Implications for contralateral bone-conducted transmission as measured by cochlear vibrations.

HYPOTHESIS: The velocity response at the contralateral cochlea from bone-conducted (BC) stimulation depends on the stimulation position.
BACKGROUND: BC sound transmission in the human skull is complex and differs from air-conducted sound. BC sound stimulates both cochleae with different amplitudes and time delays influencing hearing perception in a way that is not completely understood. One important parameter is the stimulation position on the human skull.
METHOD: By applying BC stimulation at 8 positions on both sides of 7 human cadaver skulls, the contralateral velocity response of the cochlear promontory was investigated in the frequency range of 0.1 to 10 kHz. Using previous data from ipsilateral stimulation, the transcranial transmission (TT) and effects of bilateral stimulation to one cochlea was calculated.
RESULTS: The contralateral transmission from the 8 positions showed small differences, but the TT showed a generally increased cochlear separation when the stimulation position approached the cochlea. The effect of simultaneous bilateral stimulation was calculated, showing a low-frequency negative effect for correlated signals, whereas uncorrelated signals gave 3-dB gain. At higher frequencies, there was less interaction of the combined stimulation because of the greater intercochlear separation. Also, the greatest time difference between ipsilateral transmission and contralateral transmission was at positions close to the cochlea.
CONCLUSION: The stimulation position only slightly affects the amplitude and phase of the contralateral cochlear velocity response. However, because of the great influence from the ipsilateral transmission, a position close to the cochlea would be beneficial for patients with bilateral BC hearing aids.