POOR SACCADIC CONTROL CORRELATES WITH DYSLEXIA

A large group of subjects, either average readers or reading/spelling disabled subjects (n = 185; age between 8-25 years; M = 13+/-4 years), were tested in various standardized cognitive tasks including reading /spelling assessment and in non-cognitive saccadic eye movement tasks. Dyslexics were separated into a subgroup (D1) with deficits in the se rial auditory short-term memory and a subgroup (D2) with an isolated l ow achievement in reading/writing. Control subjects had no relevant co gnitive deficit of any type. Saccadic eye movements were measured in a single target and in a sequential-target task. A significant correlat ion was found between abnormal saccadic control and reading disability . The two dyslexic groups showed only slight differences. As compared to the control group, the mean values of the standard deviations of th e saccadic reaction times (SRT) and the amount of late saccades (SRT > 700) were significantly increased in both dyslexic groups and especia lly in group D1 who also showed an increased amount of anticipatory sa ccades. The number of express saccades (SRT = 80-134 ms) was increased , but not significantly, in D2 dyslexics. Both dyslexic groups produce d significantly more regressive saccades in the sequential-target task . The correlation between saccadic variables and ''reading factor'' wa s 0.4. Significant deviations from normal performance of the saccadic variables were found in an estimated 50% of the dyslexics as compared to 20% of the control subjects. In spite of their worse level in sacca dic control, dyslexics also developed with age in the eye movement per formance as the control subjects did. Yet, the development was slower in group D1. It is suggested that reading process and saccade system a re both controlled by visuo-spatial attention and fixation systems tha t maybe impaired or develop slowly in many dyslexic subjects. (C) 1998 Elsevier Science Ltd. All rights reserved.