# Functional Roles of Saccades for a Hand Movement
## 3 Results  
### Hitting Task  
Monkeys had to control its hand quickly and precisely to execute the task. For _Monkey H_, the radii of the home and target points were set at 0.012 and 0.014 m, respectively, and the task time was designated at 0.75 s. This subject completed a total of 780 trials, of which 401 (52%) were successful. For _Monkey U_, the radii of the home and target points were both set at 0.006 m, and the task time was designated at 0.54 s. This subject completed a total of 560 trials, of which 403 (58%) were successful. Although success ratio of the task was more or less the same in the two monkeys, movement characteristics of the hand in the hitting task varied depending on the monkey. _Monkey U_ showed better performance than _Monkey H_ with smaller target diameter, shorter movement time (0.47 \(\pm\) 0.04 s vs 0.33 \(\pm\) 0.03 s) (mean \(\pm\) standard deviation [SD]), higher peak hand velocity (0.54 \(\pm\) 0.05 m/s vs 0.90 \(\pm\) 0.07 m/s) (mean \(\pm\) SD), all of which indicate more precise feedforward control of the hand. In the following analysis, trials that did not reach 0.125 m in y-coordinate within the designated task time were excluded; 762 trials in Monkey H and 556 trials in Monkey U were analyzed as a result. In almost all of the hitting task trials, both monkeys made a saccade directly to the target point upon its appearance and prior to the onset of hand movement. Subsequently, two different types of gaze control were observed. In one type, the monkey kept its gaze on the target point until the cursor hit it (Figure 2A), and in the other type, the monkey returned its gaze from the target to the cursor and made a couple of saccades around the cursor while following its movement (Figure 2B). LS and CS that execute these two types of gaze control were analyzed. We defined LS and CS as follows: LS was a saccade that directed the gaze in the range from 0.135 to 0.200 m in y-coordinate; CS was one that was not LS and difference of gaze position directed by that and the cursor position was within \(\pm\) 0.050 m in y-coordinate. The LS was observed in both monkeys in more than 90% of the trials. Inversely correlated with the performance of the task, interestingly, the number of the CS in Monkey H was prominently higher than that in Monkey U (Table 1). Therefore, the CS in Monkey H was analyzed in the following analysis. In the trial with the CS in Monkey H, the subject executed the CS along the hand trajectory (Figure 3A). The gaze points following the CS in Monkey H seemed bimodally distributed in the direction of cursor movement with a border point at 0.11 m from the center of the start point (Figure 3B), which roughly corresponded to the cursor velocity maximum point. We tentatively classified the CS into two types; the CS in hand acceleration phase (CSa) and the CS in hand deceleration phase (CSd). To analyze the relationship between the CS and precision of the hand movement, the hitting task trials were grouped into three; those without the CS, those with CSa, and those with CSd, and precision of the hand movement among the groups was compared. The trials with CSd showed a tendency towards larger variance of the movement than the other two groups from the middle phase of the movement and showed significantly larger variance than those without the CS both at the middle (F-test, p = 0.0144) and deceleration (F-test, p = 0.0158) phases of the movement (Figure 4A). Although there was no significant difference in the variance of Dm among the three groups, we found that only the subsequent trials to ones with CSd showed a tendency towards smaller variance than the current trials (Figure 4B). This was indicative of influence of CSd on improvement of precision of