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PubMed1981

Off-frequency listening: effects on psychoacoustical tuning curves obtained in simultaneous and forward masking.

Off-frequency listening--the use of information in different frequency regions to improve performance in masking tasks--has been reported to influence psychoacoustical tuning curves measured in simultaneous masking. The present experiment was designed to establish whether suppression has an effect on off-frequency listening. Psychoacoustical tuning curves were obtained from three observers in both simultaneous and forward masking. A probe level of 10 dB SL and narrowband noise maskers were used. After obtaining the conventional tuning curves, a fixed masker, at either 1.8 or 2.2 kHz and 10 dB below the corresponding tuning-curve threshold, was added to limit off-frequency listening. In both simultaneous and forward masking, the branch of the tuning curve opposite the fixed masker was shifted downwards, but to a greater extent in forward masking. The suppression of the probe in tuning curves measured in simultaneous masking appears to constrain off-frequency listening. In forward-masking tuning curves, off-frequency listening has a greater effect, and may explain to a large extent the very sharp tuning which has been found in some studies.

Auditory Threshold↗
PubMed1981

Some results on lateral suppression obtained in a partial-masking lateralization paradigm.

Results were reported of psychophysical forward-masking experiments using a lateralization method. A general interpretation of masking was given, considering masking to be the combined result of three different mechanisms: the overlap mechanism, the adaptation mechanism, and the suppression mechanism. The aim of this study was to demonstrate the use of the lateralization method in a masking experiment. Masking was measured in a band-widening experiment using a test tone frequency of 3 kHz, which is the center frequency of the masking noise. It was found that the effect of the suppression mechanism depends in a complex way on the difference between masker level and test tone level, as does the bandwidth at which maximum masking occurs. These level effects could be described qualitatively by means of nonlinear excitation patterns.

Acoustic Stimulation↗
PubMed1983

Growth of forward masking for sinusoidal and noise maskers as a function of signal delay; implications for suppression in noise.

The first two experiments were designed to determine whether mutual suppression in broadband noise increases in strength with increasing overall level. In experiment I masking functions (signal threshold versus masker level) were measured in forward masking as a function of the delay time of a 10-ms signal, both for a broadband noise masker (low-pass filtered at 8 kHz) and for sinusoidal maskers at 1, 2 and 4 kHz. In the latter case the signal frequency equaled the masker frequency. For short signal delays the masking functions were steeper for the sinusoidal masker than for the noise masker. At longer delays the slopes for both masker types decreased and the slopes for the two masker types became more nearly equal. In experiment II we investigated the effect of gating a low-level noise cue with the sinusoidal masker. At the longer signal delays the masking functions had equal slopes for the broadband noise masker and the sinusoidal masker with cue. At short signal delays the masking functions for sinusoidal maskers may be "artificially" steepened, since the subject lacks an effective cue to distinguish the signal from the masker. The equal slopes at longer delays indicate that mutual suppression of the components within a broadband noise does not increase in strength with increasing overall level. In experiment III we attempted to estimate the magnitude of mutual suppression in a broadband noise by comparing masking functions for a broadband noise and for a noise whose bandwidth was 20% of the center frequency. The suppression was estimated to be about 2 dB at 4 kHz and 8 dB at 2 kHz. A simple mathematical expression, suggested by Jesteadt et al. [J. Acoust. Soc. Am. 71, 950-962 (1982)], was found to give an accurate description of the amount of masking produced by the broadband masker as a function of masker level and signal delay.

Auditory Threshold↗
PubMed1986

Effects of forward and simultaneous masking on intensity discrimination.

Experiments on intensity discrimination determine the size of the smallest detectable increment added to a fixed pedestal. This paper examines the effects of a masker which either precedes the pedestal (forward masking) or is simultaneous with the pedestal. The increment and pedestal were 1-kHz tones masked in forward masking by pure tones and in simultaneous masking by a broadband noise. Simultaneous masking by the broadband noise eliminates the "near miss" to Weber's law, and thus degrades intensity discrimination at high pedestal levels. Forward masking by the pure tone also degrades intensity discrimination, which may, in part, be explained by the elimination of the near miss. However, the effect on intensity discrimination in some cases is greater in forward than in simultaneous masking, suggesting that some additional process (e.g., adaptation) is involved.

Adult↗
PubMed1986

Temporal effects in masking and their influence on psychophysical tuning curves.

Psychophysical tuning curves (PTCs) were obtained in simultaneous and forward masking for a 20-ms, 1000-Hz signal presented at 10 dB SL. The signal was presented at the beginning of, at the temporal center of, at the end of, or immediately following a 400-ms masker. The first experiment was done in quiet; the second experiment was done in the presence of two bands of noise on either side of 1000 Hz. The results were similar in quiet and in noise. In simultaneous masking, the PTCs were broadest for the signal at masker onset, and generally sharpest for the signal at temporal center; the differences were largest on the high-frequency side. In most cases, there was virtually no difference in Q10 between the forward-masking PTC and the simultaneous-masking PTC with the signal temporally centered, although the high-frequency slope was always steeper in forward masking. These results indicate that, at least for brief signals, frequency selectivity measured with simultaneous-masking PTCs and the degree of sharpening revealed in forward-masking PTCs depend upon the temporal position of the signal within the simultaneous masker.

Adult↗
PubMed1987

The effect of broadband noise on the human brain-stem auditory evoked response. IV. Additivity of forward-masking and rate-induced wave V latency shifts.

The additivity of forward masking and repetitive stimulation effects on wave V of the brain-stem auditory evoked response (BAER) was investigated. The effects of repetitive stimulation were evaluated for a stimulus train (called the adaptation series), with a 12.5-ms within-train interclick interval. The forward masker was a 100-ms, 80-dB SPL broadband noise with forward-masker intervals ranging from 12.5-87.5 ms. Forward masking and repetitive stimulation increased the latency of wave V of the BAER. The combined forward masking/adaptation series produced less wave V latency shift than the summed individual effects. Forward masking reduced wave V amplitude at brief forward masker intervals, while repetitive stimulation did not affect wave V amplitude. Wave V amplitude was decreased for the combined forward masking/adaptation series, and the time course of amplitude recovery of the combination was prolonged compared to the forward masking alone condition. The nonadditivity of forward masking and rate effects on wave V latency is similar to that found for repetitive stimulation and simultaneous masking [Burkard and Hecox, J. Acoust. Soc. Am. 74, 1204-1213 (1983)]. These findings are consistent with the position that forward masking and rate effects on wave V latency are produced by overlapping mechanisms.

Adult↗
PubMed1987

The temporal course of masking and the auditory filter shape.

Recent experiments have shown that frequency selectivity measured in tone-on-tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched-noise masker, threshold was measured for a 20-ms signal presented at the beginning, the temporal center, or the end of the 400-ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1-kHz signal frequency. Growth-of-masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched-noise experiment in terms of the level of a flat-spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high-frequency side, which was attributed to suppression.

Adult↗
PubMed1987

A release from masking by continuous, random, notched noise.

Thresholds for 10-ms sinusoids simultaneously masked by bursts of bandpass noise centered on the signal frequency were measured for a wide range of signal frequencies and noise levels. Thresholds were defined as the signal power relative to the masker power at the output of an auditory filter centered on the signal frequency. It was found that the presentation of a continuous random noise, with a spectral notch centered on the signal frequency, produced a reduction in signal thresholds of up to 11 dB. A notched noise spectrum level of 0-5 dB above that of the masker proved most effective in producing a masking release, as measured by a reduction in masked threshold. A release from masking of up to 7 dB could be obtained with a continuous bandpass noise. The most effective spectrum level of this noise was 5 dB below that of the masker. The effect of the continuous notched noise was to reduce signal-to-masker ratios at threshold to about 0 dB, regardless of the threshold in the absence of continuous noise. Thus the greatest release from masking occurred when "unreleased" thresholds were highest. The release from masking is almost complete within 320 ms of notched noise onset, and persists for about 160 ms after notched noise offset, regardless of notched noise level. The phenomenon is similar in many ways to the "overshoot" effect reported by Zwicker [J. Acoust. Soc. Am. 37, 653-663 (1965)]. It is argued that both effects can be largely attributed to peripheral short-term adaptation, a mechanism which is also believed to be involved in forward masking.

Auditory Fatigue↗
PubMed1988

Masked cochlear whole-nerve response intensity functions altered by electrical stimulation of the crossed olivocochlear bundle.

Cochlear whole-nerve response (CAP) intensity functions were recorded from the guinea pig round window. The intensity functions were obtained in the presence of masking noise, with electrical stimulation of the crossed olivocochlear bundle (COCB), and the combination of masking noise and COCB stimulation. Electrical stimulation of the COCB produced the expected reduction of CAP magnitude for low- to moderate-intensity tone bursts. Masking noise produced reductions in CAP magnitude over the whole signal intensity range used in this study. The combination of electrical stimulation of the COCB with the masking noise produced CAP magnitude changes graded between reduction and enhancement dependent on signal and masker levels. In general, at low signal levels, the masked CAP magnitude is reduced compared to the masked alone condition. At high signal levels, the masked CAP is increased in magnitude. These results confirm and extend the earlier observations of Nieder and Nieder [Exp. Neurol. 28, 174-188 (1970); also, Nature 227, 184-185 (1970)].

Animals↗
PubMed1988

Interpreting measures of frequency selectivity: is forward masking special?

In a previous article [Lutfi, J. Acoust. Soc. Am. 76, 1045-1050 (1984)], the following relation was used to predict measures of frequency selectivity obtained in forward masking from measures obtained in simultaneous masking: F(g) = G + H(g) - H(0), where, for a given masker level, F is the amount of forward masking (in dB) as a function of signal-masker frequency separation (g), H is the amount of simultaneous masking, and G is the amount of forward masking for g = 0. In the present study, the relation was tested for a wider range of signal and masker frequencies, masker levels, and signal delays. The relation described thresholds from all conditions well with the inclusion of one free parameter lambda corresponding to a constant frequency increment, F(g) = G + H(g + lambda) - H(lambda). The parameter lambda was required to account for observed shifts in the frequency of maximum forward masking. It is argued that a single tuning mechanism can account for commonly observed differences between simultaneous- and forward-masked measures of frequency selectivity.

Adult↗
PubMed1991

Binaural modulation masking.

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.

Acoustic Stimulation↗
PubMed1991

Free-field release from masking.

Free-field release from masking was studied as a function of the spatial separation of a signal and masker in a two-interval, forced-choice (2IFC) adaptive paradigm. The signal was a 250-ms train of clicks (100/s) generated by filtering 50-microseconds pulses with a TDH-49 speaker (0.9 to 9.0 kHz). The masker was continuous broadband (0.7 to 11 kHz) white noise presented at a level of 44 dBA measured at the position of the subject's head. In experiment I, masked and absolute thresholds were measured for 36 signal source locations (10 degree increments) along the horizontal plane as a function of seven masking source locations (30 degree increments). In experiment II, both absolute and masked thresholds were measured for seven signal locations along three vertical planes located at azimuthal rotations of 0 degrees (median vertical plane), 45 degrees, and 90 degrees. In experiment III, monaural absolute and masked thresholds were measured for various signal-masker configurations. Masking-level differences (MLDs) were computed relative to the condition where the signal and mask were in front of the subjects after using absolute thresholds to account for differences in the signal's sound-pressure level (SPL) due to direction. Maximum MLDs were 15 dB along the horizontal plane, 8 dB along the vertical, and 9 dB under monaural conditions.

Adult↗
PubMed1997

Forward-masked intensity discrimination: duration effects and spectral effects.

Three experiments were completed to examine the effect of masker duration and spectrum on forward-masked intensity discrimination. Four listeners participated in each experiment. Intensity discrimination was measured in quiet and in the presence of forward maskers using adaptive forced-choice procedures. The standard duration was either short (10 ms) or long (250 ms) in experiment 1 and short (10 ms) in experiment 2. The standard always occurred 100 ms after the offset of the masker. In the first experiment employing 1.0-kHz maskers and standards, a short duration masker (10 ms) produced more masking than a long duration masker (250 ms). A mid-level elevation of the Weber fraction was observed for all conditions. To ensure that the results of experiment 1 were not influenced by off-frequency listening, the second experiment employed a broadband noise masker. As before, a short duration (10 ms) masker produced more masking than a long duration masker (100 ms) and a mid-level elevation of Weber fractions was observed. This outcome is inconsistent with a peripheral sensory effect for which an increase in masker duration should result in a greater amount of adaptation, and, as a consequence more masking. A third experiment employing a broadband noise masker and standard showed the greatest amount of masking for low-level standards, but only when the duration of the masker and standard was short. This result is similar to one seen for a single listener in the first experiment for short duration tonal maskers and standards. For this listener, a second tone presented at 4.133 kHz presented simultaneously with the 1.0 kHz masker reduced significantly the amount of masking for low-level standards, but the mid-level elevation of the Weber fraction remained. Taken together, these results suggest that perceptual similarity plays a role in forward-masked intensity discrimination but does not account entirely for the mid-level elevation of the Weber fraction.

Adult↗
PubMed1998

Masking by sinusoidally amplitude-modulated tonal maskers.

In experiment 1, masking patterns were obtained with a tonal masker that was sinusoidally amplitude modulated (SAM) at a rate of 8 Hz and a depth (m) of 1.0. The signal was centered at a masker peak or masker valley. Masker frequency (fm) was 750, 1350, or 2430 Hz, and signal frequency (fs) ranged from 0.8 to 1.62 fm. Thresholds were generally higher for a signal in a masker peak than in a masker valley. The magnitude of this peak-to-valley (PV) difference was governed by fs/fm, rather than by fs, and was largest for fs > fm. The PV differences were smallest at the lowest fm, at least when fs > fm. In experiment 2, growth-of-masking functions were measured (fm = 1350 Hz, fs = 1.44fm). The masker was modulated at a depth (m) of 1.0, 0.75, or 0.50. These thresholds were compared with those obtained with an unmodulated masker in forward or simultaneous masking. The comparisons suggest that thresholds for a signal at a peak of an 8-Hz SAM masker are due to simultaneous masking, while those in a valley are due primarily to forward masking when m = 1.0 or simultaneous masking when m = 0.75 or 0.50. For these masker depths, the PV difference first increased but then decreased as masker level increased from 60 to 90 dB SPL. This was a consequence of the slope of the masking function for peak placement changing from a value greater than 2.0 to a value of 1.0 at the highest signal levels (an effect that was also observed with the unmodulated simultaneous masker), a result that may be understood in terms of basilar membrane nonlinearity.

Adult↗
PubMed1992

Should we judge a mask by its cover?

BACKGROUND: A table of the approximate ranges of inspired oxygen delivered at given oxygen flow rates is often given on the packaging of oxygen masks. A study was carried out to check the inspired oxygen concentration given by one of the new masks, which has been designed to be used with or without the Venturi attachment as a result of the proposal to use it without the Venturi attachment as a general purpose mask for emergency use. METHODS: Measurements were made at resting respiratory rate and 26 breaths/min in 12 normal subjects. Continuous oxygen and carbon dioxide concentrations were recorded at the lips with a mass spectrometer, and inspired oxygen concentrations were calculated from end tidal values by means of the alveolar gas equation. Measurements were made at oxygen flow rates of 2, 4, and 6 l/min for the mask alone and at 2 and 4 l/min with both the 24% and the 28% Venturi attachments. RESULTS: Without the Venturi attachment the mask gave average inspired oxygen concentrations 8-10% greater than are stated on the packaging at oxygen flow rates of 2, 4, and 6 l/min at resting respiratory rates of 8-20 breaths/min, some individuals receiving 30% more than expected. Addition of the interchangeable Venturi attachments designed to give 24% and 28% inspired oxygen delivered average concentrations within 2% of the expected concentrations, no individual receiving more than 5% above the expected concentrations. CONCLUSIONS: The labelling on the packaging of oxygen masks may lead to inappropriate use by those not expert in prescribing oxygen therapy. Caution is still needed when a single multipurpose mask is being selected for emergency use, where accurate delivery of low concentrations of oxygen is vital for some patients.

Adult↗
PubMed1979

Cutaneous masking. I. Psychophysical observations on interactions of multipoint stimuli in man.

1. Psychophysical masking of cutaneous sensation at the locus of punctate test stimulation has been quantitatively examined with phasic mechanical and brief air-pulse stimuli using a conditioning-test stimulus paradigm. 2. Masking was maximal at the minimal interstimulus distance effective with this paradigm, varies inversely with interstimulus distance, and is demonstrable with the conditioning and test stimuli up to 10 cm apart on the forearm. 3. The degree of masking was found to be a direct function of the relative intensity of the conditioning stimulus with respect to the test stimulus. 4. Variations in the interstimulus interval permitted an investigation of the temporal features of cutaneous masking. It was detectable from 10 ms before to 70 ms after conditioning stimulation. Maximum masking occurred when the test stimulus was delivered about 10 ms following conditioning stimulus onset. 5. We also noted the much less marked, but still significant, enhancement phenomenon, in which weak conditioning stimuli, at just-threshold intensity levels, lowered the detection threshold for sensation at the test stimulus locus. We found this enhancement of sensation to have the same spatial distribution as did masking, but a much reduced time course. It began with the test stimulus presented simultaneously with the conditioning stimulus, peaked with 10--15 ms interstimulus intervals, but decayed in less than 40 ms. 6. Since psychophysical experiments often form the framework for the understanding of physiologic processes, it is suggested that these behavioral determinations of enhancement and masking may be correlated with the electrophysiologic properties of excitation and inhibition in neurons of the major primary somatic pathways of the central nervous system.

Animals↗
PubMed1993

Antimasking effects of the olivocochlear reflex. I. Enhancement of compound action potentials to masked tones.

1. The effects of olivocochlear (OC) feedback on signal processing in the cochlea were studied by comparing responses seen with and without a contralateral noise or by comparing responses seen before and after cutting the OC bundle (OCB). Adding and subtracting a contralateral noise is a convenient, reversible way of changing the level of OC feedback; however, it fully reveals only the contribution of the contralaterally responsive efferent fibers. Cutting the OCB can reveal the full contribution of all fibers in the OCB; however, the manipulation can only be performed once per experiment. 2. The amplitude of the compound action potential (CAP), recorded from anesthetized or decerebrate cats in response to tone pips, could be increased by addition of contralateral noise at moderate sound pressure levels. These enhancement phenomena were most easily demonstrable when the tone pips were masked by ipsilateral broadband noise; however, in some animals CAP enhancement was seen in the absence of ipsilateral maskers. Enhancement-in-quiet may arise because of internal masking from animal-generated noise. All contralateral-noise enhancement disappeared when the OCB was cut. 3. Enhancement effects of contralateral noise could be seen in both simultaneous and forward-masking paradigms. Enhancement was largest for high-frequency tone pips (8-16 kHz) and could be demonstrated over a wide range of tone-pip levels and ipsilateral-masker levels. Suppression of CAP by the contralateral noise was often seen for lower tone-pip frequencies (2-8 kHz) and lower tone-pip intensities. These trends may be understood in the context of known properties of OC peripheral effects and known properties of physiological masking. 4. Cutting the OCB resulted in a decrease in CAP amplitudes to masked tone pips. When CAP was measured to tone pips presented in equilevel, binaural noise, OCB section resulted in a decrease in CAP amplitudes equivalent to at least a 6-dB decrease in signal-to-noise ratio. Such antimasking effects of an intact OCB were seen in both simultaneous and forward-masking paradigms. 5. Present evidence suggests that all these antimasking effects can be explained on the basis of activation of the medial OC fibers to the outer hair cells. By suppressing responses to continuous noise backgrounds, the OC reflex may enhance responses to transient masked stimuli by decreasing the level of adaptation in auditory nerve fibers. Such effects of the OC reflex should improve discrimination of transient signals presented in a continuous noise background.

Acoustic Stimulation↗
PubMed2000

Neural responses in the inferior colliculus to binaural masking level differences created by inverting the noise in one ear.

We have measured the responses of inferior colliculus neurons in the anesthetized guinea pig to signals which in human psychophysical experiments reveal a release of masking as a result of binaural processing (the binaural masking level difference: BMLD). More specifically we have used diotic tones at 500 Hz (So) masked by noise that is either identical at the two ears (No) or inverted in one ear (Npi). This combination of signals and noise maskers produces a prominent masking release in humans such that the So signal is about 6-12 dB more detectable in the presence of the Npi noise than the No noise. Low-frequency inferior colliculus neurons are sensitive to the interaural delay of the masking noise and generally respond most to the components nearest their best frequency. Since most inferior colliculus neurons have peaks in their delay functions close to zero interaural time delay this means that while No noise is effective in driving the unit, Npi noise is much less effective. As the level of an So tone was progressively increased in the presence of No and Npi noises, the first response could be either an increase or a decrease in the activity due to the noise. However, because Npi generated little or no activity itself, the predominant response to the So tone was an increase in discharge in this condition. Masked thresholds were defined as the point at which the standard separation D (related to the d' of signal detection theory) = 1 in either direction. BMLDs were measured in single neurons and in the majority of units were in a direction consistent with the psychophysical observations irrespective of the direction of the discharge rate change that occurred at threshold. The lowest masked thresholds always occurred at or near the signal frequency of 500 Hz. An average value of the single unit BMLD around 500 Hz was 3.6 dB (NoSo vs. NpiSo) compared with 6.6 dB for the NoSo versus NoSpi BMLD we had previously reported. This lower magnitude is consistent with the hierarchy of human psychophysical BMLDs.

Acoustic Stimulation↗