title: Inner Ear Excitation in Normal and Postmastoidectomy Participants by Fluid Stimulation in the Absence of Air- and Bone-Conduction Mechanisms.
abstract: BACKGROUND
  Hearing can be induced not only by airborne sounds (air conduction [AC]) and by the induction of skull vibrations by a bone vibrator (osseous bone conduction [BC]), but also by inducing vibrations of the soft tissues of the head, neck, and thorax. This hearing mode is called soft tissue conduction (STC) or nonosseous BC.
  
  
  OBJECTIVE
  This study was designed to gain insight into the mechanism of STC auditory stimulation.
  
  
  METHODS
  Fluid was applied to the external auditory canal in normal participants and to the mastoidectomy common cavity in post-radical mastoidectomy patients. A rod coupled to a clinical bone vibrator, immersed in the fluid, delivered auditory frequency vibratory stimuli to the fluid. The stimulating rod was in contact with the fluid only. Thresholds were assessed in response to the fluid stimulation.
  
  
  METHODS
  Eight ears in eight normal participants and eight ears in seven post-radical mastoidectomy patients were studied.
  
  
  METHODS
  Thresholds to AC, BC, and fluid stimulation were assessed. The postmastoidectomy patients were older than the normal participants, with underlying sensorineural hearing loss (SNHL). Therefore, the thresholds to the fluid stimulation in each participant were corrected by subtracting his BC threshold, which expresses any underlying SNHL.
  
  
  RESULTS
  Hearing thresholds were obtained in each participant, in both groups in response to the fluid stimulation at 1.0 and 2.0 kHz. The fluid thresholds, corrected by subtracting the BC thresholds, did not differ between the groups at 1.0 kHz. However, at 2.0 kHz the corrected fluid thresholds in the mastoidectomy patients were 10 dB lower (better) than in the normal participants.
  
  
  CONCLUSIONS
  Since the corrected fluid thresholds at 1.0 kHz did not differ between the groups, the response to fluid stimulation in the normal participants at least at 1.0 kHz was probably not due to vibrations of the tympanic membrane and of the ossicular chain induced by the fluid stimulation, since these structures were absent in the mastoidectomy patients. In addition, the fluid in the external canal (normal participants) and the absence of the tympanic membrane and the ossicular chain (mastoidectomy patients) induced a conductive hearing loss (threshold elevation to air-conducted sounds coming from the bone vibrator), so that AC mechanisms were probably not involved in the thresholds to the fluid stimulation. In addition, as a result of the acoustic impedance mismatch between the fluid and skull bone, the audio-frequency vibrations induced in the fluid at threshold would probably not lead to vibrations of the bony wall of the meatus, so that hearing by osseous BC is not likely. Therefore, it seems that the thresholds to the fluid stimulation, in the absence of AC and of osseous BC, represent an example of STC, which is an additional mode of auditory stimulation in which the cochlea is activated by fluid pressures transmitted along a series of soft tissues, reaching and exciting the inner ear directly. STC can explain the mechanism of several auditory phenomena.
authors.affiliation: Department of Otorhinolaryngology/Head & Neck Surgery, Hebrew University School of Medicine-Hadassah Medical Center, Jerusalem 91120, Israel.
authors.forename: Ofri
authors.initials: O
authors.lastname: Ronen
authors.affiliation: Speech & Hearing Center, Hebrew University School of Medicine-Hadassah Medical Center, Jerusalem 91120, Israel.
authors.forename: Miriam
authors.initials: M
authors.lastname: Geal-Dor
authors.affiliation: Department of Otorhinolaryngology/Head & Neck Surgery, Hebrew University School of Medicine-Hadassah Medical Center, Jerusalem 91120, Israel.
authors.forename: Michal
authors.initials: M
authors.lastname: Kaufmann-Yehezkely
authors.affiliation: Department of Otolaryngology and Head and Neck Surgery, Shaare Zedek Medical Center, Jerusalem 91031, Israel.
authors.forename: Ronen
authors.initials: R
authors.lastname: Perez
authors.affiliation: Department of Communication Disorders, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv 52621, Israel.
authors.forename: Shai
authors.initials: S
authors.lastname: Chordekar
authors.affiliation: Speech & Hearing Center, Hebrew University School of Medicine-Hadassah Medical Center, Jerusalem 91120, Israel.|Department of Communication Disorders, Hadassah Academic College, Jerusalem 91010, Israel.
authors.forename: Cahtia
authors.initials: C
authors.lastname: Adelman
authors.affiliation: Department of Medical Neurobiology (Physiology), Institute for Medical Research, Israel-Canada, Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.
authors.forename: Haim
authors.initials: H
authors.lastname: Sohmer
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods*; D000328:Adult; D001299:Audiometry / Q000379:methods*; D001309:Auditory Threshold / Q000502:physiology; D001844:Bone Conduction / Q000502:physiology; D016022:Case-Control Studies; D007758:Ear, Inner / Q000503:physiopathology; D016057:Evoked Potentials, Auditory, Brain Stem / Q000502:physiology*; D005260:Female; D006314:Hearing Loss, Conductive / Q000175:diagnosis* / Q000534:rehabilitation / Q000601:surgery*; D006801:Humans; D008297:Male; D000074406:Mastoidectomy / Q000379:methods; D008875:Middle Aged; D011379:Prognosis; D012016:Reference Values