Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “MASKS”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,027 records · Page 57Linked to original sources
PubMed1992

Frequency discrimination in forward and backward masking.

Frequency difference limens for pure tones preceded by a forward masker or followed by a backward masker were obtained across a wide range of signal levels. Relkin and Doucet [Hear. Res. 55, 215-222 (1991)] have shown that at a masker-signal delay of 100 ms, the thresholds of high-SR (spontaneous rate) auditory-nerve fibers are recovered, while the low-SR fiber thresholds are not. Therefore, forward-masked frequency discrimination potentially offers a method to investigate the role of low-SR fibers in the coding of frequency. It has been shown that when an intense forward masker is presented 100 ms before a pure-tone signal, intensity difference limens are elevated for mid-level signals [Zeng et al., Hear. Res. 55, 223-230 (1991)]. However, Plack and Viemeister [J. Acoust. Soc. Am. 92, 3097-3101 (1992)] have shown that a similar elevation in the intensity difference limen is obtained under conditions of backward masking, where selective adaptation of the auditory neurons would not be expected to occur. A condition of backward-masked frequency discrimination was therefore included to investigate the role of interference resulting from adding additional stimuli to a discrimination task. For signals at 1000 and 6000 Hz, there was no effect of a forward masker upon frequency difference limens. For the backward-masked conditions, an elevation of the frequency difference limen was observed at all signal levels, demonstrating that the effects of forward and backward maskers upon frequency discrimination are dissimilar and suggesting that cognitive effects are present in backward-masked discrimination tasks.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory Perception↗
PubMed1993

Two experiments on the temporal boundaries for the nonlinear additivity of masking.

The present paper describes the results from two experiments which explored the temporal boundary between overlapping and nonoverlapping maskers and its effects on the additivity of masking. In the first experiment, detection thresholds for a short-duration 1000-Hz signal were measured in the presence of two equal-duration broadband maskers which varied in degree of temporal overlap. Following complete overlap of maskers, the temporal separation of masker onsets was systematically varied to create conditions ranging from partially overlapping simultaneous masking to combined forward and backward masking. The signal was always temporally centered between the onset of the first masker and the offset of the second masker. Nonlinear additivity of masking occurred for the majority of subjects when maskers and signal did not overlap, whereas linear additivity resulted for all subjects when the maskers and signal overlapped. In the second experiment, two separate forward maskers were used so that masker/masker overlap could be manipulated independent from masker/signal overlap. Maskers were changed gradually from temporally overlapping, or concurrent, forward maskers to sequential forward maskers. Results for all subjects showed nonlinear additivity for all combined-masker conditions. Together, these two experiments indicated that nonlinear masking additivity is observed when the signal does not overlap temporally with the maskers. However, when maskers and signal overlap temporally (and spectrally), linear additivity is observed.

Acoustic Impedance Tests↗
PubMed1994

Comodulation masking release for elderly listeners with relatively normal audiograms.

Thresholds for the detection of a 1.0-kHz pure tone were measured in modulated and unmodulated noise for a group of nine elderly listeners, aged 62-83, with normal or near-normal audiograms, and a reference group of seven younger normal-hearing listeners. The masker was either a band of noise 128 Hz wide [approximately one equivalent rectangular bandwidth (ERB) centered at 1.0 kHz], or a band of noise 1505 Hz wide (approximately five ERBs above and five ERBs below 1.0 kHz). The noise was either unmodulated or was square-wave modulated at rates of 10, 20, 30, 40, and 50 Hz. A measure of comodulation masking release (CMR) was derived from the masking data. Because wider filters have been found to be associated with reduced CMR, auditory filter shapes were also estimated for the elderly listeners. The filters were found to be normal at the center frequencies of 800, 1000, and 2000 Hz ruling out possible confounding effects of impaired auditory filters on CMR. Masked detection thresholds were generally significantly higher for the elderly as compared to the younger listeners for both unmodulated and modulated noise. Thresholds were lower in modulated as compared to unmodulated noise masking for both elderly and young listeners. For both groups of listeners and for both narrow- and wideband masking, detection thresholds continued to increase as modulation rates were increased. CMR was not different for the elderly and young listeners and it was similarly decreased, for both groups, at higher rates of modulation.

Adult↗
PubMed1998

Comparison of monaural (CMR) and binaural (BMLD) masking release.

Release of masking for a sinusoidal signal of 5 kHz masked by a 25-Hz-wide noise band centered around 5 kHz was measured. The masking release was provided by a second noise band that was comodulated with the on-frequency masker band. For CMR configurations the second noise band was centered at 3 kHz and presented to the ipsi-lateral or to the contra-lateral ear. For BMLD configurations the second band was centered at 5 kHz and presented to the contra-lateral ear. In another condition the second noise band also contained the signal presented with such a phase that maximal differences in the envelope resulted. For both the CMR and the BMLD paradigm, the masking release for the latter condition was larger than for the former condition. To assess further the similarity between monaural and binaural masking release, a sinusoidal masker and either a noise or a sinusoidal signal were used. The data indicate that, at high frequencies, envelope correlation may be a valuable cue for CMR as well as for the BMLD.

Auditory Perception↗
PubMed1998

Basilar-membrane nonlinearity and the growth of forward masking.

Forward masking growth functions were measured for pure-tone maskers and signals at 2 and 6 kHz as a function of the silent interval between the masker and signal. The inclusion of conditions involving short signals and short masker-signal intervals ensured that a wide range of signal thresholds were recorded. A consistent pattern was seen across all the results. When the signal level was below about 35 dB SPL the growth of masking was shallow, so that signal threshold increased at a much slower rate than masker level. When the signal level exceeded this value, the masking function steepened, approaching unity (linear growth) at the highest masker and signal levels. The results are inconsistent with an explanation for forward-masking growth in terms of saturating neural adaptation. Instead the data are well described by a model incorporating a simulation of the basilar-membrane response at characteristic frequency (which is almost linear at low levels and compressive at higher levels) followed by a sliding intensity integrator or temporal window. Taken together with previous results, the findings suggest that the principle nonlinearity in temporal masking may be the basilar membrane response function, and that subsequent to this the auditory system behaves as if it were linear in the intensity domain.

Auditory Perception↗
PubMed1998

Release from masking due to spatial separation of sources in the identification of nonspeech auditory patterns.

A nonspeech pattern identification task was used to study the role of spatial separation of sources on auditory masking in multisource listening environments. The six frequency patterns forming the signal set were comprised of sequences of eight 60-ms tone bursts. Bursts of masking sounds were played synchronously with the signals. The main variables in the study were (1) the difference in spatial separation in the horizontal plane between signals and maskers and (2) the nature of the masking produced by the maskers. Spatial separation of signal and masker ranged from 0-180 degrees. The maskers were of two types: (1) a sequence of eight 60-ms bursts of Gaussian noise intended to produce predominantly peripherally based "energetic masking" and (2) a sequence of eight 60-ms bursts of eight-tone complexes intended to produce primarily centrally based "informational masking." The results indicated that identification performance improved with increasing separation of signal and masker. The amount of improvement depended upon the type of masker and the center frequency of the signal patterns. Much larger improvements were found for spatial separation of the signal and informational masker than for the signal and energetic masker. This was particularly apparent when the acoustical advantage of the signal-to-noise ratio in the more favorable of the two ears (the ear nearest the signal) was taken into account. The results were interpreted as evidence for an important role of binaural hearing in reducing sound source or message uncertainty and may contribute toward solving the "cocktail party problem."

Adult↗
PubMed1998

The effect of spatial separation of signal and noise on masking in the free field as a function of signal frequency and age in the mouse.

Masking of low- (4 kHz) and high-frequency (25 kHz) signals by one-octave bandpass maskers either spatially coincident with the signal or contralateral to it was examined in mice, 4-6 and 20-22 months of age, in the free field. Signals were presented 120 ms prior to a startle stimulus and differences in their inhibition of the startle reflex, relative to startle stimulus alone trials, were used to measure the severity of masking. Inhibition was reduced or eliminated by spatially coincident noise for weak but not for relatively intense signals, providing the type of "loudness recruitment" effect characteristic of human listeners in similar stimulus conditions. The spatial separation of the signal and its masker relieved this maskinglike effect for the high-frequency pair in both young and old mice. In contrast there was no beneficial effect of the shift in spatial location for the low-frequency pair at either age. This finding of masking release for high- but not low-frequency stimuli supports the hypothesis that the sound shadow provided by the head and pinna would yield a favorable signal-to-noise level difference for a contralateral masker and an ipsilateral signal only at very high frequencies in the mouse. The presence of masking release in these old mice, a first generation hybrid strain with near-normal high-frequency hearing in ABR measures, agrees with reports that the masking release resulting from a similar manipulation in aged human listeners with minimal high-frequency hearing loss is the equal of that obtained in the young listener.

Age Factors↗
PubMed1999

Masker laterality and cueing in forward-masked intensity discrimination.

Forward-masked intensity discrimination was measured as a function of level in experiments designed to reveal insights into the mechanism(s) underlying the midlevel elevation of the Weber fraction. The standard and maskers were 1.0-kHz tones that were separated by 100 ms. Performance was measured for listeners with normal hearing using an adaptive procedure. In experiment 1, intensity discrimination was measured in the presence of an ipsilateral masker (80 dB SPL), a contralateral masker (93 dB SPL), and a binaural (dichotic) masker produced by combining the ipsilateral and contralateral maskers. Listeners perceived only the contralateral masker in the binaural-masker condition. The contralateral masker produced a small midlevel elevation of the Weber fraction. The ipsilateral masker and the binaural masker produced a large, midlevel elevation of the Weber fraction. Experiment 2 found that a two-tone masker resulted in a reduction (improvement) in the Weber fraction for some conditions, but the midlevel elevation remained for all subjects in this cue-tone condition. Experiment 3 demonstrated that cross talk could not account for all of the masking observed with contralateral maskers. Taken together, the results suggest that a single complex mechanism or multiple mechanisms may be responsible for the masking seen in these experiments. On the basis of the cueing results, it is concluded that a portion of the masking is due to cognitive factors; however, a sensory mechanism cannot be ruled out for the remaining portion, based on the results of these experiments. Finally, a small but significant amount of masking due to contralateral maskers places the mechanism for this outcome central to the cochlear nucleus.

Adult↗
PubMed1999

Effects of aspirin on psychophysical measures of frequency selectivity, two-tone suppression, and growth of masking.

Three psychophysical measures of nonlinearity were evaluated before and during a course of aspirin ingestion to investigate the role of outer hair cells (OHCs) in these measures, as aspirin is thought to alter the functioning of OHCs. Six normal-hearing individuals received a moderate dose (3.9 g/day) of aspirin for four days, producing essentially flat, temporary hearing losses that ranged from 5-20 dB. The losses were about 2 dB greater for a 300-ms signal than for a 15-ms signal, indicating reduced temporal integration with aspirin. On the final three days of aspirin use, three experiments were completed; each was designed to measure one aspect of nonlinear behavior: (1) the effects of level on frequency selectivity in simultaneous masking using notched-noise maskers, (2) two-tone suppression using forward maskers at the signal frequency (fs) and suppressor tones above fs, and (3) growth-of-masking functions in forward masking using a masking tone below fs. Signal frequencies of 750 and 3000 Hz were used to evaluate the effects of aspirin at relatively low- and high-frequency regions of the cochlea. In experiment 1, aspirin broadened the auditory filters and reduced the effect of level on frequency selectivity. In experiment 2, aspirin reduced or eliminated two-tone suppression. And, in experiment 3, aspirin reduced the slopes of the growth-of-masking functions. Thus, the aspirin was effective in reducing nonlinearity in all three experiments, suggesting that these measures reflect the same (or a similar) active, nonlinear mechanism, namely the compressive nonlinearity provided by the OHCs. In all experiments, aspirin tended to have larger detrimental effects on the nonlinear measures at 3000 Hz than at 750 Hz, which can be explained in terms of greater involvement of nonlinear processing at higher frequencies. Finally, these effects of aspirin were found to be similar to those observed in preliminary measurements in two subjects with mild, permanent hearing loss.

Adult↗
PubMed1999

Informational masking by everyday sounds.

Tone-in-noise detection is severely degraded when only a few spectral components of the noise are presented at random on each trial [Neff and Green, Percept. Psychophys. 41, 409-415 (1987)]. The elevations in threshold are attributed to uncertainty regarding the noise caused by sparse sampling of noise components-informational masking is the term used to describe the result. The present experiment was undertaken to determine how informational masking is affected when sparse sampling is from a set of common everyday sounds instead of noise. On each presentation a different masker waveform of constant total power was synthesized from the magnitude and phase of a fixed number, m, of spectral components (m = 2-921 across blocks of trials). The components were selected at random from 1 of 50 common environmental sounds (e.g., baby crying, door slamming, phone ringing), or 1 of 50 samples of Gaussian noise. Masked thresholds for a 1.0-kHz signal in the presence of both types of maskers were obtained using an adaptive, two-interval, forced-choice procedure. Results with noise replicated earlier, results showing largest elevations in threshold for 10-20 sampled components. Results with everyday sounds showed a similar pattern with thresholds elevated above those for noise by as much as 10 dB at the larger values of m. The differences in masked thresholds were systematically related to differences in the ensemble variance of masker spectra, as predicted by a model previously applied to noise [Lutfi, J. Acoust. Soc. Am. 94, 748-758 (1993)]. Not predicted was the result of a subsequent trial-by-trial analysis in which 9-11 dB less masking was observed for samples from everyday sounds rated as easily recognized by listeners. The results suggest that listeners fail to exploit lawful dependencies among spectral components of everyday sounds to aid detection, unless enough information is available for the sound to be easily recognized.

Adult↗
PubMed2000

On the role of envelope fluctuation processing in spectral masking.

This study examines the role of temporal cues in spectral masking, such as beats and intrinsic envelope fluctuations. Predictions from the modulation-filterbank model developed by Dau et al. [J. Acoust. Soc. Am. 102, 2906-2919 (1997)] are compared to average masking patterns from Moore et al. [J. Acoust. Soc. Am. 104, 1023-1038 (1998)]. In these experiments, tones and narrow-band noises have been used as the signal and the masker, so that all four signal-masker combinations are considered. In addition, model predictions are compared with new experimental data in conditions of notched-noise masking, where the masker consisted of two narrow-band noises whose bandwidth and frequency separation were varied systematically. The model uses a peripheral filtering stage with linear and symmetric Gammatone filters, an adaptation stage that includes a static compressive nonlinearity for stationary input stumuli and a higher sensitivity for envelope fluctuation, and a modulation filterbank that analyzes the output for each peripheral channel. For low and medium masker levels, the model accounts very well for the masking patterns in all signal-masker conditions, as well as for the notched-noise conditions. In contrast, predictions from a version of the model that acts like an energy detector account for only some of the notched-noise data, and generally do not account for the shape of the masking patterns. For a high masker level, the simulations suggest the use of asymmetric filters, with a steeper high-frequency slope than is used in the linear model, consistent with results from previous studies. In addition, several nonlinear effects become apparent at this masker level, which cannot be accounted for by the current model.

Adult↗
PubMed2005

Contextual masking of oriented lines: interactions between surface segmentation cues.

The ability of human observers to detect and discriminate a single feature of a visual image deteriorates markedly when the targeted feature is surrounded by others of a similar kind. This perceptual masking is mirrored by the suppressive effects of surround stimulation on the responses of neurons in primary visual cortex (area V1). Both perceptual and neuronal masking effects are partially relieved, however, if the targeted image feature is distinguished from surrounding features along some dimension, such as contour orientation. Masking relief is likely to play an important role in perceptual segmentation of complex images. Because dissimilar surfaces usually differ along multiple feature dimensions, we tested the possibility that those differences may influence segmentation in an invariant manner. As expected, we found that the presence of surrounding features resulted in perceptual masking and neuronal response suppression in area V1, but that either orientation or contrast polarity differences between the target and surrounding features was sufficient to partially relieve these effects. Simultaneous differences along both dimensions, however, yielded no greater relief from masking than did either difference alone. Although the averaged neuronal effects of orientation polarity cues were thus invariant, the time course over which these effects emerged after each stimulus appearance was different for the two cues. These findings refine our understanding of the functions of nonclassical receptive fields, and they support a key role for V1 neurons in surface segmentation.

Action Potentials↗
PubMed2005

Neural correlates and mechanisms of spatial release from masking: single-unit and population responses in the inferior colliculus.

Spatial release from masking (SRM), a factor in listening in noisy environments, is the improvement in auditory signal detection obtained when a signal is separated in space from a masker. To study the neural mechanisms of SRM, we recorded from single units in the inferior colliculus (IC) of barbiturate-anesthetized cats, focusing on low-frequency neurons sensitive to interaural time differences. The stimulus was a broadband chirp train with a 40-Hz repetition rate in continuous broadband noise, and the unit responses were measured for several signal and masker (virtual) locations. Masked thresholds (the lowest signal-to-noise ratio, SNR, for which the signal could be detected for 75% of the stimulus presentations) changed systematically with signal and masker location. Single-unit thresholds did not necessarily improve with signal and masker separation; instead, they tended to reflect the units' azimuth preference. Both how the signal was detected (through a rate increase or decrease) and how the noise masked the signal response (suppressive or excitatory masking) changed with signal and masker azimuth, consistent with a cross-correlator model of binaural processing. However, additional processing, perhaps related to the signal's amplitude modulation rate, appeared to influence the units' responses. The population masked thresholds (the most sensitive unit's threshold at each signal and masker location) did improve with signal and masker separation as a result of the variety of azimuth preferences in our unit sample. The population thresholds were similar to human behavioral thresholds in both SNR value and shape, indicating that these units may provide a neural substrate for low-frequency SRM.

Acoustic Stimulation↗
PubMed1997

Responses of neurons in the inferior colliculus to binaural masking level difference stimuli measured by rate-versus-level functions.

The psychophysical detection threshold of a low-frequency tone masked by broadband noise is reduced by < or = 15 dB by inversion of the tone in one ear (called the binaural masking level difference: BMLD). The contribution of 120 low-frequency neurons (best frequencies 168-2,090 Hz) in the inferior colliculus (ICC) of the guinea pig to binaural unmasking of 500-Hz tones masked by broadband noise was examined. We measured rate-level functions of the responses to identical signals (So) and noise (No) at the two ears (NoSo) and to identical noise but with the signal inverted at one ear (NoS pi): the noise was 7-15 dB suprathreshold. The masked threshold was estimated by the standard separation, "D". The neural BMLD was estimated as the difference between the masked thresholds for NoSo and NoS pi. The presence of So and S pi tones was indicated by discharge rate increases in 55.3% of neurons. In 36.4% of neurons, the presence of So tones was indicated by an increase in discharge rate and S pi tones by a decrease. In 6.8% of neurons, both So and S pi tones caused a decrease in discharge rate. In only 1.5% of neurons was So indicated by a decrease and S pi by an increase in discharge rate. Responses to the binaural configurations were consistent with the neuron's interaural delay sensitivities; 34.4% of neurons showing increases in discharge rate to both So and S pi tones gave positive BMLDs > or = 3 dB (S pi tones were detected at lower levels than So), whereas 37.3% gave negative BMLDs > or = 3 dB. For neurons in which So signals caused an increase in the discharge rate and S pi a decrease, 72.7% gave positive BMLDs > or = 3 dB and only 4.5% gave negative BMLDs > or = 3 dB. The results suggest that the responses of single ICC neurons are consistent with the psychophysical BMLDs for NoSo versus NoS pi at 500 Hz, and with current binaural interaction models based on coincidence detection. The neurons likely to contribute to the psychophysical BMLD are those with BFs near 500 Hz, but detection of So and S pi tones may depend on different populations of neurons.

Animals↗
PubMedDate not supplied

Comparison of the OxyMask and Venturi mask in the delivery of supplemental oxygen: pilot study in oxygen-dependent patients.

BACKGROUND: The OxyMask (Southmedic Inc, Canada) is a new face mask for oxygen delivery that uses a small 'diffuser' to concentrate and direct oxygen toward the mouth and nose. The authors hypothesized that this unique design would enable the OxyMask to deliver oxygen more efficiently than a Venturi mask (Hudson RCI, USA) in patients with chronic hypoxemia. METHODS: Oxygen-dependent patients with chronic, stable respiratory disease were recruited to compare the OxyMask and Venturi mask in a randomized, single-blind, cross-over design. Baseline blood oxygen saturation (SaO2) was established breathing room air, followed in a random order by supplemental oxygen through the OxyMask or Venturi mask. Oxygen delivery was titrated to maintain SaO2 4% to 5% and 8% to 9% above baseline for two separate 30 min periods of stable breathing. Oxygen flow rate, partial pressure of inspired and expired oxygen (PO2) and carbon dioxide (PCO2), minute ventilation, heart rate, nasal and oral breathing, SaO2 and transcutaneous PCO2 were collected continuously. The study was repeated following alterations to the OxyMask design, which improved clearance of carbon dioxide. RESULTS: Thirteen patients, aged 28 to 79 years, were studied initially using the original OxyMask. Oxygen flow rate was lower, inspired PO2 was higher and expired PO2 was lower while using the OxyMask. Minute ventilation and inspired and expired PCO2 were significantly higher while using the OxyMask, whereas transcutaneous PCO2, heart rate and the ratio of nasal to oral breathing did not change significantly throughout the study. Following modification of the OxyMask, 13 additional patients, aged 18 to 79 years, were studied using the same protocol. The modified OxyMask provided a higher inspired PO2 at a lower flow rate, without evidence of carbon dioxide retention. CONCLUSIONS: Oxygen is delivered safely and more efficiently by the OxyMask than by the Venturi mask in stable oxygen-dependent patients.

Adolescent↗
PubMedDate not supplied

Contralateral masking in cochlear implant users with residual hearing in the non-implanted ear.

Contralateral masking was investigated in cochlear implant users with residual hearing in the non-implanted ear. Threshold elevations for acoustic probes were observed when electrical maskers were presented in the opposite ear. Also, threshold elevations for electrical probes were observed when acoustic contralateral maskers were presented. The amount of threshold shift expressed in decibels charge or decibels sound pressure level produced by either contralateral acoustic or electric maskers was within the range found in normal listeners for similar stimuli (i.e. 4-8 dB). There was a correlation between the sensation level of acoustic maskers and the maximum amount of masking observed which is consistent with data for normally hearing subjects. The width of the masking patterns was similar to that expected from forward masking patterns in severely sensorineurally impaired ears and implanted ears. The maximum amount of acoustic masking tended to occur for electrode positions that were more basal than expected from characteristic frequency positions. However, where a relatively high-frequency 4-kHz masker could be used, there was a good match between the characteristic frequency position of the maximum threshold elevation and that of the masker.

Adult↗
PubMed1979

Backward and forward masking in Macaca mulatta.

8 highly trained rhesus monkeys were examined on both backward and forward masking to determine the feasility of studying masking in unrestrained monkeys and of studying perceptual phenomenon without the confounding of linguistic variables. It was found that monkeys could respond well under the given conditions. The data indicated a more pronounced effect of the masking stimulus on backward masking than on forward masking.

Animals↗
PubMedDate not supplied

Diagnosis of endolymphatic hydrops by low-frequency masking.

Low-frequency masking is a new method for the diagnosis of endolymphatic hydrops. A short acoustic stimulus and a low-frequency masker tone are applied to the same ear in an adjustable phase relationship. We recorded phase-dependent masked thresholds from normal-hearing subjects, and patients with Ménière's disease and sensory hearing loss without vertigo. In normal hearing, there is a mean maximal difference in masking (modulation depth) of 28 dB between the phase delays of 0 degree and 270 degrees. In patients with sensory hearing loss without vertigo, modulation depth is reduced due to recruitment. In Ménière cases, the phase dependence may be totally absent and varies as the disease progresses. Therefore, repeated measurements of masking are required: patients and subjects with normal hearing were tested for a period of 1 year. Also, modulation depth is significantly reduced in the contralateral nonsymptomatic ears of Ménière patients. The results indicate that low-frequency masking is a quick, noninvasive and relevant method for the diagnosis of endolymphatic hydrops.

Acoustic Stimulation↗