The effect of basilar-membrane nonlinearity on the shapes of masking period patterns in normal and impaired hearing

Cochlear hearing loss and the detection of sinusoidal versus random amplitude modulation

This study assessed the effect of cochlear hearing loss on detection of random and sinusoidal amplitude modulation. Listeners with hearing loss and normal-hearing listeners (eight per group) generated temporal modulation transfer functions (TMTFs) for envelope fluctuations carried by a 2000-Hz pure tone. TMTFs for the two groups were similar at low modulation rates but diverged at higher rates presumably because of differences in frequency selectivity. For both groups, detection of random modulation was poorer than for sinusoidal modulation at lower rates but the reverse occurred at higher rates. No evidence was found that cochlear hearing loss, per se, affects modulation detection.

Behavioral measures of cochlear compression and temporal resolution as predictors of speech masking release in hearing-impaired listeners

Hearing-impaired (HI) listeners often show less masking release (MR) than normal-hearing listeners when temporal fluctuations are imposed on a steady-state masker, even when accounting for overall audibility differences. This difference may be related to a loss of cochlear compression in HI listeners. Behavioral estimates of compression, using temporal masking curves (TMCs), were compared with MR for band-limited (500-4000 Hz) speech and pure tones in HI listeners and age-matched, noise-masked normal-hearing (NMNH) listeners. Compression and pure-tone MR estimates were made at 500, 1500, and 4000 Hz. The amount of MR was defined as the difference in performance between steady-state and 10-Hz square-wave-gated speech-shaped noise. In addition, temporal resolution was estimated from the slope of the off-frequency TMC. No significant relationship was found between estimated cochlear compression and MR for either speech or pure tones. NMNH listeners had significantly steeper off-frequency temporal masking recovery slopes than did HI listeners, and a small but significant correlation was observed between poorer temporal resolution and reduced MR for speech. The results suggest either that the effects of hearing impairment on MR are not determined primarily by changes in peripheral compression, or that the TMC does not provide a sufficiently reliable measure of cochlear compression.

Temporal masking functions for listeners with real and simulated hearing loss

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal masking functions for eight listeners with mild to severe sensorineural hearing loss. Each audiometric loss was simulated in a group of age-matched normal-hearing listeners through a combination of spectrally-shaped masking noise and multi-band expansion. Temporal-masking functions were obtained in both groups of listeners using a forward-masking paradigm in which the level of a 110-ms masker required to just mask a 10-ms fixed-level probe (5-10 dB SL) was measured as a function of the time delay between the masker offset and probe onset. At each of four probe frequencies (500, 1000, 2000, and 4000 Hz), temporal-masking functions were obtained using maskers that were 0.55, 1.0, and 1.15 times the probe frequency. The slopes and y-intercepts of the masking functions were not significantly different for listeners with real and simulated hearing loss. The y-intercepts were positively correlated with level of hearing loss while the slopes were negatively correlated. The ratio of the slopes obtained with the low-frequency maskers relative to the on-frequency maskers was similar for both groups of listeners and indicated a smaller compressive effect than that observed in normal-hearing listeners.

Investigating temporal asymmetry using masking period patterns and models of peripheral auditory processing

Two experiments were conducted in conjunction with modeling to evaluate the role of peripheral nonlinearity and neural adaptation in the perception of temporally asymmetric sounds. In both experiments, maskers were broadband noises amplitude modulated with ramped and damped exponential modulators that repeated at 40 Hz. Masking period patterns (MPPs) were constructed by measuring detection threshold of a 5-ms, 1000-Hz tone burst as function of the signal's onset delay. Experiment I showed that varying modulator half-life from 1 to 16 ms led to differences in the damped and the ramped MPPs that were largest at the short half-lives and diminished at the longer half-lives. When masker level was varied (experiment II), the largest difference between ramped and damped MPPs occurred at moderate stimulus levels. Two peripheral auditory models were evaluated, one a simple auditory filter followed by a power-law nonlinearity and another, a model of auditory nerve processing [J. Acoust. Soc. Am. 126, 2390-2412 (2009)] that includes neural adaptation. Neither models predicted differences between the ramped and damped MPPs, providing indirect support that the central auditory system has a role in perceptual temporal asymmetry.

Modeling speech intelligibility in quiet and noise in listeners with normal and impaired hearing

The speech intelligibility index (SII) is an often used calculation method for estimating the proportion of audible speech in noise. For speech reception thresholds (SRTs), measured in normally hearing listeners using various types of stationary noise, this model predicts a fairly constant speech proportion of about 0.33, necessary for Dutch sentence intelligibility. However, when the SII model is applied for SRTs in quiet, the estimated speech proportions are often higher, and show a larger inter-subject variability, than found for speech in noise near normal speech levels [65 dB sound pressure level (SPL)]. The present model attempts to alleviate this problem by including cochlear compression. It is based on a loudness model for normally hearing and hearing-impaired listeners of Moore and Glasberg [(2004). Hear. Res. 188, 70-88]. It estimates internal excitation levels for speech and noise and then calculates the proportion of speech above noise and threshold using similar spectral weighting as used in the SII. The present model and the standard SII were used to predict SII values in quiet and in stationary noise for normally hearing and hearing-impaired listeners. The present model predicted SIIs for three listener types (normal hearing, noise-induced, and age-induced hearing loss) with markedly less variability than the standard SII.

On- and off-frequency forward masking by Schroeder-phase complexes

Forward masking by harmonic tone complexes was measured for on- and off-frequency maskers as a function of masker phase curvature for two masker durations (30 and 200 ms). For the lowest signal frequency (1 kHz), the results matched predictions based on the expected interactions between the phase curvature and amplitude compression of peripheral auditory filtering. For the higher signal frequencies (2 and 6 kHz), the data increasingly departed from predictions in two respects. First, the effects of the masker phase curvature became stronger with increasing masker duration, inconsistent with the expected effects of the fast-acting compression and time-invariant phase response of basilar membrane filtering. Second, significant effects of masker phase curvature were observed for the off-frequency masker using a 6-kHz signal, inconsistent with predictions based on linear processing of stimuli well below the signal frequency. New predictions were generated assuming an additional effect with a longer time constant, consistent with the influence of medial olivocochlear efferent activation on otoacoustic emissions in humans. Reasonable agreement between the predicted and the measured effects suggests that efferent activation is a potential candidate mechanism to explain certain spectro-temporal masking effects in human hearing.

Temporal masking curves for hearing-impaired listeners

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, fs, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times fs. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each fs, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for fs = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high fs.

Comparing different estimates of cochlear compression in listeners with normal and impaired hearing

A loss of cochlear compression may underlie many of the difficulties experienced by hearing-impaired listeners. Two behavioral forward-masking paradigms that have been used to estimate the magnitude of cochlear compression are growth of masking (GOM) and temporal masking (TM). The aim of this study was to determine whether these two measures produce within-subjects results that are consistent across a range of signal frequencies and, if so, to compare them in terms of reliability or efficiency. GOM and TM functions were measured in a group of five normal-hearing and five hearing-impaired listeners at signal frequencies of 1000, 2000, and 4000 Hz. Compression values were derived from the masking data and confidence intervals were constructed around these estimates. Both measures produced comparable estimates of compression, but both measures have distinct advantages and disadvantages, so that the more appropriate measure depends on factors such as the frequency region of interest and the degree of hearing loss. Because of the long testing times needed, neither measure is suitable for clinical use in its current form.

Cochlear compression: effects of low-frequency biasing on quadratic distortion product otoacoustic emission

Distortion product otoacoustic emissions (DPOAEs) are generated from the nonlinear transduction n cochlear outer hair cells. The transducer function demonstrating a compressive nonlinearity can be estimated from low-frequency modulation of DPOAEs. Experimental results from the gerbils showed that the magnitude of quadratic difference tone (QDT, f2-f1) was either enhanced or suppressed depending on the phase of the low-frequency bias tone. Within one period of the bias tone, QDT magnitudes exhibited two similar modulation patterns, each resembling the absolute value of the second derivative of the transducer function. In the time domain, the center notches of the modulation patterns occurred around the zero crossings of the bias pressure, whereas peaks corresponded to the increase or decrease in bias pressure. Evaluated with respect to the bias pressure, modulated QDT magnitude displayed a double-modulation pattern marked by a separation of the center notches. Loading/unloading of the cochlear transducer or rise/fall in bias pressure shifted the center notch to positive or negative sound pressures, indicating a mechanical hysteresis. These results suggest that QDT arises from the compression that coexists with the active hysteresis in cochlear transduction. Modulation of QDT magnitude reflects the dynamic regulation of cochlear transducer gain and compression.

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.

Temporal effects in simultaneous masking with on- and off-frequency noise maskers: effects of signal frequency and masker level

Temporal effects in simultaneous masking were measured as a function of masker level for an on-frequency broadband masker and an off-frequency narrow-band masker for signal frequencies of 750, 1730, and 4000 Hz. The on-frequency masker was 10 equivalent rectangular bandwidths (ERBs) wide and centered at the signal frequency; the off-frequency masker was 500 Hz wide and its lower frequency edge was 1.038 ERBs higher in frequency than the signal. The primary goal of the study was to determine whether previously observed differences regarding the effects of signal frequency and masker level on the temporal effect for these two different types of masker might be due to considerably different signal levels at threshold. Despite similar masked thresholds, the effects of signal frequency and masker level in the present study were different for the two masker types. The temporal effect was significant for the two highest frequencies and absent for the lowest frequency in the presence of the broadband masker, but was more or less independent of frequency for the narrow-band masker. The temporal effect increased but then decreased as a function of level for the broadband masker (at the two higher signal frequencies, where there was a temporal effect), but increased and reached an asymptote for the narrow-band masker. Despite the different effects of signal frequency and masker level, the temporal effects for both types of masker can be understood in terms of a basilar-membrane input-output function that becomes more linear during the course of masker stimulation.

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.

Cochlear nonlinearity between 500 and 8000 Hz in listeners with normal hearing

Cochlear nonlinearity was estimated over a wide range of center frequencies and levels in listeners with normal hearing, using a forward-masking method. For a fixed low-level probe, the masker level required to mask the probe was measured as a function of the masker-probe interval, to produce a temporal masking curve (TMC). TMCs were measured for probe frequencies of 500, 1000, 2000, 4000, and 8000 Hz, and for masker frequencies 0.5, 0.7, 0.9, 1.0 (on frequency), 1.1, and 1.6 times the probe frequency. Across the range of probe frequencies, the TMCs for on-frequency maskers showed two or three segments with clearly distinct slopes. If it is assumed that the rate of decay of the internal effect of the masker is constant across level and frequency, the variations in the slopes of the TMCs can be attributed to variations in cochlear compression. Compression-ratio estimates for on-frequency maskers were between 3:1 and 5:1 across the range of probe frequencies. Compression did not decrease at low frequencies. The slopes of the TMCs for the lowest frequency probe (500 Hz) did not change with masker frequency. This suggests that compression extends over a wide range of stimulus frequencies relative to characteristic frequency in the apical region of the cochlea.

Evidence that comodulation detection differences depend on within-channel mechanisms

The threshold for detecting a narrow-band noise signal in the presence of one or more masking noise bands is higher when the signal and masker bands have the same envelope (i.e., are comodulated) than when they have independent envelopes. This is called a comodulation detection difference (CDD). CDD might be caused by perceptual grouping of the signal and masker bands when they are comodulated. This hypothesis leads to the prediction that some masking should occur for comodulated bands, even when they are widely separated in frequency. An alternative hypothesis is that CDD occurs because, when the signal and masker bands are independent, the signal band can be detected in the dips of the masker envelope. This leads to the prediction that CDD should only occur when the masker produces significant excitation at the signal place. Experiment 1 tested these predictions in a paradigm similar to two-tone masking. The signal was a 20-Hz-wide noise centered at 1500 Hz, and the masker consisted of two bands of noise on either side of the signal frequency, whose envelopes were either comodulated (condition C) or uncorrelated (condition U) with the envelope of the signal band. In a third condition (S), the signal band was replaced by a sinusoid. The frequency separation between the signal and masker bands, delta f, was varied from 100 to 1400 Hz. Thresholds were very similar for conditions U and S; thresholds declined progressively as delta f increased beyond 200 Hz, and reached the absolute threshold for delta f = 1400 Hz. For values of delta f from 200 to 1000 Hz, thresholds were higher for condition C than for conditions U or S (i.e., a CDD occurred), but for delta f = 1400 Hz thresholds for condition C also reached absolute threshold. In experiment 2, delta f was fixed at 600 Hz and conditions were included where only the upper or the lower masker band was correlated with the signal band. Also, the overall level of the masker was systematically varied. The results indicate that the magnitude of CDD is determined by the comodulation of the signal band with the masker band producing the most masking. Overall, the results support an explanation based on the spread of excitation and dip listening, rather than an explanation based on perceptual grouping.

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.