Previous studies have demonstrated that the magnitudes of otoacoustic emissions (OAEs) measured during behavioral tasks requiring attention were different from OAE magnitudes measured during tasks that did not require attention. The implication of these results is that the cognitive and perceptual demands of a task can affect the first neural stage of auditory processing—the sensory receptors themselves. However, the directions of the reported attentional effects have been inconsistent, the magnitudes of the observed differences always have been small, and comparisons across studies have been made difficult by significant procedural differences. In this study, we used a nonlinear version of the stimulus-frequency OAE (SFOAE), called the nSFOAE, to measure cochlear responses from human subjects while they simultaneously performed behavioral tasks requiring selective auditory attention (dichotic or diotic listening), or relative inattention. The results indicated statistically significant systematic differences in nSFOAE magnitudes within subjects, but opposite directions of effect across subjects. Differences in the nSFOAEs measured during the dichotic- and diotic-listening conditions suggested that cochlear responses differed in these two types of selective-listening tasks. Time constants measured from functions fitted to the nSFOAE responses indicated faster efferent effects during selective listening than during inattentive listening for the majority of subjects. Furthermore, measurements in brief silent intervals after the nSFOAE stimuli indicated weaker residual nSFOAE magnitudes during selective listening than during inattentive listening for every subject. This collection of findings supports the hypothesis that auditory attentional demands can modulate cochlear processing, presumably to the benefit of the listener. / text

This thesis presents data which support cochlear involvement in attentional listening. It has been previously proposed that the descending auditory pathways, in particular the medial olivocochlear system, play a role in reducing the cochlea's response to noise in a process known as antimasking. This hypothesis was investigated in human subjects for its potential impact on the detection of signals in noise following auditory cues. Three experimental chapters (Chapters 3, 4 and 5) are described in this thesis. Experiments in the first chapter measured the effect of acoustic cues on the detection of subsequent tones of equal or different frequency. Results show that changes in the ability to detect signals following auditory cues are the result of both enhanced detection for tones at the cued frequency, and suppressed detection for tones at non-cue frequencies. Both effects were measured to be in the order of ~3 dB. This thesis has argued that the enhancement of a cued tone is the implicit result of an auditory cue, while suppression of a probe tone results from the expectation of a specific frequency based on accumulated experience of a listening task. The properties of enhancement support the antimasking hypothesis, however, the physiological mechanism for suppression is uncertain. In the second experimental chapter, auditory cues were replaced with visual cues (representing musical notes) whose pitch corresponded to the target frequency, and were presented to musician subjects who possessed absolute or relative pitch. Results from these experiments showed that a visual cue produces the same magnitude of enhancement as that produced by an acoustic cue. This finding demonstrates a cognitive influence on the detection of tones in noise, and implicates the role of higher centres such as those involved in template-matching or top-down control of the efferent pathways. The final experimental chapter repeated several of the experiments from the first chapter on subjects with various forms of hearing loss. The results indicate that subjects with an outer hair cell deficit (concomitant with a sensorineural hearing loss) do not exhibit an enhancement of cued frequencies or a suppression of unexpected frequencies to the same extent as the normal-hearing subjects. In addition, one subject with a long-standing conductive hearing loss (with normal cochlear function) produced an enhancement equivalent to that of the normalhearing subjects. These findings also support the role of the medial olivocochlear system and the outer hair cells in antimasking. It is the conclusion of this thesis that enhancement most likely results from a combination of changes in receptive field characteristics, at various levels of the auditory system. The medial olivocochlear system is likely to be involved in unmasking a portion of the signal at the cochlear level, which may be influenced by both acoustic reflex pathways or higher centres of the brain.

[Truncated abstract] The mammalian inner ear is innervated by the efferent olivocochlear system which is divided into medial and lateral systems. In anaesthetised animals, medial olivocochlear (MOC) axons can be electrically stimulated at the floor of the IVth ventricle. MOC stimulation suppresses the spontaneous activity and sound-evoked responses of primary afferents by its actions on outer hair cells. Effects of MOC stimulation have been also reported on responses of neurons in the cochlear nucleus, the first central auditory center receiving cochlear input. However, very little is known about the net results of MOC effects in higher order neurons. This issue was investigated by electrically stimulating MOC axons at the IVth ventricle and recording extracellular single unit activities in the central nucleus of the inferior colliculus (CNIC) of anaesthetised guinea pigs. For the first part of the study, anatomical and neurophysiological studies were carried out to establish that the focal midline MOC stimulation can selectively stimulate MOC axons without any current spread to adjacent ascending fibers. The MOC stimulation and CNIC recordings were then carried out in a series of experiments that included normal hearing animals, animals treated acutely with gentamicin (in which the acetylcholine-mediated peripheral suppression of the olivocochlear efferents is selectively eliminated) and partially deafened animals. ... However, in other CNIC neurons, effects could not be so explained, showing either additional suppression or even marked excitatory effects. (4) MOC stimulation also suppressed the spontaneous activity of CNIC neurons in normal hearing animals. When similar efferent stimulation was carried out in partially deafened animals, the abnormally high spontaneous activity of some CNIC neurons in the deafened frequency regions was also transiently suppressed by MOC shocks. The results from this study clearly demonstrate that the MOC system can modulate the responses of midbrain neurons in a more complex manner compared to the effects seen in the periphery. The more complex effects seen for responses to tones in quiet and in noisy background are likely to result from a complex interplay between altered afferent input in the cochlea and central circuitry. In addition, the ability of MOC efferents in suppressing the normal and abnormal spontaneous activity in the midbrain also could have implications for the role of the descending system in the pathophysiology and treatment of tinnitus.

Although the suppressive effect of the medial olivocochlear system (MOCS) on peripheral auditory active mechanisms is well documented in humans, the effect of efferent inhibition over prolonged periods of acoustic stimulation is less well documented, especially as observed in suppression of transient evoked otoacoustic emissions (TEOAE’s). The present study therefore evaluated the relationship between the duration of contralateral acoustic stimulation and the suppression of TEOAE’s in ten adults with normal hearing. TEOAE recordings with linear clicks (60 dB sound pressure level) were measured at four intervals during 15 minutes of continuous contralateral white noise (45 dB sound pressure level), followed by two post-noise recordings. An identical within-subject control condition was recorded without contralateral noise. Experimental and control measurements were repeated three times, on separate days. Results revealed significant and sustained TEOAE amplitude reduction for the entire duration of contralateral stimulation. Suppression increased across the duration of contralateral noise, but not sufficiently to be statistically significant. After noise termination, TEOAE amplitudes increased to values significantly above control recordings. The sustained suppression of TEOAE’s indicates continuous efferent inhibition over time in normal adults, with a significant increase in TEOAE amplitude after noise cessation possibly indicating increased outer hair cell responsiveness after prolonged contralateral noise. / Dissertation (MCommunication Pathology)--University of Pretoria, 2009. / Speech-Language Pathology and Audiology / Unrestricted

The study aimed to determine the relationship between the medial olivocochlear (MOC) efferent reflex strength and susceptibility to noise-induced hearing loss (NIHL). This was evaluated by measuring the efferent suppression (ES) results from the contralateral suppression of Transient Evoked Otoacoustic Emissions (TEOAEs). It was predicted by several researchers that the strength of the MOC efferent reflex could determine the susceptibility to hearing loss (HL). The prediction was that an individual with a stronger MOC efferent reflex was less susceptible to developing a HL and an individual with a weaker MOC efferent reflex was more susceptible to developing a HL. The design used in the study was a categorical independent comparative design. The independent values used in the study were the results from the contralateral suppression of the TEOAEs and the thresholds obtained from each participant’s pure tone audiogram. A quantitative research approach was used as different numerical values were collected from each participant. The numerical results obtained for each participant were objectively compared between the two identified groups. Forty-one participants between the ages of 30 and 45 years, who had been exposed to noise levels between 89.3 dBA and 101.6 dBA at a Platinum mine in the North West Province, were used for the study. Twenty participants presented thresholds within normal limits of 0 to 15 dB and 21 participants presented with a permanent minimal NIHL with thresholds of 16 to 40 dB at 3000 Hz, 4000 Hz and 6000 Hz averaged. The data was analysed using the Statistical Package for the Social Sciences (SPSS) program version 25 (IBM Inc.). Non-parametric tests were used with the Mann-Whitney U test, where the ES of the two independent groups were compared. The results showed no statistically significant difference in the ES of the normal hearing participants compared to the participants with a minimal HL. However, participants with normal hearing presented, on average, with a slightly stronger ES than the participants with a minimal HL. The lowest p-values in this study were calculated at 2000 Hz and 4000 Hz, with a p-value of 0.085 at 4000 Hz for the age category 30 to 35 years and a p-value of 0.086 at 2000 Hz for the age category 41 to 45 years. This suggests that it could be possible that the MOC reflex strength may predict the degree of HL. It is recommended that more research be done on contralateral suppression of TEOAE measurements on participants who present with permanent NIHL to possibly use the MOC reflex to predict susceptibility to HL in clinical practice. / Dissertation (MCommunication Pathology (Audiology))--University of Pretoria, 2021. / Speech-Language Pathology and Audiology / MCommunication Pathology (Audiology) / Unrestricted

La variabilité inter-individuelle en localisation auditive a été étudiée à travers plusieurs expérimentations. La première expérimentation a exploré la variabilité inter-individuelle en localisation auditive de sources sonores virtuelles (HRTFs individuelles et non individuelles) dans une population de 25 auditeurs naïfs préalablement soumis à un entraînement procédural. D'après nos résultats, la variabilité inter-individuelle dans la dimension gauche/droite pourrait être liée à une préférence individuelle vers un type d'indices parmi ceux utilisables pour la localisation dans cette dimension (indices binauraux/spectraux). Dans les dimensions haut/bas et avant/arrière, la variabilité inter-individuelle pourrait être liée à l'attention spatiale et à sa variation entre les zones de l'espace. Une seconde expérimentation réalisée chez les mêmes auditeurs a montré qu'une amélioration des capacités de localisation était possible par un entraînement par feedback visuel ou audio-moteur. Cette amélioration était souvent fonction de la performance avant entraînement, conduisant ainsi à une réduction de la variabilité inter-individuelle après entraînement. La variabilité inter-individuelle dans la dimension haut/bas après entraînement était en partie expliquée par l'activité du faisceau olivocochléaire efférent médian (FOCEM), structure du système auditif probablement impliquée dans le codage des indices spectraux de localisation. Ce résultat a été conforté par les résultats d'une troisième expérimentation qui ont révélé que les auditeurs dont la performance de localisation était la moins altérée dans le bruit étaient ceux dont le FOCEM avait été déterminé comme le plus actif. / Several experiments were performed to investigate interindividual variability in auditory localization. The first experiment explored interindividual variability in the localization of virtual sound sources (individualized and non-individualized HRTFs) in 25 naive listeners following procedural training. The results suggest that interindividual variability in localization performance in the left-right dimension stems from interindividual differences in the use of the different types of cues available for sound localization in this dimension (binaural/spectral cues). Interindividual variability in sound-localization performance in the up-down and front-back dimensions appears to be related primarily to spatial attention and to its variation across the area of space. A second experiment in the same group of listeners showed that localization performance can be improved by the provision of visual or auditory-motor feedback. The improvement depended upon pre-training performance, in such a way that interindividual variability in performance was smaller after training than before training. In addition, interindividual variability in the up-down dimension after training was found to be related to interindividual differences in the activity of the medial olivocochlear bundle (MOCB), an efferent auditory system that is currently likely thought to be involved in the processing of spectral cues for sound localization. This result was supported by the results of a third experiment which revealed that the listeners in whom localization performance was least impacted by background noise were those in whom the strongest MOCB had been measured.

<p>The neural code from the inner ear to the brain is dynamically controlled by central nervous efferent feedback to the audio-vestibular epithelium. Although such efference provides the basis for a cognitive control of our hearing and balance, we know surprisingly little about this feedback system. This project has investigated the applicability of a transgenic mouse model, expressing a fluorescent protein under the choline-acetyltransferase (ChAT) promoter, for targeting the cholinergic audio-vestibular efferent neurons in the brainstem. It was found that the mouse model is useful for targeting the vestibular efferents, which are fluorescent, but not the auditory efferents, which are not highlighted. This model enables, for the first time, physiological studies of the vestibular efferent neurons and their synaptic inputs. We next assessed the expression of the potassium channel family Kv4, known to generate transient potassium currents upon depolarization. Such potassium currents are found in auditory efferent neurons, but it is not known whether Kv4 subunits are expressed in these neurons. Moreover, it is not known if Kv4 is present and has a function in the vestibular efferent neurons. Double labelling with anti-ChAT and anti-Kv4.2 or Kv4.3 demonstrates that the Kv4.3 subunits are abundantly expressed in audio-vestibular efferents, thus indicating that this subunit is a large contributor to the excitability and firing properties of the auditory efferent neurons, and most probably also for the vestibular efferent neurons. In addition, we also unexpectedly found a strong expression of Kv4.3 in principal cells of the superior olive, the neurons which are important for sound localization.</p>

Embora hiper-reatividade auditiva (HRA) e/ou hiperacusia (HPA) sejam manifestações frequentes no Transtorno do Espectro Autista (TEA) são poucos os estudos que investigaram este sintoma com medidas fisiológicas ou exploraram os mecanismos auditivos neurais que podem estar associados a esta característica. O principal objetivo deste trabalho foi estudar a queixa subjetiva (o sintoma) e o desconforto observável (o sinal) da HPA no TEA e avaliar a relação com o efeito inibitório do complexo olivococlear medial (EICOCM). Foram recrutadas 13 crianças e adolescentes com TEA (idade média = 7,2 anos) de ambos os sexos, e 11 crianças e adolescentes com desenvolvimento típico (idade média = 7,2 anos) de ambos os sexos. Para avaliar a HPA, todos os sujeitos responderam a um questionário de caracterização desse sintoma. As medidas psicoacústicas e eletroacústicas permitiram a determinação da sensibilidade auditiva, do funcionamento coclear, do nível de desconforto auditivo e do campo dinâmico da audição. Para a avaliação da via eferente, o EICOCM foi investigado pelas emissões otoacústicas evocadas transientes com ruído contralateral. Os resultados mostraram que o grupo com TEA apresentou desconforto em intensidades mais baixas nas frequências de 1 kHz, 2 kHz e 4 kHz (1 kHz, t = 1,99, p = 0,059; 2 kHz, t = 2,16, p = 0,042; 4 kHz, t = 2,37, p = 0,028), e menor faixa dinâmica auditiva em comparação com o grupo controle, sendo que nas frequências de 0,5 kHz, 4 kHz e 8 kHz houve diferenças estatisticamente significantes (respectivamente: 0,5 kHz p = 0,03; 4 kHz p = 0,01 e 8 kHz p = 0,03). As emissões otoacústicas evocadas transientes evidenciaram amplitudes de resposta coclear semelhante entre os grupos, porém, o efeito inibitório não: o grupo com TEA apresentou valores menores em comparação ao grupo controle (F(4;76) = 3,49, p = 0,01). Houve correlação positiva entre o EICOCM e a medida do campo dinâmico da audição. Crianças e adolescentes com TEA apresentaram maior ocorrência de hiperacusia, o que pode indicar que esse sinal está relacionado a um déficit no EICOCM / Although auditory hyper-responsiveness (AHR) and/or hyperacusis is a frequent manifestation in Autism Spectrum Disorder (ASD), few studies have investigated this symptom with physiological measures or explored the neural auditory mechanisms that may be associated with this characteristic. The main objective was to study the complains (symptom) and the evident discomfort (sign) of hyperacusis in ASD and to evaluate its relationship with the inhibitory effect of the medial olivocochlear bundle (MOCB). Two groups, one of 13 children and adolescents with ASD (mean age = 7.2 years) of both sexes, and other with 11 children and adolescents typically developed (mean age = 7.2 years) of both sexes, were recruited. To assess hyperacusis, all subjects answered a questionnaire characterizing this symptom. The psychoacoustic and electroacoustic measurements allowed the determination of auditory sensitivity, cochlear functioning, level of auditory discomfort and auditory dynamic range. For the evaluation of the efferent pathway, the inhibitory effect of the MOCB was assessed by transient evoked otoacoustic emissions with contralateral noise. The results showed that the group with ASD presented discomfort at lower frequencies in the frequencies of 1, 2 and 4 kHz (1 kHz, t = 1.99, p = 0.059; 2 kHz, t = 2.16, p = 0.042; 4 kHz, t = 2.37, p = 0.028), and lower auditory dynamic range in comparison with the control group, while for the frequencies of 0, 5, 4 and 8 kHz, a statistically significant difference was found (respectively: 0.5 kHz p = 0.03; 4 kHz p = 0.01 and 8 kHz p = 0.03). Transient evoked otoacoustic emissions showed similar cochlear response amplitudes between groups, while the inhibitory effect did not: the group with ASD presented lower values in comparison to the control group (F(4;76) = 3.49, p = 0,01). There was a positive correlation between the inhibitory effect of the MOCB and the auditory dynamic range measurement. Children and adolescents with ASD had a higher occurrence of hyperacusis, which may point out that this sign is associated with a deficit in the inhibitory effect of MOCB

The neural code from the inner ear to the brain is dynamically controlled by central nervous efferent feedback to the audio-vestibular epithelium. Although such efference provides the basis for a cognitive control of our hearing and balance, we know surprisingly little about this feedback system. This project has investigated the applicability of a transgenic mouse model, expressing a fluorescent protein under the choline-acetyltransferase (ChAT) promoter, for targeting the cholinergic audio-vestibular efferent neurons in the brainstem. It was found that the mouse model is useful for targeting the vestibular efferents, which are fluorescent, but not the auditory efferents, which are not highlighted. This model enables, for the first time, physiological studies of the vestibular efferent neurons and their synaptic inputs. We next assessed the expression of the potassium channel family Kv4, known to generate transient potassium currents upon depolarization. Such potassium currents are found in auditory efferent neurons, but it is not known whether Kv4 subunits are expressed in these neurons. Moreover, it is not known if Kv4 is present and has a function in the vestibular efferent neurons. Double labelling with anti-ChAT and anti-Kv4.2 or Kv4.3 demonstrates that the Kv4.3 subunits are abundantly expressed in audio-vestibular efferents, thus indicating that this subunit is a large contributor to the excitability and firing properties of the auditory efferent neurons, and most probably also for the vestibular efferent neurons. In addition, we also unexpectedly found a strong expression of Kv4.3 in principal cells of the superior olive, the neurons which are important for sound localization.

Le système auditif ascendant est constitué d’étages multiples procédant chacun à un traitement complexe du signal, traitement qui est modulé par un système descendant, formant de multiples boucles corticales et sous-corticales de rétroaction : le système efférent auditif. La boucle la plus longue de ce système serait capable de moduler directement le fonctionnement des cellules sensorielles de l’organe périphérique de l’audition. Le but de ce travail était d’étudier pour la première fois chez le sujet sain, l’effet d’une modulation corticale expérimentale sur le fonctionnement périphérique auditif. Nous avons donc recueilli, dans le cadre d’un protocole en double aveugle contre placebo, chez 34 sujets sains, l’activité des cellules ciliées externes de l’organe de Corti (cellules sensorielles directement en contact avec les fibres nerveuses efférentes), par le recueil d’otoémissions acoustiques (OEAs), avant et après stimulation du cortex auditif par une stimulation magnétique transcrânienne répétitive (SMTr). Une diminution de l’amplitude d’un type d’OEA a été obtenue du côté controlatéral à une stimulation du cortex auditif par SMTr à 10 Hz, effet qui ne peut être expliqué seulement par une action non-spécifique liée au bruit de la SMTr mais qui reste extrêmement variable d’un sujet à un autre. De plus, nous avons montré, du côté ipsilatéral à la SMTr, une diminution de l’amplitude des OEAs immédiatement après la SMTr, et uniquement pour les fortes intensités d’utilisation de la SMTr (donc, pour des niveaux de bruit plus importants), reflétant un effet direct du bruit de la SMTr sur les mécanismes cochléaires actifs, effet présent chez nos sujets même en cas de protections auditives de bonne qualité / The ascending auditory system involves multiples stages where the auditory information is processed and modulated by a top-down influence involving multiple cortico sub-cortical loops: the efferent auditory system. It is hypothesized that the longest loop of this efferent system is able to modulate directly the sensory cells of the peripheral auditory organ. The aim of this work was to study this system, to our knowledge for the first time in healthy humans, and to show a direct cortical influence on the auditory periphery. In a double blind randomized procedure, we recorded, in 34 healthy subjects, the activity of outer hair cells of the organ of Corti (sensory cells, that are in direct synaptic contact with medial efferent fibers), by means of evoked otoacoustic emissions (OAEs), before and after auditory cortex stimulation by repetitive transcranial magnetic stimulation (rTMS). We showed an inhibitory influence of the auditory cortical stimulation by 10 Hz rTMS on OAE amplitudes, predominantly in the ear contralateral to the rTMS, effect that cannot be attributed to a non-specific effect linked to the impulse noise associated with the rTMS technique, but that remains highly variable across subjects. In addition, we showed a decrease in OAE amplitude on the ipsilateral side, immediately after the rTMS session and only for high rTMS intensities (and hence, higher noise levels), that reflect a direct influence of the rTMS noise on the auditory periphery, effect that is present even in well ear-protected subjects