Manifold reaching paradigm: how do we handle target redundancy?

Area of Science:

• Neuroscience
• Motor Control
• Biomechanics

Background:

• The central nervous system (CNS) faces the challenge of coordinating numerous degrees of freedom in the musculoskeletal system for movement.
• Previous research often used point-to-point reaching tasks, where many movement solutions exist for a single goal, to study motor redundancy.

Purpose of the Study:

• To investigate how the CNS manages extrinsic redundancy, specifically target redundancy, where the target is not uniquely defined.
• To understand the strategies employed when reaching to a manifold (e.g., a vertical bar), allowing endpoint freedom.

Main Methods:

• Subjects performed an arm reaching task towards a vertical bar, allowing them to choose any point on the bar as the target.
• Analysis focused on intersubject and intertrial consistency, movement variability, and arm trajectories.
• Stochastic optimal control models incorporating energy/smoothness costs and motor noise were used for explanation.

Main Results:

• Behavior showed intersubject and intertrial consistency despite task freedom, but with greater variability than point-to-point reaches.
• Average arm trajectories and endpoint variations were explained by stochastic optimal control models.
• Movement planning appears to address target redundancy before movement execution.

Conclusions:

• The CNS overcomes target redundancy during the planning phase of movement.
• Task-specific freedom is exploited during movement execution, consistent with stochastic optimal feedback control.
• This study illustrates how goal and movement selection problems can be resolved concurrently.