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At least 397 records · Page 22Linked to original sources
PubMed1986

Two- versus four-tone masking, revisited.

Canahl [J. Acoust. Soc. Am. 50, 471-474 (1971)] measured thresholds for a 1.0-kHz sinusoid masked either by two or by four surrounding tones. He reported four-tone masked thresholds that exceeded, by 5-7.5 dB, the energy sum of the masking produced by the individual tone pairs. The present paper reports on a series of experiments investigating the effects of several factors on this 5-7.5 dB "excess" masking. In each experiment, thresholds for a 1.0-kHz 250-ms sinusoid were measured as a function of the overall level of two or four equal amplitude sinusoids with frequencies arithmetically centered around 1.0 kHz. For conditions similar to those of the Canahl experiment, 5-6 dB of excess masking was obtained independent of the level of the masking tones. Randomly varying overall level across presentations had no effect on the excess masking. The excess masking was reduced or eliminated when the masking tones were generated using an amplitude modulation technique, when they were gated on and off with the signal, or when their waveshapes were fixed across trials. Canahl's result may reflect listeners' ability to detect the signal as a change in the waveshape of the multitone masker.

Attention↗
PubMed1991

Forward masking by maskers of uncertain frequency content.

Large amounts of simultaneous masking can be produced when the frequency content of multicomponent maskers is changed with each presentation. This study examined whether such masker uncertainty also influences forward masking. The forward maskers were a 1000-Hz sinusoid, a broadband noise, and tonal complexes composed of 2-100 sinusoids drawn at random from a 300 to 3000-Hz range. A 10-ms signal at 1000 Hz followed the offset of 200-ms, 60 to 80-dB maskers with delays of 4-32 ms. For comparison, simultaneous-masked thresholds were measured for a 200-ms signal at 1000 Hz for a subset of conditions or taken from an earlier study. Unlike simultaneous masking, forward masking showed little effect of masker uncertainty. As confirmed by fitting a three-parameter descriptive model of forward masking, the pattern of results was similar for the sinusoidal, noise, and multicomponent forward maskers and there was no reduction in masking with decreased masker uncertainty. Performance in simultaneous masking improved to equal that in forward masking when the signal was shortened from 200 to 10 ms. Thus temporal disparities between masker and signal can offset the effects of spectral uncertainty and help separate the relative contribution of peripheral versus more central processes.

Acoustics↗
PubMed1994

Psychophysical and physiological forward masking studies: probe duration and rise-time effects.

To determine if the amount of forward masking observed in single auditory-nerve fibers of the chinchilla was sufficient to account for the amount of masking observed behaviorally in humans, thresholds for the detection of several types of probe signals following a forward masker were measured in both behavioral and physiological experiments. It is necessary to use the same probe stimulus characteristics in both behavioral and physiological experiments to make valid comparisons between the two measures of forward masking. Experiment 1 was behavioral and used human listeners. Two types of probe signals were used, differing in overall duration and rise/fall times. The two probes yielded different growth of masking functions. These results further demonstrate that forward masking characteristics in behavioral experiments are critically dependent upon the parameters of the probe signal used. Experiment 2 was physiological and used chinchillas. The amount of masking for high-level maskers using the same two probe types in a physiological "forced-choice" experiment are presented. In contrast with the behavioral results, there was little or no difference in the amounts of masking observed for the two probes. In addition, the amounts of masking observed in some auditory-nerve fibers were considerably smaller than those observed behaviorally. These experiments further demonstrate that the amount of forward masking observed in single auditory-nerve fibers is inconsistent with that observed behaviorally.

Acoustic Stimulation↗
PubMed1995

Pure tone masking patterns in adult chickens before and after recovery from acoustic trauma.

Three female adult chickens were trained to detect pure tones using a positive reinforcement technique and an adaptive threshold tracking procedure. Masked thresholds for tone bursts were measured in the presence of pure-tone maskers at 500, 1000, and 2000 Hz. Masker levels of 40-, 60-, and 70-dB sound-pressure level (SPL) were employed. Masking was maximal at the frequency of the masker. Tone-on-tone masking patterns were symmetrical at low masker levels, but at higher masker levels, more masking was evident at frequencies above the masker than below the masker. Distinct low threshold notches were frequently observed above and below the peak in the masking pattern. After the normal masking patterns had been measured, the chickens were exposed to a 525-Hz traumatizing tone at 120 dB SPL for 48 h and then allowed to recover. After thresholds had returned to their pre-exposure levels, tone-on-tone masking patterns were remeasured. The post-exposure masking patterns were virtually identical to the pre-exposure masking patterns.

Animals↗
PubMed1996

Release from upward spread of masking in regions of high-frequency hearing loss.

The upward spread of masking was compared for 500-Hz quasifrequency-modulated (QFM) and sinusoidally amplitude-modulated (SAM) maskers. The modulation rate was 20 Hz. These maskers had identical magnitude spectra but different envelopes, which were relatively flat for the QFM masker and strongly fluctuating for the SAM masker. At signal frequencies more than an octave above the masker, masked thresholds for the SAM masker were lower than for the QFM masker, revealing "masking release" (QFM-SAM masked threshold differences) exceeding 30 dB in normal-hearing ears. In ears with high-frequency sensorineural hearing loss, but normal hearing in the region of the masker, masking release was markedly reduced or completely absent in regions of hearing loss. The data were evaluated with a model of masking based on the linearized response growth (LRG) of basilar membrane transfer functions associated with cochlear damage in animals. The LRG model predicted more gradual slopes of the growth of masking and reduced amount of masking in regions of hearing loss. The reduced masking release seen in regions of hearing loss could be largely accounted for by a more rapid growth of response to the probe tone in regions of hearing loss.

Auditory Perception↗
PubMed1998

Interactions of forward and simultaneous masking in intensity discrimination.

Intensity coding mechanisms are explored in a paradigm involving both forward and simultaneous masking. For intensity discrimination of 1000-Hz pure tone in quiet, a near-miss to Weber's law is observed. However, as more stimulus components are added to this relatively simple experiment, interactions among components produce a more complex pattern of results. An intense forward masker, while not causing any threshold shift for the test tone, produces a nonmonotonic intensity discrimination function ["the midlevel hump," Zeng et al., Hearing Res. 55, 223-230 (1991)]. The midlevel hump can be removed by the presence of additional notched noise [Plack and Viemeister, J. Acoust. Soc. Am. 92, 1902-1910 (1992)] or narrow-band noise whose level is increased along with the test tone's standard level. The same midlevel hump can also be enhanced by a fixed-low-level notched noise or a high-level, high-pass noise which causes minimal masking at the test frequency. Interactions of forward masking and simultaneous masking present a serious problem for a clear interpretation of these results. For example, the notched noise was originally intended to restrict off-frequency listening, but on-frequency masking compromised this original purpose and confounded the interpretation of the notched noise effects. By measuring systematically the growth-of-masking functions, the present study identified various interactions of forward and simultaneous masking and clarified the role of off-frequency listening in forward-masked intensity discrimination. Both peripheral and central mechanisms may have contributed to the occurrence, reduction and enhancement of the midlevel hump under these masking conditions.

Adult↗
PubMed1998

Influence of centrifugal pathways on forward masking of ventral cochlear nucleus neurons.

When responses to one part of a sequence of auditory signals reduce the responses to a subsequent portion of the signal, "forward masking" results. Although forward masking occurs in the auditory nerve, that observed in the ventral cochlear nucleus (VCN) more closely resembles psychophysical forward masking. In contrast to the auditory nerve in which the amount of forward masking is proportional to the amount of excitation produced by the masker, most VCN neurons show a poor correlation between forward masking and excitation produced by the masker, indicating a more complex interaction between responses to adjacent signals. This study tested the hypothesis that one component of forward masking is produced by inputs from centrifugal neural connections to the VCN. The centrifugal pathways were interrupted with knife-cut lesions medial to the CN. Responses of single units obtained 60 minutes after the lesions were compared to those obtained before the lesions. In primarylike, sustained chopper and on units the lesions resulted in a reduction in forward masking and enhanced recovery. In contrast, lesions resulted in increased masking in primarylike-notch and low-intensity chopper units. The relationship between masker-elicited excitation and forward masking became more monotonic for transient choppers and on units, approaching that observed for auditory nerve fibers. These effects are probably the result of removal of both inhibitory and excitatory inputs, ultimately reflecting a balance of excitation and inhibition to each neural population in the VCN.

Animals↗
PubMed1998

Masking patterns for sinusoidal and narrow-band noise maskers.

The masking patterns produced by narrow-band maskers can show distinct irregularities. These experiments attempted to clarify the relative importance of factors contributing to these irregularities. A three-alternative adaptive forced-choice method with feedback was used, to promote use of the optimal detection cues. The masker and signal were either a sinusoid or a band of noise that was 80 Hz wide, giving four possible combinations of masker and signal type. In experiment 1, masking patterns were measured for maskers centered at 1 kHz, for all combinations of masker and signal type (tone or noise). The masking patterns showed irregularities (dips or "shoulders") above the masker frequency, and the irregularities were larger for the sinusoidal than for the noise masker. Experiment 2 was similar to experiment 1, except that low-pass noise was added to mask combination products. For the noise masker, the low-pass noise slightly increased thresholds, and largely eliminated the irregularities in the patterns, but for the tone masker, the irregularities persisted. Experiment 3 used a noise signal with tone and noise maskers centered at 250, 1000, and 4000 Hz. The tone masker produced less masking than the noise masker for masker-signal frequency separations of 150-250 Hz, regardless of masker frequency. Experiment 4 used an additional masker tone to introduce beats similar to those produced by the interaction of the signal and (main) masker, and to mask combination products. This largely eliminated the dips in the masking patterns for both the noise and tone maskers. Experiment 5 used an additional pair of high-frequency tones to introduce beats, with similar results. We conclude that temporal fluctuations (beats) have a strong influence on the masking patterns for sinusoidal maskers, for masker-signal frequency separations up to a few hundred Hz. Beats may also have some influence on the masking patterns for noise maskers. The detection of combination products also plays a role.

Auditory Perception↗
PubMed1999

Forward masking among infant and adult listeners.

Psychophysical forward-masked thresholds were estimated for 3- and 6-month-old infants and for adults. Listeners detected a repeated 1000-Hz probe, with 16-ms rise time, no steady-state duration, and 16-ms fall time. Unmasked thresholds were determined for one group of listeners who were trained to respond when they heard the probe but not at other times. In the masking conditions, each tone burst was preceded by a 100-ms broadband noise masker at 65 dB SPL. Listeners were trained to respond when they heard the probe and masker, but not when they heard the masker alone. The masker-probe interval, delta t, was either 5, 10, 25, or 200 ms. Four groups of subjects listened in the masked conditions, each at one value of delta t. Each listener attempted to complete a block of 32 trials including four probe levels chosen to span the range of expected thresholds. "Group" thresholds, based on average psychometric functions, as well as thresholds for individual listeners, were estimated. Both group and individual thresholds declined with delta t, as expected, for both infants and adults. Infants' masked thresholds were higher than those of adults, and comparison of masked to unmasked thresholds suggested that infants demonstrate more forward masking than adults, particularly at short delta t. Forward masking appeared to have greater effects on 3-month-olds' detection than on either 6-month-olds' or adults'. Compared to adults, 6-month-olds demonstrated more forward masking only for delta t of 5 ms. Thus, susceptibility to forward masking may be nearly mature by 6 months of age.

Adolescent↗
PubMed2005

Neonatal resuscitation 1: a model to measure inspired and expired tidal volumes and assess leakage at the face mask.

BACKGROUND: Neonatal resuscitation is a common and important intervention, and adequate ventilation is the key to success. In the delivery room, positive pressure ventilation is given with manual ventilation devices using face masks. Mannequins are widely used to teach and practise this technique. During both simulated and real neonatal resuscitation, chest excursion is used to assess tidal volume delivery, and leakage from the mask is not measured. OBJECTIVE: To describe a system that allows measurement of mask leakage and estimation of tidal volume delivery. METHODS: Respiratory function monitors, a modified resuscitation mannequin, and a computer were used to measure leakage from the mask and to assess tidal volume delivery in a model of neonatal resuscitation. RESULTS: The volume of gas passing through a flow sensor was measured at the face mask. This was a good estimate of the tidal volume entering and leaving the lung in this model. Gas leakage between the mask and mannequin was also measured. This occurred principally during inflation, although gas leakage during deflation was seen when the total leakage was large. A volume of gas that distended the mask but did not enter the lung was also measured. CONCLUSION: This system can be used to assess the effectiveness of positive pressure ventilation given using a face mask during simulated neonatal resuscitation. It could be useful for teaching neonatal resuscitation and assessing ventilation through a face mask.

Education, Medical, Graduate↗
PubMed2003

Nebuliser hood compared to mask in wheezy infants: aerosol therapy without tears!

BACKGROUND: Small volume nebulisers (SVNs) with masks commonly provide aerosol therapy for infants with lung diseases. However, infants and toddlers are often disturbed by and thus reject masks. AIMS: To compare the lung deposition efficiency of the "usual" SVN aerosol mask and a prototype hood attached to an SVN. The advantage of the hood is that no mask is needed and medication can readily be administered during sleep. METHODS: 99mTc salbutamol solution was administered at random by SVN plus mask or hood to 14 wheezy infants (mean age 8 (SD 5) months). The dose and distribution of salbutamol were evaluated using gamma scintigraphy. Clinical response, tolerability by the infants, and parent preference were also compared. RESULTS: Mean total lung deposition was 2.6% with the hood and 2.4% with the mask (p > 0.05). Variability with the mask was greater than with the hood (coefficient of variation (CoV) 54% v 39%). Both treatments provided similar clinical benefit and side effects as reflected in improved oxygen saturation, reduced respiratory frequency, and increased heart rate. Infants accepted the hood better than the mask and there was a positive correlation between poor acceptance and upper airways and stomach deposition for both treatment modalities. Parents preferred the hood treatments. CONCLUSIONS: Aerosol therapy by hood is as efficient as by mask but provides a better therapeutic index. It is much better tolerated by infants and preferred by parents. Hood nebulisation is a simple and patient friendly mode of aerosol therapy in wheezy infants.

Administration, Inhalation↗
PubMedDate not supplied

Outer hair cell activity is not required for the generation of the forward masking curve.

Forward masking of the auditory brainstem response (ABR) was achieved by increasing the time interval from 0 to 12 ms between the masker offset and the probe onset. The forward masking response demonstrated a near linear function with an approximate 3.0-dB increase in masking threshold for every millisecond interval increase in the control guinea pig. The slope of the masking curve at selected frequencies together with the quantification of hair cell loss through the analysis of cochlear surface morphology was studied before and after chemical insult. The intracochlear infusion of sodium salicylate caused an approximately 45-dB threshold shift of the ABR whereas the slope of the forward masking curve was not significantly different from the control values at the tested frequencies (1, 4, and 8 kHz). Systemic kanamycin administration (400 mg/kg body weight for 9 consecutive days) caused a permanent ABR threshold shift of 43-63 dB at 1, 4, and 8 kHz. The slope of the forward masking curve was not significantly different at 1 kHz despite significant outer hair cell loss. The slope of the forward masking curve at 4 and 8 kHz showed significant reductions at the time intervals between 0 and 4 ms. Analysis of the kanamycin-treated cochleae revealed not only significant outer hair cell loss throughout the cochlea but significant inner hair cell and inner pillar cell loss in the basal end of the cochlea. The results suggest that the outer hair cells are not needed for maintaining a normal forward masking curve, whereas the slope of the forward masking curve is sensitive to alterations induced to either the inner hair cells or the inner pillar cells.

Animals↗
PubMedDate not supplied

Dichoptic visual masking reveals that early binocular neurons exhibit weak interocular suppression: implications for binocular vision and visual awareness.

Visual masking effects are illusions in which a target is rendered invisible by a mask, which can either overlap or not overlap the target spatially and/or temporally. These illusions provide a powerful tool to study visibility and consciousness, object grouping, brightness perception, and much more. As such, the physiological mechanisms underlying the perception of masking are critically important to our understanding of visibility. Several models that require cortical circuits have been proposed previously to explain the mysterious spatial and timing effects associated with visual masking. Here we describe single-unit physiological experiments from the awake monkey that show that visual masking occurs in at least two separate and independent circuits, one that is binocular and one that is monocular (possibly even subcortical), without feedback from higher-level visual brain areas. These and other results together fail to support models of masking that require circuits found only in the cortex, but support our proposed model that suggests that simple ubiquitous lateral inhibition may itself be the fundamental mechanism that explains visual masking across multiple levels in the brain. We also show that area V1 neurons are dichoptic in terms of excitation, but monoptic in terms of inhibition. That is, responses within area V1 binocular neurons reveal that excitation to monocular targets is inhibited strongly only by masks presented to the same eye, and not by masks presented to the opposite eye. These results lead us to redefine the model for the first stage of binocular processing in the visual system, and may be crucial to interpreting the effects of other similar binocular and dichoptic stimulation paradigms, such as the binocular rivalry family of illusions.

Action Potentials↗
PubMed1999

The neurophysiology of backward visual masking: information analysis.

Backward masking can potentially provide evidence of the time needed for visual processing, a fundamental constraint that must be incorporated into computational models of vision. Although backward masking has been extensively used psychophysically, there is little direct evidence for the effects of visual masking on neuronal responses. To investigate the effects of a backward masking paradigm on the responses of neurons in the temporal visual cortex, we have shown that the response of the neurons is interrupted by the mask. Under conditions when humans can just identify the stimulus, with stimulus onset asynchronies (SOA) of 20 msec, neurons in macaques respond to their best stimulus for approximately 30 msec. We now quantify the information that is available from the responses of single neurons under backward masking conditions when two to six faces were shown. We show that the information available is greatly decreased as the mask is brought closer to the stimulus. The decrease is more marked than the decrease in firing rate because it is the selective part of the firing that is especially attenuated by the mask, not the spontaneous firing, and also because the neuronal response is more variable at short SOAs. However, even at the shortest SOA of 20 msec, the information available is on average 0.1 bits. This compares to 0.3 bits with only the 16-msec target stimulus shown and a typical value for such neurons of 0.4 to 0.5 bits with a 500-msec stimulus. The results thus show that considerable information is available from neuronal responses even under backward masking conditions that allow the neurons to have their main response in 30 msec. This provides evidence for how rapid the processing of visual information is in a cortical area and provides a fundamental constraint for understanding how cortical information processing operates.

Animals↗
PubMed1979

Spatial frequency masking in human vision: binocular interactions.

Binocular contrast interactions in human vision were studied psychophysically. Thresholds were obtained for sinewave grating stimulation of the right eye in the presence of simultaneous masking gratings presented to the right eye (monocular masking) or left eye (dichoptic masking). In the first experiment, thresholds were measured at 0.25, 1.0, 4.0, and 16.0 cycle per degree (cpd) as a function of the contrast of masking gratings of identical frequency and phase. Thresholds rose nonmonotonically with masking contrast. At medium and high contrast levels, dichoptic masking was more effective in elevating contrast thresholds than monocular masking, and approached Weber's Law behavior. In the second experiment, spatial frequency tuning functions were obtained for test gratings at five spatial frequencies, by measuring threshold elevation as a function of the spatial frequency of constant-contrast masking gratings. At 1.0, 4.0, and 16.0 cpd, the tuning functions peaked at the test frequencies. The dichoptic tuning functions had a bandwidth of about 1 octave between half-maximum points, narrower than +/- 1 octave bandwidths of the monocular tuning functions. At 0.125 and 0.25 cpd, the tuning functions were broader and exhibited a shift in peak masking to frequencies above the test frequencies.

Adult↗
PubMed1980

Contrast masking in human vision.

Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.

Adult↗
PubMed1986

[Filter efficiency of commercial face masks in capturing particles and airborne bacteria].

The filter efficiency of seven kinds of commercial face mask for particles and airborne bacteria was tested in the wash room of a laboratory animal facility. The filter efficiency of the masks was 19 to 50%, as measured by the weight of particles with diameters below 10 micron, 22 to 71% for particles of the 0.3 micron level, 47 to 90% for the 1 micron level, and 90 to 99.6% for the 5 micron level. The filter efficiency for airborne bacteria was 35 to 81%. Among these even masks tested, glasswool surgery masks, three-sheet synthetic fiber masks with and without charcoal, and 28-sheet gauze masks with glass filter showed generally high efficiency, and single-sheet synthetic fiber masks, 18-sheet of gauze masks and gas masks showed low efficiency.

Animal Husbandry↗
PubMed1977

Backward recognition masking in relative pitch judgments.

Backward recognition masking refers to interference of a second masking tone with recognition of a target tone presented earlier in time. The degree of interference has been found to decrease as the length of the silent interval separating the two tones increases. These results have been interpreted as representing interference of the masking tone upon the preperceptual storage and perceptual resolution of the target. It is logically possible, however, that the masking tone does not interfere with perceptual resolution but interferes with comparison of the target to a long-term memory representation. The current research was designed to provide a critical test of this alternative hypothesis by modifying the backward recognition masking task. Subjects determined whether the masking tone was higher or lower in pitch than the target tone. The frequencies of the target and masking tones varied randomly across trials. This ensured that the task could not be performed by comparing the target to a representation in long-term memory. Nevertheless, masking was obtained in this task, arguing against the comparison argument and in favor of the perceptual resolution interpretation. Given that masking was obtained under both ipsilateral and contralateral presentation of the tones, the results argue for a central preperceptual auditory storage that holds information after the inputs from the two ears are combined in the auditory system.

Functional Laterality↗