Spatial separation benefit for unaided and aided listening.

OBJECTIVES: The benefit attributable to spatial separation of speech and noise was measured as a function of low-pass cutoff frequency with and without bilateral hearing aids. DESIGN: Fourteen younger and 10 older adults with normal hearing and 12 older adults with mild-to-moderate sloping high frequency hearing loss were included to assess the effects of age and hearing loss. Subjects with hearing loss were provided commercially available bilateral hearing aids. Consonant recognition was measured at 70 dB SPL in a background of speech-shaped noise at 66 dB SPL. Speech and noise were low-pass filtered at 1.7, 3.4, and 7.1 kHz. Nonsense syllables were always at 0° and noise was at either 0° or 90°. Speech and noise spectra for all conditions were digitally recorded using a probe microphone placed in each ear canal of each subject. Spectra and levels of speech, and quiet thresholds for narrowband noises, were used to calculate the Articulation Index and provide predictions of unaided and aided (hearing-impaired only) consonant recognition, spatial benefit, and hearing aid benefit for each condition. Subjective ratings of workload (NASA Task Load Index) were obtained for all unaided and aided measures of speech recognition. RESULTS: Consonant recognition in noise improved for all groups with speech and noise spatially separated and with the addition of high-frequency speech information. Scores were poorer overall for the older adults with hearing loss than for the other groups. For normal-hearing subjects, observed scores and spatial benefit were better than predicted. For hearing-impaired subjects, scores did not significantly improve with hearing aids, even with higher frequencies and spatial separation, and were poorer than predicted especially for aided listening. Similar to subjects with normal hearing, spatial benefit for hearing-impaired subjects was larger than predicted. CONCLUSIONS: Younger and older adults with normal hearing benefited from spatial separation of speech and noise sources to a greater extent than predicted based on simple audibility. Thus, no age-related deficits in the use of interaural difference cues were observed. Although hearing aid benefit was negligible, perceived listening effort was lower aided than unaided, especially with spatial separation. Articulation Index predictions revealed that speech audibility was generally restored with hearing aids across a wide bandwidth of speech, especially in the far ear. Thus, reduced audibility was not a primary factor in limited hearing aid benefit, suggesting that peripheral, central-auditory and/or cognitive changes may have played a role. In contrast, unaided and aided spatial benefit was better than predicted, and spatial benefit was slightly larger with hearing aids than without. Thus, these older adults with hearing loss using bilateral hearing aids were able to take advantage of binaural cues to improve consonant recognition in noise.

Level-dependent changes in perception of speech envelope cues.

Level-dependent changes in temporal envelope fluctuations in speech and related changes in speech recognition may reveal effects of basilar-membrane nonlinearities. As a result of compression in the basilar-membrane response, the "effective" magnitude of envelope fluctuations may be reduced as speech level increases from lower level (more linear) to mid-level (more compressive) regions. With further increases to a more linear region, speech envelope fluctuations may become more pronounced. To assess these effects, recognition of consonants and key words in sentences was measured as a function of speech level for younger adults with normal hearing. Consonant-vowel syllables and sentences were spectrally degraded using "noise vocoder" processing to maximize perceptual effects of changes to the speech envelope. Broadband noise at a fixed signal-to-noise ratio maintained constant audibility as speech level increased. Results revealed significant increases in scores and envelope-dependent feature transmission from 45 to 60 dB SPL and decreasing scores and feature transmission from 60 to 85 dB SPL. This quadratic pattern, with speech recognition maximized at mid levels and poorer at lower and higher levels, is consistent with a role of cochlear nonlinearities in perception of speech envelope cues.

Individual and level-dependent differences in masking for adults with normal and impaired hearing.

Simultaneous, on-frequency masking is commonly assumed to be linear with increasing noise intensity. However, some evidence suggests that, expressed in terms of signal-to-noise ratio changes with background level changes, masking slopes can vary from 0 dB/dB. These results and evidence from a large sample of subjects with normal and impaired hearing demonstrate level-dependent changes in masking, large individual differences in masking among subjects with similar thresholds in quiet, and significant correlations of masking slope with other estimates of auditory function measured in the same backgrounds.

Individual differences in behavioral estimates of cochlear nonlinearities.

Psychophysical methods provide a mechanism to infer the characteristics of basilar membrane responses in humans that cannot be directly measured. Because these behavioral measures are indirect, the interpretation of results depends on several underlying assumptions. Ongoing uncertainty about the suitability of these assumptions and the most appropriate measurement and compression estimation procedures, and unanswered questions regarding the effects of cochlear hearing loss and age on basilar membrane nonlinearities, motivated this experiment. Here, estimates of cochlear nonlinearities using temporal masking curves (TMCs) were obtained in a large sample of adults of various ages whose hearing ranged from normal to moderate cochlear hearing loss (Experiment 1). A wide range of compression slopes was observed, even for subjects with similar ages and thresholds, which warranted further investigation (Experiment 2). Potential sources of variance contributing to these individual differences were explored, including procedural-related factors (test-retest reliability, suitability of the linear-reference TMC, probe sensation levels, and parameters of TMC fitting algorithms) and subject-related factors (age and age-related changes in temporal processing, strength of cochlear nonlinearities estimated with distortion-product otoacoustic emissions, estimates of changes in cochlear function from damage to outer hair cells versus inner hair cells). Subject age did not contribute significantly to TMC or compression slopes, and TMC slopes did not vary significantly with threshold. Test-retest reliability of TMCs suggested that TMC masker levels and the general shapes of TMCs did not change in a systematic way when re-measured many weeks later. Although the strength of compression decreased slightly with increasing hearing loss, the magnitude of individual differences in compression estimates makes it difficult to determine the effects of hearing loss and cochlear damage on basilar membrane nonlinearities in humans.

Level-dependent changes in detection of temporal gaps in noise markers by adults with normal and impaired hearing.

Compression in the basilar-membrane input-output response flattens the temporal envelope of a fluctuating signal when more gain is applied to lower level than higher level temporal components. As a result, level-dependent changes in gap detection for signals with different depths of envelope fluctuation and for subjects with normal and impaired hearing may reveal effects of compression. To test these assumptions, gap detection with and without a broadband noise was measured with 1, 000-Hz-wide (flatter) and 50-Hz-wide (fluctuating) noise markers as a function of marker level. As marker level increased, background level also increased, maintaining a fixed acoustic signal-to-noise ratio (SNR) to minimize sensation-level effects on gap detection. Significant level-dependent changes in gap detection were observed, consistent with effects of cochlear compression. For the flatter marker, gap detection that declines with increases in level up to mid levels and improves with further increases in level may be explained by an effective flattening of the temporal envelope at mid levels, where compression effects are expected to be strongest. A flatter effective temporal envelope corresponds to a reduced effective SNR. The effects of a reduction in compression (resulting in larger effective SNRs) may contribute to better-than-normal gap detection observed for some hearing-impaired listeners.

Spatial benefit of bilateral hearing AIDS.

OBJECTIVES: To assess the extent to which hearing aids improve spatial benefit by restoring the availability of interaural difference cues, the benefit attributable to spatial separation of speech and babble with and without bilateral hearing aids was measured as a function of low-pass cutoff frequency. DESIGN: Twenty-one older adults with sloping high-frequency hearing loss were provided commercially available bilateral hearing aids. After a 3 to 6 month acclimatization period, speech levels corresponding to 50% correct recognition of sentences from the Hearing in Noise Test (HINT) were measured in a 65-dB SPL babble, with speech and babble low-pass filtered at 1.8, 3.6, and 5.6 kHz. Sentences were always at 0 degrees azimuth, and babble was at either 0 degrees or 90 degrees . Speech and babble spectra for all conditions were digitally recorded using a probe microphone placed in each ear canal of each subject. Spectra and levels of speech and babble and unaided thresholds for narrowband noises were used to calculate the aided audibility index and provide predictions of unaided and aided thresholds for HINT sentences, hearing aid benefit, and spatial benefit for each cutoff frequency. In addition, subjects' willingness to tolerate background noise with and without amplification was measured in the spatially coincident and spatially separated conditions using the Acceptable Noise Level (ANL) procedure. RESULTS: Thresholds for HINT sentences in babble and ANL improved significantly when aided and when speech and babble were spatially separated. Specifically, hearing aid benefit improved significantly as cutoff frequency increased from 1.8 to 3.6 kHz but only when speech and babble were spatially separated; likewise, spatial benefit improved significantly from 1.8 to 3.6 kHz but only in the aided condition. No further improvement in hearing aid or spatial benefit was observed when cutoff frequency was increased from 3.6 to 5.6 kHz, although improvement in hearing aid benefit was predicted. CONCLUSIONS: Hearing aid benefit, although significant, was poorer than predicted, suggesting that these older adults with high-frequency hearing loss did not take full advantage of the increase in audible speech information provided by amplification. Hearing aid benefit was also limited because hearing aids for some subjects did not restore speech audibility across the full bandwidth of speech. Unaided and aided spatial benefit was significantly greater than predicted, and aided spatial benefit was greater than unaided. This suggests that these older adults were able to take advantage of interaural level and time difference cues to improve speech recognition in babble and that they benefited from these cues to a greater extent with than without bilateral hearing aids. Finally, in contrast to results of previous studies, ANL may vary for an individual depending on the listening condition.

At the heart of the ventral attention system: the right anterior insula.

The anterior insula has been hypothesized to provide a link between attention-related problem solving and salience systems during the coordination and evaluation of task performance. Here, we test the hypothesis that the anterior insula/medial frontal operculum (aI/fO) provides linkage across systems supporting task demands and attention systems by examining the patterns of functional connectivity during word recognition and spatial attention functional imaging tasks. A shared set of frontal regions (right aI/fO, right dorsolateral prefrontal cortex, bilateral anterior cingulate) were engaged, regardless of perceptual domain (auditory or visual) or mode of response (word production or button press). We present novel evidence that: (1) the right aI/fO is functionally connected with other frontal regions implicated in executive function and not just brain regions responsive to stimulus salience; and (2) that the aI/fO, but not the ACC, exhibits significantly correlated activity with other brain regions specifically engaged by tasks with varying perceptual and behavioral demands. These results support the hypothesis that the right aI/fO aids in the coordination and evaluation of task performance across behavioral tasks with varying perceptual and response demands.

Factors affecting the benefits of high-frequency amplification.

PURPOSE: This study was designed to determine the extent to which high-frequency amplification helped or hindered speech recognition as a function of hearing loss, gain-frequency response, and background noise. METHOD: Speech recognition was measured monaurally under headphones for nonsense syllables low-pass filtered in one-third-octave steps between 2.2 and 5.6 kHz. Adults with normal hearing and with high-frequency thresholds ranging from 40 to 80 dB HL listened to speech in quiet processed with an identical "nonindividualized" gain-frequency response. Hearing-impaired participants also listened to speech in quiet and noise processed with gain-frequency responses individually prescribed according to the National Acoustic Laboratories-Revised (NAL-R) formula. RESULTS: Mean speech recognition generally increased significantly with additional high-frequency speech bands. The one exception was that hearing-impaired participants' recognition of speech processed by the nonindividualized response did not improve significantly with the addition of the highest frequency band. Significantly larger increases in scores with increasing bandwidth were observed for speech in noise than quiet. CONCLUSIONS: Given that decreases in scores with additional high-frequency speech bands for individual participants were relatively small and few and did not increase with quiet thresholds, no evidence of a degree of hearing loss was found above which it was counterproductive to provide amplification.

Binaural advantage for younger and older adults with normal hearing.

PURPOSE: Three experiments measured benefit of spatial separation, benefit of binaural listening, and masking-level differences (MLDs) to assess age-related differences in binaural advantage. METHOD: Participants were younger and older adults with normal hearing through 4.0 kHz. Experiment 1 compared spatial benefit with and without head shadow. Sentences were at 0 degrees, and speech-shaped noise was at 0 degrees, 90 degrees, or +/-90 degrees. Experiment 2 measured binaural benefit with the near ear unplugged compared with plugged for sentences at 0 degrees and masker at 90 degrees. Experiment 3 measured MLDs under earphones for 0.5-kHz pure tones in Gaussian and low-noise noise, and spondees in speech-shaped noise. RESULTS: Spatial-separation benefit for speech did not differ significantly for younger and older adults but was smaller than predicted by an audibility-based model for older adults and larger than predicted for younger adults. Binaural listening benefit was observed for younger participants only. Tonal MLDs suggested that listeners benefit from interaural difference cues during noise dips for signals out of phase. Neither tonal nor speech MLDs differed significantly between younger and older participants. CONCLUSION: Binaural processing of sentences revealed some age-related deficits in the use of interaural difference cues, whereas no deficits were observed for more simple detection or recognition tasks.

Age-related effects on word recognition: reliance on cognitive control systems with structural declines in speech-responsive cortex.

Speech recognition can be difficult and effortful for older adults, even for those with normal hearing. Declining frontal lobe cognitive control has been hypothesized to cause age-related speech recognition problems. This study examined age-related changes in frontal lobe function for 15 clinically normal hearing adults (21-75 years) when they performed a word recognition task that was made challenging by decreasing word intelligibility. Although there were no age-related changes in word recognition, there were age-related changes in the degree of activity within left middle frontal gyrus (MFG) and anterior cingulate (ACC) regions during word recognition. Older adults engaged left MFG and ACC regions when words were most intelligible compared to younger adults who engaged these regions when words were least intelligible. Declining gray matter volume within temporal lobe regions responsive to word intelligibility significantly predicted left MFG activity, even after controlling for total gray matter volume, suggesting that declining structural integrity of brain regions responsive to speech leads to the recruitment of frontal regions when words are easily understood.

Longitudinal changes in speech recognition in older persons.

Recognition of isolated monosyllabic words in quiet and recognition of key words in low- and high-context sentences in babble were measured in a large sample of older persons enrolled in a longitudinal study of age-related hearing loss. Repeated measures were obtained yearly or every 2 to 3 years. To control for concurrent changes in pure-tone thresholds and speech levels, speech-recognition scores were adjusted using an importance-weighted speech-audibility metric (AI). Linear-regression slope estimated the rate of change in adjusted speech-recognition scores. Recognition of words in quiet declined significantly faster with age than predicted by declines in speech audibility. As subjects aged, observed scores deviated increasingly from AI-predicted scores, but this effect did not accelerate with age. Rate of decline in word recognition was significantly faster for females than males and for females with high serum progesterone levels, whereas noise history had no effect. Rate of decline did not accelerate with age but increased with degree of hearing loss, suggesting that with more severe injury to the auditory system, impairments to auditory function other than reduced audibility resulted in faster declines in word recognition as subjects aged. Recognition of key words in low- and high-context sentences in babble did not decline significantly with age.

Speech recognition in noise: estimating effects of compressive nonlinearities in the basilar-membrane response.

OBJECTIVES: This experiment was designed to estimate effects of cochlear nonlinearities on tonal and speech masking for individuals with normal hearing who have a range of quiet thresholds. Physiological and psychophysical evidence indicates that for signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower stimulus levels and compressed at medium to higher stimulus levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, the hypothesis tested was that masker effectiveness would change as a function of stimulus level consistent with the underlying basilar membrane response. Specifically, with a fixed-level speech signal and a speech-shaped masker that ranges from low to higher levels, the resulting response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels. This would result in relatively less effective masking at higher masker levels. It was further hypothesized that the transition from linear to compressed responses to both tones and maskers would occur at higher levels for listeners with higher quiet thresholds than for listeners with lower quiet thresholds. DESIGN: Tonal thresholds and speech recognition in noise were measured as a function of masker level. A 10-msec, 2.0-kHz tone was presented in a lower frequency masker ranging from 40 to 85 dB SPL. Moderate-level speech was presented in interrupted noise at six levels ranging from 47 to 77 dB SPL. To minimize differences in speech audibility that could arise during the "off" periods of the interrupted noise, a low-level steady-state "threshold-matching noise" was also present during measurement of speech recognition. Subjects were 30 adults with normal hearing with a 20-dB range of average quiet thresholds. RESULTS: Tonal breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were significantly correlated with quiet thresholds, whereas slopes measured above the breakpoints were not. Speech recognition in noise was consistent with the hypothesis that the response of the basilar membrane to the masker was linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. That is, at lower masker levels, as masker level increased, mean observed speech scores declined as predicted using the articulation index, an audibility-based model. With further increases in masker level, mean scores declined less than predicted. Moreover, for subjects with higher quiet thresholds, masker effectiveness remained constant for a wider range of masker levels than for subjects with lower quiet thresholds, consistent with the hypothesis that the transition from linear to compressed responses occurred at higher levels. Finally, significant negative correlations were obtained between individual subjects' tonal and speech measures. CONCLUSIONS: Results from tonal and speech tasks were consistent with basilar membrane nonlinearities and consistent with changes in nonlinearities with minor threshold elevations, providing support for their role in the understanding of speech in noise with increases in noise level.

Estimates of basilar-membrane nonlinearity effects on masking of tones and speech.

OBJECTIVE: The aim of this experiment was to assess the contribution of cochlear nonlinearities to speech recognition in noise for individuals with normal hearing and a range of quiet thresholds. For signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower signal levels and compressed at medium to higher signal levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, for moderate-level speech and a lower frequency masker, the response to the masker grows linearly whereas the response to the speech is compressed, which may result in changes in the effectiveness of the masker on speech recognition with increases in masker level. To test this hypothesis, observed speech-recognition scores were compared with scores predicted using an audibility-based model, which did not include nonlinear effects that may influence masker effectiveness. DESIGN: Growth of simultaneous masking was measured for moderate-level bandpass-filtered nonsense syllables and for 350-msec pure tones at frequencies within the speech passband. Masker frequencies were within (on-frequency) or below (off-frequency) the speech passband. Estimates of basilar-membrane nonlinearities were derived from growth-of-masking functions for 10-msec, 2.0- and 4.0-kHz tones in narrowband, off-frequency maskers presented simultaneously. Subjects were 26 adults with normal hearing with approximately a 20-dB range of average quiet thresholds. RESULTS: Breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were strongly associated with quiet thresholds but slopes measured above the breakpoints were independent of quiet thresholds. Individual differences were substantially larger for off-frequency masking of pure tones and speech than for on-frequency masking of pure tones and speech. Using an audibility-based predictive model, the change in speech audibility resulting from the compressed response to speech with increasing off-frequency masker level (and the resulting decline in scores) was well predicted from nonlinear growth of masking for pure tones measured in the same off-frequency masker. However, absolute speech-recognition predictions were generally inaccurate and were a function of how well pure-tone signal levels at masked threshold estimated masker effectiveness for speech. That is, subjects with lower off-frequency masked thresholds had less accurate predictions of speech recognition in off-frequency maskers. CONCLUSIONS: Large individual differences in off-frequency masking of pure tones and speech are consistent with the assumption that small changes in the shape of the basilar-membrane input-output function result in large changes in the amount of off-frequency masking but small (if any) changes in on-frequency masking where the signal and masker are subject to a similar compression. Growth of off-frequency masking of pure tones and speech were correlated with each other, consistent with the underlying basilar-membrane response, and consistent with changes in breakpoints for subjects with normal hearing and a range of quiet thresholds. These results provide support for a role of nonlinear effects in the understanding of speech in noise.

Spectral and threshold effects on recognition of speech at higher-than-normal levels.

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.

Word recognition in noise at higher-than-normal levels: decreases in scores and increases in masking.

Under certain conditions, speech recognition in noise decreases above conversational levels when signal-to-noise ratio is held constant. The current study was undertaken to determine if nonlinear growth of masking and the subsequent reduction in "effective" signal-to-noise ratio accounts for this decline. Nine young adults with normal hearing listened to monosyllabic words at three levels in each of three levels of a masker shaped to match the speech spectrum. An additional low-level noise equated audibility by producing equivalent masked thresholds for all subjects. If word recognition was determined entirely by signal-to-noise ratio and was independent of overall speech and masker levels, scores at a given signal-to-noise ratio should remain constant with increasing level. Masked pure-tone thresholds measured in the speech-shaped maskers increased linearly with increasing masker level at lower frequencies but nonlinearly at higher frequencies, consistent with nonlinear growth of upward spread of masking that followed the peaks in the spectrum of the speech-shaped masker. Word recognition declined significantly with increasing level when signal-to-noise ratio was held constant which was attributed to nonlinear growth of masking and reduced "effective" signal-to-noise ratio at high speech-shaped masker levels, as indicated by audibility estimates based on the Articulation Index.

Recognition of filtered words in noise at higher-than-normal levels: decreases in scores with and without increases in masking.

To examine spectral effects on declines in speech recognition in noise at high levels, word recognition for 18 young adults with normal hearing was assessed for low-pass-filtered speech and speech-shaped maskers or high-pass-filtered speech and speech-shaped maskers at three speech levels (70, 77, and 84 dB SPL) for each of three signal-to-noise ratios (+8, +3, and -2 dB). An additional low-level noise produced equivalent masked thresholds for all subjects. Pure-tone thresholds were measured in quiet and in all maskers. If word recognition was determined entirely by signal-to-noise ratio, and was independent of signal levels and the spectral content of speech and maskers, scores should remain constant with increasing level for both low- and high-frequency speech and maskers. Recognition of low-frequency speech in low-frequency maskers and high-frequency speech in high-frequency maskers decreased significantly with increasing speech level when signal-to-noise ratio was held constant. For low-frequency speech and speech-shaped maskers, the decline was attributed to nonlinear growth of masking which reduced the "effective" signal-to-noise ratio at high levels, similar to previous results for broadband speech and speech-shaped maskers. Masking growth and reduced "effective" signal-to-noise ratio accounted for some but not all the decline in recognition of high-frequency speech in high-frequency maskers.

Recovery from prior stimulation: masking of speech by interrupted noise for younger and older adults with normal hearing.

In a previous study [Dubno et al, J. Acoust. Soc. Am. 111, 2897-2907 (2002)], older subjects benefitted less than younger subjects from momentary improvements in signal-to-noise ratio when listening to speech in interrupted maskers. It has been hypothesized that the benefit derived from interrupted maskers may be related to recovery from forward masking, i.e., the recovery of a response to a suprathreshold signal from prior stimulation by a masker. The effect of interrupted maskers on speech recognition may be well suited to test hypotheses regarding recovery from prior stimulation, given that both involve the perception of signals following a masker. Here, younger and older adults with normal but not identical audiograms listened to nonsense syllables at moderate and high levels in a speech-shaped noise that was modulated by a 2-, 10-, 25-, or 50-Hz square wave. An additional low-level noise was always present that was shaped to produce equivalent masked thresholds for all subjects. To assess recovery from forward masking, forward-masked thresholds were measured at 0.5 and 4.0 kHz as a function of the delay between the speech-shaped masker and the signal. Speech recognition in interrupted noise was poorer for older than younger subjects. Small but consistent age-related differences were observed in the decrease in score with interrupted noise relative to the score without interrupted noise. Forward-masked thresholds of older subjects were higher than those of younger subjects, but there were no age-related differences in the amount of forward masking or in simultaneous masking. Negative correlations were observed between speech-recognition scores in interrupted noise and forward-masked thresholds. That is, the benefit derived from momentary improvements in speech audibility in an interrupted noise decreased as forward-masked thresholds increased. Stronger correlations with forward masking were observed for the higher frequency signal, for higher noise interruption rates, and when the signal-to-noise ratio was poor. Comparisons of speech-recognition scores at moderate and high levels for younger and older subjects were not consistent with the hypothesis of an age-related difference in the contribution of low-spontaneous-rate fibers to speech recognition in interrupted noise.

Benefit of modulated maskers for speech recognition by younger and older adults with normal hearing.

To assess age-related differences in benefit from masker modulation, younger and older adults with normal hearing but not identical audiograms listened to nonsense syllables in each of two maskers: (1) a steady-state noise shaped to match the long-term spectrum of the speech, and (2) this same noise modulated by a 10-Hz square wave, resulting in an interrupted noise. An additional low-level broadband noise was always present which was shaped to produce equivalent masked thresholds for all subjects. This minimized differences in speech audibility due to differences in quiet thresholds among subjects. An additional goal was to determine if age-related differences in benefit from modulation could be explained by differences in thresholds measured in simultaneous and forward maskers. Accordingly, thresholds for 350-ms pure tones were measured in quiet and in each masker; thresholds for 20-ms signals in forward and simultaneous masking were also measured at selected signal frequencies. To determine if benefit from modulated maskers varied with masker spectrum and to provide a comparison with previous studies, a subgroup of younger subjects also listened in steady-state and interrupted noise that was not spectrally shaped. Articulation index (AI) values were computed and speech-recognition scores were predicted for steady-state and interrupted noise; predicted benefit from modulation was also determined. Masked thresholds of older subjects were slightly higher than those of younger subjects; larger age-related threshold differences were observed for short-duration than for long-duration signals. In steady-state noise, speech recognition for older subjects was poorer than for younger subjects, which was partially attributable to older subjects' slightly higher thresholds in these maskers. In interrupted noise, although predicted benefit was larger for older than younger subjects, scores improved more for younger than for older subjects, particularly at the higher noise level. This may be related to age-related increases in thresholds in steady-state noise and in forward masking, especially at higher frequencies. Benefit of interrupted maskers was larger for unshaped than for speech-shaped noise, consistent with AI predictions.

A potential role for thalamocingulate circuitry in human maternal behavior.

BACKGROUND: Little is known about the regional brain basis of human maternal behavior. To understand this better, we have been examining brain activity in mothers listening to infant cries. METHODS: We measured functional Magnetic Resonance Imaging brain activity in healthy, breastfeeding first-time mothers with young infants while they listened to infant cries, white noise control sounds, and a rest condition. Based on the thalamocingulate theory of maternal behavior and pilot work, we hypothesized that the cingulate, medial thalamus, medial prefrontal cortex, and right orbitofrontal cortex would display more activity with infant cries than with white noise (comparison 1) and would uniquely activate with the cries, meaning that these regions would display activity with cry minus rest but not with white noise minus rest (comparison 2). RESULTS: In hypothesized regions, the group displayed more activity in the medial thalamus, medial prefrontal and right orbitofrontal cortices with both comparisons. The anterior and posterior cingulate cortex displayed more activity only with comparison 1. In non-hypothesized brain regions, several other structures thought important in rodent maternal behavior displayed activity with both comparisons including the midbrain, hypothalamus, dorsal and ventral striatum, and vicinity of the lateral septal region. CONCLUSIONS: Our results partially support our hypotheses and are generally consistent with neuroanatomical studies of rodent maternal behavior.

Recognition of low-pass-filtered consonants in noise with normal and impaired high-frequency hearing.

People with high-frequency hearing loss often complain of difficulty understanding speech, particularly in noisy environments. The reduction in audible high-frequency speech information provides one explanation. In addition, high-frequency hearing loss may reduce the contribution from the "tails" of high-frequency auditory nerve fibers, resulting in diminished availability of lower frequency speech cues. This study was designed to determine if high-frequency hearing loss results in speech-understanding deficits beyond those accounted for by reduced high-frequency speech information. Recognition of speech, both low-pass filtered and unfiltered, was measured for subjects with normal hearing and those with hearing loss limited to high frequencies. Nonsense syllables were presented in three levels of noise that was spectrally shaped to match the long-term spectrum of the speech. Scores for subjects with impaired high-frequency hearing were significantly poorer than scores for subjects with normal hearing. In the case of the low-pass-filtered speech, performance differences between groups could not be attributed to differences in speech audibility, as high-frequency speech cues were absent for all subjects. These results are consistent with the hypothesis that high-frequency fibers encode useful low-frequency speech information.