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PubMed2003

Auditory masking in three pinnipeds: aerial critical ratios and direct critical bandwidth measurements.

This study expands the limited understanding of pinniped aerial auditory masking and includes measurements at some of the relatively low frequencies predominant in many pinniped vocalizations. Behavioral techniques were used to obtain aerial critical ratios (CRs) within a hemianechoic chamber for a northern elephant seal (Mirounga angustirostris), a harbor seal (Phoca vitulina), and a California sea lion (Zalophus californianus). Simultaneous, octave-band noise maskers centered at seven test frequencies (0.2-8.0 kHz) were used to determine aerial CRs. Narrower and variable bandwidth masking noise was also used in order to obtain direct critical bandwidths (CBWs). The aerial CRs are very similar in magnitude and in frequency-specific differences (increasing gradually with test frequency) to underwater CRs for these subjects, demonstrating that pinniped cochlear processes are similar both in air and water. While, like most mammals, these pinniped subjects apparently lack specialization for enhanced detection of specific frequencies over masking noise, they consistently detect signals across a wide range of frequencies at relatively low signal-to-noise ratios. Direct CBWs are 3.2 to 14.2 times wider than estimated based on aerial CRs. The combined masking data are significant in terms of assessing aerial anthropogenic noise impacts, effective aerial communicative ranges, and amphibious aspects of pinniped cochlear mechanics.

Acoustic Stimulation↗
PubMed2004

A positron emission tomography study of the neural basis of informational and energetic masking effects in speech perception.

Positron emission tomography (PET) was used to investigate the neural basis of the comprehension of speech in unmodulated noise ("energetic" masking, dominated by effects at the auditory periphery), and when presented with another speaker ("informational" masking, dominated by more central effects). Each type of signal was presented at four different signal-to-noise ratios (SNRs) (+3, 0, -3, -6 dB for the speech-in-speech, +6, +3, 0, -3 dB for the speech-in-noise), with listeners instructed to listen for meaning to the target speaker. Consistent with behavioral studies, there was SNR-dependent activation associated with the comprehension of speech in noise, with no SNR-dependent activity for the comprehension of speech-in-speech (at low or negative SNRs). There was, in addition, activation in bilateral superior temporal gyri which was associated with the informational masking condition. The extent to which this activation of classical "speech" areas of the temporal lobes might delineate the neural basis of the informational masking is considered, as is the relationship of these findings to the interfering effects of unattended speech and sound on more explicit working memory tasks. This study is a novel demonstration of candidate neural systems involved in the perception of speech in noisy environments, and of the processing of multiple speakers in the dorso-lateral temporal lobes.

Acoustic Stimulation↗
PubMed2005

Combining energetic and informational masking for speech identification.

This study examined combinations of energetic and informational maskers in speech identification. Speech targets and maskers (speech or noise) were processed and filtered into sets of 15 narrow frequency bands. The target was the sum of eight randomly selected bands. More masking occurred for speech maskers than for spectrally matched noise maskers regardless of whether the masker bands overlapped the target bands. The greater effect of the speech maskers was interpreted as due to informational masking. When the masker was comprised of nonoverlapping bands of speech, the addition of bands of noise overlapping the speech masker, but not the speech target, reduced the overall amount of masking. Surprisingly, presenting the noise to the ear contralateral to the target and masker produced an even greater release from masking. The contralateral noise was apparently sufficient to cause a slight change in the image of the ipsilateral speech masker, possibly pulling it away from the target enough to allow the focus of attention on the target. This finding is consistent with the interpretation that in some conditions small binaural differences may be sufficient to cause, or significantly strengthen, the perceptual segregation of sounds.

Acoustic Stimulation↗
PubMed2006

Effects of reverberation and masking on speech intelligibility in cochlear implant simulations.

Two experiments investigated the impact of reverberation and masking on speech understanding using cochlear implant (CI) simulations. Experiment 1 tested sentence recognition in quiet. Stimuli were processed with reverberation simulation (T=0.425, 0.266, 0.152, and 0.0 s) and then either processed with vocoding (6, 12, or 24 channels) or were subjected to no further processing. Reverberation alone had only a small impact on perception when as few as 12 channels of information were available. However, when the processing was limited to 6 channels, perception was extremely vulnerable to the effects of reverberation. In experiment 2, subjects listened to reverberated sentences, through 6- and 12-channel processors, in the presence of either speech-spectrum noise (SSN) or two-talker babble (TTB) at various target-to-masker ratios. The combined impact of reverberation and masking was profound, although there was no interaction between the two effects. This differs from results obtained in subjects listening to unprocessed speech where interactions between reverberation and masking have been shown to exist. A speech transmission index (STI) analysis indicated a reasonably good prediction of speech recognition performance. Unlike previous investigations, the SSN and TTB maskers produced equivalent results, raising questions about the role of informational masking in CI processed speech.

Acoustics↗
PubMed1980

Low-frequency hearing loss: perception of filtered speech, psychophysical tuning curves, and masking.

Four subjects with low-frequency hearing loss were evaluated to determine whether their responses to low-frequency stimulation might be the result of stimulation of nerve fibers with higher characteristic frequencies. Two masking paradigms were employed to indirectly investigate the contribution of high-frequency nerve fibers to the detection of low-frequency stimuli: (1) masking of a low-level, fixed-frequency probe by a variable-frequency pure-tone masker (psychophysical tuning curve) and (2) masking of pure tones by a high-level, fixed-frequency pure-tone masker. Low-frequency remote masking by tones and displacements in the tips of tuning curves to higher frequencies were interpreted as evidence that low-frequency signals near threshold were being detected by high-frequency fibers in three of the subjects. Three subjects were also tested with high-pass, low-pass, and unfiltered speech both in quiet and in the presence of a high-pass noise masker. Results were interpreted as showing a relatively small contribution of high-frequency fibers to the perception of low-frequency speech.

Adolescent↗
PubMed1981

Relation between pitch shifts and MMF shifts in forward masking.

Thresholds for brief sinusoidal signals were determined as a function of signal frequency for fixed sinusoidal maskers in a forward masking task. In some cases the frequency at which maximum masking occurred (MMF) was slightly above or below the masker frequency. In a second experiment subjects were required to adjust the frequency of the signals that its pitch matched that of the "masker." It was found that the frequency required for equal pitch correspond closely to the MMF. A possible explanation is that differences in quality between signal and masker (such as pitch differences) provide a cue for detection in forward masking which is not available when the signal and masker are very similar in quality. Thus maximum masking occurs when the signal and masker are most similar in quality.

Acoustics↗
PubMed1982

Frequency discrimination at 1200 Hz in the presence of high-frequency masking noise.

To evaluate the possible contribution of high-frequency hearing to frequency discrimination at lower frequencies, pure-tone frequency discrimination tasks at 1200 Hz, with and without high-frequency masking noise, were performed by two highly practiced normal-hearing listeners. Signal levels ranged from 10 to 80 dB SPL. Spectra of the three high-frequency masking noises ranged from 1.8 to 2.1 kHz, from 2.8 to 3.1 kHz, and from 4 to 8 kHz. The frequency difference limen (DLF) intensity functions, describing DLFs as a function of signal level which were obtained in the presence of high-frequency masking noise, were essentially the same as the DLF intensity functions obtained without masking noise. These results indicate that high-frequency hearing is not necessary for acute frequency discriminations at low and at middle frequencies.

Adult↗
PubMed1983

Upward shifts in the masking pattern with increasing masker intensity.

Masking patterns obtained with forward-masking paradigms and relatively intense maskers sometimes have their peaks at the masker frequency and sometimes at a frequency well above it. Here it is shown that which outcome is obtained depends upon certain temporal parameters of the procedure. Specifically, the masking pattern for a 2000-Hz tone showed a gradual shift toward higher frequencies as masker intensity was increased from 65 to 95 dB SPL when long signals (about 50 ms) and long masker-to-signal intervals (about 50 ms) were used, but the effect was absent or smaller when the signals and intervals were short. This shift did not occur with a 750-Hz masker. Upward shifts in the masking pattern with increasing masker intensity are in accord with the view that the peak of displacement of the traveling-wave envelope migrates basally with increasing intensity--an idea that has frequently been suggested as an explanation of the so-called half-octave shift so routinely seen in auditory fatigue experiments.

Acoustic Stimulation↗
PubMed1984

Dependence of post-masking on masker duration and its relation to temporal effects in loudness.

Temporal masking of tones by noise was investigated using a post-(forward-) masking paradigm. The masker level and duration were varied. For every masker level employed, the rate of decay of masking was found to depend on the duration of the masker. Specific loudness values were computed from the forward masking functions and an electronic device which simulates these loudness functions is presented. In the simulation, a series of integrators or a single integration with one long time constant was utilized. The effect of the proposed integrators on loudness meters is discussed.

Adult↗
PubMed1984

Sinusoidal and noise maskers in simultaneous and forward masking.

We compare psychophysical tuning curves obtained with sinusoidal and narrow-band (50-Hz wide) noise maskers in both simultaneous and forward masking. In one experiment, we examine the effects of different combinations of duration and intensity of the 1-kHz sinusoidal signal. In a second experiment, we compare tuning curves obtained with a sinusoidal signal to those obtained with a noise signal. In both experiments, a narrow-band noise is a more effective simultaneous masker than a sinusoid for masker frequencies near the signal frequency. We argue that this is probably due to the use of different detection cues in the presence of sinusoidal and noise maskers, and that the greater masking produced by the noise is not simply due to its greater variability. As observed in other studies, tuning curves are narrower in forward masking than in simultaneous masking.

Auditory Perception↗
PubMed1985

Pitch shift of pure and complex tones induced by masking noise.

Psychoacoustic experiments were performed to measure the pitch-shift effects of pure and complex tones resulting from the addition of a masking noise to the tonal stimuli. Harmonic residue tones with either two or three harmonics and a fundamental frequency of 200 Hz were chosen as test tones. The pitch shifts of virtual and spectral pitches of the residue tones were measured as a function of the intensity of a low-pass noise with 600-Hz cutoff frequency. The SPL of this noise varied between 30 and 70 dB. In another experiment, the pitch shifts of single pure tones corresponding to the frequencies and SPLs of the harmonics of the residue tones were measured using the same masking noise. The results from five subjects for the harmonic residue tones show only a weak dependence of pitch shift on masking noise intensity. This dependence exists for both spectral and virtual pitches. In the case of single pure tones, pitch shift depends more distinctly on noise intensity. Pitch shifts of up to 5% were found in the range of noise intensity investigated. The magnitude of pitch shift shows pronounced interindividual differences, but the direction of the shift effect is always the same. In all cases pitch increases with higher masking noise levels.

Adult↗
PubMed1985

Auditory brain stem responses from human infants: pure-tone masking profiles for clicks and filtered clicks.

The effects of simultaneous pure-tone maskers on ABR wave V latency and amplitude were examined in three-month-old infants as a means of delineating the frequency specificity of these responses in the immature auditory system. Masking profiles at two intensities (60 and 40 dBn HL) were obtained for click, as well as 4000- and 1000-Hz filtered-click stimuli. Infant profiles, obtained by measuring both latency and amplitude shifts as a result of the discrete-frequency maskers, were compared to adult data obtained under an identical masking paradigm. Both latency and amplitude analyses showed masking profiles for infants which reveal greater low-frequency contribution to responses than found in adult profiles. Additionally, the infant profiles reveal clear differences in the degree of high-frequency spread of masking when comparisons are made to the adult data.

Auditory Perception↗
PubMed1987

Recognition of nonsense syllables by hearing-impaired listeners and by noise-masked normal hearers.

In the present study, speech-recognition performance was measured in four hearing-impaired subjects and twelve normal hearers. The normal hearers were divided into four groups of three subjects each. Speech-recognition testing for the normal hearers was accomplished in a background of spectrally shaped noise in which the noise was shaped to produce masked thresholds identical to the quiet thresholds of one of the hearing-impaired subjects. The question addressed in this study is whether normal hearers with a hearing loss simulated through a shaped masking noise demonstrate speech-recognition difficulties similar to those of listeners with actual hearing impairment. Regarding overall percent-correct scores, the results indicated that two of the four hearing-impaired subjects performed better than their corresponding subgroup of noise-masked normal hearers, whereas the other two impaired listeners performed like the noise-masked normal listeners. A gross analysis of the types of errors made suggested that subjects with actual and simulated losses frequently made different types of errors.

Adolescent↗
PubMed1987

Binaural versus monaural loudness: supersummation of tone partially masked by noise.

A series of three experiments used the method of magnitude estimation to examine binaural summation of the loudness of a 1000-Hz tone heard in the quiet and against various backgrounds of masking noise. In the quiet, binaural loudness as measured in sones, is twice monaural loudness. Two conditions of noise masking acted to increase the ratio of binaural/monaural loudness in sones above 2:1--that is, to produce supersummation. (1) When tone was presented to both ears, but masking noise to just one ear (dichotic stimulation), the loudness of the binaural tone was 30%-35% greater than the sum of the loudness of the monaural components. This increase in summation provides a suprathreshold analog to increases in threshold sensitivity observed with dichotic stimulation (masking-level differences). (2) Supersummation was also evident when tone and noise alike were presented to both ears (diotic stimulation); here, the binaural tone's loudness was 10%-25% greater than the sum of the monaural components. The increase in summation with diotic stimulation may be related to the characteristics of binaural summation of the noise masker itself.

Dominance, Cerebral↗
PubMed1988

Combined monaural and binaural masking release.

Stimulus conditions were examined where both across-frequency [comodulation masking release (CMR)] and across-ear [binaural masking-level difference (BMLD)] cues were available, as well as conditions where only one of these cue types was available. The main goal of the study was to determine how the two types of cues combine. The effects of comodulation were assessed either by modulating a masking noise and manipulating its bandwidth (experiment 1) or by using two comodulated narrow bands of noise separated in frequency (experiment 2). The masker was always No, and the 500-Hz pure-tone signal was either So or S pi. The effect of the frequency of modulation was examined either by changing the frequency of the modulating stimulus (experiment 1) or by changing the bandwidth of the comodulated narrow-band noise (experiment 2). Four of six subjects showed greater masking release when both BMLD and CMR cues were available than for either type of cue alone, whereas the other two subjects did not show an ability to combine the two cues for additional advantage. For the subjects who were able to combine the two types of cue, the additional advantage was greater for low frequencies of modulation. The results indicate that one component of CMR may be based upon across-frequency envelope comparisons at a stage of processing after binaural analysis.

Adolescent↗
PubMed1988

A reexamination of forward masking in the auditory nerve.

Forward masking, as measured behaviorally, is defined as an increase in a signal's detection threshold resulting from a preceding masker. Previously, forward masking in the auditory nerve has been measured as a reduction in the neural response to a signal when preceded by a masker. However, detection threshold depends on both the magnitude of the response to the signal and the variance of the response. Thus changes in detectability cannot be inferred from response reduction alone. Relkin and Pelli (1987) have described a two-interval forced-choice procedure that may be used to measure the threshold for the detection of a probe signal in recordings of spike counts in single auditory neurons. These methods have been used to study the forward masking of characteristic frequency probe tones by characteristic frequency maskers as masker intensity was varied. Although the masker does reduce the detectability of the probe tone, it was found that the threshold shifts are much less than those observed behaviorally, particularly for intense maskers. In part, the small threshold shifts can be attributed to the reduction in response variance following the masker, which is the result of the adaptation of spontaneous activity. These results imply that behavioral forward masking must result from suboptimal processing of spike counts from auditory neurons at a location central to the auditory nerve.

Animals↗
PubMed1989

Models of the additivity of masking.

Three models of masking additivity are reviewed, which are referred to as the high-compression model [M. J. Penner, J. Acoust. Soc. Am. 67, 608-616 (1980); M. J. Penner and R. M. Shiffrin, J. Acoust. Soc. Am. 67, 617-627 (1980)], the power-law model [R. A. Lutfi, J. Acoust. Soc. Am. 73, 262-267 (1983); 80, 422-428 (1986)], and the modified power-law model with compressed internal noise [Humes et al., J. Acoust. Soc. Am. 83, 188-202 (1988)]. While the high-compression model was derived from data for two or more nonsimultaneous maskers and the power-law model was derived from data for two or more simultaneous maskers, the modified power-law model can be applied to both cases. The modified power-law model assumes that the threshold in quiet is equivalent to a masked threshold resulting from an internal noise that is continually present. Additional assumptions concern the interaction of two maskers prior to the addition of the masking effects. Most of the data on the additivity of masking are well described by the modified power-law model, regardless of the nature of the maskers. Thus the model provides a good description of data for combined simultaneous maskers and combined nonsimultaneous maskers, a task heretofore requiring the use of at least two separate and independently developed models.

Auditory Threshold↗