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Saccade Control in Reading

Vision and Saccades: General

In the centre of the retina of the human eye is a special spot called the fovea. This is the area with the highest resolution and the highest concentration of receptors and nerve cells. The fovea is the sharpest point of visual acuity. The areas of the eye further away from the fovea are less focused with less light, consequently peripheral vision is less sharp. Saccade control is the ability of the eye(s) to move quickly from one point of interest to the next after an appropriate time of fixation (100 to 300 msec). To obtain a complete picture of the visual field a normal adult has to perform between 3-5 saccades or "snap-shots" per second, including the corresponding breaks (periods of no eye movements), in order to bring all the visual field into focus. Details are captured in these serial images. The brain organises this temporal sequence so that it appears to us as an unbroken image. The visual system not only creates these sequences by means of controlled saccades, it also must be able to cope with the speed of sequences of images.

This scanning of the visual field by saccades is learned during the first years of life. The corresponding developmental process functions without conscious awareness and is stimulated learning as the young child explores the environment. Babies are limited in their ability to focus and to discriminate shapes and objects, including the recognition of their mothers! A child has to learn how to how to fixate precisely on a small item. The stability of gaze control is a complex challenge to the visual system. The ability to select one item amongst several is a further development. In order to change the direction of sight quickly and accurately the stability of gaze control has to be finely integrated with the visual optomotor functions. In our brains we use the so called optomotor cycle.


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The optomotor cycle consists of 3 components: the reflex (generating saccades), the fixation (suppressing saccades), and the voluntary conscious control. The cycle works for reading under control of the linguistic processes by the frontal lobe component.



Visual Processing and Saccade Control in Reading

Learning to read is an exceptionally complicated task for visual processing and visual optomotor functions. Children have to recognise letters, chain them into words, chain words into sentences and sentences into paragraphs. In learning to read the eyes must be able to focus (form saccades) letter-by- letter, word-by-word and line-by-line. If an error is made, because word or phrase cannot be recognized in a single gaze, then the person has to backtrack and fix or focus again, in order to re-track the line correctly. Errors may be caused by too long or too short reaction times. If saccades occur too quickly insufficient information may be taken in and letters, words or phrases may be misread or skipped. If there is binocular instability, i.e. if one or both eyes drift from the point of focus In different directions, reading may be both tiring and frustrating.

In contrast fluent readers perform an almost automatic and reliable cycle of saccades and fixation. They are also able intervene in this automatic process by voluntary action. They can decide to pause over a long or difficult word, i.e. to focus longer or fixate, in order to decipher it more carefully. For reading the eyes require two intricately integrated functions. First, there are the reflexive saccades to scan the visual field, and secondly there must be a strong voluntary system to control the optomotor cycle of fixation. Both the reflexes and the voluntary control are essential for reading. These dual processes take time and practice to develop.


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When reading text presented on top of the figure the eyes start In the middle of the first word and jumps to the next word, when the reading of first word is completed. Longer words need two or even three landings of the eyes. The fixation periods are of different duration depending on several linguistic factors, e.g. the familiarity of the word. The blue line shows the trace of eye movements typical for this sentence. Note: when repeating to read the text the eyes not necessarily take exactly the same steps. Depending on the landing point and the success of identifying the word under consideration, the next step my be different.

If the development of these visual and optomotor processes is deficient then challenging tasks, such as reading, can only be learned with great difficulty. There may symptoms of dyslexia, however, the cause may not be inadequate processing of acoustic signals, but insufficient saccade control. In this case intensive training in reading skills has no affect; saccade control must be improved first.


Assessment of Saccade Control

These two aspects of saccade control, the reflexive and the voluntary, are assessed at BlickLabor with imple standardized tests. These tests are not language based and do not involve reading tasks.

In the Prosaccade Overlap Task the subject focuses on a flashing point (fixates on a central stimulus) until a new flashing point appears to the left or right. This requires a saccadic movement to the new target. The individual being tested follows his natural inclination to look to the target. The reaction times, the accuracy in sighting and the number of unnecessary retro-saccades are used to analyse fixation, reflexive saccades and binocular stability.

The Anti-Gap Task assesses the subjectís voluntary control of eye movement. The person focuses on a flashing point as before, but, when the new target appears the person is instructed to look away, i.e. to move the eyes (perform a saccade) in the opposite direction. The subject has to make to conscious choice to move his focus away from the new target. The errors and reaction times provide information about the subjectís voluntary saccade control.

About 3000 dyslexic children and teenagers from age 8 to 18 years were assessed on the Procsaccade Overlap and Anti-Gap Tasks.

Note: Dyslexia is characterized by reading difficulties and normal or above average intelligence.) The number of boys in the dyslexic group was much higher than the number of girls (75%).

The results show that saccade control and reading abilities depend on similar brain functions and show a parallel development. In other words, there is a correlation between the development of saccade control and reading ability. Secondly, the group of dyslexic children performed significantly lower in saccade control, i.e. the children with reading difficulties also had developmental deficits in saccade control. The figure depicts a variable describing the performance of the antisaccade task: the percentage of trials, In which the subject did not reach the opposite side, even after two saccades. The deficits of the dyslexic subjects become evident by the different age curves In the figure.

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The percentage of misses (the eyes did not reach the opposite side as require for the antisaccade task) shows a different age development In normal control subjects (green) and the dyslexic subjects (red). The right side shows the percentage of the dyslexic subjects, who failed the range of the controls by more than one standard deviation.


A later study showed that specific training rectifies these deficits and transfers to improves to school progress. In other words, improvement in auditory and visual processing transfers to reading, writing and/or arithmetic skills. Studies show that some children show progress even before the completion of training. In other cases parents report improvement afterwards.

There are more variables that con be derived from the measurement of the eye movements: fixation can be examined for monocular and binocular stability. Differences are found in control subjects as compared to dyslexics.

When daily training of saccade control and fixation is successfully completed improvements of reading can be observed.


Please note: There are many causes of reading, writing and arithmetic difficulties. It is unlikely there is one solution for all of them. Some approaches may be effective with some children but not with others. The assessment of auditory and visual processing is a key strand in strategy to overcome learning difficulties.

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