Transmission pathways of vibratory stimulation as measured by subjective thresholds and distortion-product otoacoustic emissions.

To clarify the contribution of the skull contents to the transmission of bone vibratory stimuli, and to examine the characteristics of such stimuli, we compared auditory thresholds and distortion-product otoacoustic emission (DPOAE) levels with a bone vibrator placed on various sites of the head, including the eye. The best audiometric thresholds and the highest DPOAE levels were obtained with the vibrator placed on the mastoid of the measuring side, or on the "ultrasound-window" of the temple. The audiometric thresholds obtained with the bone vibrator on the eye were similar to those of the forehead, and about 10 dB higher than at the best sites. DPOAEs were clearly present when elicited by a combination of air-conducted stimuli presented through an insert earphone and with the bone vibrator placed on the eye. These results indicate that vibratory sounds can be transmitted through the skull contents to the inner ear. The intracranial transmission pathway of the vibratory stimuli may play a significant role, particularly at low frequencies, and possibly also when the vibratory stimuli are applied on the skull bone.