Sylviane Chéry-Croze

Psychoacoustic characterization of the tinnitus spectrum: implications for the underlying mechanisms of tinnitus.

In this study, an original psychometric procedure was used in order to characterize in more detail than in previous studies the different perceptual components of tinnitus, i.e. auditory sensations which are perceived in the absence of a corresponding external acoustic stimulus. Ten subjects with chronic tinnitus were asked to rate on a numeric scale the contribution of elementary pitch sensations evoked by isolated frequency components to their overall tinnitus sensation. The resulting ‘internal tinnitus spectra’, which represented the estimated perceptual contribution to the tinnitus sensation as a function of frequency over a large range of frequencies, were found to occupy a wide frequency range corresponding largely to that at which hearing thresholds were abnormally elevated. In most cases, they exhibited a broad peak falling within the hearing loss range. This pattern of result suggests that in subjects with high-frequency hearing loss, tinnitus sensations, when present, resemble those evoked by high-frequency noise bands with, in some cases, a superimposed tonal-like pitch. These results confirm and extend earlier results in the literature and agree with the patients’ reports; their practical implications for the design of future studies on tinnitus and theoretical implications for the understanding of the neurophysiological mechanisms underlying tinnitus are discussed. The results of an additional experiment showed that the internal tinnitus spectrum could be altered by perceptual training in a fine frequency discrimination task with tones in the frequency range of the main peak of the tinnitus spectrum.

Auditory efferents involved in speech-in-noise intelligibility.

FOLLOWING studies proposing that medial olivocochlear efferents might be involved in the processing of complex signals in noise, we tested the involvement of efferent feedback in speech-in-noise intelligibility. Two approaches were used: measures of speech-in-noise intelligibility in vestibular neurotomized patients with cut efferents and comparison with normal hearing subjects; and correlations between effectiveness of olivocochlear feedback, assessed by contralateral suppression of otoacoustic emissions and speech-in-noise intelligibility in normal subjects. Contralateral noise improved speech-in-noise intelligibility in normal ears. This improvement, which was almost absent in de-efferented ears of vestibular neurotomized patients, was correlated with the strength of the olivocochlear feedback. Together, these results suggest that olivocochlear efferents play an antimasking role in speech perception in noisy environments.

Evidence of a medial olivocochlear involvement in contralateral suppression of otoacoustic emissions in humans

Otoacoustic emissions (OAEs) evoked by click stimuli were recorded in both ears of 20 normal human subjects, in the presence and absence of a contralateral masking broad band noise. No difference in the amplitude of OAE suppression was noted between the first tested ear and the second one. In addition, 20 pathological subjects were tested according to the same protocol. Ten of them belonged to a group of patients whose vestibular nerve was sectioned on one side to relieve incapacitating vertigo and thus represented a group in whom olivocochlear efferents were severed. A great reduction of suppression observed in the operated ear suggested that olivocochlear efferent fibers are necessary to obtain a full suppressive effect. Three of the pathological subjects were patients who had undergone a decompression of the facial nerve which necessitated the same surgical approach as vestibular neurotomy, but without any section of vestibular fibers. This surgical control group demonstrated that the surgical act by itself cannot explain the difference observed in the neurotomized group. Finally, seven of the pathological subjects were patients with Bells palsy, which paralyses the facial nerve and abolishes the stapedial reflex. No suppression difference was observed between healthy ears and ears without stapedial reflex. Therefore, it appeared that the stapedial reflex was not involved in the contralateral suppression of EOAEs. However, as the tensor tympani muscle remained functional in these patients, its involvement in the suppressive effect cannot be excluded.

Effect of contralateral sound stimulation on the distortion product 2f1−f2 in humans: Evidence of a frequency specificity

The frequency characteristics of the suppression by contralateral stimulations, of the cubic 2f1-f2 distortion products (DPOAEs), were studied during 63 sessions performed in 39 humans with normal hearing. Each session consisted of exposure to five successive series of randomized contralateral NBN frequencies centered between 0.25 and 8 kHz, while measuring 2f1-f2 set at one of the four studied frequencies, i.e., 1, 2, 3 or 5 kHz. For each value of 2f1-f2, analysis of variance and Students t-test were used in order to indicate and, if necessary, to localize, a possible significant frequency specificity. Results showed that the suppression exerted on 2f1-f2 DPOAEs by contralateral sound stimulation, is frequency specific, at least for middle frequencies of 1 and 2 kHz. The frequency specificity is specially marked when 2f1-f2 = 1 kHz with contralateral NBN central frequencies around 1 kHz. When 2f1-f2 = 2 kHz, central frequencies which induced a significant suppressive effect include frequencies around 2 kHz and also lower frequencies around 1 kHz. These observations support the interpretation that the suppression of the DPOAE is mediated by the medial olivo-cochlear efferent system.

Enriched acoustic environment rescales auditory sensitivity.

Loudness perception may be controlled by a central gain, possibly dependent on the mean level of the acoustic environment. Owing to hearing loss, for instance, a decrease in sensory inputs could increase this central gain and cause an auditory hypersensitivity or hyperacusis. According to this model, individuals with hyperacusis, provided with an enriched acoustic environment specifically designed to compensate for the decrease in sensory inputs, should show an improvement in their hyperacusis. This study showed that such an enriched acoustic environment indeed decreased auditory hypersensitivity: stimuli initially considered as being too loud became comfortable after a few weeks of acoustic stimulation. Therefore, this original approach could provide a solution to the problem of hyperacusis.

Psychopathological Profile of Tinnitus Sufferers: Evidence Concerning the Relationship between Tinnitus Features and Impact on Life

This study involving 281 French tinnitus sufferers sought to investigate possible correlations between psychopathological profile and scores obtained from three tinnitus questionnaires. The patients all completed a French version of the Mini-Mult – a shortened Minnesota Multiphasic Personality Inventory – and French translations of three questionnaires designed especially for the clinical assessment of tinnitus: Tinnitus Reaction Questionnaire (TRQ), Subjective Tinnitus Severity Scale (STSS) and Tinnitus Handicap Questionnaire (THQ). Significant correlations were found (p < 0.0001) between scores on various Mini-Mult scales and total or factor 1 THQ and total TRQ scores. No significant correlation was found between the STSS and any Mini-Mult score.

Medial olivo-cochlear system and tinnitus

A possible role of the efferent system in the mechanisms of tinnitus generation has been put forward by several authors. A simple method for studying the functioning of this system is to compare the amplitudes of otoemissions with and without mild contralateral stimulation. In a recent communication, Veuillet et al. (1991) reported that, on the side of tinnitus, the efferent system of patients suffering from unilateral tinnitus seems to be less efficient than on the other side. The results presented here correspond to those obtained in the very first tinnitus patients submitted to a research protocol exploring systematically the efferent system. When possible, the effectiveness was tested in two ways: globally, using evoked otoacoustic emissions (EOAEs) and, very precisely in the frequency zone of the tinnitus, using cubic distortion product 2f1-f2. Preliminary results obtained in bilateral and unilateral tinnitus sufferers show that a majority of them exhibits, at least in the proximity of the tinnitus, a lack of effectiveness of the efferent system. In some of them, instead of the suppressive effect, an increasing one was even observed.

Is the test of medial efferent system function a relevant investigation in tinnitus

The present results are drawn from preliminary observations made in 28 patients suffering from unilateral, permanent, non-pulsatile tinnitus, who were systematically submitted to an investigation of their otoacoustic emissions in both ears. Spontaneous otoemissions (SOAE) were carefully looked for, before recording of transient evoked otoemissions (TEOAE). The effectiveness of the medial olivo-cochlear efferent system (MOC) was also tested. The comparison between TEOAE input/output curves obtained with and without contralateral stimulation allowed a global assessment of the functioning of MOC. Additional information relating to the MOC functioning at the precise frequency of tinnitus and/or around it, was available from the comparison of analogous curves obtained using distortion product otoacoustic emissions (DPOAE) instead of TEOAEs. The results show that: (1) when the amplitude of otoacoustic emissions differs between the two ears, the tinnitus is generally lateralized on the side where amplitude was the lowest, which seems to be related to a greater hearing loss; (2) no general law concerning tinnitus can be drawn from the global testing of MOC functioning; the only result is that the slope of TEOAE input/output function appears as a deciding factor for efficiency of MOC, the least efficiency almost always occurring on the side where the slope is greater, and (3) even in the case of proved global effectiveness of MOC activation, there is, at least in the vicinity and in at least one ear, a part of the cochlea where its functioning is altered. A better comprehension of the mechanisms underlying this symptom will probably come from exhaustive exploration of individual cases rather than a rougher investigation of large samples in which different aetiologies are likely to be mixed.

Inflammation of the colonic wall induced by formalin as a model of acute visceral pain.

Acute inflammation of the sigmoid wall was induced by perendoscopic injection of formalin (50 microliters, 5%) under brief anesthesia in rats. The procedure was followed by behavioral patterns that significantly differed from those in animals injected with isotonic saline instead of formalin. Analysis of the formalin-induced behaviors allowed for the calculation of a pain score that evolved in a biphasic manner along the 3 h of test. The score was dose-dependently reduced by morphine (0.5-4 mg/kg), and the analgesic effect of the largest morphine dose was abolished by naloxone (2.4 mg/kg). These results suggest that formalin into the sigmoid colon is a new model of visceral pain, presumably through direct irritation at injection site and/or localized acute inflammation of the intestinal wall.

Contralateral suppression of transiently evoked otoacoustic emissions and tinnitus

The present paper reports individual data obtained in three different patients who consulted for unilateral tinnitus in the Department of Otolaryngology. After pure tone and high-frequency audiometry, Audioscan audiometry was recorded, and tinnitus measurement which comprised a determination of pitch and loudness matches. Spontaneous, transient evoked and 2f1-f2 distortion product otoacoustic emissions at 65 dB SPL stimulus intensity were then determined. The functioning of the medial olivocochlear system (MOC) was also tested from a comparison between OAE input/output curves obtained in the presence and absence of 30 dB SL contralateral stimulation by a broadband noise: MOC global effectiveness was assessed through transient evoked emissions while those concerning distortion product emissions allowed a precise testing at the tinnitus frequency itself. The examples here displayed illustrate the diversity of results that can be found in such investigations, thereby preventing a general law to be established from the global testing of MOC functioning. On the contrary, the local testing at the precise frequency of tinnitus revealed the existence of an alteration of MOC functioning in at least one ear as shown by either a weak, null or inverse effect of contralateral stimulation. A better understanding of the sensorineural forms of tinnitus should come from the extensive investigation of MOC efficiency along the basilar membrane when routine clinical investigations lead one to suspect an involvement of this system, due to discordant damaging between inner and outer hair cells. Such studies would allow one to test the validity of hypotheses which invoke MOC-related mechanisms as an essential link for the emergence and persistence of tinnitus.