Second-order modulation detection thresholds for pure-tone and narrow-band noise carriers

Probing temporal modulation detection in white noise using intrinsic envelope fluctuations: A reverse-correlation study

Part of the detrimental effect caused by a stationary noise on sound perception results from the masking of relevant amplitude modulations (AM) in the signal by random intrinsic envelope fluctuations arising from the filtering of noise by cochlear channels. This study capitalizes on this phenomenon to probe AM detection strategies for human listeners using a reverse correlation analysis. Eight normal-hearing listeners were asked to detect the presence of a 4-Hz sinusoidal AM target applied to a 1-kHz tone carrier using a yes-no task with 3000 trials/participant. All stimuli were embedded in a white-noise masker. A reverse-correlation analysis was then carried on the data to compute "psychophysical kernels" showing which aspects of the stimulus' temporal envelope influenced the listener's responses. These results were compared to data simulated with different implementations of a modulation-filterbank model. Psychophysical kernels revealed that human listeners were able to track the position of AM peaks in the target, similar to the models. However, they also showed a marked temporal decay and a consistent phase shift compared to the ideal template. In light of the simulated data, this was interpreted as an evidence for the presence of phase uncertainty in the processing of intrinsic envelope fluctuations.

Frequency selectivity in the modulation domain estimated using forward masking: Effects of masker modulation depth and masker-signal delay

The threshold for detecting amplitude modulation (AM) of a sinusoidal or noise carrier is elevated when the signal AM is preceded by masker AM applied to the same carrier. This effect, called AM forward masking, shows selectivity in the AM domain, consistent with the existence of a modulation filter bank (MFB). In this paper we explore the effect of two factors that can influence AM forward masking, using an 8-kHz sinusoidal carrier and a range of masker AM frequencies, f_m, both below and above the signal AM frequency, f_s, of 40 Hz. The first factor was the time delay, t_d, between the end of the masker AM and the start of the signal AM. The second was the AM depth, m, of the masker, which was either 1 or 0.25. The AM forward masking patterns in all conditions showed tuning in the AM domain; signal thresholds were highest when f_m was close to f_s. The amount of AM forward masking decreased with increasing t_d in a similar way for all f_m, so the shapes of the masking patterns did not change markedly with t_d. Remarkably, the amount of AM forward masking decreased by only about 3 dB (a non-significant effect) when the masker m was decreased from 1 to 0.25. This result appears to be inconsistent with an explanation of AM forward masking in terms of adaptation in a MFB or in terms of a sliding temporal integrator.

Development of temporal auditory processing in childhood: Changes in efficiency rather than temporal-modulation selectivity

The ability to detect amplitude modulation (AM) is essential to distinguish the spectro-temporal features of speech from those of a competing masker. Previous work shows that AM sensitivity improves until 10 years of age. This may relate to the development of sensory factors (tuning of AM filters, susceptibility to AM masking) or to changes in processing efficiency (reduction in internal noise, optimization of decision strategies). To disentangle these hypotheses, three groups of children (5-11 years) and one of young adults completed psychophysical tasks measuring thresholds for detecting sinusoidal AM (with a rate of 4, 8, or 32 Hz) applied to carriers whose inherent modulations exerted different amounts of AM masking. Results showed that between 5 and 11 years, AM detection thresholds improved and that susceptibility to AM masking slightly increased. However, the effects of AM rate and carrier were not associated with age, suggesting that sensory factors are mature by 5 years. Subsequent modelling indicated that reducing internal noise by a factor 10 accounted for the observed developmental trends. Finally, children's consonant identification thresholds in noise related to some extent to AM sensitivity. Increased efficiency in AM detection may support better use of temporal information in speech during childhood.

Cascaded Amplitude Modulations in Sound Texture Perception

Sound textures, such as crackling fire or chirping crickets, represent a broad class of sounds defined by their homogeneous temporal structure. It has been suggested that the perception of texture is mediated by time-averaged summary statistics measured from early auditory representations. In this study, we investigated the perception of sound textures that contain rhythmic structure, specifically second-order amplitude modulations that arise from the interaction of different modulation rates, previously described as “beating” in the envelope-frequency domain. We developed an auditory texture model that utilizes a cascade of modulation filterbanks that capture the structure of simple rhythmic patterns. The model was examined in a series of psychophysical listening experiments using synthetic sound textures—stimuli generated using time-averaged statistics measured from real-world textures. In a texture identification task, our results indicated that second-order amplitude modulation sensitivity enhanced recognition. Next, we examined the contribution of the second-order modulation analysis in a preference task, where the proposed auditory texture model was preferred over a range of model deviants that lacked second-order modulation rate sensitivity. Lastly, the discriminability of textures that included second-order amplitude modulations appeared to be perceived using a time-averaging process. Overall, our results demonstrate that the inclusion of second-order modulation analysis generates improvements in the perceived quality of synthetic textures compared to the first-order modulation analysis considered in previous approaches.

Perception of speech modulation cues by 6-month-old infants

PURPOSE

The capacity of 6-month-old infants to discriminate a voicing contrast (/aba/-/apa/) on the basis of amplitude modulation (AM) cues and frequency modulation (FM) cues was evaluated.

METHOD

Several vocoded speech conditions were designed to either degrade FM cues in 4 or 32 bands or degrade AM in 32 bands. Infants were familiarized to the vocoded stimuli for a period of either 1 or 2 min. Vocoded speech discrimination was assessed using the head-turn preference procedure.

RESULTS

Infants discriminated /aba/ from /apa/ in each condition; however, familiarization time was found to influence strongly infants' responses (i.e., their preference for novel vs. familiar stimuli).

CONCLUSIONS

Six-month-old infants do not require FM cues and can use the slowest (< 16 Hz) AM cues to discriminate voicing. Moreover, 6-month-old infants can use AM cues extracted from only 4 broad-frequency bands to discriminate voicing.

Lateralization based on interaural differences in the second-order amplitude modulator

Second-order amplitude modulation is a relatively slow variation of the modulation depth of a first-order amplitude modulation with higher frequency. In contrast to first-order modulation, which appears as a physical component in the stimulus spectrum after half-wave rectification, second-order modulation is not necessarily demodulated by the auditory periphery. For binaural processing of second-order amplitude modulated stimuli it is unknown whether interaural time differences (ITDs) in the second-order modulation result in a lateralized percept. Thus, second-order modulation can serve as a tool to investigate whether demodulation of interaurally delayed components is a prerequisite for lateralization. In most of the psychoacoustic experiments presented here, a 25 Hz sinusoidally amplitude-modulated (SAM) 160 Hz tone was either transposed to 4 kHz by half-wave rectifying this SAM waveform before multiplication with a 4 kHz tone (TSAM), or by adding an offset before multiplication (SAMAM). The experiments revealed an inability to lateralize the SAMAM based on ITDs in the 25 Hz component, whereas subjects could lateralize the TSAM. Given that only the TSAM results in a demodulated 25 Hz component after peripheral auditory processing, this result supports the hypothesis that demodulation is a prerequisite for lateralization, which has consequences for temporal modulation processing in models of binaural interaction.

Sound texture perception via statistics of the auditory periphery: evidence from sound synthesis

Rainstorms, insect swarms, and galloping horses produce "sound textures"--the collective result of many similar acoustic events. Sound textures are distinguished by temporal homogeneity, suggesting they could be recognized with time-averaged statistics. To test this hypothesis, we processed real-world textures with an auditory model containing filters tuned for sound frequencies and their modulations, and measured statistics of the resulting decomposition. We then assessed the realism and recognizability of novel sounds synthesized to have matching statistics. Statistics of individual frequency channels, capturing spectral power and sparsity, generally failed to produce compelling synthetic textures; however, combining them with correlations between channels produced identifiable and natural-sounding textures. Synthesis quality declined if statistics were computed from biologically implausible auditory models. The results suggest that sound texture perception is mediated by relatively simple statistics of early auditory representations, presumably computed by downstream neural populations. The synthesis methodology offers a powerful tool for their further investigation.

Neural representations of complex temporal modulations in the human auditory cortex

Natural sounds such as speech contain multiple levels and multiple types of temporal modulations. Because of nonlinearities of the auditory system, however, the neural response to multiple, simultaneous temporal modulations cannot be predicted from the neural responses to single modulations. Here we show the cortical neural representation of an auditory stimulus simultaneously frequency modulated (FM) at a high rate, f(FM) approximately 40 Hz, and amplitude modulation (AM) at a slow rate, f(AM) <15 Hz. Magnetoencephalography recordings show fast FM and slow AM stimulus features evoke two separate but not independent auditory steady-state responses (aSSR) at f(FM) and f(AM), respectively. The power, rather than phase locking, of the aSSR of both decreases with increasing stimulus f(AM). The aSSR at f(FM) is itself simultaneously amplitude modulated and phase modulated with fundamental frequency f(AM), showing that the slow stimulus AM is not only encoded in the neural response at f(AM) but also encoded in the instantaneous amplitude and phase of the neural response at f(FM). Both the amplitude modulation and phase modulation of the aSSR at f(FM) are most salient for low stimulus f(AM) but remain observable at the highest tested f(AM) (13.8 Hz). The instantaneous amplitude of the aSSR at f(FM) is successfully predicted by a model containing temporal integration on two time scales, approximately 25 and approximately 200 ms, followed by a static compression nonlinearity.

Comparison of level discrimination, increment detection, and comodulation masking release in the audio- and envelope-frequency domains

In general, the temporal structure of stimuli must be considered to account for certain observations made in detection and masking experiments in the audio-frequency domain. Two such phenomena are (1) a heightened sensitivity to amplitude increments with a temporal fringe compared to gated level discrimination performance and (2) lower tone-in-noise detection thresholds using a modulated masker compared to those using an unmodulated masker. In the current study, translations of these two experiments were carried out to test the hypothesis that analogous cues might be used in the envelope-frequency domain. Pure-tone carrier amplitude-modulation (AM) depth-discrimination thresholds were found to be similar using both traditional gated stimuli and using a temporally modulated fringe for a fixed standard depth (ms = 0.25) and a range of AM frequencies (4-64 Hz). In a second experiment, masked sinusoidal AM detection thresholds were compared in conditions with and without slow and regular fluctuations imposed on the instantaneous masker AM depth. Release from masking was obtained only for very slow masker fluctuations (less than 2 Hz). A physiologically motivated model that effectively acts as a first-order envelope change detector accounted for several, but not all, of the key aspects of the data.

Cues for masked amplitude-modulation detection

The ability of psychoacoustic models to predict listeners' performance depends on two key stages: preprocessing and the generation of a decision variable. The goal of the current study was to determine the perceptually relevant decision variables in masked amplitude-modulation detection tasks in which the modulation depth of the masker was systematically varied. Potential cues were made unreliable by roving the overall modulation depth from trial to trial or were reduced in salience by equalizing the envelope energy of the standard and target after the signal was added. Listeners' performance was significantly degraded in both paradigms compared to the baseline (fixed-level modulation masker) condition, which was similar to those used in previous studies of masking in the envelope-frequency domain. Although this observation was broadly consistent with a simple long-term envelope power-spectrum model, there were several aspects of the data that were not. For example, the steep rate of change in threshold with masker depth and the fact that an optimal amount of envelope noise could enhance performance were not predicted by decision variables calculated directly from the stimulus envelope. A physiologically based processing model suggested a realistic nonlinear mechanism that could give rise to these second-order features of the data.

Effect of modulation maskers on the detection of second-order amplitude modulation with and without notched noise

The mechanisms underlying the detection of second-order amplitude modulation (AM) were explored. The detectability of second-order AM (fixed depth for each subject) was measured for first- and second-order modulation rates of 16 and 2 Hz, respectively (slow-rate pair), and 50 and 10 Hz, respectively (fast-rate pair), with no masker, a low-band modulation masker (centered at 2 or 10 Hz), and a high-band modulation masker (centered at 16 or 50 Hz). This was done in the absence and presence of an audio-frequency notched noise centered at the carrier frequency of 4000 Hz. Both modulation maskers were "low-noise" noises, to prevent overmodulation. In the absence of notched noise, both modulation maskers impaired performance for the slow-rate pair, but only the low-band masker impaired performance for the fast-rate pair. When notched noise was present, the low-band masker had no significant effect for either rate pair and the high-band masker had an effect only for the slow-rate pair. These results suggest that second-order AM detection is mediated both by an envelope distortion component at the second-order rate and by slow fluctuations in the output of a modulation filter tuned to the first-order rate. When notched noise is present, the distortion component plays little role.

Perception of the envelope-beat frequency of inharmonic complex temporal envelopes

Listeners can hear slow sinusoidal variations in the depth of sinusoidally amplitude-modulated (SAM) stimuli. Here, the SAM stimulus of frequency f(m) acts as the carrier, and the slow variation in depth of frequency f'm (referred to as "second-order" amplitude modulation) corresponds to a beat in the temporal envelope. Recent studies have suggested that second-order amplitude modulation perception is based on a modulation-distortion component or the "venelope" (the Hilbert envelope of the ac-coupled Hilbert envelope), both occurring at the envelope-beat frequency f'm. This was tested by transposing to the modulation domain the matching paradigm used by Schouten et al. [J. Acoust. Soc. Am. 34, 1418-1424 (1962)]. Listeners estimated the envelope-beat frequency evoked by a 5-Hz, second-order SAM white noise with f(m) either an integer multiple of f'm or shifted in frequency to make the complex envelope inharmonic. The results indicate that the perception of the envelope-beat frequency was affected by these shifts when f(m) < or = 20 Hz. This suggests that, at least at low modulation frequencies, the perceived envelope beat is not determined by a modulation-distortion or venelope component, but rather relies on the time intervals between the main peaks of the first-order envelope.

Masking release for consonant features in temporally fluctuating background noise

Consonant identification was measured for normal-hearing listeners using Vowel-Consonant-Vowel stimuli that were either unprocessed or spectrally degraded to force listeners to use temporal-envelope cues. Stimuli were embedded in a steady state or fluctuating noise masker and presented at a fixed signal-to-noise ratio. Fluctuations in the maskers were obtained by applying sinusoidal modulation to: (i) the amplitude of the noise (1st-order SAM masker) or (ii) the modulation depth of a 1st-order SAM noise (2nd-order SAM masker). The frequencies of the amplitude variation fm and the depth variation f'm were systematically varied. Consistent with previous studies, identification scores obtained with unprocessed speech were highest in an 8-Hz, 1st-order SAM masker. Reception of voicing and manner also peaked around fm=8 Hz, while the reception of place of articulation was maximal at a higher frequency (fm=32 Hz). When 2nd-order SAM maskers were used, identification scores and received information for each consonant feature were found to be independent of f'm. They decreased progressively with increasing carrier modulation frequency fm, and ranged between those obtained with the steady state and the 1st-order SAM maskers. Finally, the results obtained with spectrally degraded speech were similar across all types of maskers, although an 8% improvement in the reception of voicing was observed for modulated maskers with fm < 64 Hz compared to the steady-state masker. These data provide additional evidence that listeners take advantage of temporal minima in fluctuating background noises, and suggest that: (i) minima of different durations are required for an optimal reception of the three consonant features and (ii) complex (i.e., 2nd-order) envelope fluctuations in background noise do not degrade speech identification by interfering with speech-envelope processing.

Modulation masking produced by second-order modulators

Recent studies suggest that an auditory nonlinearity converts second-order sinusoidal amplitude modulation (SAM) (i.e., modulation of SAM depth) into a first-order SAM component, which contributes to the perception of second-order SAM. However, conversion may also occur in other ways such as cochlear filtering. The present experiments explored the source of the first-order SAM component by investigating the ability to detect a 5-Hz, first-order SAM probe in the presence of a second-order SAM masker beating at the probe frequency. Detection performance was measured as a function of masker-carrier modulation frequency, phase relationship between the probe and masker modulator, and probe modulation depth. In experiment 1, the carrier was a 5-kHz sinusoid presented either alone or within a notched-noise masker in order to restrict off-frequency listening. In experiment 2, the carrier was a white noise. The data obtained in both carrier conditions are consistent with the existence of a modulation distortion component. However, the phase yielding poorest detection performance varied across experimental conditions between 0 degrees and 180 degrees, confirming that, in addition to nonlinear mechanisms, cochlear filtering and off-frequency listening play a role in second-order SAM perception. The estimated magnitude of the modulation distortion component ranges from 5%-12%.

Estimation of the level and phase of the simple distortion tone the modulation domain

These experiments were designed to test the idea that nonlinearities in the auditory system can introduce a distortion component into the internal representation of the envelope of a sound, and to estimate the phase of the hypothetical distortion component. In experiment 1, a two-alternative forced-choice (2AFC) task with feedback was used to measure psychometric functions for detecting 5-Hz probe modulation of a 4-kHz sinusoidal carrier in the presence of a masker modulator with components at 50 and 55 Hz (m=0.3 for each component). Performance was measured as a function of the relative phase, delta[symbol see text], of the probe relative to the "venelope" (envelope of the envelope) of the masker. Performance was poorest for delta[symbol see text]= 135 degrees. In experiment 2, delta[symbol see text] was fixed at 135 degrees, m was set to 0.48 for each masker component, and psychometric functions for detecting probe modulation were measured using a 2AFC task without feedback. For small probe modulation depths (m approximately 0.03), the detectability index, d', was consistently negative, consistent with the existence of a weak distortion product which can "cancel" the probe modulation. The distortion component for the conditions of the experiment was estimated to have a phase of about -25 degrees relative to the venelope.

Effects of amplitude compression on first- and second-order modulation detection thresholds in cochlear implant listeners

The aim of this study was to examine the effects of instantaneous non-linear amplitude mapping on the detection of single-component and multicomponent temporal envelopes. To address this issue, first- and second-order amplitude modulation detection thresholds were measured in four cochlear implant users with the intervention of the compression device of the implant processor. The compression device is set to produce either a strongly or a weakly logarithmic mapping of stimulus amplitude to electrical amplitude. 'First-order' modulation detection thresholds indicate the ability of listeners to detect sinusoidal amplitude modulation (SAM) applied to a white noise carrier; they are measured as a function of the rate of that modulation, fm. 'Second-order' modulation detection thresholds indicate the ability to detect sinusoidal modulation applied to the depth of a sinusoidally amplitude-modulated signal (here, a 16-Hz sinusoidally amplitude-modulated white noise); they are measured as a function of the rate of the modulation applied to the modulation depth (referred to as fm'). In each task, stimuli are transformed by the implant processor and are presented through one electrode at approximately the same level. The results show that, in cochlear implant listeners, both first- and second-order modulation detection thresholds measured at the lower rates (< or =7 Hz) decrease slightly by about 3-6dB when the stronger compression is used. No effect of compression is observed at higher rates. These results suggest that instantaneous logarithmic amplitude mapping has beneficial- but limited-effects on the detection of single-component and multicomponent temporal envelopes. These results are discussed in light of current models of temporal envelope processing.

Modulation masking produced by complex tone modulators

Thresholds were measured for detecting sinusoidal amplitude modulation in the presence of a complex-tone masker modulation. Both modulations were applied to the same sinusoidal carrier. Two different masker modulations were used: (i) a pair of components beating at the difference frequency and (ii) a three-tone complex producing a sinusoidal amplitude modulation of the modulation depth at the difference frequency between adjacent components. Both maskers show a periodicity in the waveform that is not contained in the envelope spectrum itself but can be observed when the envelope of the envelope, referred to as the "venelope" [Ewert et al., J. Acoust. Soc. Am. 112, 2921-2931 (2002)], is calculated. For a signal frequency equal to the masker-venelope periodicity, modulation depth at threshold was measured as a function of the signal phase relative to the phase of the masker-venelope component. Signal frequencies of 5, 30, and 90 Hz were used. It was found that masking was phase dependent for all three signal frequencies. Thresholds were lower for the in-phase condition, where maxima in the signal waveform coincided with maxima in the masker-venelope waveform, than for the antiphase condition. The maximum threshold difference was 15 dB. The results are in contrast to recent data [Moore et al., J. Acoust. Soc. Am. 106, 908-918 (1999)], where lowest thresholds were reported for the antiphase condition in a similar experiment. The present data are in line with the idea that a nonlinearity prior to a modulation filterbank extracts the venelope of the masker modulator. However, a compressive nonlinearity such as that associated with the processing on the basilar membrane cannot account for the empirical findings, since it predicts the opposite phase effect.

Testing the concept of a modulation filter bank: the audibility of component modulation and detection of phase change in three-component modulators

Two experiments were performed to test the concept that the auditory system contains a "modulation filter bank" (MFB). Experiment 1 examined the ability to "hear out" the modulation frequency of the central component of a three-component modulator applied to a 4-kHz sinusoidal carrier. On each trial, three modulated stimuli were presented. The modulator of the first stimulus contained three components. Within a run the frequencies of the outer two components were fixed and the frequency of the central ("target") component was drawn randomly from one of five values. The modulators of second and third stimuli contained one component. One had a frequency equal to that of the target and the other had a frequency randomly selected from one of the other possible values. Subjects indicated whether the target corresponded to the second or third stimulus. Scores were around 80% correct when the components in the three-component modulator were widely spaced and when the frequencies of the target and comparison differed sufficiently. Experiment 2 examined the ability to hear a change in the relative phase of the components in a three-component modulator with harmonically spaced components, using a 31FC task. The frequency of the central component, f(c), was either 50 or 100 Hz. Scores were 80%-90% correct when the component spacing was < or = 0.5 f(c), but decreased markedly for greater spacings. Performance was only slightly impaired by randomizing the overall modulation depth from one stimulus to the next. The results of both experiments are broadly consistent with what would be expected from a MFB with a Q value of 1 or slightly less.

Effect of a noise modulation masker on the detection of second-order amplitude modulation

Amplitude modulation waveforms can contain complex patterns of modulation frequency and depth that are characteristic of many biologically relevant sounds. To investigate the mechanisms involved in the processing of such patterns, we measured detection thresholds for second-order amplitude modulation (AM), a sinusoidal AM in which AM depth varies with time at frequency f(m)'. Second-order AM generates sidebands in the modulation spectrum on either side of the frequency components introduced by the first-order AM. Previous masking studies suggested that a distortion product located at f(m)' contributes to the detection of second-order AM. This hypothesis was tested by masking the putative distortion product using a noise modulation masker centred on (1) the second-order modulation frequency (f(m)'=2 Hz) and (2) the first-order modulation frequency (f(m)=16 Hz). The second-order AM was applied to a 5-kHz pure-tone carrier. Increasing the depth of a 2-Hz-wide noise modulator masker centred on 2 Hz had little effect on detection thresholds for second-order AM, but increased detection thresholds for 2-Hz first-order AM six-fold. Increasing the depth of an 8-Hz-wide noise modulator masker centred on 16 Hz increased detection thresholds for both first- and second-order AM three-fold. These results show that the detection of the second-order AM, when f(m)' is 2 Hz, is not dependent on the detection of modulation at f(m)' but is dependent on the detection of modulation components centred on f(m).

Effect of cochlear damage on the detection of complex temporal envelopes

Recent studies have demonstrated that the detection of complex temporal envelopes relies - at least partially - on the perception of a distortion component generated by a peripheral (cochlear) and/or central (post-cochlear) non-linearity. In the present study, first- and second-order amplitude modulation (AM) detection thresholds were obtained in normally hearing (NH) and hearing-impaired (HI) listeners using a 2-kHz pure-tone carrier. In both groups of listeners, first-order AM detection thresholds were measured for AM rates fm ranging between 4 and 87 Hz, and second-order AM detection thresholds were measured for second-order AM rates fm' ranging between 4 and 23 Hz, using a fixed first-order 'carrier' AM rate fm of 64 Hz. When the sound pressure level was adjusted in order to yield equal detectability in both groups for the 64-Hz first-order carrier modulation, (i) first-order AM detection thresholds for the HI listeners were normal at fm=87 Hz, and better-than-normal at fm=4 and 16 Hz, and (ii) second-order AM detection thresholds were identical at all modulation rates in NH and HI listeners. Similar results were obtained when the audibility of the 2-kHz pure-tone carrier was equated for both groups, i.e. when listeners were tested at the same sensation level. These results demonstrate clearly that cochlear damage has no effect on the detection of complex temporal envelopes, and indicate that the distortion component must be generated by a more central non-linearity than cochlear compression, transduction, or short-term adaptation.

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.