Force Field Studies in Motor Control

Force field experiments have been a popular technique used for determining the mechanisms underlying motor planning, execution, and learning in the human motor system. In these experiments, robotic manipulanda apply controlled, extraneous forces/torques either at the hand or individual joints while the subjects carry out movement tasks, such as point-to-point reaching movements or continuous patterns. However, because of the mechanical constraints of the manipulanda used, these experiments have been limited to 2-DOF movements, focusing on shoulder and elbow joints, and thus not allowing for any spatial redundancy in a movement. By utilizing a 7-DOF exoskeleton, our experimental platform allows us to explore a wider variety of movements, including tasks in full 3-D space with the major 7-DOF of the human arm, and because of the inherent redundancy in these movements we can specifically examine issues such as inverse kinematics and redundancy resolution in human arm control.

Online Movement Correction

In double-step target-displacement protocol we investigate how an unexpected upcoming new target modifies ongoing discrete movements. Interesting observations in literature are: the initial direction of the movement, the spatial path of the movement to the second target, and the amplification of the speed in the second movement. Experimental data show that the above properties are influenced by the movement reaction time and the interstimulus interval between the onset of the first and second target. In the present study, we use DMPs to reproduce in simulation a large number of target-switching experimental data from the literature and to show that online correction and the observed target switching phenomena can be accomplished by changing the goal state of an on-going DMP, without the need to switch to different movement primitives or to re-plan the movement.

Discrete-Rhythmic Movement Interactions

We investigate the interaction of discrete and rhythmic movements in single and two joint experiments. In previous studies of single-joint movement tasks two measures of interaction were identified: 1) Initiation of a discrete movement superimposed to an on-going rhythmic movement is constrained to a particular phase window and 2) The ongoing rhythmic movement is disrupted during the discrete initiation, i.e. phase resetting. The goal of our research is to determince whether the interactions happens at a higher cerebral (i.e., planning) or a lower muscular/spinal (i.e. execution) level. In our psychophysical experiments we use Sensuit to record joint angle position while performing rhythmic and discrete tasks. We are using a simplified spinal cord model to study the effect of co-occurance of the discrete and rhythmic movements in a single joint.

Contact persons: Michael Mistry, Heiko Hoffmann Peyman Mohajerian, Stefan Schaal

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