Sinusoidal Maskers Research Articles

Is off-frequency overshoot caused by adaptation of suppression?

This study is concerned with the mechanism of off-frequency overshoot. Overshoot refers to the phenomenon whereby a brief signal presented at the onset of a masker is easier to detect when the masker is preceded by a “precursor” sound (which is often the same as the masker). Overshoot is most prominent when the masker and precursor have a different frequency than the signal (henceforth referred to as “off-frequency overshoot”). It has been suggested that off-frequency overshoot is based on a similar mechanism as “enhancement,” which refers to the perceptual pop-out of a signal after presentation of a precursor that contains a spectral notch at the signal frequency; both have been proposed to be caused by a reduction in the suppressive masking of the signal as a result of the adaptive effect of the precursor (“adaptation of suppression”). In this study, we measured overshoot, suppression, and adaptation of suppression for a 4-kHz sinusoidal signal and a 4.75-kHz sinusoidal masker and precursor, using the same set of participants. We show that, while the precursor yielded strong overshoot and the masker produced strong suppression, the precursor did not appear to cause any reduction (adaptation) of suppression. Predictions based on an established model of the cochlear input–output function indicate that our failure to obtain any adaptation of suppression is unlikely to represent a false negative outcome. Our results indicate that off-frequency overshoot and enhancement are likely caused by different mechanisms. We argue that overshoot may be due to higher-order perceptual factors such as transient masking or attentional diversion, whereas enhancement may be based on mechanisms similar to those that generate the Zwicker tone.

On the near non-existence of "pure" energetic masking release for speech.

Stone et al. [(2012). J. Acoust. Soc. Am. 132, 317-326] showed that a masker constructed to produce a near-constant envelope at the output of each auditory filter reduced speech intelligibility less than maskers of the same mean level with fluctuating envelopes, produced by 100% sinusoidal amplitude modulation (SAM) at 8 Hz. Here, this effect was explored for a range of SAM rates from 1 to 81 Hz. Speech was filtered into 28 channels. A sinusoidal masker centered on each channel was added to the channel signal. The maskers were either unmodulated or had 100% SAM. In most conditions, even-numbered channels were presented to one ear and odd-numbered channels to the other. The signal-to-masker ratio was adapted to measure the Speech Reception Threshold (SRT) corresponding to 50% correct. The fluctuating masker benefit (FMB), the difference in SRT between the SAM and unmodulated masker, was negative for all SAM frequencies except 1 Hz. Due to the different slopes of the psychometric functions, when SRTs were inferred for more realistic performance levels, 74% or more, FMB was zero or negative for all SAM rates. It is concluded that a positive FMB, when it occurs, is a release from modulation and not energetic masking.

Psychophysical tuning curves for frequencies below 100 Hz

Psychophysical tuning curves (PTCs) were measured for sinusoidal signals with frequency f(s) = 31.5, 40, 50, 63, and 80 Hz, using sinusoidal and narrowband-noise maskers. For the former, conditions were included where a pair of beating tones was added to reduce the use of cues related to beats. Estimates of each subject's middle-ear transfer function (METF) were obtained from equal-loudness contours measured from 20 to 160 Hz. With decreasing f(s), the PTCs became progressively broadened and markedly asymmetrical, with shallow upper skirts and steep lower skirts. For the sinusoidal maskers, the tips were more irregular than for narrowband-noise maskers or when beating tones were added. For f(s) = 31.5 and 40 Hz, the tips of the PTCs always fell above f(s). Allowing for the METF so as to infer underlying filter shapes resulted in flatter lower skirts, especially below 40 Hz, and reduced the frequency at the tips for f(s) between 31.5 and 50 Hz; however, the tips did not fall below 40 to 50 Hz. The bandwidths of the PTCs increased with decreasing f(s) below 80 Hz. However, bandwidths remained roughly constant if the METF was included as part of auditory filtering for frequencies below 40 Hz.

Auditory steady-state evoked potentials vs. compound action potentials for the measurement of suppression tuning curves in the sedated dog puppy

Auditory steady-state evoked potential (ASSEP) tuning curves were compared to compound action potential (CAP) tuning curves, both measured at 2 Hz, using sedated beagle puppies. The effect of two types of masker (narrowband noise and sinusoidal) on the tuning curve parameters was assessed. Whatever the masker type, CAP tuning curve parameters were qualitatively and quantitatively similar to the ASSEP ones, with a similar inter-subject variability, but with a greater incidence of upward tip displacement. Whatever the procedure, sinusoidal maskers produced sharper tuning curves than narrow-band maskers. Although these differences are not likely to have significant implications for clinical work, from a fundamental point of view, their origin requires further investigations. The same amount of time was needed to record a CAP and an ASSEP 13-point tuning curve. The data further validate the ASSEP technique, which has the advantages of having a smaller tendency to produce upward tip shifts than the CAP technique. Moreover, being non invasive, ASSEP tuning curves can be easily repeated over time in the same subject for clinical and research purposes.SumarioSe compararon las curvas de afinamiento de los potenciales evocados auditivos de estado estable (ASSEP) con las curvas de afinamiento de los potenciales compuestos de acción (CAD), ambas medidas a 2 Hz, utilizando cachorros de Beagle sedados. Se evaluó el efecto de dos tipos de enmascarador (ruido de banda estrecha y ruido sinusoidal) sobre los parámetros de la curva de afinamiento. Sin importar el tipo de enmascarador, los parámetros de la curva de afinamiento de los CAP fueron cuantitativa y cualitativamente similares a los de los ASSEP, con una variabilidad inter-sujeto similar, pero con una incidencia mayor de desplazamiento de la punta ascendente. Sin importar el procedimiento, los enmascaradores sinusoidales produjeron curvas de afinamiento más agudas que los enmascaradores de banda estrecha. Aunque estas diferencias no parece que puedan tener implicaciones significativas para el trabajo clínico, desde un punto de vista fundamental, su origen requiere de mayores investigaciones. Se requirió la misma cantidad de tiempo para registrar una curva de afinamiento de 13 puntos de CAP y de ASSEP. La información validó más aún la técnica de los ASSEP, que tiene la ventaja de mostrar una menor tendencia a producir desplazamientos de punta ascendente que la técnica de CAP. Más aún, por ser no invasivas, las curvas de afinamiento de los ASSEP pueden ser repetidas fácilmente en el tiempo, en el mismo sujeto, para propósitos clínicos y de investigación.

Slopes of psychometric functions for forward-masked tones with and without a suppressor

The compressive nonlinearity defined by the basilar membrane (BM) input-output (I/O) function is evident in the slopes of psychometric functions for forward-masked tones. A suppressor tone may reduce compression by linearizing the compressive portion of the BM I/O function, or reduce cochlear gain without affecting the value of maximum compression. The present study compared slopes of psychometric functions obtained for a fixed low-level signal in the presence of a variable masker with or without a suppressor gated with the masker. Thresholds and psychometric functions were obtained for four normal-hearing listeners using a 2I-AFC task. The sinusoidal signal was a low-level 4-kHz tone (0-ms steady state, 2-ms cos2 ramps) masked by a sinusoidal masker (100-ms steady state, 2-ms cos2 ramps). The maskers were either on frequency (4 kHz) or off frequency (2.2 kHz) and were presented at masker-signal intervals of 20-100 ms. Initial analyses suggest that as masker levels increase, the slopes of psychometric functions become increasingly shallow (on-frequency masker) or remain relatively steep (off-frequency masker). The effect of a suppressor on the slopes of the psychometric functions appears to be minimal. Results will be discussed in the context of a cochlear model of peripheral nonlinearity.

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

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

The effects of hearing loss on growth-of-masking functions for sinusoidal and complex-tone maskers with differing phase spectra

Growth-of-masking (GOM) functions in forward masking (0-ms masker-signal delay) were measured for normally hearing (NH) and hearing-impaired (HI) listeners using as maskers complex tones (harmonics 1–40, 100-Hz fundamental frequency) with components starting in cosine or random phase, and on-frequency sinusoids. The signal was a 20-ms sinusoid, usually with a frequency of 1 or 2 kHz. It is argued that differences in the slopes of the GOM functions for the random- and cosine-phase maskers provide a measure of the strength of compression in the cochlea. For the NH listeners and some of the HI listeners, the slopes were significantly greater for the random- than for the cosine-phase maskers, and for these listeners the slopes for the complex-tone maskers were less than for the sinusoidal maskers. For the remaining HI listeners, the slopes of the GOM functions were similar for all masker types. It is argued that these listeners had almost complete loss of cochlear compression. The GOM functions for the sinusoidal maskers had slopes between 0.45 and 0.78 and were typically in the range 0.6–0.7. The finding of slopes below one for listeners in whom cochlear compression was probably absent is not consistent with linear-integrator models of forward masking.

Psychophysical estimates of cochlear tuning using simultaneous and nonsimultaneous masking

This study attempts to estimate the tuning of an auditory filter, through both simultaneous and forward masking experiments with a notched‐noise masker or a sinusoidal masker, to investigate how the effects of suppression affect estimates of frequency selectivity. In these experiments, masked thresholds of the short probe signal (5‐ms ramps, no steady state) centered in the masker or delayed from the masker were measured for normal‐hearing subjects as a function of the signal frequency (0.5, 1.0, 2.0, and 4.0 kHz), signal level (10, 20, and 30 dB SL), signal delay, and notch‐width conditions in the notched‐noise masker, or the masker frequency in the sinusoidal masker. ERBs were calculated from derived auditory filters to compare the cochlear tuning. The estimated tunings became much sharper as the center frequency was increased. Moreover, the difference between the tunings of the filter at lower frequencies with either form of masking tended to be smaller than that at higher frequencies. The tuning may be affected by residual effects of suppression due to lower masker components below the signal frequency. [Work supported by special coordination funds for promoting science and technology of young researchers from the Japanese Ministry of Education.]

Detecting dead regions using psychophysical tuning curves: A comparison of simultaneous and forward masking

A dead region (DR) is a region of the cochlea where there are no functioning inner hair cells and/or neurones. We compared the edge frequencies, fe, of DRs estimated using four methods: the TEN(HL) test; psychophysical tuning curves (PTCs) measured in simultaneous masking (320-Hz wide noise masker) using a ‘fast’ method (sweeping masker) and a ‘classical’ method; and PTCs measured in forward masking (sinusoidal masker) using a ‘classical’ method. Fourteen subjects with high-frequency DRs were tested. For measurement of PTCs, the signal frequency was chosen to fall inside the DR; the tip frequencies of the PTCs were taken as indicating the values of fe. The values of fe obtained from the PTCs in forward and simultaneous masking (both fast and classical methods) were similar and were usually close to, but somewhat above, the values of fe estimated from the TEN(HL) test. Fast PTCs measured in simultaneous masking are recommended for use in clinical practice, as they give a precise estimate of fe and are quick to administer.

Factors affecting psychophysical tuning curves for normally hearing subjects

These experiments were conducted to clarify the influence of beats and combination products on psychophysical tuning curves (PTCs) for normally hearing subjects. PTCs for 1- and 4-kHz sinusoidal signals were determined using as maskers a sinusoidal tone and 80-, 160-, and 320-Hz wide bands of noise. PTCs obtained using the sinusoidal masker showed distinct irregularities, particularly for masker frequencies close to the signal frequency. The PTCs determined for the noise maskers were more regular. The broader the masker, the more regular were the shapes of the PTCs. To reduce the detectability of beats produced by the interaction of the signal and masker, a pair of low-frequency tones, called “Modulation detection interference (MDI) tones”, was used to introduce beats at the same rate. The MDI tones reduced the threshold level of the sinusoidal masker by up to 20 dB for frequencies within 300 Hz of the signal frequency; a similar but smaller effect was found when an 80-Hz wide masker was used. Adding a lowpass filtered (LF) noise to the sinusoidal or narrowband noise masker did not affect the low-frequency sides of the PTCs, suggesting no influence of combination products. The LF noise did affect the high-frequency sides of the PTCs, but this can be attributed to it reducing off-frequency listening. To achieve a PTC whose shape around the tip is minimally affected by beats, we propose using a noise masker with a bandwidth approximately equal to the bandwidth of the auditory filter for which the PTC is measured.

Spectro-temporal processing in the envelope-frequency domain

The frequency selectivity for amplitude modulation applied to tonal carriers and the role of beats between modulators in modulation masking were studied. Beats between the masker and signal modulation as well as intrinsic envelope fluctuations of narrow-band-noise modulators are characterized by fluctuations in the "second-order" envelope (referred to as the "venelope" in the following). In experiment 1, masked threshold patterns (MTPs), representing signal modulation threshold as a function of masker-modulation frequency, were obtained for signal-modulation frequencies of 4, 16, and 64 Hz in the presence of a narrow-band-noise masker modulation, both applied to the same sinusoidal carrier. Carrier frequencies of 1.4, 2.8, and 5.5 kHz were used. The shape and relative bandwidth of the MTPs were found to be independent of the signal-modulation frequency and the carrier frequency. Experiment 2 investigated the extent to which the detection of beats between signal and masker modulation is involved in tone-in-noise (TN), noise-in-tone (NT), and tone-in-tone (TT) modulation masking, whereby the TN condition was similar to the one used in the first experiment. A signal-modulation frequency of 64 Hz, applied to a 2.8-kHz carrier, was tested. Thresholds in the NT condition were always lower than in the TN condition, analogous to the masking effects known from corresponding experiments in the audio-frequency domain. TT masking conditions generally produced the lowest thresholds and were strongly influenced by the detection of beats between the signal and the masker modulation. In experiment 3, TT masked-threshold patterns were obtained in the presence of an additional sinusoidal masker at the beat frequency. Signal-modulation frequencies of 32, 64, and 128 Hz, applied to a 2.8-kHz carrier, were used. It was found that the presence of an additional modulation at the beat frequency hampered the subject's ability to detect the envelope beats and raised thresholds up to a level comparable to that found in the TN condition. The results of the current study suggest that (i) venelope fluctuations play a similar role in modulation masking as envelope fluctuations do in spectral masking, and (ii) envelope and venelope fluctuations are processed by a common mechanism. To interpret the empirical findings, a general model structure for the processing of envelope and venelope fluctuations is proposed.

The relative role of beats and combination tones in determining the shapes of masking patterns: II. Hearing-impaired listeners

Masking patterns were measured for hearing-impaired subjects with varying degrees of hearing loss. In one set of conditions, three subjects were tested using narrowband noise (‘noise’) and sinusoidal (‘tone’) maskers and narrowband noise signals. The maskers had centre frequencies of 0.25, 0.5, 1.0 and 4.0 kHz and levels of 60, 80 and 100 dB SPL. Masking patterns for both the noise and tone maskers showed irregularities (‘dips’), especially for signal frequencies up to 500 Hz above the masker frequency. The irregularities occurred for all masker levels and for all subjects for at least one masker frequency and they occurred for a relatively constant range of masker-signal frequency separations, suggesting that they were the result of beat detection. In another set of conditions, masking patterns were measured using two subjects, for a 2.0-kHz tone masker with a level of 100 dB SPL and tone and noise signals. For the tone masker alone (baseline condition), the masking patterns again exhibited prominent dips above, and sometimes below, the masker frequency. The addition of a lowpass noise to the masker, intended to mask combination tones, had little effect for one subject. For the other subject, who had near-normal absolute thresholds at low frequencies, the noise elevated thresholds for masker-signal frequency separations between 500 and 1500 Hz. For this subject, an extra tone with a frequency equal to the masker-signal frequency separation, added in place of the lowpass noise, had a very similar effect to that produced by the lowpass noise, suggesting that he was detecting a simple difference tone in the baseline condition. The addition of a pair of high-frequency tones (MDI tones – intended to reduce the detectability of beats) to the masker elevated thresholds for signal frequencies from 1500 to 2500 Hz for one subject and from 1500 to 3500 Hz for another subject. The addition of lowpass noise and MDI tones to the masker produced masking patterns very similar to those observed when the MDI tones alone were added to the masker. Overall, the results suggest that the irregularities in the masking patterns were caused mainly by the detection of beats and not by the detection of combination tones.

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.

The relative role of beats and combination tones in determining the shapes of masking patterns at 2 kHz: I. Normal-hearing listeners

Masking patterns for a 2-kHz sinusoidal masker at 45, 65 or 85 dB SPL were measured for three normal-hearing subjects, using a 3-AFC method with feedback (condition 1). The patterns showed distinct irregularities, particularly at the highest masker level. In condition 2, a lowpass noise was added to mask combination tones. The noise increased thresholds mainly for the 85 dB masker, for signal frequencies of 2.3–3.0 kHz. In condition 3, a pair of high-frequency tones (‘modulation detection interference (MDI) tones’) was used to introduce beats at the same rate as produced by the interaction of the masker and signal. Thresholds were higher than for condition 1, particularly for signal frequencies adjacent to the masker frequency. In condition 4, the lowpass noise was presented simultaneously with the MDI tones. Thresholds were well predicted as a combination of the effects of the lowpass noise and the MDI tones. In condition 5, a pair of low-frequency MDI tones was added to the masker. The thresholds had the same overall pattern as in condition 4. We conclude that the shapes of masking patterns measured using a 2-kHz masker are influenced by the detection of beats for masker–signal frequency separations up to at least 300 Hz and by the detection of combination tones for separations between 300 and 1000 Hz.

The effects of aspirin on a psychophysical estimate of basilar membrane compression

It has been suggested that the amount of basilar membrane (BM) compression to tones at characteristic frequency can be estimated psychophysically by examining the slope of a growth-of-masking function for a masker that is lower in frequency than the signal [A. J. Oxenham and C. J. Plack, J. Acoust. Soc. Am. 101, 3666–3675]. In this study, growth-of-forward masking functions were measured before and during administration of a moderate dose of aspirin that caused 5–15 dB of temporary hearing loss. The level of a 200-ms sinusoidal masker required to mask a 5-ms, 4-kHz sinusoidal signal presented 2 ms after the offset of the masker was determined for signal levels from 30 to 90 dB SPL. The masker was either the same frequency, or lower in frequency than the signal (signal-masker ratios of 1.2 to 1.8). High-pass noise was presented continuously to prevent off-frequency listening. In general, the aspirin, by presumably altering outer hair cell function, resulted in growth-of-masking functions (masker level versus signal level) for the off-frequency maskers that were steeper by as much as a factor of 2. These results support the argument that the slopes of the functions reflect BM compression, and that aspirin reduces that compression. [Work supported by NIDCD.]

Modeling masking patterns for sinusoidal and narrow-band noise maskers

In a recent paper by Moore, Alcántara, and Dau [J. Acoust. Soc. Am. 104, 1023–1038 (1998)] masking patterns for sinusoidal and narrow-band noise were presented. Data were collected for all four masker-signal combinations. The same subjects participated in the different experimental conditions and the same experimental procedure was used in each case. Despite the influence of combination products, a strong influence of temporal fluctuations on the masking patterns for sinusoidal maskers was found for masker-signal frequency separations up to a few hundred Hz. The present study shows model predictions obtained with the modulation-filterbank model [Dau et al., J. Acoust. Soc. Am. 102, 2892–2905 (1997)]. This model is able to use envelope fluctuations, for example, beats, as a detection cue, but it does not contain a nonlinear peripheral filter stage. Thus the model predictions offer the possibility to separate between effects caused by temporal effects like beating and those caused by combination products. As in the experimental data of Moore et al., less masking was found for the tone-on-tone masking condition than for the tone-in-noise masking condition, for tone-masker frequency separations lower than 250 Hz. The asymmetry of masking between the signal-tone masker-noise condition and the signal-noise masker-tone condition can also be accounted for quantitatively by the model.

Masking patterns for sinusoidal and narrow-band noise maskers.

The masking patterns produced by narrow-band maskers can show distinct irregularities. These experiments attempted to clarify the relative importance of factors contributing to these irregularities. A three-alternative adaptive forced-choice method with feedback was used, to promote use of the optimal detection cues. The masker and signal were either a sinusoid or a band of noise that was 80 Hz wide, giving four possible combinations of masker and signal type. In experiment 1, masking patterns were measured for maskers centered at 1 kHz, for all combinations of masker and signal type (tone or noise). The masking patterns showed irregularities (dips or "shoulders") above the masker frequency, and the irregularities were larger for the sinusoidal than for the noise masker. Experiment 2 was similar to experiment 1, except that low-pass noise was added to mask combination products. For the noise masker, the low-pass noise slightly increased thresholds, and largely eliminated the irregularities in the patterns, but for the tone masker, the irregularities persisted. Experiment 3 used a noise signal with tone and noise maskers centered at 250, 1000, and 4000 Hz. The tone masker produced less masking than the noise masker for masker-signal frequency separations of 150-250 Hz, regardless of masker frequency. Experiment 4 used an additional masker tone to introduce beats similar to those produced by the interaction of the signal and (main) masker, and to mask combination products. This largely eliminated the dips in the masking patterns for both the noise and tone maskers. Experiment 5 used an additional pair of high-frequency tones to introduce beats, with similar results. We conclude that temporal fluctuations (beats) have a strong influence on the masking patterns for sinusoidal maskers, for masker-signal frequency separations up to a few hundred Hz. Beats may also have some influence on the masking patterns for noise maskers. The detection of combination products also plays a role.

Temporal integration at 6 kHz as a function of masker bandwidth.

Thresholds were measured for a 6-kHz sinusoidal signal presented within a 500-ms masker. The masker was either a bandpass Gaussian noise of varying bandwidth, or a sinusoid of the same frequency as the signal. The spectrum level of the noise masker was kept constant at 20 dB SPL, and the level of the sinusoidal masker was 40 dB SPL. Thresholds for signal durations between 2 and 300 ms were measured for masker bandwidths ranging from 60 to 12,000 Hz. The masker was spectrally centered around 6 kHz. For masker bandwidths less than 600 Hz, the slope of the temporal integration function decreased with decreasing masker bandwidth. The results are not consistent with current models of temporal integration or temporal resolution. It is suggested that the results at narrow bandwidths can be understood in terms of changes in the power spectrum of the stimulus envelope or modulation spectrum. According to this view, the onset and offset ramps of the signal introduce detectable high-frequency components into the modulation spectrum, which provide a salient cue in narrowband maskers. For broadband maskers, these high-frequency components are masked by the inherent rapid fluctuations in the masker envelope. Additionally, for signal durations between 7 and 80 ms, signal thresholds decreased by up to 5 dB as the masker bandwidth increased from 1200 to 12,000 Hz. The mechanisms underlying this effect are not yet fully understood.

The role of spread excitation and suppression in simultaneous masking.

This experiment was intended to clarify the relative role of spread of excitation and suppression in simultaneous masking, for masker frequencies just below and well below the signal frequency. The experiment had two stages. In stage 1, growth-of-masking functions were measured in simultaneous masking for a 2200-Hz sinusoidal signal and a sinusoidal masker with frequency of either 1800 Hz or 500 Hz. Straight lines fitted to these data were used to determine masker levels that would give 10, 20, and 30 dB of masking. In stage 2, thresholds for detecting a brief 2200-Hz signal were measured using forward masking. It was reasoned that the threshold of the signal would give an indication of the amount of excitation evoked by the masker in the frequency region of the signal. Three forward maskers were used: (1) a 2200-Hz sinusoid at 10, 20, or 30 dB sensation level (SL); (2) a 2200-Hz sinusoid at the same levels as in (1) together with a sinusoid with frequency 500 or 1800 Hz at a level just sufficient to mask the 2200-Hz sinusoid. We refer to this as the "combined masker," (3) a 500-Hz or 1800-Hz sinusoid at the same levels as in (2) above. The 1800-Hz combined masker produced slightly less forward masking than the 2200-Hz masker (1), which might be explained in terms of suppression or as perceptual cueing. Both the 1800-Hz combined masker and the 1800-Hz component alone (3) gave significant amounts of forward masking (up to 18 dB), indicating that these maskers produced substantial excitation at 2200 Hz. This is consistent with the idea that the simultaneous masking of the 2200-Hz component in stage 1 was produced by spread of excitation rather than by suppression. The 500-Hz combined masker produced much less forward masking than the 2200-Hz component alone, indicating strong suppression of the 2200-Hz component of the combined masker by the 500-Hz component. However, both the 500-Hz combined masker and the 500-Hz component alone produced some forward masking. This is not consistent with the idea that masking of the 2200-Hz component in stage 1 (simultaneous masking) was produced solely by suppression.

Spread of excitation produced by maskers with damped and ramped envelopes

Three experiments compared the ability of pairs of modulated narrow-band sounds with identical power spectra to mask signals above, below, and at their center frequency. In the first experiment, forward-masked psychophysical tuning curves (PTCs) were measured using a 1600-Hz 10-ms signal and 400-ms ‘‘damped’’ and ‘‘ramped’’ sinusoidal maskers. The damped maskers were produced by repeating a 25-ms sinusoid which had been shaped by an exponentially decaying function with a half-life of 4 ms. The ramped maskers were simply time reversals of the damped maskers. When the masker frequency was below that of the signal, ramped maskers were more effective than damped maskers, whereas the reverse was true for maskers above the signal. Experiment 2 measured thresholds for a 400-ms 1000-Hz damped or ramped sinusoidal signal, in the presence of a simultaneous masker with the same envelope shape, as a function of the masker frequency. Again, ramped maskers were more effective below the signal frequency, and damped maskers more effective above it. Experiment 3 was similar to experiment 2 except that the masker was a damped or ramped narrow-band noise, and the signal was an unmodulated sinusoid. The pattern of results was generally similar to that obtained in experiments 1 and 2. The results of all three experiments are inconsistent with the ‘‘traditional’’ method of calculating excitation patterns, which is based on the power spectrum of the stimulus, and also differ from the predictions of a recent model of auditory processing [R. D. Patterson, J. Acoust. Soc. Am. 96, 1409–1418 (1994)]. It is suggested that changes to the peripheral filtering stage of Patterson’s model may provide a useful first step toward accounting for the data.