Effects of low-pass noise masking on auditory event-related potentials to speech

OBJECTIVE: This study investigated the effects of decreased audibility in low-frequency spectral regions, produced by low-pass noise masking, on cortical event-related potentials (ERPs) to the speech sounds /ba/ and /da/. DESIGN: The speech sounds were presented to normal-hearing adults (N = 10) at 65- and 80-dB peak-to-peak equivalent SPL while they were engaged in an active condition (pressing a button to deviant sounds) and a passive condition (ignoring the stimuli and reading a book). Broadband masking noise was simultaneously presented at an intensity sufficient to mask the response to the 65-dB speech sounds and subsequently low-pass filtered. The conditions were quiet (no masking), low-pass noise cutoff frequencies of 250, 500, 1000, 2000, and 4000 Hz, and broadband noise. RESULTS: As the cutoff frequency of the low-pass noise masker was raised, ERP latencies increased and amplitudes decreased. The low-pass noise affected N1 differently than the other ERP or behavioral measures, particularly for responses to 80-dB speech stimuli. N1 showed a smaller decrease in amplitude and a smaller increase in latency compared with the other measures. Further, the cutoff frequency where changes first occurred was different for N1. For 80-dB stimuli, N1 amplitudes showed significant changes when the low-pass noise masker cutoff was raised to 4000 Hz. In contrast, d', MMN, N2, and P3 amplitudes did not change significantly until the low-pass noise masker was raised to 2000 Hz. N1 latencies showed significant changes when the low-pass noise masker was raised to 1000 Hz, whereas RT, MMN, N2, and P3 latencies did not change significantly until the low-pass noise masker was raised to 2000 Hz. No significant differences in response amplitudes were seen across the hemispheres (electrode sites C3M versus C4M) in quiet, or in masking noise. CONCLUSIONS: These results indicate that decreased audibility, resulting from the masking, affects N1 in a differential manner compared with MMN, N2, P3, and behavioral measures. N1 indexes the presence of audible stimulus energy, being present when speech sounds are audible, whether or not they are discriminable. MMN indexes stimulus discrimination at a pre-attentive level. It was present only when behavioral measures indicated the ability to differentiate the speech sounds. N2 and P3 also were present only when the speech sounds were behaviorally discriminated. N2 and P3 index stimulus discrimination at a conscious level. These cortical ERP in low-pass noise studies provide insight into the changes in brain processes and behavioral performance that occur when audibility is reduced, such as with low frequency hearing loss.     

The effects of masking on the activation of auditory-associated cortex during speech listening in white noise

CONCLUSIONS: Noise-induced masking has different effects on the two hemispheres during speech listening. Auditory-associated cortices in the left hemisphere were more affected by masking than the right side. However, activation of primary and secondary auditory cortices was not affected in both sides under the masking with high signal to noise ratio. OBJECTIVES: The purpose of this study was to investigate the effects of masking on the central auditory system during speech listening in white noise. MATERIALS AND METHODS: Twelve healthy young subjects with normal hearing participated in this study. Functional magnetic resonance imaging (fMRI) was performed while subjects were listening to speech sounds alone and speech plus white noise binaurally. RESULTS: In humans, the activation of several regions including the middle parts of the superior and middle temporal gyri, parahippocampal gyrus, cuneus and thalamus of the left hemisphere was significantly reduced under the masking paradigm with +5 dB signal to noise ratio. In addition, reduced activation was also found at the lingual gyrus, anterior and middle parts of the superior temporal gyrus (STG), uncus, fusiform gyrus, and inferior frontal gyrus of the right hemisphere during masking.     

Interactive effects of high-pass filtering and masking noise on word recognition

OBJECTIVES: This study sought to examine the word recognition performance in noise of individuals with a simulated low-frequency hearing loss. The goal was to understand how low-frequency hearing impairment affects performance on tasks that challenge temporal processing skills. METHODS: Twenty-two normal-hearing young adults participated. Monosyllabic words were presented in continuous and interrupted noise at 3 signal-to-noise ratios of -10, 0, and +10 dB. High-pass filtering of the stimuli at 3 different cutoff frequencies (ie, 1,000, 1,250, and 1,500 Hz) simulated the low-frequency hearing impairment. RESULTS: In general, performance decreased with increasing cutoff frequency, was higher for more favorable signal-to-noise ratios, and was superior in the interrupted condition relative to the continuous noise condition. One important revelation was that the magnitude of the performance superiority observed in the interrupted noise condition did not diminish with high-pass filtering; ie, the release from masking in interrupted noise was preserved. CONCLUSIONS: The results of the present study complement previous findings in which this paradigm was used with low-pass filtering to simulate a high-frequency hearing loss. That is to say, low-frequency hearing channels are inherently poorer than high-frequency channels in temporal resolution.     

Effects of noise and filtering on the intelligibility of speech produced during simultaneous communication

This study investigated the effects of noise and filtering on the intelligibility of speech produced during simultaneous communication (SC). Four normal hearing, experienced sign language users were recorded under SC and speech alone (SA) conditions speaking Boothroyd’s forced-choice phonetic contrast material designed for measurement of speech intelligibility. Twenty-four normal hearing listeners audited the speech samples produced by the four speakers under the SC and SA conditions, three listeners in noise and three listeners in filtered listening conditions for each of the four speakers. Although results indicated longer sentence durations for SC than SA, the data showed no difference in the intelligibility of speech produced during SC versus speech produced during SA under either the noise or filtered listening condition, nor any difference in pattern of phonetic contrast recognition errors between the SA and SC speech samples in either listening condition. This conclusion is consistent with previous research indicating that temporal alterations produced by SC do not produce degradation of temporal or spectral cues to speech intelligibility or disruption of the perception of specific English phoneme segments.

Learning outcomes

As a result of this activity, the participant will be able to (1) describe simultaneous communication; (2) explain the role of simultaneous communication in communication with children who are deaf; (3) discuss methods of measuring speech intelligibility under filtered and noise conditions; and (4) specify the ability of listeners to perceive speech produced during simultaneous communication under noise and filtered listening conditions.     

Binaural masking level difference for speech signals in noise

This study investigates the binaural masking level difference in both detection and recognition of speech embedded in noise when the signal is phase-shifted 180 degrees between the left and the right ear. Swedish spondaic words masked by noise werc presented binaurally to 10 normally hearing subjects. The results are presented in terms of speech detection threshold, speech recognition threshold, a psychometric function of speech recognition versus signal-to-noise ratio, and speech intelligibility as a function of binaural intelligibility level difference. Both speech detection threshold and speech intelligibility improved when the speech signal was phase-shifted 180 degrees. The binaural intelligibility level difference ranged from 5.7 to 7.7 dB.     

Reverberation, masking, filtering, and level effects on speech recognition performance

The purpose of this investigation was to describe the interactive effects of four signal modifications typically encountered in everyday communication settings. These modifications included reverberation, masking, filtering, and fluctuation in speech intensity. The relationship between recognition performance and spectral changes to the speech signal due to the presence of these signal alterations was also studied. The interactive effects of these modifications were evaluated by obtaining indices of nonsense syllable recognition ability from normally hearing listeners for systematically varied combinations of the four signal parameters. The results of this study were in agreement with previous studies concerned with the effect of these variables in isolation on speech recognition ability. When present in combination, the direction of each variable's effect on recognition performance is maintained; however, the magnitude of the effect increases. The results of this investigation are reasonably accounted for a spectral theory of speech recognition.     

Effect of venting the ear mold on speech discrimination in masking noise]

In this study the importance of in situ measurement during the fitting of a hearing aid is clearly emphasized. Reliable evaluation of the real ear gain can be achieved only with this method, and hence the assessment of the advantages of earmold modifications on the hearing aid. In particular, the gain can be evaluated by the person fitting the aid. The insertion of a vent into an earmold raises the listening comfort of the hearing-impaired person. The speech intelligibility in background noise may also improve. We analysed the effect of earmold venting on speech intelligibility under different background noise conditions. We found that venting improves the speech intelligibility, especially in background noise simulating modulated speech. Our example clearly demonstrates the importance of an exact control of the hearing aid fitting by the physician. In one case a vent ended at the ear canal wall so that no improvement of hearing comfort could be expected. A new earmold was made and the effect on insertion gain was demonstrated when enlarging the vent step by step.     

Effects of reverberation and filtering on speech rate judgment

This study investigated the effects of listening condition on speech rate judgment. Four listening conditions, in which a single sentence was presented at 21 speech rates ranging from 90 wpm to 250 wpm, were incorporated. These conditions included non-degraded, reverberation, band-pass filtered, and low-pass filtered conditions, each of which was selected to simulate listening conditions one might encounter in daily life. The participants were 20 young adults (20 to 40 years) with normal hearing. They were asked to make judgments of the rates of speech randomly presented in the four listening conditions using an equal-interval 5-step scale from too slow through too fast. Overall, speech rate was judged to be faster in the reverberant condition than in the other three conditions. These findings may have implications for auditory rehabilitation and counseling.     

Effects of reverberation and filtering on speech rate judgment

This study investigated the effects of listening condition on speech rate judgment. Four listening conditions, in which a single sentence was presented at 21 speech rates ranging from 90 WPM to 250 WPM, were incorporated. These conditions included non-degraded, reverberation, band-pass filtered, and low-pass filtered conditions, each of which was selected to simulate listening conditions one might encounter in daily life. The participants were 20 young adults (20 to 40 years) with normal hearing. They were asked to make judgments of the rates of speech randomly presented in the four listening conditions using an equal-interval 5-step scale from too slow through too fast. Overall, speech rate was judged to be faster in the reverberant condition than in the other three conditions. These findings may have implications for auditory rehabilitation and counseling.     

Acoustic hemifields in the spatial release from masking of speech by noise

The Hearing-in-Noise Test (HINT) is able to measure the benefit to speech intelligibility in noise conferred when the noise masker is displaced 90 degrees in eccentricity from a speech source located at zero degrees azimuth. Both psychoacoustic and neurophysiological data suggest that the perceptual benefit of the 90-degree azimuth separation would be greatest if the speech and noise were presented in different acoustic hemifields, and would be smallest if the two sources were in the same acoustic hemifield. The present study tested this hypothesis directly in ten normal-hearing adult listeners. Using the HINT stimuli, we confirmed the hypothesis. Release from masking scores averaged 8.61 dB for "between-hemifield" conditions, 6.05 dB for HINT conditions, and 1.27 dB for "within-hemifield" conditions, even though all stimulus configurations retained a 90-degree angular separation of speech and noise. These data indicate that absolute separation of speech and noise alone is insufficient to guarantee a significant release from masking, and they suggest that what matters is the location of the stimulus elements relative to the left and right spatial perceptual channels.     

Babble and random-noise masking of speech in high and low context cue conditions

"Perceptual" masking of speech by multitalker speech (babble) has been widely reported but poorly quantified. Furthermore, the validity of the construct of perceptual masking is questionable. This report describes an experiment using a newly standardized test of speech perception in noise (SPIN) with both babble and spectrally matched random-noise maskers. Classical psychophysical ogive curves were used to model speech recognition as a function of signal-to-noise ratio (S/N). The two maskers yielded speech recognition functions of the same steepness but different locations on the S/N axis. The high-context items of SPIN yielded speech recognition curves with steeper slope and different locations on the S/N axis than the low-context items. These data are used to argue that perceptual masking was not documented (under certain assumptions) and that the superior masking of babble may be explained in purely acoustical terms. Speculations are offered about the possible acoustical differences that could be responsible for the differences in masking effect.     

The effects of noise masking and required accuracy on speech errors, disfluencies, and self-repairs.

The covert repair hypothesis views disfluencies as by-products of covert self-repairs applied to internal speech errors. To test this hypothesis we examined effects of noise masking and accuracy emphasis on speech error, disfluency, and self-repair rates. Noise reduced the numbers of disfluencies and self-repairs but did not affect speech error rates significantly. With accuracy emphasis, speech error rates decreased considerably, but disfluency and self-repair rates did not. With respect to these findings, it is argued that subjects monitor errors with less scrutiny under noise and when accuracy of speaking is unimportant. Consequently, covert and overt repair tendencies drop, a fact that is reflected by changes in disfluency and self-repair rates relative to speech error rates. Self-repair occurrence may be additionally reduced under noise because the information available for error detection--that is, the auditory signal--has also decreased. A qualitative analysis of self-repair patterns revealed that phonemic errors were usually repaired immediately after their intrusion.     

Intelligibility characteristics of superior esophageal speech presented under various levels of masking noise.

Broad-band masking of speech was used to assess the effects that broad-band masking noise had upon the recognition of consonants and vowels produced by esophageal speakers. Procedures were developed to compare the articulation functions of superior esophageal speech with those of normal speech under comparable levels of masking noise. Within the range of speech-to-noise ratios studied, articulation functions for vowels were essentially the same for esophageal and normal talkers (4% per dB). With respect to consonants, the intelligibility scores for esophageal speech were 12 to 14% lower than for normal speech under adverse noise conditions. Gains in the consonant articulation functions were 2.5%/dB and 4%/dB for normal and esophageal talkers, respectively. For adverse noise conditions, the lowered consonant scores for esophageal speakers were the result of poorer than normal intelligibility for liquid-glides and nasal and, secondarily, for stop consonants. Additional differences between the intelligibility characteristics of esophageal and normal speech were found in work-position and voicing features.     

Most comfortable loudness for pure tones and speech in the presence of masking noise.

Pure-tone and speech most-comfortable-loudness (MCL) levels were determined in noise and quiet. A Bekesy-type tracking task was used to determine the MCLs under all conditions. The subjects (N = 12) tracked their MCLs for five frequencies (250, 500, 1000, 20008, and 4000 Hz) in the presence of four white-noise levels (55, 65, 75, and 85 dB SPL) and in quiet. Speech MCLs were tracked under the identical noise conditions. The results indicated that the mean MCLs for tones between 500 and 4000 Hz, and for the speech stimuli, systematically increased as a function of the masking level. The rate of this increase was 7 dB per 10-dB noise increment for both stimulus modes when the level of the noise exceeded 65 dB SPL. At levels below 65 dB SPL, however, pure-tone MCL increases were only about 3 dB per 10-dB noise increment, while the slope of the speech MCL function remained relatively unchanged. The variability of MCL judgments for both stimulus modes decreased as a function of increased masking.     

Effects of noise and noise suppression on speech perception by cochlear implant users.

The recognition of phonemes in consonant-vowel-consonant words, presented in speech-shaped random noise, was measured as a function of signal to noise ratio (S/N) in 10 normally hearing adults and 10 successful adult users of the Nucleus cochlear implant. Optimal scores (measured at a S/N of +25 dB) were 98% for the average normal subject and 42% for the average implantee. Phoneme recognition threshold was defined as the S/N at which the phoneme recognition score fell to 50% of its optimal value. This threshold was -2 dB for the average normal subject and +9 dB for the average implantee. Application of a digital noise suppression algorithm (INTEL) to the mixed speech plus noise signal had no effect on the optimal phoneme recognition score of either group or on the phoneme recognition threshold of the normal group. It did, however, improve the phoneme recognition threshold of the implant group by an average of 4 to 5 dB. These findings illustrate the noise susceptibility of Nucleus cochlear implant users and suggest that single-channel digital noise reduction techniques may offer some relief from this problem.     

Modulation detection, modulation masking, and speech understanding in noise in the elderly.

Temporal processing of suprathreshold sounds was examined in a group of young normal-hearing subjects (mean age of 26.0 years), and in three groups of older subjects (mean ages of 54.3, 64.8, and 72.2 years) with normal hearing or mild sensorineural hearing loss. Three experiments were performed. In the first experiment (modulation detection), subjects were asked to detect sinusoidal amplitude modulation (SAM) of a broadband noise, for modulation frequencies ranging from 2-1024 Hz. In the second experiment (modulation masking), the task was to detect a SAM signal (modulation frequency of 8 Hz) in the presence of a 100%-modulated SAM masker. Masker modulation frequency ranged from 2-64 Hz. In the final experiment, speech understanding was measured as a function of signal-to-noise ratio in both an unmodulated background noise and in a SAM background noise that had a modulation frequency of 8 Hz and a modulation depth of 100%. Except for a very modest correlation between age and modulation detection sensitivity at low modulation frequencies, there were no significant effects of age once the effect of hearing loss was taken into account. The results of the experiments suggest, however, that subjects with even a mild sensorineural hearing loss may have difficulty with a modulation masking task, and may not understand speech as well as normal-hearing subjects do in a modulated noise background.     

Effect of white noise "masking" on vestibular evoked potentials recorded using different stimulus modalities.

Short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses (L-VsEPs) are initiated in the otolith organs (saccule and utricle). Some of the saccule afferents have been reported to respond not only to linear acceleration, but also to high intensity acoustic stimuli. If so, the L-VsEP recorded from the saccule (elicited with the stimulus orientated relative to the head so as to optimally activate the saccule, i.e. stimulus in the vertical plane, Z-VsEP) should be reduced during high intensity broad band noise (BBN) "masking". Conversely, the utricular afferents have been reported to be less auditory-sensitive. Therefore, an L-VsEP which is mainly utricular in origin (stimulus in the horizontal plane, X-VsEP) should be less affected by this noise "masking". This was investigated in rats by recording X-VsEPs and Z-VsEPs and angular VsEPs (A-VsEPs), originating in the lateral semi-circular canals, before, during and after exposure to short duration, high intensity (113 dB SPL) BBN. This intensity completely masked auditory nerve evoked responses. The Z-VsEP did appear to be slightly more affected by the noise "masking" than the X-VsEP, implying the presence of more auditory-sensitive elements in the saccule. The A-VsEP was also affected by the BBN. The overall effect was relatively small (on average, 10-25% depression of the first wave of the different VsEPs). The responses showed recovery 5 min later.     

Effects of room reverberation and noise on speech discrimination by the elderly

Speech discrimination performance was measured in sound field for 10 young normal-hearing subjects, 10 elderly 'normal-hearing' subjects with a mean pure-tone average of 9.9 dB HTL, and 10 elderly hearing-impaired subjects with a mean pure-tone average of 48.5 dB HTL. Speech discrimination abilities were assessed in quiet and noise (S/N = +10 dB) in a sound suite and under two levels of reverberation in a reverberant room (RT = 0.59 and 1.56 s). Results indicated that the elderly 'normal-hearing' and young normal-hearing subjects have similar speech discrimination performance in the sound suite for both the quiet and noise conditions. In addition, performance by these two groups was almost identical under both levels of reverberation in quiet. However, when noise was added to the reverberant conditions, performance by the elderly 'normal-hearing' subjects was significantly poorer than that obtained by the young normal-hearing subjects. The elderly hearing-impaired subjects yielded speech discrimination scores that were significantly poorer than the elderly 'normal-hearing' subjects and the young normal-hearing subjects for all of the listening conditions.