Jonathan H. Siegel

Ear‐canal standing waves and high‐frequency sound calibration using otoacoustic emission probes

Low‐noise microphones designed to measure otoacoustic emissions from the human ear canal typically sample the sound field in the canal some 15–20 mm away from the eardrum. The input sound levels to the middle ear are usually defined as the sound‐pressure level ‘‘at the eardrum.’’ However, the well‐known phenomenon of standing waves produces a spatially nonuniform pressure for frequencies above 2–3 kHz. This phenomenon is demonstrated using physical simulation with a commonly used emission probe and the interpretation of the results are confirmed by simple acoustical theory. In addition, it is shown large (±20 dB) errors in the estimated eardrum sound pressure result from varying the position of the sound source in the canal. Theory, physical simulations and measurements in human ear canals are used to characterize the errors using various calibration procedures commonly employed in otoacoustic emission studies. It is predicted that the reliability of measurements of high‐frequency stimulated otoacoustic e...

Spontaneous synaptic potentials from afferent terminals in the guinea pig cochlea

Records of spontaneous activity from units likely to be radial afferents were analyzed to find the origin of spontaneous action potentials in single auditory nerve fibers. Single synaptic events (excitatory postsynaptic potentials or EPSPs) nearly all triggered action potentials (spikes). An abrupt increase in slope during the rising phase of the EPSP often signalled the initiation of an action potential. Synaptic potentials that did not trigger spikes occurred frequently during the refractory period. These events sometimes appeared to be composed of subunits. Refractoriness appears to be the primary reason these EPSPs were ineffective. Distributions of the onset slopes of postsynaptic potentials were highly skewed. Skewing was not a consequence of refractoriness, but most likely because the amplitude distribution of spontaneous potentials is not gaussian.

Sound calibration and distortion product otoacoustic emissions at high frequencies

Distortion product otoacoustic emissions offer the potential for assessing inner ear function at high frequencies. However, commonly employed methods for calibrating the acoustic system used in these studies can lead to errors of +/- 20 dB or more in the estimate of eardrum sound pressure levels above 2-3 kHz [Siegel, J. Acoust. Soc. Am. 95, 2589-2597 (1994)]. We assessed the magnitude of these errors by measuring the distortion product emission 2f1-f2 (f1 < f2) in human subjects using either of two microphone locations to calibrate the stimulus. Either the emission probe microphone itself or a probe tube positioned near the eardrum were used in calibrations. The emissions collected with f1 in the vicinity of 5-7 kHz showed a pronounced peak in level relative to other stimulus frequencies when the emission probe was used for calibration. The peak at 5-7 kHz disappeared when the probe tube near the eardrum was used for calibration. The discrepancy in emission levels between the two calibration procedures was as large as 20 dB. The difference is attributed to the presence of standing waves in the ear canal. The systematic errors in estimating eardrum sound pressure level using the emission probe microphone undoubtedly contribute to the variability of emission measurements for high-frequency stimuli.

Behavioral hearing thresholds between 0.125 and 20 kHz using depth-compensated ear simulator calibration.

Objectives: The purpose of this study was to obtain behavioral hearing thresholds for frequencies between 0.125 and 20 kHz from a large population between 10 and 65 yr old using a clinically feasible calibration method expected to compensate well for variations in the distance between the eardrum and an insert-type sound source. Previous reports of hearing thresholds in the extended high frequencies (>8 kHz) have either used calibration techniques known to be inaccurate or specialized equipment not suitable for clinical use. Design: Hearing thresholds were measured from 352 human subjects between 10 and 65 yr old having clinically normal-hearing thresholds (<20 dB HL) up to 4 kHz. An otoacoustic emission probe fitted with custom sound sources was used, and the stimulus levels individually tailored on the basis of an estimate of the insertion depth of the measurement probe. The calibrated stimulus levels were determined on the basis of measurements made at various depths of insertion in a standard ear simulator. Threshold values were obtained for 21 frequencies between 0.125 and 20 kHz using a modified Békésy technique. Forty-six of the subjects returned for a second measurement months later from the initial evaluation. Results: In agreement with previous reports, hearing thresholds at extended high frequencies were found to be sensitive to age-related changes in auditory function. In contrast with previous reports, no gender differences were found in average hearing thresholds at most evaluated frequencies. Two aging processes, one faster than the other in time scale, seem to influence hearing thresholds in different frequency ranges. The standard deviation (SD) of test–retest threshold difference for all evaluated frequencies was 5 to 10 dB, comparable to that reported in the literature for similar measurement techniques but smaller than that observed for data obtained using the standard clinical procedure. Conclusions: The depth-compensated ear simulator-based calibration method and the modified Békésy technique allow reliable measurement of hearing thresholds over the entire frequency range of human hearing. Hearing thresholds at the extended high frequencies are sensitive to aging and reveal subtle differences, which are not evident in the frequency range evaluated regularly (⩽8 kHz). Previously reported gender-related differences in hearing thresholds may be related to ear-canal acoustics and the calibration procedure and not because of differences in hearing sensitivity.

Auditory filter shapes and high-frequency hearing in adults who have impaired speech in noise performance despite clinically normal audiograms

Some individuals complain of hearing difficulties in the presence of background noise even in the absence of clinically significant hearing loss (obscure auditory dysfunction). Previous evidence suggests that these listeners have impaired frequency resolution, but there has been no thorough characterization of auditory filter shapes in this population. Here, the filter shapes of adults (n = 14) who self-reported speech recognition problems in noise and performed poorly on a sentence-in-noise perception test despite having clinically normal audiograms were compared to those of controls (n = 10). The filter shapes were evaluated using a 2-kHz probe with a fixed level of 30, 40, or 50 dB sound pressure level (SPL) and notched-noise simultaneous maskers that were varied in level to determine the masker level necessary to just mask the probe. The filters of the impaired group were significantly wider than those of controls at all probe levels owing to an unusual broadening of the upper slope of the filter. In addition, absolute thresholds were statistically indistinguishable between the groups at the standard audiometric frequencies, but were elevated in the impaired listeners at higher frequencies. These results strengthen the idea that this population has a variety of hearing deficits that go undetected by standard audiometry.

Spike activity recorded from the organ of Corti

During microelectrode passes through the inner hair cell region of the organ of Corti, spike activity is frequently encountered. We have obtained both extracellular and intracellular records from afferent terminals or dendrites. The response characteristics of all-or-none spike trains mimic those recorded from primary auditory axons. EPSPs may be identified, either associated with the spikes or in the absence of a spike. Inferences may be made about the nature of neurotransmitter quantum release from inner hair cells.

Factors that influence rate-versus-intensity relations in single cochlear nerve fibers of the gerbil.

The relationship between characteristic frequency (CF) and rate-intensity curve shape was examined in 144 cochlear nerve fibers obtained from 39 Mongolian gerbils. Quasi-steady-state firing rates were measured in response to tone bursts at the CF. From each intensity curve, estimates of slope, firing rate at saturation, and dynamic range were derived using nonlinear curve fitting. Saturation firing rate was depressed for stimuli with a high duty cycle, especially for units with low rates of spontaneous discharge. The distributions of slope and saturation firing rate differed for fibers with CFs above and below 3 to 4 kHz. The interrelation of slope, dynamic range, maximum driven rate, and spontaneous firing rate was also different for fibers with CFs above and below this band. This mid-CF transition is discussed in terms of possible longitudinal changes in the function of the gerbil cochlea.

Distortion-product otoacoustic emissions measured at high frequencies in humans

Distortion-product otoacoustic emissions (DPOAEs) elicited with stimulus frequencies less than or equal to 8 kHz have been used in hearing clinics to assess whether the middle ear and cochlea are normal, but high-frequency hearing (>4 kHz) is most vulnerable to cochlear pathology. It might prove useful to measure DPOAEs with even higher frequency stimuli (>8 kHz), but there have been few reports of such studies in humans. DPOAEs have been measured in other mammals to the upper range of hearing sensitivity. The purpose of this study was to compare some characteristics of DPOAEs in human subjects elicited with high-frequency stimuli with those that have been extensively measured with lower-frequency stimuli. The primary goal was to establish if the same phenomenon responsible for the behavior of low-frequency DPOAEs is responsible for the behavior of high-frequency DPOAEs. Specifically, the DPOAE level with stimuli varied from 2 to 20 kHz, growth functions of DPOAEs, effects of varying the primary frequency ratio (f2/f1) on the DPOAE level, and DPOAE group delay were determined. Because the behaviors appeared to vary smoothly with stimulus frequency, the study suggests that emissions measured from 2 to 20 kHz were the product of the same biological process.

Cochlear basal and apical differences reflected in the effects of cooling on responses of single auditory nerve fibers.

Responses of single auditory nerve fibers in the Mongolian gerbil were examined before and during rapid, moderate cooling of the cochlea. Reducing cochlear temperature from 35-39 degrees C to 29-32 degrees C led to stable, reversible changes in spontaneous firing rates (SRs), and responses to tonebursts, as characterized by frequency tuning curves and rate-versus-intensity curves. The nature and extent of effects of cooling were strongly linked to characteristic frequency (CF). Rate thresholds at the CF were increased by 0-15 dB for fibers with CFs below 8 kHz, and by 10-30 dB for higher CFs. Although SRs were generally reduced, the percent reduction in SR was striking CF dependent. For fibers with CFs below 4 kHz, the reduction did not exceed 50% of the initial SR. For higher CFs, the reduction was always greater than 50%. The effects of cooling on intensity curve shape differed qualitatively for fibers with CFs below and above 3-4 kHz. The slope of the curve was reduced by an average of 50% for lower CFs, but on average was unchanged for higher CFs. Cooling-related increases in CF threshold probably reflect impairment of active mechanical processes. The CF dependence of these increases suggests either that active mechanical processes are more impaired in the cochlear base relative to the apex, or that they play a more crucial role in determining sensitivity in the base. The CF-dependent changes in SR and in the shape of rate-intensity curves caused by cooling correspond to an enhancement of basal/apical differences seen at normal temperatures. They are best explained by longitudinal gradients in the properties of the inner hair cells and their afferent synapses. Basal and apical differences in the distribution of SRs and in supra-threshold response properties suggest that stimulus coding strategies differ between low and high frequency regions of the cochlea.

The effects of moderate cooling on gross cochlear potentials in the gerbil: Basal and apical differences

Changes in the threshold of the compound action potential (CAP) response in the gerbil to low- and high-frequency tonebursts were monitored during uniform cooling of the cochlea by 7-8 degrees C below normal body temperature. Recordings of the endocochlear potential (EP), cochlear microphonic (CM), and summating potentials (SP) were also obtained from the base and apex of the cochlea under the same conditions. Cooling-related changes in the CAP, as well as the CM and SP response obtained near the best frequency of the recording location, were greater in the base than in the apex. In contrast, reductions in the EP appeared uniform throughout the cochlea. Thus the greater vulnerability of CAP thresholds in the base does not result from a greater vulnerability of the stria vascularis in this region. Our results suggest that the enhanced susceptibility to cooling of the CAP in the cochlear base reflects changes in hair cell mechanisms.