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.     

Effects of peripheral nonlinearity on psychometric functions for forward-masked tones

Psychometric functions (PFs) for forward-masked tones were obtained for conditions in which signal level was varied to estimate threshold at several masker levels (variable-signal condition), and in which masker level was varied to estimate threshold at several signal levels (variable-masker condition). The changes in PF slope across combinations of masker frequency, masker level, and signal delay were explored in three experiments. In experiment 1, a 2-kHz, 10-ms tone was masked by a 50, 70 or 90 dB SPL, 20-ms on-frequency forward masker, with signal delays of 2, 20, or 40 ms, in a variable-signal condition. PF slopes decreased in conditions where signal threshold was high. In experiments 2 and 3, the signal was a 4-kHz, 10-ms tone, and the masker was either a 4- or 2.4-kHz, 200-ms tone. In experiment 2, on-frequency maskers were presented at 30 to 90 dB SPL in 10-dB steps and off-frequency maskers were presented at 60 to 90 dB SPL in 10-dB steps, with signal delays of 0, 10, or 30 ms, in a variable-signal condition. PF slopes decreased as signal level increased, and this trend was similar for on- and off-frequency maskers. In experiment 3, variable-masker conditions with on- and off-frequency maskers and 0-ms signal delay were presented. In general, the results were consistent with the hypothesis that peripheral nonlinearity is reflected in the PF slopes. The data also indicate that masker level plays a role independent of signal level, an effect that could be accounted for by assuming greater internal noise at higher stimulus levels.     

A variant temporal-masking-curve method for inferring peripheral auditory compression

Recent studies have suggested that the degree of on-frequency peripheral auditory compression is similar for apical and basal cochlear sites and that compression extends to a wider range of frequencies in apical than in basal sites. These conclusions were drawn from the analysis of the slopes of temporal masking curves (TMCs) on the assumption that forward masking decays at the same rate for all probe and masker frequencies. The aim here was to verify this conclusion using a different assumption. TMCs for normal hearing listeners were measured for probe frequencies (f(P)) of 500 and 4000 Hz and for masker frequencies (f(M)) of 0.4, 0.55, and 1.0 times the probe frequency. TMCs were measured for probes of 9 and 15 dB sensation level. The assumption was that given a 6 dB increase in probe level, linear cochlear responses to the maskers should lead to a 6 dB vertical shift of the corresponding TMCs, while compressive responses should lead to bigger shifts. Results were consistent with the conclusions from earlier studies. It is argued that this supports the assumptions of the standard TMC method for inferring compression, at least in normal-hearing listeners.     

Simultaneous masking by gated and continuous sinusoidal maskers

Simultaneous masking of a 20-ms, 1-kHz signal was investigated using 50-ms gated and continuous sinusoidal maskers with frequencies below, at, and above 1 kHz. Gated maskers can produce considerably (5-20 dB) more masking than continuous maskers, and this difference does not appear to result from the spread of energy produced by gating either the masker or the signal. For masker frequencies below the signal frequency, this difference in masking is primarily due to the detection of the cubic difference tone in the continuous condition. For masker frequencies at and above the signal frequency, the difference appears to be an important property of masking. Implications of this frequency-dependent effect for measures of frequency selectivity are discussed.     

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.     

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

Masking period patterns (MPPs) were measured in listeners with normal and impaired hearing using amplitude-modulated tonal maskers and short tonal probes. The frequency of the masker was either the same as the frequency of the probe (on-frequency masking) or was one octave below the frequency of the probe (off-frequency masking). In experiment 1, MPPs were measured for listeners with normal hearing using different masker levels. Carrier frequencies of 3 and 6 kHz were used for the masker. The probe had a frequency of 6 kHz. For all masker levels, the off-frequency MPPs exhibited deeper and longer valleys compared with the on-frequency MPPs. Hearing-impaired listeners were tested in experiment 2. For some hearing-impaired subjects, masker frequencies of 1.5 kHz and 3 kHz were paired with a probe frequency of 3 kHz. MPPs measured for listeners with hearing loss had similar shapes for on- and off-frequency maskers. It was hypothesized that the shapes of MPPs reflect nonlinear processing at the level of the basilar membrane in normal hearing and more linear processing in impaired hearing. A model assuming different cochlear gains for normal versus impaired hearing and similar parameters of the temporal integrator for both groups of listeners successfully predicted the MPPs.     

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.     

Release from masking caused by envelope fluctuations

This paper examines how short-term energy fluctuations in a masker affect the thresholds for tones at frequencies above those of the masker. Two equally intense tones at 1060 and 1075 Hz produce up to 25 dB less masking than does a 1075-Hz tone set to the overall level of the two-tone complex. At wider frequency separations, two-tone complexes also produce less masking than the pure tone. These results indicate that envelope fluctuations in a masker, whose spectrum is confined to a single critical band, may result in release from masking. The release from masking probably is related to the comodulation masking release reported by Hall et al. [J. Acoust. Soc. Am. 76, 50-56 (1984b)] for modulated-noise maskers with bandwidths greater than one critical band. Further measurements with maskers, whose intensity level in the critical band around 1 kHz was 90 dB SPL, show similar masking by a pure tone and a 625- to 1075-Hz bandpass noise, but less masking by narrow-band noises. These results are inconsistent with a simple frequency selective energy-detector model and indicate that the auditory system can use periods of low masker energy as brief as a few ms to enhance detection of a tone. The results also imply that the upward spread of excitation is best represented by masking patterns for noises with bandwidths of several critical bands.     

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.     

Additivity of simultaneous masking

Simultaneous masking functions (signal level at threshold versus masker level) were obtained for equally intense maskers presented individually and in pairs. The signal was a 2.0-kHz sinusoid. The pairs of maskers were (1) two sinusoids with frequencies 1.9 and 2.1 kHz, (2) two narrow bands of noise (50 Hz wide) centered at 1.9 and 2.1 kHz, (3) two narrow bands of noise (50 Hz wide) centered at 1.8 and 1.9 kHz, and (4) the 1.9-kHz sinusoid combined with the narrow band of noise centered at 2.1 kHz. The pairs of maskers produced anywhere from 10 to 17 dB of masking beyond that predicted from the simple sum of the masking produced by the individual maskers. The amount of this "additional masking" was independent of masker level. Adding a continuous low level background noise reduced the amount of additional masking only slightly (approximately 5 dB). The data are consistent with a model in which the effects of the maskers are summed after undergoing independent compressive transformations.     

Effects of stimulus level on forward-masked psychophysical tuning curves in quiet and in noise

Forward-masked psychophysical tuning curves were obtained from normal-hearing listeners at different probe levels in quiet and in a broadband background noise. In quiet, tuning-curve shape changed with probe level. For six listeners, tuning curves became broader with increasing probe level, primarily due to a decrease in the low-frequency slopes. For one listener, tuning curves became narrower with increasing probe level. The addition of a background noise, which was presented continuously at a level 10 dB below the noise level required to mask the probe tone, reduced the masker levels required to mask the probe tone. The reduction was greater near the tip of the tuning curve than on the tail, so that tuning curves in background noise were narrower than those obtained in quiet. Tuning curves with comparable masker levels near the tip of the tuning curve (Lmtip) were similar in shape, regardless of probe level or whether tuning curves were obtained in quiet or noise. Comparisons of tuning-curve characteristics derived by fitting tuning curves with least-squares procedures, indicated that low-frequency slopes decreased with Lmtip. As a consequence, Q10 dB values decreased with Lmtip. These results are consistent with the interpretation that tuning-curve shapes are determined by the intensities of the maskers required to mask the probe tone. The addition of a background noise restricted (partially masked) the excitation pattern of the probe so that lower masker intensities were required to "forward mask" the probe tone, and narrower tuning curves resulted from less intense markers. The results are well described by a two-process model of auditory excitation patterns.     

Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.     

The danger of using narrow-band noise maskers to measure "suppression"

These experiments investigated whether perceptual cueing plays a role in the "unmasking" effects which have been observed in forward masking for narrow-band noise maskers and brief signals. The forward masking produced by a 100-Hz-wide noise masker at a level of 60 dB SPL was measured for a 1-kHz sinusoidal signal with a raised-cosine envelope and a duration of 10 ms at the 6-dB-down points, both for the masker alone, and with various components added to the masker (and gated synchronously with the masker). Unmasking was found to occur even for components which were extremely unlikely to produce a significant suppression of the masker: these included a 75-dB SPL 4-kHz sinusoid, a 50-dB SPL 1.4-kHz sinusoid, a noise low-pass filtered at 4 kHz with a spectrum level of 0 dB, and a noise low-pass filtered at 4 kHz with a spectrum level of 20 dB presented in the opposite ear to the masker-plus-signal. It is concluded that perceptual cueing can play a significant role in producing unmasking for brief signals following narrow-band noise maskers, and that it is unwise to interpret the unmasking solely in terms of suppression.     

Frequency and level dependent masking of the multiple auditory steady-state response in the bottlenose dolphin (Tursiops truncatus)

The potential for interactions between steady-state evoked responses to simultaneous auditory stimuli was investigated in two bottlenose dolphins (Tursiops truncatus). Three experiments were conducted using either a probe stimulus (probe condition) or a probe in the presence of a masker (probe-plus-masker condition). In the first experiment, the probe and masker were sinusoidal amplitude-modulated (SAM) tones. Probe and masker frequencies and masker level were manipulated to provide variable masking conditions. Probe frequencies were 31.7, 63.5, 100.8, and 127.0 kHz. The second experiment was identical to the first except only the 63.5 kHz probe was used and maskers were pure tones. For the third experiment, thresholds were measured for the probe and probe-plus-masker conditions using two techniques, one based on the lowest detectable response and the other based on a regression analysis. Results demonstrated localized masking effects where lower frequency maskers suppressed higher frequency probes and higher amplitude maskers produced a greater masking effect. The pattern of pure tone masking was nearly identical to SAM tone masking. The two threshold estimates were similar in low masking conditions, but in high masking conditions the lowest detectable response tended to overestimate thresholds while the regression-based analysis tended to underestimate thresholds.     

Physiological responses to the pulsation threshold paradigm. II: Representations of high-pass noise in average rate measures of auditory-nerve fiber discharge

In a companion article [L. I. Hellstrom, J. Acoust. Soc. Am. 85, 230-242 (1989)], it was shown that psychophysical pulsation threshold masking patterns (PTPs) for high-pass noise maskers are not a simple transformation of the profile of activity evoked in the auditory nerve by the masker. In this article, PTPs are compared with neural representations in which interactions of masker and probe are considered. It is hypothesized that, at pulsation threshold, some criterion value of rate change occurs when the stimulus switches from masker to probe. The iso-rate probe level, defined for single auditory-nerve fibers, is the probe level at which this rate change is zero. Iso-rate probe levels are lowest when probe frequency equals best frequency (BF) of the fiber. Profiles of iso-rate probe level versus BF (equal to probe frequency) are qualitatively similar to PTPs but differ quantitatively, e.g., in the rate of growth of probe level with masker level (1.2 dB/dB for PTPs, 0.54 dB/dB for iso-rate profiles). Quantitative differences can be further reduced by requiring a positive rate criterion. These results suggest that PTPs are not solely a reflection of the internal representation of the masker, but reflect responses to the probe tone as well.     

The effect of a precursor on growth of forward masking

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

The effect of masker spectral asymmetry on overshoot in simultaneous masking

"Overshoot" is a simultaneous masking phenomenon: Thresholds for short high-frequency tone bursts presented shortly after the onset of a broadband masker are raised compared to thresholds in the presence of a continuous masker. Overshoot for 2-ms bursts of a 5000-Hz test tone is described for four subjects as a function of the spectral composition and level of the masker. First, it was verified that overshoot is largely independent of masker duration. Second, overshoot was determined for a variety of 10-ms masker bursts composed of differently filtered uniform masking noise with an overall level of 60 dB SPL: unfiltered, high-pass (cutoff at 3700 Hz), low-pass (cutoff at 5700 Hz), and third-octave-band-(centered at 5000 Hz) filtered uniform masking noises presented separately or combined with different bandpass maskers (5700-16000 Hz, 5700-9500 Hz, 8400-16000 Hz) were used. Third, masked thresholds were measured for maskers composed of an upper or lower octave band adjacent to the third-octave-band masker as a function of the level of the octave band. All maskers containing components above the critical band of the test tone led to overshoot; no additional overshoot was produced by masker components below it. Typical values of overshoot were on the order of 12 dB. Overshoot saturated when masker levels were above 60 dB SPL for the upper octave-band masker. The standard neurophysiological explanation of overshoot accounts only partially for these data. Details that must be accommodated by any full explanation of overshoot are discussed.     

Failure to obtain comodulation masking release with frequency-modulated maskers

These experiments were intended to determine whether comodulation masking release (CMR) occurs for maskers that are modulated in frequency rather than in amplitude. In experiment I, thresholds for a sinusoidal signal were measured in the presence of two continuous sinusoidal maskers: one was centered at the signal frequency (1.0 kHz), and the other was positioned at flanking frequencies ranging from 0.5 to 2.0 kHz. The two maskers were frequency modulated (FM) by the same low-pass-noise modulator (correlated condition) or by independent noise modulators (uncorrelated condition). Thresholds were the same for the correlated and uncorrelated maskers, i.e., no CMR occurred. This was also true when the flanking band was presented in the ear opposite to that containing the signal and the on-frequency masking band. In experiment II, 25-Hz-wide noise maskers were used. The on-frequency band was sinusoidally frequency modulated, while the off-frequency band either had the same FM or no FM. Thresholds were similar for the two conditions, again indicating that no CMR occurred. The results suggest that, unlike amplitude modulation, correlated FM of the masker in different frequency bands does not give rise to a release from masking.     

Comodulation masking release using SAM tonal complex maskers: effects of modulation depth and signal position

The purpose of this investigation was to examine two stimulus parameters that were reasoned to be of importance to comodulation masking release (CMR). The first was the degree of fluctuation, or depth of modulation, in the masker bands, and the second was the temporal position of the signal with respect to the modulations of the masker. The investigation began by demonstrating the efficacy of sinusoidally amplitude-modulated (SAM) tonal complex maskers in eliciting CMR. "Nine-band" maskers, 650 ms in duration, were constructed by adding together nine SAM tones spaced at 100-Hz intervals from 300 to 1100 Hz. The rate of modulation for each SAM tone was 10 Hz, and the depth of modulation was 100%. Using such maskers, it was shown that when the on-frequency SAM tone had a modulation depth of 100%, the threshold for a 250-ms, 700-Hz tone improved monotonically as the modulation depths of the flanking SAM tones increased from 0% to 100%. When the on-frequency SAM tone had a modulation depth of 63%, some listeners performed optimally when the flanking SAM tones also exhibited a modulation depth of 63%, whereas others performed best when the flankers had modulation depths of 100%. With regard to signal position, a typical CMR effect was observed when the signal, consisting of a train of three 50-ms, 700-Hz tone bursts, was placed in the dips of the on-frequency masker. However, when the signal was placed at the peaks of the envelope, an increase in masking was observed for a comodulated masker.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of masker harmonicity on informational masking

Detection thresholds for a tone in an unfamiliar tonal pattern can be greatly elevated under conditions of masker uncertainty [Neff and Green, Percept. Psychophys. 41, 409-415 (1987); Oh and Lutfi, J. Acoust. Soc. Am. 101, 3148 (1997)]. The present experiment was undertaken to determine whether harmonicity of masker tones can reduce the detrimental effect of masker uncertainty. Inharmonic maskers were comprised of m=2-49 frequency components selected at random on each presentation within 100-10000 Hz, excluding frequencies between 920-1080. Harmonic maskers were comprised of frequency components selected at random within this same range, but constrained to have a fundamental frequency of 200 Hz. For inharmonic maskers the signal was a 1000-Hz tone. For harmonic-maskers the signal was a tone whose frequency was either harmonically (1000 Hz) or inharmonically (1047 Hz) related to the masker. In all conditions the amount of masking was greatest for m = 20-40 components. At this point, harmonic maskers with harmonic signal produced an average of 9-12 dB less masking than inharmonic maskers. Harmonic maskers with inharmonic signal produced an average of 16-20 dB less masking.