Spectral modulation detection as a function of modulation frequency, carrier bandwidth, and carrier frequency region

The present study investigates the nature of spectral envelope perception using a spectral modulation detection task in which sinusoidal spectral modulation is superimposed upon a noise carrier. The principal goal of this study is to characterize spectral envelope perception in terms of the influence of modulation frequency (cycles/octave), carrier bandwidth (octaves), and carrier frequency region (defined by lower and upper cutoff frequencies in Hz). Spectral modulation detection thresholds measured as a function of spectral modulation frequency result in a spectral modulation transfer function (SMTF). The general form of the SMTF is bandpass in nature, with a minimum modulation detection threshold in the region between 2 to 4 cycles/octave. SMTFs are not strongly dependent on carrier bandwidth (ranging from 1 to 6 octaves) or carrier frequency region (ranging from 200 to 12 800 Hz), with the exception of carrier bands restricted to very low audio frequencies (e.g., 200-400 Hz). Spectral modulation detection thresholds do not depend on the presence of random level variations or random modulation phase across intervals. The SMTFs reported here and associated excitation pattern computations are considered in terms of a linear systems approach to spectral envelope perception and potential underlying mechanisms for the perception of spectral features.     

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.     

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.     

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.     

Binaural modulation masking

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.     

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.     

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.     

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.     

Simultaneous masking and unmasking with bandlimited noise

This study examines how simultaneous masking of a tone by bandlimited noise may be affected by nonlinear interactions among spectral components of the noise. Simultaneous masking patterns (signal threshold versus signal frequency) were obtained with three types of maskers: (A) a narrow-band noise, 50 Hz wide with variable center frequency fv, (B) pairs of narrow-band noises, each band 50 Hz wide with center frequencies fl and fu, and (C) wide-band noise formed by filling the spectral gap between the two bands of (B). The variable frequency fv was set to 1.0, 1.1, 1.2, and 1.3 kHz: fl was fixed at 1.0 kHz, and fu had values of 1.1, 1.2, and 1.3 kHz. In most conditions, the two-band maskers and the wideband maskers produced more masking than would be predicted from the masking produced by the single narrow-band maskers. For certain signal frequencies below the maskers, adding noise to fill the spectral gap of the two-band masker actually resulted in a 3- to 15-dB release from masking. These results reveal factors that may operate to confound modern measures of frequency selectivity.     

Psychophysical tuning curves measured in simultaneous and forward masking

The level of a masker necessary to mask a probe fixed in frequency and level was determined as a function of masker frequency using a two-interval forced-choice technique. Both simultaneous- and forward- masking techniques were used. Parameters investigated include the level of the probe tone and the frequency of the probe tone. The general form of the psychophysical tuning curves obtained in this way is quite similar to that of single-neurone tuning curves, when low-level probe tones are used. However, the curves obtained to forward masking generally show sharper tips and steeper slopes than those found in simultaneous masking, and they are also generally sharper than neurophysiological tuning curves. For frequencies of the masker close to that of the probe a simultaneous masker was sometimes less effective than a forward masker. The results are discussed in relation to possible lateral suppression effects in simultaneous masking, and in relation to the observer's use of pitch cues in forward masking. It is concluded that neither the simultaneous-masking curves nor the forward-masking curves are likely to give an accurate representation of human neural tuning curves.     

Physiological mechanisms of psychophysical masking: observations from auditory-nerve fibers

Masking might be due either to the spread of the excitation produced by the masker to the place of the tone signal along the cochlea or to the suppression of the response to the signal by the masker. In order to identify the contributions of these two mechanisms to tone-on-tone masking, masked thresholds of auditory-nerve fibers were measured in anesthetized cats using the same stimulus paradigms and detection criteria as in psychophysics. Suppressive masking was identified by comparing thresholds for simultaneous masking with those for a nonsimultaneous masking technique resembling pulsation thresholds. These nonsimultaneous thresholds do not include the contribution of suppression to masking because suppression only occurs for stimuli that overlap in time. For each masker and signal frequency, the fibers with the lowest (or "best") masked thresholds had characteristic frequencies (CF) slightly on the opposite side of the masker frequency with respect to the signal frequency, consistent with the psychophysical phenomenon of off-frequency listening. Patterns of best masked thresholds against signal frequency resembled psychophysical masking patterns in that they showed a maximum for signal frequencies close to the masker, and a skew toward high frequencies. Masking was found to be both excitatory and suppressive, with the relative contribution of the two mechanisms depending on the frequency separation between signal and masker. Suppressive masking was large for signal frequencies well above the masker. For these conditions, simultaneous thresholds grew more rapidly with masker level than did nonsimultaneous thresholds, suggesting that the upward spread of masking is largely due to the growth of suppression rather than to that of excitation.     

Amplitude modulation sensitivity as a mechanism increment detection

Detectability of a tonal signal added to a tonal masker increases with increasing duration ("temporal integration"), up to some maximum duration. Initially assumed to be some form of energy integration over time, this phenomenon is now often described as the result of a statistical "multiple looks" process. For continuous maskers, listeners may also use a mechanism sensitive to changes in stimulus intensity, possibly a result of inherent sensitivity to amplitude modulation (AM). In order to examine this hypothesis, change detection was investigated in the presence of AM maskers presented at either the same carrier frequency as the target signal or at a distant frequency. The results are compatible with the hypothesis that listeners detect intensity increments by using change-detection mechanisms (modeled here as the outputs of a bank of modulation filters) sensitive to envelope modulation at both low (4-16 Hz) and high (around 100 Hz) rates. AM masking occurred even when the masker was at a carrier frequency more than two octaves above that of the signal to be detected. This finding is also compatible with the hypothesis that similar mechanisms underlie sensitivity to AM (where across-frequency masking is commonly shown) and detection of intensity increments.     

The temporal evolution of masking and frequency selectivity in the goldfish (Carassius auratus)

The temporal evolution of masking and frequency selectivity was studied in the goldfish using classical respiratory conditioning and a tracking psychophysical procedure. The temporal position of a brief tonal signal within a longer duration, tonal masker has little or no effect on signal detectability when the frequency of the masker is less than or equal to that of the signal. For masker frequencies above that of the signal, signal detectability improves as the signal onset is delayed relative to that of the masker. These patterns of tone-on-tone masking are quite similar to those observed for humans. These temporal masking patterns are qualitatively similar in shape to the peristimulus-time histogram profiles of the low-frequency saccular fibers thought to be used in this task. Frequency- and time-dependent changes in signal detectability result in specific changes in the sharpness of psychophysical tuning curves (PTC). In general, PTCs determined for signals occurring at masker onset are the most broadly tuned, and PTCs determined in forward masking are the most sharply tuned. The PTCs for signals temporally centered in the masker are intermediate. These results suggest that temporal tone-on-tone masking patterns and the temporal evolution of psychophysical tuning curves result from the response properties of peripheral auditory-nerve fibers.     

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.     

Interpreting measures of frequency selectivity: is forward masking special?

In a previous article [Lutfi, J. Acoust. Soc. Am. 76, 1045-1050 (1984)], the following relation was used to predict measures of frequency selectivity obtained in forward masking from measures obtained in simultaneous masking: F(g) = G + H(g) - H(0), where, for a given masker level, F is the amount of forward masking (in dB) as a function of signal-masker frequency separation (g), H is the amount of simultaneous masking, and G is the amount of forward masking for g = 0. In the present study, the relation was tested for a wider range of signal and masker frequencies, masker levels, and signal delays. The relation described thresholds from all conditions well with the inclusion of one free parameter lambda corresponding to a constant frequency increment, F(g) = G + H(g + lambda) - H(lambda). The parameter lambda was required to account for observed shifts in the frequency of maximum forward masking. It is argued that a single tuning mechanism can account for commonly observed differences between simultaneous- and forward-masked measures of frequency selectivity.     

Modulation cues influence binaural masking-level difference in masking-pattern experiments

Binaural masking patterns show a steep decrease in the binaural masking-level difference (BMLD) when masker and signal have no frequency component in common. Experimental threshold data are presented together with model simulations for a diotic masker centered at 250 or 500 Hz and a bandwidth of 10 or 100 Hz masking a sinusoid interaurally in phase (S(0)) or in antiphase (S(π)). Simulations with a binaural model, including a modulation filterbank for the monaural analysis, indicate that a large portion of the decrease in the BMLD in remote-masking conditions may be due to an additional modulation cue available for monaural detection.     

Modulation masking produced by beating modulators.

This study examined whether "modulation masking" could be produced by temporal similarity of the probe and masker envelopes, even when the masker envelope did not contain a spectral component close to the probe frequency. Both masker and probe amplitude modulation were applied to a single 4-kHz sinusoidal or narrow-band noise carrier with a level of 70 dB SPL. The threshold for detecting 5-Hz probe modulation was affected by the presence of a pair of masker modulators beating at a 5-Hz rate (40 and 45 Hz, 50 and 55 Hz, or 60 and 65 Hz). The threshold was dependent on the phase of the probe modulation relative to the beat cycle of the masker modulators; the threshold elevation was greatest (12-15 dB for the sinusoidal carrier and 9-11 dB for the noise carrier, expressed as 20 log m) when the peak amplitude of the probe modulation coincided with a peak in the beat cycle. The maximum threshold elevation of the 5-Hz probe produced by the beating masker modulators was 7-12 dB greater than that produced by the individual components of the masker modulators. The threshold elevation produced by the beating masker modulators was 2-10 dB greater for 5-Hz probe modulation than for 3- or 7-Hz probe modulation. These results cannot be explained in terms of the spectra of the envelopes of the stimuli, as the beating masker modulators did not produce a 5-Hz component in the spectra of the envelopes. The threshold for detecting 5-Hz probe modulation in the presence of 5-Hz masker modulation varied with the relative phase of the probe and masker modulation. The pattern of results was similar to that found with the beating two-component modulators, except that thresholds were highest when the masker and probe were 180 degrees out of phase. The results are consistent with the idea that nonlinearities within the auditory system introduce distortion in the internal representation of the envelopes of the stimuli. In the case of two-component beating modulators, a weak component is introduced at the beat rate, and it has an amplitude minimum when the beat cycle is at its maximum. The results could be fitted well using two models, one based on the concept of a sliding temporal integrator and one based on the concept of a modulation filter bank.     

The effect of modulation coherence on signal threshold in frequency-modulated noise bands

A series of four experiments was undertaken to ascertain whether signal threshold in frequency-modulated noise bands is dependent upon the coherence of modulation. The specific goal was to determine whether a masking release could be obtained with frequency modulation (FM), analogous to the comodulation masking release (CMR) phenomenon observed with amplitude modulation (AM). It was hypothesized that an across-frequency grouping process might give rise to such an effect. In experiments 1-3, maskers were composed of three noise bands centered on 1600, 2000, and 2400 Hz; these were either comodulated or noncomodulated with respect to both FM and AM. In experiment 1, the modulation was sinusoidal, and the signal was a 2000-Hz pure tone; in experiment 2, the modulation was random, and the signal was an FM noise band centered on 2000 Hz. The results obtained showed that, given sufficient width of modulation, thresholds were lower in a coherent FM masker than in an incoherent FM masker, regardless of the pattern of AM or signal type. However, thresholds in multiband maskers were usually elevated relative to that in a single-band masker centered on the signal. Experiment 3 demonstrated that coherent FM could be discriminated from incoherent FM. Experiment 4 gave similar patterns of results to the respective conditions of experiments 2 and 3, but for an inharmonic masker with bands centered on 1580, 2000, and 2532 Hz. While within-channel processes could not be entirely excluded from contributing to the present results, the experimental conditions were designed to be minimally conducive to such processes.     

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.     

Similarity, uncertainty, and masking in the identification of nonspeech auditory patterns

This study examined whether increasing the similarity between informational maskers and signals would increase the amount of masking obtained in a nonspeech pattern identification task. The signals were contiguous sequences of pure-tone bursts arranged in six narrow-band spectro-temporal patterns. The informational maskers were sequences of multitone bursts played synchronously with the signal tones. The listener's task was to identify the patterns in a 1-interval 6-alternative forced-choice procedure. Three types of multitone maskers were generated according to different randomization rules. For the least signal-like informational masker, the components in each multitone burst were chosen at random within the frequency range of 200-6500 Hz, excluding a "protected region" around the signal frequencies. For the intermediate masker, the frequency components in the first burst were chosen quasirandomly, but the components in successive bursts were constrained to fall in narrow frequency bands around the frequencies of the components in the initial burst. Within the narrow bands the frequencies were randomized. This masker was considered to be more similar to the signal patterns because it consisted of a set of narrow-band sequences any one of which might be mistaken for a signal pattern. The most signal-like masker was similar to the intermediate masker in that it consisted of a set of synchronously played narrow-band sequences, but the variation in frequency within each sequence was sinusoidal, completing roughly one period in a sequence. This masker consisted of discernible patterns but not patterns that were part of the set of signals. In addition, masking produced by Gaussian noise bursts--thought to produce primarily peripherally based "energetic masking"--was measured and compared to the informational masking results. For the three informational maskers, more masking was produced by the maskers comprised of narrow-band sequences than for the masker in which the frequencies were not constrained to narrow bands. Also, the slopes of the performance-level functions for the three informational maskers were much shallower than for the Gaussian noise masker or for no masker. The findings provided qualified support for the hypothesis that increasing the similarity between signals and maskers, or parts of the maskers, causes greater informational masking. However, it is also possible that the greater masking was a consequence of increasing the number of perceptual "streams" that had to be evaluated by the listener.