Abstract
Purpose: Dyslexia is the most common developmental reading disorder that affects language skills. Latent strabismus (heterophoria) has been suspected to be causally involved. Even though phoria correction in dyslexic children is commonly applied, the evidence in support of a benefit is poor. In order to provide experimental evidence on this issue, we simulated phoria in healthy readers by modifying the vergence tone required to maintain binocular alignment.Methods: Vergence tone was altered with prisms that were placed in front of one eye in 16 healthy subjects to induce exophoria, esophoria, or vertical phoria. Subjects were to read one paragraph for each condition, from which reading speed was determined. Text comprehension was tested with a forced multiple choice test. Eye movements were recorded during reading and subsequently analyzed for saccadic amplitudes, saccades per 10 letters, percentage of regressive (backward) saccades, average fixation duration, first fixation duration on a word, and gaze duration.Results: Acute change of horizontal and vertical vergence tone does neither significantly affect reading performance nor reading associated eye movements.Conclusion: Prisms in healthy subjects fail to induce a significant change of reading performance. This finding is not compatible with a role of phoria in dyslexia. Our results contrast the proposal for correcting small angle heterophorias in dyslexic children.
Highlights
Dyslexia is a developmental reading disorder that affects language skills
Prisms in healthy subjects fail to induce a significant change of reading performance
Our results contrast the proposal for correcting small angle heterophorias in dyslexic children
Summary
Dyslexia is a developmental reading disorder that affects language skills. It is the most common neurobehavioral disorder in children with a prevalence between 5 and 20% of school-aged children in the United States (Handler and Fierson, 2011). It is characterized by difficulties with decoding, fluent word recognition, rapid automatic naming, and/or reading-comprehension skills (Handler and Fierson, 2011). Whether altered eye movements are the cause or rather the consequence of dyslexia is still debated (Livingstone et al, 1991; Skottun and Skoyles, 2006; Stoodley and Stein, 2013)
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