Effects of hearing loss on echo thresholds

OBJECTIVE: The purpose of this investigation was to determine the effects of hearing loss on the perception of echoes. DESIGN: Echo thresholds were measured for eight listeners with normal hearing and nine listeners with impaired hearing. Pairs of 4-msec noise bursts were presented to each listener with onset-to-onset delays ranging from 2 to 16 msec. Echo thresholds were obtained at signal presentation levels of 10, 20, 30, 40, and 50 dB SL. RESULTS: Results revealed differences between the psychometric functions of the two subject groups. Psychometric functions of the subjects with impaired hearing indicated higher echo thresholds than for the subjects with normal hearing. In addition, echo thresholds at 10 dB SL were significantly higher than echo thresholds measured at 40 dB SL for both subject groups. CONCLUSION: Listeners with impaired hearing exhibit higher echo thresholds than listeners with normal hearing. The higher echo thresholds for listeners with impaired hearing may account, at least in part, for difficulty on tasks such as localization in everyday listening environments.     

High-level psychophysical tuning curves: forward masking in normal-hearing and hearing-impaired listeners.

Forward-masked psychophysical tuning curves (PTCs) were obtained for 1000-Hz probe tones at multiple probe levels from one ear of 26 normal-hearing listeners and from 24 ears of 21 hearing-impaired listeners with cochlear hearing loss. Comparisons between normal-hearing and hearing-impaired PTCs were made at equivalent masker levels near the tips of PTCs. Comparisons were also made of PTC characteristics obtained by fitting each PTC with three straight-line segments using least-squares fitting procedures. Abnormal frequency resolution was revealed only as abnormal downward spread of masking. The low-frequency slopes of PTCs from hearing-impaired listeners were not different from those of normal-hearing listeners. That is, hearing-impaired listeners did not demonstrate abnormal upward spread of masking when equivalent masker levels were compared. Ten hearing-impaired ears demonstrated abnormally broad PTCs, due exclusively to reduced high-frequency slopes in their PTCs. This abnormal downward spread of masking was observed only in listeners with hearing losses greater than 40 dB HL. From these results, it would appear that some, but not all, cochlear hearing losses greater than 40 dB HL influence the sharp tuning capabilities usually associated with outer hair cell function.     

Auditory brainstem responses predict auditory nerve fiber thresholds and frequency selectivity in hearing impaired chinchillas

Noninvasive auditory brainstem responses (ABRs) are commonly used to assess cochlear pathology in both clinical and research environments. In the current study, we evaluated the relationship between ABR characteristics and more direct measures of cochlear function. We recorded ABRs and auditory nerve (AN) single-unit responses in seven chinchillas with noise-induced hearing loss. ABRs were recorded for 1-8 kHz tone burst stimuli both before and several weeks after 4 h of exposure to a 115 dB SPL, 50 Hz band of noise with a center frequency of 2 kHz. Shifts in ABR characteristics (threshold, wave I amplitude, and wave I latency) following hearing loss were compared to AN-fiber tuning curve properties (threshold and frequency selectivity) in the same animals. As expected, noise exposure generally resulted in an increase in ABR threshold and decrease in wave I amplitude at equal SPL. Wave I amplitude at equal sensation level (SL), however, was similar before and after noise exposure. In addition, noise exposure resulted in decreases in ABR wave I latency at equal SL and, to a lesser extent, at equal SPL. The shifts in ABR characteristics were significantly related to AN-fiber tuning curve properties in the same animal at the same frequency. Larger shifts in ABR thresholds and ABR wave I amplitude at equal SPL were associated with greater AN threshold elevation. Larger reductions in ABR wave I latency at equal SL, on the other hand, were associated with greater loss of AN frequency selectivity. This result is consistent with linear systems theory, which predicts shorter time delays for broader peripheral frequency tuning. Taken together with other studies, our results affirm that ABR thresholds and wave I amplitude provide useful estimates of cochlear sensitivity. Furthermore, comparisons of ABR wave I latency to normative data at the same SL may prove useful for detecting and characterizing loss of cochlear frequency selectivity.     

Auditory-filter characteristics for listeners with real and simulated hearing impairment

Functional simulation of sensorineural hearing impairment is an important research tool that can elucidate the nature of hearing impairments and suggest or eliminate compensatory signal-processing schemes. The objective of the current study was to evaluate the capability of an audibility-based functional simulation of hearing loss to reproduce the auditory-filter characteristics of listeners with sensorineural hearing loss. The hearing-loss simulation used either threshold-elevating noise alone or a combination of threshold-elevating noise and multiband expansion to reproduce the audibility-based characteristics of the loss (including detection thresholds, dynamic range, and loudness recruitment). The hearing losses of 10 listeners with bilateral, mild-to-severe hearing loss were simulated in 10 corresponding groups of 3 age-matched normal-hearing listeners. Frequency selectivity was measured using a notched-noise masking paradigm at five probe frequencies in the range of 250 to 4000 Hz with a fixed probe level of either 70 dB SPL or 8 dB SL (whichever was greater) and probe duration of 200 ms. The hearing-loss simulation reproduced the absolute thresholds of individual hearing-impaired listeners with an average root-mean-squared (RMS) difference of 2.2 dB and the notched-noise masked thresholds with an RMS difference of 5.6 dB. A rounded-exponential model of the notched-noise data was used to estimate equivalent rectangular bandwidths and slopes of the auditory filters. For some subjects and probe frequencies, the simulations were accurate in reproducing the auditory-filter characteristics of the hearing-impaired listeners. In other cases, however, the simulations underestimated the magnitude of the auditory bandwidths for the hearing-impaired listeners, which suggests the possibility of suprathreshold deficits.     

Effects of cochlear impairment and equivalent-threshold masking on psychoacoustic tuning curves.

Psychoacoustic tuning curves in simultaneous masking were measured in three groups: listeners with hearing impairment of primarily cochlear origin, normal listeners with equivalent-threshold masking (ETM) and normal listeners tested in the quiet. ETM was produced by presenting a continuous back-ground noise that was spectrally shaped to yield masked thresholds within 3 dB of each impaired listener's quiet thresholds. For the tuning curves, the level of a 50-Hz-wide noise band necessary to mask a 10-dB SL tone was measured for six masker frequencies. Results under ETM indicate that the shape of the tuning curve depends not only on masked threshold at the probe frequency but also on the spectral shape of the masker. Tuning curves are abnormally broad in some impaired listeners even when the effects of their elevated thresholds, audiometric configuration and altered intensity perception are taken into account by comparisons with normal listeners with ETM.     

The influence of cochlear hearing loss and probe tone level on compound action potential tuning curves in humans

The effect of cochlear hearing loss and of probe tone level on slopes and sharpness of compound action potential tuning curves was investigated. Thirty-one simultaneously masked isoreduction (50%) tuning curves were determined in 26 adults with cochlear hearing losses up to 60 dB. Probe tone frequency was 2 or 3 kHz. Probe tone level was chosen as close as possible to the action potential threshold, usually within 30 dB. In 5 cases a second tuning curve was determined at a 20–30 dB higher probe tone level in order to differentiate between effects of hearing loss and of probe tone level itself on decrease of selectivity. Tuning was analysed in terms of high- and low-frequency slopes of the tuning curves, both in the steepest parts near the tip and overall, and in terms of Q_10dB. Slopes and tuning quality diminished with increasing hearing loss up to 60 dB. Part of the decrease in Q₁₀ could be attributed to increased probe tone level, implying that frequency selectivity is also a level-dependent property. In the same group of subjects so called ‘narrow-band’ (or ‘derived response’) compound action potential latencies were determined at 90 dB per SPL and a derived frequency similar to the probe tone in the tuning curve experiments. Narrow band latencies did not change significantly out of the normal range (2 periods) with increasing hearing loss. This implies that narow band latencies are not related to hearing loss, but reflect only the probe-level dependent impulse response delay. Analysis shows that it is possible to derived Q_10dB from narrow band latencies with probe level as a parameter.     

Interaction of click intensity and cochlear hearing loss on auditory brain stem response wave V latency.

Auditory brain stem responses (ABRs) to 95, 80, 60, 40, and 30 dB nHL clicks were retrospectively studied from 103 patients (194 ears) with various degrees of cochlear impairment. Hearing loss and sample size were balanced across gender. Results indicate that the slope of the wave V latency versus 4000 Hz hearing loss function doubles as click intensity is decreased from 80 (0.01 msec/dB HL) to 60 nHL (0.02 msec/dB HL). Overall results indicate a slope increase of 0.0004 msec for each decibel decrease in click intensity from 95 to 30 dB nHL. Intersubject variability increased with increased hearing loss and/or decreased stimulus intensity. The effects of hearing loss on wave V latency are minimal, and intersubject variability is less if high-intensity clicks (greater than or equal to 95 dB nHL) are used. No differences in the effects of hearing loss on wave V latency were seen between males and females. Latency corrections for cochlear hearing loss should, therefore, consider stimulus intensity.     

Use of low-pass noise in word-recognition testing.

This study attempted to determine whether word-recognition scores obtained in noise were more sensitive to the presence of a hearing loss than recognition scores obtained in quiet. Subjects with normal hearing, high-frequency cochlear hearing loss, and flat cochlear hearing loss were tested in quiet and in the presence of a 500-Hz low-pass noise. Two signal-to-noise conditions were employed, -4 dB and -12 dB. Words were presented at 40dB SL in one experiment and at 96 dB SPL for normal-hearing subjects in a second experiment. The results indicated that, while the word-recognition scores of groups were similar in quiet, the more negative the signal-to-noise ratio, the greater the separation of group scores, with hearing-impaired subjects having poorer recognition scores than normal-hearing subjects. When the speech and noise were presented at high SPLs, however, the normal-hearing subjects had poorer word recognition than those with flat cochlear losses. The results are interpreted as indicating greater spread of masking in normal-hearing than hearing-impaired subjects at high sound pressure levels.     

Frequency selectivity and speech discrimination in sensorineural hearing loss

Two supposed measures of auditory frequency selectivity--the critical band (CB) in loudness summation and the psychoacoustic tuning curve (PTC), Both measured at 1 kHz--were compared with the capacity for speech discrimination in patients with various cochlear disorders and a relatively flat audiometric pattern. The CB in loudness summation was correlated neither to the degree of hearing loss nor to the speech discrimination score. In contrast, the PTC changed with increasing hearing loss in the same manner as the electrophysiological tuning curve (FTC), i.e. rapidly deteriorating beyond normal limit values when the hearing loss exceeded 30--40 dB (HL). Nearly the same dependency of the degree of the hearing loss was demonstrated for the speech discrimination score (determined in noise and after filtering of the signal) and a significant correlation was present between this score and cochlear tuning, as expressed by the PTC. It is proposed that the PTC is a more valid measure of auditory frequency selectivity than the CB in loudness summation. If this is accepted the results seem to support the hypothesis of impaired frequency selectivity as a major cause for deteriorated speech discrimination in patients with cochlear disorders.     

Temporal gap detection in sensorineural and simulated hearing impairments

The objectives of this study were to assess the effect of the configuration of a hearing loss on gap detection and to determine if hearing impairment affects temporal resolution, per se. The minimum detectable gap duration, MDG, in a low-pass (cut-off at 7 kHz) noise was measured monaurally as a function of sound pressure level in six listeners with normal hearing, seven with hearing impairments of primarily cochlear origin, and eight with impairments simulated by masking. The impaired listeners' MDGs at 80 and 90 dB vary from about 3.5 ms (equal to the normal MDG) to about 8 ms and show little correlation with their average HL. At lower levels, the MDG is enlarged for all impaired listeners owing to the decreased SL of the noise. Most of the enlargement of the MDG could be reproduced by presenting a normal listener with a masking noise spectrally shaped to simulate the impaired listener's audiogram. However, at high levels, some impaired listeners performed worse than their simulated-loss counterparts, indicating that temporal resolution per se may be reduced in some, but not all, impaired listeners.     

Homogeneity of the 18 QuickSIN lists

The purpose of this study was to determine the list equivalency of the 18 QuickSIN (Quick Speech in Noise test) lists. Individuals with normal hearing (n = 24) and with sensorineural hearing loss (n = 72) were studied. Mean recognition performances on the 18 lists by the listeners with normal hearing were 2.8 to 4.3 dB SNR (signal-to-noise ratio), whereas the range was 10.0 to 14.3 dB SNR for the listeners with hearing loss. The psychometric functions for each list showed high performance variability across lists for listeners with hearing loss but not for listeners with normal hearing. For listeners with hearing loss, Lists 4, 5, 13, and 16 fell outside of the critical difference. The data from this study suggest nine lists that provide homogenous results for listeners with and without hearing loss. Finally, there was an 8.7 dB difference in performances between the two groups indicating a more favorable signal-to-noise ratio required by the listeners with hearing loss to obtain equal performance.     

Growth of Loudness in Listeners with Cochlear Hearing Losses: Recruitment Reconsidered

This article examines how loudness grows with increasing intensity near threshold in five listeners with hearing losses of predominantly cochlear origin. It provides evidence against the pervasive and long-held notion that such listeners show abnormally rapid loudness growth near their elevated thresholds. As in a previous study for listeners with normal hearing, loudness functions near threshold were derived from loudness matches between a pure tone and four- or ten-tone complexes using a simple model of loudness summation. This study assumed that the loudness function had the same form for any component of a complex, but a scale factor that depended on the amount of hearing loss allowed the loudness at threshold to vary with frequency. The best-fitting loudness functions had low-level local exponents [i.e., slopes of the low-level loudness function plotted as log(loudness) versus log(intensity)] that were within the normal range. At 0 dB SL, the average local exponents were 1.26 for the listeners with hearing losses compared with 1.31 for normal listeners, which indicates that loudness near threshold grows at similar rates in normal listeners and listeners with hearing losses. The model also indicated that, on average, the loudness at threshold doubled for every 16 dB of hearing loss. The increased loudness at threshold, together with somewhat enlarged local exponents around 20 dB SL, accounts for the near-normal loudness often obtained for high-SPL tones in listeners with hearing losses. Such loudness functions are consistent with the steep functions shown by classical data on loudness matches between tones for which thresholds are normal and tones for which thresholds are elevated. Thus, the present data indicate that an abnormally large loudness at an elevated threshold is likely to be a better definition of recruitment than the classical definition of it as an abnormally rapid growth of loudness above an elevated threshold.     

Role of probe tone parameters in frequency selectivity for normal listeners

The objective of this research was a determination of the role of probe tone parameters in frequency selectivity. Frequency selectivity is the ability of the auditory system to analyze a complex sound into its frequency components, and is typically evaluated by measuring the psychophysical tuning curve. Generally this entails documenting the effect of varying the center frequency of a narrowband noise masker for a given pure tone probe, 10 dB above threshold. In the present study, a new approach was taken. A family of tuning curves were generated for a series of tone probes which differed in frequency, duration and intensity, but which remained constant in energy. The results for three normal-hearing subjects indicated that the critical parameter for the shape of the tuning curve was the energy of the probe. Decreasing the energy below values generally tested resulted in aberrant shapes across subjects, which bore some resemblance to those documented in hearing-impaired listeners. Temporal summation was suggested as the unifying principle.     

Residual frequency selectivity in the profoundly hearing-impaired listener

The extent to which auditory frequency analysis is retained in profoundly hearing-impaired listeners has major implications for hearing aid design. We have measured simplified psychoacoustic tuning curves in nine such listeners, using sinusoidal probes at 125 and 250 Hz, and 80-Hz wide narrow-band noise maskers. Two listeners showed PTCs at 125 and 250 Hz whose shapes were independent of probe frequency and parallel to their absolute thresholds, indicating the complete absence of frequency selectivity. Seven listeners showed evidence of frequency selectivity at 125 or 250 Hz or at both frequencies; at 250 Hz, frequency selectivity was evident in the six listeners whose 250-Hz hearing level was 95 dB or less, but not in the listeners with 250 Hz hearing levels above 95 dB. Where conventional 'v'-shaped PTCs were observed, estimated 3-dB auditory filter bandwidths were two to three times larger than those typically found in normal listeners. Notched-noise masking results at 250 Hz from the least hearing-impaired listener gave an estimated 3-dB bandwidth in reasonable agreement with that from the same listener's PTC data. Listeners who retain some frequency selectivity are able to make some use of first formant information in vowel identification, and preliminary results from one patient showed the ability to distinguish a variety of noise spectra. Both of these abilities could be of potential importance as a basis for the recoding of speech spectral patterning through stimulation matched to the listener's residual frequency selectivity.     

Upward spread of masking in normal and abnormal ears

Upward spread of masking was studied for normals and sensorineurally hearing-impaired subjects with high-frequency hearing loss. Hearing-impaired listeners were recruited in such a way as to present normal hearing on the frequency of the masker, that is a narrow band of noise centered at 1 000 Hz. Levels of the masker were set at 70, 80 and 90 dB, respectively. Results first indicated the presence of a relationship between masked and elevated absolute thresholds for a masker level of 70 dB. At masker levels of 80 and 90 dB, hearing-impaired listeners showed excessive upward spread of masking in spite of normal hearing sensitivity at the masker frequency: with 80 and 90 dB of noise, upward spread of masking grew, respectively, 2.6 and 1.6 times faster than in normals. Furthermore, excessive upward spread of masking was shown to progress as a function of hearing loss. Results were interpreted as additional evidence of abnormal frequency selectivity in sensorineurally hearing-impaired listeners.     

Temporal modulation transfer functions for band-limited noise in subjects with cochlear hearing loss.

The modulation depth required for the detection of sinusoidal amplitude modulation was measured as a function of modulation rate, giving temporal modulation transfer functions (TMTFs). The carrier was a one-octave wide noise centred at 2 kHz, and it was presented in an unmodulated background noise lowpass filtered at 5 kHz. Three subjects with unilateral cochlear hearing loss were tested. For each subject, the normal ear was tested both at the same sound pressure level (SPL) and at the same sensation level (SL) as the impaired ear. The TMTFs were essentially the same for the normal and impaired ears, both at equal SPL and at equal SL. The better ears of three subjects with bilateral cochlear losses were also tested. Again, TMTFs were essentially the same as obtained for normal ears. These results suggest that temporal resolution is not necessarily adversely affected by cochlear hearing loss, at least as measured by this task.     

Measurement of the temporal-modulation transfer function for a single listener with cochlear hearing loss and left-hemisphere damage

The modulation depth required for the detection of sinusoidal amplitude-modulation applied to a white noise carrier was measured as a function of modulation frequency, giving temporal modulation transfer functions (TMTFs). Five adult listeners with normal hearing (mean age 52 years), five elderly listeners with moderate cochlear hearing loss (mean age 66 years) and a single elderly listener (aged 73 years) with moderate cochlear hearing loss and left-hemisphere damage were tested in the right ear at 50 dB SL. The five elderly listeners were matched in audiogram with the brain-damaged listener. Modulation detection was systematically poorer than normal in the five elderly listeners with cochlear hearing loss. However, their TMTFs were lowpass in shape, as for the five normal-hearing adult listeners. Modulation detection was much poorer in the elderly listener with cochlear hearing loss and left-hemisphere damage compared to the five normal-hearing adults and the five elderly listeners with cochlear hearing loss. Moreover, modulation detection was poorer at 4, 64 and 128 Hz than at 8, 16 and 32 Hz in the brain-damaged listener, giving his TMTF a bandpass appearance. These results are in agreement with the hypothesis that the main factors limiting the ability to detect changes in the temporal-envelope of sounds are located at a central (retro-cochlear) level of the auditory system rather than at a peripheral (cochlear) level. They also suggest that the TMTF approach may prove useful in distinguishing peripheral and central hearing losses.     

Débordement de masquage normal et anormal sur les aigus

Upward Spread of Masking in Normal and Impaired Ears

Upward spread of masking was studied for normals and sensorineurally hearing-impaired subjects with high-frequency hearing loss. Hearing-impaired listeners were recruited in such a way as to present normal hearing on the frequency of the masker, that is a narrow band of noise centered at 1 000 Hz. Levels of the masker were set at 70, 80 and 90 dB, respectively. Results first indicated the presence of a relationship between masked and elevated absolute thresholds for a masker level of 70 dB. At masker levels of 80 and 90 dB, hearing-impaired listeners showed excessive upward spread of masking in spite of normal hearing sensitivity at the masker frequency: with 80 and 90 dB of noise, upward spread of masking grew, respectively, 2.6 and 1.6 times faster than in normals. Furthermore, excessive upward spread of masking was shown to progress as a function of hearing loss. Results were interpreted as additional evidence of abnormal frequency selectivity in sensorineurally hearing-impaired listeners.     

Critical bandwidth in loudness summation in sensorineural hearing loss.

Critical band estimation was performed in thirty-three patients with sensorineural hearing loss by the method of loudness summation, using noise bands centred around 1 kHZ. In 17 patients the hearing loss was of hereditary origin, in 16 patients of various, mostly unknown, origin. The normal loudness difference between broad band noise and narrow band noise decreased with increasing hearing loss, most pronounced in recruiting ears. Judged individually, 1/3 of the patients appeared to have a widened critical band, but several biases appeared to be responsible for these results. The pooled data indicated a normal critical band, both in patients with hearing losses less than 50 dB HL and in patients with hearing losses greater than or equal to 50 dB HL. Based on this finding, the validity of the critical band as a measure of the frequency selectivity of the ear is discussed and a theory is proposed for the anatomical and functional correlate of the critical band in loudness summation.     

Comparing normal hearing and hearing-impaired subject's performance on the Northwestern Auditory Test Number 6, California Consonant Test, and Pascoe's High-Frequency Word Test

The Northwestern Auditory Test No. 6, the California Consonant Test, and Pascoe's High-Frequency Test were presented to two groups of listeners. One group consisted of 12 patients with normal hearing up to 2 kHz accompanied by a high-frequency loss. The second group consisted of persons with normal hearing. There were significant differences between the two groups of listeners in overall discrimination scores, among the four intensity levels (10, 20, 30, 40 dB) and the three speech discrimination tests. When the data from the hearing-impaired subjects were analyzed, it was found that these individuals obtained higher scores on the NU 6 material at 20, 30, and 40 dB SL when compared with the California Consonant Test and Pascoe's test. The results from the Newmann-Kuel multicomparison test indicated no statistical significance between the results of the California test and Pascoe's test; however, the distribution of scores at 40 dB was different. Results of this study suggested that the NU 6 test was not sufficiently sensitive to detect consonant confusions in individuals with high-frequency hearing loss.