Low-frequency Tone Bursts Research Articles

Round Window Electrocochleography to Low Frequency Tones in Pediatric Cochlear Implant Recipients with and Without Auditory Neuropathy Spectrum Disorder: Separating Hair Cell and Neural Contributions Using a Computational Model.

Characterize the contribution of the auditory nerve neurophonic (ANN) to electrocochleography (ECochG) of pediatric cochlear implant (CI) recipients with and without auditory nerve spectrum disorder (ANSD). ECochG is an emerging technique for predicting outcomes in CI recipients. Its utility may be increased by separating the cochlear microphonic (CM), produced by hair cells, from the ANN, the evoked potential correlate of neural phase-locking, which are mixed in the ongoing portion of the response to low frequency tone bursts. Responses to tone bursts of different frequency and intensities were recorded from the round window of pediatric CI recipients. Separation of the CM and ANN was performed using a model of the underlying processes that lead to the shapes of the observed waveforms. Preoperative mean pure tone amplitudes of the included ANSD (n = 36) and non-ANSD subjects (n = 123), were similar (89.5 and 93.5, p = 0.1). Total of 1,024 ECochG responses to frequency and intensity series were recorded. The mean correlation ( r ) between the input and the modeled signals was 0.973 ± 0.056 (standard deviation). The ANN magnitudes were higher in the ANSD group (ANOVAs, F = 26.5 for frequency and 21.9 for intensity, df's = 1, p 's < 0.001). However, its relative contribution to the overall signal was lower (ANOVAs, F = 25.8 and 12.1, df = 1, p 's < 0.001). ANN was detected in low frequency ECochG responses but not high frequency responses in both ANSD and non-ANSD subjects. ANSD subjects, evidence of neural contribution in responses to low frequency stimuli was highly variable and often comparable to signals recorded in non-ANSD subjects. The computational model revealed that on average the ANN comprised a lower proportion of the overall signal than in non-ANSD subjects.

Auditory nerve phase-locked response recorded from normal hearing adults using electrocochleography

Objective The auditory nerve overlapped waveform response (ANOW), a new measure that can be recorded non-invasively from humans, holds promise for providing more accurate assessment of low frequency hearing thresholds than currently used objective measures. This research aims to investigate the robustness and the nature of the ANOW response in humans. Design Repeated within-session recordings of the ANOW response using low-frequency Tone Bursts (TBs) were obtained at multiple stimulus levels. ANOW’s absolute amplitude and phase locking value (PLV) measures were analysed to obtain normative data and to test the reliability of the ANOW response. Study sample Thirteen normal hearing adults within the age range of 25 to 40 years. Results ANOW response was obtained to both 250 Hz and 500 Hz TBs and was traced down to 30–40 dB nHL. ANOW response showed significantly higher amplitude and stronger phase locking using 250 Hz TB compared to 500 Hz TB. High degree of test retest reliability of the ANOW response was found using 250 Hz TB at presentation levels higher than 40 dB nHL. Conclusions ANOW response is recordable noninvasively using low-frequency TBs and shows higher robustness as the stimulus frequency decreases.

Towards optimal selection of stimuli polarity method for effective evoking auditory brainstem responses.

Towards eliminating stimulus artifacts, alternating polarity stimuli have been widely adopted in eliciting the auditory brainstem response. However, considering the difference in the physiologic basis of the positive and negative polarity stimuli on the auditory system, it is unclear whether alternating polarity stimuli would adversely affect the auditory brainstem response characteristics. This research proposes a new polarity method for stimulus artifacts elimination, Sum polarity, that separately utilized the rarefaction and condensation stimuli and then summed the two evoked responses. We compared the waveform morphology and latencies of the auditory brainstem responses evoked by familiar stimuli (including click, tone-burst, and chirp) with different polarity methods in normal-hearing subjects to investigate the new method's effectiveness. The experimental results showed that alternating polarity of the click and chirp had little effect on the auditory brainstem response. In contrast, alternating polarity affected the waveform morphology and latencies of the auditory brainstem responses to the low-frequency tone-burst, with the effect decreasing as the stimulus frequency increased. These results demonstrated the performance of any polarity method is related to the characteristics of the stimulus signal itself, and no polarity method is optimal for all types of stimuli. Based on the analysis of experimental results, a fixed polarity and alternating polarity were recommended for the click and chirp auditory brainstem responses, respectively. Furthermore, considering the apparent latency differences between the responses to opposite polarity stimuli, the Sum polarity was suggested for the tone-burst auditory brainstem responses. Moreover, this work verified the feasibility of the Sum polarity, which offers another choice for eliminating stimulus artifacts in an evoked potential acquisition.

Band-Limited Chirp-Evoked Compound Action Potential in Guinea Pig: Comprehensive Neural Measure for Cochlear Implantation Monitoring.

Patients with severely impaired high-frequency hearing and sufficient residual low-frequency hearing can be provided with a cochlear implant (CI), thereby facilitating ipsilateral electric and acoustic stimulation with established advantages over electric stimulation alone. However, partial or complete hearing loss often occurred after implantation due to, inter alia, acute mechanical trauma to cochlear structures during electrode insertion. Possibilities of intraoperative monitoring using electrocochleography (ECochG) have recently been studied in CI patients, primarily using the ongoing response to low-frequency tone bursts consisting of the cochlear microphonic (CM) and the auditory nerve neurophonic. By contrast, the transient neural response to tone bursts, that is, compound action potential (CAP), was generally less detectable or less sensitive as a monitoring measure, thus falling short of providing useful contribution to electrocochleography analysis. In this study, we investigate using chirps to evoke more robust CAP responses in a limited frequency band by synchronizing neural firing, and thereby improving CAP sensitivity to mechanical trauma in a guinea pig model of cochlear implantation. Stimuli were band-limited between 100 Hz and 10 kHz to investigate their frequency range selectivity as a preliminary model for low-frequency hearing. They were constructed by adding a harmonic series either with zero phase delay (click) or by adjusting the phase delay at a rate that is inversely related to a traveling wave delay model (chirp), with three different parameters to examine level-dependent delay compression. The amplitude spectrum was thus identical between stimuli with differences only in phase. In Experiment 1, we compared input-output functions recorded at the round window in normal-hearing guinea pigs and implemented a high-pass noise masking paradigm to infer neural contribution to the CAP. In Experiment 2, guinea pigs were implanted with a custom-built CI electrode using a motorized micromanipulator. Acute mechanical trauma was simulated during the electrode insertion. At each insertion step, CAP and CM responses were measured at the round window for the following stimuli: broad-band click, band-limited click, and band-limited chirps (3 parameters), and tone bursts at frequencies 1, 2, 4, and 8 kHz. Chirps compared with the equal-band click showed significantly lower thresholds and steeper slopes of sigmoid-fitted input-output functions. The shorter chirp evoked significantly larger amplitudes than click when compared at equal sensation level. However, the click evoked larger amplitudes than chirps at higher levels and correspondingly achieved larger saturation amplitudes. The results of the high-pass noise masking paradigm suggest that chirps could efficiently synchronize neural firing in their targeted frequency band, while the click recruited more basal fibers outside its limited band. Finally, monitoring sensitivity during electrode insertion, defined as relative amplitude change per unit distance, was higher for chirp-evoked CAP and tone burst-evoked CM, but smaller for CAP responses evoked by clicks or tone bursts. The chirp was shown to be an efficient stimulus in synchronizing neural firing for a limited frequency band in the guinea pig model. This study provides a proof of principle for using chirp-evoked CAP as a comprehensive neural measure in CI patients with residual hearing.

Ocular vestibular evoked myogenic potential (VEMP) reveals mesencephalic HTLV-1-associated neurological disease.

PurposeVestibular Myogenic Evoked Potential (VEMP) evaluates vestibulo-ocular and vestibulo-collic reflexes involved in the function of the otolithic organs and their afferent pathways. We compared the results of cervical and ocular VEMP in HTLV-1 associated myelopathy (HAM) and HTLV-1-asymptomatic infection.Participants and methodsThis cross-sectional study included 52 HTLV-1-infected individuals (26 HAM and 26 asymptomatic carriers) and 26 seronegative controls. The groups were similar regarding age and gender. Participants underwent simultaneous ocular and cervical VEMP. The stimulus to generate VEMP was a low-frequency tone burst sound tone burst, with an intensity of 120 decibels normalized hearing level, bandpass filter from 10 to 1,500 Hertz (Hz), with 100 stimuli at 500 Hz and 50 milliseconds recording time. The latencies of the electrophysiological waves P13 and N23 for cervical VEMP and N10 and P15 waves for ocular VEMP were compared among the groups. The absence or delay of the electrophysiological waves were considered abnormal results.ResultsOcular VEMP was similar among the groups for N10 (p = 0.375) and different for P15 (p≤0.001). Cervical VEMP was different for P13 (p = 0.001) and N23 (p = 0.003). About ocular VEMP, in the HTLV-1-asymptomatic group, normal waves were found in 23(88.5%) individuals; in HAM group, normal waves were found in 7(26.9%). About cervical VEMP, 18(69.2%) asymptomatic carriers presented normal waves and only 3(11.5%) patients with HAM presented normal waves. Abnormalities in both VEMPs were found in 1(3.8%) asymptomatic carrier and in 16(61.5%) patients with HAM.ConclusionNeurological impairment in HAM was not restricted to the spinal cord. The mesencephalic connections, tested by ocular VEMP, have been also altered. Damage of the oculomotor system, responsible for eye stabilization during head and body movements, may explain why dizziness is such a frequent complaint in HAM.

Cochlear compound action potentials from high-level tone bursts originate from wide cochlear regions that are offset toward the most sensitive cochlear region.

Little is known about the spatial origins of auditory nerve (AN) compound action potentials (CAPs) evoked by moderate to intense sounds. We studied the spatial origins of AN CAPs evoked by 2- to 16-kHz tone bursts at several sound levels by slowly injecting kainic acid solution into the cochlear apex of anesthetized guinea pigs. As the solution flowed from apex to base, it sequentially reduced CAP responses from low- to high-frequency cochlear regions. The times at which CAPs were reduced, combined with the cochlear location traversed by the solution at that time, showed the cochlear origin of the removed CAP component. For low-level tone bursts, the CAP origin along the cochlea was centered at the characteristic frequency (CF). As sound level increased, the CAP center shifted basally for low-frequency tone bursts but apically for high-frequency tone bursts. The apical shift was surprising because it is opposite the shift expected from AN tuning curve and basilar membrane motion asymmetries. For almost all high-level tone bursts, CAP spatial origins extended over 2 octaves along the cochlea. Surprisingly, CAPs evoked by high-level low-frequency (including 2 kHz) tone bursts showed little CAP contribution from CF regions ≤ 2 kHz. Our results can be mostly explained by spectral splatter from the tone-burst rise times, excitation in AN tuning-curve "tails," and asynchronous AN responses to high-level energy ≤ 2 kHz. This is the first time CAP origins have been identified by a spatially specific technique. Our results show the need for revising the interpretation of the cochlear origins of high-level CAPs-ABR wave 1. NEW & NOTEWORTHY Cochlear compound action potentials (CAPs) and auditory brain stem responses (ABRs) are routinely used in laboratories and clinics. They are typically interpreted as arising from the cochlear region tuned to the stimulus frequency. However, as sound level is increased, the cochlear origins of CAPs from tone bursts of all frequencies become very wide and their centers shift toward the most sensitive cochlear region. The standard interpretation of CAPs and ABRs from moderate to intense stimuli needs revision.

Monitoring of the Inner Ear Function During and After Cochlear Implant Insertion Using Electrocochleography.

To preserve residual hearing during cochlear implant (CI) surgery, it is desirable to use intraoperative monitoring of inner ear function (cochlear monitoring), especially during electrode insertion. A promising method is electrocochleography (ECochG). Within this project, the relations between ongoing responses (ORs), recorded extra- and intracochlearly (EC and IC), and preservation of residual hearing were investigated. Before, during, and after insertion of hearing preservation electrodes, intraoperative ECochG recordings were performed EC using a cotton wick electrode and after insertion also IC using the CI electrode (MED-EL) and a research software tool. The stimulation was delivered acoustically using low frequency tone bursts. The recordings were conducted in 10 adult CI recipients. The amplitudes of IC ORs were detected to be larger than EC ORs. Intraoperative EC thresholds correlated highly to preoperative audiometric thresholds at 1000 Hz, IC thresholds highly at 250 Hz and 500 Hz. The correlations of both intraoperative ECochG recordings to postoperative pure tone thresholds were low. When measured postoperatively at the same appointments, IC OR thresholds correlated highly to audiometric pure tone thresholds. For all patients, it was possible to record ORs during or directly after electrode insertion. Consequently, we conclude that we did not observe any cases with severe IC trauma. Delayed hearing loss could not be predicted with our method. Nevertheless, intraoperative ECochG recordings are a promising tool to gain further insight into mechanisms impacting residual hearing. Postoperatively recorded IC OR thresholds seem to be a reliable tool for frequency specific hearing threshold estimation.

Auditory Evoked Potentials of the Loggerhead Sea Turtle (Caretta caretta)

Much of the published research on the auditory ability of sea turtles centers on anatomy, and from these anatomical studies, we hypothesize that the loggerhead sea turtle's (Caretta caretta) hearing range centers around low-frequency sounds. To validate this hypothesis, we examined thresholds of juvenile loggerheads by collecting auditory evoked potentials derived from two types of vibrational stimuli: low-frequency clicks and tone bursts. The broadband click elicited clear auditory brainstem responses (ABRs) that could be tracked to threshold; a mean click threshold for 35 juvenile loggerheads was -10.8 dB re: Ig rms ? 2.3 dB SD. ABR recordings from the low frequency tone bursts were similar in form to the click data and indicated the range of effective hearing to be from at least 250 Hz to 750 Hz. The most sensitive threshold was recorded at the lowest frequency tested, 250 Hz, with a mean threshold of -26.3 dB re: Ig rms ? 2.3 dB SD. Historically, ABRs have been used as a technique to evaluate audition and acoustic threshold, and we feel confident that the procedures developed here are both valid and effective when used on sea turtles.

Hearing sensitivity in the Greater Prairie Chicken (Tympanuchus cupido)

As their scientific name implies, Greater Prairie Chickens (Tympanuchus cupido) are well known for their vocalizations, particularly those produced by males during courtship. In this report, the auditory sensitivity of a small cohort of Greater Prairie Chickens inhabiting the grasslands of southeastern Nebraska is presented as part of an effort to assess the capacity of wind turbine farm generated noise to mask male courtship vocalizations. Birds were captured from breeding grounds (leks) between March and June of 2014 and sensitivity to tone-burst stimuli was assessed using the auditory brainstem response. While response waveforms were typical of responses observed in other avian species, as was the bandwidth of audibility curves, preliminary data suggest the possibility that Greater Prairie Chickens may be unusually sensitive to low frequency tone-bursts in the vicinity of the dominant spectral component of the “booming” call, a prominent, intense vocalization that males produce during courtship. As part of the larger study that is designed to evaluate the potential impact of wind turbine farm operation on reproductive success, demography and overall survival rates, the potential masking influence of noise produced by turbine operations at wind farms will be discussed in relation to sensitivity findings.

Determining auditory-evoked activities from multiple cells in layer 1 of the dorsal cortex of the inferior colliculus of mice by in vivo calcium imaging

Layer 1 of the dorsal cortex of the inferior colliculus (DCIC) is distinguished from other layers by its cytoarchitecture and fiber connections. However, the information of the sound types represented in layer 1 of the DCIC remains unclear because placing electrodes on such thin structures is challenging. In this study, we utilized in vivo calcium imaging to assess auditory-evoked activities in multiple cells in layer 1 of DCIC and to characterize sound stimuli producing strong activity. Most cells examined showed strong responses to broad-band noise and low-frequency tone bursts of high sound intensity. In some cases, we successfully obtained frequency response areas, which are receptive fields to tone frequencies and intensities, and ~30% of these showed V-shape tunings. This is the first systematic study to record auditory responses of cells in layer 1 of DCIC. These results indicate that cells in this area are selective to tones with low frequency, implying the importance of such auditory information in the neural circuitry of layer 1 of DCIC.

Reproducibility (test–retest) of vestibular evoked myogenic potential

IntroductionThere is still no consensus in the literature as to the best acoustic stimulus for capturing vestibular evoked myogenic potential (VEMP). Low-frequency tone bursts are generally more effective than high-frequency, but recent studies still use clicks. Reproducibility is an important analytical parameter to observe the reliability of responses. ObjectiveTo determine the reproducibility of p13 and n23 latency and amplitude of the VEMP for stimuli with different tone-burst frequencies, and to define the best test frequency. MethodsCross-sectional cohort study. VEMP was captured in 156 ears, on the sternocleidomastoid muscle, using 100 tone-burst stimuli at frequencies of 250, 500, 1000, and 2000Hz, and sound intensity of 95dB nHL. Responses were replicated, that is, recorded three times on each side. ResultsNo significant difference was observed for p13 and n23 latencies of the VEMP, captured at three moments with tone-burst stimuli at 250, 500, and 1000Hz. Only the frequency of 2000Hz showed a difference between captures of this potential (p<0.001). p13 and n23 amplitude analysis was also similar in the test–retest for all frequencies analyzed. Conclusionp13 and n23 latencies and amplitudes of VEMP for tone-burst stimuli at frequencies of 250, 500, and 1000Hz are reproducible.

Effects of the intensity of masking noise on ear canal recorded low-frequency cochlear microphonic waveforms in normal hearing subjects

Compared to auditory brainstem responses (ABRs), cochlear microphonics (CMs) may be more appropriate to serve as a supplement to the test of otoacoustic emissions (OAEs). Researchers have shown that low-frequency CMs from the apical cochlea are measurable at the tympanic membrane using high-pass masking noise. Our objective is to study the effect of such noise at different intensities on low-frequency CMs recorded at the ear canal, which is not completely known. Six components were involved in this CM measurement including an ear canal electrode (1), a relatively long and low-frequency toneburst (2), and high-pass masking noise at different intensities (3). The rest components include statistical analysis based on multiple human subjects (4), curve modeling based on amplitudes of CM waveforms (CMWs) and noise intensity (5), and a technique based on electrocochleography (ECochG or ECoG) (6). Results show that low-frequency CMWs appeared clearly. The CMW amplitude decreased with an increase in noise level. It decreased first slowly, then faster, and finally slowly again. In conclusion, when masked with high-pass noise, the low-frequency CMs are measurable at the human ear canal. Such noise reduces the low-frequency CM amplitude. The reduction is noise-intensity dependent but not completely linear. The reduction may be caused by the excited basal cochlea which the low-frequency has to travel and pass through. Although not completely clear, six mechanisms related to such reduction are discussed.

Effects of Stimulus Intensity on Low-Frequency Toneburst Cochlear Microphonic Waveforms.

This study investigates changes in amplitude and delays in low-frequency toneburst cochlear microphonic (CM) waveforms recorded at the ear canal in response to different stimulus intensities. Ten volunteers aged 20-30 were recruited. Low-frequency CM waveforms at 500 Hz in response to a 14-ms toneburst were recorded from an ear canal electrode using electrocochleography techniques. The data was statistically analyzed in order to confirm whether the differences were significant in the effects of stimulus intensity on the amplitudes and delays of the low-frequency CM waveforms. Electromagnetic interference artifacts can jeopardize CM measurements but such artifacts can be avoided. The CM waveforms can be recorded at the ear canal in response to a toneburst which is longer than that used in ABR measurements. The CM waveforms thus recorded are robust, and the amplitude of CM waveforms is intensity-dependent. In contrast, the delay of CM waveforms is intensity-independent, which is different from neural responses as their delay or latency is intensity-dependent. These findings may be useful for development of the application of CM measurement as a supplementary approach to otoacoustic emission (OAE) measurement in the clinic which is severely affected by background acoustic noise. The development of the application in the assessment of low-frequency cochlear function may become possible if a further series of studies can verify the feasibility, but it is not meant to be a substitute for audiometry or OAE measurements. The measurement of detection threshold of CM waveform responses using growth function approach may become possible in the clinic. The intensity-independent nature of CMs with regards to delay measurements may also become an impacting factor for differential diagnoses and for designing new research studies.

Augmented Ocular Vestibular Evoked Myogenic Potentials to Air-Conducted Sound in Large Vestibular Aqueduct Syndrome

To demonstrate the value of recording air-conducted ocular Vestibular Evoked Myogenic Potentials (oVEMP) in a patient with bilaterally enlarged vestibular aqueducts. Cervical VEMP and oVEMP were recorded from a patient presenting with bilateral hearing loss and imbalance, attributable to large vestibular aqueduct syndrome. The stimuli were air-conducted tone bursts at octave frequencies from 250 to 2000 Hz. Amplitudes and thresholds were measured and compared with the normal response range of 32 healthy control subjects. oVEMP reflexes demonstrated pathologically increased amplitudes and reduced thresholds for low-frequency tone bursts. Cervical VEMP amplitudes and thresholds were within normal limits for both ears across all frequencies of stimulation. This study is the first to describe the augmentation of AC oVEMPs in an adult with large vestibular aqueduct syndrome.

Mapping auditory nerve firing density using high-level compound action potentials and high-pass noise masking

Future implementation of regenerative treatments for sensorineural hearing loss may be hindered by the lack of diagnostic tools that specify the target(s) within the cochlea and auditory nerve for delivery of therapeutic agents. Recent research has indicated that the amplitude of high-level compound action potentials (CAPs) is a good predictor of overall auditory nerve survival, but does not pinpoint the location of neural damage. A location-specific estimate of nerve pathology may be possible by using a masking paradigm and high-level CAPs to map auditory nerve firing density throughout the cochlea. This initial study in gerbil utilized a high-pass masking paradigm to determine normative ranges for CAP-derived neural firing density functions using broadband chirp stimuli and low-frequency tonebursts, and to determine if cochlear outer hair cell (OHC) pathology alters the distribution of neural firing in the cochlea. Neural firing distributions for moderate-intensity (60 dB pSPL) chirps were affected by OHC pathology whereas those derived with high-level (90 dB pSPL) chirps were not. These results suggest that CAP-derived neural firing distributions for high-level chirps may provide an estimate of auditory nerve survival that is independent of OHC pathology.

Détection du micro-endommagement dans le tissu osseux trabéculaire par une méthode d’acousto-élasticité dynamique

Osteoporosis affects the whole skeleton strength leading to an actual fracture risk. Associated to a high level of morbidity, it is a critical public health issue. During the last decades, several X-rays and ultrasound devices have been developed to help the clinicians in their diagnosis. They are essentially related to bone mass while several factors are responsible for this pathology. Among them, microdamage has an interesting place. Nonlinear acoustic methods have already shown their ability to detect microcracks in different geophysical and industrial materials. We present a dynamic acousto-elastic testing (interaction of a very low frequency tone burst and high frequency pulses) to measure viscoelastic and dissipative nonlinearities. Applied to trabecular bone samples (human defatted calcanei), this method has shown a high level of nonlinearity in very dense regions of the calcanei. However, native microdamage is often at the limit of the method sensibility. To better evaluate this parameter, we have performed acousto-elastic measurements before and after inducing microdamage thanks to a compression fatigue test. We observed higher nonlinearity after induced damage. Moreover, the level of acoustic nonlinearity has been correlated to actual damage accumulation thanks to confocal microscopy analysis. However, acousto-elasticity measurements repeatability is weak. Correct reproducibility is observed for low level of nonlinearity (low level of microcracks). For high level of nonlinearity, quite different nonlinear behaviors are usually observed from one measurement to the next one.

Binaural image position distributions for phase-shifted low frequency tone bursts

This experiment was designed to yield precise measures of the statistical properties of perceived sound images. Results are reported for listeners' judgments of intracranial sound image lateral positions in response to binaural tone burst stimuli (250 Hz, 50 ms) with varying interaural phase differences, conditional on the absence or presence of a (left or right) reference monaural tone burst (also 250 Hz, 50 ms) ending 500 ms prior to the test signal. The monaural-reference shifted the position distributions toward the opposite side of the head. The position distribution variance and skewness depended on the mean of the position distribution, not on the interaural phase difference of the stimulus. The standard deviation increased as the mean moved laterally from midline. Near the midline the position distributions were skewed ipsilaterally. Near either ear they were skewed toward the midline. The results suggest that the most important noise limiting performance originates central to brainstem coincidence detector networks.

Comparison of vestibular evoked myogenic potentials elicited by click and short duration tone burst stimuli

Vestibular evoked myogenic potentials are short latency electrical impulses that are produced in response to higher level acoustic stimuli. They are used clinically to diagnose sacculocollic pathway dysfunction. This study aimed to compare the vestibular evoked myogenic potential responses elicited by click stimuli and short duration tone burst stimuli, in normal hearing individuals. Seventeen subjects participated. In all subjects, we assessed vestibular evoked myogenic potentials elicited by click and short duration tone burst stimuli. The latency of the vestibular evoked myogenic potential responses (i.e. the p13 and n23 peaks) was longer for tone burst stimuli compared with click stimuli. The amplitude of the p13-n23 waveform was greater for tone burst stimuli than click stimuli. Thus, the click stimulus may be preferable for clinical assessment and identification of abnormalities as this stimulus has less variability, while a low frequency tone burst stimulus may be preferable when assessing the presence or absence of vestibular evoked myogenic potential responses.

Development of Low-Frequency Tone Burst versus the Click Auditory Brainstem Response

Often ABR threshold testing employs clicks to assess high-frequency hearing, and low-frequency tone bursts to assess low-frequency sensitivity. While a maturation effect has been shown for click stimuli, similar data are lacking for low-frequency toneburst stimuli. Thus, 305 infants ranging in conceptional age (CA) from 33 weeks to 74 weeks were tested. Absolute latencies were measured for wave V at 55, 35, and 25 dB nHL in response to a click and for wave V500 in response to a 500 Hz tone burst. Major wave latency in response to 500 Hz tone bursts decreases with age and do not stabilize by 70 weeks CA. Likewise, waves III and V latencies in response to clicks decrease with age, as has been reported by others, and do not stabilize by 70 weeks CA. Wave I latency produced by clicks did not decrease with age, being mature by 33 weeks CA.

Acoustically responsive fibers in the vestibular nerve of the cat

Recordings were made from single afferent fibers in the inferior vestibular nerve, which innervates the saccule and posterior semicircular canal. A substantial portion of the fibers with irregular background activity increased their firing in response to moderately intense clicks and tones. In responsive fibers, acoustic clicks evoked action potentials with minimum latencies of < or = 1.0 msec. Fibers fell into two classes, with the shortest latency either to condensation clicks (PUSH fibers) or to rarefaction clicks (PULL fibers). Low-frequency (800 Hz) tone bursts at moderately high sound levels (> 80 dB SPL) caused synchronization of spikes to preferred phases of the tone cycle. PUSH and PULL fibers had preferred response phases approximately 180 degrees apart. These two response classes are consistent with fibers that innervate hair cells having opposite morphological polarizations, an arrangement found in the saccule. With low-frequency tone bursts, sound levels of > or = 90 dB SPL evoked increases in mean spike rate. Spike rates increased monotonically with sound level without saturating at levels < or = 115 dB SPL. Contraction of the middle-ear muscles decreased responses to sound, consistent with the sound transmission path being through the middle ear. Several fibers were labeled with biocytin and traced. All labeled fibers had bipolar cell bodies in the inferior vestibular ganglion with peripheral processes extending toward the saccular nerve and central processes in the vestibular nerve. Two fibers were traced to the saccular epithelium. One fiber was traced centrally and arborized extensively in vestibular nuclei and a region ventromedial to the cochlear nucleus. Our results confirm and extend previous suggestions that the mammalian saccule responds to sound at frequencies and levels within the normal range of human hearing. We suggest a number of auditory roles that these fibers may play in the everyday life of mammals.