Acoustic correlates of tonal tinnitus

Whistling While it Works: Spontaneous Otoacoustic Emissions and the Cochlear Amplifier

Perhaps the most striking evidence for active processes operating within the inner ears of mammals and non-mammals alike is their ability to spontaneously produce sound. Predicted by Thomas Gold in 1948, some 30 years prior to their discovery, the narrow-band sounds now known as spontaneous otoacoustic emissions (SOAEs) remain incompletely understood, their origins controversial. Without a single equation in the main text, we review the essential concepts underlying the "local-" and "global-oscillator" frameworks for understanding SOAE generation. Comparing their key assumptions and predictions, we relate the two frameworks to unresolved questions about the biophysical mechanisms of cochlear amplification.

Distortion Product Emissions in Humans

Multifrequency and multicomponent evaluations of aural acoustic immittance, including tympanometry and acoustic reflex testing, were performed on 44 normal ears to examine the influence of middle ear functioning on the generation and detection of distortion product otoacoustic emissions (DPEs). In the same ears, the prevalence and parametric features of spontaneous and stimulus frequency emissions were also assessed so that their relationship to the detection “thresholds” and amplitudes of corresponding DPEs could be determined. The general outcome was that none of the examined features of acoustic immittance provided an explanation for the discrete, low-amplitude DPE regions observed in about one third of normal ears. Moreover, the presence of typical spontaneous and stimulus frequency emissions in these same “irregular” ears indicated that emission generation and reverse cochlear transmission were also operating normally within these regions of reduced DPEs. Consequently, other, as yet undetermined influences appear to contribute to the DPE variability noted in some ears. Finally, the simultaneous presence of stimulus frequency emissions, but not spontaneous emissions, appeared to reduce the detection “thresholds” and increase the amplitudes of low-frequency DPEs.

Effects of contralateral acoustic stimulation on spontaneous otoacoustic emissions and hearing threshold fine structure

Medial olivocochlear (MOC) influence on cochlear mechanics can be noninvasively, albeit indirectly, explored via the effects of contralateral acoustic stimulation (CAS) on otoacoustic emissions. CAS-mediated effects are particularly pronounced for spontaneous otoacoustic emissions (SOAEs), which are typically reduced in amplitude and shifted upward in frequency by CAS. We investigated whether similar frequency shifts and magnitude reductions were observed behaviorally in the fine structure of pure-tone hearing thresholds, a phenomenon thought to share a common underlying mechanism with SOAEs. In normal-hearing listeners, fine-resolution thresholds were obtained over a narrow frequency range centered on the frequency of an SOAE, both in the absence and presence of 60-dB SPL broadband CAS. While CAS shifted threshold fine structure patterns and SOAEs upward in frequency by a comparable amount, little reduction in the presence or depth of fine structure was observed at frequencies near those of SOAEs. In fact, CAS typically improved thresholds, particularly at threshold minima, and increased fine structure depth when reductions in the amplitude of the associated SOAE were less than 10 dB. Additional measurements made at frequencies distant from SOAEs, or near SOAEs that were more dramatically reduced in amplitude by the CAS, revealed that CAS tended to elevate thresholds and reduce threshold fine structure depth. The results suggest that threshold fine structure is sensitive to MOC-mediated changes in cochlear gain, but that SOAEs complicate the interpretation of threshold measurements at nearby frequencies, perhaps due to masking or other interference effects. Both threshold fine structure and SOAEs may be significant sources of intersubject and intrasubject variability in psychoacoustic investigations of MOC function.

Are human spontaneous otoacoustic emissions generated by a chain of coupled nonlinear oscillators?

Spontaneous otoacoustic emissions (SOAEs) are generated by self-sustained cochlear oscillators. Properties of a computational model for a linear array of active oscillators with nearest neighbor coupling are investigated. The model can produce many experimentally well-established properties of SOAEs.

Classification of tinnitus

Classification in general serves a number of purposes. In relation to tinnitus there are particular problems because the cause in only rarely known and the mechanism is not known at all. This means that firm classifications tend to be misleading and serve little purpose other than to delude the doctor and the patient. The aim in this paper therefore is to examine the various ways in which information (whether scientific or clinical) about tinnitus can be organized so as to help diagnosis, management or research towards recognition of the fundamental abnormalities. Attempts at classification based on the site of the lesion and on speculation about its cause are described. Classification is analysed according to (1) patients' own reports, (2) doctors' clinical observations, (3) various techniques of measurement, (4) associated phenomena and (5) results of treatment. From these various aspects a pattern emerges which, though incomplete, at least exposes rather than obscures the gaps in our knowledge.

Pathophysiology of tinnitus

Guided by findings from neural imaging and population responses in humans, where tinnitus is well characterized, several morphological and physiological substrates of tinnitus in animal studies are reviewed. These include changes in ion channels, receptor systems, single unit firing rate, and population responses. Most findings in humans can be interpreted as resulting from increased neural synchrony.

Spontaneous basilar membrane oscillation and otoacoustic emission at 15 kHz in a guinea pig

A spontaneous otoacoustic emission (SOAE) measured in the ear canal of a guinea pig was found to have a counterpart in spontaneous mechanical vibration of the basilar membrane (BM). A spontaneous 15-kHz BM velocity signal was measured from the 18-kHz tonotopic location and had a level close to that evoked by a 14-kHz, 15-dB SPL tone given to the ear. Lower-frequency pure-tone acoustic excitation was found to reduce the spontaneous BM oscillation (SBMO) while higher-frequency sound could entrain the SBMO. Octave-band noise centered near the emission frequency showed an increased narrow-band response in that frequency range. Applied pulses of current enhanced or suppressed the oscillation, depending on polarity of the current. The compound action potential (CAP) audiogram demonstrated a frequency-specific loss at 8 and 12 kHz in this animal. We conclude that a relatively high-frequency spontaneous oscillation of 15 kHz originated near the 15-kHz tonotopic place and appeared at the measured BM location as a mechanical oscillation. The oscillation gave rise to a SOAE in the ear canal. Electric current can modulate level and frequency of the otoacoustic emission in a pattern similar to that for the observed mechanical oscillation of the BM.

A behavioral paradigm to judge acute sodium salicylate-induced sound experience in rats: a new approach for an animal model on tinnitus

Behavioral conditioning studies on rats have been proven to be a valid animal model for the evaluation of acute and chronic phantom auditory experience (tinnitus). We developed an animal model for short-term, acute induced phantom auditory sensations in rats. Rats were trained in a conditioning chamber to actively access a liquid feeder whenever a constant white noise sound was present. During silence, no reward was given. Fulfilling the demands of animal protection laws for maximal avoidance of pain and fear, punitive paradigms were maximally reduced. After 15-17 learning sessions, all animals performed more accesses to the reward feeder during periods of sound than during periods of silence. Tinnitus was induced by the administration of sodium salicylate (350 mg/kg body weight) given 3 h before testing. The feeder access activity of a rat treated with salicylate was significantly increased during periods of silence, indicating a phantom auditory experience. The presumptive auditory experience was estimated to be comparable to a white noise sound of about 30 dB SPL rms. The activity increase was less pronounced for lower doses of sodium salicylate (150 mg/kg body weight) and was not found in animals trained on a dark-light paradigm, as expected. As the learning sessions of the operant conditioning were performed without pharmacological treatment, unintentional drug effects, for example, on learning and motivation of a rat were minimized in this behavioral paradigm. Furthermore, the behavioral changes reported here were shown to be a specific drug effect evoking a phantom auditory experience of a rat and cannot be explained by unspecific drug effects on motor activity, motivation, learning or hearing loss. The conditional paradigm is discussed in the context of its potential as a model for testing drugs that may have a therapeutic value in tinnitus research.

Mechanisms of tinnitus

The generation of tinnitus is a topic of much scientific enquiry. This chapter reviews possible mechanisms of tinnitus, whilst noting that the heterogeneity observed within the human population with distressing tinnitus means that there may be many different mechanisms by which tinnitus can occur. Indeed, multiple mechanisms may be at work within one individual. The role of the cochlea in tinnitus is considered, and in particular the concept of discordant damage between inner and outer hair cells is described. Biochemical models of tinnitus pertaining to the cochlea and the central auditory pathway are considered. Potential mechanisms for tinnitus within the auditory brain are reviewed, including important work on synchronised spontaneous activity in the cochlear nerve. Whilst the number of possible mechanisms of tinnitus within the auditory system is considerable, the identification of the physiological substrates underlying tinnitus is a crucial element in the design of novel and effective therapies.

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.

Temperature dependency of the frequency and level of a spontaneous otoacoustic emission during fever

Using a method of heterodyne mixing with display of beats on a chart recorder, measurements were made of the frequency and level of a spontaneous otoacoustic emission in a human subject on 17 occasions during a period of 13 consecutive days. Temperature measurement was obtained with a thermometer measuring infra-red radiation from the ear-drum, indicative of body core temperature. In the latter half of this period there was variation of body set point temperature due to fever, and it was found that there was clear evidence of inverse relationship between temperature and both the frequency and level of the emission. The occurrence of fever was due to simple urinary tract infection and was considered to be without ototoxic implication; there was no associated worsening of pure tone threshold nor change in measurements of middle ear function.

Ipsilateral suppression effects on transient evoked otoacoustic emission

The spectral properties of click- and tone-evoked otoacoustic emission (OAE) under ipsilateral simultaneous tonal masking conditions as well as the changes of click-evoked OAE under ipsilateral forward masking by clicks were studied in normal-hearing subjects. It was found that (i) transiently evoked OAE (TEOAE) spectra consist of a number of peaks individual to the subject and distributed along a sufficiently wide cochlear region; (ii) each spectral peak is excited only when the stimulus energy was concentrated within the frequency range covering the frequency of the particular peak; (iii) different TEOAE spectral peaks can be masked independently under simultaneous tonal masking conditions; (iv) the TEOAE tuning curve shape is more closely related to the spectrum of TEOAE but not to that of the stimulus; (v) forward masking of TEOAE generation is most pronounced during the first few milliseconds after masker click onset and become significantly smaller with the longer latency. It is suggested that: (i) TEOAE is generated by a number of local generators individually distributed along the cochlear partition within a sufficiently wide region and characterized by different power; (ii) possible efferent effects in the ipsilateral TEOAE suppression are negligible as compared to the TEOAE reduction of exclusively cochlear origin.

Influence of general anesthesia on transiently evoked otoacoustic emissions in humans

The influence of general anesthesia (GA) on transiently evoked otoacoustic emissions (TEOAEs) was studied in 19 normally hearing women undergoing surgery. Emissions were measured on the day before the operation, after premedication but before the beginning of the operation, and during and after the operation. There were no significant differences in TEOAE amplitude or in reproducibility between results obtained the day before the operation and after premedication. Ten patients received nitrous oxide (N2O) during GA (N2O group), and 9 patients did not (non-N2O group). The amplitude of TEOAEs was reduced during GA in 9 of 10 patients in the N2O group and in 7 of 9 patients in the non-N2O group. However, the average decrease of amplitude after the first 10 minutes was greater in the N2O group (4 +/- 3.4 dB) than in the non-N2O group (0.18 +/- 1.4 dB). The corresponding mean reproducibility of the response decreased in 9 of 10 patients of the N2O group (29% +/- 24%) and was nearly unchanged in the non-N2O group (2.3% +/- 7.2%). The time course of the amplitude reduction was similar in both groups. The smallest amplitudes were reached on an average by 19.3 +/- 11.4 minutes in the N2O group and by 17 +/- 13.6 minutes in the non-N2O group. Preoperative and postoperative TEOAEs were comparable in level and reproducibility. Differential frequency effects imply a middle ear effect for the greater reduction of TEOAE amplitudes in the N2O group due to gas diffusion into the middle ear.(ABSTRACT TRUNCATED AT 250 WORDS)

Study of the so-called cochlear mechanical tinnitus

Stimulated oto-acoustic emission (OAE) was investigated in 225 ears with normal hearing and c5 dip hearing loss. Continual OAE persisting longer than 6 ms against the sound stimulation of relatively low intensities was frequently found in ears affected by tinnitus but without hearing loss and in the tinnitus ears with c5 dip hearing loss. The results suggest that there is an abnormality of cochlear micromechanics in these ears which might be related to the 'cochlear mechanical tinnitus' described by Kemp & Wilson in 1981.