Effects of midline sagittal location on bone-conducted cervical and ocular vestibular evoked myogenic potentials.

We have investigated the effectiveness of two bone-conducted (BC) stimuli in producing vestibular evoked myogenic potentials (VEMPs) following stimulation along midsagittal skull sites. Twenty subjects (mean age 24 yr, range: 18-34 yr; 6 men; 14 women) were studied using a smoothed impulse and a 500-Hz tone burst applied to Nz, Fpz, AFz, Fz, FCz, and Cz with both compressive and rarefactive onset phases. Cervical (cVEMPs) and ocular VEMPs (oVEMPs) were recorded as well as linear acceleration in three axes. cVEMPs evoked by 500 Hz showed no change in response polarity to either stimulus location or phase. cVEMPs evoked by the impulsive stimulus showed larger initial peak amplitudes at AFz and Fz using compressive stimuli and differences in initial peak latency between the two phases. In contrast, amplitude, latency, and response polarity for oVEMPs were markedly affected by stimulus location and phase, which were similar for both BC stimuli, with little correlation with induced acceleration of the head. Latencies were earliest at AFz and Fz where compressive onset stimuli evoked an initial negativity (average latency 8.6-11.0 ms). At other sites compressive onset stimuli usually evoked oVEMPs with an initial positivity. We conclude that both 500 Hz and impulsive stimuli are effective means of evoking cVEMPs and oVEMPs from mid sagittal skull sites. The effects depend upon both location and phase and differ for oVEMPs and cVEMPs. Initial negativities for oVEMPs following compressive stimuli were most consistently obtained using the AFz and Fz sites. We investigated the effect of stimulus location and phase (compressive and rarefactive) in the midsagittal plane for the cVEMP and oVEMP evoked by bone-conducted (BC) 500 Hz and BC impulsive stimuli. For cVEMPs, location effects were limited but were observed for BC impulses. For oVEMPs, both stimuli affected amplitude, latency, and polarity, depending on stimulus location and phase. Compressive stimuli at Fz and AFz evoked early negative oVEMPs most reliably.