Spectral contributions to the benefit from spatial separation of speech and noise

Speech recognition in noise improves when speech and noise sources are separated in space. This benefit has two components whose effects are strongest in different frequency regions: (1) interaural level differences (e.g., head shadow), which are largest at higher frequencies, and (2) interaural time differences, which have their greatest contribution at lower frequencies. Binaural interactions enhance the separation of signals from noise through the use of these interaural differences. Here, the benefit attributable to spatial separation was measured as a function of the low- and high-pass cutoff frequency of speech and noise. Listeners were younger adults with normal hearing, older adults with normal hearing, and older adults with hearing loss. Binaural thresholds for narrowband noises were measured in quiet and in a speech-shaped masker as a function of masker low-pass cutoff frequency. Speech levels corresponding to 50% correct recognition of sentences from the Hearing in Noise Test (HINT) were measured in a 65-dB SPL speech-shaped noise. Thresholds for narrowband noises and for speech were measured with two loudspeaker configurations: (1) signals and speech-shaped noise at 0 degrees azimuth (in front of the listener) and (2) signals at 0 degrees azimuth and speech-shaped noise at 90 degrees azimuth (at the listener's side). The criterion measure was spatial separation benefit, or the difference in thresholds for the two conditions. Benefit of spatial separation for unfiltered speech averaged 6.1 dB for younger listeners with normal hearing, 4.9 dB for older listeners with normal hearing, and 2.7 dB for older listeners with hearing loss. Benefit was differentially affected by low-pass and high-pass filtering, suggesting a trade-off of the contributions of higher frequency interaural level differences and lower frequency interaural timing cues. As expected, older listeners with hearing loss benefited little from the improved signal-to-noise ratios in the higher frequencies resulting from head shadow, but showed some benefit from lower frequency cues. Spatial benefit for older listeners with normal hearing was reduced relative to benefit for younger listeners. This result may be related to older listeners' elevated thresholds at frequencies above 6.0 kHz.

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.

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.

Auditory brainstem responses in younger and older adults for broadband noises separated by a silent gap

Wave V of the auditory brainstem response was measured to two 50-ms broadband noise bursts separated by silent gaps of varied duration (4, 8, 32, or 64 ms) for younger and older adults with normal hearing. All subjects had measurable wave V responses to the first noise burst. However, for the second noise burst, three of eight older adults did not have responses with gap durations of 4 and 8 ms, and one of eight younger adults did not have a measurable response with a gap duration of 4 ms. When responses were present for older adults, latencies were similar to those of younger subjects but amplitudes were smaller. These results suggest age-related deficits in gap detection at the level of the brainstem in a group of aged subjects with no threshold elevation. Results are similar to those of Boettcher et al. (1996) using an identical paradigm in young and aged Mongolian gerbils.

Psychophysical suppression effects for tonal and speech signals

This experiment assessed the benefits of suppression and the impact of reduced or absent suppression on speech recognition in noise. Psychophysical suppression was measured in forward masking using tonal maskers and suppressors and band limited noise maskers and suppressors. Subjects were 10 younger and 10 older adults with normal hearing, and 10 older adults with cochlear hearing loss. For younger subjects with normal hearing, suppression measured with noise maskers increased with masker level and was larger at 2.0 kHz than at 0.8 kHz. Less suppression was observed for older than younger subjects with normal hearing. There was little evidence of suppression for older subjects with cochlear hearing loss. Suppression measured with noise maskers and suppressors was larger in magnitude and more prevalent than suppression measured with tonal maskers and suppressors. The benefit of suppression to speech recognition in noise was assessed by obtaining scores for filtered consonant-vowel syllables as a function of the bandwidth of a forward masker. Speech-recognition scores in forward maskers should be higher than those in simultaneous maskers given that forward maskers are less effective than simultaneous maskers. If suppression also mitigated the effects of the forward masker and resulted in an improved signal-to-noise ratio, scores should decrease less in forward masking as forward-masker bandwidth increased, and differences between scores in forward and simultaneous maskers should increase, as was observed for younger subjects with normal hearing. Less or no benefit of suppression to speech recognition in noise was observed for older subjects with normal hearing or hearing loss. In general, as suppression measured with tonal signals increased, the combined benefit of forward masking and suppression to speech recognition in noise also increased.

Psychophysical suppression measured with bandlimited noise extended below and/or above the signal: effects of age and hearing loss

The objectives of this study were to measure suppression with bandlimited noise extended below and above the signal, at lower and higher signal frequencies, between younger and older subjects, and between subjects with normal hearing and cochlear hearing loss. Psychophysical suppression was assessed by measuring forward-masked thresholds at 0.8 and 2.0 kHz in bandlimited maskers as a function of masker bandwidth. Bandpass-masker bandwidth was increased by introducing noise components below and above the signal frequency while keeping the noise centered on the signal frequency, and also by adding noise below the signal only, and above the signal only. Subjects were younger and older adults with normal hearing and older adults with cochlear hearing loss. For all subjects, suppression was larger when noise was added below the signal than when noise was added above the signal, consistent with some physiological evidence of stronger suppression below a fiber's characteristic frequency than above. For subjects with normal hearing, suppression was greater at higher than at lower frequencies. For older subjects with hearing loss, suppression was reduced to a greater extent above the signal than below and where thresholds were elevated. Suppression for older subjects with normal hearing was poorer than would be predicted from their absolute thresholds, suggesting that age may have contributed to reduced suppression or that suppression was sensitive to changes in cochlear function that did not result in significant threshold elevation.

Forward- and simultaneous-masked thresholds in bandlimited maskers in subjects with normal hearing and cochlear hearing loss

Forward- and simultaneous-masked thresholds were measured at 0.5 and 2.0 kHz in bandpass maskers as a function of masker bandwidth and in a broadband masker with the goal of estimating psychophysical suppression. Suppression was operationally defined in two ways: (1) as a change in forward-masked threshold as a function of masker bandwidth, and (2) as a change in effective masker level with increased masker bandwidth, taking into account the nonlinear growth of forward masking. Subjects were younger adults with normal hearing and older adults with cochlear hearing loss. Thresholds decreased as a function of masker bandwidth in forward masking, which was attributed to effects of suppression; thresholds remained constant or increased slightly with increasing masker bandwidth in simultaneous masking. For subjects with normal hearing, slightly larger estimates of suppression were obtained at 2.0 kHz rather than at 0.5 kHz. For hearing-impaired subjects, suppression was reduced in regions of hearing loss. The magnitude of suppression was strongly correlated with the absolute threshold at the signal frequency, but did not vary with thresholds at frequencies remote from the signal. The results suggest that measuring forward-masked thresholds in bandlimited and broadband maskers may be an efficient psychophysical method for estimating suppression.

The amplitude-modulation following response in young and aged human subjects

The amplitude-modulation following response (AMFR) is a steady-state auditory response which may be an objective measure of intensity discrimination. Aged subjects with normal hearing have poorer intensity discrimination for low-frequency tones measured behaviorally, which would predict poorer AMFRs for low-frequency carriers. Experiment 1 was designed to assess age-related differences in AMFR characteristics. Response amplitudes were not significantly different among the young and aged groups for either carrier frequency (520 or 4000 Hz) or modulation depth (0--100%). Response phase did not vary systematically among groups. These results suggest that the AMFR may not be directly comparable to behavioral measures of intensity discrimination in aged subjects with normal hearing. To assess the contribution of high-frequency hearing loss on the AMFR in aged subjects, Experiment 2 compared AMFR amplitudes in aged subjects and in young subjects under the condition of high-pass masking. The amplitude of the AMFR was reduced at 520 Hz for both aged subjects and masked young subjects compared to unmasked young subjects, suggesting that reduced amplitudes in aged subjects with high-frequency hearing loss were associated with threshold elevations. Furthermore, the results suggest that the base of the cochlea contributes to the AMFR for low carrier frequencies.

Feasibility of using fMRI to study mothers responding to infant cries

While parenting is a universal human behavior, its neuroanatomic basis is currently unknown. Animal data suggest that the cingulate may play an important function in mammalian parenting behavior. For example, in rodents cingulate lesions impair maternal behavior. Here, in an attempt to understand the brain basis of human maternal behavior, we had mothers listen to recorded infant cries and white noise control sounds while they underwent functional MRI (fMRI) of the brain. We hypothesized that mothers would show significantly greater cingulate activity during the cries compared to the control sounds. Of 7 subjects scanned, 4 had fMRI data suitable for analysis. When fMRI data were averaged for these 4 subjects, the anterior cingulate and right medial prefrontal cortex were the only brain regions showing statistically increased activity with the cries compared to white noise control sounds (cluster analysis with one-tailed z-map threshold of P < 0.001 and spatial extent threshold of P < 0.05). These results demonstrate the feasibility of using fMRI to study brain activity in mothers listening to infant cries and that the anterior cingulate may be involved in mothers listening to crying babies. We are currently replicating this study in a larger group of mothers. Future work in this area may help (1) unravel the functional neuroanatomy of the parent-infant bond and (2) examine whether markers of this bond, such as maternal brain response to infant crying, can predict maternal style (i.e., child neglect), offspring temperament, or offspring depression or anxiety.

Experience with a yes-no single-interval maximum-likelihood procedure

The report in 1993 by Green [J. Acoust. Soc. Am. 93, 2096-2105 (1993)] describing the application of a new psychophysical method requiring few trials and little time to measure auditory thresholds has generated considerable interest among experimentalists. The procedure uses a single-interval stimulus presentation, requests a yes-no decision by subjects, and implements a maximum-likelihood calculation to determine the next trial stimulus level within an adaptive track, as well as the final threshold estimate. Data are presented here describing separate experiences with this procedure in two laboratories in both detection and discrimination tasks. Issues addressed include comparisons with more traditional psychophysical methods, variability in threshold estimates, experimental time required, and possible minor modifications to improve the basic procedure. Results using this procedure are comparable in terms of variability of estimates to those emerging from more lengthy procedures. However, because it may be difficult for some listeners to maintain a consistent criterion and because attentional lapses may be costly, experimenters must be willing to monitor performance closely and repeat some tracks in cases where excessively high variability is noted. Further, this procedure may not be suitable for tasks for which the form of the psychometric function is not well-established. Modifications allowing a variable slope parameter in the maximum-likelihood evaluations of psychometric functions may be of benefit.

Use of context by young and aged adults with normal hearing

Word recognition in sentences with and without context was measured in young and aged subjects with normal but not identical audiograms. Benefit derived from context by older adults has been obscured, in part, by the confounding effect of even mildly elevated thresholds, especially as listening conditions vary in difficulty. This problem was addressed here by precisely controlling signal-to-noise ratio across conditions and by accounting for individual differences in signal-to-noise ratio. Pure-tone thresholds and word recognition were measured in quiet and threshold-shaped maskers that shifted quiet thresholds by 20 and 40 dB. Word recognition was measured at several speech levels in each condition. Threshold was defined as the speech level (or signal-to-noise ratio) corresponding to the 50 rau point on the psychometric function. As expected, thresholds and slopes of psychometric functions were different for sentences with context compared to those for sentences without context. These differences were equivalent for young and aged subjects. Individual differences in word recognition among all subjects, young and aged, were accounted for by individual differences in signal-to-noise ratio. With signal-to-noise ratio held constant, word recognition for all subjects remained constant or decreased only slightly as speech and noise levels increased. These results suggest that, given equivalent speech audibility, older and younger listeners derive equivalent benefit from context.

Gender-specific effects of drugs on hearing levels of older persons

As part of a study of human presbyacusis, a questionnaire on medicinal drug usage was given to 357 subjects (184 females, 173 males). Previous results from 211 subjects showed gender effects, that is, for males, none of the drugs had any measurable effects on hearing, whereas women taking calcium channel blockers (CCBs) had hearing levels 12 dB better than women not taking them; women taking beta adrenergic medication had hearing levels 20 dB poorer, and women taking antihistamine/cold preparations had hearing levels 9 dB poorer. Results from the original 211 subjects were confirmed when the sample size was increased from 211 to 357 subjects only for the beta adrenergic medications. Results for antihistamine/cold preparation medications showed small effects only for female subjects. Data from 13 additional female subjects who used CCBs showed hearing levels 10-14 dB poorer than predicted from the original data. Male data were consistent in both samples. The inconsistency for females could reflect sampling error. A more likely possibility is that since the original 10 subjects using CCBs had a mean age of 72 yr and the second sample of 13 had a mean age of 79.5 yr, poorer hearing levels might be anticipated because of the difference in chronological age and possibly duration of drug usage.

Determination of optimal data placement for psychometric function estimation: a computer simulation

Psychometric functions are used to relate the responses of a subject to physical stimuli in a variety of psychophysical tasks. However, it is time consuming to obtain data to determine a psychometric function if many stimulus levels and many trials are required. A computer simulation was conducted to determine the minimum number of data points needed for such a determination. The computer simulation also determined the optimal placements of the stimuli and the number of trials per datum point for psychometric function determinations. Results indicate that a 2-point sampling method with 30-50 trials per point at optimal locations can produce a psychometric function with accurate spread and threshold estimates in a yes-no paradigm. However, the 4-point sampling method yields statistically smaller variances of the estimates. For the 2-alternative forced-choice paradigm, at least 120 trials per point are needed for the 2-point sampling method's estimated parameters to differ from the known parameter values by less than 5%. The simulation results suggest that 3-alternative or 4-alternative forced-choice is preferable to 2-alternative. Furthermore, when a criterion-free paradigm is not required, the yes-no paradigm is a better procedure than m-alternative forced-choice for obtaining the corresponding psychometric function because of smaller standard deviation of the estimates and smaller number of trials/point required.

Psychometric functions for gap detection in noise measured from young and aged subjects

Psychometric functions for gap detection of temporal gaps in wideband noise were measured in a "yes/no" paradigm from normal-hearing young and aged subjects with closely matched audiograms. The effects of noise-burst duration, gap location, and uncertainty of gap location were tested. A typical psychometric function obtained in this study featured a steep slope, which was independent of most experimental conditions as well as age. However, gap thresholds were generally improved with increasing duration of the noise burst for both young and aged subjects. Gap location and uncertainty had no significant effects on the thresholds for the young subjects. For the aged subjects, whenever the gap was sufficiently away from the onset or offset of the noise burst, detectability was robust despite uncertainty about the gap location. Significant differences between young and aged subjects could be observed only when the gap was very close to the signal onset and offset.

Analysis of blood chemistry and hearing levels in a sample of older persons

OBJECTIVE

As part of an ongoing study of presbyacusis, the relationship between blood chemistry levels and hearing levels was investigated. Previous reports often used small sets of blood chemistry measures, and results were inconclusive. This experiment examined hearing levels and 27 measures of blood chemistry using various univariate and multivariate statistical procedures.

DESIGN

Blood from 89 female and 128 male human subjects was collected. Subjects' ages ranged from 60 to 82 yr, and hearing levels ranged from normal to moderate/severe. Subjects with a history of middle ear disease were excluded. Electrolyte panel (Na, K, Cl, CO2, Ca, urea nitrogen, glucose, creatinine, and Mg), hematology panel (WBC, RBC, Hgb, hematocrit, platelet, etc.), serum lipids (total cholesterol, low-density lipoprotein [LDL], and high-density lipoprotein [HDL]), immunoglobulins (IgG, IgA, IgM, and IgE), and thyroxine were analyzed using univariate and multivariate statistical procedures.

RESULTS

Blood chemistry levels of most subjects were within normal ranges as defined by our laboratory. Correlation between blood chemistry measures and pure-tone averages (PTAs) ranged from minimal to low. Results of factor analysis, discriminant analysis, and canonical analysis showed that combining blood chemistry measures from the same panel still could not predict PTA effectively. One exception to this was a gender-specific effect of cholesterol. Hearing levels of women with high LDL/HDL ratios were 5 dB better than those of women with low LDL/HDL ratios. The comparable difference in men was only 1 dB.

CONCLUSION

Results suggest that blood chemistry measures that are primarily within the normal range have very little value in predicting pure-tone thresholds in older subjects.

Interaction of noise-induced hearing loss and presbyacusis

A medical-legal and scientific topic of longstanding interest is the interaction between presbyacusis and noise-induced permanent threshold shift (NIPTS). Current medical-legal practices as well as international standard, ISO 1999 International Organization for Standards: Acoustics: Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment. ISO 1999. Geneva, International for Standards, 1990 assume that NIPTS and hearing loss caused by the aging process add in dB. Results of laboratory studies with animals are inconsistent in their support of the "additivity assumption". When intense, short-duration exposures are used, the predictions of the combined effects of noise and age are too large. The additivity model appears to be supported with long-duration exposures, but we question the accuracy of such predictions. The animal studies reviewed here suggest that the allocation of hearing loss in an older individual into a noise component and an aging component is much more complex than "additivity in dB".

Gender-specific effects of medicinal drugs on hearing levels of older persons

As part of a large-scale study of presbyacusis, responses to a medicinal drug questionnaire from 85 female and 126 male human subjects were analyzed. Medicinal drugs were divided into 35 categories based on their pharmacologic effects. Subjects' ages ranged from 60 to 82 years. At least 10% of subjects reported taking drugs in 14 of 35 categories. Results were significantly different between female and male subjects. In men, none of the 14 categories showed a statistically significant relation to the pure-tone average (PTA) of 500, 1000, 2000, and 4000 Hz. In women, 3 of the 14 categories showed a statistically significant relation to the PTA. First, the average PTA of female subjects taking beta-adrenergic medication was 20 dB higher (poorer) than those not taking beta-adrenergic medication. Second, women taking antihistamine/cold preparations had an average PTA 9 dB higher (poorer) than those not taking antihistamine/cold preparations. Third, the average PTA of women taking calcium-channel blockers (CCBs) was 12 dB lower (better) than those not taking CCBs. In men, however, these drugs produced effects on the PTA of less than 3 dB. Differences between women and men were not explainable by differences in age or hearing level.

Frequency and intensity discrimination measured in a maximum-likelihood procedure from young and aged normal-hearing subjects

A maximum-likelihood method was applied in measurements of frequency and intensity discrimination for aged and young normal-hearing subjects with closely matched audiograms. This method was preferred over other psychophysical procedures because it is efficient and controls experimental variance, features that are highly desirable for testing aged subjects. In order to implement the method, psychometric functions for each task were also measured from young subjects using a constant-stimuli procedure. For the young subjects, the differential thresholds obtained from these two procedures were generally comparable. Further, both sets of data were consistent with previous literature, indicating that the maximum-likelihood method was successfully applied for frequency and intensity discrimination. A frequency-dependent difference between young and aged subjects in both frequency and intensity discrimination was observed. Even with closely matched audiograms, aged subjects demonstrated poorer discrimination abilities than young subjects. The age-related difference was always largest at 500 Hz and decreased as frequency increased.

Estimating parameters for psychometric functions using the four-point sampling method

Although a psychometric function describing a subject's responses to some physical stimuli is of considerable value, characterizing such functions is time consuming and, hence, is not carried out routinely in psychophysical experiments. A principal reason for the lack of efficiency in characterizing a psychometric function is the use of sampling methods that either converge on a single point on the psychometric function, such as the PEST method, or which distribute observations uniformly over a wide range, such as the constant stimuli method. As an alternative, a multimodal four-point sampling method has been proposed [C. F. Lam, J. H. Mills, and J. R. Dubno, J. Acoust. Soc. Am. 99, 3689-3693 (1996)]. A psychometric function is then fitted to the four points (each with several trials) to estimate the threshold and slope parameters of the psychometric function. Adaptive methods, such as the up-down methods [H. Levitt, J. Acoust. Soc. Am. 49, 467-477 (1971)], can be used to provide good initial estimates of the threshold and spread parameters of a psychometric function described by a logistic function. In ongoing studies of age-related changes in auditory masking and discrimination, this new four-point sampling method has been applied to determine psychometric functions for absolute thresholds as a function of duration, thresholds in simultaneous and forward masking, frequency discrimination, and intensity discrimination in both young and aged human subjects. Results indicate that a reduction in data collection time of about 50% with no increase in variance can be achieved. This increase in efficiency applies to simple detection tasks by normal hearing subjects as well as to complex discrimination tasks by older subjects with hearing loss.

Age-related and gender-related changes in monaural speech recognition

Previous studies of older listeners suggest age-related declines in speech recognition. However, the interpretation of these results is not straightforward because auditory thresholds, which account for the largest proportion of the variance in speech-recognition scores, also vary considerably with age. Here, effects of age, gender, and auditory thresholds on several measures of speech recognition were assessed for a large sample of individuals enrolled in a longitudinal study of age-related hearing loss. Participants ranged in age from 55-84 years. They were evaluated with a battery of conventional audiometric measures and speech-recognition materials, including NU-6 monosyllabic words, Synthetic Sentence Identification sentences, and high-context and low-context sentences from the Speech Perception in Noise test. Two analyses were conducted to assure that changes in speech-recognition scores with age were examined independently of age-related changes in auditory thresholds. In the first analysis, no significant differences in speech recognition were observed for individuals in three age groups (55-64, 65-74, 75-84 years) who were selected so that average puretone thresholds for the groups were within 5 dB. In the second analysis, using partial correlations to adjust both score and age for their association with average thresholds, significant declines with age were observed for males in maximum word recognition, maximum synthetic sentence identification, and keyword recognition in high-context sentences. For females, no significant changes in speech recognition with age were observed for any test.

Interaction of noise-induced permanent threshold shift and age-related threshold shift

Current medical-legal practices as well as an international standard (ISO 1999) assume the permanent threshold shifts produced by exposure to noise add (in dB) to the threshold shifts caused by increased chronological age (presbyacusis). This assumption, known as the additivity rule, was tested in an animal model. Mongolian gerbils, born and raised in a quiet vivarium, were exposed at age 18 months to a 3.5-kHz pure tone for 1 h at 113 dB SPL. At 6-weeks post-exposure, permanent threshold shifts in the exposed ear were approximately 20 dB in the 4- to 8-kHz region. Thresholds in the nonexposed, control ear were unaffected by the exposure. Animals were then allowed to age in the quiet vivarium until age 36 months and then were retested. Thus in a given animal, aging-only effects were assessed in one ear (internal control) and noise-plus-aging effects were assessed in the other (test) ear. A second control was mean age-related threshold shift measured in 48 gerbils who were born and raised in the quiet vivarium. This group is referred to as a non-noise-exposed population (population control). Using the additivity rule, predictions with either the internal or population control significantly overestimated noise-plus-aging effects. Use of the ISO 1999 compression factor reduced the overestimations by 0-5 dB. The intensity rule produced the most accurate predictions. These results suggest that the interaction of noise-induced permanent threshold shift and age-related threshold shift is not straightforward and that current medical-legal methods using the additivity rule overestimate the contribution of "noise effects".

Extended high-frequency thresholds in older adults

Most measures of auditory sensitivity at extended high frequencies (frequencies greater than 8 kHz) have been obtained from listeners with normal hearing less than 40 years of age. The purpose of this study was (a) to measure thresholds at frequencies above 8 kHz in older listeners who, as a group, have elevated thresholds at lower frequencies, and (b) to assess test-retest reliability, age and gender effects, and the influence of thresholds below 8 kHz. Extended high-frequency (EHF) thresholds were measured for 162 older listeners (60-79 years) using a commercially available high-frequency audiometer, with a frequency range of 8 to 18 kHz and an intensity range of 0 to 110 dB SPL. Thresholds were measured once at the beginning of a 1- to 2-hour test session and then remeasured at the end of the test session. EHF thresholds of older listeners with normal hearing at conventional audiometric frequencies were substantially higher than the thresholds reported for younger listeners, with normal hearing by Dreschler and van der Hulst (1987). EHF thresholds of older listeners with hearing loss at conventional audiometric frequencies were further elevated as compared to older listeners with normal hearing. Differences in EHF thresholds between females and males were either not present or were reduced when gender differences in conventional audiometric thresholds were taken into account. No significant differences were seen in thresholds at 8 kHz and higher between the 60- to 69-and 70- to 79-year-old age groups. Results also indicated that thresholds above 8 kHz can be measured in older listeners within a clinically acceptable +/- 10 dB test-retest range.

Masking by ipsilateral and contralateral maskers

Contralateral masking occurs when the threshold of a signal in one ear is elevated by the presence of a masker in the other, contralateral ear. The classic data and theory on contralateral masking were provided by Zwislocki [J. Acoust. Soc. Am. 52, 644-659 (1972)] who observed a 3- to 18-dB threshold shift (masking) for a gated pure-tone signal in one ear when a gated pure-tone masker was presented via insert earphones to the other ear. Zwislocki referred to this phenomenon as "central masking." Here, using two psychophysical methods (Yes-No; two-interval forced-choice), Zwislocki's original results, obtained with other psychophysical methods, were successfully replicated. Similar results using several psychophysical methods suggest that contralateral masking is indicative of a sensory phenomenon rather than observer bias and other response proclivities. In a second experiment, psychophysical tuning curves were obtained using either an ipsilateral masker or a contralateral masker. Tuning curves obtained with a contralateral masker had steeper slopes on both the low- and high-frequency sides than tuning curves obtained with an ipsilateral masker. Thus, although substantially smaller in effect than ipsilateral masking, contralateral masking is more sharply tuned. The sharp tuning of contralateral masking reflects a greater compression of the input/output functions for contralateral masking than for ipsilateral masking. The closest correspondence between the tuning curves reported here for contralateral masking and those predicted by Zwislocki's theory and data (on central masking) occurred for tuning curves where the ratio of driven activity to spontaneous activity was about six. A remaining issue is the role, if any, of the efferent auditory system, especially the olivocochlear bundle, in threshold shifts measured using the Zwislocki (central masking) paradigm.

Placement of observations for the efficient estimation of a psychometric function

Psychometric functions for different psychophysical tasks describe the relationship between physical stimuli and subjects' responses. Although a psychometric function is of considerable value, characterizing the function is time consuming and, hence, is not carried out routinely in psychophysical experiments. A principal reason for the lack of efficiency in characterizing the psychometric function is the use of unimodal (e.g., Gaussian and uniform) sampling methods. As an alternative, a multimodal four-point sampling method is proposed. A psychometric function is then fitted to the four data points (each with several trials) to estimate the threshold and slope parameters of the psychometric function. Results from three examples demonstrate that a 60% savings in data-collection time can be achieved.

Growth of low-pass masking of pure tones and speech for hearing-impaired and normal-hearing listeners

Tonal thresholds and consonant recognition were measured in low-pass maskers as a function of masker bandwidth and spectrum level. Thresholds and consonant-recognition scores were obtained for normal-hearing subjects, and for pairs of normal-hearing subjects (who listened in threshold-equating background noise) and hearing-impaired subjects. Consonant-recognition scores were compared to scores predicted by a modified articulation index. Mean thresholds measured in low-pass maskers were higher for hearing-impaired than for normal-hearing subjects for signal frequencies above the masker. Slopes of functions relating thresholds for signals above the masker to masker spectrum level were not significantly different between hearing-impaired and normal-hearing subjects listening in spectrally shaped broadband noise (SSBB), but were shallower than slopes of masking functions for normal-hearing subjects listening without SSBB. Slopes of masking functions for signals within the masker were equivalent for all subjects. Slopes of functions relating consonant recognition to masker spectrum level were similar within subject pairs, whereas, in some cases, slopes of functions relating consonant recognition to speech level were shallower for hearing-impaired subjects than for their normal-hearing counterparts. Although greater improvement in consonant recognition with speech level was predicted for hearing-impaired than for normal-hearing subjects, on average, less improvement with speech level was observed. Shallower slopes of functions relating score to speech level observed for some hearing-impaired listeners may result from more shallow growth of speech sensation levels in spectral regions above the low-pass masker.

Masking of auditory brainstem responses in young and aged gerbils

Auditory brainstem responses (ABR) were recorded in the presence of low-pass (1 kHz cutoff) or high-pass (8 kHz cutoff) filtered noise in young (4-8 month) and aged (36 month) gerbils. For low-pass maskers, aged gerbils had higher masked thresholds at 2 and 4 kHz than young subjects. This was true for all aged subjects, including those with quiet thresholds similar to those of young controls. For high-pass masking, the majority of aged subjects had higher masked thresholds at 2 and 4 kHz than young controls; however, aged subjects with relatively normal quiet thresholds had masked thresholds similar to those of young subjects. A modified power-law (MPL) model was used to predict masked thresholds for aged subjects. Thresholds measured in the presence of low-pass noise were higher than predicted in many of the aged subjects, particularly those with near-normal quiet thresholds. In contrast, thresholds measured in the presence of the high-pass masker were similar to the predicted thresholds. These results suggest that: (a) excess masking occurred in aged subjects for low-pass, but not high-pass, maskers; (b) the excess masking occurred independently of quiet thresholds; and (c) excess upward spread of masking was related to the spectrum of the masker and not the 2 and 4 kHz regions of the auditory periphery.

Masked thresholds and consonant recognition in low-pass maskers for hearing-impaired and normal-hearing listeners

Thresholds and consonant recognition were measured in six low-pass maskers as a function of masker bandwidth for hearing-impaired subjects and for normal-hearing subjects listening in spectrally shaped broadband noise (SSBB). SSBB was adjusted such that thresholds in that masker for a normal-hearing listener were equal to a hearing-impaired listener's absolute thresholds. Thresholds measured in low-pass maskers were higher for hearing-impaired than for normal-hearing subjects for signal frequencies both within and outside masker passbands, although threshold differences were larger for signal frequencies outside masker passbands. Slopes of functions relating consonant recognition to speech level were not significantly different between groups, due to the presence of SSBB for the normal-hearing listeners. However, 25% of observed scores for hearing-impaired listeners, compared to only 5% of observed scores for normal-hearing listeners, were significantly poorer than predicted by the articulation index (AI), when AIs were computed using subjects' absolute thresholds. Better correspondence between observed and predicted scores in low-pass maskers was achieved when AIs were derived empirically from thresholds measured in each low-pass masker. Hence poorer-than-predicted consonant recognition scores in low-pass maskers were accounted for by higher-than-normal thresholds in those maskers.

Confidence limits for maximum word-recognition scores

Clinical judgments are often made regarding whether maximum word-recognition scores (PBmax) are appropriate in relation to degree of sensorineural hearing loss. In order to determine if word recognition is significantly poorer than expected, it is necessary to consider the lower boundary of PBmax associated with a particular degree of hearing loss for speech materials commonly used to measure word recognition. The purpose of this experiment was to define a confidence limit for PBmax from Northwestern University Test #6 (NU-6) word-recognition scores obtained from a large group of young and aged subjects with confirmed cochlear hearing loss. Word-recognition scores at several speech levels were obtained from 407 ears with a wide range of pure-tone averages. Because the characteristics of the distribution of maximum scores are not known, a procedure was developed using computer simulations to approximate the distribution of word-recognition scores corresponding to PBmax and determine the 95% confidence limit (CL). Results of the simulation were confirmed by comparing means and standard deviations of PBmax derived from experimental and simulation data. Percentages of young and aged subjects with scores outside the 95% CL are equal to their proportions in the entire subject sample. If PBmax determined from a score-level psychometric function is poorer than the 95% CL, PBmax may be considered "disproportionately" poor in relation to the degree of hearing loss. One score measured at a single arbitrary suprathreshold level that is poorer than the 95% CL suggests that the score may underestimate PBmax and that word recognition should be measured at additional levels to obtain a more reasonable estimate of the listener's maximum word-recognition score.

Frequency selectivity and consonant recognition for hearing-impaired and normal-hearing listeners with equivalent masked thresholds

Thresholds in notched-noise maskers (NN) and narrow-band maskers (NB) were measured for normal-hearing and hearing-impaired subject pairs who were listening in a background of spectrally shaped broadband noise (SSBB). Consonant recognition was also measured in SSBB. SSBB was adjusted so that thresholds in that noise for each normal-hearing/hearing-impaired subject pair were equal. Threshold and signal-level differences between subject pairs were minimized with the addition of threshold-elevating SSBB, and the presence of a noise background for both groups provided a comparable listening environment for all subjects. At signal frequencies outside masker passbands, thresholds in NN and NB for hearing-impaired subjects were higher than for normal-hearing subjects, although threshold differences were much smaller than observed between normal-hearing and hearing-impaired subjects without SSBB. No consistent differences in consonant recognition measured in SSBB were observed between groups. The pattern of results is comparable to that observed in a previous experiment [Dubno and Schaefer, J. Acoust. Soc. Am. 91, 2110-2121 (1992)] in which thresholds in NN and NB, and consonant recognition, for hearing-impaired listeners were compared to results obtained for normal-hearing subjects, but only normal-hearing subjects listened in SSBB. Using a modified power-law model of masking additivity, thresholds in combined-masker conditions were estimated. Masking effects for spectrally overlapping maskers were similar for normal-hearing and hearing-impaired listeners and suggest that residual differences between subject groups are not due to the presence of an additional background noise.

Minimal upward spread of masking: correlations with speech and auditory brainstem response masked thresholds

Much less than predicted upward spread of masking, termed undermasking, was previously noted in three elderly listeners, who are the focus of this report. Three aspects of undermasking were studied: Reliability, facilitation of speech recognition in noise, and correlation with physiological measurements. The masker was a 90-dB-SPL 1-kHz low-pass-filtered noise. Reliability of undermasking was assessed from serial measurements made over an approximately 2-year interval. Masked speech thresholds were measured with an adaptive procedure for two types of stimuli: spondaic words and the California Consonant Test. Subjects with undermasking had substantially lower masked speech thresholds than either audiometrically matched elderly subjects or young, normal-hearing subjects. It appears that undermasking is a stable characteristic that provides the listener with exceptionally good communication abilities in noise. Finally, a strong positive correlation was observed between the masked auditory brainstem response (wave V) threshold and the masked behavioral threshold, suggesting that the mechanism producing undermasking is in the auditory periphery.

Comparison of frequency selectivity and consonant recognition among hearing-impaired and masked normal-hearing listeners

Frequency selectivity and consonant recognition were determined for normal-hearing and hearing-impaired listeners using techniques that facilitate comparisons of performance among listeners whose absolute thresholds vary in magnitude and configuration. First, for each of six subjects with cochlear hearing loss, masked thresholds in notched noise and narrow-band-noise maskers were obtained and compared to results for three normal-hearing listeners whose thresholds were precisely matched to the impaired listeners' by masking with spectrally shaped broadband noise. Second, for hearing-impaired listeners and their masked normal-hearing controls, measurements of consonant recognition were obtained at several speech-presentation levels selected on the basis of articulation-index predictions to assure equal speech-spectrum audibility across listeners. The results suggest that frequency selectivity is poorer for hearing-impaired listeners than for masked normal-hearing listeners, even when thresholds among subjects are equated, but the deviation from normal frequency selectivity is smaller than estimated from comparisons with normal-hearing listeners in quiet. Critical ratios for hearing-impaired listeners are equivalent to normal. Although frequency selectivity is reduced, there is no consistent difference in consonant recognition between hearing-impaired subjects and masked normal-hearing subjects, when performance is assessed under conditions that assure equal speech-spectrum audibility across subjects.

Frequency selectivity for hearing-impaired and broadband-noise-masked normal listeners

When evaluating frequency selectivity, it is difficult to determine if the deviation from normal performance observed for hearing-impaired listeners reflects abnormal cochlear function or normal, level-dependent changes in frequency selectivity. This experiment was designed to investigate the dependence of frequency-selectivity measures on threshold and signal level. Auditory-filter characteristics, critical ratios, forward-masked psychophysical tuning curves, and narrowband-noise masking patterns were obtained from normal-hearing listeners in quite and in the presence of broadband noise at five levels. These findings were then compared to previously obtained frequency-selectivity measurements for subjects with mild-to-moderate sensorineural hearing loss whose absolute thresholds are comparable to the normal-hearing listeners' masked thresholds. The results suggest that frequency selectivity deterioriates as threshold increases for all subjects. However, the poor frequency selectivity exhibited by hearing-impaired listeners may not be explained entirely by the effects of threshold and signal level. Nevertheless, because frequency selectivity is poorer in the normal auditory system at higher stimulus levels, the deviation from normal performance observed for hearing-impaired listeners may not be as large as previously suggested.

Auditory filter characteristics and consonant recognition for hearing-impaired listeners

To examine the association between frequency resolution and speech recognition, auditory filter parameters and stop-consonant recognition were determined for 9 normal-hearing and 24 hearing-impaired subjects. In an earlier investigation, the relationship between stop-consonant recognition and the articulation index (AI) had been established on normal-hearing listeners. Based on AI predictions, speech-presentation levels for each subject in this experiment were selected to obtain a wide range of recognition scores. This strategy provides a method of interpreting speech-recognition performance among listeners who vary in magnitude and configuration of hearing loss by assuming that conditions which yield equal audible spectra will result in equivalent performance. It was reasoned that an association between frequency resolution and consonant recognition may be more appropriately estimated if hearing-impaired listeners' performance was measured under conditions that assured equivalent audibility of the speech stimuli. Derived auditory filter parameters indicated that filter widths and dynamic ranges were strongly associated with threshold. Stop-consonant recognition scores for most hearing-impaired listeners were not significantly poorer than predicted by the AI model. Furthermore, differences between observed recognition scores and those predicted by the AI were not associated with auditory filter characteristics, suggesting that frequency resolution and speech recognition may appear to be associated primarily because both are degraded by threshold elevation.

Stop-consonant recognition for normal-hearing listeners and listeners with high-frequency hearing loss. II: Articulation index predictions

Articulation index (AI) theory was used to evaluate stop-consonant recognition of normal-hearing listeners and listeners with high-frequency hearing loss. From results reported in a companion article [Dubno et al., J. Acoust. Soc. Am. 85, 347-354 (1989)], a transfer function relating the AI to stop-consonant recognition was established, and a frequency importance function was determined for the nine stop-consonant-vowel syllables used as test stimuli. The calculations included the rms and peak levels of the speech that had been measured in 1/3 octave bands; the internal noise was estimated from the thresholds for each subject. The AI model was then used to predict performance for the hearing-impaired listeners. A majority of the AI predictions for the hearing-impaired subjects fell within +/- 2 standard deviations of the normal-hearing listeners' results. However, as observed in previous data, the AI tended to overestimate performance of the hearing-impaired listeners. The accuracy of the predictions decreased with the magnitude of high-frequency hearing loss. Thus, with the exception of performance for listeners with severe high-frequency hearing loss, the results suggest that poorer speech recognition among hearing-impaired listeners results from reduced audibility within critical spectral regions of the speech stimuli.

Auditory brain stem evoked response characteristics in developing infants

The auditory brain stem response (ABR) of a single group of developing normal infants was examined longitudinally, from newborn through 6 months of age. A sufficiently broad range of stimulus variables was included to ensure that the auditory system was adequately sampled in order to demonstrate developmental principles. Findings indicate that there are no differences in wave V latency-intensity functions between infants and adults. For waves I, III, and V, absolute and interwave latency-repetition rate functions differ between infants and adults and undergo systematic changes throughout the first 6 months of life. The most dramatic ABR changes (between any two sequential test sessions in infants) occurred between the ages of newborn and 2 weeks, with less pronounced ABR changes beyond 2 weeks of age. The pattern of latency change for wave I was different from that for waves III and V. After 2 weeks of age, wave I latency was the same as the adult value at all repetition rates. In contrast, waves III and V were characterized by decreasing latency throughout the follow-up period. A curvilinear developmental model provided a satisfactory fit to ABR latency data.

Auditory brain stem evoked response characteristics in the full-term newborn infant

The auditory brain stem response to click stimuli was investigated in a group of 50 full-term healthy newborns, as well as in a group of 20 older children and adults. The stimulus parameters of click level and click repetition rate were varied systematically to quantify the characteristics of the auditory brain stem response in the full-term newborn infant. The results reveal increased latencies for waves I, III, and V for all conditions among the newborns, relative to the older age group. The results suggest that the neurological system is the primary source of differences between newborns and older subjects, but do not rule out the possibility that external ear, middle ear, or cochlear mechanisms may also contribute to the differences observed.

Comparison of speech recognition-in-noise and subjective communication assessment

A comparison was made between speech recognition performance in conditions of quiet and babble (Speech Perception in Noise Test) and items from a self-assessment scale concerned with communication ability in quiet and noise (Understanding Speech section of Hearing Performance Inventory). Performance on both the speech recognition and self-assessment tests differentiated between normal listeners and individuals with mild-to-moderate sensorineural hearing loss. For the hearing-impaired group, correlations between speech recognition scores and ratings on the self-assessment items were poor, suggesting that performance measured with these tests have only a weak relationship.

Effects of age and mild hearing loss on speech recognition in noise

Using an adaptive strategy, the effects of mild sensorineural hearing loss and adult listeners' chronological age on speech recognition in babble were evaluated. The signal-to-babble ratio required to achieve 50% recognition was measured for three speech materials presented at soft to loud conversational speech levels. Four groups of subjects were tested: (1) normal-hearing listeners less than 44 years of age, (2) subjects less than 44 years old with mild sensorineural hearing loss and excellent speech recognition in quiet, (3) normal-hearing listeners greater than 65 with normal hearing, and (4) subjects greater than 65 years old with mild hearing loss and excellent performance in quiet. Groups 1 and 3, and groups 2 and 4 were matched on the basis of pure-tone thresholds, and thresholds for each of the three speech materials presented in quiet. In addition, groups 1 and 2 were similar in terms of mean age and age range, as were groups 3 and 4. Differences in performance in noise as a function of age were observed for both normal-hearing and hearing-impaired listeners despite equivalent performance in quiet. Subjects with mild hearing loss performed significantly worse than their normal-hearing counterparts. These results and their implications are discussed.

Suggestions for optimizing reliability with the synthetic sentence identification test

The reliability of the Synthetic Sentence Identification (SSI) test was determined for a group of 33 listeners with mild-to-moderate sensorineural hearing loss. Randomizations of the 10-item test were administered 12 times (six trials on each of two days) under identical listening conditions. Lists were presented monaurally, under earphones, at 90 dB SPL with a cafeteria background noise set at a signal-to-noise ratio (S/N) estimated (by adaptive procedure for each subject) to obtain 50% performance. Average scores ranged from 48.8% for Trial 1 to 67.9% for Trial 9. Reliability of the 12 measurements was determined by Pearson correlation, analyses of variance, and by testing individual differences in scores against a theoretical statistical distribution. Results evidenced large intra-subject variability, especially for the initial trials conducted in each testing session. However, if sufficient practice trials are presented each day under relatively difficult listening conditions, the scores on subsequent lists tend to remain relatively stable. Suggested procedures to optimize reliability are included.

A procedure for quantifying the effects of noise on speech recognition

This paper describes the results of two experiments in which speech recognition performance was determined for listeners with sensorineural hearing loss, while listening in babble. Adaptive strategies were used in both experiments to measure the signal-to-babble ratio required to achieve a preselected level of performance at several speech presentation levels encountered in normal conversation or when listening through an amplification system. The results suggest that the proposed adaptive strategy may provide a practical method by which the relative effects of competition on speech recognition may be quantified in an individual listener.

Evaluation of hearing-impaired listeners using a Nonsense-syllable Test. II. Syllable recognition and consonant confusion patterns

Syllable recognition ability and consonant confusion patterns were evaluated for 38 listeners with mild-to-moderate sensorineural hearing loss using the closed-set Nonsense-Syllable Test (NST). Performance for these materials varies as a function of consonant voicing, the position of the consonant in the syllable, and the accompanying vowel. Scores for listeners with steeply sloping audiometric configurations were consistently poorer than those for listeners with gradually sloping or flat audiograms. Consonant confusion analyses revealed place of articulation errors to be the most frequent, regardless of the listener's audiometric configuration. Analysis of consonant confusion patterns indicates the existence of a systematic relationship between consonant confusions and audiometric configuration. The NST findings are discussed in terms of the test's potential use and are compared to the results of existing confusion analyses.

Evaluation of hearing-impaired listeners using a Nonsense-Syllable Test. I. Test reliability

The reliability of a closed-set Nonsense-Syllable Test was determined on a group of 38 listeners with mold-to-moderate sensorineural hearing loss. Eight randomizations of the 91-item test (four trials on each of two days) were presented monaurally, under earphones, at 90 dB SPL with a cafeteria background noise set at a +20-dB S/N ratio. Performance under these conditions ranged from 21.4 to 91.2%, reflecting the wide range of syllable-recognition ability of these subjects. Reliability of the eight measurements was determined by analysis of variance and analysis of covariance structure (parallel-test modelling) for the entire test and each of 11 subtests. Overall and individual subject results failed to show any systematic differences in scores over eight trials. Likewise, no significant differences were found in performance on individual syllables, nor were changes in the relative occurrence of specific syllable confusions noted. The test is highly reliable when evaluating hearing-impaired subjects, and thus is appropriate for use in investigations where identical items are administered under multiple experimental conditions.

Detection of tones in band-reject noise

Three experiments were conducted to investigate the detectability of tonal signals simultaneously presented with computer-generated, long duration (.5 sec), band-reject maskers. In Experiment 1, detectability of a tone of 500, 1000, or 2000 Hz presented at 50 dB SPL was measured as a function of the width of a spectral "notch" symmetrically placed around the tone. A narrow notch resulted in decreased detectability relative to the wide band (zero-notch width) control condition. Further increases in notch width resulted in increased detectability until subjects approached errorless performance. In Experiment 2, psychometric functions were obtained for a 1000-Hz tone presented in four notch-noise conditions. The slopes of the psychometric functions were consistently steeper in the band-reject conditions relative to the zero notch-width condition. These slope differences result in relationships between threshold and notch width that depend upon the region of the psychometric function from which the threshold estimates are obtained. In Experiment 3, the decrease in performance associated with the notch was found to increase in proportion to notch depth. The data suggest that spectral edges placed in the immediate vicinity of a tone reduce the detectability of the tonal signal. This "edge effect" is discussed in relation to two-tone inhibition that has been observed in the auditory nerve.

Predicting consonant confusions from acoustic analysis

Acoustic measurements of nonsense syllables in quiet and in noise were used to predict the pattern of consonant confusions made under those conditions. Eleven sets of nonsense syllables were presented to normal-hearing subjects in quiet and at a +5 dB speech-to-noise ratio, at five speech levels. A set of acoustic characteristics of the speech stimuli were chosen for analysis and measured using digital processing techniques. Results of the recognition task revealed significant effects of consonant voicing, position and vowel context on syllable recognition. The performance-intensity function of the quiet condition rises more steeply than the function obtained in noise. The effect of noise on consonant recognition is dependent upon the manner in which the consonant is produced, and the location of maximum constriction. Differences in the absolute values of the acoustic parameters of syllable pairs were used to predict their percentage of confusion. A set of acoustic variables was isolated which was found to be the best predictor of confusion percentages. Although the sets of acoustic variables were different for various syllable types and test conditions, three variables (consonant energy, consonant spectral peaks, consonant-to-noise ratio) were used in a majority of the predictions.

Speech recognition performance at loudness discomfort level

The purpose of this experiment was to determine the relationship between the loudness discomfort level (LDL) and maximum speech recognition performance for listeners with sensorineural hearing loss. Performance-intensity functions were obtained for 30 listeners with mild to moderate sensorineural hearing impairment at five speech levels from -18 to +4 dB re: the LDL. Recognition scores were obtained for three speech materials. For all speech materials, recognition scores at levels below the LDL were equivalent to or higher than scores obtained at and above the LDL. These results support the contention that the LDL represents a sound pressure level above which no improvement in speech recognition occurs, and thus, might reasonably be used as the appropriate level for the saturation sound pressure level of a hearing aid.

Acoustic-reflex thresholds for noise stimuli

The effect of noise bandwidth on the acoustic-reflex threshold was studied in 12 normal-hearing adult subjects by independently varying the cutoff frequencies of low-pass (fL = 100 Hz) and high-pass (fH = 6000 Hz) computer-generated noises. Adjusting the cutoff frequency of the low-pass or high-pass activating stimuli over the 700-6000 Hz range resulted in a change in the reflex threshold of 4 dB/octave. As the cutoff frequency of the low-pass noise was reduced below 700 Hz, more rapid increases in reflex thresholds occurred. For high-pass stimuli, very small changes in reflex thresholds were noted as the lower cutoff frequency was increased from 100 to 700 Hz. The data suggest that between 700 and 6000 Hz equal spectral regions of a noise stimulus contribute equally to the reflex threshold, and that energy below 700 Hz contributes little to the sensitivity of the reflex.

Comments on the acoustic-reflex response for bone-conducted signals

Previous studies which have measured acoustic-reflex responses to bone-conducted signals have not effectively differentiated reflex responses from artifacts. A convenient method for identifying such artifacts was developed and employed on some acoustic-reflex measures for bone-conducted signals. The findings indicated that artifacts result when a frequently-used acoustic admittance meter (Grason-Stadler 1720B) and a conventional bone vibrator were used to measure reflex responses for tonal and noise-activating signals. It was suggested that the method be employed in future studies which investigate the acoustic reflex in response to bone-conducted signals.