## 4 Discussion
### PD Patients Rely More on Visual Feedback with Increasing Shape Complexity  
Previous studies reported that PD patients benefit from the use of external cues [21] since their movements rely on visual feedback [17]. Regarding the tasks performed using a habitual motor plan, we found that:  
* For simple tasks, the performance difference between the H and PD groups observed in the free condition does not become smaller in the guided condition. In contrast, the discrepancy between groups in terms of handwriting fluency (normalized jerk) increases in the guided condition. * For complex tasks, the difference of performance (in terms of velocity and acceleration) between groups in the guided condition is reduced compared to free condition. Indeed, we observed that, in the guided condition, the performance of H subjects has a larger decay compared to that of PD patients, and that the performance of patients becomes not significantly different from that of healthy subjects. Visual cues represent a disturbing factor for the execution of automated movements, whereas they are beneficial for movements based on the visual feedback [17]. Obtained results suggest that, at least in the early stage of the disease, movements performed by patients for producing simple habitual shapes, still show some automatisms, and slightly rely on visual feedback. Indeed, we observed that performance decay from free to guided conditions of PD patients is similar to that observed in H subjects. Conversely, for more complex tasks, performance decay is worse in H subjects than PD patients, suggesting that, as the complexity of the task increases (in terms of shape), PD patients tend to rely more on the visual feedback. As regarding tasks performed through a novel motor plan, we found no significant difference in performance between free and guided conditions for both groups. This result was expected, since the sequence of movements involved in this task is mainly novel (non-automated) for both groups (although at stroke level movements can be considered skilled), and therefore requires more the use of visual and proprioceptive feedback both for PD patients and H subjects. However, the H subjects seem to have more troubles in contrasting the automatisms, especially for simple handwriting patterns, where the difference between groups in the guided condition is reduced compared to the free condition. Increasing Task Shape Complexity Affects the Performance of Healthy Participants More than the Performance of PD Patients  
Our protocol contains tasks at different level of complexity, both in terms of shape of the drawing pattern and the automaticity of the drawing movement. We assumed that performing loops of alternating sizes increases control complexity compared to performing separate loops of equal sizes. Analysis of the measured parameters showed that, when using the habitual motor plan, the performance of healthy participants significantly decreased both in terms of writing velocity and acceleration as the shape complexity increases, regardless of whether the task is performed freely or through guiding dots. In contrast, the performance of PD patients does not change significantly when increasing shape complexity, both in the free and in the guided conditions. Our results are in accordance with the observations in [17], showing that performance of PD patients was only slightly influenced by the difficulty of the task, which consisted of movements based on visual feedback, and therefore depended only little on the complexity of the shape of the writing pattern. On the other hand, performance obtained by H subjects, that had to retrieve and instantiate a more complex motor program, was significantly affected. Our results suggest that using a handwriting test that includes different levels of shape complexity, could provide clues about the presence of PD, even in its early stage, e.g., when significant changes in velocity and acceleration are not observed with increased shape complexity of the task.