Suppression and the upward spread of masking

Auditory enhancement in younger and older listeners with normal and impaired hearinga)

Auditory enhancement is a spectral contrast aftereffect that can facilitate the detection of novel events in an ongoing background. A single-interval paradigm combined with roved frequency content between trials can yield as much as 20 dB enhancement in young normal-hearing listeners. This study compared such enhancement in 15 listeners with sensorineural hearing loss with that in 15 age-matched adults and 15 young adults with normal audiograms. All groups were presented with stimulus levels of 70 dB sound pressure level (SPL) per component. The two groups with normal hearing were also tested at 45 dB SPL per component. The hearing-impaired listeners showed very little enhancement overall. However, when tested at the same high (70-dB) level, both young and age-matched normal-hearing listeners also showed substantially reduced enhancement, relative to that found at 45 dB SPL. Some differences in enhancement emerged between young and older normal-hearing listeners at the lower sound level. The results suggest that enhancement is highly level-dependent and may also decrease somewhat with age or slight hearing loss. Implications for hearing-impaired listeners may include a poorer ability to adapt to real-world acoustic variability, due in part to the higher levels at which sound must be presented to be audible.

Feasibility of Diagnosing Dead Regions Using Auditory Steady-State Responses to an Exponentially Amplitude Modulated Tone in Threshold Equalizing Notched Noise, Assessed Using Normal-Hearing Participants

The aim of this study was to assess feasibility of using electrophysiological auditory steady-state response (ASSR) masking for detecting dead regions (DRs). Fifteen normally hearing adults were tested using behavioral and electrophysiological tasks. In the electrophysiological task, ASSRs were recorded to a 2 kHz exponentially amplitude-modulated tone (AM2) presented within a notched threshold equalizing noise (TEN) whose center frequency (CFNOTCH) varied. We hypothesized that, in the absence of DRs, ASSR amplitudes would be largest for CFNOTCH at/or near the signal frequency. In the presence of a DR at the signal frequency, the largest ASSR amplitude would occur at a frequency (fmax) far away from the signal frequency. The AM2 and the TEN were presented at 60 and 75 dB SPL, respectively. In the behavioral task, for the same maskers as above, the masker level at which an AM and a pure tone could just be distinguished, denoted AM2ML, was determined, for low (10 dB above absolute AM2 threshold) and high (60 dB SPL) signal levels. We also hypothesized that the value of fmax would be similar for both techniques. The ASSR fmax values obtained from grand average ASSR amplitudes, but not from individual amplitudes, were consistent with our hypotheses. The agreement between the behavioral fmax and ASSR fmax was poor. The within-session ASSR-amplitude repeatability was good for AM2 alone, but poor for AM2 in notched TEN. The ASSR-amplitude variability between and within participants seems to be a major roadblock to developing our approach into an effective DR detection method.

Auditory filter shapes derived from forward and simultaneous masking at low frequencies: Implications for human cochlear tuning

Behavioral forward-masking thresholds with a spectrally notched-noise masker and a fixed low-level probe tone have been shown to provide accurate estimates of cochlear tuning. Estimates using simultaneous masking are similar but generally broader, presumably due to nonlinear cochlear suppression effects. So far, estimates with forward masking have been limited to frequencies of 1 kHz and above. This study used spectrally notched noise under forward and simultaneous masking to estimate frequency selectivity between 200 and 1000 Hz for young adult listeners with normal hearing. Estimates of filter tuning at 1000 Hz were in agreement with previous studies. Estimated tuning broadened below 1000 Hz, with the filter quality factor based on the equivalent rectangular bandwidth (Q_ERB) decreasing more rapidly with decreasing frequency than predicted by previous equations, in line with earlier predictions based on otoacoustic-emission latencies. Estimates from simultaneous masking remained broader than those from forward masking by approximately the same ratio. The new data provide a way to compare human cochlear tuning estimates with auditory-nerve tuning curves from other species across most of the auditory frequency range.

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.

Modeling potential masking of echolocating sperm whales exposed to continuous 1-2 kHz naval sonar

Modern active sonar systems can (almost) continuously transmit and receive sound, which can lead to more masking of important sounds for marine mammals than conventional pulsed sonar systems transmitting at a much lower duty cycle. This study investigated the potential of 1-2 kHz active sonar to mask echolocation-based foraging of sperm whales by modeling their echolocation detection process. Continuous masking for an echolocating sperm whale facing a sonar was predicted for sonar sound pressure levels of 160 dB re 1 μPa², with intermittent masking at levels of 120 dB re 1 μPa², but model predictions strongly depended on the animal orientation, harmonic content of the sonar, click source level, and target strength of the prey. The masking model predicted lower masking potential of buzz clicks compared to regular clicks, even though the energy source level is much lower. For buzz clicks, the lower source level is compensated for by the reduced two-way propagation loss to nearby prey during buzzes. These results help to predict what types of behavioral changes could indicate masking in the wild. Several key knowledge gaps related to masking potential of sonar in echolocating odontocetes were identified that require further investigation to assess the significance of masking.

Auditory enhancement under forward masking in normal-hearing and hearing-impaired listeners

A target within a spectrally notched masker can be enhanced by a preceding copy of the masker. Enhancement can also increase the effectiveness of the target as a forward masker. Enhancement has been reported in hearing-impaired listeners under simultaneous but not forward masking. However, previous studies of enhancement under forward masking did not fully assess the potential effect of differences in sensation level or spectral resolution between the normal-hearing and hearing-impaired listeners. This study measured enhancement via forward masking in hearing-impaired and age-matched normal-hearing listeners with different spectral notches in the masker, to account for potential differences in frequency selectivity, and with levels equated by adding a background masking noise to equate both sensation level and sound pressure level or by reducing the sound pressure level of the stimuli to equate sensation level. Hearing-impaired listeners showed no significant enhancement, regardless of spectral notch width. Normal-hearing listeners showed enhancement at high levels, but showed less enhancement when sensation levels were reduced to match those of the hearing-impaired group, either by reducing sound levels or by adding a masking noise. The results confirm a lack of forward-masked enhancement in hearing-impaired listeners but suggest this may be partly due to reduced sensation level.

Simultaneous masking between electric and acoustic stimulation in cochlear implant users with residual low-frequency hearing

Ipsilateral electric-acoustic stimulation (EAS) is becoming increasingly important in cochlear implant (CI) treatment. Improvements in electrode designs and surgical techniques have contributed to improved hearing preservation during implantation. Consequently, CI implantation criteria have been expanded toward people with significant residual low-frequency hearing, who may benefit from the combined use of both the electric and acoustic stimulation in the same ear. However, only few studies have investigated the mutual interaction between electric and acoustic stimulation modalities. This work characterizes the interaction between both stimulation modalities using psychophysical masking experiments and cone beam computer tomography (CBCT). Two psychophysical experiments for electric and acoustic masking were performed to measure the hearing threshold elevation of a probe stimulus in the presence of a masker stimulus. For electric masking, the probe stimulus was an acoustic tone while the masker stimulus was an electric pulse train. For acoustic masking, the probe stimulus was an electric pulse train and the masker stimulus was an acoustic tone. Five EAS users, implanted with a CI and ipsilateral residual low-frequency hearing, participated in the study. Masking was determined at different electrodes and different acoustic frequencies. CBCT scans were used to determine the individual place-pitch frequencies of the intracochlear electrode contacts by using the Stakhovskaya place-to-frequency transformation. This allows the characterization of masking as a function of the difference between electric and acoustic stimulation sites, which we term the electric-acoustic frequency difference (EAFD). The results demonstrate a significant elevation of detection thresholds for both experiments. In electric masking, acoustic-tone thresholds increased exponentially with decreasing EAFD. In contrast, for the acoustic masking experiment, threshold elevations were present regardless of the tested EAFDs. Based on the present findings, we conclude that there is an asymmetry between the electric and the acoustic masker modalities. These observations have implications for the design and fitting of EAS sound-coding strategies.

Procedural Factors That Affect Psychophysical Measures of Spatial Selectivity in Cochlear Implant Users

Behavioral measures of spatial selectivity in cochlear implants are important both for guiding the programing of individual users’ implants and for the evaluation of different stimulation methods. However, the methods used are subject to a number of confounding factors that can contaminate estimates of spatial selectivity. These factors include off-site listening, charge interactions between masker and probe pulses in interleaved masking paradigms, and confusion effects in forward masking. We review the effects of these confounds and discuss methods for minimizing them. We describe one such method in which the level of a 125-pps masker is adjusted so as to mask a 125-pps probe, and where the masker and probe pulses are temporally interleaved. Five experiments describe the method and evaluate the potential roles of the different potential confounding factors. No evidence was obtained for off-site listening of the type observed in acoustic hearing. The choice of the masking paradigm was shown to alter the measured spatial selectivity. For short gaps between masker and probe pulses, both facilitation and refractory mechanisms had an effect on masking; this finding should inform the choice of stimulation rate in interleaved masking experiments. No evidence for confusion effects in forward masking was revealed. It is concluded that the proposed method avoids many potential confounds but that the choice of method should depend on the research question under investigation.

Automated screening for high-frequency hearing loss

Objective:

Hearing loss at high frequencies produces perceptual difficulties and is often an early sign of a more general hearing loss. This study reports the development and validation of two new speech-based hearing screening tests in English that focus on detecting hearing loss at frequencies above 2000 Hz.

Design:

The Internet-delivered, speech-in noise tests used closed target-word sets of digit triplets or consonant–vowel–consonant (CVC) words presented against a speech-shaped noise masker. The digit triplet test uses the digits 0 to 9 (excluding the disyllabic 7), grouped in quasi-random triplets. The CVC test uses simple words (e.g., “cat”) selected for the high-frequency spectral content of the consonants. During testing, triplets or CVC words were identified in an adaptive procedure to obtain the speech reception threshold (SRT) in noise. For these new, high-frequency (HF) tests, the noise was low-pass filtered to produce greater masking of the low-frequency speech components, increasing the sensitivity of the test for HF hearing loss. Individual test tokens (digits, CVCs) were first homogenized using a group of 10 normal-hearing (NH) listeners by equalizing intelligibility across tokens at several speech-in-noise levels. Both tests were then validated and standardized using groups of 24 NH listeners and 50 listeners with hearing impairment. Performance on the new high frequency digit triplet (HF-triplet) and CVC (HF-CVC) tests was compared with audiometric hearing loss, and with that on the unfiltered, broadband digit triplet test (BB-triplet) test, and the ASL (Adaptive Sentence Lists) speech-in-noise test.

Results:

The HF-triplet and HF-CVC test results (SRT) both correlated positively and highly with high-frequency audiometric hearing loss and with the ASL test. SRT for both tests as a function of high-frequency hearing loss increased at nearly three times the rate as that of the BB-triplet test. The intraindividual variability (SD) on the tests was about 2.1 (HF-triplet) and 1.7 (HF-CVC) times less than that for the BB-triplet test. The effect on the HF-triplet test of varying presentation method (professional or cheap headphones and loudspeakers) was small for the NH group and somewhat larger, but nonsignificant for the hearing-impaired group. Test repetition produced a moderate, significant learning effect for the first and second retests, but was small and nonsignificant for further retesting. The learning effect was about two times larger for the HF-CVC test than for the HF-triplet test. The sensitivity of both new tests for high-frequency hearing loss was similar, with an 87% true-positive and 7% false-positive ratio for detecting an average high-frequency hearing loss of 20 dB or more.

Conclusions:

The new HF-triplet and HF-CVC tests provide a sensitive and accurate method for detecting high-frequency hearing loss. The tests may signal developing hearing impairment at an early stage. The HF-triplet is preferred over the HF-CVC test because of its smaller learning effect, smaller error rate, greater simplicity, and lower cultural dependency.

We have developed 2 new simple, internet-delivered, speech-innoise tests for high-frequency (HF) hearing loss. They may be used at home or in the clinic/lab as screening tests or as outcome measures for hearing rehabilitation. The new tests, HF-triplet and HFCVC (Consonant-Vowel-Consonant), both showed better sensitivity for detecting HF hearing loss (HL) than the standard broadband BBtriplet test. The accuracy of the HF-triplet was 2.5 times that of the BB-triplet test. A small training effect for the HF-triplet was about half that of the HF-CVC. The HF-triplet out performed the HF-CVC test in both accuracy and test repeatability and is recommended for screening HFHL.

Place specificity measured in forward and interleaved masking in cochlear implants

Interleaved masking in cochlear implants is analogous to acoustic simultaneous masking and is relevant to speech processing strategies that interleave pulses on concurrently activated electrodes. In this study, spatial decay of masking as the distance between masker and probe increases was compared between forward and interleaved masking in the same group of cochlear implant users. Spatial masking patterns and the measures of place specificity were similar between forward and interleaved masking. Unlike acoustic hearing where broader tuning curves are obtained in simultaneous masking, the type of masking experiment did not influence the measure of place specificity in cochlear implants.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Simultaneous masking additivity for short Gaussian-shaped tones: spectral effects

Laback et al. [(2011). J. Acoust. Soc. Am. 129, 888-897] investigated the additivity of nonsimultaneous masking using short Gaussian-shaped tones as maskers and target. The present study involved Gaussian stimuli to measure the additivity of simultaneous masking for combinations of up to four spectrally separated maskers. According to most basilar membrane measurements, the maskers should be processed linearly at the characteristic frequency (CF) of the target. Assuming also compression of the target, all masker combinations should produce excess masking (exceeding linear additivity). The results for a pair of maskers flanking the target indeed showed excess masking. The amount of excess masking could be predicted by a model assuming summation of masker-evoked excitations in intensity units at the target CF and compression of the target, using compressive input/output functions derived from the nonsimultaneous masking study. However, the combinations of lower-frequency maskers showed much less excess masking than predicted by the model. This cannot easily be attributed to factors like off-frequency listening, combination tone perception, or between-masker suppression. It was better predicted, however, by assuming weighted intensity summation of masker excitations. The optimum weights for the lower-frequency maskers were smaller than one, consistent with partial masker compression as indicated by recent psychoacoustic data.

Effects of the selective disruption of within- and across-channel cues to comodulation masking release

In many experiments on comodulation masking release (CMR), both across- and within-channel cues may be available. This makes it difficult to determine the mechanisms underlying CMR. The present study compared CMR in a flanking-band (FB) paradigm for a situation in which only across-channel cues were likely to be available [FBs placed distally from the on-frequency band (OFB)] and a situation where both across- and within-channel cues might have been available (proximally spaced FBs, for which larger CMRs have previously been observed). The use of across-channel cues was selectively disrupted using a manipulation of auditory grouping factors, following Dau et al. [J. Acoust. Soc. Am. 125, 2182-2188(2009)] and the use of within-channel cues was selectively disrupted using a manipulation called "OFB reversal," following Goldman et al. [J. Acoust. Soc. Am. 129, 3181-3193 (2011)]. The auditory grouping manipulation eliminated CMR for the distal-FB configuration and reduced CMR for the proximal-FB configuration. This may indicate that across-channel cues are available for proximal FB placement. CMR for the proximal-FB configuration persisted when both manipulations were used together, which suggests that OFB reversal does not entirely eliminate within-channel cues.

Ipsilateral masking between acoustic and electric stimulations

Residual acoustic hearing can be preserved in the same ear following cochlear implantation with minimally traumatic surgical techniques and short-electrode arrays. The combined electric-acoustic stimulation significantly improves cochlear implant performance, particularly speech recognition in noise. The present study measures simultaneous masking by electric pulses on acoustic pure tones, or vice versa, to investigate electric-acoustic interactions and their underlying psychophysical mechanisms. Six subjects, with acoustic hearing preserved at low frequencies in their implanted ear, participated in the study. One subject had a fully inserted 24 mm Nucleus Freedom array and five subjects had Iowa/Nucleus hybrid implants that were only 10 mm in length. Electric masking data of the long-electrode subject showed that stimulation from the most apical electrodes produced threshold elevations over 10 dB for 500, 625, and 750 Hz probe tones, but no elevation for 125 and 250 Hz tones. On the contrary, electric stimulation did not produce any electric masking in the short-electrode subjects. In the acoustic masking experiment, 125-750 Hz pure tones were used to acoustically mask electric stimulation. The acoustic masking results showed that, independent of pure tone frequency, both long- and short-electrode subjects showed threshold elevations at apical and basal electrodes. The present results can be interpreted in terms of underlying physiological mechanisms related to either place-dependent peripheral masking or place-independent central masking.

Within-channel cues to comodulation masking release for single and symmetrically placed pairs of flanking bands

Comodulation masking release (CMR) as measured in a flanking-band (FB) paradigm is often larger when the FBs are close to the signal frequency, f(s), than when they are remote from f(s), an effect which may be partly due to the use of within-channel cues. Schooneveldt and Moore [J. Acoust. Soc. Am. 85, 262-272 (1989)] reported that, for f(s) = 1000 Hz, this effect was larger when a single FB was used than when there were two FBs symmetrically placed about f(s), and proposed that there are within-channel cues that are available for a single FB, but not for a symmetrically placed pair of FBs. The present study replicated and extended their study. Although CMR was larger for two symmetrically placed FBs than for a single FB, the effect of FB proximity to f(s) did not differ for the two cases. The results do not support the idea that there are additional within-channel cues that are available for a single FB. Changes in the regularity of temporal fine structure and changes in the prevalence of low-amplitude envelope portions are both plausible within-channel cues.

Suppression and comodulation masking release in normal-hearing and hearing-impaired listeners

The detectability of a sinusoidal signal embedded in a masker at the signal frequency can be improved by simultaneously presenting additional maskers in off-frequency regions if the additional maskers and the on-frequency masker component have the same temporal envelope. This effect is commonly referred to as comodulation masking release (CMR). Recently, it was hypothesized that peripheral nonlinear processes such as suppression may play a role in CMR over several octaves when the level of the off-frequency masker component is higher than the level of the on-frequency masker component. The aim of the present study was to test this hypothesis by measuring suppression and CMR within the same subjects for various frequency-level combinations of the off-frequency masker component. Experimental data for normal-hearing listeners show a large overlap between the existence regions for suppression and CMR. Hearing-impaired subjects with a sensorineural hearing loss show, on average, negligible suppression and CMR. The data support the hypothesis that part of the CMR in experiments with large spectral distances and large level differences between the masker components is due to the nonlinear processing at the level of the cochlea.

Spectral profile cues in comodulation masking release

Previous work on spectral shape discrimination has shown that detection of a level increment in one tone of a tonal complex is dependent on spectral position, with thresholds forming a "bowl" pattern for components spanning 200 to 5000 Hz [Green, D. M., (1988). Profile Analysis: Auditory Intensity Discrimination (Oxford University Press, New York)]. The current study examined whether a similar bowl occurs for comodulation masking release, a paradigm in which dynamic spectral cues could be used to detect an added signal. Maskers were logarithmically spaced 15-Hz-wide bands of noise. The signal was a tone or a copy of the on-signal masker band. When the masker was composed of one or more random bands, thresholds were relatively consistent across frequency. When the masker was a set of comodulated bands, thresholds for both signal types formed a bowl, but the minimum threshold occurred at a higher signal frequency for the tonal than for the narrowband noise signal. Results for additional conditions indicate that spectral effects depend on both absolute frequency and relative frequency of the signal within the masker. Data collected with flanking maskers presented contralateral to the signal and on-signal masker indicate that peripheral effects may play a role in threshold elevation at high signal frequencies with narrowband noise signals.

The role of suppression in psychophysical tone-on-tone masking

This study tested the hypothesis that suppression contributes to the difference between simultaneous masking (SM) and forward masking (FM). To obtain an alternative estimate of suppression, distortion-product otoacoustic emissions (DPOAEs) were measured in the presence of a suppressor tone. Psychophysical-masking and DPOAE-suppression measurements were made in 22 normal-hearing subjects for a 4000-Hz signal/f(2) and two masker/suppressor frequencies: 2141 and 4281 Hz. Differences between SM and FM at the same masker level were used to provide a psychophysical estimate of suppression. The increase in L(2) to maintain a constant output (L(d)) provided a DPOAE estimate of suppression for a range of suppressor levels. The similarity of the psychophysical and DPOAE estimates for the two masker/suppressor frequencies suggests that the difference in amount of masking between SM and FM is at least partially due to suppression.

Reflex control of the human inner ear: a half-octave offset in medial efferent feedback that is consistent with an efferent role in the control of masking

The high sensitivity and frequency selectivity of the mammalian cochlea is due to amplification produced by outer hair cells (OHCs) and controlled by medial olivocochlear (MOC) efferents. Data from animals led to the view that MOC fibers provide frequency-specific inhibitory feedback; however, these studies did not measure intact MOC reflexes. To test whether MOC inhibition is primarily at the frequency that elicits the MOC activity, acoustically elicited MOC effects were quantified in humans by the change in otoacoustic emissions produced by 60-dB SPL tone and half-octave-band noise elicitors at different frequencies relative to a 40-dB SPL, 1-kHz probe tone. On average, all elicitors produced MOC effects that were skewed (elicitor frequencies -1 octave below the probe produced larger effects than those -1 octave above). The largest MOC effects were from elicitors below the probe frequency for contra- and bilateral elicitors but were from elicitors centered at the probe frequency for ipsilateral elicitors. Typically, ipsilateral elicitors produced larger effects than contralateral elicitors and bilateral elicitors produced effects near the ipsi+contra sum. Elicitors at levels down to 30-dB SPL produced similar patterns. Tuning curves (TCs) interpolated from these data were V-shaped with Q10s approximately 2. These are sharper than MOC-fiber TCs found near 1 kHz in cats and guinea pigs. Because cochlear amplification is skewed (more below the best frequency of a cochlear region), these data are consistent with an anti-masking role of MOC efferents that reduces masking by reducing the cochlear amplification seen at 1 kHz.

Factors contributing to comodulation masking release with dichotic maskers

Detection threshold for a pure tone signal centered in a narrow band of noise may be reduced by inclusion of additional flanking masker bands, provided that they share coherent amplitude modulation (AM) across frequency. This comodulation masking release (CMR) associated with coherent AM across frequency is often much smaller if the signal and on-signal masker are presented to one ear and the flanking masker band(s) are presented contralaterally. An experiment was carried out to explore the role of peripheral effects (e.g., suppression) and central effects (e.g., grouping) in this finding. As frequently reported, CMR was smaller when two or more flanking maskers were presented contralaterally to the signal than when presented ipsilaterally. An intermediate condition, where a subset of flanking maskers was presented to each ear, provided comparable benefit to presenting all flankers ipsilateral to the signal. This result suggests that central effects may play a significant role in the reduced dichotic CMR under some conditions.

Discrimination of the spectral structures of sound signals on the background of interference

Presentation of test signals consisting of sounds with rippled spectra allowed measurements of the frequency-resolving ability (FRA) of hearing to be performed in humans without using frequency-dependent masking techniques. This allowed studies of changes in FRA in the presence of noise interference. In conditions of diotic presentation (to both ears in parallel) of the test signal and noise, FRA decreased significantly if the interference frequency was lower or equal to the test signal frequency. The relationship between this effect and the sound intensity and noise:signal ratio varied for low-frequency noise and for noise at the same frequency as the test signal, which indicates that these two types of interference have different mechanisms. However, in both cases, noise of sufficient intensity led to a complete inability to discriminate the fine spectral structure of the test signal. In dichotic presentation (test signal to one ear, noise to the other), noise had virtually no effect on FRA over a wide range of test signal and noise frequencies and noise:signal ratios. Thus, there was essentially complete dichotic release of FRA from the effects noise, which has potential to be used in constructing prosthetic hearing devices.

Peripheral and central aspects of auditory across-frequency processing

Many natural sounds such as, e.g., speech show common level fluctuations across frequency. It is generally assumed that the auditory system uses this spectro-temporal information to group the frequency components into auditory objects although the exact physiological mechanism is still not fully understood. The aim of the present study is to disentangle the relative contribution of peripheral and central aspects of this across-frequency processing using psychophysical experiments and modelling. The study focuses on two different psychophysical phenomena which are thought to be related to the ability to compare information across frequency: comodulation masking release (CMR), i.e., a release from masking of a sinusoidal signal due to the addition of a comodulated off-frequency masker component to the masker component at the signal frequency, and comodulation detection difference (CDD), i.e., the reduced ability of the auditory system to detect a masked signal if masker and signal share the same envelope. The comparison between model predictions and experimental results indicates that a considerable amount of these effects can be accounted for by peripheral processing alone. This is confirmed by experimental results with confounding across-frequency information about the grouping of the different frequencies into auditory objects.

Two-tone suppression of stimulus frequency otoacoustic emissions

Stimulus frequency otoacoustic emissions (SFOAEs) measured using a suppressor tone in human ears are analogous to two-tone suppression responses measured mechanically and neurally in mammalian cochleae. SFOAE suppression was measured in 24 normal-hearing adults at octave frequencies (f(p)=0.5-8.0 kHz) over a 40 dB range of probe levels (L(p)). Suppressor frequencies (f(s)) ranged from -2.0 to 0.7 octaves re: f(p), and suppressor levels ranged from just detectable suppression to full suppression. The lowest suppression thresholds occurred for "best" f(s) slightly higher than f(p). SFOAE growth of suppression (GOS) had slopes close to one at frequencies much lower than best f(s), and shallow slopes near best f(s), which indicated compressive growth close to 0.3 dBdB. Suppression tuning curves constructed from GOS functions were well defined at 1, 2, and 4 kHz, but less so at 0.5 and 8.0 kHz. Tuning was sharper at lower L(p) with an equivalent rectangular bandwidth similar to that reported behaviorally for simultaneous masking. The tip-to-tail difference assessed cochlear gain, increasing with decreasing L(p) and increasing f(p) at the lowest L(p) from 32 to 45 dB for f(p) from 1 to 4 kHz. SFOAE suppression provides a noninvasive measure of the saturating nonlinearities associated with cochlear amplification on the basilar membrane.

The effect of masker level uncertainty on intensity discrimination

Thresholds were measured for detection of an increment in level of a 60-dB SPL target tone at 1 kHz, either in quiet or in the presence of maskers at 0.5 and 2 kHz. Interval-by-interval level rove applied independently to remote masker tones substantially elevated thresholds compared to intensity discrimination in quiet, an effect on the order of 10+dB [10 log(DeltaII)]. Asynchronous onset and stimulus envelope mismatches across frequency reduced but did not eliminate masking. A preinterval cue to signal frequency had no effect, but cuing masker frequency reduced thresholds, whether or not masker level was also cued. About 1 to 2 dB of threshold elevation in these conditions can be attributed to energetic masking. Decreasing the overall presentation level and increasing masker separation essentially eliminates energetic masking; under these conditions masker level rove elevates thresholds by approximately 7 dB when the target and masker tones are gated synchronously. This masking persists even when the flanking masker tones are presented contralateral to the target. Results suggest that observers tend to listen synthetically, even in conditions when this strategy reduces sensitivity to the intensity increment.

Temporal integration and compression near absolute threshold in normal and impaired ears

The decrease in absolute threshold with increasing stimulus duration (often referred to as "temporal integration") is greater for listeners with normal hearing than for listeners with sensorineural hearing loss. It has been suggested that the difference is related to reduced basilar-membrane (BM) compression in the impaired group. The present experiment tested this hypothesis by comparing temporal integration and BM compression in normal and impaired ears at low levels. Absolute thresholds were measured for 4, 24, and 44 ms pure-tone signals, with frequencies (f(s)) of 2 and 4 kHz. The difference between the absolute thresholds for the 4 and 24 ms signals was used as a measure of temporal integration. Compression near threshold was estimated by measuring the level of a 100 ms off-frequency (0.45f(s)) pure-tone forward masker required to mask a 44 ms pure-tone signal presented at sensation levels of 5 and 10 dB. There was a significant negative correlation between amount of temporal integration and absolute threshold. However, there was no correlation between absolute threshold and compression at low levels; both normal and impaired ears showed a nearly linear response. The results suggest that the differences in integration between normal and impaired ears cannot be explained by differences in BM compression.

Speech Recognition in Noise: Estimating Effects of Compressive Nonlinearities in the Basilar-Membrane Response

OBJECTIVES

This experiment was designed to estimate effects of cochlear nonlinearities on tonal and speech masking for individuals with normal hearing who have a range of quiet thresholds. Physiological and psychophysical evidence indicates that for signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower stimulus levels and compressed at medium to higher stimulus levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, the hypothesis tested was that masker effectiveness would change as a function of stimulus level consistent with the underlying basilar membrane response. Specifically, with a fixed-level speech signal and a speech-shaped masker that ranges from low to higher levels, the resulting response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels. This would result in relatively less effective masking at higher masker levels. It was further hypothesized that the transition from linear to compressed responses to both tones and maskers would occur at higher levels for listeners with higher quiet thresholds than for listeners with lower quiet thresholds.

DESIGN

Tonal thresholds and speech recognition in noise were measured as a function of masker level. A 10-msec, 2.0-kHz tone was presented in a lower frequency masker ranging from 40 to 85 dB SPL. Moderate-level speech was presented in interrupted noise at six levels ranging from 47 to 77 dB SPL. To minimize differences in speech audibility that could arise during the "off" periods of the interrupted noise, a low-level steady-state "threshold-matching noise" was also present during measurement of speech recognition. Subjects were 30 adults with normal hearing with a 20-dB range of average quiet thresholds.

RESULTS

Tonal breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were significantly correlated with quiet thresholds, whereas slopes measured above the breakpoints were not. Speech recognition in noise was consistent with the hypothesis that the response of the basilar membrane to the masker was linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. That is, at lower masker levels, as masker level increased, mean observed speech scores declined as predicted using the articulation index, an audibility-based model. With further increases in masker level, mean scores declined less than predicted. Moreover, for subjects with higher quiet thresholds, masker effectiveness remained constant for a wider range of masker levels than for subjects with lower quiet thresholds, consistent with the hypothesis that the transition from linear to compressed responses occurred at higher levels. Finally, significant negative correlations were obtained between individual subjects' tonal and speech measures.

CONCLUSIONS

Results from tonal and speech tasks were consistent with basilar membrane nonlinearities and consistent with changes in nonlinearities with minor threshold elevations, providing support for their role in the understanding of speech in noise with increases in noise level.

Estimates of basilar-membrane nonlinearity effects on masking of tones and speech

OBJECTIVE

The aim of this experiment was to assess the contribution of cochlear nonlinearities to speech recognition in noise for individuals with normal hearing and a range of quiet thresholds. For signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower signal levels and compressed at medium to higher signal levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, for moderate-level speech and a lower frequency masker, the response to the masker grows linearly whereas the response to the speech is compressed, which may result in changes in the effectiveness of the masker on speech recognition with increases in masker level. To test this hypothesis, observed speech-recognition scores were compared with scores predicted using an audibility-based model, which did not include nonlinear effects that may influence masker effectiveness.

DESIGN

Growth of simultaneous masking was measured for moderate-level bandpass-filtered nonsense syllables and for 350-msec pure tones at frequencies within the speech passband. Masker frequencies were within (on-frequency) or below (off-frequency) the speech passband. Estimates of basilar-membrane nonlinearities were derived from growth-of-masking functions for 10-msec, 2.0- and 4.0-kHz tones in narrowband, off-frequency maskers presented simultaneously. Subjects were 26 adults with normal hearing with approximately a 20-dB range of average quiet thresholds.

RESULTS

Breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were strongly associated with quiet thresholds but slopes measured above the breakpoints were independent of quiet thresholds. Individual differences were substantially larger for off-frequency masking of pure tones and speech than for on-frequency masking of pure tones and speech. Using an audibility-based predictive model, the change in speech audibility resulting from the compressed response to speech with increasing off-frequency masker level (and the resulting decline in scores) was well predicted from nonlinear growth of masking for pure tones measured in the same off-frequency masker. However, absolute speech-recognition predictions were generally inaccurate and were a function of how well pure-tone signal levels at masked threshold estimated masker effectiveness for speech. That is, subjects with lower off-frequency masked thresholds had less accurate predictions of speech recognition in off-frequency maskers.

CONCLUSIONS

Large individual differences in off-frequency masking of pure tones and speech are consistent with the assumption that small changes in the shape of the basilar-membrane input-output function result in large changes in the amount of off-frequency masking but small (if any) changes in on-frequency masking where the signal and masker are subject to a similar compression. Growth of off-frequency masking of pure tones and speech were correlated with each other, consistent with the underlying basilar-membrane response, and consistent with changes in breakpoints for subjects with normal hearing and a range of quiet thresholds. These results provide support for a role of nonlinear effects in the understanding of speech in noise.

The future of hearing aid technology

Hearing aids have advanced significantly over the past decade, primarily due to the maturing of digital technology. The next decade should see an even greater number of innovations to hearing aid technology, and this article attempts to predict in which areas the new developments will occur. Both incremental and radical innovations in digital hearing aids will be driven by research advances in the following fields: (1) wireless technology, (2) digital chip technology, (3) hearing science, and (4) cognitive science. The opportunities and limitations for each of these areas will be discussed. Additionally, emerging trends such as connectivity and individualization will also drive new technology, and these are discussed within the context of the areas given here.

The effects of low- and high-frequency suppressors on psychophysical estimates of basilar-membrane compression and gain

Physiological studies suggest that the increase in suppression as a function of suppressor level is greater for a suppressor below than above the signal frequency. This study investigated the pattern of gain reduction underlying this increase in suppression. Temporal masking curves (TMCs) were obtained by measuring the level of a 2.2-kHz sinusoidal off-frequency masker or 4-kHz on-frequency sinusoidal masker required to mask a brief 4-kHz sinusoidal signal at 10 dB SL, for masker-signal intervals of 20-100 ms. TMCs were also obtained in the presence of a 3- or 4.75-kHz sinusoidal suppressor gated with the 4-kHz masker, for suppressor levels of 40-70 dB SPL. The decrease in gain (increase in suppression) as a function of suppressor level was greater with a 3-kHz suppressor than with a 4.75-kHz suppressor, in line with previous findings. Basilar membrane input-output (I/O) functions derived from the TMCs showed a shift to higher input (4-kHz masker) levels of the low-level (linear) portion of the I/O function with the addition of a suppressor, with partial linearization of the function, but no reduction in maximum compression.

Role of suppression and retro-cochlear processes in comodulation masking release

Recent physiological studies suggest that comodulation masking release (CMR) could be a consequence of wideband inhibition at the level of the cochlear nucleus. The present study investigates whether the existence region of psychophysical CMR is comparable to the inhibitory areas of units showing a physiological correlate of CMR. Since the inhibitory areas are similar to suppressive regions at the level of the basilar membrane, the amount of CMR that can be accounted for by suppression was determined by predicting the data with a model incorporating a peripheral nonlinearity. A CMR of up to 6 dB could still be experimentally observed for a flanking band (FB) four octaves below the on-frequency masker (OFM). For FB frequencies below the OFM, the suggested model predicts CMR equal to the measured CMR for high levels of the FB. The model underestimates the magnitude of CMR for midlevels of the FB, indicating that suppression alone cannot account for CMR. The data are consistent with the hypothesis that wideband inhibition plays a role in CMR.

The role of suppression in the upward spread of masking

The upward spread of masking refers to the higher growth rate of masking for maskers lower in frequency than the signal, compared to maskers at the signal frequency (Wegel RL, Lane CE. The auditory masking of one pure tone by another and its possible relation to the dynamics of the inner ear. Physics Rev. 23:266-285, 1924; Egan JP, Hake HW. On the masking pattern of a simple auditory stimulus. J. Acoust. Soc. Am. 22:622-630, 1950; Delgutte B. Physiological mechanisms of psychophysical masking: Observations from auditory-nerve fibres. J. Acoust. Soc. Am. 87:791-809, 1990a, Delgutte B. Two-tone rate suppression in auditory-nerve fibres: Dependence on suppressor frequency and level. Hear Res. 49:225-246, 1990b). The upward spread of simultaneous masking may arise from a combination of excitatory and suppressive effects. In this study, growth of masking functions were obtained for a 4-kHz signal masked by an on-frequency (4 kHz) or off-frequency (2.4 kHz), simultaneous or forward masker, in the presence of a notched noise with a center frequency of 4 kHz presented to restrict off-frequency listening. Compression was estimated from the slopes of the off-frequency growth of masking functions. Suppression was estimated by comparing the off-frequency simultaneous- and forward-masked growth of masking functions. Results showed that, for midlevel signals (35-60 dB SPL), the compression exponent estimated from simultaneous and forward masking averaged 0.31 and 0.26, respectively. The maximum amount of suppression (defined as the decrease in the basilar-membrane response to the signal) was variable, ranging from about 6 to 17 dB across subjects. Despite the substantial reduction in the response to the signal, the results suggest that suppression has a minimal effect on the slope of the masking function at mid levels. Rather, upward spread of masking seems to be mainly determined by the compressive basilar-membrane response to the signal in relation to the linear response to the lower-frequency masker.

Rippled-spectrum resolution dependence on masker-to-probe ratio

Resolution of rippled sound spectrum (probe) in the presence of additional noise band (masker) was studied as a function of masker-to-probe ratio and sound level in normal listeners. The probe bands were 0.5-oct wide (ERB) centered at 2 kHz; the masker band either coincided with the probe (on-frequency masker), or was 3/4 octaves below (low-frequency masker), or 3/4 octaves above the probe (high-frequency masker). Ripple-density resolution in the probe band was measured by finding the highest ripple density at which an interchange of ripple peaks and valleys was detectable (the phase-reversal test). (i) The effect of the low-frequency masker increased (resolution decreased) when masker-to-probe ratio changed from -25 dB to +20 dB; the effect increased (resolution decreased) with sound level increase. (ii) The effect of the on-frequency masker steeply increased (resolution abruptly decreased) when masker-to-probe ratio exceeded 0 dB; the effect was little dependent on sound level. (iii) The high-frequency masker was little effective unless the masker-to-probe ratio reached 30-40 dB; the effect increased (resolution decreased) with sound level decrease. Thus, different position of the masker band relative to the probe resulted in qualitatively different kinds of spectrum-pattern resolution dependence on both the masker-to-probe ratio and sound level.

Level dependence of auditory filters in nonsimultaneous masking as a function of frequency

Auditory filter bandwidths were measured using nonsimultaneous masking, as a function of signal level between 10 and 35 dB SL for signal frequencies of 1, 2, 4, and 6 kHz. The brief sinusoidal signal was presented in a temporal gap within a spectrally notched noise. Two groups of normal-hearing subjects were tested, one using a fixed masker level and adaptively varying signal level, the other using a fixed signal level and adaptively varying masker level. In both cases, auditory filters were derived by assuming a constant filter shape for a given signal level. The filter parameters derived from the two paradigms were not significantly different. At 1 kHz, the equivalent rectangular bandwidth (ERB) decreased as the signal level increased from 10 to 20 dB SL, after which it remained roughly constant. In contrast, at 6 kHz, the ERB increased consistently with signal levels from 10 to 35 dB SL. The results at 2 and 4 kHz were intermediate, showing no consistent change in ERB with signal level. Overall, the results suggest changes in the level dependence of the auditory filters at frequencies above 1 kHz that are not currently incorporated in models of human auditory filter tuning.

Some problems in the measurement of the frequency-resolving ability of hearing

Despite the detailed development of masking methods for measurement of the frequency selectivity of hearing, these measurements are hardly used for diagnostic purposes because they are time-consuming and because of the uncertain extrapolation of the results to the perception of complex spectral patterns. A method for the direct measurement of the spectral resolving ability of hearing using test signals with rippled spectra is proposed. These measurements showed 1) that the resolving ability of the auditory system in terms of discriminating complex spectra is greater than that suggested by the acuity of auditory frequency filters; 2) that changes in the acuity of frequency auditory filters associated with sound intensity hardly affect the ability to resolve complex spectra; 3) that the effects of interference on frequency-resolving ability do not lead to decreases in the spectral contrast of signals due to superimposition of noise.

Transient-evoked stimulus-frequency and distortion-product otoacoustic emissions in normal and impaired ears

Transient-evoked stimulus-frequency otoacoustic emissions (SFOAEs), recorded using a nonlinear differential technique, and distortion-product otoacoustic emissions (DPOAEs) were measured in 17 normal-hearing and 10 hearing-impaired subjects using pairs of tone pips (pp), gated tones (gg), and for DPOAEs, continuous and gated tones (cg). Temporal envelopes of stimulus and OAE waveforms were obtained by narrow-band filtering at the stimulus or DP frequency. Mean SFOAE latencies in normal ears at 2.7 and 4.0 kHz decreased with increasing stimulus level and were larger at 4.0 kHz than latencies in impaired ears. Equivalent auditory filter bandwidths were calculated as a function of stimulus level from SFOAE latencies by assuming that cochlear transmission is minimum phase. DPOAE latencies varied less with level than SFOAE latencies. The ppDPOAEs often had two (or more) peaks separated in time with latencies consistent with model predictions for distortion and reflection components. Changes in ppDPOAE latency with level were sometimes explained by a shift in relative amplitudes of distortion and reflection components. The pp SFOAE SPL within the main spectral lobe of the pip stimulus was higher for normal ears in the higher-frequency half of the pip than the lower-frequency half, which is likely an effect of basilar membrane two-tone suppression.

Searching for the optimal stimulus eliciting auditory brainstem responses in humans

This study examines auditory brainstem responses (ABR) elicited by rising frequency chirps. Two chirp stimuli were developed and designed such as to compensate for cochlear travel-time differences across frequency, in order to maximize neural synchrony. One chirp, referred to as the O-chirp, was based on estimates of human basilar membrane (BM) group delays derived from stimulus-frequency otoacoustic emissions (SFOAE) at a sound pressure level of 40 dB [Shera and Guinan, in Recent Developments in Auditory Mechanics (2000)]. The other chirp, referred to as the A-chirp, was derived from latency functions fitted to tone-burst-evoked ABR wave-V data over a wide range of stimulus levels and frequencies [Neely et al., J. Acoust. Soc. Am. 83(2), 652-656 (1988)]. In this case, a set of level-dependent chirps was generated. The chirp-evoked responses, particularly wave-V amplitude and latency, were compared to click responses and to responses obtained with the original chirp as defined in Dau et al. [J. Acoust. Soc. Am. 107(3), 1530-1540 (2000)], referred to here as the M-chirp since it is based on a (linear) cochlea model. The main hypothesis was that, at low and medium stimulation levels, the O- and A-chirps might produce a larger response than the original M-chirp whose parameters were essentially derived from high-level BM data. The main results of the present study are as follows: (i) All chirps evoked a larger wave-V amplitude than the click stimulus indicating that for the chirps a broader range of spectral components contributes effectively to the ABR. (ii) Only small differences were found between the O-chirp and M-chirp responses at low and medium levels. This indicates that SFOAE may not provide a robust estimate of BM group delay, particularly at low frequencies, or that frequency-dependent neural delays exist which are not reflected in the design of these chirps. (iii) The A-chirp produced the largest responses, particularly at low stimulation levels. This chirp might therefore be valuable for clinical applications, particularly in tests where the click stimulus has been used so far.

Rippled-spectrum resolution dependence on level

Rippled-density resolution of a rippled sound spectrum (probe band) in both the presence and absence of another band (masker) was studied as a function of sound level in normal listeners. The resolvable ripple density in the probe band was measured by finding the highest ripple density at which an interchange of ripple peak and valley positions was detectable (the phase-reversal test). Probe bands were 0.5 oct wide with center frequencies of 1, 2, and 4 kHz. In the control condition (no masker), the ripple-density resolution was almost independent of sound level within a range of 40-90 dB SPL. When an on-frequency masker coincided with the probe band (that resulted in reduced ripple depth), resolution decreased slightly relative to the control condition but remained little dependent on level. With an off-frequency low-side masker, the ripple-density resolution was a little less than in the control but almost independent of level within a range of 40-60 dB SPL and progressively decreased with level increase from 70 to 90 dB SPL. The dependence on level was qualitatively similar at all probe frequencies and at various widths and positions of the low-side off-frequency masker band.

The temporal effect with notched-noise maskers: analysis in terms of input-output functions

This study examines whether a temporal masking effect may be consistent with a decrease in gain at the masker frequency during the course of the masker. Threshold level of a long-duration notched-noise masker needed to mask a 1- or 4-kHz signal was measured for three conditions: a short-duration signal with a short delay or a long delay from masker onset, and a long-duration signal. The difference between threshold for the long-delay signal and the short-delay signal was defined as the temporal effect. The size of the temporal effect depended on signal frequency, signal level, and masker notch width. Filters estimated from the data had narrower bandwidths for the long-delay condition than for the short-delay condition or the long-duration condition, which seems inconsistent with the hypothesis of a decrease in gain. However, modeling of the data in terms of basilar-membrane input-output functions is consistent with a decrease in gain in the masker frequency region during the course of the masker. For a notch width of 0.0 the results are consistent with a decrease in gain at the signal frequency. For a relative notch width of 0.4, the decrease in gain at the masker frequency may cause a decrease in the suppression of the signal. This decrease in suppression could explain the decrease in filter bandwidth with signal delay.

Acoustic noise concerns in functional magnetic resonance imaging

Magnetic resonance (MR) acoustic scanner noise may negatively affect the performance of functional magnetic resonance imaging (fMRI), a problem that worsens at the higher field strengths proposed to enhance fMRI. We present an overview of the current knowledge on the effects of confounding acoustic MR noise in fMRI experiments. The principles and effectiveness of various methods to reduce acoustic noise in fMRI are discussed, practical considerations are addressed and recommendations are made.

Maturation of cochlear nonlinearity as measured by distortion product otoacoustic emission suppression growth in humans

The growth of distortion product otoacoustic emission (DPOAE) suppression follows a systematic, frequency-dependent pattern. The pattern is consistent with direct measures of basilar-membrane response growth, psychoacoustic measures of masking growth, and measures of neural rate growth. This pattern has its basis in the recognized nonlinear properties of basilar-membrane motion and, as such, the DPOAE suppression growth paradigm can be applied to human neonates to study the maturation of cochlear nonlinearity. The objective of this experiment was to investigate the maturation of human cochlear nonlinearity and define the time course for this maturational process. Normal-hearing adults, children, term-born neonates, and premature neonates, plus a small number of children with sensorineural hearing loss, were included in this experiment. DPOAE suppression growth was measured at two f2 frequencies (1500 and 6000 Hz) and three primary tone levels (55-45, 65-55, and 75-65 dB SPL). Slope of DPOAE suppression growth, as well as an asymmetry ratio (to compare slope for suppressor tones below and above f2 frequency), were generated. Suppression threshold was also measured in all subjects. Findings indicate that both term-born neonates and premature neonates who have attained term-like age, show non-adult-like DPOAE suppression growth for low-frequency suppressor tones. These age effects are most evident at f2 = 6000 Hz. In neonates, suppression growth is shallower and suppression thresholds are elevated for suppressor tones lower in frequency than f2. Additionally, the asymmetry ratio is smaller in neonates, indicating that the typical frequency-dependent pattern of suppression growth is not present. These findings suggest that an immaturity of cochlear nonlinearity persists into the first months of postnatal life. DPOAE suppression growth examined for a small group of hearing-impaired children also showed abnormalities.

Estimates of human cochlear tuning at low levels using forward and simultaneous masking

Auditory filter shapes were derived from psychophysical measurements in eight normal-hearing listeners using a variant of the notched-noise method for brief signals in forward and simultaneous masking. Signal frequencies of 1, 2, 4, 6, and 8 kHz were tested. The signal level was fixed at 10 dB above absolute threshold in the forward-masking conditions and fixed at either 10 or 35 dB above absolute threshold in the simultaneous-masking conditions. The results show that filter equivalent rectangular bandwidths (ERBs) are substantially narrower in forward masking than has been found in previous studies using simultaneous masking. Furthermore, in contrast to earlier studies, the sharpness of tuning doubles over the range of frequencies tested, giving Q(ERB) values of about 10 and 20 at signal frequencies of 1 and 8 kHz, respectively. It is argued that the new estimates of auditory filter bandwidth provide a more accurate estimate of human cochlear tuning at low levels than earlier estimates using simultaneous masking at higher levels, and that they are therefore more suitable for comparison to cochlear tuning data from other species. The data may also prove helpful in defining the parameters for nonlinear models of human cochlear processing.

Psychophysical evidence for auditory compression at low characteristic frequencies

Psychophysical estimates of compression often assume that the basilar-membrane response to frequencies well below characteristic frequency (CF) is linear. Two techniques for estimating compression are described here that do not depend on this assumption at low CFs. In experiment 1, growth of forward masking was measured for both on- and off-frequency pure-tone maskers for pure-tone signals at 250, 500, and 4000 Hz. The on- and off-frequency masking functions at 250 and 500 Hz were just as shallow as the on-frequency masking function at 4000 Hz. In experiment 2, the forward masker level required to mask a fixed low-level signal was measured as a function of the masker-signal interval. The slopes of these functions did not differ between signal frequencies of 250 and 4000 Hz for the on-frequency maskers. At 250 Hz, the slope for the 150-Hz masker was almost as steep as that for the on-frequency masker, whereas at 4000 Hz the slope for the 2400-Hz masker was much shallower than that for the on-frequency masker. The results suggest that there is substantial compression, of around 0.2-0.3 dB/dB, at low CFs in the human auditory system. Furthermore, the results suggest that at low CFs compression does not vary greatly with stimulation frequency relative to CF.

Evidence of upward spread of suppression in DPOAE measurements

Measurements of DPOAE level in the presence of a suppressor were used to describe a pattern that is qualitatively similar to population studies in the auditory nerve and to behavioral studies of upward spread of masking. DPOAEs were measured in the presence of a suppressor (f3) fixed at either 2.1 or 4.2 kHz, and set to each of seven levels (L3) from 20 to 80 dB SPL. In the presence of a fixed f3 and L3 combination, f2 was varied from about 1 oct below to at least 1/2 oct above f3, while L2 was set to each of 6 values (20-70 dB SPL). L1 was set according to the equation L1 = 0.4L2 + 39 [Janssen et al., J. Acoust. Soc. Am. 103, 3418-3430 (1998)]. At each L2, L1 combination, DPOAE level was measured in a control condition in which no suppressor was presented. Data were converted into decrements (the amount of suppression, in dB) by subtracting the DPOAE level in the presence of each suppressor from the DPOAE level in the corresponding control condition. Plots of DPOAE decrements as a function of f2 showed maximum suppression when f2 approximately = f3. As L3 increased, the suppressive effect spread more towards higher f2 frequencies, with less spread towards lower frequencies relative to f3. DPOAE decrement versus L3 functions had steeper slopes when f2 > f3, compared to the slopes when f2 < f3. These data are consistent with other findings that have shown that response growth for a characteristic place (CP) or frequency (CF) depends on the relation between CP or CF and driver frequency, with steeper slopes when driver frequency is less than CF and shallower slopes when driver frequency is greater than CF. For a fixed amount of suppression (3 dB), L3 and L2 varied nearly linearly for conditions in which f3 approximately = f2, but grew more rapidly for conditions in which f3 < f2, reflecting the basal spread of excitation to the suppressor. The present data are similar in form to the results observed in population studies from the auditory nerve of lower animals and in behavioral masking studies in humans.

Revised estimates of human cochlear tuning from otoacoustic and behavioral measurements

We develop an objective, noninvasive method for determining the frequency selectivity of cochlear tuning at low and moderate sound levels. Applicable in humans at frequencies of 1 kHz and above, the method is based on the measurement of stimulus-frequency otoacoustic emissions and, unlike previous noninvasive physiological methods, does not depend on the frequency selectivity of masking or suppression. The otoacoustic measurements indicate that at low sound levels human cochlear tuning is more than twice as sharp as implied by standard behavioral studies and has a different dependence on frequency. New behavioral measurements designed to minimize the influence of nonlinear effects such as suppression agree with the emission-based values. A comparison of cochlear tuning in cat, guinea pig, and human indicates that, contrary to common belief, tuning in the human cochlea is considerably sharper than that found in the other mammals. The sharper tuning may facilitate human speech communication.

Quantifying the implications of nonlinear cochlear tuning for auditory-filter estimates

The relation between auditory filters estimated from psychophysical methods and peripheral tuning was evaluated using a computational auditory-nerve (AN) model that included many of the response properties associated with nonlinear cochlear tuning. The phenomenological AN model included the effects of dynamic level-dependent tuning, compression, and suppression on the responses of high-, medium-, and low-spontaneous-rate AN fibers. Signal detection theory was used to evaluate psychophysical performance limits imposed by the random nature of AN discharges and by random-noise stimuli. The power-spectrum model of masking was used to estimate psychophysical auditory filters from predicted AN-model detection thresholds for a tone signal in fixed-level notched-noise maskers. Results demonstrate that the role of suppression in broadening peripheral tuning in response to the noise masker has implications for the interpretation of psychophysical auditory-filter estimates. Specifically, the estimated psychophysical auditory-filter equivalent-rectangular bandwidths (ERBs) that were derived from the nonlinear AN model with suppression always overestimated the ERBs of the low-level peripheral model filters. Further, this effect was larger for an 8-kHz signal than for a 2-kHz signal, suggesting a potential characteristic-frequency (CF) dependent bias in psychophysical estimates of auditory filters due to the increase in strength of cochlear nonlinearity with increases in CF.

Towards a measure of auditory-filter phase response

This study investigates how the phase curvature of the auditory filters varies with center frequency (CF) and level. Harmonic tone complex maskers were used, with component phases adjusted using a variant of an equation proposed by Schroeder [IEEE Trans. Inf. Theory 16, 85-89 (1970)]. In experiment 1, the phase curvature of the masker was varied systematically and sinusoidal signal thresholds were measured at frequencies from 125 to 8000 Hz. At all signal frequencies, threshold differences of 20 dB or more were observed between the most effective and least effective masker phase curvature. In experiment 2, the effect of overall masker level on masker phase effects was studied using signal frequencies of 250, 1000, and 4000 Hz. The results were used to estimate the phase curvature of the auditory filters. The estimated relative phase curvature decreases dramatically with decreasing CF below 1000 Hz. At frequencies above 1000 Hz, relative auditory-filter phase curvature increases only slowly with increasing CF, or may remain constant. The phase curvature of the auditory filters seems to be broadly independent of overall level. Most aspects of the data are in qualitative agreement with peripheral physiological findings from other mammals, which suggests that the phase responses observed here are of peripheral origin. However, in contrast to the data reported in a cat auditory-nerve study [Carney et al., J. Acoust. Soc. Am. 105, 2384-2391 (1999)], no reversal in the sign of the phase curvature was observed at very low frequencies. Overall, the results provide a framework for mapping out the phase curvature of the auditory filters and provide constraints on future models of peripheral filtering in the human auditory system.

Psychophysical suppression effects for tonal and speech signals

This experiment assessed the benefits of suppression and the impact of reduced or absent suppression on speech recognition in noise. Psychophysical suppression was measured in forward masking using tonal maskers and suppressors and band limited noise maskers and suppressors. Subjects were 10 younger and 10 older adults with normal hearing, and 10 older adults with cochlear hearing loss. For younger subjects with normal hearing, suppression measured with noise maskers increased with masker level and was larger at 2.0 kHz than at 0.8 kHz. Less suppression was observed for older than younger subjects with normal hearing. There was little evidence of suppression for older subjects with cochlear hearing loss. Suppression measured with noise maskers and suppressors was larger in magnitude and more prevalent than suppression measured with tonal maskers and suppressors. The benefit of suppression to speech recognition in noise was assessed by obtaining scores for filtered consonant-vowel syllables as a function of the bandwidth of a forward masker. Speech-recognition scores in forward maskers should be higher than those in simultaneous maskers given that forward maskers are less effective than simultaneous maskers. If suppression also mitigated the effects of the forward masker and resulted in an improved signal-to-noise ratio, scores should decrease less in forward masking as forward-masker bandwidth increased, and differences between scores in forward and simultaneous maskers should increase, as was observed for younger subjects with normal hearing. Less or no benefit of suppression to speech recognition in noise was observed for older subjects with normal hearing or hearing loss. In general, as suppression measured with tonal signals increased, the combined benefit of forward masking and suppression to speech recognition in noise also increased.