Abstract
BackgroundEstimates of the prevalence of developmental dyslexia in the general population range from 5% to as many as 10%. Symptoms include reading, writing, and language deficits, but the severity and mix of symptoms can vary widely across individuals. In at least some people with dyslexia, the structure and function of the cerebellum may be disordered. Saccadic adaptation requires proper function of the cerebellum and brainstem circuitry and might provide a simple, noninvasive assay for early identification and sub-phenotyping in populations of children who may have dyslexia.MethodsChildren between the ages of 7 and 15 served as participants in this experiment. Fifteen had been diagnosed with developmental dyslexia and an additional 15 were typically developing children. Five of the participants diagnosed with dyslexia were also diagnosed with an attention deficit hyperactivity disroder and were excluded from further analyses. Participants performed in a saccadic adaptation task in which visual errors were introduced at the end of saccadic eye movements. The amplitudes of primary saccades were measured and plotted as a function of the order in which they occurred. Lines of best fit were calculated. Significant changes in the amplitude of primary saccades were identified.Results12/15 typically developing children had significant adaptation of saccade amplitude in this experiment. 1/10 participants with dyslexia appropriately altered saccade amplitudes to reduce the visual error introduced in the saccade adaptation paradigm.ConclusionsProper cerebellar function is required for saccadic adaptation, but in at least some children with dyslexia, cerebellar structure and function may be disordered. Consistent with this hypothesis, the data presented in this report clearly illustrate a difference in the ability of children with dyslexia to adapt saccade amplitudes in response to imposed visual errors. Saccadic adaptation might provide a noninvasive assay for early identification of dyslexia. Future work will determine whether reduced saccadic adaptation is pervasive in dyslexia or whether this identifies a sub-phenotype within the larger population of people identified with reading and language deficits.
Highlights
Estimates of the prevalence of developmental dyslexia in the general population range from 5% to as many as 10%
We investigated saccadic adaptation in people with dyslexia to determine if, in response to persistent visual error, they are able to alter the amplitude of saccades
Within the first 10 or so trials, the mean amplitude of saccades made to this same target begin to decline as a result of the systematic visual error introduced after the saccade ends
Summary
Estimates of the prevalence of developmental dyslexia in the general population range from 5% to as many as 10%. A large population study demonstrated reduced performance in an antisaccade task [8], and in a study that employed a doublestep eye movement task, increased latencies of orthogonal (but not co-linear) second saccades were observed in dyslexic participants [13]. While these studies provide clues about neural mechanisms that may be altered in dyslexia, neither anti-saccade deficits nor problems in the context of a double-step task are proximal causes of reading disturbances in children with dyslexia. It is instructive that reading without needing to make saccades can increase reading rates by 30–40% without degrading comprehension [15], suggesting that even in typically developing children saccades may limit reading rates
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