Motor commands contribute to human position sense

Area of Science:

• Neuroscience
• Sensory Physiology
• Motor Control

Background:

• The contribution of afferent (sensory) and efferent (motor command) signals to proprioception has been debated for over a century.
• Existing views suggest that perceived limb position changes are minimal during attempted contractions of paralyzed muscles.

Purpose of the Study:

• To re-examine the role of efferent outflow signals in position sense.
• To investigate perceived limb position changes during complete paralysis.

Main Methods:

• Six healthy subjects performed a wrist position-matching task without vision.
• Perceived wrist position was assessed before and during induced paralysis and anesthesia of the right arm.
• Subjects attempted to flex or extend the paralyzed limb.

Main Results:

• Subjects accurately matched passive wrist positions without vision.
• Paralyzing and anesthetizing the right arm induced a 'phantom' hand sensation.
• Attempting to move the paralyzed limb resulted in a perceived wrist position shift exceeding 20 degrees.
• The magnitude of this position illusion increased with greater effort during paralysis.
• The illusion was independent of the method used to induce paralysis.

Conclusions:

• Efferent outflow signals play a definitive role in proprioception.
• This study provides the first conclusive evidence for the influence of motor commands on the sense of limb position.