Bone conduction: an explanation for this phenomenon comprising complex mechanisms

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. Therefore, further investigation of bone conduction mechanisms is required.

Directional sensitivity of bone conduction stimulation on the otic capsule in a finite element model of the human temporal bone

Sound transmission to the human inner ear by bone conduction pathway with an implant attached to the otic capsule is a specific case where the cochlear response depends on the direction of the stimulating force. A finite element model of the temporal bone with the inner ear, no middle and outer ear structures, and an immobilized stapes footplate was used to assess the directional sensitivity of the cochlea. A concentrated mass represented the bone conduction implant. The harmonic analysis included seventeen frequencies within the hearing range and a full range of excitation directions. Two assessment criteria included: (1) bone vibrations of the round window edge in the direction perpendicular to its surface and (2) the fluid volume displacement of the round window membrane. The direction of maximum bone vibration at the round window edge was perpendicular to the round window. The maximum fluid volume displacement direction was nearly perpendicular to the modiolus axis, almost tangent to the stapes footplate, and inclined slightly to the round window. The direction perpendicular to the stapes footplate resulted in small cochlear responses for both criteria. A key factor responsible for directional sensitivity was the small distance of the excitation point from the cochlea.

The Audiometric Assessment of the Effectiveness of Surgical Treatment of Otosclerosis Depending on the Preoperative Incidence of Carhart's Notch

Objective: The presence of Carhart's notch at 2000 Hz in otosclerosis links the changed bone conduction for this frequency with the otosclerotic process occurring in the oval window. The aim of this study is to perform an audiometric assessment of the effectiveness of surgical treatment of otosclerosis depending on the incidence of Carhart's notch. Methods: The analysis included 116 patients treated surgically for the first time due to otosclerosis. Patients were divided into 4 groups depending on the occurrence of Carhart's notch, determined by pure-tone audiometry (PTA) before the surgery and 36 months afterward. The mean value of bone conduction thresholds was calculated for 500 Hz, 1000 Hz, 2000 Hz, and 3000 Hz in the groups in which the Cahart's notch was observed. This value of bone conduction (BC) was a reference point for further analysis in patients who had no preoperative or postoperative Carhart's notch. Results: The analysis indicated that Cahart's notch in preoperative PTA is a statistically significant improvement factor for average BC. It was found that over a longer observation period, the presence of Carhart's notch has adverse effects on the size of the postoperative air-bone gap, and consequently on hearing improvement after surgical treatment. A comparison between patients from the two groups without preoperative Carhart's notch found that no beneficial effects of the surgery on speech comprehension were observed regarding high-level sensorineural hearing loss (SNHL). Conclusions: (1) In a long-term observation post-stapedotomy, average BC values were found to improve. Nevertheless, the improvement is less evident in patients with preoperative Carhart's notch. (2) Disappearance of Cahart's notch after surgical treatment of otosclerosis is a good prognosis of improvement in speech audiometry. (3) Deep SNHL in the absence of Carhart's notch in PTA constitutes a bad prognostic factor for improvement in speech audiometry in patients qualified for surgical treatment of otosclerosis.

Noise-induced hearing loss in the contralateral ear during otologic and neurotologic surgeries

OBJECTIVE

Noise-induced hearing loss in the non-surgical ear during otologic/neurotologic surgery has not been well studied. The purpose of this study was to evaluate changes in hearing that may occur in the contralateral (i.e., non-surgical) ear after various otologic/neurotologic surgeries due to noise generated by drills. We hypothesized that otologic/neurotologic surgeries, longer in duration, would suggest longer drilling times and result in decreased hearing in the contralateral ear as evidenced by a change post-operative pure tone air conduction thresholds when compared to pre-operative thresholds.

METHODS

A retrospective chart review at a tertiary referral center. Adult patients (18-75 years old) who underwent otologic/neurotologic surgeries from May 1, 2016 through May 1, 2021 were considered for inclusion. Surgeries included vestibular schwannoma resection (translabyrinthine, middle cranial fossa, or retrosigmoid approaches), endolymphatic sac/shunt and labyrinthectomy for Meniere's disease, and tympanomastoid surgery for middle ear pathology (e.g., cholesteatoma). Patient characteristics obtained through record review included age, sex, surgical procedure, pre-operative and post-operative audiometric thresholds and word recognition scores (WRS) for the contralateral ear, and duration of surgery.

RESULTS

No significant differences were observed for change in audiometric thresholds in the contralateral ear for any surgery when considering individual frequencies. Additionally, no significant change in WRS was observed for any surgical approach.

CONCLUSIONS

The risk of hearing loss in the non-surgical ear during various otologic/neurotologic surgeries appears to be minimal when measured via routine clinical tests.

Estimates of interaural attenuation in children and the implications for masking in clinical audiometry

OBJECTIVE

The aim of this study was to provide estimates of interaural attenuation (IA) in children, under clinical test conditions for supra-aural and insert earphones.

DESIGN

This was a retrospective review of clinical audiograms for children aged 8 months to 16 years.

STUDY SAMPLE

There were between 2 and 21 subjects, depending on the transducer and stimulus frequency.

RESULTS

For insert earphones, younger age groups had smaller IA estimates (mean 60 dB, minimum 40 dB) compared to older children (mean 78 dB, minimum 60 dB). The insert IA estimates for older children were similar to published adult IA data. There was no significant effect of age on the children's estimated IA for supra-aural earphones.

CONCLUSIONS

Under the clinical conditions of this study, cross-hearing should be considered when the difference between the better ear and poorer ear not-masked air conduction thresholds are ≥ 40 dB for inserts with foam tips in children under 13 years. Smaller estimates of IA in younger children compared to older children may be due to difficulties achieving deep insertion of foam tips in smaller ears and less cooperative subjects under these conditions. Limitations of the study, including lack of bone conduction threshold data, are discussed.

Bone Conduction Cervical Vestibular Evoked Myogenic Potentials as an Alternative in Children with Middle Ear Effusion

OBJECTIVE

To compare the amplitude ratio and P-wave latency of cervical vestibular evoked myogenic potentials (c-VEMPs) for bone conduction (BC) and air conduction (AC) stimulation in children with otitis media with effusion (OME).

MATERIAL AND METHODS

This is an observational study of a cohort of 27 children and 46 ears with OME. The c-VEMP amplitude ratio and P-wave latency were compared between BC and AC in children with OME and healthy age-matched children.

RESULTS

The c-VEMP response rate in children with OME was 100% when using BC stimulation and 11% when using AC stimulation. The amplitude ratio for BC was significantly higher in the OME group than the age-matched healthy control group (p = 0.004). When focusing on ears with an AC c-VEMP response (n = 5), there was a significant difference in the amplitude ratio between the AC and BC stimulation modes, but there was no significant difference in the AC results between the OME group and the age-matched control group.

CONCLUSIONS

BC stimulation allows for reliable vestibular otolith testing in children with middle ear effusion. Given the high prevalence of OME in children, clinicians should be aware that recording c-VEMPs with AC stimulation may lead to misinterpretation of otolith dysfunction in pediatric settings.

Inter-aural separation during hearing by bilateral bone conduction stimulation

Cross-head transmission inherent in bone conduction (BC) hearing is one of the most important factors that limit the performance of BC binaural hearing compared to air conduction (AC) binaural hearing. In AC, cross-head transmission is imperceptible leading to a clear understanding of the nature and position of the sound source(s). In this study, the prominence of cross-head transmission in BC hearing is addressed using the fact that ipsilateral cochlear excitation can be canceled by controlled bilateral BC stimulation. A cancellation experiment was conducted on twenty participants with normal hearing at thirteen third-octave frequencies between 250 and 4000 Hz. Both stationary and transient BC stimulation at the mastoid was used. The technique employed multiple stages of masking enabling adjustments of the stimulation level and phase until the tones got canceled in the ipsilateral ear. In addition, the ear canal sound pressure was obtained for ipsilateral and contralateral BC stimulation in isolation, and with bilateral BC stimulation at perceptual cancellation. The inter-aural level differences of both the types of stimulations were found to be the same. Crosstalk was found to be the lowest around 2 kHz and the highest around 1 kHz. The unwrapped inter-aural phase difference from stationary signal cancellation showed an overall increase with frequency starting at around no difference (35°) at 250 Hz to reach 607° at 4 kHz. Cycle-adjusted inter-aural time difference was very low (61 µs) at 250 Hz and increased to 1.1 ms at 800 Hz before falling to 0.6 ms at 4 kHz. It was also found that the ear canal sound pressure was not cancelled at the same phase as the sound in the cochlea.

Audiological outcome after stapes surgery in relation to prosthesis type

PURPOSE

Different techniques are used to fix crimp and CliP® Piston stapes prostheses to the long process of the incus (LPI). The CliP® Piston provides a stiff connection in contrast to the static bended loop of the crimp prosthesis, which imitates the physiological incudostapedial joint (ISJ) and thereby potentially leads to different hearing outcome.

METHODS

In a retrospective single-center study of German-speaking one hundred and ninety patients who underwent stapes surgery CliP® Piston or crimp prostheses between the years of 2016 and 2019 by the same surgeon and in the same setting. Pre- and postoperative bone- (BC) and air-conduction (AC) pure-tone thresholds, pre- and postoperative air-bone gap (ABG) for 0.5, 1, 1.5, 2, 3, 4 kHz and the surgery time were examined.

RESULTS

The postoperative bone conduction thresholds were significantly lower in the frequencies between 0.5 and 3 kHz and the mean ABG was < 10 dB in most cases independent of the prosthesis used. Crimp prosthesis showed a significantly better closure of the ABG at 0.5 kHz.

CONCLUSIONS

The audiological outcome after stapes surgery is dependent on the type of prosthesis used, as reflected by the frequency-specific air-bone gap. The better ABG closure with the crimp prosthesis might be the result of the connection to the LPI imitating the physiological ISJ. The crimp prosthesis may be the better choice if use of hearing aids is expected postoperatively.

The sound stimulation method and EEG change analysis for development of digital therapeutics that can stimulate the nervous system: Cortical activation and drug substitution potential

INTRODUCTION

The purpose of this study is to propose a treatment method and the effect on the nervous system of digital therapeutics, which is a new treatment method to replace surgery and drug prescription for the treatment and prevention of diseases.

METHODS

The 20 subjects who participated in the experiment, including men and women, had an average age of 26 ± 2.40 years. The proposed treatment method used three types of sound stimulation and air or bone conduction sound transmission methods to induce total of 6-time EEG electroencephalogram(EEG) changes. EEG was measured with 200 sampling rate each in the P4, Cz, F8 and T7 channel located in the parietal, central, frontal and temporal lobes, respectively, according to the 10/10 system. A total of 2 min of data were created by extracting EEG signals with less noise from the measured data and the extracted data were applied with a 1-40 Hz Butterworth filter and a 50 Hz notch filter with a quality factor of 30. After that, EEG are subdivided into delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), and gamma (30-45 Hz) bands. Finally, EEG changes in response to sound stimuli were analyzed using power spectral density and T-test validation in the frequency band.

RESULTS

When a sound stimulus of less than 1 KHz was stimulated by air conduction, brainstem activation was induced and the reticular activation system was activated. In addition, a great potential for replacing drugs was confirmed by inducing changes in the nervous system similar to drugs used for sedation.

CONCLUSION

These results will be able to expand the concept of digital therapeutics, and it is expected that it will be developed as a safer treatment method that can replace surgery and drugs.

The outer ear pathway during hearing by bone conduction

There have been conflicting reports in the literature about the importance of the induced ear canal sound pressure for the perception of bone-conducted (BC) sound. Here we investigated this by comparing the ear canal sound pressure at threshold for air-conducted (AC) and BC stimulation. Twenty-one adults with subjectively normal hearing function participated. They were tested for their hearing thresholds in the frequency range 250 Hz to 12.5 kHz with AC and BC stimulation and the ear canal sound pressure within 5 mm of the eardrum was obtained with probe tube microphones. Contralateral masking used with BC stimulation shifted the hearing threshold by 5 to 10 dB due to central masking effects. When the ear canal sound pressures at threshold were investigated, the results indicate that the ear canal component for hearing BC sound is around 10 dB below other contributors at frequencies below 2 kHz and similar to other important contributors at frequencies between 2 and 4 kHz. At frequencies above 4 kHz, the contribution from the ear canal sound pressure on BC hearing declines and was around 40 dB below other contributors at 12.5 kHz. The contribution of the ear canal sound pressure in the mid-frequency region is facilitated by the ear canal resonance occurring in this frequency area. The results were similar irrespective of stimulation position. The study also revealed problems estimating the force out of BC transducers caused by a shift in resonance frequency when the artificial mastoid impedance deviates from the impedance of human mastoids. The current study indicates that model predictions have underestimated the contribution from the ear canal sound pressure on BC hearing by around 10 dB.

Bone conducted responses in the neonatal rat auditory cortex

Rats are born deaf and start hearing at the end of the second postnatal week, when the ear canals open and low-intensity sounds start to evoke responses in the auditory cortex. Here, using μECoG electrode arrays and intracortical silicon probe recordings, we found that bone-conducted (BC) sounds evoked biphasic responses in the auditory cortex starting from postnatal day (P) 8. The initial phase of these responses, generated by thalamocortical input, was followed by intracortical propagation within supragranular layers. BC-evoked responses co-localized with the responses evoked by electrical stimulation of the cochlea and the deepest layers of the inferior colliculus prior to onset of low-threshold hearing (P13), as well as with the responses evoked by high-frequency (30 kHz) low-intensity (70 dB) air-conducted sounds after that. Thus, BC signals reach high-frequency processing regions of the auditory cortex well before the onset of low-threshold hearing, reflecting early integrity of the auditory system.

A Comparison of Intracochlear Pressures During Ipsilateral and Contralateral Stimulation With a Bone Conduction Implant

OBJECTIVES

To compare contralateral to ipsilateral stimulation with percutaneous and transcutaneous bone conduction implants.

BACKGROUND

Bone conduction implants (BCIs) effectively treat conductive and mixed hearing losses. In some cases, such as in single-sided deafness, the BCI is implanted contralateral to the remaining healthy ear in an attempt to restore some of the benefits provided by binaural hearing. While the benefit of contralateral stimulation has been shown in at least some patients, it is not clear what cues or mechanisms contribute to this function. Previous studies have investigated the motion of the ossicular chain, skull, and round window in response to bone vibration. Here, we extend those reports by reporting simultaneous measurements of cochlear promontory velocity and intracochlear pressures during bone conduction stimulation with two common BCI attachments, and directly compare ipsilateral to contralateral stimulation.

METHODS

Fresh-frozen whole human heads were prepared bilaterally with mastoidectomies. Intracochlear pressure (PIC) in the scala vestibuli (PSV) and tympani (PST) was measured with fiber optic pressure probes concurrently with cochlear promontory velocity (VProm) via laser Doppler vibrometry during stimulation provided with a closed-field loudspeaker or a BCI. Stimuli were pure tones between 120 and 10,240 Hz, and response magnitudes and phases for PIC and VProm were measured for air and bone conducted sound presentation.

RESULTS

Contralateral stimulation produced lower response magnitudes and longer delays than ipsilateral in all measures, particularly for high-frequency stimulation. Contralateral response magnitudes were lower than ipsilateral response magnitudes by up to 10 to 15 dB above ~2 kHz for a skin-penetrating abutment, which increased to 25 to 30 dB and extended to lower frequencies when applied with a transcutaneous (skin drive) attachment.

CONCLUSIONS

Transcranial attenuation and delay suggest that ipsilateral stimulation will be dominant for frequencies over ~1 kHz, and that complex phase interactions will occur during bilateral or bimodal stimulation. These effects indicate a mechanism by which bilateral users could gain some bilateral advantage.

Sensory Stimulation in the NICU Environment: Devices, Systems, and Procedures to Protect and Stimulate Premature Babies

Prematurity deprives infants of the prenatal sensory stimulation essential to their correct development; in addition, the stressful environment of the NICU impacts negatively on their growth. The purpose of this review was to investigate the effects of NICU noise pollution on preterm infants and parents. We focused on the systems and projects used to control and modulate sounds, as well as on those special devices and innovative systems used to deliver maternal sounds and vibrations to this population. The results showed beneficial effects on the preterm infants in different areas such as physiological, autonomic, and neurobehavioral development. Although most of these studies highlight positive reactions, there is also a general acknowledgement of the current limits: small and heterogeneous groups, lack of structured variable measurements, systematic control groups, longitudinal studies, and normative values. The mother's presence is always preferred, but the use of music therapy and the devices analyzed, although not able to replace her presence, aim to soften her absence through familiar and protective stimuli, which is a very powerful aid during the COVID-19 pandemic.

Neural competition between concurrent speech production and other speech perception

Understanding others' speech while individuals simultaneously produce speech utterances implies neural competition and requires specific mechanisms for a neural resolution given that previous studies proposed opposing signal dynamics for both processes in the auditory cortex (AC). We here used neuroimaging in humans to investigate this neural competition by lateralized stimulations with other speech samples and ipsilateral or contralateral lateralized feedback of actively produced self speech utterances in the form of various speech vowels. In experiment 1, we show, first, that others' speech classifications during active self speech lead to activity in the planum temporale (PTe) when both self and other speech samples were presented together to only the left or right ear. The contralateral PTe also seemed to indifferently respond to single self and other speech samples. Second, specific activity in the left anterior superior temporal cortex (STC) was found during dichotic stimulations (i.e. self and other speech presented to separate ears). Unlike previous studies, this left anterior STC activity supported self speech rather than other speech processing. Furthermore, right mid and anterior STC was more involved in other speech processing. These results signify specific mechanisms for self and other speech processing in the left and right STC beyond a more general speech processing in PTe. Third, other speech recognition in the context of listening to recorded self speech in experiment 2 led to largely symmetric activity in STC and additionally in inferior frontal subregions. The latter was previously reported to be generally relevant for other speech perception and classification, but we found frontal activity only when other speech classification was challenged by recorded but not by active self speech samples. Altogether, unlike formerly established brain networks for uncompetitive other speech perception, active self speech during other speech perception seemingly leads to a neural reordering, functional reassignment, and unusual lateralization of AC and frontal brain activations.

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.

RESULTS

BC thresholds were normal (≤15 dB HL, i.e., better) in 19 (30%) radical mastoidectomy patients and in 19 (8%) subtotal petrosectomy patients at each of the frequencies assessed (0.5, 1.0, 2.0, and 4.0 kHz).

CONCLUSION

Normal BC thresholds seen in many patients following mastoidectomy and petrosectomy may be induced by a non-osseous mechanism, and the onset ("threshold") of the classical osseous BC mechanisms may be somewhat higher.

To Hear or Not to Hear: Sound Availability Modulates Sensory-Motor Integration

When we walk in place with our eyes closed after a few minutes of walking on a treadmill, we experience an unintentional forward body displacement (drift), called the sensory-motor aftereffect. Initially, this effect was thought to be due to the mismatch experienced during treadmill walking between the visual (absence of optic flow signaling body steadiness) and proprioceptive (muscle spindles firing signaling body displacement) information. Recently, the persistence of this effect has been shown even in the absence of vision, suggesting that other information, such as the sound of steps, could play a role. To test this hypothesis, six cochlear-implanted individuals were recruited and their forward drift was measured before (Control phase) and after (Post Exercise phase) walking on a treadmill while having their cochlear system turned on and turned off. The relevance in testing cochlear-implanted individuals was that when their system is turned off, they perceive total silence, even eliminating the sounds normally obtained from bone conduction. Results showed the absence of the aftereffect when the system was turned off, underlining the fundamental role played by sounds in the control of action and breaking new ground in the use of interactive sound feedback in motor learning and motor development.