Rapid Multimuscle Cortical Mapping of the Upper Limb: Activity Reveals Intralimb Gradients

Area of Science:

• Neuroscience
• Motor Control
• Neurophysiology

Background:

• Transcranial magnetic stimulation (TMS) mapping is vital for understanding corticospinal pathways but is often too slow for clinical use.
• Existing protocols struggle to comprehensively map upper limb motor representations, especially for proximal muscles.

Purpose of the Study:

• To develop and validate a rapid, multimuscle TMS mapping protocol for the upper limb.
• To investigate the effects of stimulation intensity and voluntary muscle activity on cortical map properties and motor evoked potential (MEP) latency.

Main Methods:

• Nineteen healthy adults underwent neuronavigated TMS mapping of eight upper limb muscles using a rapid random walk protocol.
• Cortical maps were acquired at varying intensities, both at rest and during low-level voluntary contraction.
• Map metrics (area, volume, center of gravity) and MEP latency were analyzed.

Main Results:

• Stable somatotopic organization was observed, with proximal muscles mapped more medially than distal muscles.
• Higher stimulation intensity expanded distal muscle maps; voluntary activity enhanced proximal muscle representations, indicating a proximal-distal gradient.
• Motor evoked potential latencies were shorter for proximal muscles and during activity, with voluntary contraction altering spatial latency gradients.

Conclusions:

• Rapid, multimuscle TMS mapping is feasible and provides clinically relevant metrics for the entire upper limb.
• A functional dissociation exists between proximal and distal motor representations.
• Combining resting and active mapping conditions is essential for a comprehensive assessment of corticospinal integrity.