Masker Onset Research Articles

The Influence of the Masker on the Localization of the Moving Signal in the Horizontal Plane

The effect of the masker on the localization of the moving signal was investigated in the free field conditions. The experiments were carried out in an anechoic chamber. Sound signals were presented from loudspeakers located on a semicircular arc in the horizontal plane. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. The signal and the mask were uncorrelated stimuli and were created from two independent noise bursts. The stationary masker was always on the right at an angle of 15°. The moving signals traveled to or from the masker along two paths located at two places (–86° to –52° and –52° to –18°). The signal and the masker of 1-s duration each were presented either simultaneously or with a delay of the signal onset relative to the masker onset. The delay varied from 1 to 40 ms and 1200 ms. The subjects evaluated the start and end points of the trajectory of the moving sounds. Localization data for a moving signal under masking conditions were compared with spatial estimates of the same signal when presented in isolation (without a masker). Localization of the start and end points of the signal in masking condition was compared with localization of the moving source alone. Results showed that the masker affected the start and end points of the signal trajectory. The shift depended on the direction of movement. The starting points were always shifted in the direction of motion of the signal. The end points were shifted in the opposite direction.

The Influence of the Masker on the Localization of the Moving Signal in the Horizontal Plane

The effect of the masker on the localization of the moving signal was investigated in the free field conditions. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. The signal and the mask were uncorrelated stimuli and were created from two independent noise bursts. The stationary masker was always on the right at an angle of 15 degrees. The moving signals traveled to or from the masker along two paths located at two places (–86°…–52° and –52°…–18°). The signal and the masker of 1 s duration each were presented either simultaneously or with a delay of the signal onset relative to the masker onset. The delay varied from 1 to 40 ms and 1200 ms. The subjects localized the start and end points of the trajectory of the moving sounds. Localization of the start and end points of the signal in masking condition was compared with localization of the moving source alone. Results showed that the masker affected the start and end points of the signal trajectory. The shift depended on the direction of movement. The starting points were always shifted in the direction of motion of the signal. The end points were shifted in the opposite the direction.

Localization of Correlated and Uncorrelated Audio Signals in the Horizontal Plane under Masking Conditions

The effect of the masker on the localization of the signal was investigated in the free field conditions. Bandpass noise bursts (5–18 kHz) were used to create a signal and a masker. In the case of correlated stimuli, the same noise burst served both as a masker and a signal. In the case of uncorrelated stimuli, the signal and the masker were created from two different noise bursts. The masker was always on the right at an angle of 15 degrees. The signal was presented in one of three positions on the left: –18, –52, –86 degrees. The signal and the masker of 1 s duration each were presented either simultaneously or with a shift of the signal onset relative to the masker onset. The delay varied from 1 to 1200 ms. Perceived position of signals under masking conditions were compared with a single presentation of the signal. It is shown that under the masking conditions the perceived position of the signal shifted towards the masker, and the perceived position of the masker shifted towards the signal. The shift value decreased with increasing delay between the signal and the masker and with increasing angular distance between them. The mutual influence of the signal and the masker was more pronounced for correlated stimuli than for uncorrelated ones.

Effects of task and language nativeness on the Lombard effect and on its onset and offset timing.

This study focuses on the differences in speech sound pressure levels (here, called speech loudness) of Lombard speech (i.e., speech produced in the presence of an energetic masker) associated with different tasks and language nativeness. Vocalizations were produced by native speakers of Japanese with normal hearing and limited English proficiency while performing four tasks: dialog, a competitive game (both communicative), soliloquy, and text passage reading (noncommunicative). Relative to the native language (L1), larger loudness increments were observed in the game and text reading when performed in the second language (L2). Communicative tasks yielded louder vocalizations and larger increments of speech loudness than did noncommunicative tasks regardless of the spoken language. The period in which speakers increased their loudness after the onset of the masker was about fourfold longer than the time in which they decreased their loudness after the offset of the masker. Results suggest that when relying on acoustic signals, speakers use similar vocalization strategies in L1 and L2, and these depend on the complexity of the task, the need for accurate pronunciation, and the presence of a listener. Results also suggest that speakers use different strategies depending on the onset or offset of an energetic masker.

Masking of short tones in noise: Evidence for envelope-based, rather than energy-based detection.

The "temporal effect" in simultaneous masking may be characterized by better probe detection thresholds for a short, tonal probe presented at the temporal center of a masker compared to at the onset of a masker. Energy-based models of masking have been used to interpret the temporal effect as evidence that the gain of the auditory system decreases during acoustic stimulation. This study shows that masking from temporal-envelope fluctuations of a precursor or from a temporal gap between stimuli violates the assumptions of energy-based models and complicates the interpretation of temporal effects in terms of a reduction in gain. Detection thresholds were measured for a 6-ms, 4000-Hz probe preceded by a narrowband precursor and presented 2-, 197-, or 392-ms after the onset of a narrowband masker. The delay between the precursor offset and masker onset ranged from -2 to 250 ms. Probe thresholds were elevated in the presence of precursors with fluctuating compared to flattened temporal envelopes and when a temporal gap was inserted between the precursor and masker. The results suggest that the interpretation and design of temporal-effect studies should consider the masking effects of temporal-envelope fluctuations. These findings are consistent with speech-perception experiments that show masking from temporal-envelope fluctuations.

Partial loudness at masker onset indicates temporal effects at supra-threshold levels

This study examines temporal effects both at threshold and at supra-threshold levels. The level needed to detect a short-duration 4.0-kHz signal was measured for signals presented with different onset delays relative to a 300-ms broadband noise masker: 100 ms and 5 ms before the onset of the masker and 5 ms and 100 ms after the onset of the masker. Loudness matches between the signal in quiet and the signal at the same four onset delays were obtained for five presentation levels of the short-duration signal and for three masker levels. The temporal effect was defined as the level difference between the signals near masker onset and the signals well before or well after masker onset, needed to reach threshold and/or achieve equal loudness. Both at threshold and at supra-threshold levels temporal effects were observed consistent with a decrease in gain at the masker frequency during the course of the masker. The temporal effect was not restricted to simultaneous masking, but was also found for backward masking. In both cases the temporal effects were stronger at supra-threshold levels than at threshold. This may be caused by a transient effect at masker onset. The almost simultaneous onset of the signal and the masker makes it difficult for subjects to separate signal from the masker, especially when the signal level is close to masked threshold.

Effects of Masker Envelope Fluctuations on the Temporal Effect.

Under certain conditions, detection thresholds in simultaneous masking improve when the onset of a short sinusoidal probe is delayed from the onset of a long masker. This improvement, known as the temporal effect, is largest for broadband maskers and is smaller or absent for narrowband maskers centered on the probe frequency. This study tests the hypothesis that small or absent temporal effects for narrowband maskers are due to the inherent temporal envelope fluctuations of Gaussian noise. Temporal effects were measured for narrowband noise maskers with fluctuating ("fluctuating maskers") and flattened ("flattened maskers") temporal envelopes as a function of masker level (Exp. I) and in the presence of fluctuating and flattened precursors (Exp. II). The temporal effect was absent for fluctuating narrowband maskers and as large as ~ 7dB for flattened narrowband maskers. The AC-coupled power of the temporal envelopes of precursors and maskers accounted for 94% of the variance in probe detection thresholds measured with fluctuating and flattened precursors and maskers. These results suggest that masker temporal envelope fluctuations contribute to the temporal effect and should be considered in future modeling efforts.

Effects of age and hearing loss on overshoot.

The detection of a brief, sinusoidal probe in a long broadband, simultaneous masker improves as the probe is delayed from the masker's onset. This improvement ("overshoot") may be mediated by a reduction in cochlear amplifier gain over the timecourse of the masker via the medial olivocochlear (MOC) reflex. Overshoot was measured in younger adults with normal hearing and in older adults with normal and impaired hearing to test the hypothesis that aging and cochlear hearing loss result in abnormal overshoot, consistent with changes in certain structures along the MOC pathway. Overshoot decreased with increasing quiet probe thresholds and was only minimally influenced by increasing age. Marked individual differences in overshoot were observed due to differences in masking thresholds for probes presented near the masker's onset. Model simulations support the interpretation that reduced overshoot in hearing-impaired listeners is due to limited cochlear amplifier gain and therefore less gain to adjust over the timecourse of the masker. Similar overshoot among younger and older adults with normal hearing suggests that age-related changes to mechanisms underlying overshoot do not result in significant differences in overshoot among younger and older adults with normal hearing.

Does the degree of linguistic experience (native versus nonnative) modulate the degree to which listeners can benefit from a delay between the onset of the maskers and the onset of the target speech?

Background noise has a greater adverse effect on word recognition when people are listening in their second language (L2) as opposed to their first language (L1). The present study investigates the extent to which linguistic experience affects the ability of L2 listeners to benefit from a delay between the onset of a masker and the onset of a word. In a previous study (Ben-David, Tse & Schneider, 2012), word recognition thresholds for young L1s were found to improve with the increase in the delay between the onset of a masker (either a stationary noise or a babble of voices) and the onset of a word. The investigators interpreted this result as reflecting the ability of L1 listeners to rapidly segregate the target words from a masker. Given stream segregation depends, in part, on top-down knowledge-driven processes, we might expect stream segregation to be more “sluggish” for L2 listeners than for L1 listeners, especially when the masker consists of a babble of L2 voices. In the present study, we compared the ability of native English speakers to those who had either recent or long-term immersion in English as L2, to benefit from a delay between masker onset and word onset for English words. Results show that thresholds were higher for the two L2s groups than for the L1s. However, the rate at which word recognition improved with word-onset delay was unaffected by linguistic status, both when words were presented in noise, and in babble. Hence, for young listeners, stream segregation appears to be independent of linguistic status, suggesting that bottom-up sensory mechanisms play a large role in stream segregation in this paradigm. The implications of a failure of older L1 listeners (in Ben-David et al.) to benefit from a word-onset delay when the masker is a babble of voices are discussed.

Neural Mechanisms for Acoustic Signal Detection under Strong Masking in an Insect.

Communication is fundamental for our understanding of behavior. In the acoustic modality, natural scenes for communication in humans and animals are often very noisy, decreasing the chances for signal detection and discrimination. We investigated the mechanisms enabling selective hearing under natural noisy conditions for auditory receptors and interneurons of an insect. In the studied katydid Mecopoda elongata species-specific calling songs (chirps) are strongly masked by signals of another species, both communicating in sympatry. The spectral properties of the two signals are similar and differ only in a small frequency band at 2 kHz present in the chirping species. Receptors sharply tuned to 2 kHz are completely unaffected by the masking signal of the other species, whereas receptors tuned to higher audio and ultrasonic frequencies show complete masking. Intracellular recordings of identified interneurons revealed two mechanisms providing response selectivity to the chirp. (1) Response selectivity is when several identified interneurons exhibit remarkably selective responses to the chirps, even at signal-to-noise ratios of -21 dB, since they are sharply tuned to 2 kHz. Their dendritic arborizations indicate selective connectivity with low-frequency receptors tuned to 2 kHz. (2) Novelty detection is when a second group of interneurons is broadly tuned but, because of strong stimulus-specific adaptation to the masker spectrum and "novelty detection" to the 2 kHz band present only in the conspecific signal, these interneurons start to respond selectively to the chirp shortly after the onset of the continuous masker. Both mechanisms provide the sensory basis for hearing at unfavorable signal-to-noise ratios. Significance statement: Animal and human acoustic communication may suffer from the same "cocktail party problem," when communication happens in noisy social groups. We address solutions for this problem in a model system of two katydids, where one species produces an extremely noisy sound, yet the second species still detects its own song. Using intracellular recording techniques we identified two neural mechanisms underlying the surprising behavioral signal detection at the level of single identified interneurons. These neural mechanisms for signal detection are likely to be important for other sensory modalities as well, where noise in the communication channel creates similar problems. Also, they may be used for the development of algorithms for the filtering of specific signals in technical microphones or hearing aids.

Modeling individual differences in overshoot: Effects of age, hearing loss, and efferent feedback

The detection of a short sinusoidal probe in simultaneous masking improves as the probe's onset is delayed from the masker's onset. This “overshoot” may be mediated by the medial olivocochlear (MOC) reflex, whose pathway includes spiral ganglion neurons (SGNs), olivocochlear neurons and the outer hair cells (OHCs). Overshoot was measured in younger adults with normal hearing, older adults with normal hearing, and older adults with hearing loss to test the hypothesis that overshoot decreases as components in the MOC reflex pathway are compromised. Overshoot was significantly reduced in older adults, but only those with hearing loss, which is consistent with overshoot depending primarily on the status of the OHCs and only minimally influenced by age-related reductions in SGNs. Thresholds measured when the probe was near the masker’s onset showed large differences across listeners, resulting in appreciable individual differences in overshoot. Simulations were generated from a computational model of the auditory system to quantify the contributions of cochlear hearing loss, MOC reflex strength, and detection efficiency to individual differences in overshoot. Preliminary results suggest that cochlear hearing loss and detection efficiency explain the largest portion of the variance in overshoot among adults with normal and impaired hearing. [Work supported by NIH/NIDCD.]

Strategies adopted by talkers faced with fluctuating and competing-speech maskers

Studying how interlocutors exchange information efficiently during conversations in less-than-ideal acoustic conditions promises to both further the understanding of links between perception and production and inform the design of human-computer dialogue systems. The current study explored how interlocutors' speech changes in the presence of fluctuating noise. Pairs of talkers were recorded while solving puzzles cooperatively in quiet and with modulated-noise or competing speech maskers whose silent intervals were manipulated to produce either temporally sparse or dense maskers. Talkers responded to masked conditions by both increasing the amount of speech produced and locally changing their speech activity patterns, resulting in a net reduction in the proportion of speech in temporal overlap with the maskers, with larger relative reductions for sparse maskers. An analysis of talker activity in the vicinity of masker onset and offset events showed a significant reduction in onsets following masker onsets, and a similar increase in onsets following masker offsets. These findings demonstrate that talkers are sensitive to masking noise and respond to its fluctuations by adopting a "wait-and-talk" strategy.

Central masking with bilateral cochlear implants

Across bilateral cochlear implants, contralateral threshold shift has been investigated as a function of electrode difference between the masking and probe electrodes. For contralateral electric masking, maximum threshold elevations occurred when the position of the masker and probe electrode was approximately place-matched across ears. The amount of masking diminished with increasing masker-probe electrode separation. Place-dependent masking occurred in both sequentially implanted ears, and was not affected by the masker intensity or the time delay from the masker onset. When compared to previous contralateral masking results in normal hearing, the similarities between place-dependent central masking patterns suggest comparable mechanisms of overlapping excitation in the central auditory nervous system.

Does it take older adults longer than younger adults to perceptually segregate a speech target from a background masker?

Older adults often find it more difficult than younger adults to attend to a target talker when there are other people talking. One possible reason for this difficulty is that it may take them longer to perceptually segregate the target speech from competing speech. This study investigated age-related differences in the time it takes to segregate target speech from either a speech spectrum noise masker or a babble masker (many people talking simultaneously). Specifically, we employed five different delays (0.1 s–1.1 s) between masker onset and target speech onset. Four signal-to-masker ratios were employed at each delay to determine the 50% thresholds for word recognition accuracy when target words were masked by either speech spectrum noise or multi-talker babble. Thresholds for word recognition decreased exponentially as a function of the masker-word-onset delay, at the same rate for younger and older adults, when the masker was speech spectrum noise. When the masker was babble, thresholds for younger adults decreased exponentially with delay at the same rate as they did when the masker was speech spectrum noise. The word recognition thresholds for older adults, however, did not appear to change over the range of delays explored in this study. In addition, the average difference between word recognition thresholds for younger and older adults (younger adult thresholds < older adult thresholds) was significantly larger when the masker was babble than when it was noise. These results indicate that older adults are as fast as younger adults at separating speech from a steady-state noise masker, but are not as capable as younger adults of taking advantage of the delayed onset of the speech target when the masker is babble. The potential contributions of age-related sensory and cognitive declines to these stream segregation effects are discussed. Finally, we conclude that age-related differences in the timeline for stream segregation contribute to the difficulties older adults experience in listening to speech in a background of babble.

Evaluating Adaptation and Olivocochlear Efferent Feedback as Potential Explanations of Psychophysical Overshoot

Masked detection threshold for a short tone in noise improves as the tone's onset is delayed from the masker's onset. This improvement, known as "overshoot," is maximal at mid-masker levels and is reduced by temporary and permanent cochlear hearing loss. Computational modeling was used in the present study to evaluate proposed physiological mechanisms of overshoot, including classic firing rate adaptation and medial olivocochlear (MOC) feedback, for both normal hearing and cochlear hearing loss conditions. These theories were tested using an established model of the auditory periphery and signal detection theory techniques. The influence of several analysis variables on predicted tone-pip detection in broadband noise was evaluated, including: auditory nerve fiber spontaneous-rate (SR) pooling, range of characteristic frequencies, number of synapses per characteristic frequency, analysis window duration, and detection rule. The results revealed that overshoot similar to perceptual data in terms of both magnitude and level dependence could be predicted when the effects of MOC efferent feedback were included in the auditory nerve model. Conversely, simulations without MOC feedback effects never produced overshoot despite the model's ability to account for classic firing rate adaptation and dynamic range adaptation in auditory nerve responses. Cochlear hearing loss was predicted to reduce the size of overshoot only for model versions that included the effects of MOC efferent feedback. These findings suggest that overshoot in normal and hearing-impaired listeners is mediated by some form of dynamic range adaptation other than what is observed in the auditory nerve of anesthetized animals. Mechanisms for this adaptation may occur at several levels along the auditory pathway. Among these mechanisms, the MOC reflex may play a leading role.

The relationship between quiet threshold and the forward-masking temporal effect.

Research in our laboratory has shown that the temporal effect (TE) in simultaneous masking is consistent with a decrease in gain, possibly mediated by the medial olivocochlear reflex (MOCR). The TE in simultaneous masking is a decrease in threshold signal-to-masker ratio for a signal at masker onset when a precursor is added. This work has been extended to a forward-masking paradigm. A growth of masking (GOM) function is measured with a short-duration off-frequency masker (which should not activate the MOCR) and a 4-kHz signal. Then the masker level is fixed at a point on the lower leg of the GOM function, and threshold is measured with a long-duration precursor which is intended to activate the MOCR. The estimated input-output function is compared for a precursor at and well below the signal frequency. The difference in thresholds is the TE, which is also a measure of the change in gain. In simultaneous masking, the size of the TE decreases with increasing quiet threshold. In the present study, this relationship was examined for the forward-masking TE. The TE was measured as a function of precursor level for listeners who had a range of quiet thresholds, including listeners with mild cochlear hearing impairment. [Research supported by a grant from NIH(NIDCD) R01 DC008327.]

The influence of masker fringe on sound localization in noise.

Several studies have shown that detection of a dichotic target signal in a concurrent diotic noise masker improves when the masker onset occurs prior to that of the target and/or the masker duration extends beyond the target offset (so‐called masker fringe; McFadden, 1966; Trahiotis et al. 1972). It has been argued that this improved performance results from the fact that the masker fringe provides a baseline set of interaural parameters against which a change in parameters, associated with the signal onset or offset, can be detected. As the duration of the fringe increases, more time is available to estimate these baseline parameters, thus improving signal detectability. However, little is known about the relation between masker fringe and spatial hearing. In this study, the localization of a 250‐ms click train signal presented in a broadband directional noise masker was measured as a function of the duration of the masker fringe and the signal‐to‐noise ratio. The results indicate that the presence of masker fringe improves localization performance relative to the case in which the masker and signal are pulsed on and off simultaneously. Moreover, the benefit of masker fringe increases with decreasing signal‐to‐noise ratio. [Work supported by a grant from AFOSR.]

Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoot.

Behavioral threshold for a tone burst presented in a long-duration noise masker decreases as the onset of the tone burst is delayed relative to masker onset. The threshold difference between detection of early- and late-onset tone bursts is called overshoot. Although the underlying mechanisms are unclear, one hypothesis is that overshoot occurs due to efferent suppression of cochlear nonlinearity [von Klitzing, R., and Kohlrausch, A. (1994). J. Acoust. Soc. Am. 95, 2192-2201]. This hypothesis was tested by using overshoot conditions to elicit stimulus-frequency otoacoustic emissions (SFOAEs), which provide a physiological measure of cochlear nonlinearity. SFOAE and behavioral thresholds were estimated using a modified maximum-likelihood yes-no procedure. The masker was a 400-ms "frozen" notched noise. The signal was a 20-ms, 4-kHz tone burst presented at 1 or 200 ms after the noise onset. Behavioral overshoot results replicated previous studies, but no overshoot was observed in SFOAE thresholds. This suggests that either efferent suppression of cochlear nonlinearity is not involved in overshoot, or a SFOAE threshold estimation procedure based on stimuli similar to those used to study behavioral overshoot is not sensitive enough to measure the effect.

Role of attention in overshoot: Frequency certainty versus uncertainty

Overshoot, the elevation in the threshold for a brief signal that comes on close to masker onset, was measured with signal frequency certain (same frequency on every trial) or uncertain (randomized over trials). In broadband noise, thresholds were higher 2 ms after masker onset than 200 ms later, by 9 dB with frequency certainty, by 6-7 dB with uncertainty. In narrowband noise centered on the signal frequency, thresholds at 2 ms were not elevated with certainty, but were elevated 4-5 dB with uncertainty. Thus, frequency uncertainty leads to less overshoot in broadband noise, to more overshoot in narrowband noise. Reduced overshoot in broadband noise may come about because the masker, given its many frequencies, disrupts focusing at onset as much under certainty as uncertainty. Once the initial disruption dissipates, threshold is lower with certainty so overshoot is greater. In contrast, a narrowband noise with frequencies only near the signal does not disrupt focusing when the signal frequency is known beforehand, so overshoot is absent. When frequency is uncertain, the narrowband noise serves to focus attention on the signal frequency; as this requires time, detection near noise onset is poorer than later on, so overshoot is present.

The relationship between precursor level and the temporal effect.

Previous studies have suggested that temporal effects in masking may be consistent with a decrease in cochlear gain. One paradigm used to show this is to measure the level of a long-duration masker required to just mask a short-duration tone that occurs near masker onset. The temporal effect is revealed when the signal is detected at a lower signal-to-noise ratio following preceding stimulation (either an extension of the masker or a separate precursor). The present study examined whether this effect depends on precursor level. The signal was a 10-ms, 4-kHz tone. The masker was 200 ms. A fixed-level precursor had the same frequency characteristics as the masker, and was 205 ms. The masker and precursor had either no notch or a wide notch about the signal frequency. For a given precursor level, the growth of masker level with signal level was determined. These data were used to estimate input-output functions. The results are consistent with a graded decrease in gain at the signal frequency when there is no notch in the masker and precursor, and a graded decrease in suppression when there is a large notch. These results could be consistent with the action of the medial olivocochlear reflex.