Issues Concerning the Mechanisms of Bone Conduction.

Air and bone conduction thresholds are used to differentiate between conductive and sensori-neural hearing losses because bone conduction is thought to bypass the conductive apparatus, directly activating the inner ear. However, the suggested bone conduction mechanisms involve the outer and middle ears. Also, normal bone conduction thresholds have been reported in cases of lesions to the conduction pathway.

Soft Tissue Conduction Activates the Auditory Pathway in the Brain.

Soft tissue conduction is a mode of hearing which differs from air and bone conduction since the soft tissues of the body convey the audio-frequency vibrations to the ear. It is elicited by inducing soft tissue vibrations with an external vibrator applied to sites on the body or by intrinsic vibrations resulting from vocalization or the heartbeat. However, the same external vibrator applied to the skin sites also excites cutaneous mechanoreceptors, and attempts have been made to assist patients with hearing loss by audio-tactile substitution.

How Is the Cochlea Activated in Response to Soft Tissue Auditory Stimulation in the Occluded Ear?

Soft tissue conduction is an additional mode of auditory stimulation which can be initiated either by applying an external vibrator to skin sites not overlying skull bone such as the neck (so it is not bone conduction) or by intrinsic body vibrations resulting, for example, from the heartbeat and vocalization. The soft tissue vibrations thereby induced are conducted by the soft tissues to all parts of the body, including the walls of the external auditory canal. In order for soft tissue conduction to elicit hearing, the soft tissue vibrations which are induced must penetrate into the cochlea in order to excite the inner ear hair cells and auditory nerve fibers.

The Effect of Soft Tissue Stimulation on Skull Vibrations and Hearing Thresholds in Humans.

Hypothesis: Hearing via soft tissue stimulation involves an osseous pathway.

Background: A recent study that measured both hearing thresholds and skull vibrations found that vibratory stimulation of soft tissue led to hearing sensation that correlated with skull vibrations, supporting the hypothesis of an osseous pathway. It is possible, however, that a lower application force of the vibrator on the stimulated soft tissue would not be sufficient to elicit skull vibration suggesting hearing via a nonosseous pathway.

Occlusion Effect in Response to Stimulation by Soft Tissue Conduction-Implications.

To gain insight into the broader implications of the occlusion effect (OE-difference between unoccluded and occluded external canal thresholds), the OE in response to pure tones at 0.5, 1.0, 2.

Hearing at threshold intensities: by slow mechanical traveling waves or by fast cochlear fluid pressure waves.

The three modes of auditory stimulation (air, bone and soft tissue conduction) at threshold intensities are thought to share a common excitation mechanism: the stimuli induce passive displacements of the basilar membrane propagating from the base to the apex (slow mechanical traveling wave), which activate the outer hair cells, producing active displacements, which sum with the passive displacements. However, theoretical analyses and modeling of cochlear mechanics provide indications that the slow mechanical basilar membrane traveling wave may not be able to excite the cochlea at threshold intensities with the frequency discrimination observed. These analyses are complemented by several independent lines of research results supporting the notion that cochlear excitation at threshold may not involve a passive traveling wave, and the fast cochlear fluid pressures may directly activate the outer hair cells: opening of the sealed inner ear in patients undergoing cochlear implantation is not accompanied by threshold elevations to low frequency stimulation which would be expected to result from opening the cochlea, reducing cochlear impedance, altering hydrodynamics.

Implications from cochlear implant insertion for cochlear mechanics.

It is usually thought that the displacements of the two inner ear windows induced by sound stimuli lead to pressure differences across the basilar membrane and to a passive mechanical traveling wave progressing along the membrane. However, opening a hole in the sealed inner ear wall in experimental animals is surprisingly not accompanied by auditory threshold elevations. It has also been shown that even in patients undergoing cochlear implantation, elevation of threshold to low-frequency acoustic stimulation is often not seen accompanying the making of a hole in the wall of the cochlea for insertion of the implant.

Audiogram in Response to Stimulation Delivered to Fluid Applied to the External Meatus.

Background And Objectives: Hearing can be elicited in response to vibratory stimuli delivered to fluid in the external auditory meatus. To obtain a complete audiogram in subjects with normal hearing in response to pure tone vibratory stimuli delivered to fluid applied to the external meatus. Subjects and.

Implications for Bone Conduction Mechanisms from Thresholds of Post Radical Mastoidectomy and Subtotal Petrosectomy Patients.

Objectives: To assess bone conduction (BC) thresholds following radical mastoidectomy and subtotal petrosectomy, in which the tympanic membrane and the ossicular chain, responsible for osseous BC mechanisms, are surgically removed. The removal of the tympanic membrane and the ossicular chain would reduce the contributions to BC threshold of the following four osseous BC mechanisms: the occlusion effect of the external ear, middle ear ossicular chain inertia, inner ear fluid inertia, and distortion (compression-expansion) of the walls of the inner ear.

Materials And Methods: BC thresholds were determined in 64 patients who underwent radical mastoidectomy and in 248 patients who underwent subtotal petrosectomy.

Does hearing in response to soft-tissue stimulation involve skull vibrations? A within-subject comparison between skull vibration magnitudes and hearing thresholds.

Hearing can be elicited in response to bone as well as soft-tissue stimulation. However, the underlying mechanism of soft-tissue stimulation is under debate. It has been hypothesized that if skull vibrations were the underlying mechanism of hearing in response to soft-tissue stimulation, then skull vibrations would be associated with hearing thresholds.

Soft Tissue Conduction: Review, Mechanisms, and Implications.

Soft tissue conduction (STC) is a recently explored mode of auditory stimulation, complementing air (AC) and bone (BC) conduction stimulation. STC can be defined as the hearing induced when vibratory stimuli reach skin and soft tissue sites not directly overlying skull bone such as the head, neck, thorax, and body. Examples of STC include the delivery of vibrations to the skin of parts of the body by a clinical bone vibrator, hearing underwater sounds and free field air sounds, while AC hearing is attenuated by earplugs.

Inner Ear Excitation in Normal and Postmastoidectomy Participants by Fluid Stimulation in the Absence of Air- and Bone-Conduction Mechanisms.

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.

Purpose: This study was designed to gain insight into the mechanism of STC auditory stimulation.

Soft tissue conduction as a possible contributor to the limited attenuation provided by hearing protection devices.

Context: Damage to the auditory system by loud sounds can be avoided by hearing protection devices (HPDs) such as earmuffs, earplugs, or both for maximum attenuation. However, the attenuation can be limited by air conduction (AC) leakage around the earplugs and earmuffs by the occlusion effect (OE) and by skull vibrations initiating bone conduction (BC).

Aims: To assess maximum attenuation by HPDs and possible flanking pathways to the inner ear.

Fluid stimulation elicits hearing in the absence of air and bone conduction--An animal study.

Conclusion Cochlea can be directly excited by fluid (soft-tissue) stimulation. Objective To determine whether there is no difference in auditory-nerve-brainstem evoked response (ABR) thresholds to fluid stimulation between normal and animal models of post radical-mastoidectomy, as seen in a previous human study. Background It has been shown in humans that hearing can be elicited with stimulation to fluid in the external auditory meatus (EAM), and radical-mastoidectomy cavity.

Experimental Analysis of the Mechanism of Hearing under Water.

The mechanism of human hearing under water is debated. Some suggest it is by air conduction (AC), others by bone conduction (BC), and others by a combination of AC and BC. A clinical bone vibrator applied to soft tissue sites on the head, neck, and thorax also elicits hearing by a mechanism called soft tissue conduction (STC) or nonosseous BC.

Bone Conduction Thresholds without Bone Vibrator Application Force.

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).

Relation between Body Structure and Hearing during Soft Tissue Auditory Stimulation.

Hearing is elicited by applying the clinical bone vibrator to soft tissue sites on the head, neck, and thorax. Two mapping experiments were conducted in normal hearing subjects differing in body build: determination of the lowest soft tissue stimulation site at which a 60 dB SL tone at 2.0 kHz was effective in eliciting auditory sensation and assessment of actual thresholds along the midline of the head, neck, and back.

Air conduction, bone conduction, and soft tissue conduction audiograms in normal hearing and simulated hearing losses.

Background: In order to differentiate between a conductive hearing loss (CHL) and a sensorineural hearing loss (SNHL) in the hearing-impaired individual, we compared thresholds to air conduction (AC) and bone conduction (BC) auditory stimulation. The presence of a gap between these thresholds (an air-bone gap) is taken as a sign of a CHL, whereas similar threshold elevations reflect an SNHL. This is based on the assumption that BC stimulation directly excites the inner ear, bypassing the middle ear.

Investigation of the mechanism of soft tissue conduction explains several perplexing auditory phenomena.

Soft tissue conduction (STC) is a recently expounded mode of auditory stimulation in which the clinical bone vibrator delivers auditory frequency vibratory stimuli to skin sites on the head, neck, and thorax. Investigation of the mechanism of STC stimulation has served as a platform for the elucidation of the mechanics of cochlear activation, in general, and to a better understanding of several perplexing auditory phenomena. This review demonstrates that it is likely that the cochlear hair cells can be directly activated at low sound intensities by the fluid pressures initiated in the cochlea; that the fetus in utero, completely enveloped in amniotic fluid, hears by STC; that a speaker hears his/her own voice by air conduction and by STC; and that pulsatile tinnitus is likely due to pulsatile turbulent blood flow producing fluid pressures that reach the cochlea through the soft tissues.

The mechanism of direct stimulation of the cochlea by vibrating the round window.

Background: Active middle ear implants such as the vibrant sound bridge (VSB) have been placed on the round window (RW) in patients with conductive or mixed hearing loss, with satisfactory hearing results. Several observations show that the mechanism of RW stimulation is not completely understood. The purpose of the present study was to compare different coupling procedures between the transducer and the RW in order to contribute to an understanding of the mechanism of RW stimulation.

Reflections on the role of a traveling wave along the basilar membrane in view of clinical and experimental findings.

Air conduction (AC) is accompanied by displacements of the two cochlear windows, bulk fluid flow between them, a pressure difference across the basilar membrane, leading to a passive traveling wave along the membrane, which activates the cochlear amplifier and enhances the displacements. AC interacts with bone conduction (BC) stimulation, so that it has been assumed that BC stimulation also involves a passive traveling wave. However, several clinical conditions and experimental manipulations provide evidence that a passive traveling wave may not be involved in BC stimulation at low intensities.

Air, bone and soft tissue excitation of the cochlea in the presence of severe impediments to ossicle and window mobility.

Clinical conditions have been described in which one of the two cochlear windows is immobile (otosclerosis) or absent (round window atresia), but nevertheless bone conduction (BC) thresholds are relatively unaffected. To clarify this apparent paradox, experimental manipulations which would severely impede several of the classical osseous mechanisms of BC were induced in fat sand rats, including discontinuity or immobilization of the ossicular chain, coupled with window fixation. Effects of these manipulations were assessed by recording auditory nerve brainstem evoked response (ABR) thresholds to stimulation by air conduction (AC), by osseous BC and by non-osseous BC (also called soft tissue conduction-STC) in which the BC bone vibrator is applied to skin sites.

Effect on auditory threshold of air and water pockets bordering the pathway between soft tissue stimulation sites and the cochlea.

Objectives: Auditory sensation can be elicited by applying a bone conduction vibrator to skin sites on the head, neck, and thorax over soft tissues. This is called soft tissue conduction (STC). We hypothesized that introducing substances with acoustic impedances that sharply deviate from those of soft tissues, such as air pockets, into the soft tissues beneath soft tissue stimulation sites would have an effect on the auditory threshold to stimulation at skin sites over soft tissue.

Mutual masking in the cochlea by bone conduction stimulation and by soft tissue conduction stimulation in an animal model.

Background: Soft tissue conduction (STC) is a recently described mode of auditory stimulation in which vibrations induced by a clinical bone vibrator applied to soft tissue sites on the head, neck, and thorax of human subjects reach the cochlea and elicit auditory sensation. In humans, STC stimulation interacts with air conduction stimulation and with bone conduction (BC) stimulation in several ways, e.g.