Mechanical and neural interactions between binaurally applied sounds in cat cochlear nerve fibers

Contralateral sound stimulation suppresses the compound action potential from the auditory nerve in humans

BACKGROUND

In the past decade, contralateral sound suppression of otoacoustic emissions has been extensively used to study the role of the medial olivocochlear efferent in humans. In most studies, the suppressive effect of contralateral sound stimulation was not greater than 2 to 4 dB. However, the relation between the degree of otoacoustic emission reduction and the neural auditory threshold is unknown.

METHODS

The current study investigates the effect of contralateral sound stimulation by measuring compound action potential response from the auditory nerve during retrosigmoid surgery in humans.

RESULTS

Although only a small number of subjects responded to contralateral sound stimulation, we report that efferent activation by contralateral sound stimulation results in 10 dB effective neural attenuation.

CONCLUSIONS

Together with previous otoacoustic emission measurements in humans, this result demonstrates that the suppressive effect of contralateral noise suppression is greater when measured with compound action potential than otoacoustic emissions, and that contralateral sound suppressive effect is at least as strong in humans as in animals.

Delay and temporal integration in medial olivocochlear bundle activation in humans

OBJECTIVE

Contralateral suppression of the transient-evoked otoacoustic emissions (TEOAEs) provides a means of studying auditory efferent function, but the temporal dynamics of the reflex are not fully understood. The most fundamental parameter is the time-course of activation of contralateral suppression. The stimulus parameters are likely to be important; this may include temporal dynamics of the suppressor itself. This investigation thus was devoted to the further study of 1) delay of contralateral suppression of TEOAEs-effect of delay of the ipsilateral probe-and 2) temporal variation of the suppressor-effect of amplitude modulation of the contralateral noise stimulus.

DESIGN

Measurements were made in three samples of normal-hearing subjects (N(total) = 71), employing well-established methods of TEOAE assessment.

RESULTS

Statistically significant contralateral suppression occurred some 60 msec after onset of the contralateral noise; thereafter, the effect was essentially constant (i.e., to >180 msec). The results for click delays less than 60 msec, nevertheless, were systematic and readily fitted by a sloping straight line (dB suppression versus time) reminiscent of the concept of threshold power integration. The onset of suppression may thus be characterized by a time constant. The delay of suppression also was found to be reduced by contralateral amplitude-modulated noise.

CONCLUSIONS

These findings reinforce a growing consensus in the literature that, despite initiation perhaps some milliseconds after onset of the contralateral stimulus, there is a substantial delay, i.e., in the tens of milliseconds, before maximal suppression is achieved. The exact time constant of this effect appears to depend upon the combination of probe and suppressor levels, including the temporality of the suppressor. These factors are likely to delimit the role/influence of this reflex in real-world function, favoring perhaps more-or-less sustained suppression that is activated in a time-varying sound environment.

Development of human cochlear active mechanism asymmetry: involvement of the medial olivocochlear system?

To study the functional development of the medial olivocochlear system, transient-evoked otoacoustic emission suppression experiments were conducted in 73 ears of 38 pre-term and 11 full-term neonates. The continuous contralateral stimulation was a broad band white noise, presented at 70 dB SPL. Efferent suppression was determined by subtracting the without-contralateral stimulation condition from the with-contralateral stimulation condition. Across this population, a mean suppression effect of contralateral stimulation on transient-evoked otoacoustic emissions was found, with most of the suppression effect observed after 8 ms. The amount of suppression is linearly, positively correlated with the conceptional age. In the subgroup of bilaterally tested neonates, the suppression of transient-evoked otoacoustic emissions is similar in the right ear and the left ear in subjects whose conceptional age is less than 36 weeks and significantly higher in the right ear than in the left ear in older neonates. This last observation was seen at frequencies where transient-evoked otoacoustic emission amplitudes became higher in the right ear than in the left ear as the conceptional age increased, a finding already reported in adults. This study shows that the functional adult pattern of the medial efferent system, probably involved in the detection of signals in noise such as speech sounds, seems to appear gradually in neonates and represents one of the several arguments in favor of functional auditory lateralization in humans, with a right ear advantage.

Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study

Although the chinchilla is widely used as a model for auditory research, little is known about the distribution and morphology of its olivocochlear neurons. Here, we report on the olivocochlear neurons projecting to one cochlea, as determined by single and double retrograde fluorescent tracer techniques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast Blue) into scala tympani or as a control, a unilateral injection into the middle ear cavity. The results indicate that there are similarities as well as significant differences between the chinchilla and other species of rodents in the distributions of their olivocochlear neurons. Based on three well-labelled cases, there was a mean total of 1168 olivocochlear neurons in the chinchilla. Of these, the majority (mean 787) were small, lateral olivocochlear neurons found almost exclusively within the ipsilateral lateral superior olivary nucleus. The next largest group consisted of a mean of 280 medial olivocochlear neurons virtually all of which were located in the dorsomedial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were more predominantly crossed in their projections (4:1) than in any known species. The smallest group of olivocochlear neurons (mean 101) consisted of larger lateral olivocochlear neurons (shell neurons) which were located on the margins of the superior olivary nucleus and which projected mainly (2.2:1) ipsilaterally. Double retrograde labelling was observed only in medial olivocochlear neurons and occurred in only 1-2% of these cells. The results confirm previous findings which indicated a relative paucity of fibers belonging to the uncrossed as compared to the crossed olivocochlear bundle. This, together with the strong apical bias of the uncrossed projection reported previously, offers possible explanations for the apparent absence of efferent-mediated suppressive effects of contralateral acoustic stimulation in this species. Regarding the lateral olivocochlear system, the chinchilla is shown to possess both intrinsic and shell neurons, as in the rat.

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.

Effect of contralateral acoustic stimulation on the growth of click-evoked otoacoustic emissions in humans

Input-output (I/O) functions of click-evoked otoacoustic emissions (CEOAEs) were obtained over a 12 dB range for 64 normally hearing adult listeners with and without contralateral broadband noise (BBN). Contralateral acoustic stimulation (CAS) is a convenient way of suppressing responses to ipsilateral stimuli, probably acting via the medial olivocochlear system (MOCS). The present study shows that this contralateral sound suppression of CEOAEs is largest at low stimulus levels. In fact, the curves obtained under CAS approach the curves obtained without CAS as stimulus level rises. I/O slope analysis for the whole study population (n = 64) showed a slight but significant rise in slope with BBN, which may be interpreted as I/O function decompression. A loss of contralateral suppression effect at high ipsilateral stimulus levels was found in both very low and very high amplitude CEOAE subjects, despite the fact that I/O slopes differed between these two groups, whereas rise in slope under contralateral stimulation failed to be found for these same 2 groups of 16 subjects each. These findings clearly indicate that the MOCS is mostly functional at low sound levels, and suggest that the study of CEOAE I/O slope alteration under CAS may help specify one form of MOCS action on cochlear functioning.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

Contralateral auditory stimulation and otoacoustic emissions: a review of basic data in humans

The influence of contralateral auditory stimulation on otoacoustic emissions (OAE), spontaneous OAE, evoked OAE and acoustic distortion products, can be summarized as follows: (1) alteration (mainly a decrease) of OAE amplitude; (2) alteration of response spectrum (upward shift frequency of SOAE); (3) alteration of phase; (4) effect dependent on intensity of contralateral stimulation; (5) effect inversely dependent on intensity of ipsilateral stimulation; (6) frequency specificity of the suppressive effect. Involvement of the medial olivocochlear bundle is highly probable but one cannot exclude a double pathway including also the acoustic reflex.

Development of contralateral suppression of the VIIIth nerve compound action potential (CAP) in the Mongolian gerbil

We studied whether same-frequency contralateral tones of 65 dB pSPL (peak Sound Pressure Level) suppress the VIIIth nerve compound action potential (CAP) evoked by 40-45 dB pSPL tone pips in the Mongolian gerbil from 22 to 92 days after birth (DAB). The primary stimuli were tone pips of 1, 2, 4, 8, and 10 kHz; only the 1 kHz CAP amplitude was suppressed significantly by tones of the same frequency. The suppression was seen at 22 DAB, and underwent little relative change with development.

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.

Contralateral sound suppresses distortion product otoacoustic emissions through cholinergic mechanisms

Presentation of an acoustic signal to one ear can suppress sound-evoked activity recorded at the opposite ear. The suppression appears to be mediated by medial olivocochlear (MOC) efferent neurons synapsing with outer hair cells (OHCs) and acting through the MOC neurotransmitter, acetylcholine (ACh). The purpose of the present investigation was to study the suppression of distortion product otoacoustic emissions (DPOAEs) by contralateral sound and to examine whether the suppression could be blocked by known antagonists of olivocochlear (OC) efferent activity. Urethane-anesthetized guinea pigs were used. Perilymph spaces of ipsilateral cochleae were alternately perfused with artificial perilymph and drugs at 2.5 microliters/min for 10 min. After each period of perfusion, DPOAEs were measured before, during and after contralateral wideband noise (WBN) stimulation. Pre-perfusion, contralateral WBN attenuated the ipsilateral DPOAEs between 1-3 dB. This suppression was blocked reversibly by strychnine (10 microM), curare (10 microM) and atropine (20 microM), known antagonists of OC efferent activity. These results confirm the findings of Puel and Rebillard (1990) that contralateral WBN can suppress DPOAEs in anesthetized guinea pigs. Furthermore, results suggest that this efferent control of the cochlear mechanical response can either be mediated by both nicotinic and muscarinic cholinergic receptors, or that a single receptor with as yet undescribed structure and pharmacology mediates effects seen.

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.

Use of otoacoustic emissions to explore the medial olivocochlear system in humans

The association between contralateral stimulation and otoacoustic emissions makes it possible to study the medial olivocochlear system in humans. Characterization of this functional exploration and clinical applications are discussed.

Alternating current induced otoacoustic emissions in the guinea pig

Injection of alternating current (AC) into the scala media of the guinea pig cochlea induced otoacoustic emissions (OAEs) at the frequency of the AC fundamental, together with harmonic and intermodulation distortion products. Although the waveform of the injected ACs was distorted, probably due to nonlinear polarization of the metal electrodes, and was composed of the fundamental plus distortion products of every order, only a few of the lowest order distortion products were selectively emitted with the fundamental. AC injection at a basal site extended the high frequency limit of OAEs. Electrical stimulation of the crossed olivocochlear bundle inhibited the sideband emissions with little change in the fundamental. OAE was reduced reversibly by temporary impairment of the cochlea due to exposure to fatiguing sound, by intravenous application of furosemide and by temporary anoxia. Irreversible reduction resulted from intracochlear perfusion with excess K+ solution, acoustic trauma and cardiac arrest. These facts imply that AC-induced OAE is not an artifact generated electrically; rather, such emissions originate in the cochlea and normal metabolic activity in the cochlea is essential. A proposed mechanism of generation includes two components: 1) electromechanical transduction from AC to mechanical vibration in the cochlea and 2) a distortion-producing process; the contribution of each component to the receptor mechanism is discussed.

Changes in spontaneous otoacoustic emissions produced by acoustic stimulation of the contralateral ear

Spontaneous otoacoustic emissions (SOAEs) were measured in human ear canals before, during and after presentation of tonal stimuli to the contralateral ear. Stimuli were presented in 1/8 octave steps from 2 octaves below to 1 octave above the SOAE frequency at sound levels below the observed contralateral acoustic reflex threshold. For certain conditions there was an abrupt upward frequency shift at stimulus onset. For a fixed level the effect was frequency selective; the maximum frequency shift was obtained with tones approximately 1/2 octave below the SOAE. SOAE amplitude usually decreased but in some cases increased or remained unchanged. When amplitude changes were observed, the maximum shifts were observed for tones at or near the SOAE frequency. Changes in SOAEs were not observed for stimulus levels below 60 dB SPL. The effect is believed to be mediated by medial efferent neurons of the uncrossed olivocochlear bundle which arise in the medial region of the superior olivary complex and terminate on outer hair cells (OHCs). These results support those models which attribute SOAE generation to OHCs, and are indicative of an efferent influence on cochlear mechanics. A simple model is presented that proposes that efferent activity alters the tuning of the emission generator by causing changes in OHC membrane conductance.

Effects of contralateral sound on auditory-nerve responses. I. Contributions of cochlear efferents

The response properties of single auditory-nerve fibers in barbiturate-anaesthetized cats were recorded with and without simultaneous presentation of sound to the contralateral ear. The tendons to the middle ear muscles on both sides were cut before all experiments, and contralateral stimuli were restricted to levels below the threshold for crosstalk to the ipsilateral ear. Contralateral tones and broad-band noise were found to suppress the responses of auditory-nerve afferents to ipsilateral tones at their characteristic frequency (CF), but not to tones off CF. The suppression due to contralateral sound required approximately 100-200 ms to develop and to decay. When the contralateral stimuli were tones at the CF, the strongest suppression was observed in low- and medium-spontaneous-rate units with CFs between 1 and 2 kHz. The suppressive effect of contralateral sound completely disappeared immediately after severing the entire olivocochlear bundle (OCB) within the internal auditory meatus. the completeness of the OCB cuts was assessed histologically. Most of the suppressive effect remained after lesions to the OCB in the floor of the IVth ventricle which eliminated the crossed olivocochlear projection but spared most of the uncrossed projection. It is argued that this suppressive effect of contralateral sound is mediated by the uncrossed olivocochlear efferents to the outer hair cells.

Effects of contralateral sound on auditory-nerve responses. II. Dependence on stimulus variables

The suppression by moderate-level contralateral sound of auditory-nerve-fiber responses to ipsilateral stimuli at the characteristic frequency (CF) was studied in barbiturate-anesthetized cats. The dependence of suppression strength on ipsilateral and contralateral stimulus variables, including level, frequency, bandwidth, and timing relationships, was investigated. The principal findings were: (1) Contralateral-sound suppression is greatest when the ipsilateral stimulus level is within the dynamic range of the unit. (2) When the contralateral stimuli are tones, suppression is greatest when the contralateral tone frequency is at or near CF. (3) Units with CFs above 3-4 kHz are only weakly suppressed by contralateral CF tones but more strongly suppressed by contralateral broad-band noise. (4) Continuous contralateral stimuli are significantly more effective suppressors than are gated stimuli. The characteristics of contralateral-sound suppression are compared with the physiology and anatomy of the uncrossed medial olivocochlear efferents, the subset of efferents which are the primary mediators of the effect.