.01, MSE = .39, h2p ¼ :78, p < .001 was significant, replicating the advantage of the structured over the random arrangement, and so was the main effect of memory load F(7,266) = 151.09, MSE = .39, h2p ¼ :23, p < .001, showing that in cases where performance did not differ between Levels 1 and 2, the same applied when Level 2 was compared with Level 3, but performance at Level 3 was lower than at Level 1 (Mdiff = .14, p < .05). Subsequent levels differed significantly from the first three (p < .001) and with respect to each other (p < .001), suggesting a linear decline with increasing amounts of information to recall. The configuration group interaction F(1,38) = 10.70, MSE = .39, h2p ¼ :22, p < .01 was also significant, and so was the interaction between configuration and memory load F(7,266) = 17.69, MSE = .24, h2p ¼ :32, p < .001. Both were qualified by the third-order group configuration memory load interaction F(7,266) = 2.82, MSE = .24, h2p ¼ :07, p < .01. Post hoc comparisons with the Tukey method, adjusted for multiple comparisons with Bonferroni corrections, showed that DS individuals differed from TD children in the pattern condition at Levels 6 and 7 (Mdiff = .626, p < .01 and Mdiff = .821, p < .01, respectively); and the differences were marginally significant at Levels 4, (Mdiff = .200, p = .088), 5 (Mdiff = .300, p = .087) and 8 (Mdiff = .468, p = .098) as well. It also emerged that both groups took advantage of the structured condition from Level 4 onwards (p < .01 for all comparisons) and that increasing amounts of information to recall coincided with a decreasing performance. Two separate, repeated measures ANOVA for group memory load were run to further elucidate the results, distinguishing by condition (pattern vs. random). In the pattern condition, the effect of group was significant, F(1,38) = 8.52, MSE = 1.09, h2p ¼ :18, p < .01, with the TD group performing better than the group of individuals with DS; so was the main effect of memory load, F(7,266) = 31.33, MSE = .21, h2p ¼ :45, p < .001, a greater memory load impairing the performance of both groups with a quadratic trend. The group memory load interaction was also significant F(7,266) = 3.85, MSE = .21, h2p ¼ :09, p < .01, replicating the previously-reported results. When the random condition was considered, only the main effect of memory load was significant, F(7,268) = 151.49, MSE = .18, h2p ¼ :79, p < .001, with performance decreasing linearly with the increase in the amount of information to recall. 4. Discussion and conclusion The aim of this study was to shed light on how VSWM functions in individuals with DS by analyzing the role of presentation format and item arrangement. Previous results focusing on the distinction between simultaneous and sequential components of VSWM highlighted that DS individuals performed as well as TD children matched for mental age in spatial-sequential tasks,
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but not in spatial-simultaneous tasks (Lanfranchi et al., 2009). Their greater difficulties in spatial-simultaneous WM tasks remained even when the information to be recalled was presented in a structured format (Carretti & Lanfranchi, 2010). The results of the present study confirmed that individuals with DS may find spatial-simultaneous processing difficult, paradoxically in the more advantageous condition, i.e. when the items to remember are arranged in a pattern. In contrast with the findings reported by Lanfranchi et al. (2009), the two groups did not differ in the random condition. It should be noted, however, that the differences between the TD and DS groups in the Lanfranchi et al. paper appeared to be stronger in the active version of the spatial-simultaneous task (though no direct comparison was drawn by the authors), which was not used in the present study. The size of the matrices was also larger in the present study, making it difficult to compare the two reports directly. Consistently with Carretti and Lanfranchi (2010), our DS sample took less advantage of the structured arrangement of the information than the TD children of matching mental age, although both groups performed better in the pattern than in the random condition. A strategic deficit in individuals with DS could partly account for this result, i.e. they could be unable to use patterns to help them recall positions in spatial-simultaneous tasks. This would be consistent with other studies using verbal short-term memory tasks and showing that individuals with DS do not spontaneously use memory strategies such as rehearsal (e.g. Hulme & Mackenzie, 1992), though it has been demonstrated that their use of rehearsal can be improved by specific training (e.g. Broadley & MacDonald, 1993). It should be noted, however, that no differences were seen for the spatial-sequential task, in with both groups performed equally well. This finding partially rules out any role of strategies in explaining the difficulties encountered by individuals with DS, since the advantage they gained from a pattern configuration in the spatial-sequential tasks was much the same as in the TD group. Another possibility is that individuals with DS performed less well than TD children in the pattern condition of the spatial-simultaneous task due to visual perception problems. They could suffer from visual crowding, for example, making it difficult for them to distinguish between different items of information. Previous research showed that individuals with DS take a global approach to analyzing visuospatial stimuli and often make mistakes that indicate a failure to perceive or faulty perception of details in visuospatial tasks (Bellugi, Lichtenberger, Jones, Lai, & St George, 1999). A possible explanation for our present results might therefore lie in the DS individuals’ difficulties in recalling the exact location of the perceived pattern in the matrices, i.e. they can see the patterns just as well as the TD children, but this does not necessarily mean they can remember them. It is worth adding that our analysis of memory load revealed a deterioration in the DS individuals’ performance when 6 or 7 cells had to be recalled in 4 4 matrices, so it would seem that they no longer take advantage of their ability to perceive the information as a whole when this information exceeds their memory capacity. The role of perception in VSWM performance has already been explored. To give an example, Vicari et al. (2006) demonstrated that individuals with DS no longer differed in spatial and visual tasks from TD children matched for mental age when measures of visual and spatial perception were taken into account. The spatial task they used was more complex than the one used in the present study, however, since participants were asked to remember the positions of non-verbalizable geometrical figures, which added a further source of difficulty. In fact, Vicari et al. found differences between the DS and TD groups even though the positions of the figures they used were presented sequentially (as in our spatial-sequential task), suggesting that no direct comparisons can be drawn between their results and ours. From a theoretical point of view, the present results support the importance of distinguishing between different components in VSWM, as suggested by Pazzaglia and Cornoldi (1999), and demonstrated in individuals of various ages (e.g. Mammarella, Borella, Pastore, & Pazzaglia, in press; Mammarella et al., 2008). In fact, we found that individuals with DS performed just as well as TD children of the same mental age in spatial-sequential tasks, both in the random and in the pattern condition, while the same was not the case for spatial-simultaneous tasks, giving the impression that the processing of spatial-sequential and spatial-simultaneous stimuli differs not only in typically-developing populations, but also in individuals with DS. In conclusion, our results clearly showed that the profile of individuals with DS in tasks assessing VSWM is not homogeneous. Individuals with DS demonstrated a more limited advantage in spatial-simultaneous tasks using structured material, while this was not the case when spatial-sequential tasks were considered. Future research should try to shed light on the factors that might account for these results, from the most basic (relating to visual perception skills) to the most complex (relating to the use of strategies). 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Contents lists available at SciVerse ScienceDirect
Research in Developmental Disabilities
Spatial-simultaneous and spatial-sequential working memory in individuals with Down syndrome: The effect of configuration Barbara Carretti a,*, Silvia Lanfranchi b, Irene C. Mammarella b a b
Department of General Psychology, University of Padova, Padova, Italy Department of Developmental Psychology and Socialization, University of Padova, Padova, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 July 2012 Received in revised form 12 September 2012 Accepted 14 September 2012 Available online 1 November 2012
Earlier research showed that visuospatial working memory (VSWM) is better preserved in Down syndrome (DS) than verbal WM. Some differences emerged, however, when VSWM performance was broken down into its various components, and more recent studies revealed that the spatial-simultaneous component of VSWM is more impaired than the spatial-sequential one. The difficulty of managing more than one item at a time is also evident when the information to be recalled is structured. To further analyze this issue, we investigated the advantage of material being structured in spatial-simultaneous and spatial-sequential tasks by comparing the performance of a group of individuals with DS and a group of typically-developing children matched for mental age. Both groups were presented with VSWM tasks in which both the presentation format (simultaneous vs. sequential) and the type of configuration (pattern vs. random) were manipulated. Findings indicated that individuals with DS took less advantage of the pattern configuration in the spatial-simultaneous task than TD children; in contrast, the two groups’ performance did not differ in the pattern configuration of the spatial-sequential task. Taken together, these results confirmed difficulties relating to the spatial-simultaneous component of VSWM in individuals with DS, supporting the importance of distinguishing between different components within this system. The findings are discussed in terms of factors influencing this specific deficit. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: Down syndrome Visuospatial working memory Configuration Spatial-simultaneous Spatial-sequential
1. Introduction Down syndrome (DS) is due to abnormalities on chromosome 21 and it is the most common cause of intellectual disability (Kittler, Krinsky-McHale, & Devenny, 2008). The IQ generally ranges between 25 and 70, with few individuals reaching a mental age beyond 7–8 years. The cognitive functioning of individuals with DS is characterized by speech and language impairments (Chapman & Hesketh, 2000) and their difficulties are often greater in expressive language than in language comprehension. Their non-verbal skills are usually less impaired than their verbal abilities and much the same pattern is observable in their performance in working memory (WM) tasks. In particular, several studies found that children (Jarrold & Baddeley, 1997), adolescents (Hulme & Mackenzie, 1992; Marcell & Weeks, 1988), and adults (Kittler, KrinskyMcHale, & Devenny, 2004; Kittler et al., 2008; Numminen, Service, Ahonen, & Ruoppila, 2001) with DS had a weaker verbal
* Corresponding author. E-mail address: [email protected] (B. Carretti). 0891-4222/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ridd.2012.09.011
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WM than individuals of the same mental age with severe intellectual disabilities or typical development (TD). On the other hand, most of the research on visuospatial working memory (VSWM) suggested that individuals with DS obtain the same scores as TD children matched for mental age (Jarrold & Baddeley, 1997; Laws, 2002; Numminen et al., 2001). It is important to remember, however, that VSWM is not conceived as a unitary system. According to the Logie (1995) model, it consists of a visual store (known as the visual cache) and a rehearsal mechanism (known as the inner scribe). Consistent with this distinction, there is a large body of evidence to support a dissociation between visual and spatial memory, based on studies using the paradigm of selective interference (Della Sala, Gray, Baddeley, Allamano, & Wilson, 1999; Klauer & Zhao, 2004; Quinn & McConnell, 1996) and on neuropsychological (Carlesimo, Perri, Turriziani, Tomaiuolo, & Caltagirone, 2001; Farah, Hammond, Levine, & Calvanio, 1988; Luzzatti, Vecchi, Agazzi, Cesa-Bianchi, & Vergani, 1998), and developmental studies (Gathercole & Pickering, 2000; Hamilton, Coates, & Heffernan, 2003; Logie & Pearson, 1997; Pickering, Gathercole, Hall, & Lloyd, 2001; Pickering, Gathercole, & Peaker, 1998). Lecerf and de Ribaupierre (2005) considered three VSWM components rather than two, including an extra-figural encoding responsible for anchoring objects in relation to an external frame of reference, and an intra-figural encoding based on the relations of each item within a pattern, further broken down into pattern encoding (leading to a global visual image), and path encoding (leading to spatial-sequential positions). Pazzaglia and Cornoldi (1999) and Mammarella, Pazzaglia, and Cornoldi (2008) likewise suggested distinguishing between three components: a visual component in charge of processing shapes and colors, and two kinds of spatial component, both involved in memorizing patterns of spatial locations but presenting them in a different format and consequently using different spatial processes, simultaneous in one case, sequential in the other. Evidence collected with various groups of children supported the distinction between visual and spatial-simultaneous processes (Mammarella, Cornoldi, & Donadello, 2003) and between spatial-simultaneous and spatialsequential processes (Mammarella et al., 2006). The possibility of the pattern of performance differing depending on the VSWM components considered also emerges from the literature on individuals with DS. For instance, an early study by Ellis, Woodley Zanthos, and Dulaney (1989) found that individuals with DS have an unimpaired spatial memory, but an impaired visual memory. A subsequent study by Laws (2002) found that individuals with DS performed significantly better than TD controls matched for receptive vocabulary in the Corsi blocks task (testing spatial working memory), while the two groups’ performance did not differ significantly in a memory for color task (testing visual working memory). Vicari, Bellucci, and Carlesimo (2006) reported individuals with DS having a worse spatial and visual working memory (WM) span than TD individuals, but the difference disappeared after controlling for the role of perceptual and visual abilities. More recently, Lanfranchi, Carretti, Spano`, and Cornoldi (2009) examined the performance of individuals with DS in spatial WM tasks distinguished in terms of presentation format (simultaneous vs. sequential) and demand for controlled attention. The results reported by Lanfranchi et al. (2009) showed that individuals with DS fared worse than TD children of the same mental age on spatial-simultaneous WM tasks, but not on spatial-sequential tasks, thus supporting the distinctions in VSWM drawn by Pazzaglia and Cornoldi (1999). Lanfranchi et al. tentatively interpreted the DS individuals’ worse performance on spatial-simultaneous WM tasks as being due to the difficulty of processing more than one item at a time. This hypothesis finds support in the results of a study by Carretti and Lanfranchi (2010), who showed that individuals with DS were unable to take advantage of structured materials in spatial-simultaneous WM tasks. The task they used contained a confounder, however: the characteristics of the structured pattern were not appropriately controlled because some, but not all, of the pattern could lead to a phonological recoding of the stimuli. This is a point of particular importance because one of the explanations for their results concerned the different use of strategies by DS individuals and TD children. To study these aspects in depth, we analyzed in the present study whether individuals with DS are as able as TD children of the same mental age to take advantage of structured material in spatial-simultaneous and spatial-sequential tasks. Two versions of spatial-sequential and spatial-simultaneous tasks were devised, i.e. pattern configurations (in which the filled cells were grouped to create a pattern), and random configurations (in which the filled cells were separated by spaces and did not create patterns). Unlike the material used by Carretti and Lanfranchi (2010), all the patterns were controlled to avoid the use of phonological recoding strategies. In particular, patterns similar to the shapes of letters were avoided. The main aim of the present study was therefore to analyze whether individuals with DS have a general problem in using structured material (i.e. pattern configurations) to memorize information, or whether this problem applies specifically to spatial-simultaneous WM tasks. Previous findings suggested that structured materials are generally easier to recall than non-structured materials. For example, Attneave (1955) found that memory performance for regular figures was better than for irregular figures. To be more specific, he varied the symmetry in the stimulus, observing memory performance in immediate reproduction, delayed reproduction and recognition paradigms. He demonstrated that symmetrical patterns were always remembered better than asymmetrical patterns containing the same number of cells, and that random patterns were more difficult to remember. More recently, Kemps (2001) examined quantitative and structural complexity using a spatial-sequential task (i.e. the Corsi blocks task) and demonstrated that memory span correlated inversely with the number of blocks in a sequence (quantitative complexity), but it improved when the blocks were put in a matrix rather than a random pattern (structural complexity). Judging from these results, TD children will plausibly take advantage of structured materials in both spatial-simultaneous and spatial-sequential tasks, while we would expect less of an improvement in the performance of individuals with DS, particularly in the spatial-simultaneous tasks (in which they revealed greater difficulties) (Carretti & Lanfranchi, 2010; Lanfranchi et al., 2009).
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Table 1 Participants’ characteristics. Down syndrome
Typical development
M
SD
M
SD
PPVT-R
Raw score Mental age*
56.20 61.90
24.38 22.00
58.00 61.20
18.85 20.00
CPM
Raw score Mental age*
14.05 59.70
4.20 16.00
15.15 64.20
1.98 7.00
Note. PPTV-R, Peabody Picture Vocabulary Test – Revised; CPM, Raven’s colored matrices. * Age is expressed in months.
2. Method 2.1. Participants Twenty individuals with DS took part in the study. Their mean chronological age was 14 years and 2 months (SD = 29 months, range 9 years and 5 months, 17 years and 11 months), and their mean mental age was 5 years and 2 months (SD = 62 months). A control group consisted of 20 TD children with a mean chronological age of 5 years and 5 months (SD = 5 months). The two groups were matched one to one on the raw scores they obtained with the Peabody Picture Vocabulary Test – Revised (PPVT-R, Dunn & Dunn, 1997) (see Table 1). In addition, since individuals with DS show a discrepancy between their verbal and nonverbal abilities, the Raven’s Colored Progressive Matrices (CPM, Raven, Court, & Raven, 1998) were administered to both groups to measure their nonverbal abilities. The two groups’ performance was comparable in both the PPTV-R (F < 1) and the Raven F(1,39) = 1.12, p = .29. 2.2. Materials Visuospatial memory task. The task consisted of a series of matrices presented on a computer screen in which an increasing number of cells were filled. The size of the matrices varied with the number of items to recall (3 4 matrices for levels 1, 2, 3 and 4; 4 4 for levels 5, 6 and 7; and 4 5 for level 8). Each cell in the matrix measured 3 cm 3 cm. Two trials were presented for each level of difficulty. Each task progressed from the shortest lists (containing 1 position to recall) to the longest (maximum 8 positions). To avoid generating frustration, the task was terminated if a participant failed on both lists of the same length, in which case it was assumed that the remaining items would not be recalled correctly. The following conditions were manipulated: (1) the arrangement of the filled cells either in a visual configuration (or pattern), or at random; and (2) the presentation format, which could be simultaneous (requiring the recall of simultaneously presented information) or sequential (involving the recall of items presented one at a time). In the simultaneous condition (see Fig. 1 for an example of the stimuli), each matrix was displayed on a computer screen for 5000 ms, then participants were asked to recall the location of the filled cells. In the sequential condition, each position was displayed for 1000 ms, preceded and followed by an empty matrix of 500 ms. For both conditions, a blue screen appeared for 1000 ms to signal the end of the stimuli’s presentation, then participants were asked to remember the positions without any order constraints, and to point to them on a blank grid. For all conditions, the partial credit unit score method (see Conway et al., 2005) was used to calculate the final score: each position correctly recalled was awarded 1 point, and the mean proportion of the recalled items was calculated on the maximum number of positions presented. 2.3. Procedure During a first session, participants completed the CPM test and PPVT-R. The raw score for the latter test obtained by each individual with DS was used to identify the matching child in the control group, while the CPM scores were used as a measure of the two groups’ non-verbal abilities. Participants subsequently completed the VSWM task. All tasks were administered individually in two sessions with an interval of approximately 1 week between them, each session lasting approximately 25 min. In the second session, the presentation order of the tasks was counterbalanced across participants. 3. Results A 2 2 2 repeated measures ANOVA was performed on the proportion of information recalled, with group (DS vs. TD group) as the between-group variable, and presentation format (spatial-sequential vs. spatial-simultaneous) and configuration (pattern vs. random) as within-group factors (see Fig. 2). The results showed a significant main effect of presentation format F(1,38) = 120.78, MSE = .009, h2p ¼ :76, p < .001, performance being better in the spatial-simultaneous task than in the spatial-sequential one. The main effect of configuration was also significant F(1,38) = 102.72, MSE = .008, h2p ¼ :73, p < .001 with better performances in the pattern than in the random condition. A significant interaction emerged
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Fig. 1. Examples of items in the spatial-simultaneous task. The pattern configuration is shown on the left, the random one on the right.
Fig. 2. Mean proportion of positions recalled by DS and TD individuals as a function of presentation format (simultaneous vs. sequential) and configuration (random vs. pattern). Error bars represent standard errors of the mean.
for the group x presentation format F(1,38) = 7.02, MSE = .009, h2p ¼ :16, p < .05. Post hoc comparisons with the Tukey method, adjusted for multiple comparisons with Bonferroni corrections, indicated that: DS individuals performed marginally less well than the TD group in spatial-simultaneous tasks (Mdiff = .07, p = .065); and both groups did better in the spatial-simultaneous task than in the spatial-sequential one (p < .001 for both groups). The presentation format configuraconfiguration interaction was also significant, F(1, 38) = 45.25, MSE = .005, h2p ¼ :54, p < .001, the pattern condition coinciding with a better performance than the random condition in both spatial-simultaneous and spatial-sequential tasks (for all comparisons, p < .001). The third-order group presentation format configuration interaction F(1,38) = 11.22, MSE = .005, h2p ¼ :23, p < .001 was significant too (see Fig. 2). Post hoc comparisons with the Tukey method adjusted for multiple comparisons with Bonferroni corrections, showed that the DS individuals’ performance was only worse than the TD children’s in the spatialsimultaneous task in the pattern condition (Mdiff = .167, p < .01). Both groups took advantage of the pattern configuration in spatial-sequential and spatial-simultaneous tasks (for all comparisons, p < .001). No other differences were significant. To better understand the results, 2 separate ANOVA were conducted (one for spatial-sequential and one for spatialsimultaneous tasks) 2 (group: DS vs. TD) 2 (configuration: pattern vs. random). The results for the proportion of correctly-recalled information in the simultaneous task showed a main effect of configuration, F(1,38) = 120.59, MSE = .012, h2p ¼ :76, p < .001 (the pattern condition coinciding with a better performance than the random condition), and a significant group configuration interaction, F(1,38) = 8.89, MSE = .012, h2p ¼ :19, p < .01, with the TD group outperforming the group of individuals with DS in the pattern condition (Mdiff = .167 p < .01). Both groups’ performance was better in the pattern than in the random condition, however (TD Mdiff = .347, p < .001; DS Mdiff = .199, p < .001). The effect of group was not significant.
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Fig. 3. Performance of DS and TD individuals as a function of condition (random vs. pattern) and memory load in spatial-simultaneous tasks. Error bars represent standard errors of the mean.
The ANOVA on the spatial-sequential task revealed a main effect of configuration, F(1,38) = 16.06, MSE = .008, h2p ¼ :29, p < .001, with an advantage for the pattern condition. No other effects were significant. 3.1. Spatial-simultaneous task: the effect of configuration as a function of memory load The changes occurring across the different levels of difficulty were analyzed to better understand the results obtained in the spatial-simultaneous task. To do so, a 2 8 repeated measures ANOVA was run on the score obtained at each trial with group (DS vs. TD) as the between-group variable, and memory load (1 vs. 2 vs. 3 vs. 4 vs. 5 vs. 6 vs. 7 vs. 8) and configuration (pattern vs. random) as within-group factors (see Fig. 3). The main effect of configuration F(1,38) = 134.01, MSE = .39, h2p ¼ :78, p < .001 was significant, replicating the advantage of the structured over the random arrangement, and so was the main effect of memory load F(7,266) = 151.09, MSE = .39, h2p ¼ :23, p < .001, showing that in cases where performance did not differ between Levels 1 and 2, the same applied when Level 2 was compared with Level 3, but performance at Level 3 was lower than at Level 1 (Mdiff = .14, p < .05). Subsequent levels differed significantly from the first three (p < .001) and with respect to each other (p < .001), suggesting a linear decline with increasing amounts of information to recall. The configuration group interaction F(1,38) = 10.70, MSE = .39, h2p ¼ :22, p < .01 was also significant, and so was the interaction between configuration and memory load F(7,266) = 17.69, MSE = .24, h2p ¼ :32, p < .001. Both were qualified by the third-order group configuration memory load interaction F(7,266) = 2.82, MSE = .24, h2p ¼ :07, p < .01. Post hoc comparisons with the Tukey method, adjusted for multiple comparisons with Bonferroni corrections, showed that DS individuals differed from TD children in the pattern condition at Levels 6 and 7 (Mdiff = .626, p < .01 and Mdiff = .821, p < .01, respectively); and the differences were marginally significant at Levels 4, (Mdiff = .200, p = .088), 5 (Mdiff = .300, p = .087) and 8 (Mdiff = .468, p = .098) as well. It also emerged that both groups took advantage of the structured condition from Level 4 onwards (p < .01 for all comparisons) and that increasing amounts of information to recall coincided with a decreasing performance. Two separate, repeated measures ANOVA for group memory load were run to further elucidate the results, distinguishing by condition (pattern vs. random). In the pattern condition, the effect of group was significant, F(1,38) = 8.52, MSE = 1.09, h2p ¼ :18, p < .01, with the TD group performing better than the group of individuals with DS; so was the main effect of memory load, F(7,266) = 31.33, MSE = .21, h2p ¼ :45, p < .001, a greater memory load impairing the performance of both groups with a quadratic trend. The group memory load interaction was also significant F(7,266) = 3.85, MSE = .21, h2p ¼ :09, p < .01, replicating the previously-reported results. When the random condition was considered, only the main effect of memory load was significant, F(7,268) = 151.49, MSE = .18, h2p ¼ :79, p < .001, with performance decreasing linearly with the increase in the amount of information to recall. 4. Discussion and conclusion The aim of this study was to shed light on how VSWM functions in individuals with DS by analyzing the role of presentation format and item arrangement. Previous results focusing on the distinction between simultaneous and sequential components of VSWM highlighted that DS individuals performed as well as TD children matched for mental age in spatial-sequential tasks,
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but not in spatial-simultaneous tasks (Lanfranchi et al., 2009). Their greater difficulties in spatial-simultaneous WM tasks remained even when the information to be recalled was presented in a structured format (Carretti & Lanfranchi, 2010). The results of the present study confirmed that individuals with DS may find spatial-simultaneous processing difficult, paradoxically in the more advantageous condition, i.e. when the items to remember are arranged in a pattern. In contrast with the findings reported by Lanfranchi et al. (2009), the two groups did not differ in the random condition. It should be noted, however, that the differences between the TD and DS groups in the Lanfranchi et al. paper appeared to be stronger in the active version of the spatial-simultaneous task (though no direct comparison was drawn by the authors), which was not used in the present study. The size of the matrices was also larger in the present study, making it difficult to compare the two reports directly. Consistently with Carretti and Lanfranchi (2010), our DS sample took less advantage of the structured arrangement of the information than the TD children of matching mental age, although both groups performed better in the pattern than in the random condition. A strategic deficit in individuals with DS could partly account for this result, i.e. they could be unable to use patterns to help them recall positions in spatial-simultaneous tasks. This would be consistent with other studies using verbal short-term memory tasks and showing that individuals with DS do not spontaneously use memory strategies such as rehearsal (e.g. Hulme & Mackenzie, 1992), though it has been demonstrated that their use of rehearsal can be improved by specific training (e.g. Broadley & MacDonald, 1993). It should be noted, however, that no differences were seen for the spatial-sequential task, in with both groups performed equally well. This finding partially rules out any role of strategies in explaining the difficulties encountered by individuals with DS, since the advantage they gained from a pattern configuration in the spatial-sequential tasks was much the same as in the TD group. Another possibility is that individuals with DS performed less well than TD children in the pattern condition of the spatial-simultaneous task due to visual perception problems. They could suffer from visual crowding, for example, making it difficult for them to distinguish between different items of information. Previous research showed that individuals with DS take a global approach to analyzing visuospatial stimuli and often make mistakes that indicate a failure to perceive or faulty perception of details in visuospatial tasks (Bellugi, Lichtenberger, Jones, Lai, & St George, 1999). A possible explanation for our present results might therefore lie in the DS individuals’ difficulties in recalling the exact location of the perceived pattern in the matrices, i.e. they can see the patterns just as well as the TD children, but this does not necessarily mean they can remember them. It is worth adding that our analysis of memory load revealed a deterioration in the DS individuals’ performance when 6 or 7 cells had to be recalled in 4 4 matrices, so it would seem that they no longer take advantage of their ability to perceive the information as a whole when this information exceeds their memory capacity. The role of perception in VSWM performance has already been explored. To give an example, Vicari et al. (2006) demonstrated that individuals with DS no longer differed in spatial and visual tasks from TD children matched for mental age when measures of visual and spatial perception were taken into account. The spatial task they used was more complex than the one used in the present study, however, since participants were asked to remember the positions of non-verbalizable geometrical figures, which added a further source of difficulty. In fact, Vicari et al. found differences between the DS and TD groups even though the positions of the figures they used were presented sequentially (as in our spatial-sequential task), suggesting that no direct comparisons can be drawn between their results and ours. From a theoretical point of view, the present results support the importance of distinguishing between different components in VSWM, as suggested by Pazzaglia and Cornoldi (1999), and demonstrated in individuals of various ages (e.g. Mammarella, Borella, Pastore, & Pazzaglia, in press; Mammarella et al., 2008). In fact, we found that individuals with DS performed just as well as TD children of the same mental age in spatial-sequential tasks, both in the random and in the pattern condition, while the same was not the case for spatial-simultaneous tasks, giving the impression that the processing of spatial-sequential and spatial-simultaneous stimuli differs not only in typically-developing populations, but also in individuals with DS. In conclusion, our results clearly showed that the profile of individuals with DS in tasks assessing VSWM is not homogeneous. Individuals with DS demonstrated a more limited advantage in spatial-simultaneous tasks using structured material, while this was not the case when spatial-sequential tasks were considered. Future research should try to shed light on the factors that might account for these results, from the most basic (relating to visual perception skills) to the most complex (relating to the use of strategies). 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