Investigating memory functions in dyslexia and other specific learning disorders

Although linguistic deficits are frequently reported in children with dyslexia, the role of memory mechanisms underlying these impairments have yet to be clearly defined. It is still unclear whether the main reason for dyslexia is due to a phonological impairment, or more specific memory dysfunctions, such as deficits in memory encoding or memory retrieval. The purpose of this study was to try to determine, or rule out, the role of memory functions in children with dyslexia and mixed learning disorder (MLD). Thus, 54 children (aged 8 to 12 years old) were recruited and divided into three groups; children diagnosed with dyslexia, MLD, and healthy controls. We assessed children's fluid intelligence, working memory, short-term and long-term object recognition memory, digit span, and reading speed. There was no significant difference between control and dyslexia groups in terms of their fluid intelligence scores, on the other hand the fluid intelligence scores of MLD group was significantly lower than both dyslexia and control groups. Besides, the diagnosed groups showed significantly poorer performance on working memory, object recognition memory, digit span and reading speed than controls. Discriminant function analysis indicated that significant predictors for distinguishing the three groups are object recognition, forward digit span, and working memory, respectively. The lower performance on digit span and both verbal and non-verbal working memory tasks in dyslexic children can be evaluated as a general encoding strategy problem. We suggest that dyslexic children cannot properly encode verbal and nonverbal stimuli, and thus cannot maintain and retrieve them.