Instability of Fixation in Dyslexia: Development – Deficits – Training
Authors: Burkhart Fischer, Dipl Phys; Klaus Hartnegg, Dipl Phys
Organization: Centre of Neuroscience, Optomotor Laboratory, University of Freiburg
Journal: Optom Vis Dev 2009;40(4):221-228Abstract:
Background: While eye movements as a necessary prerequisite for natural viewing become more and more important as a part of the neurological diagnostic evaluation, the utilization of fixation is not as well established. This paper discusses two different and independent types of instability of fixation which can only be recognized by recording and analyzing specific movements of the eyes that include: (i) binocular instability (slow movements of the two eyes with different velocities mostly of opposite sign) and (ii) simple instability (small involuntary conjugate saccades (intrusions) during periods of fixating a stationary fixation point).
Methods: A prosaccade task with overlap conditions is utilized which requires periods of stationary fixation as well as saccades to a new stimulus target. This allows the quantitative determination of the appropriate variables. Results: The diagnostic data from children diagnosed with dyslexia are compared with those of age-matched control subjects. An optomotor therapy procedure with one eye covered reduced the binocular instability by 50%, while the simple instability was reduced by 20%.
Conclusion: The results indicate that the two types of fixation instability are independent from each other. Both may contribute to problems of visual processing of those with dyslexia and possible other learning problems.
Keywords: Eye movements, fixation, binocular vision, saccades, dyslexia, binocular instability, training.
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