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For all calculations involving errors, trials in which the normalised error (as described in the paragraph on data analysis) of eye or hand movements deviated more than 2 standard deviations from the mean over all subjects and conditions were excluded from the analysis.

2.4.2.1 Effect of task on mean errors

Mean directional errors for eye and hand in different tasks are listed in Table 3.

Table 3 Mean directional error in degrees of visual angle and standard deviations between subjects of eye and hand movements for different saccadic tasks (each cell represents the mean of the individual errors, N=10)

Task eye hand

mean (deg) (deg) SD mean (deg) (deg) SD

A steps (persisting target) -0.18 0.48 1.38 1.96 B steps (flashed target) -0.34 0.47 1.25 1.29

C proGap -0.14 0.50 3.75 4.13

D memory -1.66 1.46 1.77 2.93

E scanning 0.10 0.48 1.46 3.20

F antiGap -0.96 1.66 2.57 2.33

Over all conditions, mean directional error for ocular movements was -0.53 deg (sd=0.66 deg), and 2.03 deg (sd=0.97 deg) for hand movements. Thus, eye movements tend to undershoot the target, while hand movements generally overshoot the target (see Figure 4).

hand eye saccadic task d ir ect io n a l er ro r ( d eg rees visu al an g le) -3 -2 -1 0 1 2 3 4 5

steps steps (flashing) proGap memory scanning antiGap

Figure 4 Mean directional error of eye and hand for different tasks

To assess whether errors of eye and hand were organised in a pattern, directional errors (i.e. single trials) were submitted to cluster analysis. As it yielded ambiguous results that were contrary to mean latencies, no hypotheses stemming from cluster analysis were tested for directional errors. Instead, both dependent measures were investigated by means of a separate analysis of variance.

Mean directional errors were affected by both the task and the type of movement. Analysis of directional errors revealed a significant main effect for task (F=2.76, df=5, p<.05) and movement type (F=15.58, df=1, p<.01), indicating that directional errors under all conditions were higher for hand movements than for eye movements. Directional error was also affected by a two-way interaction of the factors movement type x task (F=3.21, df=5, p<.05). The pattern of dependency of the directional error on the tasks was not identical for

eye and hand movements. Whereas larger undershoot of the eye was observed in the memory and antiGap task, larger overshoot of the hand was found in both gap tasks.

Mean variable errors are plotted in Figure 5. Analysis of variable errors revealed a significant main effect only for movement type (F=59.26, df=1, p<.0001). Again, variable errors under all conditions were larger for hand movements than for eye movements. Moreover, a two-way interaction of the factors movement type x task (F=2.77, df=5, p<.05) was observed, indicating different patterns of variable error for eye and hand movements.

hand eye saccadic task vari abl e error (degrees vi sual angl e) 0 1 2 3 4 5 6 7 8

steps steps (flashing) proGap memory scanning antiGap

Figure 5 Mean variable error of eye and hand for different tasks

2.4.2.2 Effect of target flashing on directional and variable errors

As the eyes reached the target well before the hand, sufficient time was available for updating target position by vision and using it to adjust the hand’s landing point. If such an online- correction occurs, variable errors of manual movements should increase under the steps condition, when visual target information is restricted by target flashing as opposed to steps with a persistently visible target .

To determine the effects of target flashing on accuracy of ocular and manual movements, the corresponding variable errors of the step conditions with persisting and

flashed target were submitted to a 2x2 (visual target information x movement type) two- factor, within-subject analysis of variance. There was only a main effect for movement type (F=83.98, df=1, p<.0001), i.e., an overall larger variable error for hand movements. Flashing of targets had no influence on variable errors.

An analogous analysis of the directional error was performed, because other studies reported that restricted visual (foveal) target information affected the directional error (e.g., Prablanc et al. 1979; Delreux et al. 1991). Again, only a main effect for movement type was observed (F=13.15, df=1, p<.01), showing that manual directional errors were larger than ocular directional errors. Flashing of targets had no influence on directional errors.

2.4.2.3 Correlation of ocular and manual normalised errors

To investigate spatial coupling of eye and hand, mean correlations of the ocular and manual normalised errors on a trial by trial basis were calculated. Subsequently, the mean correlation for each task was calculated by averaging the z-transformed correlation coefficients of each subject. Mean correlations of eye and hand normalised errors on a trial by trial basis were not significant for any of the conditions tested (see Table 4).

Table 4 Mean trial to trial correlation and standard deviation between subjects of eye and hand

normalised errors in different saccadic tasks (each cell represents the mean of the individual correlation coefficients, N=10)

Task pearson correlation

standard deviation

A steps (persisting target) 0.09 0.26 B steps (flashed target) 0.16 0.31

C proGap 0.09 0.21

D memory 0.12 0.16

E scanning -0.03 0.37

F antiGap -0.02 0.14

(All correlations not significantly different from 0 (two-tailed), according to a t-test performed on the z-transformed correlation coefficients)

To check for effects of the task on correlations, an analysis of variance of the z- transformed correlations of ocular and manual normalised errors with task as within-subjects factor was carried out. There were no significant differences for tasks.

Contrary to latencies, correlations of normalised errors of eye and hand were not compared for correct intentional anti-movements and wrong reflexive pro-movements in the antisaccade paradigm, because there were too few trials for a valid calculation of the normalised error of wrong pro-movements.

2.4.2.4 Effect of target flashing on spatial coupling

Because restricted visual target information prevents updating, the manual movement might be executed purely on the basis of originally encoded information that is perhaps shared with the oculomotor system. We therefore determined whether the error correlation increases in tasks characterised by restricted availability of visual target information, i.e., steps with flashed target compared to steps with a persistently visible target.

The z-transformed error correlations of the step conditions with persistent and flashed target were submitted to a paired-samples t-test. No difference in error correlations was observed.