Auditory sensation via moist contact of the bone vibrator with skin at soft tissue sites

Background: Auditory sensation can be elicited not only by air conduction (AC) with an earphone and by bone conduction by applying a bone vibrator to bony sites on the head, but also by a newly described mode based on applying the bone vibrator to soft tissue sites on the head, neck, and thorax (soft tissue conduction - STC). This study was designed to assess whether it is necessary to compress the skin at the STC sites, which could induce vibrations of the underlying bone. Methods: In 15 normal-hearing subjects, thresholds were assessed with the bone vibrator in air (control for possible AC), direct contact of the bone vibrator with the mastoid and regions around the lip, and indirect contact (via a cotton wool wick, dry or wet) of the bone vibrator with sites around the lip. Results: Even though the best (lowest) thresholds were obtained with direct contact, the subjects clearly heard the sound stimulation when presented only by the gentle contact of the wick with the skin, especially when the contact site was moist. Conclusions: STC stimulation does not require vibrations of the skull bone and seems to involve the transmission of auditory frequency vibrations, through a series of soft tissues, to the inner ear.