The effect of a precursor on growth of forward masking

Subcortical auditory model including efferent dynamic gain control with inputs from cochlear nucleus and inferior colliculus

An auditory model has been developed with a time-varying, gain-control signal based on the physiology of the efferent system and subcortical neural pathways. The medial olivocochlear (MOC) efferent stage of the model receives excitatory projections from fluctuation-sensitive model neurons of the inferior colliculus (IC) and wide-dynamic-range model neurons of the cochlear nucleus. The response of the model MOC stage dynamically controls cochlear gain via simulated outer hair cells. In response to amplitude-modulated (AM) noise, firing rates of most IC neurons with band-enhanced modulation transfer functions in awake rabbits increase over a time course consistent with the dynamics of the MOC efferent feedback. These changes in the rates of IC neurons in awake rabbits were employed to adjust the parameters of the efferent stage of the proposed model. Responses of the proposed model to AM noise were able to simulate the increasing IC rate over time, whereas the model without the efferent system did not show this trend. The proposed model with efferent gain control provides a powerful tool for testing hypotheses, shedding insight on mechanisms in hearing, specifically those involving the efferent system.

The effects of broadband elicitor duration on a psychoacoustic measure of cochlear gain reduction

Physiological and psychoacoustic studies of the medial olivocochlear reflex (MOCR) in humans have often relied on long duration elicitors (>100 ms). This is largely due to previous research using otoacoustic emissions (OAEs) that found multiple MOCR time constants, including time constants in the 100s of milliseconds, when elicited by broadband noise. However, the effect of the duration of a broadband noise elicitor on similar psychoacoustic tasks is currently unknown. The current study measured the effects of ipsilateral broadband noise elicitor duration on psychoacoustic gain reduction estimated from a forward-masking paradigm. Analysis showed that both masker type and elicitor duration were significant main effects, but no interaction was found. Gain reduction time constants were ∼46 ms for the masker present condition and ∼78 ms for the masker absent condition (ranging from ∼29 to 172 ms), both similar to the fast time constants reported in the OAE literature (70-100 ms). Maximum gain reduction was seen for elicitor durations of ∼200 ms. This is longer than the 50-ms duration which was found to produce maximum gain reduction with a tonal on-frequency elicitor. Future studies of gain reduction may use 150-200 ms broadband elicitors to maximally or near-maximally stimulate the MOCR.

Magnitude of the contralateral efferent olivocochlear effect as a function of the frequency

Background

The activation of the medial olivocochlear reflex reduces the cochlear gain, which is manifested perceptually as decreased auditory sensitivity. However, it has remained unclear whether the extent of this suppression varies according to the cochlear region involved. Here we aims to assess the magnitude of contralateral efferent suppression across human cochlea, at low levels, and its impact on hearing sensitivity.

Methods

Assuming that acoustic stimulation activates the contralateral medial olivocochlear reflex, we evaluated the magnitude of the suppressive effect as a function of frequency in 17 subjects with normal hearing. Absolute thresholds were measured for bursts tones of various durations (10, 100, and 500 ms) and frequencies (250, 500, 1000, 4000, and 8000 Hz) in the presence or absence of contralateral white noise at 60 dB SPL.

Results

We found that contralateral noise raised the absolute threshold for the burst tones evaluated. The effect was greater at lower than higher frequencies (3.85 dB at 250 Hz vs. 2.22 dB at 8000 Hz).

Conclusions

Our findings suggest that in humans, the magnitude of this suppression varies according to the cochlear region stimulated, with a greater effect towards the apex (lower frequencies) than the base (higher frequencies) of the cochlea.

Behavioral Measures of Cochlear Gain Reduction Depend on Precursor Frequency, Bandwidth, and Level

Sensory systems adjust to the environment to maintain sensitivity to change. In the auditory system, the medial olivocochlear reflex (MOCR) is a known physiological mechanism capable of such adjustment. The MOCR provides efferent feedback between the brainstem and cochlea, reducing cochlear gain in response to sound. The perceptual effects of the MOCR are not well understood, such as how gain reduction depends on elicitor characteristics in human listeners. Physiological and behavioral data suggest that ipsilateral MOCR tuning is only slightly broader than it is for afferent fibers, and that the fibers feed back to the frequency region of the cochlea that stimulated them. However, some otoacoustic emission (OAE) data suggest that noise is a more effective elicitor than would be consistent with sharp tuning, and that a broad region of the cochlea may be involved in elicitation. If the elicitor is processed in a cochlear channel centered at the signal frequency, the growth of gain reduction with elicitor level would be expected to depend on the frequency content of the elicitor. In the current study, the effects of the frequency content and level of a preceding sound (called a precursor) on signal threshold was examined. The results show that signal threshold increased with increasing precursor level at a shallower slope for a tonal precursor at the signal frequency than for a tonal precursor nearly an octave below the signal frequency. A broadband noise was only slightly more effective than a tone at the signal frequency, with a relatively shallow slope similar to that of the tonal precursor at the signal frequency. Overall, these results suggest that the excitation at the signal cochlear place, regardless of elicitor frequency, determines the magnitude of ipsilateral cochlear gain reduction, and that it increases with elicitor level.

The effect of broadband elicitor laterality on psychoacoustic gain reduction across signal frequency

There are psychoacoustic methods thought to measure gain reduction, which may be from the medial olivocochlear reflex (MOCR), a bilateral feedback loop that adjusts cochlear gain. Although studies have used ipsilateral and contralateral elicitors and have examined strength at different signal frequencies, these factors have not been examined within a single study. Therefore, basic questions about gain reduction, such as the relative strength of ipsilateral vs contralateral elicitation and the relative strength across signal frequency, are not known. In the current study, gain reduction from ipsilateral, contralateral, and bilateral elicitors was measured at 1-, 2-, and 4-kHz signal frequencies using forward masking paradigms at a range of elicitor levels in a repeated measures design. Ipsilateral and bilateral strengths were similar and significantly larger than contralateral strength across signal frequencies. Growth of gain reduction with precursor level tended to differ with signal frequency, although not significantly. Data from previous studies are considered in light of the results of this study. Behavioral results are also considered relative to anatomical and physiological data on the MOCR. These results indicate that, in humans, cochlear gain reduction is broad across frequencies and is robust for ipsilateral and bilateral elicitation but small for contralateral elicitation.

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

The relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition at positive and negative signal-to-noise ratios

Active mechanisms that regulate cochlear gain are hypothesized to influence speech-in-noise perception. However, evidence of a relationship between the amount of cochlear gain reduction and speech-in-noise recognition is mixed. Findings may conflict across studies because different signal-to-noise ratios (SNRs) were used to evaluate speech-in-noise recognition. Also, there is evidence that ipsilateral elicitation of cochlear gain reduction may be stronger than contralateral elicitation, yet, most studies have investigated the contralateral descending pathway. The hypothesis that the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition depends on the SNR was tested. A forward masking technique was used to quantify the ipsilateral cochlear gain reduction in 24 young adult listeners with normal hearing. Speech-in-noise recognition was measured with the PRESTO-R sentence test using speech-shaped noise presented at -3, 0, and +3 dB SNR. Interestingly, greater cochlear gain reduction was associated with lower speech-in-noise recognition, and the strength of this correlation increased as the SNR became more adverse. These findings support the hypothesis that the SNR influences the relationship between ipsilateral cochlear gain reduction and speech-in-noise recognition. Future studies investigating the relationship between cochlear gain reduction and speech-in-noise recognition should consider the SNR and both descending pathways.

Efferent unmasking of speech-in-noise encoding?

OBJECTIVE

The medial olivocochlear (MOC) reflex provides efferent feedback from the brainstem to cochlear outer hair cells. Physiologic studies have demonstrated that the MOC reflex is involved in "unmasking" of signals-in-noise at the level of the auditory nerve; however, its functional importance in human hearing remains unclear.

DESIGN

This study examined relationships between pre-neural measurements of MOC reflex strength (click-evoked otoacoustic emission inhibition; CEOAE) and neural measurements of speech-in-noise encoding (speech frequency following response; sFFR) in four conditions (Quiet, Contralateral Noise, Ipsilateral Noise, and Ipsilateral + Contralateral Noise). Three measures of CEOAE inhibition (amplitude reduction, effective attenuation, and input-output slope inhibition) were used to quantify pre-neural MOC reflex strength. Correlations between pre-neural MOC reflex strength and sFFR "unmasking" (i.e. response recovery from masking effects with activation of the MOC reflex in time and frequency domains) were assessed.

STUDY SAMPLE

18 young adults with normal hearing.

RESULTS

sFFR unmasking effects were insignificant, and there were no correlations between pre-neural MOC reflex strength and sFFR unmasking in the time or frequency domain.

CONCLUSION

Our results do not support the hypothesis that the MOC reflex is involved in speech-in-noise neural encoding, at least for features that are represented in the sFFR at the SNR tested.

Exploring the Role of Medial Olivocochlear Efferents on the Detection of Amplitude Modulation for Tones Presented in Noise

The medial olivocochlear reflex has been hypothesized to improve the detection and discrimination of dynamic signals in noisy backgrounds. This hypothesis was tested here by comparing behavioral outcomes with otoacoustic emissions. The effects of a precursor on amplitude-modulation (AM) detection were measured for a 1- and 6-kHz carrier at levels of 40, 60, and 80 dB SPL in a two-octave-wide noise masker with a level designed to produce poor, but above-chance, performance. Three types of precursor were used: a two-octave noise band, an inharmonic complex tone, and a pure tone. Precursors had the same overall level as the simultaneous noise masker that immediately followed the precursor. The noise precursor produced a large improvement in AM detection for both carrier frequencies and at all three levels. The complex tone produced a similarly large improvement in AM detection at the highest level but had a smaller effect for the two lower carrier levels. The tonal precursor did not significantly affect AM detection in noise. Comparisons of behavioral thresholds and medial olivocochlear efferent effects on stimulus frequency otoacoustic emissions measured with similar stimuli did not support the hypothesis that efferent-based reduction of cochlear responses contributes to the precursor effects on AM detection.

Long-standing problems in speech perception dissolve within an information-theoretic perspective

An information theoretic framework is proposed to have the potential to dissolve (rather than attempt to solve) multiple long-standing problems concerning speech perception. By this view, speech perception can be reframed as a series of processes through which sensitivity to information-that which changes and/or is unpredictable-becomes increasingly sophisticated and shaped by experience. Problems concerning appropriate objects of perception (gestures vs. sounds), rate normalization, variance consequent to articulation, and talker normalization are reframed, or even dissolved, within this information-theoretic framework. Application of discriminative models founded on information theory provides a productive approach to answer questions concerning perception of speech, and perception most broadly.

The effects of preceding sound and stimulus duration on measures of suppression in younger and older adults

Despite clinically normal audiometric thresholds, some older adults may experience difficulty in tasks such as understanding speech in a noisy environment. One potential reason may be reduced cochlear nonlinearity. A sensitive measure of cochlear nonlinearity is two-tone suppression, which is a reduction in the auditory system's response to one tone in the presence of a second tone. Previous research has been mixed on whether suppression decreases with age in humans. Studies of efferent cochlear gain reduction also suggest that stimulus duration should be considered in measuring suppression. In the present study, suppression was first measured psychoacoustically using stimuli that were too short to result in gain reduction. The potential effect of efferent cochlear gain reduction was then measured by using longer stimuli and presenting tonal or noise precursors before the shorter stimuli. Younger adults (ages 19-22 yr) and older adults (ages 57+ yr) with clinically normal hearing were tested. Suppression estimates decreased with longer stimuli or preceding sound which included the signal frequency, but did not decrease with preceding sound at the suppressor frequency. On average, the older group had lower suppression than the younger group, but this difference was not statistically significant.

Evidence for Gain Reduction by a Precursor in an On-Frequency Forward Masking Paradigm

A forward masking technique was used to measure cochlear gain reduction which might be consistent with the medial olivocochlear reflex (MOCR). A 4-kHz signal was set at 20 dB SL, and an on-frequency forward masker adjusted to just mask the signal. Adding a pink noise precursor before the signal and masker increased the level of the masker needed to mask the signal, in contrast to what would be expected from theories such as additivity of masking. The magnitude and pattern of this increase was similar to the increase in signal threshold seen with an off-frequency masker following a precursor.

Effects of Masker Envelope Fluctuations on the Temporal Effect

Under certain conditions, detection thresholds in simultaneous masking improve when the onset of a short sinusoidal probe is delayed from the onset of a long masker. This improvement, known as the temporal effect, is largest for broadband maskers and is smaller or absent for narrowband maskers centered on the probe frequency. This study tests the hypothesis that small or absent temporal effects for narrowband maskers are due to the inherent temporal envelope fluctuations of Gaussian noise. Temporal effects were measured for narrowband noise maskers with fluctuating ("fluctuating maskers") and flattened ("flattened maskers") temporal envelopes as a function of masker level (Exp. I) and in the presence of fluctuating and flattened precursors (Exp. II). The temporal effect was absent for fluctuating narrowband maskers and as large as ~ 7 dB for flattened narrowband maskers. The AC-coupled power of the temporal envelopes of precursors and maskers accounted for 94 % of the variance in probe detection thresholds measured with fluctuating and flattened precursors and maskers. These results suggest that masker temporal envelope fluctuations contribute to the temporal effect and should be considered in future modeling efforts.

Psychoacoustic measurements of ipsilateral cochlear gain reduction as a function of signal frequency

Forward masking experiments at 4 kHz have demonstrated that preceding sound can elicit changes in masking patterns consistent with a change in cochlear gain. However, the acoustic environment is filled with complex sounds, often dominated by lower frequencies, and ipsilateral cochlear gain reduction at frequencies below 4 kHz is largely unstudied in the forward masking literature. In this experiment, the magnitude of ipsilateral cochlear gain reduction was explored at 1, 2, and 4 kHz using forward masking techniques in an effort to evaluate a range of frequencies in listeners with normal hearing. Gain reduction estimates were not significantly different at 2 and 4 kHz using two forward masking measurements. Although the frequency was a significant factor in the analysis, post hoc testing supported the interpretation that gain reduction estimates measured without a masker were not significantly different at 1, 2, and 4 kHz. A second experiment provided evidence that forward masking in this paradigm at 1 kHz cannot be explained by excitation alone. This study provides evidence of ipsilateral cochlear gain reduction in humans at frequencies below the 4 kHz region.

Amplitude modulation detection with a short-duration carrier: Effects of a precursor and hearing loss

This study tests the hypothesis that amplitude modulation (AM) detection will be better under conditions where basilar membrane (BM) response growth is expected to be linear rather than compressive. This hypothesis was tested by (1) comparing AM detection for a tonal carrier as a function of carrier level for subjects with and without cochlear hearing impairment (HI), and by (2) comparing AM detection for carriers presented with and without an ipsilateral notched-noise precursor, under the assumption that the precursor linearizes BM responses. Average AM detection thresholds were approximately 5 dB better for subjects with HI than for subjects with normal hearing (NH) at moderate-level carriers. Average AM detection for low-to-moderate level carriers was approximately 2 dB better with the precursor than without the precursor for subjects with NH, whereas precursor effects were absent or smaller for subjects with HI. Although effect sizes were small and individual differences were noted, group differences are consistent with better AM detection for conditions where BM responses are less compressive due to cochlear hearing loss or due to a reduction in cochlear gain. These findings suggest the auditory system may quickly adjust to the local soundscape to increase effective AM depth and improve signal-to-noise ratios.

Olivocochlear Efferents in Animals and Humans: From Anatomy to Clinical Relevance

Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. This article reviews the current knowledge on olivocochlear efferents, with emphasis on human medial efferents. The review covers (1) the anatomy and physiology of olivocochlear efferents in animals; (2) the methods used for investigating this auditory feedback system in humans, their limitations and best practices; (3) the characteristics of medial-olivocochlear efferents in humans, with a critical analysis of some discrepancies across human studies and between animal and human studies; (4) the possible roles of olivocochlear efferents in hearing, discussing the evidence in favor and against their role in facilitating the detection of signals in noise and in protecting the auditory system from excessive acoustic stimulation; and (5) the emerging association between abnormal olivocochlear efferent function and several health conditions. Finally, we summarize some open issues and introduce promising approaches for investigating the roles of efferents in human hearing using cochlear implants.

Effect of Contralateral Medial Olivocochlear Feedback on Perceptual Estimates of Cochlear Gain and Compression

The active cochlear mechanism amplifies responses to low-intensity sounds, compresses the range of input sound intensities to a smaller output range, and increases cochlear frequency selectivity. The gain of the active mechanism can be modulated by the medial olivocochlear (MOC) efferent system, creating the possibility of top-down control at the earliest level of auditory processing. In humans, MOC function has mostly been measured by the suppression of otoacoustic emissions (OAEs), typically as a result of MOC activation by a contralateral elicitor sound. The exact relationship between OAE suppression and cochlear gain reduction, however, remains unclear. Here, we measured the effect of a contralateral MOC elicitor on perceptual estimates of cochlear gain and compression, obtained using the established temporal masking curve (TMC) method. The measurements were taken at a signal frequency of 2 kHz and compared with measurements of click-evoked OAE suppression. The elicitor was a broadband noise, set to a sound pressure level of 54 dB to avoid triggering the middle ear muscle reflex. Despite its low level, the elicitor had a significant effect on the TMCs, consistent with a reduction in cochlear gain. The amount of gain reduction was estimated as 4.4 dB on average, corresponding to around 18 % of the without-elicitor gain. As a result, the compression exponent increased from 0.18 to 0.27.

The role of compression in the simultaneous masker phase effect

Peripheral compression is believed to play a major role in the masker phase effect (MPE). While compression is almost instantaneous, activation of the efferent system reduces compression in a temporally evolving manner. To study the role of efferent-controlled compression in the MPE, in experiment 1, simultaneous masking of a 30-ms 4-kHz tone by 40-ms Schroeder-phase harmonic complexes was measured with on- and off-frequency precursors as a function of masker phase curvature for two masker levels (60 and 90 dB sound pressure level). The MPE was quantified by the threshold range [min/max difference (MMD)] across the phase curvatures. For the 60-dB condition, the presence of on-frequency precursor decreased the MMD from 10 to 5 dB. Experiment 2 studied the role of the precursor on the auditory filter's bandwidth. The on-frequency precursor was found to increase the bandwidth, an effect incorporated in the subsequent modeling. A model of the auditory periphery including cochlear filtering and basilar membrane compression generally underestimated the MMDs. A model based on two-step compression, including compression of inner hair cells, accounted for the MMDs across precursor and level conditions. Overall, the observed precursor effects and the model predictions suggest an important role of compression in the simultaneous MPE.

Effect of human auditory efferent feedback on cochlear gain and compression

The mammalian auditory system includes a brainstem-mediated efferent pathway from the superior olivary complex by way of the medial olivocochlear system, which reduces the cochlear response to sound (Warr and Guinan, 1979; Liberman et al., 1996). The human medial olivocochlear response has an onset delay of between 25 and 40 ms and rise and decay constants in the region of 280 and 160 ms, respectively (Backus and Guinan, 2006). Physiological studies with nonhuman mammals indicate that onset and decay characteristics of efferent activation are dependent on the temporal and level characteristics of the auditory stimulus (Bacon and Smith, 1991; Guinan and Stankovic, 1996). This study uses a novel psychoacoustical masking technique using a precursor sound to obtain a measure of the efferent effect in humans. This technique avoids confounds currently associated with other psychoacoustical measures. Both temporal and level dependency of the efferent effect was measured, providing a comprehensive measure of the effect of human auditory efferents on cochlear gain and compression. Results indicate that a precursor (>20 dB SPL) induced efferent activation, resulting in a decrease in both maximum gain and maximum compression, with linearization of the compressive function for input sound levels between 50 and 70 dB SPL. Estimated gain decreased as precursor level increased, and increased as the silent interval between the precursor and combined masker-signal stimulus increased, consistent with a decay of the efferent effect. Human auditory efferent activation linearizes the cochlear response for mid-level sounds while reducing maximum gain.

Exploring the role of feedback-based auditory reflexes in forward masking by schroeder-phase complexes

Several studies have postulated that psychoacoustic measures of auditory perception are influenced by efferent-induced changes in cochlear responses, but these postulations have generally remained untested. This study measured the effect of stimulus phase curvature and temporal envelope modulation on the medial olivocochlear reflex (MOCR) and on the middle-ear muscle reflex (MEMR). The role of the MOCR was tested by measuring changes in the ear-canal pressure at 6 kHz in the presence and absence of a band-limited harmonic complex tone with various phase curvatures, centered either at (on-frequency) or well below (off-frequency) the 6-kHz probe frequency. The influence of possible MEMR effects was examined by measuring phase-gradient functions for the elicitor effects and by measuring changes in the ear-canal pressure with a continuous suppressor of the 6-kHz probe. Both on- and off-frequency complex tone elicitors produced significant changes in ear canal sound pressure. However, the pattern of results was not consistent with the earlier hypotheses postulating that efferent effects produce the psychoacoustic dependence of forward-masked thresholds on masker phase curvature. The results also reveal unexpectedly long time constants associated with some efferent effects, the source of which remains unknown.

Computational modeling of individual differences in behavioral estimates of cochlear nonlinearities

Temporal masking curves (TMCs) are often used to estimate cochlear compression in individuals with normal and impaired hearing. These estimates may yield a wide range of individual differences, even among subjects with similar quiet thresholds. This study used an auditory model to assess potential sources of variance in TMCs from 51 listeners in Poling et al. [J Assoc Res Otolaryngol, 13:91-108 (2012)]. These sources included threshold elevation, the contribution of outer and inner hair cell dysfunction to threshold elevation, compression of the off-frequency linear reference, and detection efficiency. Simulations suggest that detection efficiency is a primary factor contributing to individual differences in TMCs measured in normal-hearing subjects, while threshold elevation and the contribution of outer and inner hair cell dysfunction are primary factors in hearing-impaired subjects. Approximating the most compressive growth rate of the cochlear response from TMCs was achieved only in subjects with the highest detection efficiency. Simulations included off-frequency nonlinearity in basilar membrane and inner hair cell processing; however, this nonlinearity did not improve predictions, suggesting that other sources, such as the decay of masking and the strength of the medial olivocochlear reflex, may mimic off-frequency nonlinearity. Findings from this study suggest that sources of individual differences can play a strong role in behavioral estimates of compression, and these sources should be considered when using forward masking to study cochlear function in individual listeners or across groups of listeners.

Accounting for nonmonotonic precursor duration effects with gain reduction in the temporal window model

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

Contralateral efferent reflex effects on threshold and suprathreshold psychoacoustical tuning curves at low and high frequencies

Medial olivocochlear efferent neurons can control cochlear frequency selectivity and may be activated in a reflexive manner by contralateral sounds. The present study investigated the significance of the contralateral medial olivocochlear reflex (MOCR) on human psychoacoustical tuning curves (PTCs), a behavioral correlate of cochlear tuning curves. PTCs were measured using forward masking in the presence and in the absence of a contralateral white noise, assumed to elicit the MOCR. To assess MOCR effects on apical and basal cochlear regions over a wide range of sound levels, PTCs were measured for probe frequencies of 500 Hz and 4 kHz and for near- and suprathreshold conditions. Results show that the contralateral noise affected the PTCs predominantly at 500 Hz. At near-threshold levels, its effect was obvious only for frequencies in the tails of the PTCs; at suprathreshold levels, its effects were obvious for all frequencies. It was verified that the effects were not due to the contralateral noise activating the middle-ear muscle reflex or changing the postmechanical rate of recovery from forward masking. A phenomenological computer model of forward masking with efferent control was used to explain the data. The model supports the hypothesis that the behavioral results were due to the contralateral noise reducing apical cochlear gain in a frequency- and level-dependent manner consistent with physiological evidence. Altogether, this shows that the contralateral MOCR may be changing apical cochlear responses in natural, binaural listening situations.

Estimating peripheral gain and compression using fixed-duration masking curves

Estimates of human basilar membrane gain and compression obtained using temporal masking curve (TMC) and additivity of forward masking (AFM) methods with long-duration maskers or long masker-signal silent intervals may be affected by olivocochlear efferent activation, which reduces basilar membrane gain. The present study introduces a fixed-duration masking curve (FDMC) method, which involves a comparison of off- and on-frequency forward masker levels at threshold as a function of masker and signal duration, with the total masker-signal duration fixed at 25 ms to minimize efferent effects. Gain and compression estimates from the FDMC technique were compared with those from TMC (104-ms maskers) and AFM (10- and 200-ms maskers) methods. Compression estimates over an input-masker range of 40-60 dB sound pressure level were similar for the four methods. Maximum compression occurred at a lower input level for the FDMC compared to the TMC method. Estimates of gain were similar for TMC and FDMC methods. The FDMC method may provide a more reliable estimate of BM gain and compression in the absence of efferent activation and could be a useful method for estimating effects of efferent activity when used with a precursor sound (to trigger efferent activation), presented prior to the combined masker-signal stimulus.

Evaluating the effects of olivocochlear feedback on psychophysical measures of frequency selectivity

Frequency selectivity was evaluated under two conditions designed to assess the influence of a "precursor" stimulus on auditory filter bandwidths. The standard condition consisted of a short masker, immediately followed by a short signal. The precursor condition was identical except a 100-ms sinusoid at the signal frequency (i.e., the precursor) was presented before the masker. The standard and precursor conditions were compared for measurements of psychophysical tuning curves (PTCs), and notched noise tuning characteristics. Estimates of frequency selectivity were significantly broader in the precursor condition. In the second experiment, PTCs in the standard and precursor conditions were simulated to evaluate the influence of the precursor on PTC bandwidth. The model was designed to account for the influence of additivity of masking between the masker and precursor. Model simulations were able to qualitatively account for the perceptual data when outer hair cell gain of the model was reduced in the precursor condition. These findings suggest that the precursor may have reduced cochlear gain, in addition to producing additivity of masking. This reduction in gain may be mediated by the medial olivocochlear reflex.

Auditory filter tuning inferred with short sinusoidal and notched-noise maskers

The physiology of the medial olivocochlear reflex suggests that a sufficiently long stimulus (>100 ms) may reduce cochlear gain and result in broadened frequency selectivity. The current study attempted to avoid gain reduction by using short maskers (20 ms) to measure psychophysical tuning curves (PTCs) and notched-noise tuning characteristics, with a 4-kHz signal. The influence of off-frequency listening on PTCs was evaluated using two types of background noise. Iso-level curves were derived using an estimate of the cochlear input/output (I/O) function, which was obtained using an off-frequency masker as a linear reference. The influence of masker duration on PTCs was assessed using a model that assumed long maskers (>20 ms) evoked gain reduction. The results suggested that the off-frequency masker was a valid linear reference when deriving I/O functions and that off-frequency listening may have occurred in auditory filters apical to the signal place. The iso-level curves from this growth-of-masking study were consistent with those from a temporal-masking-curve study by Eustaquio-Martin and Lopez-Poveda [J. Assoc. Res. Otolaryngol. 12, 281-299. (2011)], suggesting that either approach may be used to derive iso-level curves. Finally, model simulations suggested that masker duration may not influence estimates of frequency selectivity.

What is the role of the medial olivocochlear system in speech-in-noise processing?

The medial olivocochlear (MOC) bundle reduces the gain of the cochlear amplifier through reflexive activation by sound. Physiological results indicate that MOC-induced reduction in cochlear gain can enhance the response to signals when presented in masking noise. Some previous studies suggest that this "antimasking" effect of the MOC system plays a role in speech-in-noise perception. The present study set out to reinvestigate this hypothesis by correlating measures of MOC activity and speech-in-noise processing across a group of normal-hearing participants. MOC activity was measured using contralateral suppression of otoacoustic emissions (OAEs), and speech-in-noise processing was measured by measuring the effect of noise masking on performance in a consonant-vowel (CV) discrimination task and on auditory brain stem responses evoked by a CV syllable. Whereas there was a significant correlation between OAE suppression and both measures of speech-in-noise processing, the direction of this correlation was opposite to that predicted by the antimasking hypothesis, in that individuals with stronger OAE suppression tended to show greater noise-masking effects on CV processing. The current results indicate that reflexive MOC activation is not always beneficial to speech-in-noise processing. We propose an alternative to the antimasking hypothesis, whereby the MOC system benefits speech-in-noise processing through dynamic (e.g., attention- and experience-dependent), rather than reflexive, control of cochlear gain.

Behavioral estimates of basilar-membrane compression: additivity of forward masking in noise-masked normal-hearing listeners

Cochlear hearing loss is often associated with a loss of basilar-membrane (BM) compression, which in turn may contribute to degraded processing of suprathreshold stimuli. Behavioral estimates of compression may therefore be useful as long as they are valid over a wide range of levels and frequencies. Additivity of forward masking (AFM) may provide such a measure, but research to date lacks normative data from normal-hearing (NH) listeners at high sound levels, which is necessary to evaluate data from hearing-impaired (HI) listeners. The present study measured AFM in six NH listeners for signal frequencies of 500, 1500, and 4000 Hz in the presence of background noise, designed to elevate signal thresholds to levels similar to those experienced by HI listeners. Results consistent with compressive BM responses were found for all six listeners at 500 Hz, five listeners at 1500 Hz, but only two listeners at 4000 Hz. Further measurements in the absence of background noise also indicated a lack of consistent compression at 4000 Hz at higher signal levels, in contrast to earlier results collected at lower levels. A better understanding of this issue will be required before AFM can be used as a general behavioral estimate of BM compression.

The time course of cochlear gain reduction measured using a more efficient psychophysical technique

In a previous study it was shown that an on-frequency precursor intended to activate the medial olivocochlear reflex (MOCR) at the signal frequency reduces the gain estimated from growth-of-masking (GOM) functions. This is called the temporal effect (TE). In Expt. 1 a shorter method of measuring this change in gain is established. GOM functions were measured with an on- and off-frequency precursor presented before the masker and signal, and used to estimate Input/Output functions. The change in gain estimated in this way was very similar to that estimated from comparing two points measured with a single fixed masker level on the lower legs of the GOM functions. In Expt. 2, the TE was measured as a function of precursor duration and signal delay. For short precursor durations and short delays the TE increased (buildup) or remained constant as delay increased, then decreased. The TE also increased with precursor duration for the shortest delay. The results were fitted with a model based on the time course of the MOCR. The model fitted the data well, and predicted the buildup. This buildup is not consistent with exponential decay predicted by neural adaptation or persistence of excitation.

On- and off-frequency compression estimated using a new version of the additivity of forward masking technique

On- and off-frequency compression at the 4000- and 8000-Hz cochlear places were estimated using a new version of the additivity of forward masking (AFM) technique, that measures the effects of combining two non-overlapping forward maskers. Instead of measuring signal thresholds to estimate compression of the signal as in the original AFM technique, the decrease in masker threshold in the combined-masker condition compared to the individual-masker conditions is used to estimate compression of the masker at the signal place. By varying masker frequency it is possible to estimate off-frequency compression. The maskers were 500-Hz-wide bands of noise, and the signal was a brief pure tone. Compression at different levels was estimated using different overall signal levels, or different masker-signal intervals. It was shown that the new AFM technique and the original AFM technique produce consistent results. Considerable compression was observed for maskers well below the signal frequency, suggesting that the assumption of off-frequency linearity used in other techniques may not be valid. Reducing the duration of the first masker from 200 to 20 ms reduced the compression exponent in some cases, suggesting a possible influence of olivocochlear efferent activity.

Effects of background noise level on behavioral estimates of basilar-membrane compression

Hearing-impaired (HI) listeners often show poorer performance on psychoacoustic tasks than do normal-hearing (NH) listeners. Although some such deficits may reflect changes in suprathreshold sound processing, others may be due to stimulus audibility and the elevated absolute thresholds associated with hearing loss. Masking noise can be used to raise the thresholds of NH to equal the thresholds in quiet of HI listeners. However, such noise may have other effects, including changing peripheral response characteristics, such as the compressive input-output function of the basilar membrane in the normal cochlea. This study estimated compression behaviorally across a range of background noise levels in NH listeners at a 4 kHz signal frequency, using a growth of forward masking paradigm. For signals 5 dB or more above threshold in noise, no significant effect of broadband noise level was found on estimates of compression. This finding suggests that broadband noise does not significantly alter the compressive response of the basilar membrane to sounds that are presented well above their threshold in the noise. Similarities between the performance of HI listeners and NH listeners in threshold-equalizing noise are therefore unlikely to be due to a linearization of basilar-membrane responses to suprathreshold stimuli in the NH listeners.

Precursor effects on behavioral estimates of frequency selectivity and gain in forward masking

The experiments presented in this paper explore the hypothesis that cochlear gain is reduced, in a frequency-specific manner, over the course of a sound (called a "precursor") which was designed to activate the medial olivo-cochlear reflex (MOCR). Psychophysical tuning curves (PTCs) and off-frequency growth of masking (GOM) functions were measured with two precursors. The on-frequency precursor condition, which was hypothesized to activate the MOCR at the signal frequency, produced a PTC with a lower best frequency in all subjects consistent with less gain. This same condition produced a GOM function with less gain and an elevated compression breakpoint. The data were analyzed with two models. The gain-reduction model, which assumed a change in the basilar membrane input-output function, was superior at predicting the data relative to a model of additivity of masking.