[Application of new speech intelligibility tests in silence for the assessment of acquired hearing losses]. / Der Einsatz neuer Testverfahren zur Erfassung des Sprachverstehens in Ruhe bei der Begutachtung erworbener Schwerhörigkeiten.

BACKGROUND: Previous studies comparing the Freiburg speech tests (FST) currently used in Germany to assess the severity of hearing loss with two modern speech intelligibility tests [the Göttingen sentence test (GöSa) and the monosyllabic rhyme test devised by von Wallenberg and Kollmeier (WaKo)] have indicated that a replacement of the old procedure would be possible. The current study investigates the consequences of the modern test procedures for the estimation of reduction in earning capacity resulting from hearing loss, and considers the optimal presentation levels for the WaKo test. METHODS: The FST, GöSa, and WaKo speech intelligibility tests were performed on 29 volunteers with a hearing impairment. All tests were conducted in silence. The presentation levels for the Freiburg monosyllabic test were 60, 80, and for some participants also 100 dB SPL. The WaKo test was carried out once at 45, 65, and 85 dB SPL (a reduction of 15 dB relative to the FST) and again at 40, 60 and 80 dB SPL (reduction of 20 dB relative to the FST). RESULTS: A consideration across all presentation levels revealed that the best correlation match between the two monosyllabic tests was achieved at a 20-dB reduction in presentation level for the WaKo test relative to the FST. On average, the application of modern procedures and the different options for level reduction had only a minor effect on the quantitative assessment of reduction in earning capacity.

Separating pitch chroma and pitch height in the human brain.

Musicians recognize pitch as having two dimensions. On the keyboard, these are illustrated by the octave and the cycle of notes within the octave. In perception, these dimensions are referred to as pitch height and pitch chroma, respectively. Pitch chroma provides a basis for presenting acoustic patterns (melodies) that do not depend on the particular sound source. In contrast, pitch height provides a basis for segregation of notes into streams to separate sound sources. This paper reports a functional magnetic resonance experiment designed to search for distinct mappings of these two types of pitch change in the human brain. The results show that chroma change is specifically represented anterior to primary auditory cortex, whereas height change is specifically represented posterior to primary auditory cortex. We propose that tracking of acoustic information streams occurs in anterior auditory areas, whereas the segregation of sound objects (a crucial aspect of auditory scene analysis) depends on posterior areas.

The effects of temporal asymmetry on the detection and perception of short chirps.

There is an intriguing contrast between the physiological response to short frequency sweeps in the brainstem and the perception produced by these sounds. Dau et al. (2000) demonstrated that optimised chirps with increasing instantaneous frequency (up-chirps), designed to compensate for spatial dispersion along the cochlea, enhance wave V of the auditory brainstem response (ABR), by synchronising excitation of all frequency channels across the basilar membrane. Down-chirps, that is up-chirps reversed in time, increase cochlear phase delays and therefore result in a poor ABR wave V. In this study, a set of psychoacoustical experiments with up-chirps and down-chirps has been performed to investigate how these phase changes affect what we hear. The perceptual contrast is different from what was reported at the brainstem level. It is the down-chirp that sounds more compact, despite the poor synchronisation across channels and phase delays up to 20 ms. The perceived 'compactness' of a sound is apparently more determined by the fine structure of excitation within each peripheral channel than by between-channel phase differences. This suggests an additional temporal integration mechanism at a higher stage of auditory processing, which effectively removes phase differences between channels.

Encoding of the temporal regularity of sound in the human brainstem.

We measured the neural activity associated with the temporal structure of sound in the human auditory pathway from cochlear nucleus to cortex. The temporal structure includes regularities at the millisecond level and pitch sequences at the hundreds-of-milliseconds level. Functional magnetic resonance imaging (fMRI) of the whole brain with cardiac triggering allowed simultaneous observation of activity in the brainstem, thalamus and cerebrum. This work shows that the process of recoding temporal patterns into a more stable form begins as early as the cochlear nucleus and continues up to auditory cortex.

Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. I. Fine structure and higher-order DPOAE as a function of the frequency ratio f2/f1.

Critical experiments were performed in order to validate the two-source hypothesis of distortion product otoacoustic emissions (DPOAE) generation. Measurements of the spectral fine structure of DPOAE in response to stimulation with two sinusoids have been performed with normal-hearing subjects. The dependence of fine-structure patterns on the frequency ratio f2/f1 was investigated by changing f1 or f2 only (fixed f2 or fixed f1 paradigm, respectively), and by changing both primaries at a fixed ratio and looking at different order DPOAE. When f2/f1 is varied in the fixed ratio paradigm, the patterns of 2 f1-f2 fine structure vary considerably more if plotted as a function of f2 than as a function of fDP. Different order distortion products located at the same characteristic place on the basilar membrane (BM) show similar patterns for both, the fixed-f2 and fDP paradigms. Fluctuations in DPOAE level up to 20 dB can be observed. In contrast, the results from a fixed-fDP paradigm do not show any fine structure but only an overall dependence of DP level on the frequency ratio, with a maximum for 2f1-f2 at f2/f1 close to 1.2. Similar stimulus configurations used in the experiments have also been used for computer simulations of DPOAE in a nonlinear and active model of the cochlea. Experimental results and model simulations give strong evidence for a two-source model of DPOAE generation: The first source is the initial nonlinear interaction of the primaries close to the f2 place. The second source is caused by coherent reflection from a re-emission site at the characteristic place of the distortion product frequency. The spectral fine structure of DPOAE observed in the ear canal reflects the interaction of both these sources.

Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. II. Fine structure for different shapes of cochlear hearing loss.

Distortion product otoacoustic emissions (DPOAE) were recorded from eight human subjects with mild to moderate cochlear hearing loss, using a frequency spacing of 48 primary pairs per octave and at a level L1 = L2 = 60 dBSPL and with a fixed ratio f2/f1. Subjects with different shapes of hearing thresholds were selected. They included subjects with near-normal hearing within only a limited frequency range, subjects with a notch in the audiogram, and subjects with a mild to moderate high-frequency loss. If the primaries were located in a region of normal or near-normal hearing, but DP frequencies were located in a region of raised thresholds, the distortion product 2 f1-f2 was still observable, but the DP fine structure disappeared. If the DP frequencies fell into a region of normal thresholds, fine structure was preserved as long as DPOAE were generated, even in cases of mild hearing loss in the region of the primaries. These experimental results give further strong evidence that, in addition to the initial source in the primary region, there is a second source at the characteristic place of fDP. Simulations in a nonlinear and active computer model for DPOAE generation indicate different generation mechanisms for the two components. The disappearance of DPOAE fine structure might serve as a more sensitive indicator of hearing impairment than the consideration of DP level alone.

Relations between notched-noise suppressed TEOAE and the psychoacoustical critical bandwidth.

Narrow-band transitory evoked otoacoustic emissions (TEOAE) were recorded for nine normal hearing subjects in the presence of a broadband tone complex suppressor. Introducing a spectral notch at the frequency of the narrow-band stimulus causes the suppression effect to decrease, the more so the wider the notch. This decrease in suppression permits an estimate of the size of one critical band. One advantage of this approach is that no active participation of the subjects is required. The estimated critical bandwidth is then compared with independent estimates based on a simultaneous masking experiment, using the same stimuli. The two measures of the critical bandwidth coincide well for those six subjects with spontaneous otoacoustic emissions. However, the bandwidth estimate based on the OAE measurements is too large for the other three subjects without spontaneous emissions. Simulations of the suppression effect with a driven van der Pol oscillator with moderate undamping produce critical bandwidth estimates consistent with those observed in the psychoacoustical experiments. This allows an estimate of the "effective" amount of undamping on the basilar membrane that is required to produce the critical bandwidth observable in psychoacoustic experiments.

Interaction of otoacoustic emissions with additional tones: suppression or synchronization?

The influence of an external tone on transitory evoked otoacoustic emissions (TEOAE) is investigated. Three different averaging techniques were used with the same acoustic stimulus paradigm. These techniques permitted the separation of those parts of the otoacoustic emission (OAE) that contribute to the transitory evoked otoacoustic emission and those parts of the OAE that are synchronized to the continuous tone. The experiments show that the total energy of the OAE is not reduced in the presence of an additional tone. The 'suppression' of TEOAEs is an effect of synchronization and the subsequent elimination of the 'suppressed' emission in the averaging procedure.

Detection of the acoustic reflex below 80 dB HL.

A new method for detecting the acoustic reflex that utilizes standard otoacoustic emissions recording techniques is introduced and discussed. Two successive identical tone bursts of 100 ms duration and 10 ms interstimulus interval are presented in the occluded ear canal at a repetition rate of one per second. If the acoustic reflex is elicited, the contraction of the stapedius muscle is delayed with respect to the onset of the first stimulus. Hence, the acoustic compliance in the ear canal decreases primarily during the second stimulus. The difference of the microphone signals produced by the two stimuli is computed and averaged across a certain number of repetitions of the sequence. The presentation level is increased until this difference is larger than -40 dB (with respect to the stimulus level) and if its signal-to-noise ratio exceeds 20 dB. For normal-hearing subjects, the acoustic reflex threshold measured with this method is on average 8 dB lower than in a standard clinical setup. In 5 out of the 10 tested hearing-impaired subjects, the new method could detect an acoustic reflex at one or more frequencies where no reflex was detected in the clinical setup.

Chirp evoked otoacoustic emissions.

The principles of short frequency sweeps (chirps) and their application to evoke transiently evoked otoacoustic emissions (TEOAE) are developed in comparison to using standard click stimuli. In contrast to click stimuli, chirp signals have the advantage of stimulating a freely selectable frequency range. In addition, chirp signals contain more energy than a click stimulus with the same maximum amplitude. The effects of different stimuli on TEOAE were investigated in normal hearing and hearing-impaired subjects. Using wide-band chirp signals yields a better signal-to-noise ratio compared to click stimulation. In addition, the stimulation of selected regions of the basilar membrane with frequency-limited chirps evokes TEOAE with frequency components that lie within the stimulated frequency range. The characteristic fine structure of this spectrum was found to be independent of the stimulus applied. The utilization of chirp stimuli appears to be useful for evoking TEOAE in, e.g., clinical applications.

Narrowband stimulation and synchronization of otoacoustic emissions.

Tone-burst-evoked otoacoustic emissions were obtained for several normal listeners with and without spontaneous otoacoustic emission using an optimized tone burst. The dependence of the response amplitude on stimulation level shows a linear increase below a certain value close to the threshold in quiet and levels off at higher levels exhibiting only small differences between non-linear and linear averaging modus. In addition, the latency of the response tends to decrease and the sharpness of the resonance decreases with increasing stimulation level. Synchronization tuning-curves were obtained using a fixed tone burst to elicit an evoked otoacoustic emission and a probe tone at different frequencies for synchronizing the SOAE. These synchronization tuning curves exhibit relatively sharp resonance characteristics (Q3 varying between 3 and 8) for subjects with spontaneous otoacoustic emissions and less sharp tuning (Q3 varying between 1 and 3) for subjects without spontaneous otoacoustic emissions. These experimental results could be reproduced very well with a model of a single non-linear Van-der-Pol-oscillator with the appropriate parameters. The results indicate that spontaneous otoacoustic emissions and narrow-band-evoked otoacoustic emissions are generated by the same mechanism which can be modelled as a self-sustained oscillator.

[Evoked otoacoustic emissions in adults. Criteria for evaluation in clinical use]. / Evozierte otoakustische Emissionen bei Erwachsenen. Auswertekriterien für den klinischen Einsatz.

The work here presents the first part of a prospective study regarding the clinical use of evoked otoacoustic emissions (EOAE) in adults. Sixty subjects with normal hearing and 160 patients suffering from cochlear hearing loss were tested. The results were used to develop and optimize analysis criteria for the emissions, based on their physical properties. A short-time Fourier analysis was performed so that the EOAE intensity in time and frequency domains could be observed simultaneously. A comparison of this data with the individual thresholds of hearing showed the importance of the EOAE level. However, the bandwidth of the EOAE demonstrated an even steeper transition between normal and hearing-impaired subjects. These findings suggest that this bandwidth is a better criterion for the detection of an EOAE. On the other hand, both parameters correlated weakly with the hearing threshold and the differences between subjects were very large. A prediction of hearing loss based on EOAE results is impossible with the wideband click stimulation used here.

[Evoked otoacoustic emissions as a screening test for hearing evaluation in newborn and premature infants?]. / Evozierte otoakustische Emissionen als Screeningtest für die Hörprüfung bei Neu- und Frühgeborenen?

The purpose of this study was the evaluation of evoked otoacoustic emissions (EOAE) for the screening of infant hearing. EOAE and brainstem-evoked response audiometry (BERA) were performed in 40 infants at a special care unit at the Universitäts-Klinik, Göttingen. Both examinations were performed under non-optimal conditions, reflecting a realistic screening situation. The purpose was to determine robust analysis criteria and possible error sources. A comparison of both tests shows that sensitivity and specificity are insufficient for reliable screening. The EOAE test does not yield the threshold of hearing. There is no sharp boundary with the BERA threshold for the detectability of an EOAE. Sensitivity and specificity have not been accurately determined so far due to the relatively small number of infants with impaired hearing tested. Nevertheless, the results show that EOAE should be applied as a supplement to conventional audiometry for infants. It is capable of excluding conductive and severe cochlear hearing losses and is by itself not a reliable screening test.