Dyslexia and Eye Movements

Definition

Dyslexia is a selective impairment of reading and spelling abilities without a deficit in general intelligence. A discrepancy criterion is used to assign a child as dyslexic.

At a recent Expert Colloquium in 1997, Greifswald, Germany, 10 scientists from different fields of research came to some general conclusions about the present state of research in dyslexia.

Dyslexia and Eye Movements

Dyslexia has often been considered as an impairment of language development and processing and/or of auditive differentiation abilities. Recently, however, specific visual problems, such as a defective magnocellular system, have been discussed as a possible source of dyslexia.

Problems in oculomotor control have also been considered because dyslexics show an abnormal pattern of the saccadic eye movements during reading. It remains unclear whether the abnormalities are intrinsic to the oculomotor system or a consequence of difficulties in language processing. Saccadic eye movements have been repetitively measured during non-cognitive tasks, however the finding of an increased number of regressions during saccades to sequentially presented light stimuli has remained controversial. Today we know that the voluntary component of saccade control as measured by antisaccade performance differentiates dyslexics from controls, while prosaccade performance does not.

When tested for dynamic vision by the use of the visual tasks provided by FixTrain systematic developmental deficits can be seen in dyslexia.

To overcome the developmental deficits in saccade control a specific training procedure was developped. The children were given a home training (FixTrain) by which they could practice those aspects of their saccade control system, which were out of the limits of age-matched controls. The effects of daily practice on saccade control are specific for the task that were used during the training period.

Express Saccade Makers

We have found some subjects who produce exclusively express saccades in the gap saccade task and more than 30% express saccades in the overlap task. These rare subjects may have a normal reading performance but they are found about four times more frequently among the D2 dyslexics. These express-saccade makers have difficulties in maintaning fixation and suppressing fast reflexive fast saccades to suddenly appearing stimuli in the memory-guided and the antisaccade task. In the antisaccade task, the errors of dyslexics with a normal number of express saccades in standard tasks, on the other hand, have longer latencies. A possible explanation of these results involves a deficit at one or more levels of the fixation system (see Munoz & Wurtz, J. Neurophysiol.1993), sometimes combined with a deficit in the system of visually selective attention and/or a deficit in voluntary saccade generation.

Prosaccade Performance in Dyslexia

The reaction times of saccades to a suddenly presented stimulus (prosaccades) do not systematically differentiate dyslexics from controls with the exception of the youngest group.

Antisaccade Performance in Dyslexia

Comparison of the error rate in an antisaccade gap task clearly differentiates between dyslexics and controls with the exception of the youngest group. Both groups improve with age but the development of dyslexics stays behind that of the controls.

Dynamic Vision in Dyslexia

Dynamic vision as tested during stationary fixation, prosaccade and antisaccade conditions shows systematic differences bewteen dyslexics and controls.

Effects of Daily Practice

Training effects on the performance of the antisaccades as evaluated by the percentage of misses, i.e. trials in which even after 2 saccades the subject did not reach the opposite side. The curve of the controls are shown together with the values before and after training.

Rate of Improvement in Antisaccade Performance

Improvements in the antisaccade domain are counted by the number of improved variables. If one accepts 3 or 4 improved variables as indication for an improvement the total rate of success reaches about 85%.

Age Dependence / Development

Dyslexia is often diagnosed between age 8 and 15 years. During this period a fast development of saccade control takes place. These age effects are dramatic in the percentage of involuntary saccades but less pronounced in the reaction times of normal saccades.

New Diagnostics

A quick and easy test of a subject's efficiency in reflexive and voluntary saccade control is now possible with a new small transportable instrument called Express-Eye, developed in cooperation with our lab. Training is possible with FixTrain.

Author: Dr.M.Biscaldi, Prof.Dr.B.Fischer
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