Modulation Masking Research Articles

Spectral modulation masking patterns reveal tuning to spectral envelope frequency

Auditory processing appears to include a series of domain-specific filtering operations that include tuning in the audio-frequency domain, followed by tuning in the temporal modulation domain, and perhaps tuning in the spectral modulation domain. To explore the possibility of tuning in the spectral modulation domain, a masking experiment was designed to measure masking patterns in the spectral modulation domain. Spectral modulation transfer functions (SMTFs) were measured for modulation frequencies from 0.25 to 14 cycles/octave superimposed on noise carriers either one octave (800-1600 Hz, 6400-12,800 Hz) or six octaves wide (200-12,800 Hz). The resulting SMTFs showed maximum sensitivity to modulation between 1 and 3 cycles/octave with reduced sensitivity above and below this region. Masked spectral modulation detection thresholds were measured for masker modulation frequencies of 1, 3, and 5 cycles/octave with a fixed modulation depth of 15 dB. The masking patterns obtained for each masker frequency and carrier band revealed tuning (maximum masking) near the masker frequency, which is consistent with the theory that spectral envelope perception is governed by a series of spectral modulation channels tuned to different spectral modulation frequencies.

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.

Practice effects in comodulation masking release

Substantial research has explored the acoustic factors influencing comodulation masking release (CMR), but the impact of practice has not been addressed. The current study evaluated the effect of experience on CMR thresholds. Ninety‐eight adults listened to CMR stimuli through headphones once a week for 3 weeks. CMR was measured as the difference in threshold between a reference condition with one 20‐Hz‐wide amplitude‐modulated noise band centered on a 1000‐Hz, pure‐tone signal, and a modulated masker condition having eight additional noise bands comodulated with the on‐signal band. Thresholds were estimated in a single‐interval, maximum‐likelihood procedure. Hierarchical linear modeling tested the effect of practice on thresholds and CMR. Analyses indicated that thresholds improved significantly with practice (p<0.001) for both masking conditions. On average, reference thresholds improved 5.41 dB per visit (SE=0.703), and modulated masker thresholds improved 3.15 dB per visit (SE=0.495). Total CMR decreased significantly (p<0.01) by 1.98 dB per visit (SE=0.701), most likely because performance in the reference condition improved more than performance in the modulated masker condition. Results indicate that practice improves performance in CMR masking conditions and that total CMR decreases with experience.

Modulation masking in monaural and binaural conditions

The shape of hypothetical modulation filters was measured in monaural and binaural signal presentation. A sinusoidal 4‐kHz carrier was amplitude modulated by low‐low‐noise (LNN) [A. Kohlrausch et al., Acta Acust. 83, 659–669 (1997)] and a probe tone of a frequency varying from 2 to 256 Hz. Using a three‐interval forced‐choice (3IFC) procedure the threshold of modulation by the probe tone was determined for the LNN of 16‐Hz center frequency and 8‐Hz bandwidth, and 64‐Hz center frequency and 8‐, 16‐, or 32‐Hz bandwidth. In the monaural condition, signals were presented to the right ear. In the binaural condition, modulation by the LNN was presented to the right ear, and modulation by the probe tone to the left ear. Results showed that masking in the modulation rate domain is characterized by a broad tuning with the highest values of about 17 dB for the monaural condition, and 4 to 9 dB in the binaural condition occurring for probe‐tone frequency equal the center frequency of the LNN. The large effect in bin...

Sequential amplitude‐modulation interference across auditory channels and across ears

Forward masking has been demonstrated in the amplitude‐modulation (AM) domain [M. Wojtczak and N. F. Viemeister, J. Acoust. Soc. Am. 118, 3198 (2005)] for masker and signal imposed on the same carrier. This study explores an effect of AM on detection of AM imposed sequentially on a different carrier. Data show that a 40−Hz AM (interferer) imposed on a 2−kHz carrier affects detection of a 40−Hz AM (signal) imposed on a 5.5−kHz carrier after the offset of the interferer AM. The effect is analogous (and similar in size for the shortest interferer−signal delays) to modulation detection interference for simultaneous interferer AM. In another condition, two independent noise samples were presented to opposite ears. In one ear, a 40−Hz masker AM was imposed on the carrier. In the opposite ear, a 40−Hz signal was imposed on the carrier after the offset of the masker modulation. A forward−masking effect similar to but smaller in size than that in monaural presentation was observed. The data from both experiments suggest that the forward−masking effect in the modulation domain occurs at central stages of processing, where information is combined across peripheral channels and where binaural interactions occur. [Work supported by NIH Grants DC00683 and DC006804.]

Factors affecting masking release for speech in modulated noise for normal-hearing and hearing-impaired listeners

The Speech Reception Threshold for sentences in stationary noise and in several amplitude-modulated noises was measured for 8 normal-hearing listeners, 29 sensorineural hearing-impaired listeners, and 16 normal-hearing listeners with simulated hearing loss. This approach makes it possible to determine whether the reduced benefit from masker modulations, as often observed for hearing-impaired listeners, is due to a loss of signal audibility, or due to suprathreshold deficits, such as reduced spectral and temporal resolution, which were measured in four separate psychophysical tasks. Results show that the reduced masking release can only partly be accounted for by reduced audibility, and that, when considering suprathreshold deficits, the normal effects associated with a raised presentation level should be taken into account. In this perspective, reduced spectral resolution does not appear to qualify as an actual suprathreshold deficit, while reduced temporal resolution does. Temporal resolution and age are shown to be the main factors governing masking release for speech in modulated noise, accounting for more than half of the intersubject variance. Their influence appears to be related to the processing of mainly the higher stimulus frequencies. Results based on calculations of the Speech Intelligibility Index in modulated noise confirm these conclusions.

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.

Frequency discrimination in amplitude-modulated maskers

Under some conditions, the detection threshold for a pure tone signal masked by a bandpass Gaussian noise can be reduced by the introduction of amplitude modulation (AM) to the masker. Previous data suggest that this advantage may be reduced for discrimination tasks, such as frequency discrimination of a masked tone [Hall et al., J. Acoust. Soc. Am. 102, 1746–1754]. In the present study, frequency discrimination thresholds were estimated for a 500-Hz pure tone presented at a range of levels (3, 6, and 12 dB re: masked detection threshold). The masker was either a Gaussian noise or an AM noise (bandpass filtered 50–4000 Hz), with a range of square wave AM rates (10–40 Hz). Masking release was greater for 10-Hz AM than 40-Hz AM, but frequency discrimination was better for the 10 than the 40-Hz rate when measured at equal sensation levels. This result suggests that temporal resolution and integration may have important contributions to frequency discrimination measured for tones presented in modulated maskers. [Work supported by NIH NIDCD Grant R01 DC000418.]

Speech masking release in listeners with flat hearing loss: Effects of masker fluctuation rate on identification scores and phonetic feature reception

Consonant identification was measured for a stationary and amplitude-modulated noise masker in four listeners with flat cochlear hearing loss, and four age-matched normal-hearing listeners. The masker modulation rate was systematically varied between 2 and 128 Hz. Masking release (MR), that is better identification performance in fluctuating, than in stationary noise, was highest in a masker fluctuating at 8–16 Hz in all normal-hearing listeners. In comparison, MR was only observed in two out of the four impaired listeners. In these listeners, MR was poorer than normal, and peaked at lower rates, that is 2 or 8 Hz. MR corresponded to increased reception of information for voicing, place, and manner between 2 and 64 Hz in all normal-hearing listeners. In impaired listeners, increased reception of information was mainly observed for manner, and mainly reduced for place, but these differences were not significant. For all phonetic features, MR was observed at lower masker fluctuation rates (≤32 Hz) than in normal-hearing listeners. This study therefore shows that cochlear damage affects MR, both quantitatively and qualitatively.SumarioSe midió la identificación de consonantes con un ruido enmascarador estacionario y de amplitud modulada en cuatro sujetos con pérdidas auditivas cocleares planas, y en cuatro sujetos normo-oyentes de edades similares. La tasa de modulación del enmascarador se varió sistemáticamente entre 2 y 128 Hz. La sustracción del efecto de enmascaramiento (MR), que tiene un mejor desempeño de identificación con ruido fluctuante que con ruido estacionario, fue mayor con un enmascarador que fluctuó entre 8 y 16 Hz, en todos los sujetos normales. Comparativamente, el MR sólo se observó en dos de los cuatro sujetos hipoacúsicos. En estos sujetos, el MR fue menor al normal y su pico se obtuvo a tasas bajas, esto es, de 8 a 12 Hz. El MR en sujetos normo-oyentes correspondió a un incremento en la recepción de la información en sonoridad, punto y modo de articulación entre 2 y 64 Hz. En sujetos hipoacúsicos, se observó un aumento en la recepción de información en modo, y aunque se vio reducida principalmente en cuanto al punto de articulación, estas diferencias no fueron significativas. Para todas las características fonéticas, el MR se observó a tasas más bajas de fluctuación (≤32 Hz) que en los sujetos normo-oyentes. Este estudio, por consiguiente, muestra que el daño coclear afecta el MR tanto cualitativa como cuantitativamente.

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 f m 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 f m = 8 Hz, while the reception of place of articulation was maximal at a higher frequency ( f m = 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 f m, 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 f m < 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 detection interference with periodic complex modulators

Past studies of modulation detection interference (MDI) have considered the possibility that the effect may in part relate to auditory grouping of the probe and masker. Alternatively as a cross-channel effect, MDI may be representative of informational masking. The present experiments varied modulator waveshape in attempts to distinguish the effects of auditory grouping and informational masking on MDI. In the first experiment, both the slope and duty cycle of the masker modulator varied across conditions with the task to detect either 4- or 10-Hz sinusoidal probe modulation. The effect of masker-modulator waveshape on the detection of 4-Hz probe modulation was generally consistent with change in the perceptual prominence of the masker. The 10-Hz results showed little effect of these manipulations. The second experiment utilized interrupted modulation patterns to allow for either concurrent or sequential envelope fluctuation of the probe and masker. Results showed no effect of modulation concurrency. In both experiments, significant departures from energetic masking in the modulation domain were obtained. Collectively, these results were taken to support an interpretation of the interference in terms of informational masking rather than grouping based on common modulation. [Work supported by NIDCD.]

Development of comodulation masking release

The comodulation masking release (CMR) paradigm permits assessment of across-channel auditory processing. To evaluate the development of this phenomenon, CMR was measured in 81 children (M=8.95 years) and 20 adults (M=23.3 years). A single-interval, maximum-likelihood procedure was used to estimate thresholds in a reference condition (signal plus on-signal AM masker) and a modulated masker condition (reference condition plus eight comodulated flanking bands). CMR was defined as improvement in signal detection in the modulated masker condition relative to the reference condition. Although total CMR did not change from 7 to 10 years of age, total CMR for children (4.02 dB) was significantly less than that for adults (10.85 dB), suggesting that across-channel processing develops beyond 10 years of age. In addition, thresholds for children were significantly higher than those for adults in both the reference condition (by 6.47 dB) and the modulated masker condition (by 13.91 dB), supporting the likelihood that processing efficiency also develops beyond 10 years of age. These results suggest that both processing efficiency and across-channel cues utilized in CMR are slowly developing phenomena. [Work supported by a grant from the Georgia State University Research Program Enhancement Program.]

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 in 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.

Across- and Within-Channel Envelope Interactions in Cochlear Implant Listeners

The effects of modulated maskers on detection thresholds of a 50-Hz sinusoidal amplitude modulation (SAM) in a signal carrier were measured in nine cochlear implant (CI) listeners as a function of masker envelope type and for different masker-signal electrode separations. Both signal and masker were 200-ms-long pulse trains, presented concurrently in an interleaved stimulation mode. Masker envelopes were SAM at 20, 50, (0- and pi-phase re: the signal modulator), and 125 Hz, as well as noise amplitude modulated (NAM), all with a fixed 20% modulation depth. Comparisons were made against steady-state maskers that had an amplitude equal to the mean amplitude of the modulated maskers or to their peak amplitude (SS(peak)). Modulation thresholds were larger in the presence of the dynamic maskers versus the SS(peak) maskers; however, there was significant intersubject variability in the pattern of results. Effects of relative phase between masker and signal were not consistent across subjects. Envelope masking (the dB difference in modulation detection thresholds between modulated and SS(peak) maskers) was generally larger for the lower-modulation-frequency maskers than the 125-Hz masker. The spatial distribution of masked modulation detection thresholds was found to be considerably different from spatial forward-masking patterns obtained in the same subjects. Finally, modulation thresholds measured for a very wide separation between the masker and signal showed significant envelope masking. These results suggest that, as has been shown in acoustic stimulation, central, across-channel temporal processing mechanisms also occur in electrical stimulation.

External and internal limitations in amplitude-modulation processing.

Three experiments are presented to explore the relative role of "external" signal variability and "internal" resolution limitations of the auditory system in the detection and discrimination of amplitude modulations (AM). In the first experiment, AM-depth discrimination performance was determined using sinusoidally modulated broadband-noise and pure-tone carriers. The AM index, m, of the standard ranged from -28 to -3 dB (expressed as 20 log m). AM-depth discrimination thresholds were found to be a fraction of the AM depth of the standard for standards down to -18 dB, in the case of the pure-tone carrier, and down to -8 dB, in the case of the broadband-noise carrier. For smaller standards, AM-depth discrimination required a fixed increase in AM depth, independent of the AM depth of the standard. In the second experiment, AM-detection thresholds were obtained for signal-modulation frequencies of 4, 16, 64, and 256 Hz, applied to either a band-limited random-noise carrier or a deterministic ("frozen") noise carrier, as a function of carrier bandwidth (8 to 2048 Hz). In general, detection thresholds were higher for the random- than for the frozen-noise carriers. For both carrier types, thresholds followed the pattern expected from frequency-selective processing of the stimulus envelope. The third experiment investigated AM masking at 4, 16, and 64 Hz in the presence of a narrow-band masker modulation. The variability of the masker was changed from entirely frozen to entirely random, while the long-term average envelope power spectrum was held constant. The experiment examined the validity of a long-term average quantity as the decision variable, and the role of memory in experiments with frozen-noise maskers. The empirical results were compared to predictions obtained with two modulation-filterbank models. The predictions revealed that AM-depth discrimination and AM detection are limited by a combination of the external signal variability and an internal "Weber-fraction" noise process.

Responses of dorsal cochlear nucleus neurons to signals in the presence of modulated maskers.

The detection of a signal in noise is enhanced when the masking noise is coherently modulated over a wide range of frequencies. This phenomenon, known as comodulation masking release (CMR), has been attributed to across-channel processing; however, the relative contribution of different stages in the auditory system to such across-channel processing is unknown. It has been hypothesized that wideband or lateral inhibition may underlie a physiological correlate of CMR. To further test this hypothesis, we have measured the responses of single units from the dorsal cochlear nucleus in which wideband inhibition is particularly pronounced. Using a sinusoidally amplitude-modulated tone at the best frequency of each unit as a masker, a pure-tone signal was added in the dips of the masker modulation. Flanking bands (FBs, also amplitude-modulated pure tones) were positioned to fall within the inhibitory sidebands of the receptive field of the unit. The FBs were either in phase (comodulated) or out of phase (codeviant) with the on-frequency masker. For the majority of units, the addition of the comodulated FBs produced a strong reduction in the response to the masker modulation, making the signal more salient in the post stimulus time histograms. The change in spike rate in response to the signal between the masker and signal-plus-masker conditions was greatest for the comodulated condition for 29 of 45 units. These results are consistent with the hypothesis that wideband inhibition may play a role in across-channel processing at an early stage in the auditory pathway.

The effect of modulation maskers on the detection of second-order amplitude modulation in the absence and presence of notched noise

Second-order amplitude modulation (AM) can be detected despite the absence of a component at the second-order modulation frequency in the modulation spectrum. This may depend on the detection of a modulation component at the second-order frequency produced by nonlinearities in the auditory system, or on the detection of a ‘‘beat’’ at the output of the modulation filter centered at the first-order rate. To assess these possibilities, and to explore the role of off-frequency listening, first- and second-order AM detection thresholds were determined for six listeners with normal hearing, in the absence and presence of a notched noise centered at the carrier frequency of 4 kHz. First-order thresholds were measured for f1=2, 10, 16, and 50 Hz. Second-order thresholds were measured for f1=16 and f2=2 Hz, and for f1=50 and f2=10 Hz. Then, using a second-order modulation depth giving about 80%-correct detection, performance for detecting second-order AM was measured in the presence of a modulation masker centered at either the first- or second-order rate. In the absence of notched noise, both modulation maskers impaired performance, for both second-order rates. In the presence of notched noise, for most listeners there was little effect of either modulation masker. [Work supported by MRC.]

Mechanisms underlying the auditory continuity illusion

This study investigates the auditory continuity illusion, combining psychophysics and magnetoencephalography. Stimuli consisted of amplitude-modulated (AM) noise interrupted by bursts of louder, unmodulated noise. Subjective judgments confirmed that the AM was perceived as continuous, a case of illusory continuity. Psychophysical measurements showed that the illusory modulation had little effect on the detection of a physical modulation, i.e., the illusory modulation produced no modulation masking. Duration discrimination thresholds for the AM noise segments, however, were elevated by the illusion. A whole-head magnetoencephalographic system was used to record brain activity when listeners attended passively to the stimuli. The AM noise produced a modulated magnetic activity, the auditory steady-state response. The illusory modulation did not produce such a response, instead, a possible neural correlate of the illusion was found in transient evoked responses. When the AM was interrupted by silence, oscillatory activity in the gamma-band range as well as slow evoked potentials were observed at each AM onset. In the case of the illusion, these neural responses were largely reduced. Both sets of results are inconsistent with a restoration of the modulation in the case of illusory continuity. Rather, they point to a role for onset-detection mechanisms in auditory scene analysis.