Growth of distortion product otoacoustic emissions with primary-tone level in humans.

This study addressed the hypotheses that the growth of the level of distortion product otoacoustic emissions (LDP) with primary-tone level reflects the behavior of a third-order nonlinear polynomial system, and that two sources exist for these distortion products. The results indicated that the 2f1-f2 otoacoustic emission in humans can be measured over a much larger stimulus range than reported previously, even for stimuli (L1 = L2) as low as 10 dB SPL (re 20 microPa). The input/output functions are best described as a straight line with a rate of growth of about 1 dB/dB of stimulus level. For stimulus levels at which metabolically active, nonlinear cochlear processes are in operation, the system does not behave as a simple third-order nonlinear polynomial. Small plateaus and sharp discontinuities or 'notches' can occur in the functions at stimulus levels of approximately 55 dB SPL. These characteristics are consistent with the notion of two separate sources of the LDP, one at low stimulus levels, and one at high levels. An alternative explanation is that the measured otoacoustic emission does not represent only the activity at a single location along the basilar membrane but includes the effects of interactions among similar signals arising from multiple locations, or from the original source via multiple paths.

Hepatic vein transit time of an ultrasound contrast agent: simplified procedure using pulse inversion imaging.

The aim of this study was to ascertain whether a new ultrasound technique, namely pulse inversion imaging, could assess the arrival of a contrast agent in the hepatic veins, and to describe possible advantages of this procedure in determining transit time over a previously described method based upon spectral Doppler quantification. 15 subjects were scanned using pulse inversion imaging. A bolus injection of 2.5 g Levovist (Schering AG, Berlin, Germany) 300 mg x ml(-1) was given into an antecubital vein. Median transit times of 16 s (range 14-20 s) were found in patients with liver cirrhosis (n=4), 22 s (range 16-27 s) in patients with focal liver lesions (n=8) and 31 s (range 30-32 s) in control subjects (n=3). The maximum interobserver variation was 2 s and the maximum intraobserver variation was 3 s (n=10). Transit time was assessed by both pulse inversion imaging and spectral Doppler quantification in six patients. Comparison of the two methods showed transit times within 2 s apart in five patients and within 5 s apart in one patient. In conclusion, it is possible to assess transit time using pulse inversion imaging. This method is simpler than a previously described method requiring computer analysis. Moreover, several liver veins can be assessed simultaneously. Different transit times were observed in different liver veins in two patients with liver tumours. A short transit time (<27 s) appears to be found only in patients with liver disease. After transit time assessment, it is possible to use the injected contrast agent for late phase imaging of the liver parenchyma.

A new approach for recording distortion product oto-acoustic emissions.

A system for recording of distortion product oto-acoustic emissions (DPOEs) utilizing a linear time-averaging technique is described. The main advantage of the system is the ability to obtain a high signal-to-noise ratio of the recordings. DPOEs have proven to yield important frequency-specific information on the behaviour of the outer hair cells in the human inner ear. DPOEs at nine audiometric frequencies, elicited at a stimulus intensity of 75 dB SPL were recorded from a group of 14 normal-hearing subjects in an attempt to establish normative data. An example of the relationship between the configuration of the conventional audiogram and DPOEs for a patient with a noise-induced hearing loss is presented to demonstrate the diagnostic value of the method.

Instrumentation for transient evoked otoacoustic emissions elicited by maximum length sequences.

A system for recording Transient Evoked Otoacoustic Emissions (TEOAE) for neonatal screening, utilizing Maximum Length Sequences (MLS) for the generation of stimulus signals, is described. The main advantage of the system is its ability to obtain responses within a very short period of time, compared to conventional OAE systems. The actual recording time for the MLS system is reduced by a factor of between 10 and 20 for comparable signal-to-noise ratios. Stimulus levels range from 50 to 90 dB peak equivalent SPL and available stimulus rates are 31, 62, 125, 250, 500, 1000 and 2000/sec. A dynamic noise rejection algorithm is performed during data collection, and, moreover, all sweeps (both accepted and rejected) are stored in memory so that the signal-to-noise ratio can be optimized by post-processing immediately after data collection. As a result, robust response can be achieved even if the subject is noisy during the test. A new semi-non-linear recording technique is described, combining the benefits from the conventional linear and non-linear recording methods. Examples of adult and neonatal responses under different conditions are provided to demonstrate the value of the recording principle.

Distortion product otoacoustic emissions. The influence of the middle ear transmission.

Distortion product otoacoustic emissions (DPOEs) were recorded in 8 normal hearing subjects (16 ears, mean age 37) in order to evaluate the sensitivity of the DPOE amplitudes as a function of pneumatic changes (-400 to +200 daPa) in the external ear canal. The responses were collected using time averaging and subsequent FFT-analysis of the collected data. Distortion products were measured at the following geometric mean frequencies: 1,000, 2,000, 4,000, 6,000 and 8,000 Hz. The results show that the amplitudes of the distortion products depend on an optimal transmission through the middle ear, and that measurements of DPOEs should always be preceded by determination of the middle ear pressure. The present findings are in good agreement with investigations based on evoked otoacoustic emissions published by other researchers.

Calibration of ipsilateral stimulus transducer for acoustic reflex measurements.

Pure-tone Reference Equivalent Threshold Sound Pressure Level (RETSPL) of the ipsilateral stimulus receiver for acoustic reflex measurements on Madsen Electronics type Zodiac 901 impedance audiometer is provided. The results, obtained from 20 normal-hearing subjects, are achieved by comparing hearing threshold levels measured using a TDH 39 telephone (calibrated to ISO 389) with thresholds recorded using the ipsilateral stimulus insert phone. The calibration is referenced to an IEC-711 ear simulator and comprises the following frequencies: 125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000, 8000 Hz.

Long-term test-retest reliability of category loudness scaling in normal-hearing subjects using pure-tone stimuli.

The present study investigates the test-retest reliability of category loudness scaling with pure tones for each of the scaling categories: 'very soft', 'soft', 'OK', 'loud', 'very loud' and 'too loud' at the audiometric frequencies 0.5, 1 kHz, 2 kHz and 4 kHz. Category loudness scaling at two sessions separated by between 1 and 4 weeks was obtained from 16 normal-hearing subjects who all had normal otoscopy, present acoustic reflexes at audiometric frequencies 0.5-4 kHz and middle ear pressure within +/-50 daPa. Intra-subject between-session reliability was found not to be frequency dependent, and comparison with other studies revealed that reliability is not dependent on the applied stimulus signal. Test-retest reliability varied between the different categories: In the categories 'very soft', 'loud', 'very loud' and 'too loud' the test reliability is in the same range as found for hearing thresholds determination, whereas for the 'soft' and 'OK' categories it is poorer. The greater uncertainty for intermediate levels should be considered when using category loudness scaling, e.g. for calculating hearing aid parameters.

Neonatal hearing screening using otoacoustic emissions elicited by maximum length sequences.

The aim of this study was to provide statistical information on otoacoustic emission (OAE) measurements in new-born infants, which could be useful in the interpretation of results and could add some clinical parameters that might be used for future objective and automatic response evaluations. Transient evoked OAEs (TEOAEs) using maximum length sequences (MLS) were recorded in 129 full-term new-born infants between 48 and 72 hours of age. The use of MLS enabled stimulus rates of up to 2000/sec to be utilized. The analysis comprised five main issues: (1) signal to noise ratio (S/N); (2) derived non-linear responses; (3) a combination of derived non-linear emissions called compound non-linear emissions; (4) time/frequency analysis and (5) signal power analysis. The result of the analysis were as follows. (1) The largest S/N ratio was obtained when more than 60% of the recordings were rejected. (2) The derived non-linear emissions comprised level non-linear and rate non-linear responses, the latter obtained by subtraction of recordings at fixed stimulus level, but at different stimulus rates. The rapid suppression of emission amplitude as the stimulus rate increases is the basis for this non-linear response which also features stimulus cancellation. (3) The compound non-linear emissions, consisting of a sum of derived non-linear responses may prove efficient by proper selection of stimulus parameters, but did not in the present form show the expected improvements of amplitude and stimulus cancellation. (4) The time/frequency analysis, however, based on calculations of the instantaneous frequency as a function time after stimulus onset, showed a consistent tendency of falling frequency versus time after stimulus onset as a basic characteristic feature of OAE. (5) The analysis of the emission power as a function of time after stimulus onset showed that linear recordings in new-born infants are reliable, with stimulus ringing fully decayed 3 ms after the stimulus onset. The findings may prove useful in future attempts to develop screening equipment with automatic response evaluation for new-born infants. From a clinical point of view the MLS technique was successful when measuring emissions in neonates, owing to the fast recording time and the utilization of an efficient noise rejection paradigm.

The influence of spontaneous otoacoustic emissions on the amplitude of transient-evoked emissions.

The aim of the study was to analyse whether the presence of spontaneous otoacoustic emissions (SOAEs) in a group of normal-hearing adults is related to larger transient evoked otoacoustic emission (TEOAE) amplitudes when compared to a similar adult population without SOAEs. Twenty-four normal-hearing subjects participated in the investigation. They were selected to form two groups of 12, one containing only subjects with measurable spontaneous emissions, the other of members who had no measurable spontaneous emissions. Each group comprised 7 males and 5 females. TEOAEs were recorded in both linear and non-linear mode, and equivalent sound pressure levels in different octave frequency bands were calculated. For each frequency band, the comparison of the equivalent sound pressure levels in the two selected groups showed statistically significant differences. Therefore, when normative TEOAE data were collected with the purpose of establishing a normative reference for the assessment of cochlear hearing loss, it is mandatory to search for and, when pertinent, to compensate for the presence of SOAEs.

Clinical significance of probe-tone frequency ratio on distortion product otoacoustic emissions.

Parametric variations in the ratio of primary-tone frequencies were explored to optimize the amplitude of the 2f1-f2 distortion product otoacoustic emission for clinical purposes. Ten ears from 5 normally hearing human subjects were examined with primary tones geometrically centred around the standard audiometric frequencies of 0.5, 1, 1.5, 2, 3, 4, 6 and 8 kHz. The distortion product at the frequency 2f1-f2 (f1 < f2) was measured at six probe tone frequency ratios (f2:f1) varying between 1.15 and 1.40 using equal level primaries of 75 dB SPL. The results showed that a single f2:f1 ratio between 1.20 and 1.25 provides a reasonable value for clinical use in that it optimizes the magnitude of the distortion product at 2f1-f2, provides for sufficient resolution in the test frequency range, and is applicable to the standard clinical test frequencies.

Clinical significance of relative probe-tone levels on distortion product otoacoustic emissions.

The effect of systematic variations in the relative levels (L1, L2) of two primary tones (f1, f2) on the amplitude of the distortion product otoacoustic emission (DPOAE) at 2f1-f2 and f1 < f2 was investigated in 14 ears from 7 normally hearing human subjects. The primary tones (f2:f1 = 1.23) were geometrically centred at the standard clinical frequencies of 0.5, 1, 1.5, 2, 3, 4, 6 and 8 kHz. The quantity L1-L2 was varied systematically from -10 dB through + 10 dB with L2 held constant at 75 dB SPL for negative values, L1 held constant at 75 dB SPL for positive values, and L1 = L2 = 75 dB SPL at 0 dB relative difference. The maximum amplitudes of the distortion products were generated when L1 = L2 at all geometric centre frequencies except 8 kHz. The reduction of the DPOAE with reduction of L1 was linear at a rate that gradually increased as a function of geometric mean frequency. To a lesser extent, the reduction of the DPOAE with reduction of L2 also was linear but at a rate that systematically decreased as a function of geometric mean frequency. The results suggest, that to maximize the level of the distortion product for clinical purposes, the relative levels of the primary tones should be equal to each other, at least when overall stimulus levels are around 75 dB SPL and f2:f1 = 1.23.

Loudness perception is influenced by long-term hearing aid use.

The aim of this study was to explore possible differences in the perception of loudness between long-term hearing aid full-time users and non-users. Categorical loudness scaling using pure-tone stimuli was carried out by hearing-impaired subjects. The mean levels of loudness categories at one frequency (hearing threshold: 50-75 dB HL) in a group of 18 hearing aid users (daily use < or = 15 hours per day) were compared with the corresponding levels found in 18 hearing-impaired non-users with the same distribution of hearing thresholds. The results show that, for hearing losses of 50-75 dB HL, the mean level rated as 'loud' by long-term full-time users of hearing aids is 4.5 dB above the mean level of the corresponding category rated by non-users. This difference is statistically significant (P<0.05). No significant differences were found for the lower categories. Among those subjects who had been wearing hearing aids for at least six months, no significant correlation was found between the levels of the 'loud' category and the length of time that hearing aids had been used.

The relationship between the acoustic reflex threshold and levels of loudness categories in hearing-impaired listeners.

When applied as a tool for hearing aid fitting, categorical loudness scaling (CLS) is time consuming and not feasible in all subjects. It is therefore desirable to use objective measures for accurate prediction of loudness categories among hearing-impaired individuals. The present study aimed at exploring whether loudness perception at the ART is constant with varying hearing threshold. Seventy-five subjects with various degrees of hearing impairment, measurable acoustic reflex and normal middle ear function participated. The HTLs, ARTs and the levels of six loudness categories at frequencies 0.5, 1, 2 and 4 kHz were determined for all subjects. Loudness at the ART was found to be correlated with the amount of hearing loss. On the basis of these results, it is concluded that the ART cannot be used for accurate estimation of loudness in hearing-impaired subjects.

The acoustic reflex threshold: not predictive for loudness perception in normally-hearing listeners.

The working hypothesis of an ongoing study is that the quick and reliable procedure of acoustic reflex threshold (ART) determination in conjunction with measurements of HTL may yield accurate estimates of loudness. The aim of this study was to investigate whether differences in loudness in normally-hearing subjects are reflected in the ARTs and to collect normal material with respect to pure-tone elicited ART and loudness categories. Categorical loudness scaling (CLS) and ART measurements were performed at frequencies of 0.5, 1, 2 and 4 kHz in 60 normally-hearing subjects (HTL<20 dB HL, 26 males, 34 females, aged 21-63 years) with no history or sequelae of middle ear disease. Subjects reporting disturbing tinnitus were excluded. The results show that the ART is not a predictor of individual loudness perception for normally-hearing subjects. Using a numerical scale (HTL=0, 'very soft'=5, 'soft'=15, 'OK'=25, 'loud'=35, 'very loud'=45 and 'too loud'=50) loudness for pure tones grows almost linearly at approximately 0.4 arbitrary loudness units per dB below the 'loud' category. Above the 'loud' category the slope is around 1 unit per dB. The median ART was 85 dB HL at frequencies of 0.5, 1, 2 and 4 kHz. No differences in loudness perception across frequencies were found.