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Saccade and Fixation Control

Background

Most people are not aware of the fact, that they make lots of small and fast eye movements called saccades during natural vision. The sensory surface of our eyes (the retinae) have a high resolution only in the middle (the central fovea). To really see and identify objects in our field of view we need these movements, that bring the fovea close to the object. Therefore, we do not see with our eyes but with our brains

Saccades

After the discovery of the optomotor reflex – the express saccade – our understanding of the control of saccades improved and one was able to use variables describing saccade control for diagnostic purposes.

The optomotor group of Prof. B. Fischer at the University of Freiburg developed a portable stand-alone eye tracker (ExpressEye), which also provides the visual stimuli used for eliciting controlled pro- and antisaccades on command.

Brain Research

The new methods, that can be use to day rely on the research of many groups around the world, who contribute to our understanding of vision and eye movements. About 40% of the surface of the human brain are devoted to vision and the control of eye movements.

Method used in Brain Research on Saccades

There are two similar tasks: in one the subject has to fixate a central fixation spot and to make a saccade to a newly presented stimulus at the right or left, while the central fixation spot remained visible. These saccade are called "prosaccades". The other task is called the "antisaccade" task, because the subject is instructed to make a saccade to the side opposite to a newly presented stimulus. In this task, however, the fixation spot is extinguished 200ms before the new stimulus is presented, which leads to the gap effect on saccadic reaction times. During the gap the eyes are released from fixation and the reflex movements are easy to be elicited. The task therefore challenges the fixation system (to be strong enough to suppress the reflex) and it challenges the saccade system to make a saccade to the side without a stimulus. Using these basic tasks it was possible to study the effects of attention and the significance of fixation on the occurrence of the reflex, called express saccades. The experimental results have been use to construct a model of saccade generation ( three loop model).

An introduction to the field of saccade research is available and review article has been published.

The significance of the antisaccade

The correct performance of the antisaccade task requires an intact frontal brain. Saccades controlled by the frontal and prefrontal cortex are made voluntarily and on the basis of a conscious decision. A deficit in the performance of this task points to a deficit in the frontal functions needed for voluntary saccade control and to a deficit in the coordination of cognitive functions with saccadic eye movements as are need during reading.

The Optomotor Cycle The analysis of the variables describing the different components of saccade control leads to a 3-component model illustrated by figure #1. The stop-and go-component leads to a kind of top-and-go traffic being regulated by fixation and the reflex. However, this stop-and-go must not work all by itself but should be controlled by a 3. component. It is here, where the frontal lobe function of generating antisaccades (remember: these are generated on the subjects own decision) enters the saccade control. It is particularly important during reading, when the next saccade must be generated only after the ongoing reading process for a specific word is successfully terminated.

Fig. 1 Optomotor Cycle

Age Development

The development of saccade control lasts until the age of about 20 years. The age curves show that some of the variables stay at the "best" values only for about 10 to 20 years, before they decline again, reaching small values in the range of 7 seven year old children at the age of 60 to 70 years.

Diagnosis – Developmental Deficits The application of pro- and antisaccade task in large groups of children with dyslexia or with dyscalculia or with general learning problems revealed systematic deficits in the antisaccade performance not in prosaccade performance. Figure #2 shows the percentage of uncorrected errors occurring in the antisaccade task applied in dyslexic and control subjects. The percentages of affected subjects faiki8ng the percentile of 16 are given by the numbers at the curves. Poor monocular and/or binocular fixation stability was also observed in many of these children. Children with attention deficits and children with dyscalculia also suffer from deficits in the voluntary control of saccades.

Fig. 2 Percentage of Uncorrected Errors in Anti Gap (Controls/ Dyslexics)

Training in Dyslexia When deficits in saccade are found in an individual subject, a period of training fixation and/or saccades can help to overcome this difficulty. In about 85% of the cases, this training works successfully. Three versions of the task testing dynamic vision are used to challenge (i) the fixation system (parietal cortex and superior colliculus); the reflexive saccade system (visual cortex and superior colliculus), and the antisaccade system (frontal lobe). The training program is planned individually for each child depending on the result of the analysis of the eye movements during a prosaccade task with overlap conditions and an antisaccade task with gap conditions. A special instrument called FixTrain was developed for the training at home. Figure #3 shows the improvement of the performance of the antisaccade task by daily practice at home using FixTrain. The percentage of uncorrected errors is used as a variable.

Fig. 3 Percentage of Uncorrected Errors in Anti Gap Before and After Training

Transfer to Reading Subjects, who completed the saccade and/or fixation training successfully profit by making progress in their reading skills. The details of this study are published. The result: the training group reduced their errors by almost 50%, the waiting group reached only 20% error reduction. There was no placebo effect in this study, because the effect of the fixation/saccade training depended on the tasks that were part of the training. The size ot the effect depends on age as can be seen in figure #4.

Fig. 4 Transfer of Training in Saccade Control to Reading

Literature:

Fischer B, Hartnegg K (2008)
Saccade Control in Dyslexia: Development, Deficits, Training and Transfer to Reading
Optom Vis Dev 2008:39(4):181-190
(abstract)

Dyckman KA, McDowell JE (2005)
Behavioral plasticity of antisaccade performance following daily practice.
Exp Brain Res. 2005 Mar;162(1):63-9. Epub 2004 Nov 13.
(abstract)

Biscaldi M, Fischer B, and Hartnegg K (2000)
Voluntary saccade control in dyslexia
Perception 29: 509-521
(abstract)

Fischer B, Hartnegg K (2000)
Stability of gaze control in dyslexia
Strabismus 8: 119-122
(abstract)

Fischer B, Hartnegg K (2000)
Effects of visual training on saccade control in dyslexia
Perception 29: 531-542
(abstract)

Fischer B, Biscaldi M (1999)
Saccadic Eye Movements in Dyslexia
In: Everatt J (ed) Reading and Dyslexia. Routledge, pp 91-121

Fischer B, Biscaldi M, and Gezeck S (1997)
On the development of voluntary and reflexive components in human saccade generation
Brain-Res 754: 285-297
(abstract)

A complete list of publications is available.