Evidence for apical-basal transition in the delay of the reflection components of otoacoustic emissions.

Stimulus-frequency, transient-evoked, and distortion product otoacoustic emissions (OAEs) have been measured in eight normal-hearing human ears over a wide stimulus level range, with high spectral resolution. The single-reflection component of the response was isolated using time-frequency filtering, and its average delay was measured as a function of frequency and stimulus level. The apical-basal transition was studied by fitting the average delay of the filtered single-reflection OAEs, expressed in number of cycles, to a three-slope power-law function with two knot frequencies. The results show that the scale-invariant prediction of constant dimensionless delay approximately holds only over a narrow intermediate frequency range (1-2.5 kHz). Below 1 kHz, and, to some extent, above 2.5 kHz, the dimensionless delay increases with frequency, at all stimulus levels. Comparison with the numerical simulations of a delayed-stiffness active cochlear model show that the increase of tuning with frequency reported by behavioral experiments only partly explains this result. The low-frequency scaling symmetry breaking associated with the deviation of the Greenwood tonotopic map from a pure exponential function is also insufficient to explain the steep low-frequency increase of the OAE delay. Other sources of symmetry breaking, not included in the model, could therefore play a role.

Oxidative stress biomarkers and otoacoustic emissions in humans exposed to styrene and noise.

OBJECTIVE: Evaluating the correlation between otoacoustic emission levels, styrene exposure, and oxidative stress biomarkers concentration in styrene-exposed subjects, to investigate the role of oxidative stress in outer hair cell damage. DESIGN: Distortion product otoacoustic emissions were measured in the exposed workers and in a control group. Separation between the distortion and reflection otoacoustic components was performed by time-frequency-domain filtering. The urinary concentration of the DNA and RNA oxidation products, namely 8-oxo-7,8-dihydroguanine (oxoGua), 8-oxo-7,8-dihydro-2'-deoxyguanosine (oxodGuo), and 8-oxo-7,8-dihydroguanosine (oxoGuo), were evaluated. STUDY SAMPLE: Nine subjects exposed to styrene in a fiberglass factory, eight control subjects. The two groups were statistically equivalent in mean age. RESULTS: Statistically significant differences were found in the distortion component levels between the exposed and the control group. High levels of the oxidative damage biomarkers were found in the workers exposed to high levels of styrene. Significant negative correlation was found between the otoacoustic emission distortion component levels and the concentration of the oxoGuo biomarker. CONCLUSIONS: Exposure-induced damage of the cochlear amplifier is shown in the mid-frequency range, confirming animal experiments, in which hair cells in the cochlear middle turn were damaged. Hearing damage is consistent with the outer hair cell apoptosis pathway associated with oxidative stress.

Susceptibility of linear and nonlinear otoacoustic emission components to low-dose styrene exposure.

OBJECTIVE: To investigate potential susceptibility of active cochlear mechanisms to low-level styrene exposure by comparing TEOAEs in workers and controls. DESIGN: Two advanced analysis techniques were applied to detect sub-clinical changes in linear and nonlinear cochlear mechanisms of OAE generation: the wavelet transform to decompose TEOAEs into time-frequency components and extract signal-to-noise ratio and latency of each component, and the bispectrum to detect and extract nonlinear TEOAE contributions as quadratic frequency couplings (QFCs). STUDY SAMPLE: Two cohorts of workers were examined: subjects exposed exclusively to styrene (N = 9), and subjects exposed to styrene and noise (N = 6). The control group was perfectly matched by age and sex to the exposed group. RESULTS: Exposed subjects showed significantly lowered SNR in TEOAE components at mid-to-high frequencies (above 1.6 kHz) and a shift of QFC distribution towards lower frequencies than controls. No systematic differences were observed in latency. CONCLUSION: Low-level styrene exposure may have induced a modification of cochlear functionality as concerns linear and nonlinear OAE generation mechanisms. The lack of change in latency seems to suggest that the OAE components, where generation region and latency are tightly coupled, may not have been affected by styrene and noise exposure levels considered here.

Experimental evidence for the basal generation place of the short-latency transient-evoked otoacoustic emissions.

Time-frequency analysis of the transient-evoked otoacoustic emission response was performed on a population of subjects affected by sensory-neural hearing loss characterized by a sharp audiometric profile, caused by firearm noise exposure (42 ears), and on a control population of normal-hearing subjects (84 ears). Time-frequency filtering permitted a careful evaluation of the relation between the audiometric profile and the spectral shape of the long- and short-latency otoacoustic components. Both filtered spectra closely follow the shape of the audiometric profile, with a frequency shift between them. The typical frequency shift was evaluated by averaging the otoacoustic spectra and the audiograms among groups of ears with the same cutoff frequency. Assuming that the otoacoustic emission source function depends on the local effectiveness of the cochlear amplifier, this experimental evidence suggests that the short-latency response is generated at a cochlear place displaced towards the base by about 0.5-1 mm with respect to the generation place of the long-latency component. The analysis of the control group demonstrates that, below 4 kHz, the observed effect is not dependent on the data acquisition and analysis procedure. These results confirm previous theoretical estimates and independent experimental evidence based on the measured latency difference between the two components.

Otoacoustic emission sensitivity to exposure to styrene and noise.

The ototoxic effect of the exposure to styrene is evaluated, also in the presence of simultaneous exposure to noise, using otoacoustic emissions as biomarkers of mild cochlear damage. Transient-evoked and distortion product otoacoustic emissions were recorded and analyzed in a sample of workers (15 subjects) exposed to styrene and noise in a fiberglass manufacturing facility and in a control group of 13 non-exposed subjects. Individual exposure monitoring of the airborne styrene concentrations was performed, as well as biological monitoring, based on the urinary concentration of two styrene metabolites, the Mandelic and Phenylglyoxylic acids. Noise exposure was evaluated using wearable phonometers, and hearing loss with pure tone audiometry. Due to their different job tasks, one group of workers was exposed to high noise and low styrene levels, another group to higher styrene levels, close to the limit of 20 ppm, and to low noise levels. A significant negative correlation was found between the otoacoustic emission levels and the concentration of the styrene urinary metabolites. Otoacoustic emissions, and particularly distortion products, were able to discriminate the exposed workers from the controls, providing also a rough estimate of the slope of the dose-response relation between otoacoustic levels and styrene exposure.

Localization of the Reflection Sources of Stimulus-Frequency Otoacoustic Emissions.

The generation of stimulus-frequency otoacoustic emission (SFOAE) residuals in humans is analyzed both theoretically and experimentally to investigate the relation between the frequency difference between the probe and the suppressor tone and the localization of the residual source. Experimental measurements of the SFOAE residual were performed using suppressors of increasing frequency to separate the otoacoustic response from the probe stimulus. From the response to the probe alone, the SFOAE response was also estimated, using spectral smoothing, and compared with the residuals obtained for different frequency suppressors. A nonlinear delayed-stiffness active cochlear model was used to compute the spatial distribution of the residual sources according to a recent model of the local reflectivity from roughness, as a function of the suppressor frequency. The simulations clarified the role of high-frequency suppressors, showing that in humans, with increasing suppressor frequency, the generation region of the residual is only slightly basally shifted with respect to the case of a near-frequency suppressor, near the basal edge of the peak of the resonant basilar membrane response. As a consequence, the hierarchy among different-delay components correspondingly changes, gradually favoring short-delay components, with increasing suppressor frequency. Good agreement between the experimental and theoretical dependence of the level of otoacoustic components of different delay on the frequency shift between probe and suppressor confirms the validity of this interpretation.

Cosmic rays observed by the resonant gravitational wave detector NAUTILUS

The passage of cosmic rays has been observed to excite mechanical vibrations in the resonant gravitational wave detector NAUTILUS operating at temperature of 100 mK. A very significant correlation (more than 10 standard deviations) is found.

Linear and nonlinear transient evoked otoacoustic emissions in humans exposed to noise.

Transient evoked otoacoustic emissions (TEOAEs) have been analyzed in a population of 134 ears, divided into three classes: (1) nonexposed ears in bilaterally normal hearing subjects, (2) audiometrically normal ears of subjects exposed to noise and affected by unilateral high-frequency (f>3 kHz) hearing loss in the contralateral ear, and (3) the contralateral impaired ears of the exposed subjects. The statistical distributions of global and spectral signal-to-noise ratio (SNR) were analyzed. TEOAEs were recorded both in the linear and nonlinear acquisition mode to evaluate the effectiveness of two standard averaging techniques with respect to their sensitivity to the early effects of noise exposure. Good discrimination between nonexposed and exposed ears was obtained using either the linear or the nonlinear mode. Despite its intrinsically higher SNR, the linear mode is not more sensitive than the nonlinear mode for this purpose because it is not possible to find a window for effectively cancelling the linear artifact while keeping a suitable sensitivity to the short-latency high-frequency aspect of the response. Moreover, with respect to another measurable parameter, the TEOAE latency, good discrimination is obtained only by using the nonlinear mode because, again, the linear artifact masks the high-frequency TEOAE response.

Modeling otoacoustic emissions by active nonlinear oscillators.

The phenomenology of spontaneous otoacoustic emissions (OAEs) is compared to theoretical predictions given by models in which they are produced by active nonlinear oscillators. Along with the well-known Van der Pol oscillator, a new active oscillator model is proposed and analyzed here. Numerical simulations and multi-scale analytical computation results are compared to experimental data of neonatal spontaneous and evoked OAEs. A simple analysis technique is proposed, in which the time evolution after a click stimulus of the amplitude of each spectral line corresponding to a spontaneous OAE is studied. Apart from a few stationary lines, an approximately exponential decay law, with characteristic damping coefficients in the 20-200 Hz range, was found to fit the data. These results are not compatible with a Van der Pol oscillator model, and show that some important aspects of the OAE phenomenology can be better explained by the proposed oscillator. Other interesting features of the spontaneous end evoked OAE phenomenology, such as spontaneous OAE suppression by external tones and the following recovery, as well as stimulus/response curves in the linear and nonlinear mode of acquisition, are also well reproduced by the proposed model.

On the frequency dependence of the otoacoustic emission latency in hypoacoustic and normal ears.

Experimental measurements of the otoacoustic emission (OAE) latency of adult subjects have been obtained, as a function of frequency, by means of wavelet time-frequency analysis based on the iterative application of filter banks. The results are in agreement with previous OAE latency measurements by Tognola et al. [Hear. Res. 106, 112-122 (1997)], as regards both the latency values and the frequency dependence, and seem to be incompatible with the steep 1/f law that is predicted by scale-invariant full cochlear models. The latency-frequency relationship has been best fitted to a linear function of the cochlear physical distance, using the Greenwood map, and to an exponential function of the cochlear distance, for comparison with derived band ABR latency measurements. Two sets of ears [94 audiometrically normal and 42 impaired with high-frequency (f > 3 kHz) hearing loss] have been separately analyzed. Significantly larger average latencies were found in the impaired ears in the mid-frequency range. Theoretical implications of these findings on the transmission of the traveling wave are discussed.

Spontaneous otoacoustic emissions and relaxation dynamics of long decay time OAEs in audiometrically normal and impaired subjects.

The relationship between hearing loss, detected by measuring the audiometric threshold shift, and the presence of long-lasting otoacoustic emissions, has been studied in a population of 66 adult males, by analyzing the cochlear response in the 80 ms following the subministration of a click stimulus. Most long-lasting OAEs are also recognizable as Synchronized Spontaneous OAEs (SSOAEs). The OAE characteristic decay times were evaluated according to the model by Sisto and Moleti [J. Acoust. Soc. Am. 106, 1893 (1999)]. The starting hypothesis, confirmed by the results, is that long decay time and large equilibrium amplitude are both manifestations of the effectiveness of the active feedback mechanism. The prevalence and frequency distribution of long-lasting OAEs, and of their SSOAE subset, have been separately analyzed for normal and impaired ears. No long-lasting OAE was found within the hearing loss frequency range, but several were found in impaired ears outside the hearing loss range, both at lower and higher frequencies. This result suggests that the correlation between the presence of long-lasting OAEs and good cochlear functionality be local in the frequency domain. The monitor of the OAE decay time is proposed as a new possible method for early detecting hearing loss in populations exposed to noise.

On the detection of early cochlear damage by otoacoustic emission analysis.

Theoretical considerations and experimental evidence suggest that otoacoustic emission parameters may be used to reveal early cochlear damage, even before it can be diagnosed by standard audiometric techniques. In this work, the statistical distributions of a set of otoacoustic emission parameters chosen as candidates for the early detection of cochlear damage (global and band reproducibility, response level, signal-to-noise ratio, spectral latency, and long-lasting otoacoustic emission presence) were analyzed in a population of 138 ears. These ears have been divided, according to a standard audiometric test, in three classes: (1) ears of nonexposed bilaterally normal subjects, (2) normal ears of subjects with unilateral noise-induced high-frequency hearing loss, and (3) their hearing impaired ears. For all analyzed parameters, a statistically significant difference was found between classes 1 and 2. This difference largely exceeds the difference observed between classes 2 and 3. This fact suggests that the noise exposure, which was responsible for the unilateral hearing loss, also caused subclinical damage in the contralateral, audiometrically normal, ear. This is a clear indication that otoacoustic emission techniques may be able to early detect subclinical damages.

Electromagnetic radiation from VDT units: study of the effectiveness of an active shielding device.

Measurements of extremely low frequency electromagnetic fields and low frequency magnetic fields emitted by a set of video display terminal (VDT) units are reported. The field values measured at the position normally occupied by the user are below the safety limits. This is because the field amplitudes decrease rapidly (following a 1/R3 law) with the distance from the source, as has been verified in this work. Measurements with a commercial shielding device consisting of small plastic balls filled with a water solution of rare earth elements were also performed. The only physical mechanism that could be hypothesized to produce an active suppression of the VDT field is that rare earth atoms, which probably were chosen due to their large magnetic moment, behave as oscillating magnetic dipoles capable of emitting a secondary magnetic field that, along some particular directions, has a phase that is opposite to that of the exciting field. Unfortunately, if one analyzes this mechanism quantitatively, it is easy to show that the secondary magnetic field is absolutely negligible, as was confirmed by experimental measurements performed in this study.

Increasing the bandwidth of resonant gravitational antennas: the case of explorer.

Resonant gravitational wave detectors with an observation bandwidth of tens of hertz are a reality: the antenna Explorer, operated at CERN by the ROG Collaboration, has been upgraded with a new readout. In this new configuration, it exhibits an unprecedented useful bandwidth: in over 55 Hz about its center operating frequency of 919 Hz the spectral sensitivity is better than 10(-20) Hz(-1/2). We describe the detector and its sensitivity and discuss the foreseeable upgrades to even larger bandwidths.