Olivocochlear efferent neurons originate in the brain stem and terminate in the organ of Corti, thereby allowing the central nervous system to influence the operation of the cochlea. This chapter reviews the physiology and possible functional utility of mammalian cochlear efferents. Efferent physiology in a few other hair cell systems is also reviewed for the insight provided into mammalian efferent physiology. First a historical overview is presented, and then specific topics are considered in detail.

Physiology of Olivocochlear Efferents

Middle and inner ears from human cadaver temporal bones were stimulated in the forward direction by an ear-canal sound source, and in the reverse direction by an inner-ear sound source. For each stimulus type, three variables were measured: (a) Pec--ear-canal pressure with a probe-tube microphone within 3 mm of the eardrum, (b) Vst--stapes velocity with a laser interferometer, and (c) Pv--vestibule pressure with a hydrophone. From these variables, the forward middle-ear pressure gain (M1), the cochlear input impedance (Zc), the reverse middle-ear pressure gain (M2), and the reverse middle-ear impedance (M3) are directly obtained for the first time from the same preparation. These measurements can be used to fully characterize the middle ear as a two-port system. Presently, the effect of the middle ear on otoacoustic emissions (OAEs) is quantified by calculating the roundtrip middle-ear pressure gain Gme(RT) as the product of M1 and M2. In the 2-6.8 kHz region, absolute value(Gme(RT)) decreases with a slope of -22 dB/oct, while OAEs (both click evoked and distortion products) tend to be independent of frequency; this suggests a steep slope in vestibule pressure from 2 kHz to at least 4 kHz for click evoked OAEs and to at least 6.8 kHz for distortion product OAEs. Contrary to common assumptions, measurements indicate that the emission generator mechanism is frequency dependent. Measurements are also used to estimate the reflectance of basally traveling waves at the stapes, and apically generated nonlinear reflections within the vestibule.

Measurements of human middle ear forward and reverse acoustics: Implications for otoacoustic emissions

The pressure reflectance R (omega) is the transfer function which may be defined for a linear one-port network by the ratio of the reflected complex pressure divided by the incident complex pressure. The reflectance is a function that is closely related to the impedance of the 1-port. The energy reflectance R (omega) is defined as magnitude of [R]2. It represents the ratio of reflected to incident energy. In the human ear canal the energy reflectance is important because it is a measure of the inefficiency of the middle ear and cochlea, and because of the insight provided by its simple frequency domain interpretation. One may characterize the ear canal impedance by use of the pressure reflectance and its magnitude, sidestepping the difficult problems of (a) the unknown canal length from the measurement point to the eardrum, (b) the complicated geometry of the drum, and (c) the cross-sectional area changes in the canal as a function of distance. Reported here are acoustic impedance measurements, looking into the ear canal, measured on ten young adults with normal hearing (ages 18-24). The measurement point in the canal was approximately 0.85 cm from the entrance of the canal. From these measurements, the pressure reflectance in the canal is computed and impedance and reflectance measurements from 0.1 to 15.0 kHz are compared among ears. The average reflectance and the standard deviation of the reflectance for the ten subjects have been determined. The impedance and reflectance of two common ear simulators, the Bruel & Kjaer 4157 and the Industrial Research Products DB-100 (Zwislocki) coupler are also measured and compared to the average human measurements. All measurements are made using controls that assure a uniform accuracy in the acoustic calibration across subjects. This is done by the use of two standard acoustic resistors whose impedances are known. From the experimental results, it is concluded that there is significant subject variability in the magnitude of the reflectance for the ten ear canals. This variability is believed to be due to cochlear and middle ear impedance differences. An attempt was made at modeling the reflectance but, as discussed in the paper, several problems presently stand in the way of these models. Such models would be useful for acoustic virtual-reality systems and for active noise control earphones.

Measurement of acoustic impedance and reflectance in the human ear canal

On the role of the olivocochlear bundle in hearing : 16 case studies

Mammalian spontaneous otoacoustic emissions (SOAEs) have been suggested to arise by three different mechanisms The local-oscillator model, dating back to the work of Thomas Gold, supposes that SOAEs arise through the local, autonomous oscillation of some cellular constituent of the organ of Corti (eg, the “active process” underlying the cochlear amplifier) Two other models, by contrast, both suppose that SOAEs are a global collective phenomenon—cochlear standing waves created by multiple internal reflection—but differ on the nature of the proposed power source: Whereas the “passive” standing-wave model supposes that SOAEs are biological noise, passively amplified by cochlear standing-wave resonances acting as narrow-band nonlinear filters, the “active” standing-wave model supposes that standing-wave amplitudes are actively maintained by coherent wave amplification within the cochlea Quantitative tests of key predictions that distinguish the local-oscillator and global standing-wave models are presented and shown to support the global standing-wave model In addition to predicting the existence of multiple emissions with a characteristic minimum frequency spacing, the global standing-wave model accurately predicts the mean value of this spacing, its standard deviation, and its power-law dependence on SOAE frequency Furthermore, the global standing-wave model accounts for the magnitude, sign, and frequency dependence of changes in SOAE frequency that result from modulations in middle-ear stiffness Although some of these SOAE characteristics may be replicable through artful ad hoc adjustment of local-oscillator models, they all arise quite naturally in the standing-wave framework Finally, the statistics of SOAE time waveforms demonstrate that SOAEs are coherent, amplitude-stabilized signals, as predicted by the active standing-wave model Taken together, the results imply that SOAEs are amplitude-stabilized standing waves produced by the cochlea acting as a biological, hydromechanical analog of a laser oscillator Contrary to recent claims, spontaneous emission of sound from the ear does not require the autonomous mechanical oscillation of its cellular constituents

http://web.mit.edu/apg/papers/shera-soaes-JASA03.pdf

Mammalian spontaneous otoacoustic emissions are amplitude-stabilized cochlear standing waves.

The ear-canal impedance and reflection coefficient were measured in an adult group and in groups of infants of age 1, 3, 6, 12, and 24 months over frequency range 125-10,700 Hz. The development of the external ear canal and middle ear strongly affect input impedance and reflection coefficient responses, and this development is not yet complete at age 24 months. Contributing factors include growth of the area and length of the ear canal, a resonance in the ear-canal walls of younger infants, and a probable influence of growth of the middle-ear cavities. The middle-ear compliance is lower in infants than adults, and the middle-ear resistance is higher. The power transfer into the middle ear of the infant is much less than into that of the adult. Such differences in power transfer directly influence both behavioral and physiological measurements of hearing. The difficulties of interpretation of neonatal tympanograms are shown to be a consequence of ear-canal wall vibration. Impedance and reflectance measurements in the 2-4-kHz range are recommended as a potentially useful clinical tool for circumventing these difficulties.

Ear-canal impedance and reflection coefficient in human infants and adults

A frequency-domain based system for measuring acoustic impedance and reflection coefficient is described. The calibration procedure uses a least-mean-squares approximation to the Thevenin parameters describing the source and receiver characteristics in which the data measured on closed, cylindrical tubes are matched to a viscothermal tube model. The system is intended for use in acoustical measurement in human ear canals, in which the cross-sectional area of the ear canal at the point of insertion is imprecisely known. This area is acoustically estimated from the impedance data, and the reflection coefficient is calculated in terms of this area and the impedance data. Measurements on a variety of closed tubes show the method is accurate over the frequency range investigated (less than 10.7 kHz). The time-domain reflection function is evaluated by transforming the reflection coefficient from the frequency domain, but the finite bandwidth of the measured data limits the accuracy of time-domain response measurements. The method is well suited for frequency-domain measurements in human ear canals.

Method to measure acoustic impedance and reflection coefficient

Objectives 1) To describe broad bandwidth measurements of acoustic admittance (Y) and energy reflectance (R) in the ear canals of neonates. 2) To describe a means for evaluating when a YR response is valid. 3) To describe the relations between these YR measurements and age, gender, left/right ear, and selected risk factors. Design YR responses were obtained at four test sites in well babies without risk indicators, well babies with at least one risk indicator, and graduates of neonatal intensive care units. YR responses were measured using a chirp stimulus at moderate levels over a frequency range from 250 to 8000 Hz. The system was calibrated based on measurements in a set of cylindrical tubes. The probe assembly was inserted in the ear canal of the neonate, and customized software was used for data acquisition. Results YR responses were measured in over 4000 ears, and half of the responses were used in exploratory data analyses. The particular YR variables chosen for analysis were energy reflectance, equivalent volume and acoustic conductance. Based on the view that unduly large negative equivalent volumes at low frequencies were physically impossible, it was concluded that approximately 13% of the YR responses showed evidence of improper probe seal in the ear canal. To test how these outliers influenced the overall pattern of YR responses, analyses were conducted both on the full data set (N = 2081) and the data set excluding outliers (N = 1825). The YR responses averaged over frequency varied with conceptional age (conception to date of test), gender, left/right ear, and selected risk factors; in all cases, significant effects were observed more frequently in the data set excluding outliers. After excluding outliers and controlling for conceptional age effects, the dichotomous risk factors accounting for the greatest variance in the YR responses were, in rank order, cleft lip and palate, aminoglycoside therapy, low birth weight, history of ventilation, and low APGAR scores. In separate analyses, YR responses varied in the first few days after birth. An analysis showed that the use of a YR test criterion to assess the quality of probe seal may help control the false-positive rate in evoked otoacoustic emission testing. Conclusions This is the first report of wideband YR responses in neonates. Data were acquired in a few seconds, but the responses are highly sensitive to whether the probe is fully sealed in the ear canal. A real-time acoustic test of probe fit is proposed to better address the probe seal problem. The YR responses provide information on middle-ear status that varies over the neonatal age range and that is sensitive to the presence or absence of risk factors, ear, and gender differences. Thus, a YR test may have potential for use in neonatal screening tests for hearing loss.

Identification of neonatal hearing impairment: ear-canal measurements of acoustic admittance and reflectance in neonates.

The diffuse‐field pressure transfer function from a reverberant field to the ear canal of human infants, ages 1, 3, 6, 12, and 24 months, has been measured from 125–10700 Hz. The source was a loudspeaker using pink noise, and the diffuse‐field pressure and the ear‐canal pressure were simultaneously measured using a spatial averaging technique in a reverberant room. The results in most subjects show a two‐peak structure in the 2–6‐kHz range, corresponding to the ear‐canal and concha resonances. The ear‐canal resonance frequency decreases from 4.4 kHz at age 1 month to 2.9 kHz at age 24 months. The concha resonance frequency decreases from 5.5 kHz at age 1 month to 4.5 kHz at age 24 months. Below 2 kHz, the diffuse‐field transfer function shows effects due to the torsos of the infant and parent, and varies with how the infant is held. Comparisons are reported of the diffuse‐field absorption cross section for infants relative to adults. This quantity is a measure of power absorbed by the middle ear from a diffuse sound field, and large differences are observed in infants relative to adults. The radiation efficiencies of the infant and the adult ear are small at low frequencies, near unity at midfrequencies, and decrease at higher frequencies. The process of ear‐canal development is not yet complete at age 24 months. The results have implications for experiments on hearing in infants.

Jay C. Bulen

Papers

Ear-canal impedance and reflection coefficient in human infants and adults

Method to measure acoustic impedance and reflection coefficient

Identification of neonatal hearing impairment: ear-canal measurements of acoustic admittance and reflectance in neonates.

Pressure transfer function and absorption cross section from the diffuse field to the human infant ear canal.

Voluntary contraction of middle ear muscles: effects on input impedance, energy reflectance and spontaneous otoacoustic emissions.