Masking In Normal-hearing Listeners Research Articles

Evaluation of spatial and spectro-temporal cues in release from masking in normal hearing listeners

ObjectiveNormal-hearing listeners use spatial cues to segregate target sounds from concurrent multiple sources. This phenomenon is known as the auditory spatial release of masking (SRM). Auditory spatial cues involve interaural time difference (ITD) and interaural level difference (ILD) affected by spectral and temporal features. Thus, to reveal factors influencing the SRM, the present study was conducted to investigate the effects of the frequency and ongoing duration on the lateralization (ITD and ILD). Also, this research examined the mutual effects of spatial separation, duration, and frequency on the SRM in spatial locations in virtually azimuth 0, ±45 and ±90 degrees. Methods: Twenty normal-hearing listeners were tested for ITD and ILD thresholds and auditory release of masking in spatial locations in azimuth 0, ±45 and ±90 degrees. Stochastic figure-ground (SFG) stimuli were utilized with low- and high-frequency figures and different durations. Results: ITD threshold was not affected by ongoing duration and frequency but the ILD threshold decreased with increasing the duration. A duration of 200 ms had the least SRM and further duration had roughly a flat trend. SRM was prominent in short and long durations between the spatial location in azimuth 0 and ±45 degrees but not in median durations in low-frequency figures (p < 0.05). Conclusion: it can be concluded the ILD threshold (not ITD threshold) is dependent on ongoing duration. In every spatial location in azimuth, a duration of 200 ms had the least SRM and SRM observed with variation of the location in azimuth 0 to ±45 degrees in short and long durations that is prominent for low-frequency figures.

Voice gender release from masking in cochlear implant users is correlated with binaural pitch fusion

Spatial and voice gender separation of target from masking speech leads to substantial release from masking in normal-hearing listeners. However, binaural pitch fusion is often broad in cochlear implant (CI) listeners, such that dichotic stimuli with pitches differing by up to 3-4 octaves are fused (Reiss et al., 2014). We hypothesized that broad binaural fusion could reduce a listener’s ability to separate competing speech streams with different voice pitches, and thus reduce the voice gender as well as spatial benefit for speech perception in noise. Speech reception thresholds were measured in both bilateral and bimodal CI users, using male and female target talkers at two spatial configurations (co-location and 60-degrees of target-masker separation). Binaural pitch fusion was also measured. Different-gender maskers improved target detection performance in bimodal CI users, and performance was better with female than male targets in bilateral CI users. No spatial benefit was seen in either CI group. As hypothesized, voice gender masking release was strongly correlated with binaural fusion range in bimodal CI users. These results suggest that sharp binaural fusion is necessary for maximal speech perception in noise in bimodal CI users, but does not benefit bilateral CI users. [Work supported by NIH-NIDCD grant R01 DC01337.]Spatial and voice gender separation of target from masking speech leads to substantial release from masking in normal-hearing listeners. However, binaural pitch fusion is often broad in cochlear implant (CI) listeners, such that dichotic stimuli with pitches differing by up to 3-4 octaves are fused (Reiss et al., 2014). We hypothesized that broad binaural fusion could reduce a listener’s ability to separate competing speech streams with different voice pitches, and thus reduce the voice gender as well as spatial benefit for speech perception in noise. Speech reception thresholds were measured in both bilateral and bimodal CI users, using male and female target talkers at two spatial configurations (co-location and 60-degrees of target-masker separation). Binaural pitch fusion was also measured. Different-gender maskers improved target detection performance in bimodal CI users, and performance was better with female than male targets in bilateral CI users. No spatial benefit was seen in either CI group. As...

Auditory Enhancement in Cochlear-Implant Users Under Simultaneous and Forward Masking.

Auditory enhancement is the phenomenon whereby the salience or detectability of a target sound within a masker is enhanced by the prior presentation of the masker alone. Enhancement has been demonstrated using both simultaneous and forward masking in normal-hearing listeners and may play an important role in auditory and speech perception within complex and time-varying acoustic environments. The few studies of enhancement in hearing-impaired listeners have reported reduced or absent enhancement effects under forward masking, suggesting a potentially peripheral locus of the effect. Here, auditory enhancement was measured in eight cochlear-implant (CI) users with direct stimulation. Masked thresholds were measured under simultaneous and forward masking as a function of the number of masking electrodes, and the electrode spacing between the maskers and the target. Evidence for auditory enhancement was obtained under simultaneous masking, qualitatively consistent with results from normal-hearing listeners. However, no significant enhancement was observed under forward masking, in contrast to earlier results with normal-hearing listeners. The results suggest that the normal effects of auditory enhancement are partially but not fully experienced by CI users. To the extent that the CI users' results differ from normal, it may be possible to apply signal processing to restore the missing aspects of enhancement.

Modulation masking in normal-hearing listeners and in auditory brainstem implant patients.

Recent experiments have suggested that a modulated masker can interfere with detection of a signal of similar modulation rate, in a manner analogous to the familiar critical-band concept in the frequency domain [Houtgast, J. Acoust. Soc. Am. 85, 1676–1680 (1989)]. An experiment with sinusoidally modulated maskers did not show such a critical-band effect in either normal-hearing listeners or in patients electrically stimulated on the cochlear nucleus. Maskers consisted of 500- or 1000-Hz sinusoidal carriers sinusoidally modulated at 4, 10, or 16 Hz with a modulation index of m=0.5. Thresholds for modulation depth were obtained for the same carrier modulated at signal frequencies of 1 to 32 Hz (in 1-Hz steps). Modulation masking was observed only when the masker and signal modulation frequencies were equal. The magnitude and form of the masking patterns depended strongly on the relative phase of the two modulators. Prior results might have shown critical-band type interference patterns because of the stochastic nature of the modulated maskers used. Previous and present results can be explained adequately by a short-term envelope correlation, without need to postulate a central mechanism for grouping temporal modulation patterns in the manner of a filter. [Work supported by NIH.]

Two-tone masking in normal hearing listeners.

Psychophysical measurements of two-tone masking [E. Zwicker, Acustica 4, 415-420 (1954)] were made at 0.25, 0.5, 1, 2, and 4 kHz utilizing a masker level of 62 dB SPL/tone. Fifty-eight "untrained" subjects were tested using a single run of a 4IFC adaptive procedure for each condition. Individual data were highly variable. Average data were systematic; they were analyzed using a two-line-regression procedure and the obtained critical-bandwidth estimates approximated normative values. Analysis of the literature revealed that a substantial increase of estimated critical bandwidth versus masker level occurs in two-tone masking. A portion of this increase appears artificial and stems from the relative effectiveness of the higher frequency masker tone at high masker levels. An alternative masker-frequency spacing is suggested to reduce level effects. Implications for an underlying critical-band mechanism are discussed.

Two-tone masking in normal-hearing listeners

Psychophysical measurement of two-tone masking [E. Zwicker, Acustica 4, 415-420 (1954)] were obtained at signal frequencies of 0.25, 0.5, 1, 2, and 4 kHz with seven masker separations tested at each signal frequency while utilizing a masker level of 62 dB SPL/ tone. Fifty-eight subjects were tested using a single run of a 4I-4AFC adaptive procedure for each condition. Although there were large individual differences, the average data were quite systematic. An objective technique (based on fitting two line segments to the data) was used to estimate critical bandwidths and values similar to those reported by others were obtained. These data and those of others will be discussed in terms of the effects of signal type (narrow-band noise versus tone) and masker level. A substantial increase in critical-bandwidth estimates is observed at high masker levels and may be accounted for by (a) the masking patterns produced by the individual tones of the two-tone complex, and (b) the removal of cues produced by aural nonlinearities. [Work supported by NIH.]