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PubMed2003

Noise levels within the ear and post-nasal space in neonates in intensive care.

BACKGROUND: Noise exposure in neonatal units has long been suspected of being a cause of hearing loss associated with such units. The noise intensity to which the neonate is exposed varies with the type of ventilatory support used. Also, the post-nasal space is an enclosed cavity that is close to the inner ear and an area of turbulent and hence potentially noisy airflow. AIM: To determine noise intensities within the ear and post-nasal space in neonates on different modes of ventilatory support using probe microphones, measures previously not undertaken. METHODS: A portable instrument with a probe microphone was used for the measurements. Three groups of infants were included: (a) those receiving no respiratory support (NS); (b) those receiving conventional ventilation (CV); (c) those receiving continuous positive airways pressure (CPAP) support. RESULTS: The mean in-the-ear noise intensities (at 1 kHz) were 41.7 dB SPL (NS), 39.5 dB SPL (CV), and 55.1 dB SPL (CPAP). The noise intensities in the post-nasal space in those receiving CPAP support were higher than in the other groups, reached mean levels of up to 102 dB SPL at some frequencies, and increased with increasing flow rates. CONCLUSIONS: The most important finding is the high noise intensities in the post-nasal space of those receiving CPAP support. Given the proximity of the post-nasal space to the inner ear, enough noise could be transmitted, especially in infants receiving the higher flow rates, to cause cochlear damage and hence hearing loss. It would therefore be wise, wherever possible, to avoid using the higher flow rates.

Cochlea↗
PubMed2004

Sensitivity to interaural level and envelope time differences of two bilateral cochlear implant listeners using clinical sound processors.

OBJECTIVES: To assess the sensitivity of two bilateral cochlear implant users to interaural level and time differences (ILDs and ITDs) for various signals presented through the auxiliary inputs of clinical sound processors that discard fine timing information and only preserve envelope cues. DESIGN: In a lateralization discrimination experiment, the just noticeable difference (JND) for ILDs and envelope ITDs was measured by means of an adaptive 2-AFC method. Different stimuli were used, including click trains at varying repetition rates, a speech fragment, and noise bursts. For one cochlear implant listener and one stimulus, the sensitivity to envelope ITDs was also determined with the method of constant stimuli. The dependency of ILD-JNDs on the interaural place difference was studied with stimulation at single electrode pairs by using sinusoidal input signals in combination with appropriate single-channel processor fittings. In a lateralization position experiment, subjects were required to use a visual pointer on a computer screen to indicate in-the-head positions for blocks of stimuli containing either ILD or ITD cues. All stimuli were loudness balanced (before applying ILD) and fed directly into the auxiliary inputs of the BTE processors (TEMPO+, Med-El Corp.). The automatic gain control and the processors' microphones were deactivated. RESULTS: Both cochlear implant listeners were highly sensitive to ILDs in all broadband stimuli used; JNDs approached those of normal-hearing listeners. Pitch-matched single electrode pairs showed significantly lower ILD-JNDs than pitch-mismatched electrode pairs. Envelope ITD-JNDs of cochlear implant listeners obtained with the adaptive method were substantially higher and showed a higher test-retest variability than waveform ITD-JNDs of normal-hearing control listeners and envelope ITD-JNDs of normal-hearing listeners reported in the literature for comparable signals. The envelope ITD-JNDs for the click trains were significantly lower than for the speech token or the noise bursts. The best envelope ITD-JND measured was ca. 250 mus for the click train at 100 cycles per sec. Direct measurement of the psychometric function for envelope ITD by the method of constant stimuli showed discrimination above chance level down to 150 micros. The lateralization position experiment showed that both ILDs and envelope ITDs can lead to monotonic changes in lateral percept. CONCLUSIONS: The two cochlear implant users tested showed strong effects of ILDs in various broadband stimuli with respect to JNDs as well as lateralization position. The high dependency of ILD-JNDs on the interaural pitch difference suggests the potential importance of pitch-matched assignment of electrodes in the two ears by the speech processors. Envelope ITDs appear to be more ambiguous cues than ILDs, as reflected by the higher and more variable JNDs compared with normal-hearing listeners. The envelope ITD-JNDs of cochlear implant listeners depended on the stimulus.

Acoustic Stimulation↗
PubMed1978

Low-frequency neural and cochlear-microphonic tuning curves in the gerbil.

Average tuning curves of single auditory-nerve fibers are compared with average cochlear-microphonic (CM) tuning curves corrected for electrical filtering of the cochlea. Both the neural and CM data were obtained from Mongolian gerbils (Meriones unguiculatus) with the same acoustic system and similar corrections for middle-ear effects. Under these conditions the CM tuning in the second and third cochlear turns is similar to the tuning of fibers whose characteristic frequencies (CFs) correspond to the CM best frequencies (2.5 and 0.5 dHz). Thus, little sharpening seems to take place for low CF fibers. CM tuning at the most apical electrode position is sharper than expected for frequencies below the best frequency--a result that may be due to the shunting effect of the helicotrema at low frequencies- Previous modeling results have confirmed that apical basilar-membrane tuning may be appreciable affected by the mechanical impedance of the helicotrema. This helicotrema effect may account for the nearly symmetrical shapes of neural tuning curves of low-CF fibers.

Action Potentials↗
PubMed2002

Electrical stimulation of the auditory nerve via cochlear implants in patients with auditory neuropathy.

Auditory neuropathy (AN) is a term used to describe an auditory disorder in which there is evidence of normal outer hair cell function (otoacoustic emissions and/or cochlear microphonics) and poor function of the auditory nerve (absent or highly distorted auditory brain stem response starting with wave I). Many of these patients have evidence of generalized peripheral nerve disease, leading to an assumption that the peripheral portion of the auditory nerve is the most likely site of lesion. A small group of these patients has received cochlear implants, and the majority of them achieve average to above-average performance. Although this outcome may seem incongruous with neural disease, average performance by patients with AN may be a result of the reintroduction of neural synchrony by electrical stimulation and/or the fact that most deaf patients have poor nerve survival. Although cochlear implants are promising for deaf patients with AN, more study of the disorder is needed.

Adult↗
PubMed1998

Nonlinear characteristics of electrically evoked otoacoustic emissions.

To further our knowledge of outer hair cell nonlinearities, we measured the dependence of the electrically-evoked otoacoustic emissions (EEOEs) on current level for a wide range of electrical frequencies. Alternating electrical current was delivered into the scala media of the gerbil cochlea while the EEOE was measured with a probe-tube microphone. While the EEOE scaled linearly with current level for many frequencies and current levels, notable exceptions occurred. For frequencies below 300 Hz and currents above 20-30 microA(peak), the gain (primary EEOE magnitude divided by the current level) increased abruptly. For higher frequencies, the gain often increased slightly with increasing current of up to 30-50 microA(peak), but decreased at even higher current levels. We also investigated the enhancement of the EEOE due to simultaneous acoustic stimulation. The enhancement of the EEOE was relatively insensitive to current level with little change in enhancement for current levels up to 20 microA(peak). For current levels above approximately 40 microA(peak), the enhancement decreased slightly.

Acoustic Stimulation↗
PubMed2006

Prevalence of auditory neuropathy/synaptopathy in a population of children with profound hearing loss.

OBJECTIVE: To examine the prevalence of auditory neuropathy/synaptopathy (AN/AS) in a cohort of children with profound hearing loss. METHODS: From 1997 until 2004, 5190 children, aged 1-15 years, whose hearing ability was uncertain or who had risk factors for hearing impairment were investigated with subjective and objective hearing tests. Three thousand four hundred and fifteen from these children were screened for AN/AS using pure-tone audiometry, impedance measurement, transient evoked otoacoustic emissions (TEOAE) and click-evoked auditory brainstem responses (ABR). RESULTS: From 3415 patients who participated in an ABR and TEOAE assessment, 379 children showed absent or elevated (> or = 80 dB nHL) ABR thresholds. Within this group we found 32 cases with evidence of AN/AS via visible TEOAE and/or cochlear microphonics (CM) coupled with absent ABR. In the remaining 3036 children, AN/AS, could be ruled out by means of detectable ABR-thresholds and coherent findings in pure-tone audiometry and TEOAE assessment. This results in a prevalence of AN/AS of 0.94% within the group at risk for hearing loss, compared to 8.44% among profoundly hearing impaired children. CONCLUSION: This study shows that AN/AS is a common finding in the population of hearing impaired infants. In the majority of our AN/AS children (50%, n=16), an early audiological diagnosis was made under the age of 12 months. Therefore, clinicians and other health care professionals should generally be sensitised for AN/AS in infants, so that an appropriate treatment can promptly be initiated. Further research on clinical and pathophysiological aspects is necessary to better identify and manage patients suffering from AN/AS.

Acoustic Impedance Tests↗
PubMed2000

Auditory neuropathy: case study with hyperbilirubinemia.

Auditory neuropathy (AN) has been described in the literature as presenting with a combination of audiometric findings that include elevated behavioral audiometric thresholds, auditory brainstem response findings that are not consistent with audiometric findings, poor speech recognition, and present otoacoustic emissions (OAEs) and/or cochlear microphonics. Since the availability of clinical OAE testing, AN has come to be identified with increasing frequency; however, incidence and prevalence figures are unavailable. There is a great deal of discussion about the accurate diagnosis of AN, its characteristics, and its treatment. Some of this discussion is occurring on the Internet and over the telephones. The need to continue to provide information in accessible peer-reviewed journals is paramount. Following a review of the literature, a case study is presented of a boy who was diagnosed with AN as a newborn. He experienced hyperbilirubinemia and other neonatal health complications. His educational intervention was managed elsewhere until recently. Information is presented about the progression of the case over a 5-year period that includes audiologic data and communication development results.

Audiometry, Pure-Tone↗
PubMed1982

Auditory brainstem response (ABR) to rarefaction and condensation clicks in normal and abnormal ears.

The shift in latency of wave V in ABR upon the reversal of stimulus phase was investigated in a normal-hearing reference material (65 ears) and in 20 ears of conductive hearing loss, 29 ears of steep high-frequency hearing loss and 17 ears of acoustic neurinoma or cerebellopontine angle tumor. In addition, reproducibility was studied in the reference material. The stimulus was a 2.0-kHz haversine wave at 75 or 35 dB nHL delivered at the rate of 20 Hz. 2000 responses were averaged for each stimulus phase. Electrodes were placed on the vertex and on ipsilateral mastoid. Stimulus wave form was determined in a 6-cc coupler and in a few cases with a miniature electrete microphone in the ear canal. In the reference material, a significant difference of the wave V latency between the condensation (C) and rarefaction (R) stimulus was observed. When taking the absolute difference, the mean value was 0.15 ms at 75 dB nHL. This time difference corresponds to a frequency of 3-4 kHz. In high frequency cochlear loss, the C-R difference increased in proportion to the downward shift in frequency of the high frequency slope of the audiogram. In conductive hearing loss, the C-R difference was similar to that of the reference material. In retrocochlear cases, the C-R difference was unpredictable, but was remarkably often small compared with the V latency. It was concluded that the difference in wave V latency between C and R clicks provides information about the excitation pattern in the inner ear but it is not reliable enough to give more than a rough estimate of the individual frequency of generation of ABR. The results are arguments against the use of the alternating stimulus phase as a routine procedure.

Audiometry↗
PubMed2004

A dominantly inherited progressive deafness affecting distal auditory nerve and hair cells.

We have studied 72 members belonging to a large kindred with a hearing disorder inherited in an autosomal dominant pattern. We used audiological, physiological, and psychoacoustic measures to characterize the hearing disorders. The initial phenotypic features of the hearing loss are of an auditory neuropathy (AN) with abnormal auditory nerve and brainstem responses (ABRs) and normal outer hair cell functions [otoacoustic emissions (OAEs) and cochlear microphonics (CMs)]. Psychoacoustic studies revealed profound abnormalities of auditory temporal processes (gap detection, amplitude modulation detection, speech discrimination) and frequency processes (difference limens) beyond that seen in hearing impairment accompanying cochlear sensory disorders. The hearing loss progresses over 10-20 years to also involve outer hair cells, producing a profound sensorineural hearing loss with absent ABRs and OAEs. Affected family members do not have evidence of other cranial or peripheral neuropathies. There was a marked improvement of auditory functions in three affected family members studied after cochlear implantation with return of electrically evoked auditory brainstem responses (EABRs), auditory temporal processes, and speech recognition. These findings are compatible with a distal auditory nerve disorder affecting one or all of the components in the auditory periphery including terminal auditory nerve dendrites, inner hair cells, and the synapses between inner hair cells and auditory nerve. There is relative sparing of auditory ganglion cells and their axons.

Acoustic Impedance Tests↗
PubMed1975

Peripheral specialization for fine analysis of doppler-shifted echoes in the auditory system of the "CF-FM" bat Pteronotus parnellii.

Pteronotus parnellii uses the second harmonic (61-62 kHz) of the CF component in its orientation sounds for Doppler-shift compensation. The bat's inner ear is mechanically specialized for fine analysis of sounds at about 61-62 kHz. Because of this specialization, cochlear microphonics (CM) evoked by 61-62 kHz tone bursts exhibit prominent transients, slow increase and decrease in amplitude at the onset and cessation of these stimuli. CM-responses to 60-61 kHz tone bursts show a prominent input-output non-linearity and transients. Accordingly, a summated response of primary auditory neurones (N1) appears not only at the onset of the stimuli, but also at the cessation. N1-off is sharply tuned at 60-61 kHz, while N1-on is tuned at 63-64 kHz, which is 2 kHz higher than the best frequency of the auditory system because of the envelope-distortion originating from sharp mechanical tuning. Single peripheral neurones sensitive to 61-62 kHz sounds have an unusually sharp tuning curve and show phase-locked responses to beats of up to 3 kHz. Information about the frequencies of Doppler-shifted echoes is thus coded by a set of sharply tuned neurones and also discharges phase-locked to beats. Neurones with a best frequency between 55 and 64 kHz show not only tonic on-responses but also off-responses which are apparently related to the mechanical off-transient occuring in the inner ear and not to a rebound from neural inhibition.

Action Potentials↗
PubMed1999

[Electrically evoked otoacoustic emissions and their distortion products in guinea pigs].

The objective of this study was to investigate micromechanical properties and electromotility of cochlear outer hair cells in vivo. Extracochlear electrically stimulation with sinusoid alternating current was delivered to cochlea of guinea pigs. Sound pressure level was recorded from ear canal by microphone and the electrically evoked otoacoustic emissions (EEOE) and their distortion products (DPEEOE) were analyzed with FFT spectrum analyzer. The EEOE in 3 kHz to 33 kHz were recorded from 15 guinea-pigs in 18 guinea-pigs, the transfer function was more smooth in 8 kHz to 31 kHz. The DP EEOE were very clear when F1 = 6 kHz, F2 = 7.2 kHz, and the current intensity higher than 100 microA. While the intensity increased to 300 microA, two distortion products, F2-F1 and 2(2F1-F2)-F1, could be seen in addition to 2F1-F2. The input-output function showed that EEOE and DP EEOE I/O function were linear when lower electric intensity stimulation was given, but they displayed compress non-linear features when higher intensity current was delivered. The authors conclude that EEOE and DP EEOE are expression of electrophonic hearing characteriged by wide frequency bandpass appearance, wide dynamic range and non-linear features, so they are good tools for studying the mechanical properties of outer hair cells and the integrate function of Corti's organ.

Animals↗
PubMed1996

Auditory nerve neurophonic produced by the frequency difference of two simultaneously presented tones.

When two phase-locked sinusoidal stimuli having frequencies of F1 and F2 are simultaneously introduced to the ear of the gerbil, a difference tone (DT) can be observed (DT = F2-F1, where F2 > F1) in the time-averaged electrical response recorded from the cochlear round window (RW). Tetrodotoxin (TTX), which blocks the axonal firing of the cochlear nerve fiber, greatly attenuates this DT response, suggesting it is primarily neural in origin. Alternating the polarity of a single phase-locked tone cancels out the RW cochlear microphonic (CM) from the time-averaged response, leaving a residual auditory nerve neurophonic (ANN) response if the stimulus frequency is low enough to result in phase-locked firing of cochlear nerve axons. Simultaneous presentation of 1 kHz (F1) and 2 kHz (F2) tones, each being phase-locked with alternating polarity, produces a small ANN in response to the original tones and a large time-averaged ANN in response to the DT. Even when the frequency of the individual tones is too high to support phase-locking, a large DT-ANN can also be measured in response to simultaneously presented tones. A robust time-averaged DT-ANN can be measured when the temporal and intensity relationships between F1 and F2 are varied widely, with the latency (but not amplitude) of the response following the stimulus envelope. The DT-ANN produced by pairs of tones having frequencies ranging from 500 Hz to 3.5 kHz is largest in response to a DT of approximately 700-1100 Hz. This is in contrast to the ANN generated in response to a single tone, which decreases in magnitude as the stimulus frequency increases from 500 to 1500 Hz. Robust DT-ANNs can be measured from the gerbil even when the F2 frequency is greater than 30 kHz.

Acoustic Stimulation↗
PubMed1986

Central and peripheral contributions to coding of acoustic space by neurons in inferior colliculus of cat.

Recordings of response to free-field stimuli at best frequency were made from single units in the central nucleus of the inferior colliculus of anesthetized cats. Stimulus position was varied in azimuth, and the responses of units were compared with variation in the intensity and arrival time of the sound at each ear, derived from cochlear microphonic (CM) recordings. CM recordings were made at each frequency and at every point in space for which single-unit data were collected. Interaural time difference (delay) increased monotonically, but not linearly, as the stimulus was moved away from the midline. However, a given delay did not represent a single azimuth across frequency. Low-frequency interaural intensity differences (IIDs) were monotonic across azimuth and peaked at, or near, the poles. Higher-frequency IIDs were nonmonotonic and peaked relatively close to the midline, decreasing toward the poles. Units that showed little variation in discharge across azimuth formed 28% of the sample and were classified as omnidirectional. For other units, the spike-count intensity function and the variation of the CM with azimuth were combined to form a derived monaural azimuth function. For 29% of those units showing azimuthal sensitivity, the derived monaural azimuth function matched the actual azimuth function. This suggested that these units received input from only one ear. The largest group of azimuthally sensitive units (47%) was formed from those units inferred to be IID sensitive. At higher frequencies these units displayed a peaked azimuth function paralleling the nonmonotonic relation of IID to azimuth. The proportion of inferred IID-sensitive units was close to that found in dichotic studies.

Acoustic Stimulation↗
PubMed1996

Electrically evoked cubic distortion product otoacoustic emissions from gerbil cochlea.

It has been demonstrated that electrical stimulation of the cochlear partition results in basilar membrane vibration and otoacoustic emissions. Electromotility of stimulated outer hair cells (OHCs) elicits the electrically evoked otoacoustic emissions (EEOAEs). Although electrically evoked upper and lower sideband distortion products (DPs) have been reported, electrically evoked cubic DP has not been investigated. Since the acoustically evoked cubic DP is the most commonly used otoacoustic measure of cochlear nonlinearity, this study tested whether electrical stimuli evoke a cubic DP otoacoustic emission. An electrical current containing the frequency component f1 and f2 (f1 < f2) was delivered to the round window niche of the gerbil, and electrically induced sound pressure change in the external ear canal was measured with a microphone. It was found that, in addition to f1 and f2 EEOAEs, cubic DP (2f1-f2) and other emissions at 3f1-2f2, 2f2-f1 and f2-f1 frequencies are electrically evoked. The electrically evoked cubic DP growth is similar to that of an acoustically evoked cubic DP. An electrical stimulus at f1 or f2 and an acoustic stimulus at f2 or f1 produce an identical cubic DP to that evoked by two electrical stimuli and/or two acoustic stimuli at f1 and f2 frequencies. An acoustic suppressor at a frequency near f2 can completely suppress an electrically evoked cubic DP emission. These data demonstrate that DPs can be provoked by a complex two frequency electrical current delivered to the round window niche. These stimuli elicit mechanical vibrations, from stimulated OHCs near the round window, which propagate apically toward their characteristic frequency places on the basilar membrane, and produce combination DPs. Electrically evoked cubic DPs appear to be produced by the same nonlinear mechanism that generates acoustically evoked DPs.

Acoustic Stimulation↗