Nonlinear Connectivity in the Human Stretch Reflex Assessed by Cross-Frequency Phase Coupling

Area of Science:

• Neuroscience
• Motor Control
• Systems Biology

Background:

• Neuronal communication relies on synchronized oscillatory activity.
• Nonlinearity is present in the sensorimotor system, but its connectivity is under-explored.
• Understanding nonlinear connectivity is crucial for deciphering complex neural processes.

Purpose of the Study:

• To investigate nonlinear connectivity during the human stretch reflex using neuronal synchronization.
• To estimate directional nonlinear connectivity and time delays between perturbation, brain, and muscle.
• To differentiate contributions of spinal and transcortical pathways to the stretch reflex.

Main Methods:

• Healthy participants performed wrist flexion with periodic mechanical perturbation.
• A novel cross-frequency phase coupling metric was employed to assess nonlinear connectivity.
• Source localization techniques were used to identify brain regions involved.

Main Results:

• Nonlinear phase coupling was significantly stronger from perturbation to muscle compared to perturbation to brain.
• A shorter time delay (33 ms) was observed from perturbation to muscle, aligning with spinal stretch reflex latency.
• Nonlinear coupling from brain to muscle originated from the motor cortex but had a weak effect.

Conclusions:

• The human stretch reflex exhibits nonlinear neuronal synchronization.
• Nonlinear phase coupling is predominantly mediated by the spinal reflex loop.
• Transcranial pathways contribute minimally to the nonlinear dynamics of the wrist stretch reflex.