Medial Olivocochlear Efferent System Research Articles

Psychophysical correlates of contralateral efferent suppression. I. The role of the medial olivocochlear system in "central masking" in nonhuman primates.

An extensive physiological literature, including experimental and clinical studies in humans, demonstrates that activation of the medial olivocochlear (MOC) efferent system, by either contralateral sound or electrical stimulation, can produce significant alterations in cochlear function and suggests a role for the MOC system in influencing the auditory behavior of binaural hearing. The present data are from psychophysical studies in nonhuman primates which seek to determine if the noted physiological changes in response to contralateral acoustic stimulation have a perceptual counterpart. Four juvenile Japanese macaques were trained to respond to the presence of 1-s sinusoids, presented to the test ear, in an operant reinforcement paradigm. Thresholds were compared for frequencies ranging from 1.0 to 4.0 kHz in quiet, with thresholds measured when continuous, two octave-band noise, centered on the test tone frequency, was presented in the contralateral ear. Contralateral noise was presented at levels of 10-60 dB above detection threshold for the test-tone frequency. While some variability was evident across subjects, both in the frequency distribution and magnitude (as a function of contralateral noise level), all subjects exhibited an increase, or suppression of thresholds in the presence of contralateral noise. On average, thresholds increased systematically with contralateral noise level, to a peak of 7 dB. In one subject, the threshold increase seen with contralateral noise was significantly reduced when the MOC was surgically sectioned on the floor of the IVth ventricle. The characteristics of the measured shifts in behavioral thresholds, in the presence of contralateral noise reported here, are qualitatively and quantitatively similar to both efferent physiological suppression effects and psychophysical central masking threshold shifts which have been reported previously. These data suggest that at least some aspects of "central masking" are efferent-mediated peripheral processes, and that the term "central masking" may be incorrect.

Role of the efferent medial olivocochlear system in contralateral masking and binaural interactions: an electrophysiological study in guinea pigs.

Contralateral broadband noise (BBN) elevates ipsilateral auditory thresholds (central masking) and reduces the amplitude of ipsilateral brainstem auditory evoked potentials (BAEPs). Binaural interactions are complex psychophysical phenomena, but binaural interaction components are easily extracted from BAEPs to monaural versus binaural click stimulation. However, contralateral, or binaural, acoustical stimulation is known to activate simultaneously the crossed and uncrossed medial olivocochlear (MOC) efferent systems and decrease activity in both cochleas. Particularly, contralateral BBN stimulation suppresses in part ipsilateral peripheral activity. What is the role of such contralaterally induced peripheral suppression in the overall changes in central BAEPs observed during contralateral masking or binaural stimulation? Compound action potentials (CAPs) of the auditory nerve and BAEPs were recorded simultaneously in awake guinea pigs from electrodes chronically implanted on the round window of the cochlea and the surface of the brain. Peripheral and central measures of contralateral masking and binaural interactions were obtained from responses to monaural or binaural clicks, with or without contralateral BBN, recorded before, during, and after the reversible blockade of the MOC function following a single intramuscular injection of gentamicin. Contralateral BBN effectively reduced the amplitudes of CAP and of all BAEP peaks. CAP to ipsilateral click did not, however, change significantly from monaural to binaural click stimulation; still, normal binaural interaction components developed in the BAEPs. When the medial efferent function was blocked by gentamicin, the normal contralateral BBN suppression of CAP and of the earliest BAEP peak was lost; however, the later BAEP peaks were suppressed by contralateral BBN as before gentamicin, and the central binaural interaction components were unchanged. In these experimental conditions, the MOC efferent system seems to play little role in centrally recorded contralateral masking and binaural interactions.

Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs.

The medial olivocochlear (MOC) efferent system, which innervates cochlear outer hair cells, suppresses cochlear responses. MOC-mediated suppression includes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs suggest that both slow and fast effects on cochlear responses require an initial acetylcholine activation of alpha-9 nicotinic receptors on outer hair cells and that slow effects require additional intracellular events downstream from those mediating fast effects. Gentamicin, an aminoglycoside antibiotic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without changing slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by either intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in unanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from effects of the lateral OC system.

Time course of the medial olivocochlear efferent effect on otoacoustic emissions in humans.

The time course of the medial olivocochlear efferent system has been studied in humans, using the suppressive effect of a contralateral broad-band noise (CBBN) on 2f1-f2 distortion product otoacoustic emissions (DPOAE) amplitude. DPOAE were recorded with F2 at 1.5 kHz, with a temporal resolution of 2.6 s, in the presence and absence of a 40 dB SL continuous CBBN, whose duration ranged between 30 s and 20 min. The CBBN suppressive effect on DPOAE amplitude was greatest from the first 2.6 s, and was sustained for 20 min. At the CBBN offset, when the CBBN duration was > or = 2 min, DPOAE amplitude continued to increase for > 1 min, showing an efferent effect outlasting CBBN stimulation.

Tonic efferent-induced cochlear damping in roosting and echolocating mustached bats

The activity of the medial olivocochlear (MOC) efferent system in mustached bats, Pteronotus p. parnellii, was studied by monitoring changes in the mechanical properties of the cochlea. The changing properties were expressed by the decay time (DT) of cochlear microphonic potentials produced by transient-induced ringing ( Henson et al., 1995). Tape-recorded roost noise (biosonar and communication sounds) produced sudden, marked decreases in DT when presented to the contralateral ear of animals adapted to the quiet. When the animals were first removed from their roosts the DT was relatively short (1.2–1.5 ms) but this gradually lengthened by about 0.5–1.0 ms as they rested in a quiet chamber. The time required to reach a stable, quiet-adapted state after noise exposure varied with SPL and exposure time; in many experiments recovery was in the range of 90–120 min. When quiet-adapted bats were isolated and allowed to fly and echolocate for 20 min, the DTs measured within a few minutes after the end of the flight were also short and only slowly returned to longer preflight values. The administration of a single dose of gentamicin, which blocks MOC effects, greatly reduced the amount of suppression (damping) observed after periods of noise and echolocation sound exposure. We conclude that tonic MOC activity is induced by the natural vocalizations and roost noise and this activity probably regulates and protects the highly resonant cochlear partition.

Substance P-induced inward current in identified auditory efferent neurons in rat brain stem slices.

The effects of substance P (SP) on whole cell currents were studied in neurons of the medial olivocochlear efferent system (MOCS) in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of SP (0.1-10 microM) reversibly induced an apparent inward current in 49 of 63 labeled neurons when voltage clamped at near resting voltages. This apparent inward current was consistent with the SP-induced membrane depolarization observed in current-clamp mode. The SP-induced change in current was dose dependent with a half-maximal response dose of 200 nM. It was mimicked by [Cys3,6, Tyr8, Pro9]-SP, a neurokinin (NK1) receptor selective agonist, whereas [Succinyl-Asp6, MePhe8]-SP 6-11 (Senktide), a NK3 receptor agonist, had no detectable effect. The SP effect was not blocked by 10(-6) M tetrodotoxin (TTX) and persisted when the perfusate contained 30 mM tetraethylammonium (TEA) or 100 microM Cd2+ or was in a 0-Ca solution. In a TTX-containing solution, SP caused a voltage-dependent decrease of membrane conductance, and the SP-evoked current reversed at a potential at around -105 mV. The predicted K+ equilibrium potential was -93.8 mV under the experimental conditions. The SP-induced inward current was attenuated by 66% when the perfusate contained 3 mM Cs+. We conclude that the apparent inward current is partly caused by SP decreasing an outward current normally maintained by the inward rectifier K+ channels in these cells. In the presence of Cs solution in the recording pipette and with a perfusate containing 3 mM Cs+, 0.1 mM Cd2+ and 10(-6) M TTX, a residual SP-induced inward current was observed at test voltages ranging from -120 to 40 mV. This subcomponent reversed its polarity at approximately 20 mV. This inward current was reduced substantially (but not abolished) when all NaCl in the external solution was replaced by TEA-Cl. The results indicate that SP also opens an unknown cation channel, which the available data suggests may be relatively nonselective. The results suggest that MOCS neurons are subject to modulation by SP, which depolarizes the cell membrane by decreasing the activity of inward rectifier K+ channels as well as concurrently activating a separate cation conductance. It also was found that in MOCS neurons responsive to both SP and norepinephrine, the norepinephrine effect was abolished by TTX, suggesting that an interneuronal population excited by norepinephrine converges selectively onto SP-sensitive MOCS neurons in the VNTB.

Peripheral auditory lateralization assessment using TEOAEs

Previous studies indicate a left-right asymmetry in the function of peripheral auditory system. Contralateral acoustic suppression of TEOAEs (transient evoked otoacoustic emissions) enables assessment of medial olivocochlear efferent system functioning, and has demonstrated that this system is more effective in the right than in the left ear. Moreover, TEOAE amplitudes are lower in the left than in the right ear. The aim of the present experiment was to verify firstly the absence of a relationship between medial efferent system asymmetry and TEOAE amplitude asymmetry, and secondly to study TEOAE input/output function slopes. There was no link between the asymmetries in TEOAE amplitude and in the medial efferent system functioning. Further, as previously shown, the medial olivocochlear system increased the TEOAE input/output function slopes. These TEOAE input/output function slopes seem to be consistent factors in peripheral asymmetry since the slope is lower in the right than in the left ear. Moreover, the lower the TEOAE amplitudes, the greater the TEOAE slopes. The slope asymmetry of the two ears could correspond to earlier saturation or a lower augmentation ability of the TEOAE response in the right ear, where the TEOAE amplitude is higher. This asymmetry in growth slopes reinforces the notion of peripheral auditory lateralization.

Aminoglycoside ototoxicity and the medial efferent system: I. Comparison of acute and chronic gentamicin treatments.

Recently our laboratory has demonstrated, in the guinea pig (GP), that an intramuscular (i.m.) injection of a high dose of gentamicin (GM) (150 mg/kg), can reversibly block the contralateral efferent suppression of ipsilateral cochlear activity. The aims of the present study were: (1) to investigate this effect with lower doses of GM; and (2) to find out whether this effect could constitute an anticipatory sign of ototoxicity during a chronic GM treatment (60 mg/kg i.m., 10 days). The function of the medial olivocochlear efferent system (MOES) was tested by recording the VIIIth nerve ensemble background activity (EBA) without and with contralateral low level (55 dB SPL) broadband noise stimulation. The results show a dose-dependent effect of GM on contralateral suppression, as the dose of 120 mg/kg induced a smaller blockade of the MOES, compared to 150 mg/kg, and no blockade was observed with lower doses. During the ten-day treatment no significant changes in the EBA without acoustic stimulation, nor in contralateral efferent suppression were detected. GPs monitored over several weeks after the treatment showed progressive reduction of the EBA without contralateral stimulation parallel to reduced suppression coefficients of the EBA, and CAP threshold elevations, denoting impaired cochlear function. Thus, this study demonstrated that a chronic treatment with 60 mg/kg of GM, although ototoxic, does not affect the contralateral efferent suppression, at least before the development of ototoxicity.

Fast, slow, and steady-state effects of contralateral acoustic activation of the medial olivocochlear efferent system in awake guinea pigs: action of gentamicin.

The function of the medial olivocochlear efferent system was observed in awake guinea pigs by recording, in the absence of ipsilateral external acoustic stimulation, the ensemble background activity (EBA) of the VIIIth nerve from an electrode chronically implanted on the round window of one ear. The EBA was measured by calculating the power value of the round window signal in the 0.5- to 2.5-kHz band after digital or analog (active) filtering. This EBA was compared with and without the addition of a low-level broadband noise to the opposite ear. The contralateral broadband noise (CLBN, 55 dB SPL) induced, via the efferent system, a decrease (suppression) of this EBA. With the use of noise bursts of different durations, two components in this suppression could be observed. After the onset of a 1-s CLBN, the power value of the EBA decreased rapidly by 38.0 +/- 4.2% (mean +/- SD, n = 3), with a latency of <10 ms and a decay time constant of 13.1 +/- 1.0 ms (fast effect). At the offset of the 1-s CLBN, EBA came back to prestimulation values with a similar latency and a time constant of 15.5 +/- 2.9 ms. During longer CLBN stimulation (>/=1 min), EBA presented, after the fast decrease, an additional, slower decrease of 15.6 +/- 3.1%, with a delay of 9.8 +/- 1.3 s and a decay time constant of 16.1 +/- 5.0 s (n = 12, slow effect), and then remained remarkably constant for as long as observed, i.e., >2 h (steady state). The average global suppression was thus up to 47.8 +/- 5.8% of the basal, pre-CLBN-stimulation EBA value. At the offset of the CLBN, EBA returned to pre-CLBN level with fast and slow phases, with, for the slow phase, no delay and a time constant of 32.1 +/- 8.1 s. Fast and slow changes in EBA power values were observed after a single injection of gentamicin (GM) at different doses (150, 200, and 250 mg/kg). At 150 and 200 mg/kg, GM progressively and reversibly blocked the rapid effect, but the slow component of the efferent medial suppression remained remarkably unchanged. However, at higher doses both the fast and slow suppressions were totally yet still reversibly blocked. These observations indicate that the medial olivocochlear efferent system exerts sustained influences on outer hair cells and that this effect develops in two different steps that may have different basic cellular mechanisms.

Increased amplitude of distortion product emissions in the human caused by contralateral low intensity acoustic stimulation

Distortion product otoacoustic emissions (DPOAEs) are probably based on the motile properties of the cochlear outer hair cells (OHCs), which are richly innervated by medial olivo-cochlear efferent nerves (MOCs). DPOAEs provide a stimulating model for functional exploration of the efferent system. The influences of contralateral acoustic stimulation were studied on the amplitude of ipsilateral DPOAEs at the frequency 2f1-f2. Fifteen ears of adults with normal hearing (7 women, 8 men, mean age; 23.1 +/- 4.2 years) were examined. Sound stimulation consisted of exposure to 6 successive series of contralateral pure tones at frequencies between 0.5 and 4.0 kHz and low intensities of 30 dB HL. DP grams were registered with f2 ranging from 708 to 6165 Hz. Contralateral pure tones in the frequency range above 750 Hz and with intensities of 30 dB HL increased the ipsilaterally registered DPOAE amplitudes. The results showed that this increase was frequency specific, at least for frequencies at 1 and 2 kHz. The frequency specificity was especially marked when ipsilateral f2 was near 1 kHz. Contralateral frequencies of 3 and 4 kHz induced a significant, increasing effect, including ipsilateral frequencies around f2 = 3 kHz (or 4 kHz) and also lower frequencies around f2 = 1 kHz. Our results support the concept that the observed increase in DPOAE amplitudes is mediated by the medial olivo-cochlear efferent system. The rule of OHCs as a cochlear amplifier in the presence of contralateral low-intensity sound stimulation is discussed.

Medial Olivocochlear Efferent System in Humans Studied With Amplitude-Modulated Tones

Evoked otoacoustic emissions (EOAEs) are assumed to be generated by outer hair cells (OHCs). It is now generally accepted that EOAEs represent a means of functional exploration of the active micromechanical properties of OHCs. Efferent fibers of the medial olivocochlear system (MOCS) are connected along the sides and the bases of OHCs. Some studies have shown that a suppression effect on EOAE amplitude is induced by the MOCS neurons during contralateral stimulation, presumably by modification of OHC motility. The contralateral acoustic stimuli used in experiments on the EOAE suppression effect have consisted mainly of sounds without a slow temporal fluctuation in their envelopes (broad-band noise, narrow-band noise, pure tones, or clicks). To elucidate further the parameters of MOCS activation, in the present study we looked at the contralateral suppression effect of amplitude-modulated (AM) tones. The results showed that EOAE amplitude was reduced with AM tones compared with no contralateral acoustic stimulation. The suppression effect mainly depended on three parameters. 1) Contralateral stimulation intensity: EOAE suppression occurred only with intensities > or = 40 dB SL. 2) The greater the modulation depth, the greater the suppression effect: statistical analysis showed a significant effect for 75 and 100% modulation depth. 3) The 100- and 140-Hz modulation frequencies gave the greatest suppression effect for 100 and 75% modulation depths. The suppression effect was frequency specific. The greatest decreases were observed when the carrier frequency of the contralateral AM tone was close to the frequency of the EOAE under study, i.e., 1 and 2 kHz. Acoustic cross talk and middle ear effects, which cannot be completely excluded, are discussed. However, the demonstrated frequency specificity of the EOAE suppression effect, together with observed presence of contralateral EOAE suppression in patients without stapedial reflex and the very weak intensities used (i.e., below acoustic reflex threshold), suggested that it was unlikely that the observed effects were due merely to middle ear reflexes. Our results confirm further the contralateral suppression effect on human cochlea mechanisms and show that the suppression effect can be influenced by amplitude modulations of the suppressor, characteristic of sounds in the environment.

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.

Contralateral acoustic stimulation induces a phase advance in evoked otoacoustic emissions in humans

In 28 normal-hearing human subjects, the medial olivocochlear efferent system was activated by contralateral acoustic stimulation which is able to mimic the inhibitory effects of electrical stimulation of the crossed olivocochlear bundle. A first experiment on 16 subjects demonstrated that a contralateral white noise of 35 dB SL was able to induce temporal changes on transiently evoked otoacoustic emissions in response to clicks of 63 dB SPL. These temporal changes consisted of an advance of click-evoked otoacoustic signals in 87% of cases and is referred to as phase-shift effect. The phase advance, quantified using two signal processing methods in both time and frequency domains, was found to be mainly associated with lower frequencies, with a maximal effect at 1.5 kHz and minimal effects around 3.5 and 4 kHz. In a second experiment, carried out on 12 subjects, a negative relationship was found to exist between the ipsilateral stimulation level (level of clicks ranging from 57 to 69 dB SPL) and the phase-shift effect (PSE). Specifically in the range of levels tested (25–45 dB SL), a linear relationship presenting no obvious saturation effect was observed between the contralateral level and the PSE. The PSE was examined in 6 additional subjects exhibiting pathological symptoms; 2 of 3 individuals, who had no contralateral stapedial reflexes unilaterally, showed the PSE whereas this response was reduced or absent in 3 other subjects in the ear with severed efferents associated with a vestibular neurotomy. The integrity of olivocochlear efferents was, therefore, necessary to obtain a full effect, but the absence of stapedial reflex did not prevent the effect from occurring.

Influence of auditory stimulation and visual attention on otoacoustic emissions

Transient Evoked otoacoustical Emissions (TEOAEs) express the micromechanical activity of the cochlear outer hair cells (OHCs). The inhibitory effects of contralateral acoustic stimulation and of visual attention tasks on TEOAE amplitude is well-established. Contralateral auditory stimulation and attention affect cochlear mi.cromechanics via the medial olivocochlear efferent system. The present study is a quantitative comparison of the individual and combined effects of these two inhibitory phenomena in the same subjects. TEOAEs were recorded in seven normal-hearing subjects in absence of inhibitory stimulus (S1), under contralateral 95 dB SPL white-noise stimulation (S2), during a visual attention task (S3) and with simultaneous presentation of both forms of stimulus (S4). Significant reductions in TEOAE amplitude were found with contralateral stimulation (S2) and visual attention (S3) ( p = 0.01 and 0.05 respectively), in confirmation of previous studies. The inhibitory effect of combined stimulation (S4) was found to be yet more significant ( p = 0.004) than the inhibition obtained with each stimulation presented alone.

Differential effects of visual attention on spontaneous and evoked otoacoustic emissions

A visual task decreases evoked otoacoustic emisions (TEOAE) to an interindividually variable degree. The aim of these two studies was to assess whether the presence or absence of spontaneous otoacoustic emissions (SOAE) is involved in this variability. The first experiment investigated spontaneous otoacoustic emission (SOAE) frequency shifts during a visual attention task with a Fast Fourier Transform analysis, and found no attention effect on SOAEs. Using a larger subject pool, the second experiment concerned the attention effect on transiently evoked otoacoustic emissions and SOAE amplitude, using different measurement methods with the same visual task. The sample was composed of two groups of subjects: group A comprising subjects presenting SOAEs and group B subjects without SOAEs. A decrease in EOAE amplitude for group B during attention was observed and seems to confirm a possible influence of attention on the cochlea via the medial olivocochlear efferent system (MOES). The TEOAE amplitude of group A did not vary during attention, showing the peculiarity of this population of subjects and suggesting a different cochlear functioning.

Etude des modifications temporelles des otoémissions acoustiques provoquées lors d'une stimulation acoustique controlatérale, chez l'humain normo-entendant

In 32 subjects, the medial olivo-cochlear efferent system was activated by a contralateral acoustic stimulation (CAS) which is able to mimic the inhibitory effects of electrical stimulation of the crossed olivo-cochlear bundle. A white noise CAS of 35dB SL induced temporal alterations in otocoustic emissions evoked by clicks of 63dB SPL, called phase shift effect (PSE). The PSE appeared as an advance of signals in 80% of cases and occurred after a latency of about 8ms. Two different signal processing methods (in time and frequency domains), showed that the phase shift was mainly located on lower frequencies with a maximum around 1.5kHz and a lack around 4kHz. When contralateral stimulation intensity was varying from 25 to 45 dB SPL, the PSE, increased linearly; the shape of the curve allowed to rule out a middle ear reflex. On the contrary, when ipsilateral stimulation intensity was varying from 57 to 69 dB SPL, the PSE decreased linearly, indicating that PSE originated in inner ear mechanisms. Lastly, a significant correlation had been evidenced between the amplitude of the suppressive (inhibitory) effect of CAS, and the amplitude of the phase shift effect, allowing one to suggest that both effects may have a common physiological origin

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.

Cochlear interdependence and micromechanics in Man and their relations with the activity of the medial olivocochlear efferent system (MOES)

Following stimulation of one ear with white noise (WN) or 0.5, 1 and 2 kHz tone bursts a statistically valid mean reduction in the amplitude of delayed evoked otoacoustic emissions (DEOE), elicited from the contralateral ear by bursts of the same frequencies, was observed in 10 people (19-23-years-old) with normal hearing. This reduction only appeared in response to a contralateral stimulus delivered 7, 8 and 9 ms earlier than that used to produce the DEOE. This inhibitory effect was just referable to the activity of the medial olivocochlear efferent system (MOES). This research has shown that: (i) the cochlear interdependence is linked to activation of the MOES; (ii) in man the activity of MOES is inhibitory and only appears for a stimulus of the same frequency or (for WN) including that used to elicit DEOE; (iii) the cochlear interdependence is frequency selective and the MOES thus establishes a direct functional interdependence between homologous sectors of the organs of Corti on the two sides; (iv) DEOE would appear to be no more than partly generated by outer hair cells (OHC) of the organ of Corti in relation to the frequency of the stimulus employed, thus substantiating the hypothesis that in their production the effects of an 'active' mechanism, represented by the 'slow' contractile activity of the OHC, is overlain by those of a 'passive' mechanism formed by the oscillations induced by the movements of the stapes in the basilar membrane (BM) or in the set of membranes and liquids of cochlear canal.

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

The frequency characteristics of the suppression by contralateral stimulations, of the cubic 2 f 1− f 2 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 2 f 1− f 2 set at one of the four studied frequencies, i.e., 1, 2, 3 or 5 kHz. For each value of 2 f 1− f 2, analysis of variance and Student's 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 2 f 1− f 2 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 2 f 1− f 2 = 1 kHz with contralateral NBN central frequencies around 1 kHz. When 2 f 1− f 2 = 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.

Effects of the crossed acoustic reflex on distortion-product otoacoustic emissions in awake rabbits

Recent studies in anesthetized cats suggest that contralateral-sound stimulation acts to suppress ipsilateral neural responses via the medial olivocochlear-efferent system. Activation of this descending efferent pathway presumably influences ipsilateral outer hair cell motility and, thus, cochlear micromechanics, resulting in reduced input to auditory-nerve fibers. The principal aim of the present study was to determine if contralateral-sound stimuli influence the generation of ipsilateral distortion-product otoacoustic emissions, in the ears of awake rabbits. The results showed no effects of contralateral stimuli on these emissions that could not be attributed to the crossed acoustic middle-ear reflex. The findings further indicate that distortion-product otoacoustic emission amplitudes over a wide range of frequencies can be dramatically reduced when the middle-ear reflex is activated.