Does Hearing in Response to Soft-tissue Stimulation Involve Skull Vibrations? A Within-Subject Comparison between Skull Vibration Magnitudes and Hearing Thresholds

Shai Chordekar ^1,3 Ronen Perez ² Cahtia Adelman ^3,4 Haim Sohmer ⁵ Liat Kishon-Rabin ¹

¹Department of Communication Disorders, Stanley Steyer School of Health Professions, Sackler Faculty of Medicine, Tel Aviv University
²Department of Otolaryngology and Head and Neck Surgery, Shaare Zedek Medical Center
³Speech & Hearing Center, Hebrew University School of Medicine-Hadassah Medical Center
⁴Department of Communication Disorders, Hadassah Academic College
⁵Department of Medical Neurobiology (Physiology), Institute for Medical Research - Israel-Canada, Hebrew University-Hadassah Medical School

Objective: To contribute to the understanding of the underlying mechanism of soft-tissue stimulation by assessing both skull vibrations and hearing thresholds in the same participants in response to bone (mastoid) and soft-tissue (neck) stimulation. It is hypothesized that if skull vibrations are the underlying mechanism of hearing in response to soft-tissue stimulation, then skull vibrations would be associated with hearing thresholds.

Design: The experimental group included five hearing-impaired adults in whom a bone anchored hearing aid had been implanted. Both skull vibrations (detected by a laser Doppler vibrometer focused on the implant) and hearing thresholds were assessed in response to stimuli delivered to the neck (soft tissue site) and the mastoid (bony site). Hearing thresholds were also obtained in 10 typically-hearing participants in response to stimulation at the same sites.

Results: Vibration magnitudes were detected on the implant down to 40 dB HL (mastoid stimulation) and to 50 dB HL (neck stimulation). Bone vibrations were linearly correlated to stimulus intensity for stimuli to both sites. Hearing threshold differences were similar in the two tested groups, suggesting no influence of middle ear pathology on threshold differences. At all tested frequencies, the vibration magnitude differences of mastoid stimulation and neck stimulation were similar to hearing threshold differences.

Conclusion: Bone vibration magnitude differences can adequately explain the threshold differences, and are likely responsible for the hearing sensation. Therefore, results of the present study suggest that bone conduction is the underlying mechanism of soft-tissue stimulation.