Anatomical Pathways for Saccade Control

Figure 4. The figure shows a schematic diagram of the neural system of th e control of visually guided saccades and their connections. LGN = Lateral Geniculate Nucleus; Assoc = Association Cortex; ITC = Infero-Temporal Cortex; FEF = Frontal Eye Field; PFC = Prefrontal Cortex; MT = Medio Temporal Cortex; MST = Medio-Superior-Temporral Cortex; NC = Nucleus Caudatus; SN = Substantia Nigra; Tectal = Tectum = Superior Coilliculus; BS = Brain Stem.

2.Fixation and Fixation Stability

It may come as a surprise, that a section on eye movements starts by dealing with fixation, i. e. with periods of no eye movements. It has been the problem over many years of eye movement research, that fixation was not consid ered at all as an important active function. The interest was in the moving and not in the resting (fixating) eye. Only direct neurophysiological experiments [M unoz and Wurtz, 1992] and thorough investigation of the reaction times of saccades [Mayfrank et al. 1986] provided the evidence, that fixation and sacca de generation are controlled in a mutual antagonistic way similar to the control of other body muscles. We will see, that we can observe movements of the eyes during periods where they were not supposed to move at all. It seems that there was little doubt, that almost any subject can follow the instruction ”fixate” or

Figure 5. The front view of the Express Eye designed to measure eye movements. One sees the screws for the mechanical adjustment in all 3 dimensions in front of each eye. Infrared light emitting diode and the two photocells are located behind and directed to the centre of the eye ball.

”do not move the eyes”. But this not the case: stability of fixation cannot always be guaranteed by all subjects. This section deals with the results of the corresponding analysis of eye movements.

2.1. Monocular Instability

As pointed out earlier, we have to consider two different aspects of disturbances of fixation: the first aspect are unwanted (or intrusive) sacc ades. These are mostly small conjugate saccades that take the fovea from the fixation poin t and back. This kind of disturbance is called a monocular instability, because when it occurs one sees it in both eyes at exactly the same time and by the same amount of saccade size. The disturbance remains if one closes one eye and therefore it disturbs monocular vision and it does not disturb binocular vision. This is the reason why it is called a monocular instability. Below we will also explain the binocular instability.

To measure the stability or instability of fixation due to unwanted saccades, we simply count these saccades during a short time period, when the subject is instructed to fixate a small fixation point. Such a period repeatedly occurs in both diagnostic tasks that are used for saccade analysis that are described in

section 3.2. on page 226.

The number of unwanted saccades counted during this task is used as a measure of monocular fixation instability. For each trial this number is recorde d and attributed to this trial. The mean value calculated over all trials will serve as a measure. The ideal value is zero for each individual trial and therefore the ideally fixating subject will receive also zero as a mean value.

The Fig. 6 shows the mean values of the number of intrusive (unwanted) saccades per trial as a function of age. While children at the age of 7 produce one intrusive saccade every 2 or 3 trials, adults around 25 years of age produce one intrusive saccade every 10 trials. At higher ages the number of intrusive saccades increases again. Of course, not every intrusive saccade leads to an interruption of vision and therefore one can live with a number of them without problems. But if the number of intrusive saccades is to high, visual problems may occur.

Figure 6. The curve shows the age development of the number of unwanted (intrusive) saccades per trial. The ideal value would be zero.

When we measure the monocular instability by detecting unwanted saccades, we should not forget, that there may be also another aspect of strength or weakness of fixation, which cannot be detected by looking at the moveme nts of the eyes during periods of fixation, but rather be looking at reaction time s of saccades that were required when the subject has to disengage from a visible fixation point.

2.2. Binocular Instability

To understand binocular stability we have to remember that the two eyes must be in register in order for the brain to ”see” only one image, even thoug h each eye delivers its own image. This kind of convergence of the lines of sight of the two eyes at one object is achieved by the oculomotor system. We call it motor fusion. However, even with ideal motor fusion, the two images of the two eyes will be different, because they look at a single three dimensional object from slightly different angles. The process of perceiving only one object in its three dimensions (stereo vision), is called perceptual fusion, or stereopsis.

When we talk about stereo vision (stereopsis) we mean fine stereopsis, i.e. single three-dimensional vision of objects. It is clear that we need both eyes for this kind of stereopsis. However, we also have three-dimensional vision with one eye only. The famous Necker cube shown in Fig. 7 is one of the best known examples. From the simple line drawing our brain constructs a threedimensional object. Close one eye and the percept of the cube does not change at all. This type of three-dimensional spatial vision does not need both eyes. The brain constructs a three-dimensional space within which we see objects.

In order to guarantee stable stereopsis, the two eyes must be brought in register and they have to stay in register for some time. This means that the eyes are not supposed to move independently from each other during a period of fixation of a small light spot. By recording the movements of both eyes simultaneously one has a chance to test the quality of the stability of the motor aspect of binocular vision.

The Fig. 8 illustrates the methods for determining an index of binocular stability.

Two trials from the same child are depicted. In the upper trial the left eye shows stable fixation before and after the saccade. The right eye, how ever, converges after the saccade producing a period of non-zero relative velocity. In the lower case, both eyes produce instability after the saccades. The example shows, that the instability is sometimes produced by one eye only, or by both eyes simultaneously Often it is caused in some trials by one eye, and in other trials by the other eye. Extreme dominance of one eye producing the instability was rarely seen (see below).

In the example of Fig. 8 the index of binocular instability was 22%. This means, that the two eyes were moving at different velocities during 22% of the analysed time frame. To characterize a subject's binocular stability as a whole,

Figure 7. The figure shows the famous Necker cube. Note that one sees a three-dimensional object even though the lines are not correctly connected. The percept of a three-dimensionl cube remains even when we close one eye. Note, that the lines do not really meet at the corners of the cube. Yet, our perception is stable against such disturbances and the impression of a cube is maintained.

the percent number of trials, in which this index exceeded 15% was used. The ideal observer will be assigned zero. The worst case would be assigned a value of 100%.

The Fig. 9 shows the data of binocular instability of a single subject. The upper left panel depicts the frequency of occurrence of the percentages of time, during which the eyes were not in register. The upper right panel depicts the distribution of the relative velocity of the two eyes. The scatter plot in the lower left panel displays the correlation between these variables. Ideally all data points should fall in the neighbourhood of zero. The lower right panel depicts the time development of the variable percent time of limits by showing the single values as they were obtained trial by trial from trial 1 to trial 200. This panel allows to see, whether or not fatigue has in influence on the binocular stability.

When the values of the binocular stability were compared among each other, several aspects became evident: (i) Within a single subject the values assigned to the trials can be very different. Almost perfect trials may be followed by trials with long periods of instability. This means, that the subject was not completely

Figure 8. The figure illustrates the methods for determining an index of binocular stability by analysing the relative velocity of the two eyes. Time runs horizontally. At the time of stimulus onset the subject was required to make a saccade. Up means right, down means left. Two trials from the same child are depicted. In the upper case the left eye shows stable fixation before and after the saccade. The right eye, however, converges after the saccade producing a period of non-zero relative velocity. In the lower case, both eyes produce instability after the saccades. For details see text.

unable to main the line of gaze for both eyes, but from time to time the eyes drifted against each other. (ii) There was a large interindividual scatter of the mean values even within a single age group. (iii) Even among the adult subjects large amounts of instability were observed. (iv) The test-retest reliability was reduced by effects of fatigue or general awareness of the subjects.

The Fig. 10 shows the age development of the binocular instability using data from the prosaccade task with overlap conditions.

At the beginning of school large values but also small values were obtained. There was a clear tendency towards smaller values until the adult age. How-

Figure 9. The figure shows the data of binocular instability of a single subjec t. For details see text.

ever, the ideal value of zero is not reached at any age. This means that somehow small slow movements of the two eyes in different directions during short periods of time are well tolerated by the visual system. In other words: there are subjects with considerably instable binocular fusion not complaining about visual problems. Maybe these subjects suppress the ”picture” of one eye to avoid double vision all the time at the price of a loss of fine stereo vision, their vision is monocular. Because this does not create too much of a problem in everyday life, subjects do not show up in the eye doctors praxis. Their binocular system is never checked. This situation could be regarded as similar to the case of redgreen colour blindness, which may remain undetected throughout life, because the subject has no reason to take tests of colour vision.