Fig. 1.

Robotic apparatus, measures of position sense, and exemplar task performance. a, Occluded vision condition, showing a subject in the robotic apparatus with the shields closed and fabric cover in place to prevent vision of his arms and hands. b, Normal vision condition, showing a subject in the robotic apparatus with the shields pulled back and the fabric cover removed to allow full vision of his arms and hands. c-e, Representative impairments on our measures of position sense: c, Variability (Var, right), d, Contraction/Expansion (C/E, middle), and e, Shift (right). The robot passively moved the affected arm to each of nine spatial locations (filled symbols) and the subjects actively moved their less affected arm to mirror-match each spatial location (open symbols). Each plot shows the overlap between the passive and active hands after mirror transforming data from the left side to the right side of the workspace. Ellipses around open symbols represent one standard deviation. Areas enclosed by the thick grey lines show the matching areas of passive (solid) and active (dashed) arms