title: Bone Conduction Thresholds without Bone Vibrator Application Force.
abstract: BACKGROUND
  Osseous bone conduction (BC) stimulation involves applying the clinical bone vibrator with an application force of about 5 Newton (N) to the skin over the cranial vault of skull bone (e.g., mastoid, forehead). In nonosseous BC (also called soft tissue conduction), the bone vibrator elicits hearing when it is applied to skin sites not over the cranial vault of skull bone, such as the neck.
  
  
  OBJECTIVE
  To gain insight into the mechanisms of osseous and nonosseous BC.
  
  
  METHODS
  In general, thresholds were determined with the bone vibrator applied with about 5 N force directly to osseous sites (mastoid, forehead) on the head of the participants, as classically conducted in the clinic, and again without direct physical contact (i.e., 0 N force) achieved by coupling the bone vibrator to gel as in ultrasound diagnostic imaging, on the same or nearby skin sites (nonosseous BC). The participants were equipped with earplugs to minimize air-conducted stimulation.
  
  
  METHODS
  In the first experiment, 10 normal-hearing participants were tested with stimulation (5 and 0 N) at the forehead; in the second experiment, 10 additional normal-hearing participants were tested with stimulation at the mastoid (about 5 N) and at the nearby tragus and cavum concha of the external ear (0 N).
  
  
  RESULTS
  The mean thresholds with 0 N were much better than might be expected from classical theories in response to stimulation by a bone vibrator, in the absence of any application force. The differences between the mean thresholds with the 0 N and the 5 N forces depended on condition, site, and stimulus frequency of the comparisons. The difference was 1.5 dB at 1.0 kHz on the forehead; ranged between 10 and 12.5 dB at 1.0 kHz on the cavum and tragus (versus on the mastoid) and at 2.0 and 4.0 kHz on the forehead; 17 and 19 dB at 2.0 kHz on the cavum and tragus (versus on the mastoid); reaching 32 dB only in a single condition (forehead at 0.5 kHz).
  
  
  CONCLUSIONS
  As it is unlikely that threshold intensity stimulation delivered with 0 N application force could have induced vibrations of the underlying or nearby bone, inducing osseous BC, the relatively low thresholds in the absence of any application force, together with the small differences between the thresholds with 0 N (gel/soft tissue, nonosseous) and 5 N force (osseous BC) lead to the suggestion that in most situations, the BC thresholds actually represent the nonosseous (soft tissue conduction) thresholds at the stimulation site.
authors.affiliation: Speech and Hearing Center, Hadassah University Hospital, Jerusalem, Israel.|Department of Communication Disorders, Hadassah Academic College, Jerusalem, Israel.
authors.forename: Miriam
authors.initials: M
authors.lastname: Geal-Dor
authors.affiliation: Speech and Hearing Center, Hadassah University Hospital, Jerusalem, Israel.|Department of Communication Disorders, Sackler Faculty of Medicine, Tel Aviv University, Israel.
authors.forename: Shai
authors.initials: S
authors.lastname: Chordekar
authors.affiliation: Speech and Hearing Center, Hadassah University Hospital, Jerusalem, Israel.|Department of Communication Disorders, Hadassah Academic College, Jerusalem, 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, Israel.
authors.forename: Haim
authors.initials: H
authors.lastname: Sohmer
mesh_terms: D000293:Adolescent; D000328:Adult; D001309:Auditory Threshold / Q000502:physiology*; D001844:Bone Conduction / Q000502:physiology*; D002648:Child; D003238:Connective Tissue; D005260:Female; D006257:Head; D006320:Hearing Tests; D006801:Humans; D008297:Male; D008875:Middle Aged; D010812:Physical Stimulation / Q000379:methods*; D012016:Reference Values; D014732:Vibration; D055815:Young Adult