CONTRALATERAL SUPPRESSION OF DISTORTION PRODUCT OTOACOUSTIC EMISSIONS IN CHILDREN WITH AND WITHOUT AUDITORY PROCESSING DISORDERS

This manuscript provides data on contralateral suppression of distortion product otoacoustic emissions in normal hearing children and children with Auditory Processing Disorders. Listeners included children 8 to 13 years-old. DPOAEs were elicited at three test frequencies around a narrow test ratio of f2/fi = 1.1 along with the traditional f2/fi = 1.22. Results suggest a frequency effect with suppression decreasing as f2 frequency increased, and a ratio effect with greater suppression at the narrow ratio. Additionally, no significant differences existed between normal children and previously obtained adult norms for measures of maximum suppression, mean suppression and maximum/mean suppression ratio. The APD grbup however, showed greater variance in these measures than normal children, reaching significance for maximum suppression at f2 = 3 kHz and f2/fi = 1.22. The large variance of the APD population may be of clinical interest pending a better understanding of the deficits underlying the disorder

Using otoacoustic emissions to evaluate efferent auditory function in humans

The auditory system continually adapts to changes in the acoustic environment over short periods of time. This fine-tuning of its dynamics is mediated in part by the medial olivocochlear (MOC) bundle, a neural feedback loop which aids in the regulation of cochlear micro-mechanics. The ability to measure the response of the MOC system in humans may provide significant insight into unique cochlear functions, such as its sharp frequency selectivity and wide dynamic range. In humans the efferent system can be investigated non-invasively using otoacoustic emissions (OAEs). However, how OAEs can best be used to evaluate efferent function, the pitfalls associated with such measurements, as well as the relationship between OAEs and perception are not fully understood. This dissertation presents three experiments that explore the use of OAEs to assess efferent function in humans. The first study examined the advantages of separating the major components of distortion product otoacoustic emissions (DPOAEs) when evaluating efferent function using contralateral acoustic stimulation (CAS). CAS-induced activation of the medial olivocochlear reflex (MOCR) was found to produce both reductions and enhancements of total DPOAE level. Analysis of the separated components of the DPOAE revealed that these changes could be accounted for by the contribution of an efferent-induced phase change in the reflection component of the DPOAE. In the second, analysis of DPOAE primary level and phase changes over a wide range of CAS levels used to induce MOCR revealed that middle ear muscle reflex (MEMR) activation could be simultaneously monitored. CAS levels commonly used to elicit MOCR could also elicit MEMR responses, which results in contamination of MOCR estimates. Finally, a novel technique to measure simultaneous OAEs and masked behavioral thresholds is presented and used to investigate a perceptual phenomenon thought to be associated with an efferent activation. While a direct association between physiological and behavioral masked thresholds was not observed, a strong relationship was found between the physiological measure of masked thresholds and a measure of CAS-induced efferent suppression, suggesting that although efferent-mediated suppression of basilar membrane mechanics is related to the phenomenon, more central mechanisms may be required to modulate the perceptual response

Individual Differences in Stimulus Frequency Otoacoustic Emission Phase

Otoacoustic emissions (OAEs) are sounds that originate in the cochlea and are measured in the ear canal. OAEs provide a noninvasive tool for investigating cochlear mechanics. Stimulus-frequency OAEs (SFOAEs) are evoked by presenting a single frequency tone, called a probe tone, which have an advantage over other OAEs because they are the least influenced by cochlear nonlinearities. However, because the SFOAE are generated in the cochlea with the same frequency as the stimulus, additional techniques, such as the use of suppressor tones are needed to enable separation of the probe tone from the SFOAE. The primary goal of this investigation was to explore individual differences in SFOAE phase gradient delays. These delays were hypothesized to improve estimates of cochlear health, inferred from hearing thresholds. Efficient measures of phase gradient delays can be obtained using frequency swept tones analyzed with time-frequency filtering, such as the least squares (LS) fit. The least squares fit is a time-frequency filter because the LS fit estimates coefficients for a subset of the total signal which are then used to separate and estimate signals of interest. However, the limitations of the frequency swept tone procedure and LS fit for estimating SFOAEs are not well understood. This investigation first focused on identifying limitations of such SFOAE and refining the LS fitting procedure. It was determined that including a suppressor was necessary for obtaining optimal SFOAE estimates, and the investigation shifted from further refining the LS fitting procedure to exploration of alternative time-frequency analyses which permit clearer characterization of the various latency contributions to suppressor based SFOAEs estimates. The use of a fast, continuous filtered wavelet transform provided a unique perspective on the distribution of SFOAE energy in the time-frequency domain and confirmed that SFOAEs are a sum of both long and short latency contributions. The distributions of long and short SFOAE energy explain some the discrepancies between discrete tone and swept tone SFOAEs procedures. Predicting behavioral thresholds from SFOAE phase, magnitude, or phase and magnitude combined may be misleading when the analysis is not focused around the SFOAE latency contributions from the region where SFOAEs are most affected by cochlear damage. It was revealed that more focus should be given to understanding the best ways to separate the long and the short latencies for different stimulus parameters and individuals, in order to improve sensitivity to cochlear health

Associations of the Medial Olivocochlear Reflex and Speech-In-Noise Abilities in Normal Hearing Adult Listeners: A Systematic Review

This systematic review analyzed the research concerning the medial olivocochlear reflex (MOCR) and speech-in-noise abilities in normal hearing adult listeners. In an attempt to understand the underlying difficulties in this population, the following research questions were proposed: 1) Does the research indicate that the magnitude of MOC suppression measured via OAEs is related to a normal hearing subject’s ability to recognize speech-in-noise? 2) Are MOC effects measured via OAEs lateralized? Is there a right ear advantage as suggested by Khalfa, Morlet, Micheyl, Morgon & Collet (1997)? Ten studies met the standards for inclusion for this review. Analysis of the research revealed some involvement of the MOCR in speech-in-noise abilities. However, the studies were mixed in their findings. Several studies did not find substantial correlations while others found significant favorable correlations. Interestingly, all of the studies that utilized speech-in-noise tests with words as the target stimuli found better speech recognition performance with increased MOC activity. In regards to laterality, the studies did not all point to a clear right ear advantage. The variability of the findings does not dampen the promise of potential clinical applications. Instead, they lay the groundwork for future controlled experiments that can confirm the involvement of MOCR in the discrimination of speech in the presence of background noise

Role of The Cochlea and Efferent System in Children with Auditory Processing Disorder

Auditory processing disorder (APD) is characterized by difficulty listening in noisy environments despite normal hearing thresholds. APD was previously thought to be restricted to deficits in the central auditory system. The current work sought to investigate brainstem and peripheral mechanisms that may contribute to difficulties in speech understanding in noise in children with suspected APD (sAPD). Three mechanisms in particular were investigated: cochlear tuning, efferent function, and spatial hearing. Cochlear tuning was measured using stimulus frequency otoacoustic emission (SFOAE) group delay. Results indicate that children suspected with APD have atypically sharp cochlear tuning, and reduced medial olivocochlear (MOC) functioning. Sharper-than-typical cochlear tuning may lead to increased forward masking. On the contrary, binaural efferent function probed with a forward masked click evoked OAE (CEOAE) paradigm indicated that MOC function was not different in typically developing (TD) children and children suspected with APD. A third study with multiple OAE types sought to address this contradiction. Despite numerically smaller MOC inhibition in the sAPD group, MOC function was not significantly different between the two groups. Finally, spatial release from masking, localization-in-noise and interaural time difference thresholds were compared in TD and children with sAPD. Results indicate no significant difference in spatial hearing abilities between the two groups. Non-significant findings at group level in these studies may be related to the large heterogeneity in problems associated with APD. Fragmentation of APD into deficit specific disorders may facilitate research in identification of the specific anatomical underpinnings to listening problems in APD. Prior to conducting studies in children, three studies were conducted to optimize stimulus characteristics. Results of these studies indicate that the MOC may not be especially sensitive to 100 Hz amplitude modulation, as previously reported. Click stimulus presentation rates \u3e25 Hz activate the ipsilateral MOC reflex in typical MOC assays, contaminating contralateral MOC inhibition of CEOAEs. Finally, localization-in-noise abilities of TD children are on par with adults for a white noise masker, but not for speech-babble. This finding suggests that despite maturation of physiological mechanisms required to localize in noise, non-auditory factors may restrict the ability of children in processing complex signals

Frequency specificity of contralateral, ipsilateral and bilateral medial olivocochlear acoustic reflexes in humans

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references.A variety of evidence indicates that the brain controls the gain of mechanical amplification in the cochlea in a frequency specific manner through the medial olivocochlear (MOC) efferent pathway, but the degree of MOC frequency specificity in humans is poorly understood. This thesis investigates the tuning properties of the human MOC acoustic reflex at different cochlear frequency regions and with different MOC-elicitor lateralities and frequency contents. Effects produced by the MOC reflex were quantified by the magnitude of the induced changes in stimulus frequency otoacoustic emissions (deltaSFOAEs) at probe frequencies of 0.5, 1 and 4 kHz. With MOC activity elicited by a mid-level (60 dB SPL) tone or half-octave-band of noise, significant MOC-induced deltaSFOAEs were seen over a wide range of elicitor frequencies, e.g. for elicitor frequencies at least 11/2 octaves away from each probe frequency. deltaSFOAE-versus-elicitor-frequency patterns were sometimes skewed so that elicitors at frequencies above (0.5 kHz probe) or below (1 kHz probe) the probe frequency were most effective. In contrast to the wide frequency range of MOC effects from mid-level elicitors, for 1 kHz probes MOC-effect tuning curves (TCs) were narrow with Ql0s of -2, sharper than the MOC-fiber TCs with best frequencies near I kHz in cats and guinea pigs. When MOC effects were looked at as the MOC-inhibited SFOAE relative to the original SFOAE, the SFOAE magnitude decreases and phase changes appeared to be separate functions of elicitor frequency: SFOAE magnitude inhibition was largest for on-frequency elicitors (elicitor frequencies near the probe frequency) while MOC-induced SFOAE phase leads were largest for off-frequency elicitors.(cont) One hypothesis to account for this is that on-frequency elicitors predominantly inhibit the traveling wave from the probe-tone, whereas off-frequency elicitors shift it along the frequency axis by selectively inhibiting apical or basal parts of the traveling-wave. These results are consistent with an anti-masking role of MOC efferents and suggest that MOC efferents do more than just provide feedback to a narrow frequency region around the elicitor frequency.by Watjana Lilaonitkul.Ph.D

Acoustic power flow into the ear and the auditory microstructure

An experimental technique to determine the acoustic power absorbed by the human ear at absolute threshold is described and applied to data recorded in adult subjects. A previously published method of electroacoustic probe calibration in terms of equivalent Thevenin source parameters is substantially ameliorated. Careful and detailed measurements of continuous tonal aural sound pressure (CTASP) are presented. Ear canal input impedance, reflectance and absolute power flow constituents are derived from CTASP data. Auditory microstructure, characterised by spectral periodicity, is observed and validated in CTASP, impedance, reflectance and power flow parameters at a 20 dB SPL stimulus level, but undetectable at 60 dB SPL. Periodicity in the ear canal acoustic parameters elicited at low stimulus levels is found to be commensurate with absolute threshold microstructure. An elementary analogue network model of the peripheral auditory system is formulated, enabling cochlear input impedance and reflectance to be inferred from ear canal acoustic parameters. At a 20 dB SPL stimulus level a non-zero cochlear reflectance is inferred, implying that energy propagates basally, as well as, apically. Microstructure amplitude in cochlear input impedance is shown to be 4 dB greater than that in ear canal input impedance, a consequence of decoupling of the probe from the tympanic membrane. A proportionality between transmittance and auditory sensitivity exists, implying that the ear couples more efficiently to the sound source, and consequently extracts proportionally more power, at peaks in sensitivity. However, the measured change in coupling is inadequate to wholly explain threshold microstructure. An explanation is offered by applying empirical data to a phenomenological model of power flow within the peripheral auditory system. It is argued that threshold microstructure arises predominately from a phasic interaction of the basalward and apical travelling waves effectively modifying the spatial distribution of energy within the cochlea