Abstract
Objective. Latency of eye movements depends on cortical structures while speed of execution and accuracy depends mostly on subcortical brainstem structures. Prior studies reported in dyslexic reader children abnormalities of latencies of saccades (isolated and combined with vergence); such abnormalities were attributed to deficits of fixation control and of visual attention. In this study we examine speed and accuracy characteristics of horizontal eye movements in natural space (saccades, vergence and combined movements) in dyslexic reader children. Methods. Two paradigms are tested: gap paradigm (fixation offset 200 ms prior to target onset), producing shorter latencies, in both non-dyslexic reader and dyslexic reader children and simultaneous paradigm. Seventeen dyslexic reader children (mean age: 12 ± 0.08 years) and thirteen non-dyslexic reader children (mean age: 12 ± 1 years) were tested. Horizontal eye movements from both eyes were recorded simultaneously by a photoelectric device (Oculometer, Dr. Bouis). Results. For all movements tested (saccades, vergence, isolated or combined) and for both paradigms, the mean velocity and accuracy were similar in dyslexic readers and non-dyslexic readers; no significant difference was found. Conclusion. This negative but important result, suggests no dysfunction of brainstem ocular motor circuits in dyslexic readers. It contrasts results on latencies related to visual attention dysfunction at cortical level.
Highlights
During visual exploration of the natural world, humans make saccades, vergence, and combined saccade-vergence movements
Dyslexic children were recruited from the pediatric hospital where they are referred for a complete evaluation of their dyslexia state with an extensive examination including neurological/psychological and phonological capabilities
This study shows that the mean velocity and the accuracy of eye movements in natural space in dyslexic readers are as good as in non-dyslexic children of similar age
Summary
During visual exploration of the natural world, humans make saccades, vergence, and combined saccade-vergence movements. The majority of the studies dealing with eye movements and dyslexia are limited to saccades made during reading or made towards isolated single target (tracking condition) [1]. Pavlidis [2] reported frequent regressive saccades and unstable fixation in dyslexia during reading and during sequential tracking of LEDs. Shorter saccade amplitude and longer fixation durations during reading have been reported [3, 4]. Our group [7, 8] reported in dyslexic readers poor binocular coordination of saccades and of the postsaccadic fixation period during reading single words and during tracking condition and during free exploration of artwork, suggesting that such problems occur independently from the reading process
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