Endpoints of arm movements to visual targets

Reaching out for objects with an unseen arm involves using both visual and kinesthetic information. Neither visual nor kinesthetic information is perf ect. Each is subject to both constant and variable errors. To evaluate how such errors influence performance in natural goal-directed movements, we as ked subjects to align a real 5-cm cube, which they held in their hand but c ould not see, with a three-dimensional visual simulation of such a cube. Th e simulated cube was presented at one of four target locations at the corne rs of an imaginary tetraeder. Subjects made successive, self-paced movement s between these target locations. They could not see anything except the si mulated cube throughout the experiment. Initial analysis of the spatial dis persion of movement endpoints demonstrated that the major source of errors under these conditions was visual. Further analysis of the relationship bet ween variability of the starting positions and endpoints showed that the er rors were primarily in judging the endpoint, rather than the direction or a mplitude of the required movement vector. The findings support endpoint con trol of human goal-directed movements.