The structure of working memory in young children and its relation to intelligence

Highlights

• Tested the fit of three competing theoretical models of working memory in children.

• Results suggest that working memory models are coming together on common ground.

• No evidence for a separate episodic buffer factor (Baddeley, 2000).

• Focus of attention predicted fluid reasoning and visual processing intelligence.

Abstract

This study investigated the structure of working memory in young school-age children by testing the fit of three competing theoretical models using a wide variety of tasks. The best fitting models were then used to assess the relationship between working memory and nonverbal measures of fluid reasoning (Gf) and visual processing (Gv) intelligence. One hundred sixty-eight English-speaking 7–9 year olds with typical development, from three states, participated. Results showed that Cowan’s three-factor embedded processes model fit the data slightly better than Baddeley and Hitch’s (1974) three-factor model (specified according to Baddeley, 1986) and decisively better than Baddeley’s (2000) four-factor model that included an episodic buffer. The focus of attention factor in Cowan’s model was a significant predictor of Gf and Gv. The results suggest that the focus of attention, rather than storage, drives the relationship between working memory, Gf, and Gv in young school-age children. Our results do not rule out the Baddeley and Hitch model, but they place constraints on both it and Cowan’s model. A common attentional component is needed for feature binding, running digit span, and visual short-term memory tasks; phonological storage is separate, as is a component of central executive processing involved in task manipulation. The results contribute to a zeitgeist in which working memory models are coming together on common ground (cf. Cowan, Saults, & Blume, 2014; Hu, Allen, Baddeley, & Hitch, 2016).

Introduction

Working memory is the portion of our human memory system responsible for simultaneously processing and storing incoming information. There are a number of prominent theories of working memory that differ primarily on whether working memory can be divided into domain-specific components, with unique processing and short-term storage capabilities (e.g., Alloway et al., 2006, Baddeley, 2000, Baddeley and Hitch, 1974, Shah and Miyake, 1996), or whether working memory is part of a larger, more unitary construct primarily guided by the focus of attention (e.g., Cowan, 2001, Engle, 2002). Intelligence encompasses an individual’s ability to learn, reason, adapt, understand, and overcome obstacles by thinking. Nonverbal intelligence measures assess these abilities using items that do not require overt language, and thus reduce the impact of language ability on performance. In this study we compared the statistical fit of four competing working memory models in children, including a new hybrid model, and then assessed the relationship between our best-fitting working memory models and nonverbal measures of fluid reasoning and visual processing intelligence.

There is an increased interest in the structure of working memory in children because of the central role working memory plays in learning (Alloway, 2009, Alloway et al., 2009). In the last decade alone, working memory has been investigated in children with intellectual disability (Van der Molen, 2010, Van der Molen et al., 2014), poor reading comprehension (Carretti, Cornoldi, De Beni, & Romanò, 2005), dyslexia (Jeffries & Everatt, 2004), language impairment (Gray, 2006, Leonard et al., 2007, Montgomery and Evans, 2009), autism (Gabig, 2008), attention deficit hyperactivity disorder (Alloway & Cockcroft, 2014), and fetal alcohol syndrome (Paolozza et al., 2014), as well as in children who are learning two or more languages (Blom et al., 2014, Morales et al., 2013). Because working memory is so integral to learning, it is important to determine its structure early in the elementary school years when assessment information can help lead to treatments to prevent future learning problems (Nevo & Breznitz, 2013) and when children are mature enough to complete the wide variety of experimental tasks that permit a full and fair test of working memory structure.

There is also an increased interest in the relationship between working memory and intelligence in children because different components of working memory are thought to predict different aspects of intelligence (Mackintosh & Bennett, 2003) and because some have proposed that working memory actually accounts for individual differences in fluid intelligence, which is the ability to adapt thinking to solve new problems (Conway et al., 2002, Engle et al., 1999, Oberauer et al., 2005; but see Gignac & Watkins, 2015).

A number of studies have investigated the structure of working memory in children. As shown in Table 1, seven of eight structural studies have considerable overlap in tasks. Although there were differences in the age and primary language of participants, and to some extent how working memory was assessed, results for these modeling studies were quite similar. In general, there was evidence for separate central executive, phonological, and visuospatial type factors. The exception was the study of 8–9-year-old Portuguese children by Campos, Almeida, Ferreira, and Martinez (2013). The fit for their initial confirmatory factor model, with three latent factors (phonological loop, central executive, visuospatial sketchpad), was adequate; however, there was a high correlation (.91) between the central executive and the visuospatial sketchpad factors. They concluded that a model with executive functioning and visuospatial tasks on the same factor was most parsimonious, and therefore they suggested a new two-factor structure as an alternative to the three-factor model. Consistent with this result, Michalczyk, Malstadt, Worgt, Konen, and Hasselhorn (2013) found that a three-factor model fit their data for each age group tested (5–6, 7–9, 10–12), but they reported a “remarkably high correlation between the visual-spatial sketchpad and the central executive” (.81) (p. 227), especially in the younger groups.

Of the studies in Table 1, the investigation by Hornung, Brunner, Reuter, and Martin (2011) is of particular interest because the authors pitted six competing working memory theories against each other in their study of 161 Luxemburgish or Portuguese speaking 5–7 year olds. Using two indicators for verbal simple span, two for verbal complex span, and two for visuo-spatial span, they tested (a) a unitary working memory model, (b) a two-factor model with distinct short-term memory and working memory components, (c) a two-factor model with distinct verbal and visuo-spatial working memory components, (d) a three-factor model (cf. Baddeley & Hitch, 1974) with central executive, phonological loop, and visuo-spatial sketchpad components, (e) a three-factor model (cf. Cowan, 1995a, Cowan, 1999, Cowan, 2001) with a domain-general short-term storage component reflecting the focus of attention and two domain-specific components reflecting verbal and visuo-spatial processes, and (f) a three-factor model based on adult research (cf. Unsworth & Engle, 2007) with a common short-term verbal storage component, a working memory residual component representing executive processes, and a general visuo-spatial storage component. The fit for the last three models was excellent and nearly identical, meaning that there was no clear winner. The authors acknowledged limitations in their study, including the need to administer a wider array of tasks. In particular, their battery did not include complex visuospatial tasks or tasks tapping executive function only.

Also missing from the Hornung et al. study, and from most studies of the structure of working memory in children, were tasks designed to assess episodic buffer function. Baddeley (2000) proposed that the episodic buffer is an independent working memory component with its own temporary storage capacity – a kind of ‘back-up store that is capable of supporting serial recall, and presumably of integrating phonological, visual, and possibly other types of information’ over space and time (p. 419). One study by Alloway, Gathercole, Willis, and Adams (2004) did assess episodic buffer function using two spoken sentence recall tasks. Their final model included episodic buffer, central executive, and phonological loop factors. However, they did not assess visuospatial function; thus, to our knowledge there is no structural test of Baddeley’s (2000) four-component working memory model in the research literature.

The studies discussed above raise several important questions about models of working memory. First, can the statistical fit of working memory models proposed by Baddeley and Hitch (1974) versus Cowan, 1995a, Cowan, 1999, Cowan, 2001 be differentiated, provided that a wider variety of indicators are included in the models? As shown in Table 1, we included at least three indicators for each of the four working memory factors studies.

Second, did Hornung et al. (2011) specify their models correctly? According to their representation of Cowan’s model, verbal and visuospatial storage were put on equal footing. Cowan actually thought of them differently. In his model (e.g., Cowan, 1988, Cowan, 1999), the attention-demanding nature of information storage in the focus of attention is postulated for visual information (an assumption now supported by various studies, for example in children by Ang and Lee, 2008, Ang and Lee, 2010), but attention is largely circumvented when participants can use verbal rehearsal. Cowan has also clearly acknowledged the important role of central executive processes for working memory tasks that require manipulation of information. Therefore, a more accurate three-factor representation of Cowan’s model would include as factors (1) the central executive, (2) the focus of attention, and (3) phonological storage and rehearsal.

Third, are there specific findings in addition to statistical fit that would help to adjudicate between the models? There have been reports that visuospatial working memory and central executive function are so closely related that they do not warrant separate working memory factors (e.g., Campos et al., 2013, Michalczyk et al., 2013). In the Baddeley and Hitch model, neither visuospatial nor verbal storage should be closely related to central executive processes, as they make independent contributions to performance. In contrast with the Cowan model, we would expect a close relationship (though not identical) between the focus of attention, which subsumes visuospatial working memory, and central executive function, given that the executive has control over the focus of attention; but a weaker relationship between these factors and verbal storage in situations conducive to rehearsal, given that rehearsal removes the need for much attention.

Fourth, is there evidence for the existence of an episodic buffer factor as proposed by Baddeley (2000)? There is room for debate about the way to represent the episodic buffer, but one way is to examine situations in which two different kinds of information have to be bound together.

The Cattell-Horn-Carroll (CHC) theory of human intelligence (Carroll, 1993) is a comprehensive psychometric theory of cognitive development, widely accepted as the most empirically supported theory of the structure of cognitive abilities (McGrew, 2005). Because of this empirical support, many intelligence tests are based on CHC theory. Of the 16 broad cognitive abilities described by CHC, seven have been shown to predict academic achievement, and therefore are of primary interest in children: fluid reasoning (Gf), crystallized intelligence, visual processing (Gv), auditory processing, short-term memory, long-term storage and retrieval, and processing speed (McGrew & Wendling, 2010). Of these, Gf has been the focus of working memory researchers because working memory is one of the strongest predictors of Gf in children (Engel de Abreu et al., 2010, Kuhn, in press, Shahabi et al., 2014, Swanson, 2011, Tillman et al., 2009).

In the Hornung et al. (2011) study described above, the authors examined the relationship between components in each of their six tested working memory models by adding a Gf factor to each model. Gf was represented by scores from the Raven’s Colored Progressive Matrices, a nonverbal test of intelligence (Raven, Raven, & Court, 1998). They found that the three-factor model of Baddeley and Hitch (1974) and the three-factor model of Cowan, 1995a, Cowan, 1999, Cowan, 2001 fit the data best, with nearly identical fit indices. Each of Baddeley’s working memory components had correlations of .50 or higher with Gf, but in the Cowan model the component representing shared focus of attention was more strongly correlated with Gf (r = .58) than either the domain-specific verbal factor (r = .24) or the visuo-spatial factor (r = .31). Based on these results the authors concluded that the relation between working memory and Gf was driven by short-term storage because the tasks loading on each factor required storage. This view is consistent with an earlier study by Colom, Abad, Quiroga, Shih, and Flores-Mendoza (2008), who also found that short-term storage was primarily responsible for the relationship between working memory and intelligence in 18–20 year olds, but contrasts with findings in other studies concluding that attention or cognitive control is the primary predictor of fluid reasoning in children (Cowan et al., 2006, Engel de Abreu et al., 2010).

This study had two purposes. The first was to address the unanswered question of whether Cowan, 1995a, Cowan, 1999, Cowan, 2001 or Baddeley’s three- (Baddeley & Hitch, 1974) or four-component (Baddeley, 2000) working memory models best fit the data for young school-age children. We accomplished this using a wider variety of working memory tasks than previous studies. The second was to assess the relationship between working memory factors, Gf, and Gv. to determine whether short-term storage, attention and cognitive control, or both predict Gf when Gv is also in the model. Given the possibility of the high correlations between central executive and visuospatial factors (Campos et al., 2013, Michalczyk et al., 2013), it was also of special interest to examine the somewhat parallel possibility of a close relationship between Gf and Gv.

The three working memory models we tested are presented in Fig. 1. Model 1 represents Cowan, 1988, Cowan, 1995a, Cowan, 1999, Cowan, 2001, Cowan, 2005 embedded processes model that includes central executive, focus of attention, and phonological storage-and-rehearsal factors. According to Cowan (1988), working memory includes all of the components that are used to hold information temporarily. The core of working memory is the temporarily activated portion of long-term memory that is time-limited and, within it, a focus of attention that can hold several more highly processed, integrated items at once. The central executive processes that are involved in entering information into the focus of attention and initiating mnemonic strategies also can be considered part of working memory. Early on Cowan (1995b) called the existence of the phonological loop into question, stating that it may “…be just one special application of a more general temporary information storage medium that can contain various types of stimulus features including, at the least, both acoustic and articulatory/phonological features… (p. 5).” The general storage medium to which he referred was the activated portion of long-term memory. Despite emphasizing the potential similarity between different types of activated information, though, Cowan also acknowledged that mnemonic strategies to retain information in working memory may be invoked for verbal information in that covert verbal rehearsal can make memory maintenance somewhat automatic, and thus less reliant on the focus of attention for refreshment compared to other types of information. This would be the case in adults and also in children old enough (i.e., older than about 7 years) to begin to rehearse lists of simple verbal stimuli (Cowan et al., 2005, Flavell et al., 1966, Ornstein and Naus, 1978). Cowan (1995b, p. 7) referred to the study by Guttentag (1984) as evidence that attention is not needed for verbal maintenance in adults or older children nearly as much as in children who have just learned to rehearse. In contrast, nonverbal information that cannot easily be verbally rehearsed appears to require more attention for maintenance (Camos & Barrouillet, 2011). At least in adults, there appears to be adaptive choice between attention and verbal rehearsal as means to retain information in working memory (Camos, Mora, & Oberauer, 2011); this separation between attention and verbal rehearsal should extend to the age group of our study and is quite consistent with the Cowan model.

Model 2 represents Baddeley and Hitch’s (1974) three-factor model (with further elaboration by Baddeley, 1986, Baddeley and Logie, 1999) that includes central executive, visuospatial sketchpad, and phonological loop factors. These authors viewed the central executive as an attentional control system (as does Cowan), the phonological loop as a temporary store for speech-based and pure acoustic information that could be refreshed with rehearsal, and the visuospatial sketchpad as a temporary store for visual and spatial information that could also be rehearsed by means of some kind of visual reinstatement (Baddeley, 2007).

Model 3 represents Baddeley’s (2000) four-factor model, which added an episodic buffer factor to the previous central executive, visuospatial sketchpad, and phonological loop factors. According to Baddeley (2007) he “proposed to explore the possibility that the executive had a purely attentional role, and was itself incapable of storage” (p. 12), but then needed to account for additional processing capacity observed in tasks that require both memory and processing, especially across different input codes (e.g. visual, auditory). Thus, Baddeley added the episodic buffer “…to form an interface between the three working memory subsystems and long-term memory” (p. 13). The episodic buffer was assumed to have its own temporary storage system and the capacity to bind information from visual, verbal, and perceptual codes with each other and with information held in long-term memory.

With the addition of the episodic buffer, the model of Baddeley (2000) became somewhat similar to that of Cowan, 1988, Cowan, 1999 because Baddeley’s episodic buffer took on some of the same qualities as Cowan’s focus of attention, including retention of information that is neither purely phonological nor purely visual or spatial. The models are distinguishable, however, in at least three ways. First, Cowan saw the retention of items that are visual or spatial in nature as dependent on the focus of attention because visual “rehearsal,” or refreshment, is assumed not to be semi-automated, even in adults, unlike verbal rehearsal. Therefore, in contrast with Baddeley’s models, Cowan’s model anticipates a close relation between the central executive components and visual-spatial tasks, the latter being subsumed under the focus of attention. Second, Cowan’s model also predicts that it is possible for a verbal stimulus set to be subsumed by the focus of attention when verbal rehearsal is impossible. Such is the case for running digit span, in which digits are presented in a list of unpredictable length; that unpredictability appears to make mnemonic strategies such as rehearsal futile (Cowan et al., 2005, Hockey, 1973) and does not seem to allow much updating, either (Broadway and Engle, 2010, Elosúa and Ruiz, 2008); therefore, this kind of task is quite dependent on attention at the time of recall (Bunting, Cowan, & Colflesh, 2008). Baddeley’s (2000) model would not predict that this purely verbal stimulus type would load with visual-spatial tasks under the focus of attention, but rather would predict that its phonological nature would be the overriding characteristic and thus that the task would load on the phonological loop factor. Third, although Baddeley’s episodic buffer took on some of the functions of Cowan’s focus of attention, they are not the same. According to Baddeley (2007), information that involves the binding of information from diverse sources should load on the episodic buffer. According to Cowan, both types of information should be ascribed to Cowan’s focus of attention (e.g., Cowan, 2005). Accordingly, our measures included tasks that could test these differences between the theoretical models.

After determining the best-fitting working memory model we then added subtests from the Nonverbal Scale of the Kaufman Assessment Battery for Children – Second Edition (KABC-2; Kaufman & Kaufman, 2004) to assess the relationship between working memory factors, Gf and Gv. The addition of Gv allowed us to evaluate the differential predictability of the two intelligence factors from the working memory factors.

Our study also allowed a distinction between the roles of working memory storage in general, versus the focus of attention as a storage device. If storage drives the relationship between working memory and Gf, we would expect each of the working memory factors to be significant predictors of Gf because each includes storage tasks. Conversely, if the focus of attention as a special kind of storage device drives the relationship between working memory and Gf, we would expect a stronger relationship between the focus of attention factor and Gf than between the central executive and phonological storage and rehearsal factors and Gf.