Detection of high-frequency spectral notches as a function of level

High-frequency spectral notches are important cues for sound localization. Our ability to detect them must depend on their representation as auditory nerve (AN) rate profiles. Because of the low threshold and the narrow dynamic range of most AN fibers, these rate profiles deteriorate at high levels. The system may compensate by using onset rate profiles whose dynamic range is wider, or by using low-spontaneous-rate fibers, whose threshold is higher. To test these hypotheses, the threshold notch depth necessary to discriminate between a flat spectrum broadband noise and a similar noise with a spectral notch centered at 8 kHz was measured at levels from 32 to 100 dB SPL. The importance of the onset rate-profile representation of the notch was estimated by varying the stimulus duration and its rise time. For a large proportion of listeners, threshold notch depth varied nonmonotonically with level, increasing for levels up to 70-80 dB SPL and decreasing thereafter. The nonmonotonic aspect of the function was independent of notch bandwidth and stimulus duration. Thresholds were independent of stimulus rise time but increased for the shorter noise bursts. Results are discussed in terms of the ability of the AN to convey spectral notch information at different levels.     

Spectral and level effects in noise-on-tone suppression

The effective internal level of a 1-kHz tone at 50 dB SPL was estimated by measuring the forward masking produced on a 10-ms signal tone of the same frequency. Noise containing a spectral notch was then added to the masker tone, and its influence on the effective level of the tone was measured with a variety of noise levels, notch widths, and notch shapes. In experiment 1, the masker tone was centered in the spectral notch, itself centered in a 2-kHz band of noise. As the spectrum level in the noise passbands increased from 6 dB/Hz to 36 dB/Hz, signal threshold decreased, indicating a decrease in masking by the masker tone. This "unmasking" effect of the noise was attributed to suppression of the masker tone by the components in the noise. Unmasking was greatest with the narrowest spectral notch (250 Hz), and decreased to zero as the notch widened to 1500 Hz. Compared to its level when presented alone, the effective internal level of the masker tone could be reduced by up to 30 dB (250-Hz notch, 36 dB/Hz). The relative suppressive strength of individual noise components was estimated in experiment 2, in which the 1-kHz masker tone was located at one edge of a spectral notch, rather than in the center. Noise spectrum level was fixed at 16 dB/Hz. As notch width decreased to zero, on either the high-frequency or low-frequency side of the masker tone, its effective internal level was again reduced by approximately 30 dB. In a tentative analysis, the first derivative of the smoothed threshold function was taken, to provide an estimate of the relative contributions to suppression at 1 kHz of noise components between 250 and 1740 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Effect of masker level on overshoot

Overshoot refers to the phenomenon where signal detectability improves for a short-duration signal as the onset of that signal is delayed relative to the onset of a longer duration masker. A popular explanation for overshoot is that it reflects short-term adaptation in auditory-nerve fibers. In this study, overshoot was measured for a 10-ms, 4-kHz signal masked by a broadband noise. In the first experiment, masker duration was 400 ms and signal onset delay was 1 or 195 ms; masker spectrum level ranged from - 10-50 dB SPL. Overshoot was negligible at the lowest masker levels, grew to about 10-15 dB at the moderate masker levels, but declined and approached 0 dB at the highest masker levels. In the second experiment, the masker duration was reduced to 100 ms, and the signal was presented with a delay of 1 or 70 ms; masker spectrum level was 10, 30, or 50 dB SPL. Overshoot was about 10 dB for the two lower masker levels, but about 0 dB at the highest masker level. The results from the second experiment suggest that the decline in overshoot at high masker levels is probably not due to auditory fatigue. It is suggested, instead, that the decline may be attributable to the neural response at high levels being dominated by those auditory-nerve fibers that do not exhibit short-term adaptation (i.e., those with low spontaneous rates and high thresholds).     

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.     

Effects of pure-tone forward masker duration on psychophysical measures of frequency selectivity

The effects of forward masker duration on psychophysical measures of frequency selectivity were investigated in two experiments. In both experiments, masker duration was 50 or 400 ms, signal duration was 20 ms, and there was no delay between masker offset and signal onset. In the first experiment, growth-of-masking functions were measured for a masker whose frequency was below, at, or above the 1000-Hz signal frequency. From those data, input filter patterns (IFPs) were plotted for masker levels from 40-90 dB SPL. In the second experiment, masking patterns (MPs) were measured for a 1000-Hz masker presented at 50, 70, and 90 dB SPL. Both measures of frequency selectivity (IFPs and MPs) indicate that frequency selectivity is greater for the 400-ms masker. These data suggest that there may be a sharpening of frequency selectivity with time at a stage prior to the adaptation observed in forward masking.     

Forward masking of amplitude modulation: basic characteristics

In this study we demonstrate an effect for amplitude modulation (AM) that is analogous to forward making of audio frequencies, i.e., the modulation threshold for detection of AM (signal) is raised by preceding AM (masker). In the study we focused on the basic characteristics of the forward-masking effect. Functions representing recovery from AM forward masking measured with a 150- ms 40- Hz masker AM and a 50- ms signal AM of the same rate imposed on the same broadband-noise carrier, showed an exponential decay of forward masking with increasing delay from masker offset. Thresholds remained elevated by more than 2 dB over an interval of at least 150 ms following the masker. Masked-threshold patterns, measured with a fixed signal rate (20, 40, and 80 Hz) and a variable masker rate, showed tuning of the AM forward-masking effect. The tuning was approximately constant across signal modulation rates used and consistent with the idea of modulation-rate selective channels. Combining two equally effective forward maskers of different frequencies did not lead to an increase in forward masking relative to that produced by either component alone. Overall, the results are consistent with modulation-rate selective neural channels that adapt and recover from the adaptation relatively quickly.     

The mid-level hump at 2 kHz

Shortening the duration of a Gaussian-shaped 2-kHz tone-pip causes the intensity-difference limen (DL) to depart from the "near-miss to Weber's law" and swell into a mid-level hump [Nizami et al., J. Acoust. Soc. Am. 110, 2505-2515 (2001)]. For some subjects the size of this hump approaches or exceeds the size reported for longer tones under forward masking, suggesting that forward masking might make little difference to the DL for very brief probes. To test this hypothesis, DLs were determined over 30 to 90 dB SPL for a brief Gaussian-shaped 2-kHz tone-pip. DLs were obtained first without forward masking, then with the pip placed 10 or 100 ms after a 200-ms 2-kHz tone of 50 dB SPL, or 100 ms after a 200-ms 2-kHz tone of 70 dB SPL. DLs inflated significantly under all forward-masking conditions. DLs also enlarged under an 80 dB SPL forward masker at pip delays of 4, 10, 40, and 100 ms. The peaks of the humps obtained under forward masking clustered around a sensation level (SL) that was significantly lower than the average SL for the peaks of the humps obtained without forward masking. Overall, the results do not support the neuronal-recovery-rate model of Zeng et al. [Hear. Res. 55, 223-230 (1991)], but are not incompatible with the Carlyon and Beveridge hypothesis [J. Acoust. Soc. Am. 93, 2886-2895 (1993)] that nonsimultaneous maskers corrupt the memory trace evoked by the probe.     

Upward shifts in the masking pattern with increasing masker intensity

Masking patterns obtained with forward-masking paradigms and relatively intense maskers sometimes have their peaks at the masker frequency and sometimes at a frequency well above it. Here it is shown that which outcome is obtained depends upon certain temporal parameters of the procedure. Specifically, the masking pattern for a 2000-Hz tone showed a gradual shift toward higher frequencies as masker intensity was increased from 65 to 95 dB SPL when long signals (about 50 ms) and long masker-to-signal intervals (about 50 ms) were used, but the effect was absent or smaller when the signals and intervals were short. This shift did not occur with a 750-Hz masker. Upward shifts in the masking pattern with increasing masker intensity are in accord with the view that the peak of displacement of the traveling-wave envelope migrates basally with increasing intensity--an idea that has frequently been suggested as an explanation of the so-called half-octave shift so routinely seen in auditory fatigue experiments.     

Masking patterns for synthetic vowels in simultaneous and forward masking

Two synthetic vowels /i/ and /ae/ with a fundamental frequency of 100 Hz served as maskers for brief (5 or 15 ms) sinusoidal signals. Threshold was measured as a function of signal frequency, for signals presented immediately following the masker (forward masking, FM) or just before the cessation of the masker (simultaneous masking, SM). Three different overall masker levels were used: 50, 70, and 90 dB SPL. In order to compare the data from simultaneous and forward masking, and to compensate for the nonlinear characteristics of forward masking, each signal threshold was expressed as the level of a flat-spectrum noise which would give the same masking. The internal representation of the formant structure of the vowels, as inferred from the transformed masking patterns, was enhanced in FM and "blurred" in SM in comparison to the physical spectra, suggesting that suppression plays a role in enhancing spectral contrasts. The first two or three formants were usually visible in the masking patterns and the representation of the formant structure was impaired only slightly at high masker levels. For high levels, filtering out the relatively intense low-frequency components enhanced the representation of the higher formants in FM but not in SM, indicating a broadly tuned remote suppression from lower formants towards higher ones. The relative phase of the components in the masker had no effect on thresholds in forward masking, indicating that the detailed temporal structure of the masker waveform is not important.     

The effect of pure-tone forward masking on overshoot

The overshoot effect can be reduced by temporary hearing loss induced by aspirin or exposure to intense sound. The present study simulated a hearing loss at 4.0 kHz via pure-tone forward masking and examined the effect of the simulation on threshold for a 10-ms, 4.0-kHz signal presented 1 ms after the onset of a 400-ms, broadband noise masker whose spectrum level was 20 dB SPL. Masker frequency was 3.6, 4.0, or 4.2 kHz, and masker level was 80 dB SPL. Subject-dependent delays were determined such that 10 or 20 dB of masking at 4.0 kHz was produced. In general, the pure-tone forward masker did not reduce the simultaneous-masked threshold, suggesting that elevating threshold with a pure-tone forward masker does not sufficiently simulate the effect of a temporary hearing loss on overshoot.     

Temporal effects in masking and their influence on psychophysical tuning curves

Psychophysical tuning curves (PTCs) were obtained in simultaneous and forward masking for a 20-ms, 1000-Hz signal presented at 10 dB SL. The signal was presented at the beginning of, at the temporal center of, at the end of, or immediately following a 400-ms masker. The first experiment was done in quiet; the second experiment was done in the presence of two bands of noise on either side of 1000 Hz. The results were similar in quiet and in noise. In simultaneous masking, the PTCs were broadest for the signal at masker onset, and generally sharpest for the signal at temporal center; the differences were largest on the high-frequency side. In most cases, there was virtually no difference in Q10 between the forward-masking PTC and the simultaneous-masking PTC with the signal temporally centered, although the high-frequency slope was always steeper in forward masking. These results indicate that, at least for brief signals, frequency selectivity measured with simultaneous-masking PTCs and the degree of sharpening revealed in forward-masking PTCs depend upon the temporal position of the signal within the simultaneous masker.     

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.     

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 temporal course of masking and the auditory filter shape

Recent experiments have shown that frequency selectivity measured in tone-on-tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched-noise masker, threshold was measured for a 20-ms signal presented at the beginning, the temporal center, or the end of the 400-ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1-kHz signal frequency. Growth-of-masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched-noise experiment in terms of the level of a flat-spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high-frequency side, which was attributed to suppression.     

Basilar-membrane nonlinearity estimated by pulsation threshold

The pulsation threshold technique was used to estimate the basilar-membrane (BM) response to a tone at characteristic frequency (CF). A pure-tone signal was alternated with a pure-tone masker. The frequency of the masker was 0.6 times that of the signal. For signal levels from around 20 dB above absolute threshold to 85 dB SPL, the masker level was varied to find the level at which a transition occurred between the signal being perceived as "pulsed" or "continuous" (the pulsation threshold). The transition is assumed to occur when the masker excitation is somewhat greater than the signal excitation at the place on the BM tuned to the signal. If it is assumed further that the response at this place to the lower-frequency masker is linear, then the shape of the masking function provides an estimate of the BM response to the signal. Signal frequencies of 0.25, 0.5, 1, 2, 4, and 8 kHz were tested. The mean slopes of the masking functions for signal levels between 50 and 80 dB SPL were 0.76, 0.50, 0.34, 0.32, 0.35, and 0.41, respectively. The results suggest that compression on the BM increases between CFs of 0.25 and 1 kHz and is roughly constant for frequencies of 1 kHz and above. Despite requiring a subjective criterion, the pulsation threshold measurements had a reasonably low variability. However, the estimated compression was less than in an earlier study using forward masking. The smaller amount of compression observed here may be due to the effects of off-frequency listening.     

Forward-masking properties of multicomponent signals in normal and hearing-impaired subjects

The forward-masking properties of inharmonic complex stimuli were measured both for normal and hearing-impaired subjects. The signal threshold for a 1000-Hz pure-tone probe was obtained for six different maskers, which varied in the number of pure-tone components. The masking stimuli consisted of 1, 3, 5, 7, 9, or 11 components, logarithmically spaced in frequency surrounding the signal and presented at a fixed level of 80 dB SPL per component. In most normal-hearing subjects, the threshold for the probe decreased as the number of masking components was increased, demonstrating that stimuli with more components tended to be less effective maskers. Results from hearing-impaired subjects showed no decrease in threshold with increasing number of masking components. Instead, the thresholds increased as more components were added to the first masker. These results appear to be consistent with suppression effects within the multicomponent maskers for the normal subjects and a lack of suppression effects for the hearing-impaired subjects. The results from the normal-hearing subjects are also consistent with "across-channel" cuing.     

Comparisons of frequency selectivity in simultaneous and forward masking for subjects with unilateral cochlear impairments

Two experiments are described in which frequency selectivity was estimated, in simultaneous and forward masking, for each ear of subjects with moderate (25-60 dB HL) unilateral cochlear hearing losses. In both experiments, the signal level was fixed for a given ear and type of masking (simultaneous or forward), and the masker level was varied to determine threshold, using an adaptive, two-alternative forced-choice procedure. In experiment I, the masker was a noise with a spectral notch centered at the signal frequency (either 1.0 or 1.5 kHz); threshold was determined as a function of notch width. Signal levels were chosen so that the noise level required at threshold for a notch width of zero was similar for the normal and impaired ear of each subject in both simultaneous and forward masking. The function relating threshold to notch width had a steeper slope for the normal ear than for the impaired ear of each subject. For the normal ears, these functions were steeper in forward masking than in simultaneous masking. This difference was interpreted as resulting from suppression. For the impaired ears, significant differences in the same direction were observed for three of the five subjects, but the differences were smaller. In experiment II, psychophysical tuning curves (PTCs) were determined in the presence of a fixed notched noise centered at the signal frequency (1.0 kHz). For the normal ears, the PTCs were sharper in forward masking than in simultaneous masking. For the impaired ears, the PTCs were similar in simultaneous and forward masking, but those in forward masking tended to be sharper at masker frequencies far removed from the signal frequency. Overall, the results suggest that suppression is reduced, but not completely absent in cases of moderate cochlear hearing loss.     

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.     

A release from masking by continuous, random, notched noise

Thresholds for 10-ms sinusoids simultaneously masked by bursts of bandpass noise centered on the signal frequency were measured for a wide range of signal frequencies and noise levels. Thresholds were defined as the signal power relative to the masker power at the output of an auditory filter centered on the signal frequency. It was found that the presentation of a continuous random noise, with a spectral notch centered on the signal frequency, produced a reduction in signal thresholds of up to 11 dB. A notched noise spectrum level of 0-5 dB above that of the masker proved most effective in producing a masking release, as measured by a reduction in masked threshold. A release from masking of up to 7 dB could be obtained with a continuous bandpass noise. The most effective spectrum level of this noise was 5 dB below that of the masker. The effect of the continuous notched noise was to reduce signal-to-masker ratios at threshold to about 0 dB, regardless of the threshold in the absence of continuous noise. Thus the greatest release from masking occurred when "unreleased" thresholds were highest. The release from masking is almost complete within 320 ms of notched noise onset, and persists for about 160 ms after notched noise offset, regardless of notched noise level. The phenomenon is similar in many ways to the "overshoot" effect reported by Zwicker [J. Acoust. Soc. Am. 37, 653-663 (1965)]. It is argued that both effects can be largely attributed to peripheral short-term adaptation, a mechanism which is also believed to be involved in forward masking.