Effect of contralateral auditory stimuli on active cochlear micro-mechanical properties in human subjects

The present study investigates the possibility that contralateral auditory stimulation along medial efferent system pathways may alter active cochlear micromechanics and hence affect evoked oto-acoustic emissions in humans. A first experiment, involving 21 healthy subjects showed reduction of oto-acoustic emission amplitude under low intensity contralateral white noise (from 30 dB SPL, 10 dB SL, upwards). The effect is found for intensities below the acoustic reflex threshold (85.2 dB HL). A second experiment, involving 10 of the above 21 subjects, sought to rule out any technical artefact. Recording was again carried out, but after sealing of the contralateral ear with a silicon putty plug. No contralateral intensity effect on oto-acoustic emission amplitude was found for contralateral intensities below 65 dB SPL. In subjective perception terms (dB SL) an effect was found under sealing when the sound reached or passed above the 10 dB SL level. These two findings confirm the preceding experiment. The third experiment investigated the role of transcranial transmission of the contralateral auditory stimulus. 16 subjects having total unilateral deafness and one healthy ear were tested by the same procedure as above. No fall-off in oto-acoustic emission amplitude was found for contralateral stimuli equal to or less than 80 dB SPL. There is thus a contralateral auditory stimulus effect on active cochlear micromechanics. The most appropriate explanation involves the medial cochlear efferent system, excited at brainstem level via the afferent auditory pathways. Alteration of active cochlear micromechanics seems promising at a basic level, pointing, as it does, to an interactive cochlear functioning which can be investigated by simple, non-intrusive, objective techniques which can be used with human subjects. We have here a model for functional exploration of the medial olivocochlear efferent system.

Headphone simulation of free-field listening. I: Stimulus synthesis

This article describes techniques used to synthesize headphone-presented stimuli that simulate the ear-canal waveforms produced by free-field sources. The stimulus synthesis techniques involve measurement of each subject's free-field-to-eardrum transfer functions for sources at a large number of locations in free field, and measurement of headphone-to-eardrum transfer functions with the subject wearing headphones. Digital filters are then constructed from the transfer function measurements, and stimuli are passed through these digital filters. Transfer function data from ten subjects and 144 source positions are described in this article, along with estimates of the various sources of error in the measurements. The free-field-to-eardrum transfer function data are consistent with comparable data reported elsewhere in the literature. A comparison of ear-canal waveforms produced by free-field sources with ear-canal waveforms produced by headphone-presented simulations shows that the simulations duplicate free-field waveforms within a few dB of magnitude and a few degrees of phase at frequencies up to 14 kHz.

BOLD fMRI deactivation of limbic and temporal brain structures and mood enhancing effect by transcutaneous vagus nerve stimulation

Direct vagus nerve stimulation (VNS) has proved to be an effective treatment for seizure disorder and major depression. However, since this invasive technique implies surgery, with its side-effects and relatively high financial costs, a non-invasive method to stimulate vagal afferences would be a great step forward. We studied effects of non-invasive electrical stimulation of the nerves in the left outer auditory canal in healthy subjects (n = 22), aiming to activate vagal afferences transcutaneously (t-VNS). Short-term changes in brain activation and subjective well-being induced by t-VNS were investigated by functional magnetic resonance imaging (fMRI) and psychometric assessment using the Adjective Mood Scale (AMS), a self-rating scale for current subjective feeling. Stimulation of the ear lobe served as a sham control. fMRI showed that robust t-VNS induced BOLD-signal decreases in limbic brain areas, including the amygdala, hippocampus, parahippocampal gyrus and the middle and superior temporal gyrus. Increased activation was seen in the insula, precentral gyrus and the thalamus. Psychometric assessment revealed significant improvement of well-being after t-VNS. Ear lobe stimulation as a sham control intervention did not show similar effects in either fMRI or psychometric assessment. No significant effects on heart rate, blood pressure or peripheral microcirculation could be detected during the stimulation procedure.

CONCLUSIONS

Our study shows the feasibility and beneficial effects of transcutaneous nerve stimulation in the left auditory canal of healthy subjects. Brain activation patterns clearly share features with changes observed during invasive vagus nerve stimulation.

Suppressibility of the 2f1-f2 stimulated acoustic emissions in gerbil and man

The suppression tuning properties of the oto-acoustic distortion product emission, 2f1-f2 have been measured in the ear canal of gerbil and man. The results show the acoustic response to be suppressible in a similar, frequency-dependent manner in both species. Frequencies near to those of the stimulating tones are most effective in suppressing the response. Derived iso-suppression tuning curves have Q10dB values of between 1 and 6. Suppressor tones having frequencies near to f2 (the higher frequency stimulus) make a contribution to the tuning curve which is largely independent of the stimulus intensity and the frequency ratio between the two primary tones. Suppressors having f1-associated frequencies produce a variable amount of suppression depending on the stimulus parameters chosen. No specific suppression feature could be associated with suppressors near to 2f1-f2. The frequency selectivity of the acoustic DP generation mechanism shown by this study indicates a close association with the transduction mechanism. The demonstration of comparable signals in gerbil and man facilitates the direct transfer of laboratory results to the study of human ears.

Acoustic distortion products in rabbit ear canal. II. Sites of origin revealed by suppression contours and pure-tone exposures

Previous work on acoustic distortion products (DPs) recorded from the ear canal has not established unequivocally whether emitted DPs principally reflect basilar-membrane nonlinearities at the frequency sites of the primary tones, f1 and f2, or if the DP-frequency place itself makes a significant contribution to the emitted response. Results from some studies on acoustic emissions attribute generation of the emitted DP almost exclusively to the regions of maximum primary-tone interaction, while the findings of other investigations implicate reemission of the response from the DP locus as a significant contributor to response magnitude. Using suppression, interfering tones, and temporary threshold shift (TTS) procedures, the work reported here was designed to establish more definitively the precise contributions of the basilar-membrane regions involved in generating acoustic DPs in rabbits. Suppression tuning curves and interfering-tone experiments indicated that for the DP at 2f1-f2, regions near the f1 or f2 frequencies were the major contributors to the emitted response. However, for the higher-frequency DP at 2f2-f1, the basilar-membrane region just basal to the DP site was implicated as the generator. Following brief episodes of TTS at frequencies related to either the DP or the primary tones, the locus of the exposure stimulus that most effectively reduced the magnitude of the 2f1-f2 response also implicated the region of maximal primary-tone interaction in the generation of the acoustic DP. In contrast, for the DP at 2f2-f1, basilar-membrane sites nearer the DP were identified as the primary contributors to the emitted response. Both sets of results imply that different DPs recorded from the ear canal may originate from unique regions of primary-tone interaction along the basilar membrane.

Stimulus-frequency-emission group delay: a test of coherent reflection filtering and a window on cochlear tuning

This paper tests and applies a key prediction of the theory of coherent reflection filtering for the generation of reflection-source otoacoustic emissions. The theory predicts that reflection-source-emission group delay is determined by the group delay of the basilar-membrane (BM) transfer function at its peak. This prediction is tested over a seven-octave frequency range in cats and guinea pigs using measurements of stimulus-frequency-emission (SFOAE) group delay. A comparison with group delays calculated from published measurements of BM mechanical transfer functions supports the theory at the basal end of the cochlea. A comparison across the whole frequency range based on variations in the sharpness of neural tuning with characteristic frequency (CF) suggests that the predicted relation holds in the basal-most 60% of the cochlea. At the apical end of the cochlea, however, the measurements disagree with neural and mechanical group delays. This disagreement suggests that there are important differences in cochlear mechanics and/or mechanisms of emission generation between the base and apex of the cochlea. Measurements in humans over a four-octave range indicate that human SFOAE group delays are roughly a factor of 3 longer than their counterparts in cat and guinea pig but manifest similar trends across CF. The measurements thus reveal global deviations from scaling whose form appears quantitatively similar in all three species. Interpreted using the theory of coherent reflection filtering, the group delay measurements indicate that the wavelength at the peak of the traveling wave decreases with increasing CF at a rate of roughly 25% per octave in the base of the cochlea. The measurements and analysis reported here illustrate the rich potential inherent in OAE measurements for obtaining valuable information about basic cochlear properties such as tuning.

Measurement of acoustic impedance and reflectance in the human ear canal

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 Brüel & 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.

Synaptic transmission in the auditory brainstem of normal and congenitally deaf mice

The deafness (dn/dn) mutant mouse provides a valuable model of human congenital deafness. We investigated the properties of synaptic transmission in the anteroventral cochlear nucleus (AVCN) of normal and congenitally deaf dn/dn mice. Excitatory postsynaptic currents (EPSCs) were evoked by focal stimulation of single auditory nerve fibres, and measured by whole-cell recordings from neurones in AVCN slices (mean postnatal age = P13). Absolute amplitudes of both AMPA- and NMDA-mediated components of evoked EPSCs were greater (170 %) in deaf versus control animals. Enhanced transmission in deaf mice was due to a presynaptic mechanism. Variance-mean analysis revealed that the probability of transmitter release was significantly greater in deaf (P(r) = 0.8) versus control animals (P(r) = 0.5). Following high frequency stimulation, deaf mice showed a greater depression of evoked EPSCs, and a significant increase in the frequency of delayed-release (asynchronous) miniature EPSCs (aEPSCs) (deaf 100 Hz vs. control 7 Hz). The acetoxymethyl ester of EGTA (EGTA-AM) blocked the increase in miniature aEPSCs and returned tetanic depression to control values. In deaf mice, reduction of mean P(r) using cadmium caused an expected increase in paired-pulse ratio (PPR). However, in the same cells, a similar reduction in release by EGTA-AM did not result in a change in PPR, demonstrating that a change may occur in P(r) without a concomitant change in PPR. In many respects, transmission in deaf mice was found to be remarkably similar to control mice, implying that many parameters of synaptic transmission develop normally in these animals. The two significant differences (higher P(r) and asynchronous release in deaf mice) could both be reversed by the addition of EGTA-AM, suggesting that endogenous calcium buffering may be impaired or undeveloped in the presynaptic terminals of the auditory nerve in deaf mice.

Acoustic distortion products in rabbit ear canal. I. Basic features and physiological vulnerability

In contrast to evoked otoacoustic emissions, acoustic distortion products (DPs) recorded from the ear canal are present at predictable frequencies with respect to their primary tones, f1 and f2. Such specificity may provide detailed frequency-place information concerning the functional state of limited regions of the organ of Corti following experimental intervention. However, to date, it is not clear whether emitted DPs solely reflect activity at the basilar-membrane regions of primary tones or if the remote DP site makes a significant contribution to the emitted signal measured in the ear canal. We have investigated a number of the general features of acoustic-DP generation in the rabbit so that, in later experiments, the contributions of specific basilar-membrane regions involved in generating these DPs can be identified using techniques designed to manipulate their normal properties. The first report describes the outcome of systematic manipulations of a number of stimulus conditions and alterations to the physiological state of the cochlea by exposure to fatiguing sound or anoxia. Experimental findings for the 2f1-f2 DP showed that, in general, the relations of the levels and frequency of the primary tones to DP magnitude were consistent with previously published data from other mammalian species. Additional observations for other odd-order intermodulation DPs at the 3f1-2f2 and 2f2-f1 frequencies suggested that the basic attributes of the acoustic DPs were similarly affected by systematic manipulation of the basic parameters of the primary tones and the general metabolic state of the cochlea. General anesthesia, however, did not affect DP amplitude. A companion paper describes the results of a series of subsequent experiments using response-suppression, interfering-tone, and temporary threshold shift techniques which address more directly the issue of which basilar-membrane sites contribute to the generation of different acoustic DPs.

Testing a method for quantifying the output of implantable middle ear hearing devices

This report describes tests of a standard practice for quantifying the performance of implantable middle ear hearing devices (also known as implantable hearing aids). The standard and these tests were initiated by the Food and Drug Administration of the United States Government. The tests involved measurements on two hearing devices, one commercially available and the other home built, that were implanted into ears removed from human cadavers. The tests were conducted to investigate the utility of the practice and its outcome measures: the equivalent ear canal sound pressure transfer function that relates electrically driven middle ear velocities to the equivalent sound pressure needed to produce those velocities, and the maximum effective ear canal sound pressure. The practice calls for measurements in cadaveric ears in order to account for the varied anatomy and function of different human middle ears.

Directional sensitivity of sound-pressure levels in the human ear canal

Changes in sound pressures measured in the ear canal are reported for broadband sound sources positioned at various locations about the subject. These location-dependent pressures are one source of acoustical cues for sound localization by human listeners. Sound source locations were tested with horizontal and vertical resolution of 10 degrees. Sound levels were measured with miniature microphones placed inside the two ear canals. Although the measured amplitude spectra varied with the position of the microphone in the ear canal, it is shown that the directional sensitivity at any particular frequency of the broadband stimulus is independent of microphone position anywhere within the ear canal. At any given frequency, the distribution of sound pressures as a function of sound source location formed a characteristic spatial pattern comprising one or two discrete areas from which sound sources produced maximum levels in the ear canal. The locations of these discrete areas varied in horizontal and vertical location according to sound frequency. For example, around 8 kHz, two areas of maximum sensitivity typically were found that were located laterally and were separated from each other vertically, whereas, around 12 kHz, two such areas were found located on the horizontal plane and separated horizontally. The spatial patterns of sound levels were remarkably similar among different subjects, although some frequency scaling was required to accommodate for differences in the subjects' physical sizes. Interaural differences in sound-pressure level (ILDs) at frequencies below about 8 kHz tended to increase monotonically with increasing distance of the sound source from the frontal midline and tended to be relatively constant as a function of vertical source location. At higher frequencies, however, ILDs varied both with the horizontal and with the vertical location of the sound source. At some frequencies, asymmetries between the left and right ears in a given subject resulted in substantial ILDs even for midline sound sources. These results indicate the types of horizontal and vertical spatial information that are available from sound level cues over various ranges of frequency and, within a small subject population, indicate the nature of intersubject variability.

Interrelation of different oto-acoustic emissions

Amplitude and phase of the sound pressure measured in the closed ear canal during stimulation with pure tones have been monitored as a function of frequency for subjects with and without measurable spontaneous emissions. The frequency spacing between neighboring maxima of the evoked emissions is closely related to that found between neighboring spontaneous emissions. Similar data are found with delayed evoked emissions. All three catagories , spontaneous, delayed, and synchronous evoked emissions are closely related to each other and to the fine structure of threshold in quiet.

Acoustic distortion from rodent ears: a comparison of responses from rats, guinea pigs and gerbils

The oto-acoustic emissions generated in response to two-tone stimulation have been studied in the ear canal sound pressure of three species of rodent: rat (Rattus norvegicus), guinea pig (Cavia porcellus) and Mongolian gerbil (Meriones unguiculatus). The level of acoustic intermodulation distortion evoked by equal-level stimuli at different frequencies can be related to the threshold frequency response curves obtained by other workers for these species using evoked electrical responses or behavioural techniques. Gerbils produce higher levels of distortion below 9 kHz than the rat or guinea pig. This may be due to the greater efficiency of the middle ear at low frequencies. Growth of 2f1-f2 with equal-level, widely-spaced stimuli (f2/f1 = 1.3) can be divided into two regions. The low intensity part of the curve grows with a slope of 1 and saturates above 60 dB SPL. With higher level stimuli, there is rapid growth with a slope of, or approaching 3. Differences in growth rate may distinguish a low-level, saturating response which owes its response characteristics to the activity of hair cells from a high-level response attributable to passive mechanical properties of the cochlea. A broad, low-frequency spread of distortion components is seen as the stimulus frequencies converge. Sharp interruptions in the growth curves of these components may be due to interaction between out-of-phase components of 'low' and 'high' level distortion. The distortion in the gerbil can be distinguished from that of the other two species in a number of details. Structural specialisation of the gerbil cochlea may contribute to these distinctive features.

New off-line method for detecting spontaneous otoacoustic emissions in human subjects

Spontaneous otoacoustic emissions were evaluated in 36 female and 40 male subjects. In agreement with the results of previous surveys, emissions were found to be more prevalent in female subjects and there was a tendency for the male subjects to have fewer emissions in their left ears. The digitization of five minute samples of ear canal signals, combined with sophisticated data analysis, produced a substantial reduction in the emission detection threshold. 588 emissions were detected in 72% of the subjects and 56% of the ears. Of the observed emissions, 18 could be identified with cubic distortion products of other emissions, and 11 could be identified as harmonic products (i.e., integral frequency multiples of other emissions). The large number of emissions detected (one subject had 32 in her right ear and 25 in her left) permitted evaluation of the pattern of separation of emissions. The average effective separation along the basilar membrane (according to the Greenwood frequency map) for adjacent emissions of all ears was 0.427 mm with interquartile values of 0.387 mm and 0.473 mm. The relationship between emission power, frequency, and full width at half maximum appears to be in agreement with the implications of a noise perturbed Van der Pol oscillator model of spontaneous emissions.

Head-related transfer functions of the barn owl: measurement and neural responses

Sounds arriving at the eardrum are filtered by the external ear and associated structures in a frequency and direction specific manner. When convolved with the appropriate filters and presented to human listeners through headphones, broadband noises can be precisely localized to the corresponding position outside of the head (reviewed in Blauert, 1997). Such a 'virtual auditory space' can be a potentially powerful tool for neurophysiological and behavioral work in other species as well. We are developing a virtual auditory space for the barn owl, Tyto alba, a highly successful auditory predator that has become a well-established model for hearing research. We recorded catalogues of head-related transfer functions (HRTFs) from the frontal hemisphere of 12 barn owls and compared virtual and free sound fields acoustically and by their evoked neuronal responses. The inner ca. 1 cm of the ear canal was found to contribute little to the directionality of the HRTFs. HRTFs were recorded by inserting probetube microphones to within about 1 or 2 mm of the eardrum. We recorded HRTFs at frequencies between 2 and 11 kHz, which includes the frequencies most useful to the owl for sound localization (3-9 kHz; Konishi, 1973). Spectra of virtual sounds were within +/- 1 dB of amplitude and +/- 10 degrees of phase of the spectra of free field sounds measured near to the eardrum. The spatial pattern of responses obtained from neurons in the inferior colliculus were almost indistinguishable in response to virtual and to free field stimulation.

Developmental changes in multifrequency tympanograms

The normal maturational course of tympanometric shape, static aural acoustic admittance and ear canal wall characteristics were investigated in healthy infants, who were followed at various time intervals in the first 4 months of life. Susceptance and conductance tympanograms were recorded from both ears of each subject at four probe frequencies or more. In addition, quantitative pneumatic otoscopy was performed utilizing air pressure changes of the same magnitude as those typically used in tympanometry. Results for the group were an increase in admittance magnitude with increasing age at frequencies above 226 Hz. Admittance phase angle increased with age at all frequencies, indicating a growing contribution of compliant elements in the first 4 months of life. The course of development of input admittance at the tympanic membrane differed among individual infants. Otoscopic findings indicated that external ear canal differences cannot completely account for tympanometric differences between young infants and adults.

Influence of in situ, sound-level calibration on distortion-product otoacoustic emission variability

Standing waves can cause errors during in-the-ear calibration of sound pressure level (SPL), affecting both stimulus magnitude and distortion-product otoacoustic emission (DPOAE) level. Sound intensity level (SIL) and forward pressure level (FPL) are two measurements theoretically unaffected by standing waves. SPL, SIL, and FPL in situ calibrations were compared by determining sensitivity of DPOAE level to probe-insertion depth (deep and "shallow") for a range of stimulus frequencies (1-8 kHz) and levels (20-60 dB). Probe-insertion depth was manipulated with the intent to shift the frequencies with standing-wave minima at the emission probe, introducing variability during SPL calibration. The absolute difference in DPOAE level between insertions was evaluated after correcting for an incidental change caused by the effect of ear-canal impedance on the emission traveling from the cochlea. A three-way analysis of variance found significant main effects for stimulus level, stimulus frequency, and calibration method, as well as significant interactions involving calibration method. All calibration methods exhibited changes in DPOAE level due to the insertion depth, especially above 4 kHz. However, SPL demonstrated the greatest changes across all stimulus levels for frequencies above 2 kHz, suggesting that SIL and FPL provide more consistent measurements of DPOAEs for frequencies susceptible to standing-wave calibration errors.

Modeling the identification of concurrent vowels with different fundamental frequencies

Human listeners are better able to identify two simultaneous vowels if the fundamental frequencies of the vowels are different. A computational model is presented which, for the first time, is able to simulate this phenomenon at least qualitatively. The first stage of the model is based upon a bank of bandpass filters and inner hair-cell simulators that simulate approximately the most relevant characteristics of the human auditory periphery. The output of each filter/hair-cell channel is then autocorrelated to extract pitch and timbre information. The pooled autocorrelation function (ACF) based on all channels is used to derive a pitch estimate for one of the component vowels from a signal composed of two vowels. Individual channel ACFs showing a pitch peak at this value are combined and used to identify the first vowel using a template matching procedure. The ACFs in the remaining channels are then combined and used to identify the second vowel. Model recognition performance shows a rapid improvement in correct vowel identification as the difference between the fundamental frequencies of two simultaneous vowels increases from zero to one semitone in a manner closely resembling human performance. As this difference increases up to four semitones, performance improves further only slowly, if at all.

Modeling of Sound Transmission from Ear Canal to Cochlea

A 3-D finite element (FE) model of the human ear consisting of the external ear canal, middle ear, and cochlea is reported in this paper. The acoustic-structure-fluid coupled FE analysis was conducted on the model which included the air in the ear canal and middle ear cavity, the fluid in the cochlea, and the middle ear and cochlea structures (i.e., bones and soft tissues). The middle ear transfer function such as the movements of tympanic membrane, stapes footplate, and round window, the sound pressure gain across the middle ear, and the cochlear input impedance in response to sound stimulus applied in the ear canal were derived and compared with the published experimental measurements in human temporal bones. The frequency sensitivity of the basilar membrane motion and intracochlear pressure induced by sound pressure in the ear canal was predicted along the length of the basilar membrane from the basal turn to the apex. The satisfactory agreements between the model and experimental data in the literature indicate that the middle ear function was well simulated by the model and the simplified cochlea was able to correlate sound stimulus in the ear canal with vibration of the basilar membrane and pressure variation of the cochlear fluid. This study is the first step toward the development of a comprehensive FE model of the entire human ear for acoustic-mechanical analysis.

The influence of pinnae-based spectral cues on sound localization

The role of pinnae-based spectral cues was investigated by requiring listeners to locate sound, binaurally, in the horizontal plane with and without partial occlusion of their external ears. The main finding was that the high frequencies were necessary for optimal performance. When the stimulus contained the higher audio frequencies, e.g., broadband and 4.0-kHz high-pass noise, localization accuracy was significantly superior to that recorded for stimuli consisting only of the lower frequencies (4.0- and 1.0-kHz low-pass noise). This finding was attributed to the influence of the spectral cues furnished by the pinnae, for when the stimulus composition included high frequencies, pinnae occlusion resulted in a marked decline in localization accuracy. Numerous front-rear reversals occurred. Moreover, the ability to distinguish among sounds originating within the same quadrant also suffered. Performance proficiency for the low-pass stimuli was not further degraded under conditions of pinnae occlusion. In locating the 4.0-kHz high-pass noise when both, neither, or only one ear was occluded, the data demonstrated unequivocally that the pinna-based cues of the "near" ear contributed powerfully toward localization accuracy.

Acoustic properties of different cartilage reconstruction techniques of the tympanic membrane

OBJECTIVES/HYPOTHESIS

The use of cartilage in reconstruction of the tympanic membrane has been established especially in cases such as tubal dysfunction and adhesive processes. Cartilage offers the advantage of higher mechanical stability compared with membranous transplants but may alter the acoustic transfer characteristics of the graft. Apart from material properties, it can be assumed that, also, the microsurgical reconstruction technique might influence the sound transmission properties of the reconstructed tympanic membrane. The purpose of the study was to investigate the acoustic transfer characteristics of different cartilage transplants being typically used in different reconstruction techniques of the tympanic membrane.

METHODS

Cartilage plates of different thicknesses (1.0, 0.7, 0.5, and 0.3 mm), cartilage palisades, and cartilage island transplants of varying size were investigated by means of an ear canal-tympanic membrane model. In contrast to former single-point measurements, sound-induced vibrational amplitudes of the entire transplant were measured by scanning laser Doppler vibrometry (measuring points, n = 133) (PSV-200, Polytec, Waldbronn, Germany). Frequency response functions (displacement vs. sound pressure) of all measured points were determined in the frequency range of 200 Hz to 4 kHz for the different transplants.

RESULTS

Cutting thick cartilage transplants into thin plates or palisades decreased the first resonance frequency and increased its amplitude, reflecting improved sound transmission properties of the transplant. From an acoustical point of view, the 0.5-mm cartilage plate seems preferable compared with the palisade technique. Cartilage island techniques showed vibration characteristics superior to plate or palisade techniques.

CONCLUSIONS

Apart from material characteristics, the sound transmission properties of the reconstructed tympanic membrane are strongly influenced by the reconstruction technique. The choice of the surgical technique should consider requirements based on mechanical stability and acoustic transfer characteristics of the transplant.

Mechanical and acoustical influences on spontaneous oto-acoustic emissions

Spontaneous, tone-like emissions produced by normal ears and measured in the closed outer ear canal can be affected by mechanical and acoustical events. Such effects can be measured in steady-state conditions as well as for transient stimulation, and are seen in response to the stapedius reflex, to ear canal air pressure changes, and to the presentation of external tones. Frequency and level of the emissions follow certain characteristics which are described and discussed. The emissions seem to react with 2 ms delay and with an exponential rise and decay, the time constant of which is about 13 ms.

Virtual localization improved by scaling nonindividualized external-ear transfer functions in frequency

This study examined virtual sound localization in three conditions that differed according to the directional transfer functions (DTFs) that were used to synthesize the virtual targets. The own-ear and other-ear conditions used DTFs measured from listeners' own ears and those measured from other subjects, respectively. The scaled-ear condition employed other-ear DTFs that were scaled in frequency to minimize the mismatch between spectral features in the listener's and the other subject's DTFs. All measures of localization error typically were lowest in the own-ear condition. In other-ear conditions, all error measures tended to increase in proportion to the inter-subject differences in DTFs. When spectral features in an other-ear set of DTFs fell systematically lower in frequency than in a listener's own DTFs, low frontal targets typically were reported as low in the rear, and high rear targets were reported as high in front. When spectral features in a set of DTFs fell systematically higher in frequency than in a listener's own DTFs, elevation judgements showed an upward bias. In the scaled-ear condition, all measures of performance tended to improve relative to the other-ear condition. In the majority of cases, frequency scaling more than halved the penalty for use of another subject's DTFs.

Wideband absorbance tympanometry using pressure sweeps: system development and results on adults with normal hearing

A system with potential for middle-ear screening and diagnostic testing was developed for the measurement of wideband energy absorbance (EA) in the ear canal as a function of air pressure, and tested on adults with normal hearing. Using a click stimulus, the EA was measured at 60 frequencies between 0.226 and 8 kHz. Ambient-pressure results were similar to past studies. To perform tympanometry, air pressure in the ear canal was controlled automatically to sweep between -300 and 200 daPa (ascending/descending directions) using sweep speeds of approximately 75, 100, 200, and 400 daPas. Thus, the measurement time for wideband tympanometry ranged from 1.5 to 7 s and was suitable for clinical applications. A bandpass tympanogram, calculated for each ear by frequency averaging EA from 0.38 to 2 kHz, had a single-peak shape; however, its tympanometric peak pressure (TPP) shifted as a function of sweep speed and direction. EA estimated at the TPP was similar across different sweep speeds, but was higher below 2 kHz than EA measured at ambient pressure. Future studies of EA on normal ears of a different age group or on impaired ears may be compared with the adult normal baseline obtained in this study.

Wideband reflectance tympanometry in normal adults

Acoustic impedance/reflectance measurements were made at various ear-canal pressures in 20 subjects with a clinical acoustic immittance instrument and an experimental impedance/reflectance system. Measurements were made over a frequency range of 226-2000 Hz with the clinical system and 125-11,310 Hz with the experimental system. For frequencies < or = 2.0 kHz, tympanograms obtained with the two systems are similar, with patterns that progress through the same orderly sequence with increasing frequency. Eardrum impedance measurements were also similar. There are small gender differences in middle-ear impedance. Reflectance patterns (reflectance versus frequency) at ambient ear-canal air pressure are characterized by high reflectance at low frequencies, two district minima at 1.2 and 3.5 kHz, increasing reflectance to 8.0 kHz, and decreasing reflectance above that frequency. Ear-canal pressure increases reflectance at low frequencies, decreases reflectance in the region of the minimum, and increases reflectance slightly at high frequencies. Reflectance tympanograms (reflectance versus ear-canal pressure) progress through a sequence of three patterns. At low frequencies, reflectance tympanograms are "V" shaped, indicating that pressure increases reflectance. At frequencies near the minimum reflectance, the pattern inverts, indicating that pressure decreases reflectance. At high frequencies, the patterns are flat, indicating that ear-canal pressure has little effect. Results presented for one patient suggest that reflectance tympanometry may be useful for detecting middle-ear pathology.