Reduced benefit from long-term item frequency contributes to short-term memory deficits in dyslexia

Abstract

Dyslexia, a specific difficulty in acquiring proficient reading, is also characterized by reduced short-term memory (STM) capacity. Extensive research indicates that individuals with developmental dyslexia (IDDs) benefit less from exposure, and this hampers their long-term knowledge accumulation. It is well established that long-term knowledge has a great effect on performance in STM tasks, and thus IDDs' reduced benefit of exposure could potentially reduce their relative performance in such tasks, especially when frequent items, such as digit-words, are used. In this study we used a standard, widely used, STM assessment: the Digit Span subtest from the Wechsler Adult Intelligence Scale. The task was conducted twice: in native language and in second language. As exposure to native language is greater than exposure to second language, we predicted that IDDs' performance in the task administered in native language will reveal a larger group difference as compared to second language, due to IDDs' reduced benefit of item frequency. The prediction was confirmed, in line with the hypothesis that reduced STM in dyslexia to a large extent reflects reduced benefits from long-term item frequency and not a reduced STM per se.

Keywords: Dyslexia; Individual differences; Item frequency; Long-term memory; Short-term memory.

Fig. 1

An illustration of the performance in the Digit Span task as an example of a task that benefits from exposure to its comprising items. Performance increases with every exposure, and thus it is better in the high-exposure vs. low-exposure condition. Individuals with developmental dyslexia’s (IDD) performance is reduced as compared to that of non-IDDs, and although their absolute performance improves, their relative difficulty increases with exposure. Adapted with permission from Kimel et al. (2022), made available by the Creative Commons Attribution 4.0 International License.

Fig. 2

A. Left: Digit Span Scores of Hebrew-speaking controls (blue squares) and individuals with developmental dyslexia (IDDs) (red circles) tested in Hebrew. Right: Scores of English-speaking controls (green triangles), Hebrew-speaking controls (blue squares), and IDDs (red circles) tested in English. B. Difference in scores between English and Hebrew, measuring the benefit of frequency/exposure for digit-words. Hebrew-speaking controls benefit more than Hebrew-speaking IDDs from performing the span in Hebrew, their native language. Although Hebrew-speaking IDDs’ raw scores are the same in English and Hebrew, it might still reflect a benefit in Hebrew, in which digit-words are longer than digit-words in English. C. A scatter plot of Digit Span scores in English vs. scores in Hebrew. Scores of Hebrew-speaking IDDs are highly correlated (Pearson: r = .60, p < .001; red line), whereas those of Hebrew-speaking controls are not (Pearson: r = .25, p = .179). Symbols denote individual scores. Error bars denote one SEM. The values are slightly jittered for display purposes