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PubMed1989

Acoustic-phonetic characteristics of speech produced in noise and while wearing an oxygen mask.

The present study investigated changes in the prosodic and acoustic-phonetic features of isolated words by four male talkers speaking in quite and in pink noise at a level of 95 dB SPL. Speech samples were collected both with and without an oxygen mask. Changes in duration, fundamental frequency, total energy, and formant center frequency were analyzed. In addition to the expected changes of increased pitch and amplitude associated with speaking in noise without an oxygen mask, significant effects were found (particularly in the formant center frequencies) as a result of using the oxygen mask. When the oxygen mask was employed, no further significant changes were caused by adding noise to the speaking situation.

Humans↗
PubMed1989

Comodulation masking release for various monaural and binaural combinations of the signal, on-frequency, and flanking bands.

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band. This effect is called comodulation masking release (CMR). These experiments examine two questions. (1) How does the CMR vary with the number and ear of presentation of the flanking band(s)? (2) Is it possible to obtain a CMR when a binaural masking level difference (BMLD) is already present, and vice versa? Thresholds were measured for a 400-ms signal in a continuous 25-Hz-wide noise centered at signal frequencies (fs) of 250, 1000, and 4000 Hz. This masker was presented either alone or with one or more continuous flanking bands whose envelopes were either correlated or uncorrelated with that of the on-frequency band; their frequencies ranged from 0.5fs to 1.5fs. CMRs were measured for six conditions in which the signal, the on-frequency band, and the flanking band(s) were presented in various monaural and binaural combinations. When a single flanking band was used, the CMR was typically around 2-3 dB. The CMR increased to 5-6 dB if an additional flanking band was added. The effect of the additional band was similar whether it was in the same ear as the original band or in the opposite ear. At the lowest signal frequency, a large CMR was observed in addition to a BMLD and vice versa. At the highest signal frequency, the extra release from masking was small. The results are interpreted in terms of the cues producing the CMR and the BMLD.

Acoustic Stimulation↗
PubMed1989

Forward-masking properties of multicomponent signals in normal and hearing-impaired subjects.

The forward-masking properties of inharmonic complex stimuli were measured both for normal and hearing-impaired subjects. The signal threshold for a 1000-Hz pure-tone probe was obtained for six different maskers, which varied in the number of pure-tone components. The masking stimuli consisted of 1, 3, 5, 7, 9, or 11 components, logarithmically spaced in frequency surrounding the signal and presented at a fixed level of 80 dB SPL per component. In most normal-hearing subjects, the threshold for the probe decreased as the number of masking components was increased, demonstrating that stimuli with more components tended to be less effective maskers. Results from hearing-impaired subjects showed no decrease in threshold with increasing number of masking components. Instead, the thresholds increased as more components were added to the first masker. These results appear to be consistent with suppression effects within the multicomponent maskers for the normal subjects and a lack of suppression effects for the hearing-impaired subjects. The results from the normal-hearing subjects are also consistent with "across-channel" cuing.

Adult↗
PubMed1990

Binaural forward and backward masking: evidence for sluggishness in binaural detection.

The threshold of a short interaurally phase-inverted probe tone (20 ms, 500 Hz, S pi) was obtained in the presence of a 750-ms noise masker that was switched after 375 ms from interaurally phase-inverted (N pi) to interaurally in-phase (No). As the delay between probe-tone offset and noise phase transition is increased, the threshold decays from the N pi S pi threshold (masking level difference = 0 dB) to the No S pi threshold (masking level difference = 15 dB). The decay in this "binaural" situation is substantially slower than in a comparable "monaural" situation, where the interaural phase of the masker is held constant (N pi), but the level of the masker is reduced by 15 dB. The prolonged decay provides evidence for additional binaural sluggishness associated with "binaural forward masking." In a second experiment, "binaural backward masking" is studied by time reversing the maskers described above. Again, the situation where the phase is switched from No to N pi exhibits a slower transition than the situation with constant interaural phase (N pi) and a 15-dB increase in the level of the masker. The data for the binaural situations are compatible with the results of a related experiment, previously reported by Grantham and Wightman [J. Acoust. Soc. Am. 65, 1509-1517 (1979)] and are well fit by a model that incorporates a double-sided exponential temporal integration window.

Adult↗
PubMed1991

The effect of masker spectral asymmetry on overshoot in simultaneous masking.

"Overshoot" is a simultaneous masking phenomenon: Thresholds for short high-frequency tone bursts presented shortly after the onset of a broadband masker are raised compared to thresholds in the presence of a continuous masker. Overshoot for 2-ms bursts of a 5000-Hz test tone is described for four subjects as a function of the spectral composition and level of the masker. First, it was verified that overshoot is largely independent of masker duration. Second, overshoot was determined for a variety of 10-ms masker bursts composed of differently filtered uniform masking noise with an overall level of 60 dB SPL: unfiltered, high-pass (cutoff at 3700 Hz), low-pass (cutoff at 5700 Hz), and third-octave-band-(centered at 5000 Hz) filtered uniform masking noises presented separately or combined with different bandpass maskers (5700-16000 Hz, 5700-9500 Hz, 8400-16000 Hz) were used. Third, masked thresholds were measured for maskers composed of an upper or lower octave band adjacent to the third-octave-band masker as a function of the level of the octave band. All maskers containing components above the critical band of the test tone led to overshoot; no additional overshoot was produced by masker components below it. Typical values of overshoot were on the order of 12 dB. Overshoot saturated when masker levels were above 60 dB SPL for the upper octave-band masker. The standard neurophysiological explanation of overshoot accounts only partially for these data. Details that must be accommodated by any full explanation of overshoot are discussed.

Acoustics↗
PubMed1992

Intensity discrimination in forward masking.

A nonmonotonic intensity discrimination function was recently reported in which a midlevel hump occurred for 25-ms sinusoidal standards ranging from 20 to 100 dB SPL and presented 100 ms after an intense narrow-band noise forward masker [F.-G. Zeng et al., Hear. Res. 55, 223-230 (1991)]. This paper provides additional data on how the midlevel hump is affected by three factors of forward masking: signal delay, masker level, and frequency. Specifically, just-noticeable differences (jnd's) in intensity were obtained at signal delays of 50, 200, and 400 ms. Results show that at the midlevels the forward-masked intensity jnd's did not recover to their unmasked values, even at the 400-ms signal delay. The longer the signal delay, the smaller this midlevel hump. This slow recovery of the midlevel jnd's is consistent with the finding that low-spontaneous rate (SR) neurons have a slow recovery from forward masking [E. M. Relkin and J. R. Doucet, Hear. Res. 55, 215-222 (1991)]. The large midlevel effect decreased sharply as masker level was reduced from 90 to 60 dB SPL, and disappeared for masker levels less than 40 dB SPL. A frequency selectivity effect for the large midlevel jnd effect was also observed, as maskers with frequency components 2 to 3 oct away from the signal frequency did not affect the jnd's. Overall, the present data are consistent with the hypothesis of Zeng et al. (1991) that low-SR neurons are involved in the midlevel hump of intensity discrimination in forward masking.

Adult↗
PubMed1992

Intensity discrimination under backward masking.

The Weber fraction was measured for a 25-ms sinusoidal pedestal presented 100 ms before, or 100 ms after, an intense narrow-band noise. Consistent with the finding of Zeng et al. [Hear. Res. 55, 223-230 (1991)], the forward masker caused an elevation in the Weber fraction at medium pedestal levels. Surprisingly, however, a much larger midlevel elevation was observed in the backward masking conditions; in some cases, the Weber fraction was increased by over 20 dB by the backward masker. In both masking conditions, presenting a notched noise simultaneously with the pedestal reduced the magnitude of the midlevel elevation. These results indicate that it is possible to produce large masking effects on intensity discrimination in conditions where there is no possibility of the masker affecting the representation of the pedestal at the level of the auditory nerve. This suggests that there may be "central" processes underlying the original finding of Zeng et al. Despite the similarities in the results, however, it is not certain that the elevations seen in the forward and backward masking conditions were caused by the same mechanisms.

Acoustic Stimulation↗
PubMed1994

The effect of nonsimultaneous on-frequency and off-frequency cues on the detection of a tonal signal masked by narrow-band noise.

Listeners' detection thresholds were measured for a 125-ms, 1-kHz tonal signal masked by a similarly gated 50-Hz-wide band of noise. A two-interval, adaptive, forced-choice procedure either with or without temporally surrounding cuing intervals containing 50-Hz-wide bands of noise was employed. When the cues were present, their center frequency was either 1 kHz (on-frequency) or 900 or 700 Hz (off-frequency). In the conditions of principal interest, the envelopes and phase modulations of the bands of noise were "frozen" across the four intervals that defined a trial, but were chosen randomly across trials. Thresholds were lowest with cues centered at 1 kHz and increased substantially when the center frequency of the cues was changed to 900 or 700 Hz. With cues centered at 700 Hz, performance was equivalent to that obtained without cues and with the masking noise "frozen" across the two intervals that defined a trial. A similar pattern of results was obtained with high-frequency stimuli, where sensitivity to fine-structure information is greatly reduced. Roving the level of the stimuli over a 40-dB range generally reduced sensitivity but did not greatly affect the overall pattern of the data. Thresholds obtained in the two-interval task with masking waveforms chosen randomly were compared with thresholds obtained when the masking waveform was "frozen" within, but not across trials. Differences in threshold appeared to be accounted for by the listeners' use of changes in the mean slope of the envelope of the noise produced by adding the tonal signal.

Acoustic Stimulation↗
PubMed1994

The effects of masking on vibrotactile temporal summation in the detection of sinusoidal and noise signals.

Thresholds for the detection of vibrotactile signals of varied duration applied to the thenar eminence were measured in the absence of and in the presence of a masking stimulus. Signals were 250- and 500-Hz sinusoids and noise bursts with bandwidth limited to 250-1000 Hz. The masking stimulus was either a 250-Hz sinusoid, which was presented in phase with the signal when it was sinusoidal, or noise. Changes in threshold as a function of changes in signal duration were found which were predicted accurately from Zwislocki's theory [J. Acoust. Soc. Am. 32, 1046-1060 (1960)] of temporal summation when the signal was detected by the Pacinian channel, but not when it was detected by a non-Pacinian channel (NP). However, when either the signal or the masking stimulus or both were noise, NP thresholds were affected by changes in signal duration. Only when the signal and masking stimuli were both sinusoids were NP thresholds independent of signal duration. It is concluded that signal duration effects in the NP channel are not due to temporal integration, but rather to increases in information about the signal content provided to the subject as exposure duration is increased.

Acoustic Stimulation↗
PubMed1994

Masked detection thresholds and temporal integration for noise band signals.

Masked detection thresholds were measured at a center frequency of 2500 Hz for a range of noise signal bandwidths (W = 62 to 6000 Hz) and durations (T = 10 to 480 ms) approximating that found acoustically in speech. The signals were presented in an uncorrelated 480-ms, 6000-Hz-wide masker. The masker was presented: (1) at a constant spectrum level (53 dB SPL/Hz) or (2) with the overall level varied randomly over a 50-dB range from interval to interval of a trial. Performance was disrupted in the random-level masker only for the condition where the signal and uncorrelated masker were gated on and off simultaneously and were matched spectrally. Time constants (tau) estimated from temporal integration functions fit to the masked detection threshold data were related inversely to W for W broader than the critical bandwidth. Sensitivity to the noise signals was evaluated in the context of an optimum-detection model (Green, 1960). The results did not follow the prediction of a constant bandwidth-duration (WT) product, but may be understood in terms of cues available to the listener from the relative combination of signal and masker parameters. At least three cues for detection were identified in these experiments: (1) a relative timing cue, (2) a spectral shape cue, and (3) a traditional energy cue compared across observation intervals. The relative timing cue and spectral shape cue together contributed as much as a 10- to 12-dB advantage relative to detection based on the traditional energy cue alone. A new multi-cue detection model for predicting the masked detection thresholds was derived. Predictions from the new model were highly correlated (r = 0.95) with the empirically measured masked detection thresholds.

Adult↗
PubMed1995

Individual differences in simultaneous masking with random-frequency, multicomponent maskers.

Individual differences were examined for 49 listeners in a simultaneous-masking task with random-frequency, multicomponent maskers and a 1000-Hz signal. Across conditions, the maskers were broadband noise or complexes of 2-100 sinusoids whose frequencies were randomly chosen with each presentation. Maskers were equal rms waveforms, presented at 60 dB SPL. Thresholds for the multicomponent maskers spanned a range of up to 59 dB across listeners within conditions without distinct groupings. When divided into quartiles, mean thresholds for listeners in the lowest quartile increased monotonically with the number of masker components and peaked below values produced by broadband noise. Listeners in the upper quartile had nonmonotonic functions with a broad peak around 10-20 components, with values above those produced by broadband noise. Quiet thresholds and masked thresholds for the broadband-noise masker did not distinguish "high-" from "low-threshold" groups. Lowering masker uncertainty (frequencies randomized between but not within trials) or introducing temporal differences by shortening the signal both improved performance by as much as 40 dB, dependent on the masking produced with maximum uncertainty. Training effects within experiment (excluding an initial training period) were examined for all listeners and for five listeners across two consecutive experiments. Eleven of forty-nine listeners (22%) showed systematic improvement averaging 12 dB within experiment, independent of high or low original thresholds. For the five tested longer, four improved by 5 dB or more in at least one condition, but the change in performance occurred primarily during initial training for the second experiment. The large individual differences documented present a challenge to the development of models for masking produced by masker-frequency uncertainty.

Adolescent↗
PubMed1995

Some temporal parameters in vibrotactile forward masking.

Tactile thresholds for detecting a 250-Hz signal of variable duration presented at variable times after the termination of a 250-Hz 700-ms masking stimulus were measured. It was found that the threshold shift resulting from the presentation of the masking stimulus declined as functions of the duration of the signal and the duration of the time interval between the masking stimulus and signal. Both of these effects were found to be attributable to an increase in the time interval between the offset of the masking stimulus and the offset of the signal.

Adolescent↗
PubMed1996

Directional masking of phase locking in the amphibian auditory nerve.

Directional masking in the amphibian auditory periphery was investigated by presenting frogs with a continuous tone from above and a continuous broadband noise from four different horizontal directions. This paradigm mimics the natural situation in which frogs are located in three-dimensional space and interference can and does arise from any direction. Intracellular recordings were made from single auditory-nerve fibers of the anesthetized adult leopard frogs using a dorsal approach. Vector strength (VS) and the mean preferred firing phase (MP) were measured for 94 low-frequency fibers. Thirty-six percent of the fibers demonstrated direction sensitivity of noise masking of VS. Most fibers exhibited a maximum decrease in VS at 90 degrees or 270 degrees noise incident angle and a minimum decrease in VS at 0 degrees or 180 degrees noise incident angle, suggesting higher noise susceptibility to the lateral fields than to the anterior or posterior field. Forty-nine percent of the fibers demonstrated direction sensitivity of noise masking of MP. Maximum shift in MP occurred most often at 90 degrees or 180 degrees noise incident angle, whereas minimum shift in MP occurred most frequently at 0 degrees or 270 degrees noise incident angle, suggesting higher noise susceptibility to the ipsilateral or posterior field than to the contralateral or anterior field. The difference in the directionality patterns of VS and MP suggests different mechanisms underlying noise masking of these two measures of phase locking in the amphibian auditory nerve.

Animals↗
PubMed1997

Masking by modulated and unmodulated noise: effects of bandwidth, modulation rate, signal frequency, and masker level.

The threshold for a sinusoidal signal masked by a band of noise is often times lower when the masking noise is modulated than when it is unmodulated. The difference in masked thresholds is referred to as the modulated-unmodulated difference, or MUD. These present experiments examined the effects of masker bandwidth, masker rate, and masker level on the MUD at several different signal frequencies. The MUD generally increased with increasing masker bandwidth; for masker bandwidths wider than a critical band (or an equivalent rectangular bandwidth-ERB), the results may be influenced by across-channel processes underlying comodulation masking release. The MUD for an ERB masker (MUDERB) was influenced less by masker rate than was the MUD for a broadband (BB) masker (MUDBB). The MUDERB and especially the MUDBB increased significantly with increasing masker level when the modulated masker was modulated at a depth (m) of 1.0, but not when it was modulated at a depth of 0.75. These results have significant implications for extending the MUD paradigm to hearing-impaired subjects. Finally, the MUDERB and the MUDBB increased with increasing signal frequency. This effect for the ERB masker is largely (if not completely) due to the wider absolute bandwidths at higher frequencies. The effect with the BB masker may be influenced by differences in the magnitude of suppression across frequency.

Adult↗
PubMed1998

Binaural detection as a function of interaural correlation and bandwidth of masking noise: implications for estimates of spectral resolution.

Detection thresholds were measured for an antiphasic (S pi) 500-Hz tone masked by a binaural noise, as a function of the bandwidth of the noise. Several values of interaural correlation of the masking noise were used, ranging from -1 to +1. The bandwidth dependence of the thresholds showed a pattern consistent with a 100-Hz-wide critical band for most values of interaural correlation, even for those values which resulted in a considerable binaural release of masking. Only when the interaural correlation of the masking noise was very close to, or equal to, unity was the bandwidth dependence of the thresholds in accord with an approximately 300-Hz-wide "binaural" critical band measured in previous studies. Our analysis of the data calls into question the commonly stated notion that binaural processing is characterized by a wider critical band than is monaural processing.

Auditory Perception↗
PubMed1998

Forward masked excitation patterns in multielectrode electrical stimulation.

Across-channel interactions in multi-electrode cochlear implant patients may be critical to their performance in the natural auditory environment. One measure of channel interaction can be obtained using forward masking. The patterns of threshold shift were obtained as a function of the separation between masker and probe electrode pairs in four Nucleus-implanted patients with varying levels of speech recognition. In the three subjects with better speech recognition (N4, N7, and N13), the masking patterns showed the least parameter dependence. In the subject with the lowest speech scores (N3), the masking patterns showed the greatest dependence on masker level and on probe delay. Masking as a function of masker level also reflected these differences: N3's functions showed the greatest changes for different probe delays. Similar rates of recovery were observed in the three good performers (time constant approximately 70 ms). In contrast, subject N3's recovery function shows a more rapid recovery. These data indicate that channel interaction is individually variable and parameter dependent, both of which may play a role in the perception of dynamic stimuli in the natural auditory environment.

Auditory Threshold↗
PubMed2005

Incidence of nasal trauma associated with nasal prong versus nasal mask during continuous positive airway pressure treatment in very low birthweight infants: a randomised control study.

OBJECTIVE: To compare the incidence of nasal trauma associated with the use of prong or mask during nasal continuous positive airway pressure (nCPAP) support in very low birthweight (<1501 g) infants. DESIGN: Randomised controlled clinical trial. SETTING: Tertiary care university hospital, Department of Paediatrics, Kuala Lumpur, Malaysia. METHODS: All very low birthweight infants admitted to the neonatal intensive care unit between July 2001 and December 2003 who received nCPAP through the Infant Flow Driver were randomised to the use of either nasal prong or mask. The nasal cavity of these infants was inspected daily during the first week and then weekly until they were weaned off nCPAP. RESULTS: Of the 89 infants recruited, 41 were randomised to the mask group and 48 to the prong group. There was no significant difference in the incidence of nasal trauma between the two groups (p = 0.5). The primary site of trauma was at the junction between the nasal septum and the philtrum in infants in the mask group and the walls of the nasal septum in the prong group. Logistic regression analysis showed that duration of nCPAP was the only significant risk factor associated with development of nasal injury, after birth weight, gestational age, and nasal device used had been controlled for (adjusted odds ratio 1.04; 95% confidence interval 1.01 to 1.07; p = 0.003). CONCLUSION: Irrespective of the type of nasal device used, nasal trauma is common during nCPAP treatment, which should therefore be terminated as soon as possible.

Continuous Positive Airway Pressure↗
PubMed1998

Sleep and neuromuscular disease: bilevel positive airway pressure by nasal mask as a treatment for sleep disordered breathing in patients with neuromuscular disease.

OBJECTIVE: Investigation of the therapeutic effects of bilevel positive airway pressure delivered by nasal mask in patients with neuromuscular disease. METHODS: 20 patients with neuromuscular disease were evaluated for symptoms of nocturnal sleep disruption. These symptoms included daytime tiredness, fatigue, sleepiness, and complaints of insomnia. The patients were studied with nocturnal polysomnograms and daytime multiple sleep latency tests (MSLT). Their immediate and long term responses to bilevel positive airway pressure were also investigated. The study took place at the Stanford University Sleep Disorders Clinic. Some of the polygraphic evaluations were performed with portable equipment in the patients' homes. The reported population comprised 20 patients, all of whom had progressive neuromuscular disease. Five of the patients were women. Four patients had muscular dystrophy, six had myotonic dystrophy, and two patients each had mitochondrial myopathy and glycogen storage disease. Two patients had post-traumatic lesions, one bulbar and the other phrenic. The remaining patients had vascular myopathy, unclassified myopathy, syringomyelia, and slow evolving spinocerebellar degeneration. RESULTS: 19 of the 20 patients accepted some form of non-invasive ventilation. All but one of these were initially maintained on bilevel positive airway pressure spontaneous (S) mode, although one patient required a switch to the timed (T) mode within a year. The mean expiratory positive airway pressure (EPAP) used was 4.5 with a range of 4 to 5 cm H2O. The mean inspiratory positive airway pressure (IPAP) was 11.5, range 9 to 14 cm H2O. Before treatment the MSLTs were < or = 8 minutes in 11 of the patients. The overall mean score was 8.2 (SD) 1.3 minutes. After long term treatment the mean MSLT was 12.5 (SD 2) minutes and the mean ESS score was 7 (SD 3). During the mean 3.5 years of follow up, three patients needed supplemental oxygen at a flow of 0.5 to 1.0 l/min bled into their masks. Three patients with myotonic dystrophy presented continued daytime somnolence despite apparent adequate treatment of their sleep disordered breathing. This required the addition of stimulant medication to their regimen. During this time three additional subjects had to be switched to nasal mask intermittent positive pressure ventilation delivered by traditional volume cycled home ventilator (volume controlled NIPPV). CONCLUSIONS: Bilevel positive airway pressure delivered by nasal mask may be used successfully to treat sleep disordered breathing associated with neuromuscular disease. This device can be employed to assist nocturnal ventilation by either the spontaneous or timed mode. In the United States it is less expensive and easier to institute than volume controlled NIPPV and may be as efficacious as this mode if close surveillance and regular reevaluation of the patient's status is maintained.

Adolescent↗