Are arm trajectories planned in kinematic or dynamic coordinates? An adaptation study

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Human arm movements exhibit invariant features like straight trajectories and smooth velocity profiles.
• Movement planning models are broadly categorized into kinematic and dynamic coordinate systems.
• Distinguishing between these planning frameworks is crucial for understanding motor control.

Purpose of the Study:

• To investigate whether human arm trajectories are planned in kinematic or dynamic coordinates.
• To determine the role of visual feedback in modifying perceived movement curvature and subsequent adaptation.
• To differentiate between kinematic and dynamic models of trajectory planning using artificial visual perturbations.

Main Methods:

• Studied planar two-joint arm movements under self-paced conditions.
• Introduced artificial visual feedback to alter the perceived curvature of hand trajectories.
• Applied perturbations that increased or decreased perceived curvature during sagittal and transverse movements.

Main Results:

• Increasing perceived curvature of sagittal movements led to significant corrective adaptation, making the actual movement curved.
• Increasing curvature of transverse movements resulted in significant adaptation, making the movement straighter.
• Reducing perceived curvature of transverse movements showed no significant adaptation.

Conclusions:

• Arm trajectories are planned in visually-based kinematic coordinates.
• The desired trajectory appears to be straight in visual space.
• Spatial perception via vision fundamentally influences human arm trajectory planning, challenging purely dynamic models.