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.