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.     

Effect of signal frequency and masker level on the frequency regions responsible for the overshoot effect.

Two experiments investigated the relative influence of components close to and remote from the signal frequency (fs) on the overshoot effect. Overshoot was defined as the difference in threshold between a signal presented 4 ms after, and that for one presented 300 ms after, the onset of a 350-ms masker. Experiment 1 measured the overshoot effect using both wideband and narrow-band maskers (centered on fs), at two signal frequencies and three masker levels. Experiment 2 used a masker consisting of a "middle band" (MB, centered on fs) and two flanking bands (FBs, which, when combined with the MB, produced a flat wideband spectrum). The masker was continuous except for a 300-ms interval just prior to the signal, during which either all three bands, the MB alone, or the FBs alone were turned off. The results of both experiments showed that the overshoot effect was usually determined by off-frequency components. However, the on-frequency components played a substantial role when signal-to-noise ratios (SNRs) at threshold were unusually high, such as at fs = 6500 Hz and intermediate masker levels. It is suggested that two different mechanisms contribute to the overshoot effect: an off-frequency mechanism which operates at all fs and masker levels, and an on-frequency mechanism which contributes to the overshoot effect only at high-threshold SNRs.     

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 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.     

Relative contribution of off- and on-frequency spectral components of background noise to the masking of unprocessed and vocoded speech

The present study examined the relative influence of the off- and on-frequency spectral components of modulated and unmodulated maskers on consonant recognition. Stimuli were divided into 30 contiguous equivalent rectangular bandwidths. The temporal fine structure (TFS) in each "target" band was either left intact or replaced with tones using vocoder processing. Recognition scores for 10, 15 and 20 target bands randomly located in frequency were obtained in quiet and in the presence of all 30 masker bands, only the off-frequency masker bands, or only the on-frequency masker bands. The amount of masking produced by the on-frequency bands was generally comparable to that produced by the broadband masker. However, the difference between these two conditions was often significant, indicating an influence of the off-frequency masker bands, likely through modulation interference or spectral restoration. Although vocoder processing systematically lead to poorer consonant recognition scores, the deficit observed in noise could often be attributed to that observed in quiet. These data indicate that (i) speech recognition is affected by the off-frequency components of the background and (ii) the nature of the target TFS does not systematically affect speech recognition in noise, especially when energetic masking and/or the number of target bands is limited.     

Enhancements of the edges of temporal masking functions by complex patterns of overshoot and undershoot

The purpose of this report is to present new data that provide a novel perspective on temporal masking, different from that found in the classical auditory literature on this topic. Specifically, measurement conditions are presented that minimize rather than maximize temporal spread of masking for a gated (200-ms) narrow-band (405-Hz-wide) noise masker logarithmically centered at 2500 Hz. Masked detection thresholds were measured for brief sinusoids in a two-interval, forced-choice (21FC) task. Detection was measured at each of 43 temporal positions within the signal observation interval for the sinusoidal signal presented either preceding, during, or following the gating of the masker, which was centered temporally within each 500-ms observation interval. Results are presented for three listeners; first, for detection of a 1900-Hz signal across a range of masker component levels (0-70 dB SPL) and, second, for masked detection as a function of signal frequency (fs = 500-5000 Hz) for a fixed masker component level (40 dB SPL). For signals presented off-frequency from the masker, and at low-to-moderate masker levels, the resulting temporal masking functions are characterized by sharp temporal edges. The sharpness of the edges is accentuated by complex patterns of temporal overshoot and undershoot, corresponding with diminished and enhanced detection, respectively, at both masker onset and offset. This information about the onset and offset timing of the gated masker is faithfully represented in the temporal masking functions over the full decade range of signal frequencies (except for fs=2500 Hz presented at the center frequency of the masker). The precise representation of the timing information is remarkable considering that the temporal envelope characteristics of the gated masker are evident in the remote masking response at least two octaves below the frequencies of the masker at a cochlear place where little or no masker activity would be expected. This general enhancement of the temporal edges of the masking response is reminiscent of spectral edge enhancement by lateral suppression/inhibition.     

Temporal integration in the presence of off-frequency maskers

Temporal integration was measured at a relatively low and a relatively high signal frequency under conditions of off-frequency masking. The masker was typically gated for 300 ms, and the signal was presented 70 ms after masker onset. In experiment 1, the signal frequency was 500 or 2000 Hz. Temporal integration was measured in quiet and in the presence of a masker whose frequency was lower or higher than the signal frequency. In all listening situations, there was less integration at 2000 Hz than at 500 Hz. This effect of frequency was particularly dramatic in the presence of a lower frequency masker, where there was almost no integration at 2000 Hz. Experiment 2 showed that this dramatic effect of frequency cannot be understood in terms of the underlying psychometric functions. Experiment 3 measured temporal integration at 750 and 2000 Hz for a large number of masker-signal frequency separations for both a tonal and a noise masker, and in conditions where the masker was gated or continuous. The results with the gated tonal masker largely confirmed the results of experiment 1. The results with the continuous tonal masker and the gated or continuous noise masker, however, were quite different. In those cases, the amount of temporal integration at both signal frequencies was more or less independent of the masker-signal separation; the masked temporal integration was nearly equal to the integration in quiet. Thus based on the conditions evaluated here, off-frequency masked temporal integration differs substantially from integration in quiet only for gated tonal maskers located considerably lower in frequency than the signal. It is unclear how to account for this finding, although it may be related to attentional factors.     

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.     

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.     

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.     

Growth of simultaneous masking for fm < fs: effects of overall frequency and level.

Growth-of-masking (GOM) functions were obtained in three groups of normal-hearing subjects using a simultaneous-masking paradigm. In all cases, the signal frequency (fs) was higher than the masker frequency (fm), either by a certain ratio (1.44) or by a certain distance [3 equivalent rectangular bandwidths (ERBs)]. The purpose was to evaluate the effect of overall frequency on the slope of the steep portion of the GOM function, and to evaluate the change in slope that can occur at high levels. Signal frequency ranged from 400 to 5000 Hz, and masker level ranged from 40 to 95 dB SPL. On average, the slope of the steep portion of the GOM function was about 1.4 for signal frequencies from 400 to 750 Hz, and 2.0 for signal frequencies from 1944 to 5000 Hz. This is consistent with the possibility that the cochlea may behave more linearly at the apical (low-frequency) region than at the basal (high-frequency) region. In addition, for signal frequencies at and above 750 Hz, the slope of the masking function changed from a value much greater than 1.0 to a value of 1.0 at high levels. The change in slope was better correlated with signal sensation level (i.e., amount of masking) than with either signal or masker SPL: the lack of a change at the lower signal frequencies may reflect the smaller amounts of masking there. The change to a linear growth of masking may represent a change in the response to the signal from compressive to linear at high levels.     

Masking effects in binaural detection and interaural time discrimination

This study was designed to investigate the effects of masker level and frequency on binaural detection and interaural time discrimination. Detection and interaural time discrimination of a 700-Hz sinusoidal signal were measured as a function of the center frequency and level of a narrow-band masking noise. The masker was a continuous, diotic, 80-Hz-wide noise that varied in center frequency from 250 to 1370 Hz. In the detection experiment, the signal was presented either diotically (NoSo) or interaurally phase reversed (NoS pi). In the interaural time discrimination experiment, the signal level needed to discriminate a 30-microseconds interaural delay was measured. As would be expected, the presence of the masker has a greater effect on NoSo detection than NoS pi detection, and for masker frequencies at or near the signal frequency. In contrast, interaural time discrimination can be improved by the presence of a low-level masker. Also, performance improves more rapidly as the signal/masker frequency separation increases for NoSo detection than for interaural time discrimination and NoS pi detection. For all three tasks, significant upward spread of masking occurs only at the highest masker level; at low masker levels, there is a tendency toward downward spread of masking.     

Comodulation masking release (CMR): effects of signal frequency, flanking-band frequency, masker bandwidth, flanking-band level, and monotic versus dichotic presentation of the flanking band

In experiment I, thresholds for 400-ms sinusoidal signals were measured in the presence of a continuous 25-Hz-wide noise centered at signal frequencies (fs) ranging from 250 to 8000 Hz in 1-oct steps. The masker was presented either alone or together with a second continuous 25-Hz-wide band of noise (the flanking band) whose envelope was either correlated with that of the on-frequency band or was uncorrelated; its center frequency ranged from 0.5 fs to 1.5 fs. The flanking band was presented either in the same ear (monotic condition) as the signal plus masker or in the opposite ear (dichotic condition). The on-frequency band and the flanking band each had an overall level of 67 dB SPL. The comodulation masking release, CMR (U-C), is defined as the difference between the thresholds for the uncorrelated and correlated conditions. The CMR (U-C) showed two components: a broadly tuned component, occurring at all signal frequencies and all flanking-band frequencies, and occurring for both monotic and dichotic conditions; and a component restricted to the monotic condition and to flanking-band frequencies close to fs. This sharply tuned component was small for low signal frequencies, increased markedly at 2000 and 4000 Hz, and decreased at 8000 Hz. Experiment II showed that the sharply tuned component of the CMR (U-C) was slightly reduced in magnitude when the level of the flanking band was 10 dB above that of the on-frequency band and was markedly reduced when the level was 10 dB below, whereas the broadly tuned component and the dichotic CMR (U-C) were only slightly affected. Experiment III showed that the sharply tuned component of the CMR (U-C) was markedly reduced when the bandwidths of the on-frequency and flanking bands were increased to 100 Hz, while the broadly tuned component and the dichotic CMR (U-C) decreased only slightly. The argument here is that the sharply tuned component of the monotic CMR (U-C) results from beating between the "carrier" frequencies of the two masker bands. This introduces periodic zeros in the masker envelope, which facilitate signal detection. The broadly tuned component, which is probably a "true" CMR, was only about 3 dB.     

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.     

Factors affecting thresholds for sinusoidal signals in narrow-band maskers with fluctuating envelopes

When a signal is higher in frequency than a narrow-band masker, thresholds are lower when the masker envelope fluctuates than when it is constant. This article investigates the cues used to achieve the lower thresholds, and the factors that influence the amount of threshold reduction. In experiment I the masker was either a sinusoid (constant envelope) or a pair of equal-amplitude sinusoids (fluctuating envelope) centered at the same frequency as the single sinusoid (250, 1000, 3000, or 5275 Hz). The signal frequency was 1.8 times the masker frequency. At all center frequencies, thresholds were lower for the two-tone masker than for the sinusoidal masker, but the effect was smaller at the highest and lowest frequencies. The reduced effect at high frequencies is attributed to the loss of a cue related to phase locking in the auditory nerve. The reduced effect at low frequencies can be partly explained by reduced slopes of the growth-of-masking functions. In experiment II the masker was a sinusoid amplitude modulated at an 8-Hz rate. Masker and signal frequencies were the same as for the first experiment. Randomizing the modulation depth between the two halves of a forced-choice trial had no effect on thresholds, indicating that changes in modulation depth are not used as a cue for signal detection. Thresholds in the modulated masker were higher than would be predicted if they were determined only by the masker level at minima in the envelope, and the threshold reduction produced by modulating the master envelope was less at 250 Hz than at higher frequencies. Experiments III and IV reveal two factors that contribute to the reduced release from masking at low frequencies: The rate of increase of masked threshold with decreasing duration is greater at 250 Hz than at 1000 Hz; the amount of forward masking, relative to simultaneous masking, is greater at 250 Hz than at 1000 Hz. The results are discussed in terms of the relative importance of across-channel cues and within-channel cues.     

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)     

Comodulation masking release for various monaural and binaural combinations of the signal, on-frequency, and flanking bands

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band. This effect is called comodulation masking release (CMR). These experiments examine two questions. (1) How does the CMR vary with the number and ear of presentation of the flanking band(s)? (2) Is it possible to obtain a CMR when a binaural masking level difference (BMLD) is already present, and vice versa? Thresholds were measured for a 400-ms signal in a continuous 25-Hz-wide noise centered at signal frequencies (fs) of 250, 1000, and 4000 Hz. This masker was presented either alone or with one or more continuous flanking bands whose envelopes were either correlated or uncorrelated with that of the on-frequency band; their frequencies ranged from 0.5fs to 1.5fs. CMRs were measured for six conditions in which the signal, the on-frequency band, and the flanking band(s) were presented in various monaural and binaural combinations. When a single flanking band was used, the CMR was typically around 2-3 dB. The CMR increased to 5-6 dB if an additional flanking band was added. The effect of the additional band was similar whether it was in the same ear as the original band or in the opposite ear. At the lowest signal frequency, a large CMR was observed in addition to a BMLD and vice versa. At the highest signal frequency, the extra release from masking was small. The results are interpreted in terms of the cues producing the CMR and the BMLD.     

The masking level difference for signals placed in masker envelope minima and maxima

Previous data on the masking level difference (MLD) have suggested that NoSpi detection for a long-duration signal is dominated by signal energy occurring in masker envelope minima. This finding was expanded upon using a brief 500-Hz tonal signal that coincided with either the envelope maximum or minimum of a narrow-band Gaussian noise masker centered at 500 Hz, and data were collected at a range of masker levels. Experiment 1 employed a typical MLD stimulus, consisting of a 30-ms signal and a 50-Hz-wide masker with abrupt spectral edges, and experiment 2 used stimuli generated to eliminate possible spectral cues. Results were quite similar for the two types of stimuli. At the highest masker level the MLD for signals coinciding with masker envelope minima was substantially larger than that for signals coinciding with envelope maxima, a result that was primarily due to decreased NoSpi thresholds in masker minima. For most observers this effect was greatly reduced or eliminated at the lowest masker level. These level effects are broadly consistent with the presence of physiological background noise and with a level-dependent binaural temporal window. Comparison of these results with predictions of a published model suggest that basilar-membrane compression alone does not account for this level effect.     

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.     

Modulation masking: effects of modulation frequency, depth, and phase

Modulation thresholds were measured for a sinusoidally amplitude-modulated (SAM) broadband noise in the presence of a SAM broadband background noise with a modulation depth (mm) of 0.00, 0.25, or 0.50, where the condition mm = 0.00 corresponds to standard (unmasked) modulation detection. The modulation frequency of the masker was 4, 16, or 64 Hz; the modulation frequency of the signal ranged from 2-512 Hz. The greatest amount of modulation masking (masked threshold minus unmasked threshold) typically occurred when the signal frequency was near the masker frequency. The modulation masking patterns (amount of modulation masking versus signal frequency) for the 4-Hz masker were low pass, whereas the patterns for the 16- and 64-Hz maskers were somewhat bandpass (although not strictly so). In general, the greater the modulation depth of the masker, the greater the amount of modulation masking (although this trend was reversed for the 4-Hz masker at high signal frequencies). These modulation-masking data suggest that there are channels in the auditory system which are tuned for the detection of modulation frequency, much like there are channels (critical bands or auditory filters) tuned for the detection of spectral frequency.