Chapter 10

DEVELOPMENT OF SACCADE CONTROL

Burkhart Fischer

Univ. of Freiburg, Optomotor Lab., Freiburg, Germany

Abstract

This chapter describes the development of eye movement control. We will consider, however, only those aspects of eye movements that are important for reading: stability of fixation and control of sac cades (fast eye movements from one object of interest to another). The saccadic reflex and the control of saccades by voluntary conscious decision and their role in the optomotor cycle will be explained on the basis of the reaction times and neurophysiological evidence. The diagnostic methods used in the next part of the book will be explained in this chapter. The age curves of the different variables show that the development of the voluntary component of saccade control lasts until adulthood.

1.Introduction

Saccades are fast eye movements. We make them all the time, 3 to 5 in a second. Due to these saccadic eye movements the brain receives 3 to 5 new pictures from the retina in each second. Without these ongoing sequences of saccades we would not see very much, because of the functional and anatomical structure of the retina: it contains in the middle a small area, called fovea, where the receptor cells and the other cells in the retinal layers are densely packed. It is only this small part of the retina which allows to see sharp images. What we want to see in detail and what we want to identify as an object or any other small

visual pattern, e.g. a letter, must be inspected by foveal vision. The solution for this biological demand are sequences of rapid relatively small eye movements (saccades). The time periods between saccades are 150 to 300 ms long. They are often called ”fixations”.

Usually in everyday life these saccades are made automatically, i.e. we do not have to ”think” about them and we do not have to generate each of the m by a conscious decision. But, by our own decision, we can also stop the sequence and fixate a certain small object for longer periods of times. We can also actively and voluntarily move our eyes from one place of interest to another. The situation is quite similar to our breathing: it works by itself but we can control it also voluntarily.

We are not aware of these saccades, they remain unconscious and – mos t importantly – we do not see the jumps of the retinal image. Somehow, the visual system cooperates with the saccade control centres in a perfect way to differentiate between self-induced movements of the retinal image and externally generated movements.

Only under certain somewhat artificial conditions we can see our saccade s. An example is shown by Fig. 1.

The figure shows a modification of the well known Hermann grid. As long as we look around across on this figure, we see black dots appearing sp ontaneously at the crossing lines between the black squares. The picture looks like scintillations. We notice, that there are no such black dots. To avoid this illusory black blinks, we just have to decide to stop making saccades. The reader may try be her/himself. Pick one of the white dots and maintain fixation at it. As long as one can prevent saccades, the black dots remain absent. Each saccade occurring spontaneously will create the illusion of dots again.

One can also see illusory movements, which are related to eye movements. The Fig. 2 shows an example. The movements disappear when we stop to make eye movements.

An example of a geometric illusion allows also to become aware of ones own saccadic eye movements. The Fig. 3 shows the Z-illusion.

One can see that the prolongations of the short ends of the line (upper left and lower right) will not meet the corners at the lower left and upper right. If one succeeds to prevent all saccades for some seconds (up to 10, which is a long time in this context) one will see that the lines meet the corners as they do in reality. The reader may convince her/himself by using a ruler.

As long as we do not take into account the fact that vision needs saccades,

Figure 1. Scintillations of the Herman grid due to eye movements. As long as we look around across this pattern, we see black dots jumping around. Whenever we try to look at one of them, there is no black spot. When fixating the eyes at one white spot for a few seconds black spots are no longer seen. With still longer fixation most of the white dots disappear also.

we will not understand the visual system. Any theory of vision which makes correct predictions but does not include the fast movements of the eyes can hardly be considered a valid theory.

It is interesting to note, that most visual psychophysical experiments require the subject to fixate one spot of light while being examined on their visual exp e- rience with another stimulus a distance away. The reason for this methodological detail is the sharply decreasing visual acuity from the centre to the periphery of the visual field. Unfortunately, when it came to visual illusions, fixation wa s not longer required when testing the stability of the illusory impression. The result was, that the instability of geometrical illusions, shown in the examples above, remained undiscovered until recently [Fischer et al. 2003].

In particular, when we talk about reading, eye movements are one key for the

Figure 2. The horizontal movements that one sees when looking at this figur e are obviously illusory, because nothing moves in this figure. The illusion disa p- pears, when we stop our eye movements by fixating the centre of the figure.

understanding of the reading process, which allows us to compose words from letters or from syllables. We therefore have to consider the saccade system, before we may consider its role in reading. The significance of saccadic e ye movements in reading was emphasized also by a reader model resting on the basis of experiments, where eye movements were measured while the subjects were reading text, which was manipulated in several physical and linguistic ways [Reichle et al. 2003].

In the following sections we will consider those parts of eye movement control, that play an important role for reading. The other types of eye movements (vestibular ocular compensation, optokinetic nystagmus) will be neglected all together. However, we will consider the instability of fixation due to unwanted saccades or to unwanted movements of the two eyes in different directions or with different velocities. In principal, binocular vision is not needed at all for reading, but imperfections of binocular vision, which remain undetected, may

Figure 3. Z-Illusion shows the capital letter Z. The prolongations of the short lines do not seem to hit the corners. The real geometric relations can be seen by using a ruler and draw the prolongations or they can be seen by fixating the eyes in the middle.

disturb vision and consequently reading may be difficult.

The Fig. 4 shows the most important anatomical connections that begin in the retina and end up at the eye muscles.

There are hundreds of papers on eye movements in the literature. Saccades have always received special interest [Fischer, 1987]. Here we are interested in fixation and in saccades. Today it is easy to measure eye movements in human observes. A sufficiently precise method uses infrared light reflection fr om the eyes. The data presented here are all collected by using this method. For the purpose of clinical application a special instrument was developed. It is called ExpressEye and provides the infrared light source, the light sensitive elements, the visual stimuli needed for basic tasks (with minilasers, see below), and the amplifiers. The instrument can deliver the raw eye position data trial by trial during the experiment and detect saccades and provides a statistical analysis of saccades. The front view of the system is shown in Fig. 5.The method has been described in detail elsewhere [Hartnegg and Fischer, 2002]. The raw data can be stored at the hard disc of a computer for further analysis to obtain different variables that characterize the performance of the tasks. These methods have been described in detail [Fischer et al. 1997].