Figure 1.

Experiment setup. (a) The experiment consisted of one baseline (null field) and five force field sets. During the baseline set, the first field set, and the fifth field set, subjects made movements in all 16 directions in a random order. However, center-out movements were always followed by out-center movements. The black lines indicate center-out movements and the grey lines indicate out-center movements. The out-center movements were drawn at the shifted positions for the purpose of clarity. Movements in all directions other than 0° were always in a force channel. During the second, third and fourth field sets, subjects made movements only in the 0° and 180° directions. (b) In field trials, an acceleration dependent curl field was imposed on the hand for movements toward 0°. Representative hand trajectory and the robot imposed forces are plotted here in position, velocity, and acceleration space of the hand. The example is from a representative trial in the last field set. Dots are sampled hand position every 20ms. (c) Movements in the force channel. Dots are sampled hand position (200 Hz) from single trials in the force channel from the fifth field set.