On inner ear function and the origin of oto-acoustic emissions.

The extremely low hearing threshold of the mammalian ear suggests the presence of a special amplifying mechanism, because the stereocilia of the outer hair cells (OHCs) are not likely to be sensitive enough themselves, although their mechanical embedding may provide some amplification. In the past decades, biophysicists have increasingly turned to the chaos theory for explanation. a theory the implications of which are considerable. One of its major tenets, self-organization, is not easily understood at first glance, but is easily reproducible mathematically. With self-organization, the processes involving the OHCs can readily be simulated: Self-organization can help to explain why OHCs vibrate at amplitudes much higher than those of the exciting stimulus. To further our understanding of the process of hearing, vibratory processes, which presumably occur in normal and damaged OHC clusters, are described and compared with a mathematical analysis of data sets obtained from normal subjects using an extremely sensitive microphone.

Some ideas on the origin of otoacoustic emissions.

The presence of a basal activity in the cochlea is postulated. This activity is generated by the OHCs which are made to vibrate at their intrinsic resonant frequency by Brown's motion. In keeping with the chaos theory clusters vibrating in phase at one and the same frequency aggregate in the process. These clusters alternate with one another. Minor pathology enhances OHC synchronization so that the clusters become larger. As a result, more energy is projected back into the outer ear canal. This energy is high enough for being picked up as SOAE by current measuring tools.

On mechanical preprocessing in the cochlea: the three great hearing theories combined.

Since otoacoustic emissions were discovered, the old idea of a second (active) filter in the cochlea has been pursued with considerable dedication. In this contribution a concept explaining the action of the second filter is presented, which is derived from high-frequency resonance: The outer hair cells are excited when the traveling wave breaks rather than when it reaches its peak amplitude. They vibrate at a much higher frequency, i.e. their mechano-electrical resonant frequency. The amplitude of the exciting stimulus is very small, but in keeping with the chaos theory it is amplified by self-organization to a level sufficient for physiologic excitation. This concept helps to explain the hitherto unexplained frequency changes of spontaneous otoacoustic emissions.

Pitch is influenced by differences in gas pressure between the middle ear and the external auditory canal. A tentative explanation based on a new aspect in inner ear theory.

When the pressure in the external auditory canal is changed (as during tympanometry), the pitch rises by about 6 Hz on average (at +/- 400 mm H2O). Apparently, the travelling wave breaks earlier, as impedance increases, with the sound being projected to a site farther basal. At this site a vibration at the local resonant frequency is elicited. In keeping with the chaos theory, its amplitude is amplified by self-organisation. This is a purely mechanical process which does not involve perception in terms of neural stimulation. But through this mechanical pre-processing step the amplitude becomes high enough to be recognised as a signal by the outer hair cells.

Pitch is influenced by spontaneous otoacoustic emissions.

Binaural frequency discrimination was studied in musically trained normal subjects. The frequency chosen was that of a spontaneous oto-acoustic emission (SOAE) in one ear. The other ear was free from SOAEs in the test frequency range. Subjective frequency sensation was found to drift away from the frequency of the peak of the emission: Frequencies lower than that of the emission were perceived as being lower than they actually were; frequencies higher than that of the emission were felt to be higher than they actually were (test range, emission frequency +/- 30 Hz).

A long-term observation of spontaneous oto-acoustic emissions (SOAEs).

In a follow-up study of 22 ears made after a mean interval of 68 months, spontaneous oto-acoustic emissions (SOAEs) showed marked changes. At follow-up visits, fewer SOAEs were recorded than at the first examination. As the set-up employed at follow-ups was much more sensitive, this observation gains even more weight, because--given identical cochlear structures--a greater number of SOAEs ought to have been recorded. In analogy to age-related high-frequency hearing loss, SOAEs recorded at the follow-up examination were found at lower frequencies. This was associated with yet another phenomenon: Precise frequency data (derived from a 4,000 line spectrum) available for 13 SOAEs (7 subjects) from the first examination showed that SOAE frequencies had dropped slightly in all subjects. In 5 of them the frequency drop was significant (p less than 1/1000 in each case).

The time course of spontaneous otoacoustic emissions.

Spontaneous otoacoustic emissions (SOAEs) are amplitude-modulated pure sinus tones. The character of the sinus tone, which is a rare phenomenon in nature, can be proven by resolving the shifts and drifts that more or less always occur. In this report amplitude modulation was studied by digital filtering. The SOAEs were composed of bursts which, in small emissions, appeared only a part of the time. This procedure is not only suited for studying SOAEs but also for demonstrating evoked emissions, even without averaging.

Statistical procedure for the preoperative prediction of the result of cochlear implantation.

Predicting the result of a cochlear implant preoperatively continues to be the best way to prevent an unfavourable outcome. Data from 28 cases (subdivided into two groups by their results) formed the basis for discriminant analysis using different parameters. Allocation to the two groups was found to be satisfactory. Since a jack-knifed design was used, allocation was done on the basis of data material from cases not included in this programme's analysis. Data correlation was described by an equation from which the result can be predicted with adequate accuracy using language competence, duration of deafness on the operated side, speech discrimination and threshold of acoustical sensations on promontory electrostimulation. The equation is applicable to all patients, irrespective of whether their deafness is prelingual or postlingual. Since the patient material was enlarged by 19 additional cases, we were able to confirm that the formula enables a prognosis of sufficient quality.

[Predictability of the result following cochlear implantation]. / Zur Vorhersagbarkeit des Ergebnisses nach Cochlearimplantation.

The results of cochlear implantation of 28 patients were compared with theoretical prognosis based on a mathematical formula, consisting of: speech competence, duration of deafness in the ear to be operated on, speech comprehension for numbers and the threshold for electrical stimulation using a promontorial electrode. Since the evaluation of this formula, 19 further cases have been operated on, confirming the usefulness of this prognostic scheme.

[Oto-acoustic emissions and their significance for inner ear research]. / Oto-akustische Emissionen und ihre Bedeutung für die Innenohrforschung.

In contrast to the formerly held opinion it is now clear that the spontaneous oto-acoustic emissions (OAEs) are pure sinus tones. Frequency shifting (small oscillations and a drifting in one of the two directions) causes broadening of the image in Fourier's transformation, especially when being averaged. These spontaneous OAEs can be observed in ears with minimal disturbance--often not yet detectable by means of pure tone threshold. In such ears the synchronisation of the hair-cells is out of order and this leads to the spontaneous OAEs. With increasing sensitivity of the microphones OAEs can be found in more and more ears. A connection between spontaneous and evoked OAEs can be shown when calculating a FFT from the image of an evoked emission: the spectrum is similar to that of spontaneous OAEs from the same ear. TTS situations in spontaneous OAEs: the reduction of the ability of synchronisation cancels the emission, which will reappear after some time (shifted in frequency). To explain the frequency resolution in the cochlea an active filter has been postulated and the OAEs are claimed to be a reflection of the same; in fact, they are one of the proverbial keyholes through which we might gain a glimpse of what is taking place in the cochlea.

Our present experience on spontaneous cochlear emissions.

Spontaneous otoacoustic emissions (SOAEs) consist of one or several sinusoid-like signals. Small broadening of the spectra of SOAEs, are likely due only to some small oscillations and shifts of their center frequency. Presence of a detectable SOAE seems to be attributable to hearing losses of minor severity associated with abnormal synchronization. A part from some amplitude variations, SOAEs, remain consistently the same over prolonged periods of time and for this reason have been called finger-print-like in the literature. They seem to reflect physiological and exposure-related shifts in tonal projection. SOAEs are one of the proverbial key holes through which we can have a glimpse of what is happening into the cochlea.

Spontaneous otoacoustic emissions--a comparison of the left versus the right ear.

Spontaneous cochlear emissions can vary on a daily basis and are subject to frequency shifts in the range of some thousand parts. A correlation of the shiftings on the right versus those on the left ear is significant in two cases, thus pointing to a common origin.

Binaural beats and frequency-coding.

Binaural beats were studied before and during a situation of temporary threshold shift, and no frequency shift could be found. In contrast, subjective binaural frequency comparison revealed a distinct shift. These findings demonstrate the two known modes of perception.

Frequency composition of spontaneous cochlear emissions.

Spontaneous cochlear emissions consist of one or more sinus tones. Oscillations and frequency shifts clearly widen averaged traces. Multiple peaks, if present, are spaced at intervals of 2-3% of the frequency, and increase with frequency. A frequency jump by the same amount was seen in one case. This appears to represent the distance between hair cells, and suggests that individual hair cells play a leading role in these oscillations.

Temporary threshold shift frequency profiles after low-tone exposure.

Low-tone exposure elicits four distinct dips in hearing thresholds: one octave above the exposure frequency, in the permanent threshold shift (PTS) range, at about 10-11 kHz, and at 14-15 kHz. While dips 1, 2 and 3 decay as a function of the time log, dip 4 has a much longer decay time.

On binaural beats.

Binaural beats have been investigated in normal volunteers using high-stable synthesizers. There are considerable differences between the subjective rhythm heard and the difference of the two frequencies, indicating that this dissimilarity must be caused centrally.

Pre-operative appraisal of cochlear implant results by means of electrical promontory stimulation: preliminary report.

A test procedure based on the patient's understanding of acoustic material is presented. The score obtained in this test showed a highly significant correlation with the postoperative score evaluated by independent examiners 4 weeks after stimulator adjustment.

Registration of spontaneous cochlear emissions by means of Fourier transformation.

Fourier transformation is a suitable procedure for the detection of spontaneous cochlear emissions. In the setup used, the microphone lies unsupported at the entrance to the external auditory canal; after amplification and optional filtering, the signal is processed digitally. The frequency distribution of the emissions is considerably narrow and shows alterations in frequency and intensity. Emissions seem to occur in slightly damaged cochleas.

[A method for prediction of permanent threshold shift (PTS) (author's transl)]. / Eine Methode zur Prognose des chronischen Lärmhörschadens.

A new test for noise resistance was performed in persons at the time they started on a job with a high noise level; three years later, the initial test results were compared with the chronic damage caused to their hearing by the noise to which they had been expose. In other words, the test results were correlated with the PTS which had since been acquired. The significance was found to be 5%, proving that this professional disease can be predicted. The test is based on an effect in TTS (Temporary Threshold Shift) experiments which had been overlooked so far: Exposure to a tone of low frequency caused a second TTS in the PTS frequency range. This "dip" in the audiogram is always at the same position, independent of the exposure. The amplitude of this second "dip" does not correlate with that of the well-known large frequency-dependent first "dip", which had so far been used by many authors for prognosis. The knowledge of this fact cannot have any bearing on public health until existing legislation has been modified. At present, a small percentage only of workers employed in a job involving noise are transferred; after due modification of the law involving a meaningful prognostic examination prior to their being accepted for a job, the extent of PTS in the population will depend on the number of candidates accepted for professions involving noise in relation to the total number of persons required.