Manual skill generalization enhanced by negative viscosity 1 2

18 Recent human-machine interaction studies have suggested that movement augmented with 19 negative viscosity can enhance performance and can even promote better motor learning. To test 20 this we investigated how negative viscosity influences motor adaptation to an environment where 21 forces acted only in one axis of motion. Using a force-feedback device, subjects performed free 22 exploratory movements with 1) a purely inertia generating forces proportional to hand 23 acceleration 2) negative viscosity generating destabilizing forces proportional to hand velocity, 24 or 3) a combination of the acceleration and velocity fields. After training, we evaluated each 25 subject's ability to perform circular movements in only the inertial field. Combined training 26 resulted in lowest error and revealed similar responses as Inertia training in catch trials. These 27 findings are remarkable since negative viscosity, available only during training, evidently 28 enhanced learning when combined with inertia. This success in generalization is consistent with 29 the ability of the nervous system to decompose the perturbing forces into velocity and 30 acceleration dependent components. Compared to Inertia, the Combined group exhibited a 31 broader range of speeds along the direction of maximal perturbing force. Broader exploration 32 was also correlated with better performance in subsequent evaluation trials, which suggests that 33 negative viscosity improved performance by enhancing identification of each force field. These 34 findings shed light on a new way to enhance sensorimotor adaptation through robot-applied 35