On the definition and implications of stimulus polarity for the recording of ocular vestibular evoked myogenic potentials.

Objective: This study investigates the effects of mastoid stimulus polarity on the recording of bone-conducted ocular vestibular evoked myogenic potentials (oVEMPs). The goal was to analyze how different stimulus polarities, specifically condensation and rarefaction, influence the amplitude and latency of oVEMP responses.

Methods: Monocyclic sinusoidal tone bursts at 250 Hz and 500 Hz were used to stimulate the vestibular system in 21 participants with normal hearing and normal vestibular function. The stimuli were delivered through a B250 transducer in both condensation and rarefaction polarities. The output force waveforms were measured with an artificial mastoid and the time derivative of the force were calculated as surrogate for the jerk. For the different stimulus polarities and respective signal output, i.e., acceleration and jerk, the resulting oVEMP responses were compared.

Results: Rarefaction stimuli generally produced clearer and larger n1-p1 responses compared to condensation stimuli. A pre-response n1 peak was observed for 250 Hz condensation stimulation, while 500 Hz condensation stimulation showed a secondary p1 peak following the main response. The output force and jerk, as recorded on an artificial mastoid, suggest that the inwards-directed force of the condensation phase is crucial for eliciting the oVEMP response.

Conclusion: Stimulus polarity plays a critical role in the measurement of bone-conducted oVEMPs, affecting both the timing and magnitude of the response and should thus be always checked and reported. For mastoid stimulation the most effective jerk for oVEMP recordings was achieved by rarefaction stimulation.