Many ipsilaterally-measured sonomotor reflexes are influenced by the presence of contralateral sounds. Vestibular-evoked myogenic potentials (VEMPs), thought to reflect the vestibulo-collic reflex, can be elicited by

monaural or binaural sound. The purpose of this investigation was to examine the effects of varying contralateral sound frequency on unilateral VEMP amplitude and latency characteristics. The participants were eighteen adults (4 male) with no history of vestibular or neurological disease. All participants were evaluated during monaural and binaural conditions. Monaural VEMPs were acquired with either air-conducted or bone-conducted 500-Hz tone bursts presented at 95 dB nHL and 70 dB nHL, respectively. These stimuli were simultaneously paired with 95 dB nHL contralateral tone bursts at 250, 500, 750 or 1000-Hz during the acquisition of binaural VEMPs. The results of the study indicated that air-conducted VEMP relative amplitudes in each of the four binaural conditions were significantly decreased compared to the monaural condition. Larger differences between monaural and binaural conditions were seen when contralateral sounds were < 500 Hz. However, no significant changes in relative amplitude were seen amongst the conditions for bone-conducted VEMPs. Comparisons between monaural and binaural conditions revealed no significant differences in p13 or n23 latency across the conditions for either air-conducted or bone-conducted VEMPs. An explanation for the findings of the study is that activation of the contralateral acoustic reflex during the air-conducted VEMP binaural conditions causes an attenuation of sound transmission through the middle ear and lowers effective levels of sound from reaching the vestibule. The lack of change in bone-conducted VEMP amplitudes during the binaural conditions supports the conjecture that stimulation of the vestibule through bone conduction is largely accomplished without involvement of middle-ear sound transmission.