cVEMP morphology changes with recording electrode position, but single motor unit activity remains constant

Area of Science:

• Neuroscience
• Vestibular System Physiology

Background:

• Cervical vestibular evoked myogenic potentials (cVEMPs) are used to assess vestibular function.
• Previous studies suggested potential excitatory vestibular input to the lower sternocleidomastoid (SCM) muscle based on cVEMP polarity.
• The precise nature of vestibular input to the SCM muscle remains debated.

Purpose of the Study:

• To investigate the hypothesis that the SCM muscle receives both inhibitory and excitatory vestibular inputs.
• To clarify the nature of vestibular projections to different parts of the SCM muscle.

Main Methods:

• Recorded cVEMPs using surface electrodes at multiple sites along the SCM muscle in 10 normal subjects.
• Compared various reference electrode sites (sternum, earlobes, wrist).
• Recorded single motor unit responses in the SCM muscle of 5 subjects using needle electrodes.

Main Results:

• Surface cVEMP polarity inverted from positive-negative at the midpoint to negative-positive near insertion points.
• Single motor unit recordings revealed a consistent short-latency inhibitory reflex (decreased firing) throughout the SCM muscle.
• Changing reference sites did not alter reflex polarity.

Conclusions:

• Vestibular input to the SCM muscle is predominantly inhibitory along its entire length.
• The observed surface cVEMP waveform inversion is likely an artifact of volume conduction, not a sign of excitatory input.
• This study clarifies the inhibitory nature of vestibular reflexes in the SCM muscle.