Are reaching movements planned in kinematic or dynamic coordinates?
Alice Ellmer, Heiko Hoffmann, Stefan Schaal
Computer Science and Neuroscience, University of Southern California

Whether human reaching movements are planned in kinematic (task space) or dynamic (joint or muscle space) coordinates is still a debated issue. The first hypothesis implies that a planner produces a desired end-effector position at each point in time during the reaching movement, whereas the latter hypothesis relies on the dynamics of the muscular-skeletal control system to produce a continuous end-effector trajectory.

Previous work by Wolpert et al (1995) showed that when subjects were led to believe that their straight reaching paths corresponded to curved paths as shown on a computer screen, participants adapted the true path of their hand such that they would see a straight line in visual space. These results were interpreted as supporting the stance that reaching trajectories are planned in kinematic coordinates. However, their experiment could only demonstrate that adaptation to altered paths, i.e. the position of the end-effector, did occur, but not that the precise timing of end-effector position was equally planned. Our current experiment aims at filling this gap by explicitly testing whether position over time, i.e. velocity, is a property of reaching movements that is planned in kinematic coordinates.

In the current experiment, the velocity profiles of cursor movements corresponding to the participant's hand motions were skewed either to the left or to the right. According to the adaptation paradigm, the skew of the velocity profile was introduced gradually and participants reported no awareness of any manipulation. Preliminary results indicate that the true hand motion of participants did not alter, i.e. there was no adaptation so as to counterbalance the introduced skew. However, for some participants, peak hand velocities were lowered for higher skews, which could suggest that participants interpreted the manipulation as mere noise due to variance in their own movement.

In summary, for a visuomotor transformation task, the hypothesis of a planned continuous end-effector trajectory predicts adaptation to a skewed velocity profile. The current experiment found no adaptation under such transformation. However, the possibility remains that participants interpreted deviations from their desired trajectory as noise.

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