side in only half of the trials. During the following 10 years the rate of misses drops down to almost zero. This indicates that the adult subjects in the age range between 20 and 40 years produce 20% errors, but they correct almost all of them. During years after the age of 60 the subjects begin to have more difficulties in correcting their increasing rate of errors.

Finally, we look at the reaction times of the errors shown by the upper right panel. The age curve reflects the curve for the reaction time of the prosac cades generated in the overlap condition. However, all error reaction time were shorter by about the same amount of 50 ms over the complete range of ages covered.

3.6. Left – Right Asymmetries

The question of hemispheric specialisation is asked for almost any aspect of brain functions. In the case of saccade control it might be argued, that depending on the culture writing goes from left to right, right to left, or from top to bottom. Therefore we look at the possible asymmetries of the different variables describing saccade control.

The differences between left and right variables did not show any systematic age dependence, presumably because the age dependence for the right and the left variables have the same development. Therefore we look at the total distribution of the difference values for all ages.

The Fig. 24 shows these distributions of differences for 6 variables.The upper left panels depicts the differences in the reaction time of the prosaccades with overlap conditions. The distribution looks rather symmetrical and in fact the deviation of the mean value is only 6 ms and not significantly different from zero. However, this does not indicate that there are no asymmetries. It shows, that asymmetries occur about as often in favour of the right side as they occur in favour of the left side. The standard deviation of 30 ms to either side indicates that in 32% of the cases the reaction times differ by 30 ms or more. The tendency is that reaction times are somewhat shorter for the right directed saccades as compared with the left directed saccades. This small difference maybe related to the fact that the German language is written from left to right (all data in this book comes from native German speakers).

The upper right panel depicts the differences between the percentages of express saccades made to the right and to the left. The mean value is -1.1% and not significantly different from zero. But there is a tendency to more exp ress saccades to right than to the left. The standard deviation if 12% indicating that

Figure 24. The figure shows the distributions of the left minus right differe nces of 6 variables describing saccade control.

32% of the subjects produced more then 12% of their express saccades to one side than to the other. Extreme cases can be seen within this relatively large group of normal subjects. An example can be seen in Fig. 22. The distribution of saccadic reaction times obtained with overlap conditions are shown for left

and right directed prosaccades. Almost all saccades to the left occur between 130 ms and 170 ms. These are fast regular saccades. Most saccades to the right occur between 85 ms and 140 ms. These are express saccades. The figure demonstrates an extreme case of asymmetry of prosaccades. The reaction times of the correct antisaccades in the gap condition shows a similar result: one encounters quite a number of subjects with heavy asymmetries (32% with differences of more than 45 ms), but the mean value of 5 ms is statistically not significant from zero. The correction times exhibit even stronger asymmetr ies: in 32% of the subject the differences are larger than 55 ms. The percentage of errors in the antisaccade task exhibit differences of more than 15% in 32% of the cases and the differences of the percentage of corrective saccades are larger than 28% in 32% of the subjects. From the consideration of the asymmetries in saccade control we can conclude that large asymmetries occur in quite many cases. Because the asymmetries in favour of the right or of the left side are about the same in number as well as in size, the mean value of the distribution does not deviate significantly from zero.

3.7. Correlations and Independence

Large numbers of errors in the antisaccade task are often interpreted as a consequence of a weak fixation system. This would imply that many intrusive saccades should be observed in the overlap prosaccade task (poor mono fixation stability) along with many errors in the gap antisaccade task. We can look at the possible correlation between these two measures. Fig. 25 shows the scatter plot of the data obtained from control subjects in the age range of 7 to 13 years. While the correlation coefficient indicates a positive significant correlation , the plot shows in detail, that the relation works only in one direction: High values of intrusive saccades occur along with high values of errors, but not vice versa: high values of errors may occur along with low or with high numbers of intrusive saccades. In other words: even if a subject is able to suppress intrusive saccades while fixating a small spot, he/she may not be able to suppress reflexive sac cades to a suddenly presented stimulus. But a subject, who is able to suppress the reflexive saccades, is also able to suppress intrusive saccades.

This means that the reason for many errors in the antisaccade task may be a weak fixation system, but other reasons also exits such that high errors rates may be produced even though the mono fixation stability was high.

The analysis of the relationship between errors, error correction, correction