Soft Tissue Conduction Research Articles

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, 2.0 and 4.0 kHz to two bone conduction sites (mastoid and forehead) and two soft tissue conduction (STC) sites (under the chin and at the neck) were assessed. The OE was present at the soft tissue sites and at the bone conduction sites, with no statistical difference between them. The OE was significantly greater at lower frequencies, and negligible at higher frequencies. It seems that the vibrations induced in the soft tissues (STC) during stimulation at the soft tissue sites are conducted not only to the inner ear and elicit hearing, but also reach the walls of the external canal and initiate air pressures in the occluded canal which drive the tympanic membrane and excite the inner ear, leading to hearing. Use of a stethoscope by the internist to hear intrinsic body sounds (heartbeat, blood flow) serves as a clear demonstration of STC and its relation to hearing.

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. The vibrations induced in the soft tissues are apparently transmitted along soft tissues, reaching, and exciting the ear. Further research is required to determine whether the mechanism of the final stage of STC hearing involves the excitation of the ear by eliciting inner ear fluid pressures that activate the hair cells directly, by the induction of skull bone vibrations, or by a combination of both mechanisms, depending on the magnitude of each mechanism.

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. The present study was designed to test whether underwater hearing at low intensities is by AC or by osseous BC based on bone vibrations or by nonosseous BC (STC). Thresholds of normal hearing participants to bone vibrator stimulation with their forehead in air were recorded and again when forehead and bone vibrator were under water. A vibrometer detected vibrations of a dry human skull in all similar conditions (in air and under water) but not when water was the intermediary between the sound source and the skull forehead. Therefore, the intensities required to induce vibrations of the dry skull in water were significantly higher than the underwater hearing thresholds of the participants, under conditions when hearing by AC and osseous BC is not likely. The results support the hypothesis that hearing under water at low sound intensities may be attributed to nonosseous BC (STC).

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.

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. Soft tissue conduction (STC) (also called non-osseous bone conduction) involves applying the bone vibrator to soft tissues on the head, neck and thorax, eliciting auditory sensation. STC stimulation probably does not involve a passive traveling wave. This review presents clinical conditions and experimental manipulations which assess the contributions of AC, BC and STC stimulation to the passive traveling wave. Evidence from the clinic (otosclerosis, round window atresia) and from the laboratory (holes in the wall of the inner ear, immobilization of the ossicular chain and the windows, discontinuity of the chain, measurement of basilar membrane displacements in the absence of the cochlear amplifier) lead to the conclusion that a passive basilar membrane traveling wave may not be involved in stimulation at low sound intensities. It is suggested that at low sound levels, the outer hair cell cochlear amplifier may not be activated by a passive traveling wave, but may be directly activated by the fast cochlear fluid pressures induced by AC, BC and STC stimulation. On the other hand, at high intensities, the cochlea is activated by the slow passive traveling wave.

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

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., mutual masking. This study investigated whether mutual masking between STC and BC stimulation can be demonstrated in an experimental animal. In fat sand rats, auditory nerve and brainstem evoked response to BC stimulation was recorded in the presence of noise masking presented by STC and vice versa. STC successfully masked BC, and BC also masked STC responses. Mutual masking, now demonstrated in animals, paves the way for animal experiments to clarify the pathway between the STC stimulation sites and the cochlea.

Effect on Auditory Threshold of Air and Water Pockets Bordering the Pathway between Soft Tissue Stimulation Sites and the Cochlea

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. In human subjects, we assessed the auditory threshold with a bone vibrator applied to several STC sites, especially the cheek, and to several bone conduction sites on the skull. The subjects were equipped with bilateral earplugs. The subject then filled his or her cheek with either air or water, and the auditory threshold was again determined. We also recorded the auditory brain stem response to STC stimulation under the chin in fat sand rats in the absence and presence of subcutaneous air or saline solution pockets (0.4 mL) under the chin. In humans, the threshold to stimulation on the cheek was elevated (13 to 18 dB) in the presence of an air-inflated cheek, but not with a water-filled cheek. In animals, in the presence of an air pocket, the auditory brain stem response threshold was elevated by 10 to 20 dB; no threshold change occurred with a saline solution pocket. The introduction of air (but not water) into the soft tissues beneath the soft tissue stimulation sites led to a threshold elevation in both humans and animals. This was not the case when an identical volume of water was introduced, which would also have interrupted a possible parallel bone conduction pathway. These results provide evidence that soft tissue stimulation at low intensities induces tissue vibrations that are transmitted to the cochlea along a series of soft tissues with similar acoustic impedances.

Assessment of inner ear bone vibrations during auditory stimulation by bone conduction and by soft tissue conduction

Soft tissue conduction (STC), a recently described mode of auditory stimulation elicited when the clinical bone vibrator is applied to skin sites over the head, neck, and thorax, complements air conduction (AC) and bone conduction (BC), elicited by the same vibrator. The study assessed skull bone vibrations induced during STC and BC stimulation. The experiments were conducted on fat sand rats. Thresholds of auditory nerve brainstem evoked responses (ABRs) were recorded and compared to the lowest-intensity sound stimuli that elicited vibrations at the bony vestibule of the inner ear detected by a laser Doppler vibrometer. Vibrations were detected during BC but not during STC stimulation. ABR was recorded to both STC and to BC stimulation. Low-intensity STC stimulation does not induce vibrations of the inner ear, showing that STC apparently does not involve mechanisms based on vibrations of bone.

Experimental Exploration of the Soft Tissue Conduction Pathway from Skin Stimulation Site to Inner Ear

Auditory sensation can be elicited by air conduction (AC) and by bone conduction (BC). It is also possible to elicit such responses by applying the standard clinical bone vibrator to the skin over soft tissue sites on the head, neck, or thorax of humans and animals. This mode of auditory stimulation has been called soft tissue conduction (STC). This study was designed to investigate the pathway between soft tissue sites and the ear. The air in the middle ear was replaced with saline solution in an animal with unique anatomy--the fat sand rat, in which about 70% of a thin-walled inner ear bulges into the middle ear bulla cavity--while we recorded the auditory brain stem responses (ABRs) to AC, BC, and STC stimulation. This replacement of air with saline solution led to a significant improvement in STC threshold. With AC stimulation, the ABR threshold was elevated and the latency of the first ABR wave was prolonged. Consistent changes were not seen with BC stimulation. When the air (which has a very low acoustic impedance) that normally surrounds most of the inner ear is replaced with saline solution (which has an acoustic impedance similar to that of soft tissues), the STC threshold is improved. This improvement may be due to improved transmission of acoustic energy from the soft tissues to the inner ear.

Experimental confirmation that vibrations at soft tissue conduction sites induce hearing by way of a new mode of auditory stimulation

A new mode of auditory stimulation has been demonstrated which is through soft tissue conduction (STC). It involves evoking auditory sensations by applying the clinical bone vibrator to the skin over soft tissue (not over bone) sites on the head and neck. This study was designed to show that stimulation by STC excites the cochlea in a way similar to that of air conduction (AC) and bone conduction (BC). It is shown here that auditory nerve brainstem evoked response (ABR) thresholds in mice and in the fat sand rat to AC, to BC and to STC stimulation are all elevated following administration of drugs (salicylic acid and furosemide) which depress the cochlear amplifier. In addition, the present study brings evidence that STC stimulation is not a variant of BC since the sound pressures recorded in the occluded external auditory canal (the occlusion effect) in response to STC are significantly smaller than that to BC stimulation, though both are of equal loudness. This new mode, STC, therefore appears to bypass the middle ear mechanisms and consequently may contribute to auditory diagnosis.