# Functional Roles of Saccades for a Hand Movement
## 4 Discussion
### Functional Roles of the Saccades  
We found CS during the hitting task in addition to the circle-drawing task. Although CS during the circle-drawing task was predictable considering that a similar type of saccade was observed in a previous study using a line-drawing task [20], CS during the hitting task was a novel finding. CS during the hitting task was exclusively observed in the monkey that showed lower precision of the movement than the other. While CS was found both in the hand acceleration phase and in the hand deceleration phase (CSd), precision of the middle and late phases of the movement in the trials with CSd was significantly lower than that in the trials without CSd. Since the subject had to heavily rely on feedforward control to accomplish the task because of its designated short movement time, the observed precision is considered to largely reflect that of the feedforward control component. Taking the following two results into account, therefore, we assume that CSd during the hitting task may arise from the imprecision of the feedforward control component to improve its precision. Firstly, the trials with CSd were significantly less precise than those without CSd at the middle and deceleration phases of the movement. Secondly, the initial movement direction, which represents the feedforward control component, in the subsequent trial tended to be more precise than that in the current trial with CSd, while no significant difference of the precision was found between trials without CSd. This assumption may support the previous study that showed a faster adaptation rate in a novel task in a trial with continuous visual feedback of the cursor position than in a trial with post-trial knowledge of task performance [22]. Although we could not detect any evidence suggesting contribution of CS to online error correction but find those suggesting contribution of CSd on offline error correction (\"error\" is used in terms of not systematic error but accidental error), it should be further carefully  
Figure 7: Effects of CS close to the target (CSt) on the cursor movement accuracy and precision during the circle-drawing task Monkey H are shown. They are evaluated at the cursor acceleration minimum point close to the target (**A**) Average of curvature of the cursor trajectory. An error bar represents a standard deviation. (**B**) Variance of the curvature. *** \(p<0.01\). *** \(p<0.001\). investigated which saccade, i.e. LS or CS, provides visual information for the online feedback error correction [17; 23; 24]. In the drawing task, two types of CS were also observed like in the hitting task; one occurred in early phase of the movement (CSe) and the other in late phase of the movement, especially within a phase close to the target (CSt). While the monkey that showed higher precision in the hitting task had not CSt but exclusively CSe, the one that showed lower precision had both CSt and CSe. Like CSd in the hitting task, CSt in the circle-drawing task was observed exclusively in the monkey that showed lower precision in the hitting task. Furthermore, precision at the deceleration local minimum phase of the movement close to the target (curvature was used as an index) in the trials with CSt was significantly lower than that in the trials without CSt, which coincides with the finding in the hitting task. Taking this analogy between CSd and CSt into account, we assume that CSt during the circle-drawing task may also arise from the imprecision of the feedforward controller to improve its precision. As for CSe, we presume that CSe may have a role on precise control of the hand movement because CSe was predominantly observed in the monkey that executed more precise control of the hitting task although we could not find any valuable index to evaluate the function of CSe. The present study also found LS during the circle-drawing task that directed the gaze to a point in between the start and target points although there was no explicit presentation of a via-point.