The spinal reflex cannot be perceptually separated from voluntary movements.

Area of Science:

• Neuroscience
• Motor Control
• Proprioception

Background:

• Limb movements involve both voluntary commands and involuntary spinal reflexes.
• The brain's ability to consciously perceive proprioception may depend on separating these sensory inputs.
• It remains unclear if the brain can differentiate reflexive from voluntary movement contributions.

Purpose of the Study:

• To investigate whether healthy volunteers can perceive the contribution of a spinal reflex during combined voluntary and reflexive movements.
• To determine if the brain can accurately separate sensory inputs from voluntary motor commands and involuntary spinal reflexes.

Main Methods:

• Participants performed leg movements combining voluntary control and knee-jerk reflexes (induced by patellar tendon tap).
• Perceived reflex contribution was reported by repositioning the leg to the perceived maximum reflex displacement.
• Conditions included a resting baseline, voluntary backward kicks, and voluntary forward kicks.

Main Results:

• Reflexes were accurately perceived in the resting baseline condition (near-unity slope of perceived vs. actual displacement).
• Perception accuracy (slope and regression fit) significantly decreased when voluntary backward kicks were generated concurrently with reflexes.
• In forward kicks, kinematic continuity was observed, but perceived reflexes poorly correlated with electromyography (EMG) measures, unlike the baseline.

Conclusions:

• While afferent information allows accurate reflex perception, concurrent voluntary movement significantly impairs this ability.
• The perceptual separation between voluntary and reflex movements is poor.
• The brain lacks a clear marker to distinguish voluntary from involuntary movement when both are present.