Reduced vergence response velocities in dyslexics: a preliminary report.

Dyslexia is a developmental disorder characterized by a reduced ability to recognize or comprehend written or printed words. Two clinically diagnosed dyslexics and two normals served as subjects. Both dyslexic subjects showed reduced static accommodative and vergence ranges compared with normals and numerous regressions and fixation pauses during reading and target tracking tasks. The subjects were then presented with convergent and divergent ramp disparity stimuli ranging in speed from 1.33 to 32.0 deg/s and steps of either 2 or 4 degrees amplitude. The dyslexic subjects showed reduced vergence velocities and fewer step-ramp and multiple step responses than normals. These preliminary findings suggest that certain vergence control functions may be diminished or absent in dyslexics. However, the vergence deficit may parallel dyslexia and may not be a causative factor. Thus vergence eye movement measurements may be used to provide additional insight into the components of the central deficits associated with dyslexia.