A. Morgon

Evidence of Peripheral Hearing Asymmetry in Humans: Clinical Implications

Auditory system hemispheric asymmetry in language processing is well-established, and there are many indications of lateralization as of the peripheral auditory system i.e., as of the cochlea. The left ear is more susceptible to noise damage; tinnitus is more predominant there, while spontaneous otoacoustic emissions (SOAEs) are more often found in the right ear. The present study addressed the following two questions: Does this right-ear SOAE prevalence exist as early as preterm birth? Is there any functional asymmetry in the medial olivo-cochlear (MOC) efferent system, known to modulate outer hair cell contractions? The study involved 483 preterm neonates (gestional age: 24-37 weeks) and 70 right-handed adults (age: 18-31 years). In each ear, SOAEs and evoked otoacoustic emissions (EOAEs) were recorded and analysed, and, for the adults, functional MOC system assessment was made. Results showed SOAEs and EOAE amplitude to be right-predominant and in adults a right MOC functional predominance. These results indicate peripheral auditory system lateralisation, and an early origin thereof. The MOC system being thought to play a protective role, its physiological lateralisation may be relevant to the left prevalence of tinnitus and of auditory fatigue.

Functional Maturation of Cochlear Active Mechanisms and of the Medial Olivocochlear System in Humans

The aim of this study was to investigate the functional development of cochlear active mechanisms and of the medial efferent olivocochlear system. Otoacoustic emissions (evoked and spontaneous) were recorded in 42 preterm neonates (conceptional age ranging form 33 to 39 weeks) and a control group of 20 young normal-hearing adults. Medial olivocochlear system activity was examined by coupling evoked otoacoustic emission recording to a contralateral stimulation. Otoacoustic emission recordings were carried out using the Otodynamic ILO88 software and hardware. The stimuli were unfiltered clicks and the contralateral stimulation was broad band noise of 50 and 70 dBSPL delivered by an Adam generator. The results revealed the presence of EOAEs and SOAEs from at least 33 weeks in humans, suggesting that the functional maturation of the outer hair cells is nearly complete at that age. The study further revealed that the contralateral stimulation had no effect on evoked otoacoustic emissions in preterm neonates. The lack of activity observed in medial olivocochlear system indicated functional immaturity here, at least before full-term birth.

Differential effects of ear-canal pressure and contralateral acoustic stimulation on evoked otoacoustic emissions in humans

The effect of ear canal pressure variation (ECPV) on click evoked otoacoustic emissions (EOAEs) was compared to the suppressive effect observed with contralateral acoustic stimulation (CAS) in 11 healthy subjects. Both total EOAE amplitude and amplitude of 200 Hz frequency bands (22) were analyzed. Our results revealed that the ECPV as the CAS induced a decrease of the total EOAE amplitude; these two factors showed an additive effect when they are conjoint. The study of the EOAE frequency bands showed that the majority of them decreased under CAS and ECPV; however, a few bands are not affected. Moreover, it appeared that amplitude of the EOAE frequency bands were not modified in a similar way between the two factors: indeed some bands around 4.1 kHz did not decrease either by CAS or ECPV. These results suggest that these applied factors exert different actions on EOAEs. Moreover, the lack of a decrease effect for the same bands, both with CAS and ECPV, may explain the vulnerability of some cochlear locations.

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.

Transiently evoked otoacoustic emission amplitudes change with changes of directed attention

The effect of a visual and of an auditory task on the cochlea was studied by transiently evoked otoacoustic emissions (TEOAEs). TEOAE amplitude decreased during visual and auditory attention for all 13 subjects tested. The decreased TEOAE activity was noted predominantly within the 960 to 1920 Hz range during visual attention and within the 1920-2880 Hz range during auditory attention. Selective attention, by way of the olivocochlear medial efferent system, modifies cochlear micromechanical properties. Visual and auditory attention act on different areas of the cochlear partition.

Influence of aging on active cochlear micromechanical properties and on the medial olivocochlear system in humans

Transiently evoked otoacoustic emissions (TEOAEs) without and with contralateral acoustic stimulation (CAS), as well as distortion product otoacoustic emissions (DPOAEs), were recorded on 60 subjects divided into 2 groups, the first constituted by 38 normally hearing young subjects and the second by 22 subjects aged from 70 to 88 years and with normal hearing levels in view of their age. TEOAEs and DPOAE audiograms were recorded in a third group constituted by 15 subjects with ages ranging from 6 to 57 years and with hearing levels not significantly different from those of the second group for frequencies above 500 Hz. TEOAEs were present in 100% of the subjects in the first group and in 91% in the second group, although with smaller amplitude. The fall in TEOAE amplitude under CAS was smaller in the second group. Concerning DPOAE audiograms, in the first group, a response was found in 71 to 95% of cases for frequencies ranging from 1 to 6.35 kHz, and in 7 to 37% of cases in the second group. Mean amplitudes were significantly lower in the second group for frequencies ranging from 2.83 to 5.04 kHz. group for frequencies ranging from 2.83 to 5.04 kHz. Mean thresholds ranged from 38.55 to 43.57 dB SPL for frequencies ranging from 1 to 6 kHz in the first group and from 50 to 58 dB SPL in the second group. No significant difference was found between the second and third groups concerning the amplitude and spectrum of the TEOAEs and DPOAE audiograms. Thus, age influences TEOAEs, DPOAEs and the effect of CAS on TEOAEs.(ABSTRACT TRUNCATED AT 250 WORDS)

Contralateral Suppression of Evoked Otoacoustic Emissions and Detection of a Multi-Tone Complex in Noise

Although some findings suggest that auditory efferent fibers are involved in perception in noise, their function remains controversial. The contralateral suppression of evoked otoacoustic emissions (EOAEs) has recently provided a means of exploring the medial olivocochlear system (MOCS) in humans. In an experiment based on this paradigm, the present study examined the relationships between variations of both EOAEs and detection-in-noise thresholds, induced in the same subjects by a contralateral 50-dB-SPL broad-band noise masker. EOAEs were recorded in response to a burst of a multitone complex composed of 1, 1.5 and 2-kHz components. The detection thresholds of this 3-component complex were measured at 2 ipsilateral noise levels: 50 and 70 dB SPL. The main finding was a significant correlation between EOAE suppression and threshold variations under contralateral masking. A relationship was also found between the contralateral suppression of EOAEs and threshold variation induced by the increase in ipsilateral noise level. These findings support the notion that the MOCS is involved in the detection of multicomponent stimuli in noise.

Influence of spontaneous otoacoustic emissions (SOAE) on acoustic distortion product input/output functions : does the medial efferent system act differently in the vicinity of an SOAE ?

Otoacoustic emission (OAE) generation mechanisms reside in the active micromechanical properties of the organ of Corti, and especially in the outer hair cells (OHCs). OHCs are strongly innervated by medial efferent olivo-cochlear fibres. Decrease of the intensity of transiently evoked otoacoustic emissions (TOAEs) and modification of spontaneous otoacoustic emissions (SOAEs) during acoustic stimulation of the contralateral ear have already been shown in humans. Similar results were obtained in guinea pigs with a decrease of 2F1-F2 acoustic distortion products (DPOAEs) and a suppression of the effect with sectioning of the floor of the fourth ventricle. The present study sought to investigate the influence of contralateral auditory stimulation on DPOAEs recorded in humans. It shows a decrease in DPOAE intensity for all frequencies, at levels above 45 dB SPL of contralateral broad band noise. This effect was found at levels of contralateral BBN well below the acoustic reflex threshold, and in subjects without acoustic reflex. Moreover, the influence of transcranial transmission could be ruled out since no effect was found when contralateral BBN applied to the altered ear of totally unilaterally deaf patients. Thus, the contralateral acoustic stimulation effect on DPOAEs provides a new means of functional exploration of the medial efferent system in humans. The effect obtained is more ample at low primary frequency levels. Moreover, as DPOAEs are known to be stronger and to show more irregular input/output function patterns in the vicinity of an SOAE, the influence of contralateral auditory stimulation was studied for DPOAEs recorded at 10 Hz, 50 Hz and 150 Hz from an SOAE frequency.

Spontaneous and evoked otoacoustic emissions in pre-term and full-term neonates: is there a clinical application?

In neonates and infants, hearing impairment leads to impaired language and cognitive development. For that reason, early detection of this sensory deficit is of outstanding importance, particularly in pre-term neonates, who constitute a high risk population in regard to very early acquired hearing loss. Evoked (EOAE) and spontaneous otoacoustic emission (SOAE) recording in 93 pre-term and full-term neonates revealed that this technique is potentially useful for auditory screening in neonatology units. EOAEs and SOAEs can be recorded successfully from 30 weeks of conceptional age. SOAEs were found to be prevalent in females and presented higher peak numbers in right than in left ears. Furthermore, SOAE incidence in pre-term and full-term neonates was found to be high in EOAE positive ears, associated with strong and robust EOAEs.

Sleep and active cochlear micromechanical properties in human subjects.

In this paper the effect of sleep on the cochlea is studied by transiently evoked otoacoustic emissions (TEOAEs). Amplitude increases considerably (e.g., 4 dB) and the effect of a contralateral noise on the TEOAEs decreases during the night. These modifications can be related to sleep, although there is no link to electroencephalographic sleep stage. During sleep onset, the effect of contralateral noise disappears: this could correspond to a functional rest of the auditory pathway during that period.