Binaural comodulation masking release: effects of masker interaural correlation

Supra-threshold perception and neural representation of tones presented in noise in conditions of masking release

The neural representation and perceptual salience of tonal signals presented in different noise maskers were investigated. The properties of the maskers and signals were varied such that they produced different amounts of either monaural masking release, binaural masking release, or a combination of both. The signals were then presented at different levels above their corresponding masked thresholds and auditory evoked potentials (AEPs) were measured. It was found that, independent of the masking condition, the amplitude of the P2 component of the AEP was similar for the same stimulus levels above masked threshold, suggesting that both monaural and binaural effects of masking release were represented at the level of the auditory pathway where P2 is generated. The perceptual salience of the signal was evaluated at equal levels above masked threshold using a rating task. In contrast to the electrophysiological findings, the subjective ratings of the perceptual signal salience were less consistent with the signal level above masked threshold and varied strongly across listeners and masking conditions. Overall, the results from the present study suggest that the P2 amplitude of the AEP represents an objective indicator of the audibility of a target signal in the presence of complex acoustic maskers.

Effects of forward masking on sound localization in cats: basic findings with broadband maskers

Forward masking is traditionally measured with a detection task in which the addition of a preceding masking sound results in an increased signal-detection threshold. Little is known about the influence of forward masking on localization of free-field sound for human or animal subjects. Here we recorded gaze shifts of two head-unrestrained cats during localization using a search-coil technique. A broadband (BB) noise masker was presented straight ahead. A brief signal could come from 1 of the 17 speaker locations in the frontal hemifield. The signal was either a BB or a band-limited (BL) noise. For BB targets, the presence of the forward masker reduced localization accuracy at almost all target levels (20 to 80 dB SPL) along both horizontal and vertical dimensions. Temporal decay of masking was observed when a 15-ms interstimulus gap was added between the end of the masker and the beginning of the target. A large effect of forward masking was also observed for BL targets with low (0.2-2 kHz) and mid (2-7 kHz) frequencies, indicating that the interaural timing cue is susceptible to forward masking. Except at low sound levels, a small or little effect was observed for high-frequency (7-15 kHz) targets, indicating that the interaural level and the spectral cues in that frequency range remained relatively robust. Our findings suggest that different localization mechanisms can operate independently in a complex listening environment.

Generating partially correlated noise--a comparison of methods

There are three standard methods for generating two channels of partially correlated noise: the two-generator method, the three-generator method, and the symmetric-generator method. These methods allow an experimenter to specify a target cross correlation between the two channels, but actual generated noises show statistical variability around the target value. Numerical experiments were done to compare the variability for those methods as a function of the number of degrees of freedom. The results of the experiments quantify the stimulus uncertainty in diverse binaural psychoacoustical experiments: incoherence detection, perceived auditory source width, envelopment, noise localization/lateralization, and the masking level difference. The numerical experiments found that when the elemental generators have unequal powers, the different methods all have similar variability. When the powers are constrained to be equal, the symmetric-generator method has much smaller variability than the other two.

Exploring the additivity of binaural and monaural masking release

Experiment 1 examined comodulation masking release (CMR) for a 700-Hz tonal signal under conditions of N(o)S(o) (noise and signal interaurally in phase) and N(o)S(π) (noise in phase, signal out of phase) stimulation. The baseline stimulus for CMR was either a single 24-Hz wide narrowband noise centered on the signal frequency [on-signal band (OSB)] or the OSB plus, a set of flanking noise bands having random envelopes. Masking noise was either gated or continuous. The CMR, defined with respect to either the OSB or the random noise baseline, was smaller for N(o)S(π) than N(o)S(o) stimulation, particularly when the masker was continuous. Experiment 2 examined whether the same pattern of results would be obtained for a 2000-Hz signal frequency; the number of flanking bands was also manipulated (two versus eight). Results again showed smaller CMR for N(o)S(π) than N(o)S(o) stimulation for both continuous and gated masking noise. The CMR was larger with eight than with two flanking bands, and this difference was greater for N(o)S(o) than N(o)S(π). The results of this study are compatible with serial mechanisms of binaural and monaural masking release, but they indicate that the combined masking release (binaural masking-level difference and CMR) falls short of being additive.

The role of off-frequency masking in binaural hearing

The present studies examined the binaural masking level difference (MLD) for off-frequency masking. It has been shown previously that the MLD decreases steeply with increasing spectral separation between a pure tone signal and a 10-Hz wide band of masking noise. Data collected here show that this reduction in the off-frequency MLD as a function of signal/masker separation is comparable at 250 and 2500 Hz, indicating that neither interaural phase cues nor frequency resolution are critical to this finding. The MLD decreases more gradually with spectral separation when the masker is a 250-Hz-wide band of noise, a result that implicates the rate of inherent amplitude modulation of the masker. Thresholds were also measured for a brief signal presented coincident with a local masker modulation minimum or maximum. Sensitivity was better in the minima for all NoSpi and off-frequency NoSo conditions, with little or no effect of signal position for on-frequency NoSo conditions. Taken together, the present results indicate that the steep reduction in the off-frequency MLD for a narrowband noise masker is due at least in part to envelope cues in the NoSo conditions. There was no evidence of a reduction in binaural cue quality for off-frequency masking.

Combination of masking releases for different center frequencies and masker amplitude statistics

Several masking experiments have shown that the auditory system is able to use coherent envelope fluctuations of the masker across frequency within one ear as well as differences in interaural disparity between signal and masker to enhance signal detection. The two effects associated with these abilities are comodulation masking release (CMR) and binaural masking level difference (BMLD). The aim of the present study was to investigate the combination of CMR and BMLD. Thresholds for detecting a sinusoidal signal were measured in a flanking-band paradigm at three different signal frequencies. The masker was presented diotically, and various interaural phase differences (IPDs) of the signal were used. The masker components were either multiplied or Gaussian narrowband noises. In addition, a transposed stimulus was used to increase the BMLD at a high signal frequency. For all frequencies and masker conditions, thresholds decreased as the signal IPD increased and were lower when the masker components were comodulated. The data show an addition of the monaural and binaural masking releases in decibels when masker conditions with and without comodulation and the same spectrum were compared.

Superposition of masking releases

We are constantly exposed to a mixture of sounds of which only few are important to consider. In order to improve detectability and to segregate important sounds from less important sounds, the auditory system uses different aspects of natural sound sources. Among these are (a) its specific location and (b) synchronous envelope fluctuations in different frequency regions. Such a comodulation of different frequency bands facilitates the detection of tones in noise, a phenomenon known as comodulation masking release (CMR). Physiological as well as psychoacoustical studies usually investigate only one of these strategies to segregate sounds. Here we present psychoacoustical data on CMR for various virtual locations of the signal by varying its interaural phase difference (IPD). The results indicate that the masking release in conditions with binaural (interaural phase differences) and across-frequency (synchronous envelope fluctuations, i.e. comodulation) cues present is equal to the sum of the masking releases for each of the cues separately. Data and model predictions with a simplified model of the auditory system indicate an independent and serial processing of binaural cues and monaural across-frequency cues, maximizing the benefits from the envelope comparison across frequency and the comparison of fine structure across ears.

Influences of modulation and spatial separation on detection of a masked broadband target

Experiments explored the influence of amplitude modulation and spatial separation on detectability of a broadband noise target masked by an independent broadband noise. Thresholds were measured for all combinations of six spatial configurations of target and masker and five modulation conditions. Masker level was either fixed (Experiment 1) or roved between intervals within a trial to reduce the utility of overall intensity as a cue (Experiment 2). After accounting for acoustic changes, thresholds depended on whether a target and a masker were colocated or spatially separated, but not on the exact spatial configuration. Moreover, spatial unmasking exceeded that predicted by better-ear acoustics only when modulation cues for detection were weak. Roving increased the colocated but not the spatially separated thresholds, resulting in an increase in spatial release from masking. Differences in both how performance changed over time and the influence of spatial separation support the idea that the cues underlying performance depend on the modulation characteristics of the target and masker. Analysis suggests that detection is based on overall intensity when target and masker modulation and spatial cues are the same, on spatial attributes when sources are separated and modulation provides no target glimpses, and on modulation discrimination in the remaining conditions.