Fig. 1

Experiment Setup. a Participants sat in a high backed chair with shoulder, arm and hand supported and hidden from view. The hand was mechanically coupled to the handle of a horizontal planar robot. Participants were instructed to relax the arm at all times during the experiment. The robot perturbed the hand with forces that followed a complex sum-of-sinusoids pattern during each 60 s trial. The experimenter adjusted force magnitude based on the participant’s verbal responses to questions designed to identify the psychophysical threshold of movement detection. b Hand forces applied in the x-direction (top) and y-direction (bottom) for the first five cycles of the slowest component of the sum-of-sinusoids perturbation. c Corresponding XY forces applied during the same five cycles for a neurologically intact control participant