The relationship between frequency selectivity and overshoot

Effects of ipsilateral, contralateral, and bilateral noise precursors on psychoacoustical tuning curves in humans

Cochlear tuning and hence auditory frequency selectivity are thought to change in noisy environments by activation of the medial olivocochlear reflex (MOCR). In humans, auditory frequency selectivity is often assessed using psychoacoustical tuning curves (PTCs), a plot of the level required for pure-tone maskers to just mask a fixed-level pure-tone probe as a function of masker frequency. Sometimes, however, the stimuli used to measure a PTC are long enough that they can activate the MOCR by themselves and thus affect the PTC. Here, PTCs for probe frequencies of 500 Hz and 4 kHz were measured in forward masking using short maskers (30 ms) and probes (10 ms) to minimize the activation of the MOCR by the maskers or the probes. PTCs were also measured in the presence of long (300 ms) ipsilateral, contralateral, and bilateral broadband noise precursors to investigate the effect of the ipsilateral, contralateral, and bilateral MOCR on PTC tuning. Four listeners with normal hearing participated in the experiments. At 500 Hz, ipsilateral and bilateral precursors sharpened the PTCs by decreasing the thresholds for maskers with frequencies at or near the probe frequency with minimal effects on thresholds for maskers remote in frequency from the probe. At 4 kHz, by contrast, ipsilateral and bilateral precursors barely affected thresholds for maskers near the probe frequency but broadened PTCs by reducing thresholds for maskers far from the probe. Contralateral precursors barely affected PTCs. An existing computational model was used to interpret the results. The model suggested that despite the apparent differences, the pattern of results is consistent with the ipsilateral and bilateral MOCR inhibiting the cochlear gain similarly at the two probe frequencies and more strongly than the contralateral MOCR.

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.

Adaptation to noise in normal and impaired hearing

Many aspects of hearing function are negatively affected by background noise. Listeners, however, have some ability to adapt to background noise. For instance, the detection of pure tones and the recognition of isolated words embedded in noise can improve gradually as tones and words are delayed a few hundred milliseconds in the noise. While some evidence suggests that adaptation to noise could be mediated by the medial olivocochlear reflex, adaptation can occur for people who do not have a functional reflex. Since adaptation can facilitate hearing in noise, and hearing in noise is often harder for hearing-impaired than for normal-hearing listeners, it is conceivable that adaptation is impaired with hearing loss. It remains unclear, however, if and to what extent this is the case, or whether impaired adaptation contributes to the greater difficulties experienced by hearing-impaired listeners understanding speech in noise. Here, we review adaptation to noise, the mechanisms potentially contributing to this adaptation, and factors that might reduce the ability to adapt to background noise, including cochlear hearing loss, cochlear synaptopathy, aging, and noise exposure. The review highlights few knowns and many unknowns about adaptation to noise, and thus paves the way for further research on this topic.

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.

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.

Masking of short tones in noise: Evidence for envelope-based, rather than energy-based detection

The "temporal effect" in simultaneous masking may be characterized by better probe detection thresholds for a short, tonal probe presented at the temporal center of a masker compared to at the onset of a masker. Energy-based models of masking have been used to interpret the temporal effect as evidence that the gain of the auditory system decreases during acoustic stimulation. This study shows that masking from temporal-envelope fluctuations of a precursor or from a temporal gap between stimuli violates the assumptions of energy-based models and complicates the interpretation of temporal effects in terms of a reduction in gain. Detection thresholds were measured for a 6-ms, 4000-Hz probe preceded by a narrowband precursor and presented 2-, 197-, or 392-ms after the onset of a narrowband masker. The delay between the precursor offset and masker onset ranged from -2 to 250 ms. Probe thresholds were elevated in the presence of precursors with fluctuating compared to flattened temporal envelopes and when a temporal gap was inserted between the precursor and masker. The results suggest that the interpretation and design of temporal-effect studies should consider the masking effects of temporal-envelope fluctuations. These findings are consistent with speech-perception experiments that show masking from temporal-envelope fluctuations.

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.

Partial loudness at masker onset indicates temporal effects at supra-threshold levels

This study examines temporal effects both at threshold and at supra-threshold levels. The level needed to detect a short-duration 4.0-kHz signal was measured for signals presented with different onset delays relative to a 300-ms broadband noise masker: 100 ms and 5 ms before the onset of the masker and 5 ms and 100 ms after the onset of the masker. Loudness matches between the signal in quiet and the signal at the same four onset delays were obtained for five presentation levels of the short-duration signal and for three masker levels. The temporal effect was defined as the level difference between the signals near masker onset and the signals well before or well after masker onset, needed to reach threshold and/or achieve equal loudness. Both at threshold and at supra-threshold levels temporal effects were observed consistent with a decrease in gain at the masker frequency during the course of the masker. The temporal effect was not restricted to simultaneous masking, but was also found for backward masking. In both cases the temporal effects were stronger at supra-threshold levels than at threshold. This may be caused by a transient effect at masker onset. The almost simultaneous onset of the signal and the masker makes it difficult for subjects to separate signal from the masker, especially when the signal level is close to masked threshold.

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.

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.

Temporal Effects on Monaural Amplitude-Modulation Sensitivity in Ipsilateral, Contralateral and Bilateral Noise

The amplitude modulations (AMs) in speech signals are useful cues for speech recognition. Several adaptation mechanisms may make the detection of AM in noisy backgrounds easier when the AM carrier is presented later rather than earlier in the noise. The aim of the present study was to characterize temporal adaptation to noise in AM detection. AM detection thresholds were measured for monaural (50 ms, 1.5 kHz) pure-tone carriers presented at the onset ('early' condition) and 300 ms after the onset ('late' condition) of ipsilateral, contralateral, and bilateral (diotic) broadband noise, as well as in quiet. Thresholds were 2-4 dB better in the late than in the early condition for the three noise lateralities. The temporal effect held for carriers at equal sensation levels, confirming that it was not due to overshoot on carrier audibility. The temporal effect was larger for broadband than for low-band contralateral noises. Many aspects in the results were consistent with the noise activating the medial olivocochlear reflex (MOCR) and enhancing AM depth in the peripheral auditory response. Other aspects, however, indicate that central masking and adaptation unrelated to the MOCR also affect both carrier-tone and AM detection and are involved in the temporal effects.

Assessment of Ipsilateral Efferent Effects in Human via ECochG

Development of electrophysiological means to assess the medial olivocochlear (MOC) system in humans is important to further our understanding of the function of that system and for the refinement and validation of psychoacoustical and otoacoustic emission methods which are thought to probe the MOC. Based on measurements in anesthetized animals it has been hypothesized that the MOC-reflex (MOCR) can enhance the response to signals in noise, and several lines of evidence support such a role in humans. A difficulty in these studies is the isolation of efferent effects. Efferent activation can be triggered by acoustic stimulation of the contralateral or ipsilateral ear, but ipsilateral stimulation is thought to be more effective. However, ipsilateral stimulation complicates interpretation of effects since these sounds can affect the perception of other ipsilateral sounds by mechanisms not involving olivocochlear efferents. We assessed the ipsilaterally evoked MOCR in human using a transtympanic procedure to record mass-potentials from the cochlear promontory or the niche of the round window. Averaged compound action potential (CAP) responses to masked probe tones of 4 kHz with and without a precursor (designed to activate the MOCR but not the stapedius reflex) were extracted with a polarity alternating paradigm. The masker was either a simultaneous narrow band noise masker or a short (20-ms) tonal ON- or OFF-frequency forward masker. The subjects were screened for normal hearing (audiogram, tympanogram, threshold stapedius reflex) and psychoacoustically tested for the presence of a precursor effect. We observed a clear reduction of CAP amplitude by the precursor, for different masking conditions. Even without an MOCR, this is expected because the precursor will affect the response to subsequent stimuli via neural adaptation. To determine whether the precursor also activated the efferent system, we measured the CAP over a range of masker levels, with or without precursor, and for different types of masker. The results show CAP reduction consistent with the type of gain reduction caused by the MOCR. These results generally support psychoacoustical paradigms designed to probe the efferent system as indeed activating the MOCR system, but not all observations are consistent with this mechanism.

Spatial and temporal disparity in signals and maskers affects signal detection in non-human primates

Detection thresholds for auditory stimuli (signals) increase in the presence of maskers. Natural environments contain maskers/distractors that can have a wide range of spatiotemporal properties relative to the signal. While these parameters have been well explored psychophysically in humans, they have not been well explored in animal models, and their neuronal underpinnings are not well understood. As a precursor to the neuronal measurements, we report the effects of systematically varying the spatial and temporal relationship between signals and noise in macaque monkeys (Macaca mulatta and Macaca radiata). Macaques detected tones masked by noise in a Go/No-Go task in which the spatiotemporal relationships between the tone and noise were systematically varied. Masked thresholds were higher when the masker was continuous or gated on and off simultaneously with the signal, and lower when the continuous masker was turned off during the signal. A burst of noise caused higher masked thresholds if it completely temporally overlapped with the signal, whereas partial overlap resulted in lower thresholds. Noise durations needed to be at least 100 ms before significant masking could be observed. Thresholds for short duration tones were significantly higher when the onsets of signal and masker coincided compared to when the signal was presented during the steady state portion of the noise (overshoot). When signal and masker were separated in space, masked signal detection thresholds decreased relative to when the masker and signal were co-located (spatial release from masking). Masking release was larger for azimuthal separations than for elevation separations. These results in macaques are similar to those observed in humans, suggesting that the specific spatiotemporal relationship between signal and masker determine threshold in natural environments for macaques in a manner similar to humans. These results form the basis for future investigations of neuronal correlates and mechanisms of masking.

Notched-noise precursors improve detection of low-frequency amplitude modulation

Amplitude modulation (AM) detection was measured with a short (50 ms), high-frequency carrier as a function of carrier level (Experiment I) and modulation frequency (Experiment II) for conditions with or without a notched-noise precursor. A longer carrier (500 ms) was also included in Experiment I. When the carrier was preceded by silence (no precursor condition) AM detection thresholds worsened for moderate-level carriers compared to lower- or higher-level carriers, resulting in a "mid-level hump." AM detection thresholds with a precursor were better than those without a precursor, primarily for moderate-to-high level carriers, thus eliminating the mid-level hump in AM detection. When the carrier was 500 ms, AM thresholds improved by a constant (across all levels) relative to AM thresholds with a precursor, consistent with the longer carrier providing more "looks" to detect the AM signal. Experiment II revealed that improved AM detection with compared to without a precursor is limited to low-modulation frequencies (<60 Hz). These results are consistent with (1) a reduction in cochlear gain over the course of the precursor perhaps via the medial olivocochlear reflex or (2) a form of perceptual enhancement which may be mediated by adaptation of inhibition.

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.

Auditory Time-Frequency Masking for Spectrally and Temporally Maximally-Compact Stimuli

Many audio applications perform perception-based time-frequency (TF) analysis by decomposing sounds into a set of functions with good TF localization (i.e. with a small essential support in the TF domain) using TF transforms and applying psychoacoustic models of auditory masking to the transform coefficients. To accurately predict masking interactions between coefficients, the TF properties of the model should match those of the transform. This involves having masking data for stimuli with good TF localization. However, little is known about TF masking for mathematically well-localized signals. Most existing masking studies used stimuli that are broad in time and/or frequency and few studies involved TF conditions. Consequently, the present study had two goals. The first was to collect TF masking data for well-localized stimuli in humans. Masker and target were 10-ms Gaussian-shaped sinusoids with a bandwidth of approximately one critical band. The overall pattern of results is qualitatively similar to existing data for long maskers. To facilitate implementation in audio processing algorithms, a dataset provides the measured TF masking function. The second goal was to assess the potential effect of auditory efferents on TF masking using a modeling approach. The temporal window model of masking was used to predict present and existing data in two configurations: (1) with standard model parameters (i.e. without efferents), (2) with cochlear gain reduction to simulate the activation of efferents. The ability of the model to predict the present data was quite good with the standard configuration but highly degraded with gain reduction. Conversely, the ability of the model to predict existing data for long maskers was better with than without gain reduction. Overall, the model predictions suggest that TF masking can be affected by efferent (or other) effects that reduce cochlear gain. Such effects were avoided in the experiment of this study by using maximally-compact stimuli.

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.

Effects of age and hearing loss on overshoot

The detection of a brief, sinusoidal probe in a long broadband, simultaneous masker improves as the probe is delayed from the masker's onset. This improvement ("overshoot") may be mediated by a reduction in cochlear amplifier gain over the timecourse of the masker via the medial olivocochlear (MOC) reflex. Overshoot was measured in younger adults with normal hearing and in older adults with normal and impaired hearing to test the hypothesis that aging and cochlear hearing loss result in abnormal overshoot, consistent with changes in certain structures along the MOC pathway. Overshoot decreased with increasing quiet probe thresholds and was only minimally influenced by increasing age. Marked individual differences in overshoot were observed due to differences in masking thresholds for probes presented near the masker's onset. Model simulations support the interpretation that reduced overshoot in hearing-impaired listeners is due to limited cochlear amplifier gain and therefore less gain to adjust over the timecourse of the masker. Similar overshoot among younger and older adults with normal hearing suggests that age-related changes to mechanisms underlying overshoot do not result in significant differences in overshoot among younger and older adults with normal hearing.

Medial olivocochlear efferent reflex inhibition of human cochlear nerve responses

Inhibition of cochlear amplifier gain by the medial olivocochlear (MOC) efferent system has several putative roles: aiding listening in noise, protection against damage from acoustic overexposure, and slowing age-induced hearing loss. The human MOC reflex has been studied almost exclusively by measuring changes in otoacoustic emissions. However, to help understand how the MOC system influences what we hear, it is important to have measurements of the MOC effect on the total output of the organ of Corti, i.e., on cochlear nerve responses that couple sounds to the brain. In this work we measured the inhibition produced by the MOC reflex on the amplitude of cochlear nerve compound action potentials (CAPs) in response to moderate level (52-60 dB peSPL) clicks from five, young, normal hearing, awake, alert, human adults. MOC activity was elicited by 65 dB SPL, contralateral broadband noise (CAS). Using tympanic membrane electrodes, approximately 10 h of data collection were needed from each subject to yield reliable measurements of the MOC reflex inhibition on CAP amplitudes from one click level. The CAS produced a 16% reduction of CAP amplitude, equivalent to a 1.98 dB effective attenuation (averaged over five subjects). Based on previous reports of efferent effects as functions of level and frequency, it is possible that much larger effective attenuations would be observed at lower sound levels or with clicks of higher frequency content. For a preliminary comparison, we also measured MOC reflex inhibition of DPOAEs evoked from the same ears with f2's near 4 kHz. The resulting effective attenuations on DPOAEs were, on average, less than half the effective attenuations on CAPs.

PsyAcoustX: A flexible MATLAB(®) package for psychoacoustics research

The demands of modern psychophysical studies require precise stimulus delivery and flexible platforms for experimental control. Here, we describe PsyAcoustX, a new, freely available suite of software tools written in the MATLAB(®) environment to conduct psychoacoustics research on a standard PC. PsyAcoustX provides a flexible platform to generate and present auditory stimuli in real time and record users' behavioral responses. Data are automatically logged by stimulus condition and aggregated in an exported spreadsheet for offline analysis. Detection thresholds can be measured adaptively under basic and complex auditory masking tasks and other paradigms (e.g., amplitude modulation detection) within minutes. The flexibility of the module offers experimenters access to nearly every conceivable combination of stimulus parameters (e.g., probe-masker relations). Example behavioral applications are highlighted including the measurement of audiometric thresholds, basic simultaneous and non-simultaneous (i.e., forward and backward) masking paradigms, gap detection, and amplitude modulation detection. Examples of these measurements are provided including the psychoacoustic phenomena of temporal overshoot, psychophysical tuning curves, and temporal modulation transfer functions. Importantly, the core design of PsyAcoustX is easily modifiable, allowing users the ability to adapt its basic structure and create additional modules for measuring discrimination/detection thresholds for other auditory attributes (e.g., pitch, intensity, etc.) or binaural paradigms.

Stimulus Frequency Otoacoustic Emissions Provide No Evidence for the Role of Efferents in the Enhancement Effect

Auditory enhancement refers to the perceptual phenomenon that a target sound is heard out more readily from a background sound if the background is presented alone first. Here we used stimulus-frequency otoacoustic emissions (SFOAEs) to test the hypothesis that activation of the medial olivocochlear efferent system contributes to auditory enhancement effects. The SFOAEs were used as a tool to measure changes in cochlear responses to a target component and the neighboring components of a multitone background between conditions producing enhancement and conditions producing no enhancement. In the "enhancement" condition, the target and multitone background were preceded by a precursor stimulus with a spectral notch around the signal frequency; in the control (no-enhancement) condition, the target and multitone background were presented without the precursor. In an experiment using a wideband multitone stimulus known to produce significant psychophysical enhancement effects, SFOAEs showed no changes consistent with enhancement, but some aspects of the results indicated possible contamination of the SFOAE magnitudes by the activation of the middle-ear-muscle reflex. The same SFOAE measurements performed using narrower-band stimuli at lower sound levels also showed no SFOAE changes consistent with either absolute or relative enhancement despite robust psychophysical enhancement effects observed in the same listeners with the same stimuli. The results suggest that cochlear efferent control does not play a significant role in auditory enhancement effects.

Exploring the source of the mid-level hump for intensity discrimination in quiet and the effects of noise

Intensity discrimination Weber fractions (WFs) measured for short, high-frequency tones in quiet are larger at mid levels than at lower or higher levels. The source of this "mid-level hump" is a matter of debate. One theory is that the mid-level hump reflects basilar-membrane compression, and that WFs decrease at higher levels due to spread-of-excitation cues. To test this theory, Experiment 1 measured the mid-level hump and growth-of-masking functions to estimate the basilar membrane input/output (I/O) function in the same listeners. Results showed the initial rise in WFs could be accounted for by the change in I/O function slope, but there was additional unexplained variability in WFs. Previously, Plack [(1998). J. Acoust. Soc. Am. 103(5), 2530-2538] showed that long-duration notched noise (NN) presented with the tone reduced the mid-level hump even with a temporal gap in the NN. Plack concluded the results were consistent with central profile analysis. However, simultaneous, forward, and backward NN were not examined separately, which may independently test peripheral and central mechanisms of the NN. Experiment 2 measured WFs at the mid-level hump in the presence of NN and narrowband noise of different durations and temporal positions relative to the tone. Results varied across subjects, but were consistent with more peripheral mechanisms.

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.

Effect of broadband and narrowband contralateral noise on psychophysical tuning curves and otoacoustic emissions

The relative effectiveness of narrowband and broadband noises in activating the efferent system was assessed by comparing the effect of contralateral stimulation (CS) with such sounds on psychophysical tuning curves (PTCs) determined in simultaneous masking, using signal frequencies of 1000 or 2000 Hz. To check that the CS stimuli used did activate the efferent system, distortion product otoacoustic emissions (DPOAEs) were also measured in the absence and presence of narrowband and broadband CS. The CS had no consistent effect on the masker level at the tips of the PTCs. A broadband pink noise CS consistently reduced the masker level required for threshold on both the low- and high-frequency sides of the PTCs for the 2000-Hz signal frequency. However, there were no consistent effects of the CS for any other case. The broadband pink noise CS had a greater effect in reducing DPOAE levels than the narrowband CS. The results provide psychophysical evidence supporting the idea that the efferent system is activated more effectively by a broadband than by a narrowband CS, at least for a signal frequency of 2000 Hz.

Is overshoot caused by an efferent reduction in cochlear gain?

Under certain conditions, detection of a masked tone is improved by a preceding sound ("precursor"). This phenomenon is referred to as the "temporal effect" or "overshoot". A prevalent model of overshoot, referred to as the "gain reduction model", posits that overshoot is caused by efferent reduction in cochlear gain mediated by the medial olivocochlear (MOC) bundle. The model predicts that reduction in cochlear gain will reduce masking when masking is suppressive or when masking is excitatory and the signal-to-masker ratio is high. This study was aimed at testing the validity of these predictions. It consisted of two experiments. The first experiment investigated the relative contributions of suppressive versus excitatory masking to overshoot. The signal was a short 4-kHz tone pip, and the masker and precursor were limited to contain energy either only within (-on-frequency) or only outside (off-frequency) the cochlear filter around the signal frequency. The on-frequency masker would be expected to cause mainly excitatory masking, whereas masking by the off-frequency masker would be expected to be mainly suppressive. Only the off-frequency masker and precursor yielded -significant overshoot. This suggests that measurable overshoot requires suppressive masking. The second experiment sought to quantify the effect of a precursor on cochlear -suppression more directly by measuring the amount of suppression caused by a 4.75-kHz suppressor on a lower-frequency (4-kHz) suppressee with and without a precursor present. Suppression was measured using a forward-masking paradigm. While we found large suppression and large overshoot, we found no reduction in suppression by the precursor. This is contrary to the gain reduction model. Taken together, our results indicate that measurable overshoot requires off-frequency masking and that off-frequency overshoot must be caused by a mechanism other than MOC-mediated reduction in cochlear suppression.

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.

Spectral and level effects in auditory signal enhancement

Auditory enhancement refers generally to the increased perceptual salience of a spectral region when that region is preceded by its spectral complement, e.g., reinserting a missing component in a harmonic complex makes that component "pop out." One manifestation of enhancement is the increased detectability of a signal in certain spectro-temporal configurations. In the present experiments, detection thresholds were measured for a 2-kHz signal that was masked by an inharmonic complex with a spectral notch centered at 2-kHz. When the masker was preceded by a precursor/adaptor with a spectral gap identical to that of the masker, detection thresholds were lowest when the gap width was 0.6 octave. The amount of signal enhancement, the difference in thresholds between the no-precursor and precursor -conditions, decreased for smaller and larger gap widths. In addition, this general result was robust for precursors such as band-reject noise and harmonic complexes that were different in perceptual quality from the masker. This suggests that grouping/segregation processes do not mediate enhancement as assessed here. Similarly, significant enhancement was observed with precursor-masker level differences over a 40-dB range. Overall, these results further indicate that frequency resolution is a dynamic process that depends on spectro-temporal context. They also are consistent with a mechanism involving adaptation of inhibition that likely occurs at low levels in the auditory system.

Investigating the auditory enhancement phenomenon using behavioral temporal masking patterns

A narrowband signal is subjected to less masking from a simultaneously presented notched masker if it is preceded by a precursor that occupies the same spectral region as the masker, a phenomenon referred to as enhancement. The present study investigated (i) the amount of enhancement for the detection of a narrowband noise added to a notched masker, and (ii) masking patterns associated with the detection of tone pips added to the narrowband signal. The resulting psychophysical data were compared to predictions generated using a model similar to the neural adaptation-of-inhibition model proposed by Nelson and Young [(2010b). J. Neurosci. 30, 6577-6587]. The amount of enhancement was measured as a function of the temporal separation between the precursor and masker in Experiment I, and as a function of precursor level in Experiment II. The model captured the temporal dynamics of psychophysical enhancement reasonably well for both the long-duration noise signals and the masking patterns. However, in contrast to psychophysical data which indicated reliable enhancement only when the precursor and masker shared the same levels, the model predicated enhancement at all precursor levels.

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.

Changes in amplitude and phase of distortion-product otoacoustic emission fine-structure and separated components during efferent activation

Medial olivocochlear (MOC) efferent fibers synapse directly on the outer hair cells (OHCs). Efferent activation evoked by contralateral acoustic stimulation (CAS) will affect OHC amplification and subsequent measures of distortion-product otoacoustic emissions (DPOAEs). The aim of this study was to investigate measures of total and separated DPOAEs during efferent activation. Efferent activation produces both suppression and enhancement of the total DPOAE level. Level enhancements occurred near fine-structure minima and were associated with consistent MOC evoked upward shifts in DPOAE fine-structure frequency. Examination of the phase of the separated components revealed that frequency shifts stemmed from increasing phase leads of the reflection component during CAS, while the generator component phase was nearly invariant. Separation of the two DPOAE components responsible for the fine-structure revealed more consistent reduction of the levels of both components. Using vector subtraction (which takes into account both level and phase) to estimate the changes in the unseparated DPOAE provided consistent evidence of DPOAE suppression. Including phase information provided a more sensitive, valid and consistent estimate of CAS function even if one does not know the position of the DPOAE in the fine-structure.

Evaluating adaptation and olivocochlear efferent feedback as potential explanations of psychophysical overshoot

Masked detection threshold for a short tone in noise improves as the tone's onset is delayed from the masker's onset. This improvement, known as "overshoot," is maximal at mid-masker levels and is reduced by temporary and permanent cochlear hearing loss. Computational modeling was used in the present study to evaluate proposed physiological mechanisms of overshoot, including classic firing rate adaptation and medial olivocochlear (MOC) feedback, for both normal hearing and cochlear hearing loss conditions. These theories were tested using an established model of the auditory periphery and signal detection theory techniques. The influence of several analysis variables on predicted tone-pip detection in broadband noise was evaluated, including: auditory nerve fiber spontaneous-rate (SR) pooling, range of characteristic frequencies, number of synapses per characteristic frequency, analysis window duration, and detection rule. The results revealed that overshoot similar to perceptual data in terms of both magnitude and level dependence could be predicted when the effects of MOC efferent feedback were included in the auditory nerve model. Conversely, simulations without MOC feedback effects never produced overshoot despite the model's ability to account for classic firing rate adaptation and dynamic range adaptation in auditory nerve responses. Cochlear hearing loss was predicted to reduce the size of overshoot only for model versions that included the effects of MOC efferent feedback. These findings suggest that overshoot in normal and hearing-impaired listeners is mediated by some form of dynamic range adaptation other than what is observed in the auditory nerve of anesthetized animals. Mechanisms for this adaptation may occur at several levels along the auditory pathway. Among these mechanisms, the MOC reflex may play a leading role.

Overshoot using very short signal delays

The detectability of a 10-ms tone masked by a 400-ms wideband noise was measured as a function of the delay in the onset of the tone compared to the onset of the noise burst. Unlike most studies like this on auditory overshoot, special attention was given to signal delays between 0 and 45 ms. Nine well-practiced subjects were tested using an adaptive psychophysical procedure in which the level of the masking noise was adjusted to estimate 79% correct detections. Tones of both 3.0 and 4.0 kHz, at different levels, were used as signals. For the subjects showing overshoot, detectability remained approximately constant for at least 20-30 ms of signal delay, and then detectability began to improve gradually toward its maximum at about 150-200 ms. That is, there was a "hesitation" prior to detectability beginning to improve, and the duration of this hesitation was similar to that seen in physiological measurements of the medial olivocochlear (MOC) system. This result provides further support for the hypothesis that the MOC efferent system makes a major contribution to overshoot in simultaneous masking.

Overshoot measured physiologically and psychophysically in the same human ears

A nonlinear version of the stimulus-frequency otoacoustic emission (SFOAE) was measured using stimulus waveforms similar to those used for behavioral overshoot. Behaviorally, the seven listeners were as much as 11 dB worse at detecting a brief tonal signal (4.0 kHz, 10 ms in duration) when it occurred soon after the onset of a wideband masking noise (0.1-6.0 kHz; 400 ms in duration) than when it was delayed by about 200 ms, and the nonlinear SFOAE measure exhibited a similar effect. When either lowpass (0.1-3.8 kHz) or bandpass noise (3.8-4.2 kHz) was used instead of the wideband noise, the physiological and behavioral measures again were similar. When a highpass noise (4.2-6.0 kHz) was used, the physiological and behavioral measures both showed no overshoot-like effect for five of the subjects. The physiological response to the tone decayed slowly after the termination of the noise, much like the time course of resetting for behavioral overshoot. One subject exhibited no overshoot behaviorally even though his cochlear responses were like those of the other subjects. Overall, the evidence suggests that some basic characteristics of overshoot are obligatory consequences of cochlear function, as modulated by the olivocochlear efferent system.

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.

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.

Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoot

Behavioral threshold for a tone burst presented in a long-duration noise masker decreases as the onset of the tone burst is delayed relative to masker onset. The threshold difference between detection of early- and late-onset tone bursts is called overshoot. Although the underlying mechanisms are unclear, one hypothesis is that overshoot occurs due to efferent suppression of cochlear nonlinearity [von Klitzing, R., and Kohlrausch, A. (1994). J. Acoust. Soc. Am. 95, 2192-2201]. This hypothesis was tested by using overshoot conditions to elicit stimulus-frequency otoacoustic emissions (SFOAEs), which provide a physiological measure of cochlear nonlinearity. SFOAE and behavioral thresholds were estimated using a modified maximum-likelihood yes-no procedure. The masker was a 400-ms "frozen" notched noise. The signal was a 20-ms, 4-kHz tone burst presented at 1 or 200 ms after the noise onset. Behavioral overshoot results replicated previous studies, but no overshoot was observed in SFOAE thresholds. This suggests that either efferent suppression of cochlear nonlinearity is not involved in overshoot, or a SFOAE threshold estimation procedure based on stimuli similar to those used to study behavioral overshoot is not sensitive enough to measure the effect.

Review article: review of the literature on temporal resolution in listeners with cochlear hearing impairment: a critical assessment of the role of suprathreshold deficits

A critical review of studies of temporal resolution in listeners with cochlear hearing impairment is presented with the aim of assessing evidence for suprathreshold deficits. Particular attention is paid to the roles of variables-such as stimulus audibility, overall stimulus level, and participant's age-which may complicate the interpretation of experimental findings in comparing the performance of hearing-impaired (HI) and normal-hearing (NH) listeners. On certain temporal tasks (e.g., gap detection), the performance of HI listeners appears to be degraded relative to that of NH listeners when compared at equal SPL (sound pressure level). For other temporal tasks (e.g., forward masking), HI performance is degraded relative to that of NH listeners when compared at equal sensation level. A relatively small group of studies exists, however, in which the effects of stimulus audibility and level (and occasionally participant's age) have been controlled through the use of noise-masked simulation of hearing loss in NH listeners. For some temporal tasks (including gap-detection, gap-duration discrimination, and detection of brief tones in modulated noise), the performance of HI listeners is well reproduced in the results of noise-masked NH listeners. For other tasks (i.e., temporal integration), noise-masked hearing-loss simulations do not reproduce the results of HI listeners. In three additional areas of temporal processing (duration discrimination, detection of temporal modulation in noise, and various temporal-masking paradigms), further studies employing control of stimulus audibility and level, as well as age, are necessary for a more complete understanding of the role of suprathreshold deficits in the temporal-processing abilities of HI listeners.

The effect of a precursor on growth of forward masking

This study examined the effect of an on-frequency precursor on growth-of-masking (GOM) functions measured using an off-frequency masker. The signal was a 6-ms, 4-kHz tone. A GOM function was measured using a 40-ms, 2.8-kHz tone (the off-frequency masker). GOM functions were then measured with an on-frequency, fixed level precursor presented before the off-frequency masker. The precursor was 50 or 60 dB SPL, and 160 ms in duration. For the 60-dB SPL precursor, a 40-ms duration was also used. Two-line functions were fit to the GOM data to estimate the basilar membrane input-output function. The precursors reduced the gain of the input-output function, and this decrease was graded with precursor level. Both precursor durations had the same effect on gain. Changes in masking following a precursor were larger than would be predicted by additivity of masking. The observed decrease in gain may be consistent with activation of the medial olivocochlear reflex by the precursor.

Role of attention in overshoot: frequency certainty versus uncertainty

Overshoot, the elevation in the threshold for a brief signal that comes on close to masker onset, was measured with signal frequency certain (same frequency on every trial) or uncertain (randomized over trials). In broadband noise, thresholds were higher 2 ms after masker onset than 200 ms later, by 9 dB with frequency certainty, by 6-7 dB with uncertainty. In narrowband noise centered on the signal frequency, thresholds at 2 ms were not elevated with certainty, but were elevated 4-5 dB with uncertainty. Thus, frequency uncertainty leads to less overshoot in broadband noise, to more overshoot in narrowband noise. Reduced overshoot in broadband noise may come about because the masker, given its many frequencies, disrupts focusing at onset as much under certainty as uncertainty. Once the initial disruption dissipates, threshold is lower with certainty so overshoot is greater. In contrast, a narrowband noise with frequencies only near the signal does not disrupt focusing when the signal frequency is known beforehand, so overshoot is absent. When frequency is uncertain, the narrowband noise serves to focus attention on the signal frequency; as this requires time, detection near noise onset is poorer than later on, so overshoot is present.

The relationship between precursor level and the temporal effect

Previous studies have suggested that temporal effects in masking may be consistent with a decrease in cochlear gain. One paradigm used to show this is to measure the level of a long-duration masker required to just mask a short-duration tone that occurs near masker onset. The temporal effect is revealed when the signal is detected at a lower signal-to-noise ratio following preceding stimulation (either an extension of the masker or a separate precursor). The present study examined whether this effect depends on precursor level. The signal was a 10-ms, 4-kHz tone. The masker was 200 ms. A fixed-level precursor had the same frequency characteristics as the masker, and was 205 ms. The masker and precursor had either no notch or a wide notch about the signal frequency. For a given precursor level, the growth of masker level with signal level was determined. These data were used to estimate input-output functions. The results are consistent with a graded decrease in gain at the signal frequency when there is no notch in the masker and precursor, and a graded decrease in suppression when there is a large notch. These results could be consistent with the action of the medial olivocochlear reflex.

Spectral loudness summation for sequences of short noise bursts

Recent loudness data of single noise bursts indicate that spectral loudness summation depends on signal duration. To gain insight into the mechanisms underlying this duration effect, loudness was measured as a function of signal bandwidth centered around 2 kHz for sequences of 10-ms noise bursts at various repetition rates and, for comparison, for single noise bursts of either 10- or 1000-ms duration. The test-signal bandwidth was varied from 200 to 6400 Hz. For the repeated noise bursts, the reference signal had a bandwidth of 400 Hz. For the single noise bursts, data were obtained for two reference bandwidths: 400 and 3200 Hz. In agreement with previous results, the magnitude of spectral loudness summation was larger for the 10-ms than for the 1000-ms noise bursts. The reference bandwidth had no significant effect on the results for the single noise bursts. Up to repetition rates of 50 Hz, the magnitude of spectral loudness summation for the sequences of noise bursts was the same as for the single short noise burst. The data indicate that the mechanism underlying the duration effect in spectral loudness is considerably faster than the time constant of about 100 ms commonly associated with the temporal integration of loudness.

Spectral integration and wideband analysis in gap detection and overshoot paradigms

Several listening conditions show that energy remote from a target frequency can deleteriously affect sensitivity. One interpretation of such results entails a wideband analysis involving a wide predetection filter. The present study tested the hypothesis that both temporal gap detection and overshoot results are consistent with a wideband analysis, as contrasted with statistical combination of information across independent channels. For gap detection, stimuli were random or comodulated 50-Hz-wide noise bands centered on 1000, 1932, 3569, and 6437 Hz. For overshoot, the masker was an 8-kHz low-pass filtered noise, with 5-ms tone bursts presented at the same center frequencies used for gap detection. Signals were presented with either 0- or 250-ms delay after masker onset. In each paradigm, the target was introduced at only one frequency or at all four frequencies. Results from gap detection conditions did not favor a wideband analysis interpretation: Results in the random condition were consistent with an optimal combination of cues across frequency. An across-channel interference effect was also evident when only one of the four bands contained the gap. Although results from the overshoot conditions were consistent with a wideband analysis interpretation, they were more parsimoniously accounted for in terms of statistical combination of information.

Temporal masking in electric hearing

Temporal masking can be defined as the detection threshold of a brief signal as a function of the signal delay in a relatively long masker. The temporal masking pattern in normal acoustic hearing reveals temporal edge enhancement in which the signal detection threshold is greater near the masker onset than in the steady-state portion. Both peripheral and central mechanisms appear to underlie temporal edge enhancement, but their relative contributions remain elusive. Cochlear implants bypass cochlear mechanical processing and stimulate the auditory nerve directly, thereby providing a unique opportunity to separate the peripheral mechanisms from the central mechanisms. Here, we systematically measured temporal masking in electric hearing by examining whether a brief signal was harder to detect at the onset than in the steady-state portion of a long masker (the "overshoot" effect). The signal and the masker were presented (1) either to the same electrode or to different electrodes, (2) at the same stimulation or different rates, and (3) in a simultaneous or an interleaved fashion. A consistent pattern of results was observed, depending on the stimulus configuration between the signal and the masker. Simultaneous stimulation at the same rate and with the same electrode produced no difference in sensitivity between the onset and the steady-state conditions, but interleaved stimulation at different rates or with different electrodes produced a significant difference. Unlike acoustic hearing, high masker levels produced an overshoot effect, and low masker levels produced an undershoot effect. Although the present results are consistent with the "on-frequency vs. off-frequency" hypothesis for the overshoot effect, results also suggest a central "same vs. different" mechanism underlying temporal masking. These results have practical implications for improving cochlear implant design.