The influence of carrier level and frequency on modulation and beat-detection thresholds for sinusoidal carriers

This paper is concerned with modulation and beat detection for sinusoidal carriers. In the first experiment, temporal modulation transfer functions (TMTFs) were measured for carrier frequencies between 1 and 10 kHz. Modulation rates covered the range from 10 Hz to about the rate equaling the critical bandwidth at the carrier frequency. In experiment 2, TMTFs for three carrier frequencies were obtained as a function of the carrier level. In the final experiment, thresholds for the detection of either the lower or the upper modulation sideband (beat detection) were measured for "carrier" frequencies of 5 and 10 kHz, using the same range of modulation rates as in experiment 1. The TMTFs for carrier frequencies of 2 kHz and higher remained flat up to a modulation rate of about 100-130 Hz and had similar values across carrier frequencies. For higher rates, modulation thresholds initially increased and then decreased rapidly, reflecting the subjects' ability to resolve the sidebands spectrally. Detection thresholds generally improved with increasing carrier level, but large variations in the exact level dependence were observed, across subjects as well as across carrier frequencies. For beat rates up to about 70 Hz (at 5 kHz) and 100 Hz (at 10 kHz), beat detection thresholds were the same for the upper and the lower sidebands and were about 6 dB higher than the level per sideband at the modulation-detection threshold. At higher rates the threshold for both sidebands increased, but the increase was larger for the lower sideband. This reflects an asymmetry in masking with more masking towards lower frequencies. Only at rates well beyond the maximum of the TMTF did detection for the lower sideband start to be better than that for the upper sideband. The asymmetry at intermediate frequency separations can be explained by assuming that detection always takes place in filters centered above the stimulus spectrum. The shape of the TMTF and the beat-detection data reflects a limitation in resolving fast amplitude variations, which must occur central to the inner-ear filtering. Its characteristic resembles that of a first-order low-pass filter with a cutoff frequency of about 150 Hz.     

Monaural and interaural temporal modulation transfer functions measured with 5-kHz carriers

Temporal modulation transfer functions (TMTFs) were measured for detection of monaural sinusoidal amplitude modulation and dynamically varying interaural level differences for a single set of listeners. For the interaural TMTFs, thresholds are the modulation depths at which listeners can just discriminate interaural envelope-phase differences of 0 and 180 degrees. A 5-kHz pure tone and narrowband noises, 30- and 300-Hz wide centered at 5 kHz, were used as carriers. In the interaural conditions, the noise carriers were either diotic or interaurally uncorrelated. The interaural TMTFs with tonal and diotic noise carriers exhibited a low-pass characteristic but the cutoff frequencies changed nonmonotonically with increasing bandwidth. The interaural TMTFs for the tonal carrier began rolling off approximately a half-octave lower than the tonal monaural TMTF (approximately 80 Hz vs approximately 120 Hz). Monaural TMTFs obtained with noise carriers showed effects attributable to masking of the signal modulation by intrinsic fluctuations of the carrier. In the interaural task with dichotic noise carriers, similar masking due to the interaural carrier fluctuations was observed. Although the mechanisms responsible for differences between the monaural and interaural TMTFs are unknown, the lower binaural TMTF cutoff frequency suggests that binaural processing exhibits greater temporal limitation than monaural processing.     

Temporal integration in amplitude modulation detection

Thresholds for detecting sinusoidal amplitude modulation (AM) of a wideband noise carrier were measured as a function of the duration of the modulating signal. The carrier was either; (a) gated with a duration that exceeded the duration of modulation by the combined stimulus rise and fall times; (b) presented with a fixed duration that included a 500-ms carrier fringe preceding the onset of modulation; or (c) on continuously. In condition (a), the gated-carrier temporal modulation transfer functions (TMTFs) exhibited a bandpass characteristic. For AM frequencies above the individual subject's TMTF high-pass segment, the mean slope of the integration functions was - 7.46 dB per log unit duration. For the fringe and continuous-carrier conditions [(b) and (c)], the mean slopes of the integration functions were, respectively, - 9.30 and - 9.36 dB per log unit duration. Simulations based on integration of the output of an envelope detector approximate the results from the gated-carrier conditions. The more rapid rates of integration obtained in the fringe and continuous-carrier conditions may be due to "overintegration" where, at brief modulation durations, portions of the unmodulated carrier envelope are included in the integration of modulating signal energy.     

The effects of frequency region and level on the temporal modulation transfer function

Temporal modulation transfer functions (TMTFs) were measured using narrow-band AM and QFM noises with upper spectral edges from 0.6 to 4.8 kHz, and spectrum levels of 10 and 40 dB SPL. The cutoff frequency of the TMTF increases as the upper spectral edge is increased up to 4.8 kHz at low levels, and is constant at higher levels. Sensitivity increases with bandwidth if frequency region is constant. In a second experiment, these results were compared to predictions of a model incorporating peripheral and central limitations to modulation detection. To obtain an estimate of peripheral filtering, frequency selectivity was measured using the notched-noise method, with probe frequencies and levels chosen to parallel those in the first experiment. The TMTF data were then predicted using the model. Predicted cutoff frequencies as a function of the upper spectral edge of the test stimulus were lower than but parallel to those of the subjects at the lower stimulus level. The model predicted only a slight increase in cutoff frequency with level, and thus predicted an increase in cutoff frequency with frequency region at the higher level as well, in contrast to the measured data. These results suggest that there are peripheral and central limitations to temporal resolution, but the psychoacoustically derived auditory filter may be only an indirect measure of peripheral filtering, and/or a more complex model may be needed.     

Temporal modulation transfer functions for listeners with real and simulated hearing loss

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal modulation transfer functions (TMTFs) for nine listeners with mild to profound sensorineural hearing loss. Each hearing loss was simulated in a group of three age-matched normal-hearing listeners through spectrally shaped masking noise or a combination of masking noise and multiband expansion. TMTFs were measured for both groups of listeners using a broadband noise carrier as a function of modulation rate in the range 2 to 1024 Hz. The TMTFs were fit with a lowpass filter function that provided estimates of overall modulation-depth sensitivity and modulation cutoff frequency. Although the simulations were capable of accurately reproducing the threshold elevations of the hearing-impaired listeners, they were not successful in reproducing the TMTFs. On average, the simulations resulted in lower sensitivity and higher cutoff frequency than were observed in the TMTFs of the hearing-impaired listeners. Discrepancies in performance between listeners with real and simulated hearing loss are possibly related to inaccuracies in the simulation of recruitment.     

Second-order temporal modulation transfer functions

Detection thresholds were measured for a sinusoidal modulation applied to the modulation depth of a sinusoidally amplitude-modulated (SAM) white noise carrier as a function of the frequency of the modulation applied to the modulation depth (referred to as f'm). The SAM noise acted therefore as a "carrier" stimulus of frequency fm, and sinusoidal modulation of the SAM-noise modulation depth generated two additional components in the modulation spectrum: fm-f'm and fm+f'm. The tracking variable was the modulation depth of the sinusoidal variation applied to the "carrier" modulation depth. The resulting "second-order" temporal modulation transfer functions (TMTFs) measured on four listeners for "carrier" modulation frequencies fm of 16, 64, and 256 Hz display a low-pass segment followed by a plateau. This indicates that sensitivity to fluctuations in the strength of amplitude modulation is best for fluctuation rates f'm below about 2-4 Hz when using broadband noise carriers. Measurements of masked modulation detection thresholds for the lower and upper modulation sideband suggest that this capacity is possibly related to the detection of a beat in the sound's temporal envelope. The results appear qualitatively consistent with the predictions of an envelope detector model consisting of a low-pass filtering stage followed by a decision stage. Unlike listeners' performance, a modulation filterbank model using Q values > or = 2 should predict that second-order modulation detection thresholds should decrease at high values of f'm due to the spectral resolution of the modulation sidebands (in the modulation domain). This suggests that, if such modulation filters do exist, their selectivity is poor. In the latter case, the Q value of modulation filters would have to be less than 2. This estimate of modulation filter selectivity is consistent with the results of a previous study using a modulation-masking paradigm [S. D. Ewert and T. Dau, J. Acoust. Soc. Am. 108, 1181-1196 (2000)].     

Modulation detection for amplitude-modulated bone-conducted sounds with sinusoidal carriers in the high- and ultrasonic-frequency range

Ultrasonic vibration generates a sensation of sound via bone-conduction. This phenomenon is called bone-conducted ultrasonic (BCU) hearing. Complex sounds can also be perceived by amplitude-modulating a BCU stimulus (AM-BCU). The influence of the modulation frequency on the sensitivity to detecting amplitude modulation of sinusoidal carriers of 10, 20, and 30 kHz was examined to clarify the characteristics of the perception of amplitude modulation over the sonic or audio-frequency range and the ultrasonic range. In addition, the detection sensitivity for single-sideband modulation for a 20 kHz carrier was measured. Temporal modulation transfer functions (TMTFs) obtained at each carrier frequency suggest that the auditory system has the ability to process timing information in the envelopes of AM-BCUs at lower modulation frequencies, as is the case with audio-frequency sounds. The possible influence of peripheral filtering on the shape of the TMTF at higher frequencies was examined.     

Detection and rate discrimination of amplitude modulation in electrical hearing

Three experiments were designed to examine temporal envelope processing by cochlear implant (CI) listeners. In experiment 1, the hypothesis that listeners' modulation sensitivity would in part determine their ability to discriminate between temporal modulation rates was examined. Temporal modulation transfer functions (TMTFs) obtained in an amplitude modulation detection (AMD) task were compared to threshold functions obtained in an amplitude modulation rate discrimination (AMRD) task. Statistically significant nonlinear correlations were observed between the two measures. In experiment 2, results of loudness-balancing showed small increases in the loudness of modulated over unmodulated stimuli beyond a modulation depth of 16%. Results of experiment 3 indicated small but statistically significant effects of level-roving on the overall gain of the TMTF, but no impact of level-roving on the average shape of the TMTF across subjects. This suggested that level-roving simply increased the task difficulty for most listeners, but did not indicate increased use of intensity cues under more challenging conditions. Data obtained with one subject, however, suggested that the most sensitive listeners may derive some benefit from intensity cues in these tasks. Overall, results indicated that intensity cues did not play an important role in temporal envelope processing by the average CI listener.     

Acoustic temporal modulation detection and speech perception in cochlear implant listeners

The goals of the present study were to measure acoustic temporal modulation transfer functions (TMTFs) in cochlear implant listeners and examine the relationship between modulation detection and speech recognition abilities. The effects of automatic gain control, presentation level and number of channels on modulation detection thresholds (MDTs) were examined using the listeners' clinical sound processor. The general form of the TMTF was low-pass, consistent with previous studies. The operation of automatic gain control had no effect on MDTs when the stimuli were presented at 65 dBA. MDTs were not dependent on the presentation levels (ranging from 50 to 75 dBA) nor on the number of channels. Significant correlations were found between MDTs and speech recognition scores. The rates of decay of the TMTFs were predictive of speech recognition abilities. Spectral-ripple discrimination was evaluated to examine the relationship between temporal and spectral envelope sensitivities. No correlations were found between the two measures, and 56% of the variance in speech recognition was predicted jointly by the two tasks. The present study suggests that temporal modulation detection measured with the sound processor can serve as a useful measure of the ability of clinical sound processing strategies to deliver clinically pertinent temporal information.     

Fringe effects in modulation masking.

Modulation detection thresholds (20 log ms) for a sinusoidally amplitude-modulated (SAM) noise were measured in the presence of a SAM noise masker with a modulation depth (mm) of 1.0 and a modulation frequency of 16 or 64 Hz. The signal and masker carriers were presented continuously, and the signal was modulated during one of the two 500-ms observation intervals. The masker was modulated during both observation intervals and, in some conditions, for a certain amount of time before and after signal modulation. The duration of this "fringe" ranged from 62.5 ms to continuous (masker modulated throughout the thresholds estimate). The first experiment showed that a 500-ms fringe could reduce masked thresholds by 4-6 dB, but only at low signal modulation frequencies (2-8 Hz). In the second and third experiments, it was found that the fringe had to have a duration of 500 ms and a depth of about 0.75 to be maximally effective. A final, supplementary experiment indicated that the fringe effect is not due solely to the fringe that occurs prior to the observation intervals. The results are discussed in terms of both peripheral and central auditory processing.     

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.     

Mechanisms of modulation gap detection

It has been postulated that the central auditory system contains an array of modulation filters, each responsive to a different range of modulation frequencies present at the outputs of the (peripheral) auditory filters. In the present experiments, we tested what we call the "dip hypothesis," that a gap in modulation is detected using the "dip" in the output of the modulation filter tuned to the modulator frequency. In experiment 1, the task was to detect a gap in the sinusoidal amplitude modulation imposed on a 4-kHz carrier. The modulator preceding the gap ended with a positive-going zero-crossing. There were three conditions, differing in the phase at which the modulator started at the end of the gap; zero-phase, at a positive-going zero-crossing; pi-phase, at a negative-going zero-crossing; and "preserved" phase, at the phase the modulator would have had if it had continued without interruption. Modulation frequencies were 5, 10, 20, and 40 Hz. Psychometric functions for detection of the gap were measured using a two-alternative forced-choice task. For the zero-phase and preserved-phase conditions, the detectability index, d', increased monotonically with increasing gap duration. For the pi-phase condition, performance was good (d' > 1) for small gap durations, and initially worsened with increasing gap duration, before improving again for longer gap durations. This is the pattern of results expected from the dip hypothesis, provided that the modulation filters have Q values of 2 or more. However, it is also possible that a rhythm cue was used to improve performance in the pi-phase condition for short gap durations; the introduction of the gap markedly disrupted the regular rhythm produced by the modulator peaks. In experiment 2, the rhythm cue was disrupted by varying the modulator period randomly around its nominal value, except for the modulator periods immediately before and after the gap. This markedly impaired performance, and resulted in psychometric functions that were very similar for the zero-phase and pi-phase conditions. This pattern of results is inconsistent with the dip hypothesis. For both experiments, modulation gap "thresholds" (d' approximately 1) were roughly constant when expressed as a proportion of the modulator period. Possible mechanisms of modulation gap detection are discussed and evaluated.     

Temporal resolution in frog auditory-nerve fibers

Sinusoidally amplitude-modulated (SAM) noise was monaurally presented to the neotropical frog, Eleutherodactylus coqui, while recording intracellularly from auditory-nerve fibers. Neuronal phase locking was measured to the SAM noise envelope in the form of a period histogram. The modulation depth was changed (in 10% steps) until the threshold modulation depth was determined. This was repeated for various modulation frequencies (20-1200 Hz) and different levels of SAM noise (34-64 dB/Hz). From these data, temporal modulation transfer functions (TMTFs) were produced and minimum integration time (MIT) for each auditory fiber was calculated. The median MIT was 0.42 ms (lower quartile 0.32, upper quartile 0.68 ms). A noise level-dependent effect was noted on the shape of the TMTF as well as the minimum integration time. The latter results may be explained as a loss in spectral resolution with increasing noise level, which is consistent with the correlation that was found between minimum integration time and bandwidth.     

Detection of partially filled gaps in noise and the temporal modulation transfer function

Results of experiments on the detection of silent intervals, or gaps, in broadband noise are reported for normal-hearing listeners. In some preliminary experiments, a gap threshold of about 2 ms was measured. This value was independent of the duration of the noise burst, variation of the noise level on each presentation, or the temporal position of the gap within the noise burst. In the main experiments, the thresholds for partial decrements in the noise waveform as well as brief increments were determined. As predicted by a model that assumes a single fixed peak-to-valley detection ratio, thresholds for increments are slightly higher than thresholds for decrements when the signal is measured as the change in rms noise level. A first-order model describes the temporal properties of the auditory system as a low-pass filter with a 7- to 8-ms time constant. Temporal modulation transfer functions were determined for the same subjects, and the estimated temporal parameters agreed well with those estimated from the gap detection data. More detailed modeling was carried out by simulating Viemeister's three-stage temporal model. Simulations, using an initial stage bandwidth of 4000 Hz and a 3-ms time constant for the low-pass filter, generate data that are very similar to those obtained from human subjects in both modulation and gap detection.     

Amplitude modulation thresholds in chinchillas with high-frequency hearing loss

Estimates of auditory temporal resolution were obtained from normal chinchillas using sinusoidally amplitude modulated noise. Afterwards, the animals were exposed to noise whose bandwidth was progressively increased toward the low frequencies in octave steps. The first exposure was to an octave band of noise centered at 8 kHz. Three additional octave bands of noise were subsequently added to the original exposure in order to progressively increase the extent of the high-frequency hearing loss. The first exposure produced a temporary hearing loss of 50 to 60 dB near 8 kHz and elevated the amplitude modulation thresholds primarily at intermediate (128 Hz) modulation frequencies. Successive noise exposures extended the temporary hearing loss toward lower frequencies, but there was little further deterioration in the amplitude modulation function until the last exposure when the hearing loss spread to 1 kHz. The degradation in the amplitude modulation function observed after the last exposure, however, was due to a reduction in the sensation level of the test signal rather than to a decrease in the hearing bandwidth. The results of this study suggest that the high-frequency regions of the cochlea may be important for temporal resolution.     

Modulation and gap detection for broadband and filtered noise signals

Modulation detection thresholds (as a function of sinusoidal amplitude modulation frequency) and temporal gap detection thresholds were measured for three low-pass-filtered noise signals (fc = 1000, 2000, and 4000 Hz), a high-pass-filtered noise signal (fc = 4000 Hz), and a broadband signal. The two latter noise signals were effectively low-pass filtered (fc = 6500 Hz) by the earphone. Each of the filtered signals was presented with a complementary filtered noise masker. Modulation and gap detection thresholds were lowest for the broadband and high-pass signals. Thresholds were significantly higher for the low-pass signals than for the broadband and high-pass signals. For these tasks and conditions, the high-frequency content of the noise signal was more important than was the signal bandwidth. Sensitivity (s) and time constant (tau) indices were derived from functions fitted to the modulation detection data. These indices were compared with gap detection thresholds for corresponding signals. The gap detection thresholds were correlated inversely (rho = -1.0, p less than 0.05) with s (i.e., smaller gap detection thresholds were correlated with greater sensitivity to modulation), but were not correlated significantly with tau, which was relatively invariant across signal conditions.     

Detection of combined frequency and amplitude modulation.

This article is concerned with the detection of mixed modulation (MM), i.e., simultaneously occurring amplitude modulation (AM) and frequency modulation (FM). In experiment 1, an adaptive two-alternative forced-choice task was used to determine thresholds for detecting AM alone. Then, thresholds for detecting FM were determined for stimuli which had a fixed amount of AM in the signal interval only. The amount of AM was always less than the threshold for detecting AM alone. The FM thresholds depended significantly on the magnitude of the coexisting AM. For low modulation rates (4, 16, and 64 Hz), the FM thresholds did not depend significantly on the relative phase of modulation for the FM and AM. For a high modulation rate (256 Hz) strong effects of modulator phase were observed. These phase effects are as predicted by the model proposed by Hartmann and Hnath [Acustica 50, 297-312 (1982)], which assumes that detection of modulation at modulation frequencies higher than the critical modulation frequency is based on detection of the lower sideband in the modulated signal's spectrum. In the second experiment, psychometric functions were measured for the detection of AM alone and FM alone, using modulation rates of 4 and 16 Hz. Results showed that, for each type of modulation, d' is approximately a linear function of the square of the modulation index. Application of this finding to the results of experiment 1 suggested that, at low modulation rates, FM and AM are not detected by completely independent mechanisms. In the third experiment, psychometric functions were again measured for the detection of AM alone and FM alone, using a 10-Hz modulation rate. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. Significant effects of relative modulator phase were found when detectability was relatively high. These effects were not correctly predicted by either a single-band excitation-pattern model or a multiple-band excitation-pattern model. However, the detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently.     

Noise improves modulation detection by cochlear implant listeners at moderate carrier levels

Envelope detection and processing are very important for cochlear implant (CI) listeners, who must rely on obtaining significant amounts of acoustic information from the time-varying envelopes of stimuli. In previous work, Chatterjee and Robert [JARO 2(2), 159-171 (2001)] reported on a stochastic-resonance-type effect in modulation detection by CI listeners: optimum levels of noise in the envelope enhanced modulation detection under certain conditions, particularly when the carrier level was low. The results of that study suggested that a low carrier level was sufficient to evoke the observed stochastic resonance effect, but did not clarify whether a low carrier level was necessary to evoke the effect. Modulation thresholds in CI listeners generally decrease with increasing carrier level. The experiments in this study were designed to investigate whether the observed noise-induced enhancement is related to the low carrier level per se, or to the poor modulation sensitivity that accompanies it. This was done by keeping the carrier amplitude fixed at a moderate level and increasing modulation frequency so that modulation sensitivity could be reduced without lowering carrier level. The results suggest that modulation sensitivity, not carrier level, is the primary factor determining the effect of the noise.     

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.     

Binaural processing of modulated interaural level differences

Two experiments are presented that measure the acuity of binaural processing of modulated interaural level differences (ILDs) using psychoacoustic methods. In both experiments, dynamic ILDs were created by imposing an interaurally antiphasic sinusoidal amplitude modulation (AM) signal on high-frequency carriers, which were presented over headphones. In the first experiment, the sensitivity to dynamic ILDs was measured as a function of the modulation frequency using puretone, and interaurally correlated and uncorrelated narrow-band noise carriers. The intrinsic interaural level fluctuations of the uncorrelated noise carriers raised the ILD modulation detection thresholds with respect to the pure-tone carriers. The diotic fluctuations of the correlated noise carriers also caused a small increase in the thresholds over the pure-tone carriers, particularly with low ILD modulation frequencies. The second experiment investigated the modulation frequency selectivity in dynamic ILD processing by imposing an interaurally uncorrelated bandpass noise AM masker in series with the interaurally antiphasic AM signal on a pure-tone carrier. By varying the masker center frequencies relative to the signal modulation frequency, broadly tuned, bandpass-shaped patterns were obtained. Simulations with an existing binaural model show that a low-pass filter to limit the binaural temporal resolution is not sufficient to predict the results of the experiments.