Sequence processing in developmental dyslexia

The question whether the causes of dyslexia are specific to language or found across modalities has fostered a long debate. Recently, Szmalec, Loncke, Page & Duyck (2011) showed that dyslexic adults are impaired in Hebb sequence learning, be it with verbal or visuospatial material. Here we investigated whether the impairment in sequence learning is due to an underlying problem to reliably encode ordered information in short-term memory. Administering a task that emphasizes item versus order processing, we investigated sequence processing during verbal and non-verbal short-term memory tasks in 14 good and 14 poor readers of 2nd and 4th grade of elementary school (Experiment 1) and in 24 adult dyslexics and matched controls (Experiment 2). The design comprised a 2 (verbal/nonverbal) x 2 (item/order) recognition task comprised of two order tasks, one with letters and one with nonsense figures, and two item tasks, one with nameable pictures and words, and one with a different set of nonsense figures. Results show a main effect for order that interacted with group and that did not depend on whether the material was of verbal content in both experiments. In experiment 1, poor readers as a group showed inferior performance in both order tasks, but not in item tasks. Also in the controlled adult sample who had been formally diagnosed and matched to controls, dyslexics performed less accurate on both order tasks, but at least as assurate as controls on item tasks. These findings support an integrative explanation for phonological as well as magnocellular functioning impairments and position this study within sequence learning and memory span problems reported in the literature.