Developmental Dyscalculia

We live in a world that requires us to process enormous amounts of numerical information. Operating technical devices, estimating or calculating monetary gains or costs, and time management are just a few of the daily demands that require numerical processing. It is evident that a lack of numerical fluency can exert a profoundly negative impact on individual productivity, employment, health, and finances. For example, low numerical skills are associated with lower income, higher risk of being sick, or higher risk of mortgage default (Gerardi, Goette, & Meier, 2013; Parsons & Bynner, 2005). One reason for low numeracy is developmental dyscalculia (DD), which is the topic of this issue of the Zeitschrift für Psychologie. DD is a specific learning disorder that profoundly impairs the acquisition of school-level arithmetic skills. Although the consequences of DD are at least as detrimental as those of dyslexia (Beddington et al., 2008), there is less awareness of DD in the general population, and it has long been an underresearched neurodevelopmental disorder (Bishop, 2010), although recent research activities in the field have grown substantially.

The set of papers brought together here represents some promising directions and results in DD research. Many of the empirical findings and theoretical contributions in this topical issue suggest that a more complete understanding of DD and low mathematical skills can be obtained if facets of working memory are taken into account, either as predictors in a longitudinal study, as concurrent predictors, or by actively manipulating cognitive load in dual task paradigms. Further, two studies in this issue focus on the IQ-achievement discrepancy criterion in defining DD (World Health Organization, 1993), comparing cognitive profiles of children who manifest this discrepancy to those who do not. Only few studies in the field have investigated this topic yet, such that a gap in the literature is being filled. One study addresses comorbid internalizing problems in DD, which is highly welcome as evidence on this topic is scarce. Finally, a meta-analysis on the effectiveness of interventions for children with mathematical difficulties provides new directions for how children with low numerical skills or DD can best be helped.

This topical issue starts off with a review paper by Kuhn (2015) that initially focuses on the etiology of DD, as well as its comorbidity with other neurodevelopmental disorders. Two congenital core systems of number processing, the approximate number system and the object tracking system, as well as their role in DD are then described before the paper outlines specific impairments of counting, calculation, fact retrieval, and working memory in DD. Throughout, neurobiological findings are presented and related to results from behavioral studies.

Next, the study by Klesczewski et al. (2015) presents results on working memory functioning in children with low mathematical skills (second to third grade). The authors investigated to what degree working memory in these children was affected by additional reading and spelling difficulties, as well as presence or absence of an IQ-achievement discrepancy. It was found that children with low mathematical skills have impairments in the phonological loop, the central executive, and the dynamic-spatial scribe in comparison to typical achievers. Further, children with low mathematical skills who showed an IQ-achievement discrepancy were less impaired in a matrix span task (visual working memory) than those who did not display this discrepancy. These and other results of the study help to clarify inconsistent results in the literature by taking important covariates of DD into account.

Using a large sample of (pre)adolescents, Graefen, Kohn, Wyschkon, and Esser (2015) investigated whether children with DD differed in internalizing (depressive or anxious) symptoms from typically developing subjects. Using a multi-informant approach (teachers, parents, self-reports), the main finding was that, although substantially higher levels of internalizing symptoms were reported in children with DD, these remained subclinical. Boys with DD received higher parent and teacher ratings than the control group, whereas girls with DD displayed higher self-ratings than controls. These results are important for intervention planning, in which the different emotional needs of boys and girls should be taken into account.

In a sample of 6- to 10-year-olds, Kroesbergen and van Dijk (2015) assessed visuospatial working memory and number sense for predicting mathematical ability as well as mathematical learning disability. Their key finding was that visuospatial working memory was as important as number sense in predicting mathematical ability, both for math fluency and word problems. In addition, simultaneous deficits in both visuospatial working memory and number sense were related to the lowest mathematical achievement in this study. The authors suggest that, in screening for DD, visuospatial working memory should routinely be considered in addition to number sense.

Busch, Schmidt, and Grube (2015) compared arithmetic fact retrieval in two groups of children with mathematical difficulties (with/without IQ-achievement discrepancy) to two IQ-matched control groups (third grade). Comparing two dual task conditions (articulation, tapping) with a standard condition in solving simple arithmetic tasks, the authors could show that group differences in fact retrieval could not be explained by differences in IQ. Further, results from the dual task paradigms did not support the assumption that children with mathematical difficulties used counting strategies in solving arithmetic tasks.

Friso-van den Bos et al. (2015) investigated acuity in number line estimation longitudinally from first to second grade, using number sense and working memory measured in kindergarten as early predictors. Using latent class growth models, the authors were able to identify three growth classes: at-risk for DD, catch-up, and typical. Key findings were that symbolic number sense in kindergarten predicted membership in higher-performing classes and that, of all working memory tasks, only the phonological loop was associated with higher growth in number line estimation.

Finally, a meta-analysis by Chodura, Kuhn, and Holling (2015) focusing on pre-post control group designs shows that interventions for mathematical difficulties and DD are generally effective. Taking a more nuanced view, the results point to the effectiveness of interventions in children at risk for DD. Further, interventions including direct or assisted instruction and interventions focusing on basic arithmetic were effective, among others. This study also found that an IQ-achievement discrepancy was not related to intervention outcomes.