The influence of the indicator arm on end point distribution in proprioceptive localization with multi-joint arms.

The present study attempted to demonstrate that the indicator arm influences end point distribution in contralateral multi-joint proprioceptive tasks and also that intrinsic physical characteristics of multi-joint arms (arm stiffness) may predict the error pattern. For this purpose, we carried out two types of contralateral localization tasks with multi-jointed arm movements. In the concurrent localization task, the end point distribution was significantly more elongated in the direction of the lower stiffness at each target position when based on the indicator stiffness, while in the remembered localization task, there was no significant difference between the axes. The best-fit ellipse for the end point distribution also confirmed those results. These findings may support the idea that a large part of the configuration of end point distribution could be determined by the characteristics of arm stiffness of the indicator arm in the condition without memory decay of position representation. Further, error bias of proprioceptive localization may be influenced by the combined effect between movement direction and orientation of the lower stiffness. In conclusion, this study suggests that error patterns largely reflect indicator factors such as the elastic property of the arm in multi-joint proprioceptive tasks, which have been assumed to assess the proprioceptive sense of the reference arm.