Comparing adaptation of constrained and unconstrained movements in three dimensions

This paper describes targeted reaching experiments conducted using a new augmented reality system. Combining a large-workspace immersive virtual environment with physical force feedback, the system distorted subjects' movements using a viscous curl force field. Following previous experiments using a different robot, half the subjects were constrained to horizontal, planar movements. The remaining subjects performed unconstrained movements throughout the 3D workspace. Examining after-effects as an indication of learning, we found that constrained subjects learned the force field. However, it was difficult to detect whether the unconstrained subjects learned forces of identical magnitude. Our results found that force fields strengths eliciting constrained 2D adaptation have difficulty exhibiting after-effects for unconstrained 3D movements. The increased motor variability for 3D reaching movements requires consideration for future experimental design.