Binocular coordination and dyslexia

Abstract

Developmental dyslexia is suggested to affect approximately 5-10% of the population (Habib, 2000). The most influential theory of dyslexia is the phonological-deficit theory of dyslexia (Liberman, 1973; Stanovich, 1988; Snowling, 2000). An alternative explanation is that visual deficits can lead to reading difficulties (e.g. Stein & Walsh, 1997). To date the findings are mixed regarding the extent of visual deficits within the dyslexic population. Whether these problems represent a cause, correlation or consequence of the reading difficulty also remains highly controversial.

The data presented throughout this Thesis examined the possibility that reading difficulties, associated with dyslexia, are linked to poor binocular coordination. In three experiments binocular eye movements of adults, typically developing children and children with dyslexia were measured while they read sentences or scanned dot string targets. In these experiments findings of previous binocular studies were replicated. Specifically, fixation disparity was modulated by the amplitude of the preceding saccade and the fixation position on the screen regardless of whether fixations and saccades were targeted to dots or words.

Additionally, during the dot scanning task adult’s binocular coordination was improved in relation to children’s, but no reliable differences were found between the three groups. Critically, a significantly greater magnitude of fixation disparity was found for dyslexic children compared to typically developing children and adults during the reading task alone. The existence of linguistically modulated differences in binocular coordination for dyslexic children is a novel finding. The patterns of results from the three experiments indicate that poor binocular coordination in dyslexic children is restricted to reading linguistic material. Clearly, this represents a stimulus specific deficit in regard to binocular coordination, for children with dyslexia.

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