Short-term Memory in Childhood Dyslexia: Deficient Serial Order in Multiple Modalities

Abstract

In children with dyslexia, deficits in working memory have not been well-specified. We assessed second-grade children with dyslexia, with and without concomitant specific language impairment, and children with typical development. Immediate serial recall of lists of phonological (non-word), lexical (digit), spatial (location) and visual (shape) items were included. For the latter three modalities, we used not only standard span but also running span tasks, in which the list length was unpredictable to limit mnemonic strategies. Non-word repetition tests indicated a phonological memory deficit in children with dyslexia alone compared with those with typical development, but this difference vanished when these groups were matched for non-verbal intelligence and language. Theoretically important deficits in serial order memory in dyslexic children, however, persisted relative to matched typically developing children. The deficits were in recall of (1) spoken digits in both standard and running span tasks and (2) spatial locations, in running span only. Children with dyslexia with versus without language impairment, when matched on non-verbal intelligence, had comparable serial order memory, but differed in phonology. Because serial orderings of verbal and spatial elements occur in reading, the careful examination of order memory may allow a deeper understanding of dyslexia and its relation to language impairment. Copyright © 2017 John Wiley & Sons, Ltd.

Keywords: children; dyslexia; memory; specific language impairment.

Figure 1

Illustration of trial details for three kinds of stimuli. Each sequence progresses from upper left to lower right. Lowest sequence: digit span. (Loudspeakers represent spoken digits presented while the little man appears on the screen.) Middle sequence: location span. Upper sequence: shape span. Each sequence depicts a trial with List Length 2, though the procedures as shown are otherwise descriptive of either standard or running span.

Figure 2

Scatterplots of each group (graph parameter) shotting nonverbal intelligence (x axis) as a function of the classification criterion task (y axis). In both panels, triangles represent TD children, discs represent DYS children, and plus signs represent DYS+SLI children. Left-hand panel, dyslexia classification according to the TOWRE-2; right-hand panel, additional classification of these children by language impairment according to the CELF-4. Some children fell in between on these criteria and were omitted from further testing.

Figure 3

Process of matching typically-developing children (TD group) with those with dyslexia only (DYS group), on the basis of both nonverbal intelligence (KABC-2) and a composite language score. Left-hand panel: children included in the match. Right-hand panel: children excluded by the matching process.

Figure 4

Distributions of (lax - strict serial position scoring) difference scores. Top left: standard digit span by list length for typically developing children (TD group, left side of dashed vertical line) and children with dyslexia only (DYS group, right side of dashed vertical line). Top right: same thing for standard location span. Bottom left: same thing for standard shape span. Bottom right: running digit, location, and shape spans for typically developing children (left of the dashed vertical line) and those with dyslexia only (right of the dashed vertical line). Bars demarcate the 25^th to 75^th quartiles. The small point within each rectangles is the median. Whiskers demarcate plus or minus an amount equal to the interquartile range (except that they cannot be negative for these data), circles are outliers beyond the whiskers, and asterisks are extreme values beyond the whiskers by at least twice the interquartile range. The most prominent group differences were in running spans (lower right panel) though differences also emerged in standard digit span (upper left panel).

Figure 5

Mean items correct with +95% CL in each kind of running span (X axis) using lax scoring (dark bars) and strict serial position scoring (light bars). Left-hand panel: typically developing (TD group) children. Right-hand panel: those with dyslexia (DYS group). Notice poorer performance with strict scoring in the DYS group.