[Vergence dynamics and variability of fixation disparity in school children with reading-spelling disorders]

New computer-controlled psychophysical procedures allow one to measure some temporal aspects of binocular coordination. The present study investigates whether these procedures can be operated by children and whether the tests are useful to detect previously reported impairments in dynamics and stability of vergence in children with disability in reading and writing.

To assess the variability in vergence accuracy (fixation disparity), a stationary fusion stimulus appeared on a computer monitor superimposed by short-term presentations of two dichoptic vertical nonius lines with varying horizontal offset. From the variability of the responses, a measure of vergence variability was deduced. Vergence velocity was estimated with nonius lines that appeared at a fixed delay of 400 ms after a sudden change in disparity of 30 minutes of arc. The full sample comprised 66 children, aged 7-16 years. 16 of these were not able to localize the nonius lines accurately in a control condition; therefore, these children were excluded from the following analysis.

In the remaining 50 children, a group of 30 pupils with different aspects of disability in reading and writing had a nearly significant worse vergence performance both with respect to convergence velocity and to variability of fixation disparity (as compared to a control group of n = 20). These two findings corresponded to each other since convergence velocity and variability of fixation disparity were correlated.

The results confirm previous reports and suggest further research on temporal aspects of vergence with respect to dyslexia. The computer-controlled test procedures are applicable in children of about 10 years or older.