EYE MOVEMENTS AND THEIR ROLE IN THE DIAGNOSTICS OF DYSLEXIA
Objectives: Despite a great number of studies focusing on links between eye movements, reading, and dyslexia, published in the last three decades, the role of eye movements is still unclear. The aim of this study therefore is to help clarify the role that eye movements play in dyslexia. The authors merely try to demonstrate the fact that dyslexia is far more diversified problem than generally believed, and that the problems of dyslexics reach beyond the limits of traditionally-defined language deficits stemming from damaged phoneme awareness. Proponents of the visual and magnocellular theory claim that the eye movements of dyslexics are normal, but they are unable to process visual images and spatial information as such. Supposedly eye movements alone are not the cause of poor reading. The theory of visual deficit does not deny the validity of phonological problems. A specific approach to the eye movements of dyslexics is expressed in the cerebellar theory. Its proponents note that many dyslexics have, in addition to reading and language problems described in the phonological theory, also non-linguistic problems, such as imbalance or dyscoordination of motoric capability and sensorimotoric aptitude. Cerebellum plays a significant role in controlling oculomotoric behavior. The link between cerebellar dysfunction and eye movements assumes a link between eye movements and reading, meaning that cerebellar dysfunction is consequential through eye movements to a person’s reading aptitude.
Method: The authors compared tracking saccades in Czech dyslexics (N=51; with age controls. Mean age = 9y; 6m. Their intelligence was measured by means of the WISC test, with the following mean results: IQverbal = 100 ± 11.0; IQ performance = 108 ± 11.6; IQfull = 103 ± 10.6. Their reading scores on standardized tests were at least 1,5 standard deviation below the population mean for age. Their eye movements were examined in a non-verbal, self-paced sequential task simulating movement of reader´s eyes in the text. Eye movements were recorded using infra-red limbus technique with 100 Hz temporal and 0.2° spatial resolution. Oculomotoric performance was described by measures of variability of fixation times, chaining of fixations, transition fixations in borderline areas, and reversions.
Results: The eye movements of approximately two thirds of all dyslexics were distinctly different from the controls and appeared to be heterogeneous: we divided the dyslexic group by eye movements into two subgroups, marked for our working purposes as group 1, whose eye movements were significantly below average, and group 2 whose eye movements were comparable to the control group. Both subgroups of dyslexics did not differ in terms of IQ, reading rate, percentage of reading errors, performance on phonological tests, and the IQ – reading achievement discrepancy. However, dyslexics in subgroup 1 were characterized by combined problems, i.e. substantial reading and writing errors, and phonological problems in parallel, while dyslexics in subgroup 2 were characterized by higher frequency of isolated findings (e.g. errors in dictation only, errors in reading only). The role of eye movements as a measure of the degree of gravity of dyslexic disorder was discussed.