Illusory movements of a phantom hand grade with the duration and magnitude of motor commands

The senses of limb movement and position are critical for accurate control of movement. Recent studies show that central signals of motor command contribute to the sense of limb position but it is not clear whether such signals influence the distinctly different sense of limb movement. Nine subjects participated in two experiments in which we inflated a cuff around their upper arm to produce an ischaemic block, paralysing and anaesthetising the forearm, wrist and hand. This produces an experimental phantom wrist and hand. With their arm hidden from view subjects were asked to make voluntary efforts with their blocked wrist. In the first experiment, efforts were 20 and 40% of maximum and were 2 and 4 s in duration. The second experiment used 1 and 5 s efforts of 5 and 50% of maximum. Subjects signalled perceived movements of their phantom wrist using a pointer. All subjects reported clear perceptions of movement of their phantom hand for all levels and durations of effort. On average, subjects perceived their phantom wrist to move between 16.4 +/- 3.3 deg (mean +/- 95% confidence interval (CI)) and 30.2 +/- 5.4 deg in the first experiment and between 10.3 +/- 3.5 and 38.6 +/- 6.7 deg in the second. The velocity of the movements and total displacement of the phantom graded with the level of effort, and the total displacement also graded with duration. Hence, we have shown that motor command signals have a novel proprioceptive role in the perception of movement of human joints.