Anna C. Schroder

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.

Intensity discrimination in normal‐hearing and hearing‐impaired listeners

Weber fractions (delta I/I) for gated 500-ms tones at 0.3, 0.5, 1, 2, and 3 kHz, and at levels of the standard ranging from absolute threshold to 97 dB SPL, were measured in quiet and in high-pass noise in five listeners with cochlear hearing loss and in three normal-hearing listeners. In regions of hearing loss, the Weber fractions at a given SPL were sometimes normal. When the Weber fractions were normal or near-normal, the addition of high-pass noise elevated the Weber fraction, strongly suggesting the use of spread of excitation to higher frequencies. Inversely, when the Weber fractions were elevated, the addition of high-pass noise produced no additional elevation, suggesting an inability to use spread of excitation. In general, the relative size of the Weber fractions, the effects of high-pass noise, and to a lesser extent, the dependence of the Weber fraction on level, were consistent with expectations based upon the audiometric configuration and the use of excitation spread. There were several notable inconsistencies, however, in which normal Weber fractions were seen at a frequency on the edge of a steep high-frequency loss, and in which elevated Weber fractions were observed in a flat audiometric configuration. Finally, when compared at the same SL, the Weber fraction was sometimes smaller in cochlear-impaired than in normal hearing listeners. This was true even in high-pass noise, where excitation spread was limited, and may reflect the unusually steep rate versus level functions seen in auditory nerve fibers that innervate regions of pathology.

Linearized response growth inferred from growth-of-masking slopes in ears with cochlear hearing loss

Growth of masking for OFF-frequency conditions (probe frequency above the masker spectrum) and ON-frequency conditions (probe within the masker spectrum) was investigated using simultaneous masking in three subjects with normal hearing and nine subjects with high-frequency sensorineural hearing loss. Growth-of-masking functions (probe thresholds as a function of masker intensity) for OFF-frequency conditions were obtained for probe tones placed at six frequencies above a 200-Hz-wide masker with an upper edge at 520 Hz. Growth-of-masking functions for ON-frequency conditions were obtained for probe tones placed within the 200-Hz-wide masker and for probe tones placed within 400-Hz-wide maskers with upper edges at 1040, 1300, 1627, and 2040 Hz (probe tones placed 20 Hz below the upper edge frequency). Growth-of-masking functions were fit with a power function of masker intensity added to an internal noise with intensity equal to the absolute threshold for the probe, and were well described by two free parameters and a threshold constant: the growth-of-masking slope (beta), a masking sensitivity constant (kappa) that indicated the minimum effective masker level at which masking began, and the intensity of the probe at absolute threshold (IT). For OFF-frequency masking conditions, growth-of-masking slopes (beta) decreased by a factor of 0.8 for every 10 dB of hearing loss. Comparisons with data from previous studies of upward spread of masking, and assumptions about underlying physiological mechanisms, led to the conclusion that more gradual than normal growth-of-masking slopes reflect larger (steeper) growth-of-response slopes at the probe frequency in regions of hearing loss. Derived response-growth exponents increased by a factor of 1.2 for every 10 dB of hearing loss (HL), from an exponent around 0.25 at 0 dB HL to an exponent around 1.0 at 75 dB HL (linear response growth). Masking sensitivity constants (kappa), the minimum effective masker levels, indicated that masking began at slightly higher masker levels in subjects with sensorineural hearing loss than in subjects with normal hearing. It was concluded that higher masked thresholds in regions of hearing loss were due primarily to a loss of active gain at the probe frequency and were not due to an excessive response at the probe frequency to the lower-frequency masker. For ON-frequency masking conditions, growth-of-masking slopes were not different from normal in hearing-impaired subjects. ON-frequency masking began when the effective power within an auditory filter at the probe frequency reached elevated absolute threshold at the probe frequency. Critical ratios were normal except for one subject with the most hearing loss.

Peripheral compression as a function of stimulus level and frequency region in normal-hearing listeners

Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at probe frequencies between 250 and 8000 Hz. The short probe tones were fixed in level (approximately 10-dB SPL). The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. These isoresponse TMCs were obtained for an on-frequency masker, where the masker frequency (Fm) and probe frequency (Fp) were the same, and for an off-frequency masker below the probe frequency (Fm = 0.6 Fp). Slopes of off-frequency TMCs for probe tones at 250-1000 Hz were steeper than those for probe tones between 2000 and 4000 Hz, supporting the notion that response growth for Fm = 0.6 Fp at lower probe frequencies is not linear. Therefore, a group average off-frequency TMC slope, for probe frequencies between 2 and 4 kHz, was used to calculate response growth at every probe frequency. Input/output response growth curves were derived from the TMCs, and response growth rates were calculated as a function of the masker level in individual ears. At any particular probe frequency, response growth rates varied with input level, from near 1.0 at low input levels, to <0.2 at mid levels, and back to near 1.0 at levels above 80-dB SPL. It was concluded that compression is equally strong at low and high frequencies as it is at mid frequencies.

The effect of basilar-membrane nonlinearity on the shapes of masking period patterns in normal and impaired hearing

Masking period patterns (MPPs) were measured in listeners with normal and impaired hearing using amplitude-modulated tonal maskers and short tonal probes. The frequency of the masker was either the same as the frequency of the probe (on-frequency masking) or was one octave below the frequency of the probe (off-frequency masking). In experiment 1, MPPs were measured for listeners with normal hearing using different masker levels. Carrier frequencies of 3 and 6 kHz were used for the masker. The probe had a frequency of 6 kHz. For all masker levels, the off-frequency MPPs exhibited deeper and longer valleys compared with the on-frequency MPPs. Hearing-impaired listeners were tested in experiment 2. For some hearing-impaired subjects, masker frequencies of 1.5 kHz and 3 kHz were paired with a probe frequency of 3 kHz. MPPs measured for listeners with hearing loss had similar shapes for on- and off-frequency maskers. It was hypothesized that the shapes of MPPs reflect nonlinear processing at the level of the basilar membrane in normal hearing and more linear processing in impaired hearing. A model assuming different cochlear gains for normal versus impaired hearing and similar parameters of the temporal integrator for both groups of listeners successfully predicted the MPPs.

Forward masking recovery and peripheral compression in normal‐hearing and cochlear‐impaired ears

Iso‐response temporal masking curves are obtained from normal‐hearing subjects at a probe frequency of 1000 Hz for masker frequencies between 500 and 1200 Hz. Time constants calculated from the temporal masking curves varied with masker frequency, from around 70 ms for low off‐frequency maskers (500–600 Hz) to around 36 ms for on‐frequency maskers (close to the probe frequency). Continuing Bilger’s earlier pursuits of nonlinearities in hearing, estimates of peripheral compression were calculated under the assumption that the response to a low off‐frequency masker is linear at the probe frequency place. Average compression exponents varied from close to 1.0 for remote off‐frequency maskers (both below and above the probe) to below 0.4 for on‐frequency maskers. Input–output transfer functions derived from the compression exponents were consistent with BM transfer functions recorded in animals with normal cochlear function. Comparisons of iso‐response temporal masking curves in subjects with sizable cochlear...

Masking period patterns in listeners with cochlear hearing loss

Masking period patterns (MPPs) were measured in listeners with hearing loss of cochlear origin, using a short 6‐kHz probe presented at different times within the envelope of a 100% sinusoidally amplitude‐modulated (AM) masker (4‐Hz modulation rate). The carrier frequency of the AM masker was 3 kHz (off‐frequency masking) or 6 kHz (on‐frequency masking). MPPs obtained from hearing‐impaired listeners were compared to those obtained from normal‐hearing listeners using the same stimuli. A model used previously to simulate MPPs from normal‐hearing listeners [Wojtczak et al., J. Acoust. Soc. Am. 106, 2147 (1999)] was used to predict data from the hearing‐impaired listeners. In contrast to MPPs from normal‐hearing ears, MPPs from ears with moderate to profound hearing loss did not exhibit large differences in MPP shapes for on‐ and off‐frequency maskers, i.e., they did not exhibit longer MPP valleys for the off‐frequency masking situation. Results of the modeling suggest that the similar shapes for on‐ and off‐f...

Frequency resolution and phoneme recognition by hearing‐impaired listeners

The relationship between auditory frequency resolution and speech perception was investigated in 11 hearing‐impaired listeners who differed widely in terms of hearing‐loss severity, hearing‐loss configuration, and frequency resolution abilities. First, the function relating articulation index (AI) to percent correct phoneme recognition was determined for 19 normal‐hearing listeners at a variety of speech levels and S/N ratios. Eleven hearing‐impaired subjects were then tested with the same speech materials throughout a wide range of AI conditions. Simultaneous‐masked psychophysical tuning curves were also obtained at probe frequencies of 0.5, 1, 2, and 4 kHz from all hearing‐impaired subjects. When phoneme recognition scores were weighted by AI, the performance of 10 of the 11 hearing‐impaired subjects was within normal limits. A modest correlation was observed between high‐frequency tuning curve slope and the error in the AI prediction relative to the normal AI data. Results indicate that although listen...

Temporal tuning curves and their relation to frequency tuning curves

Temporal tuning curves for a 1000‐Hz, 10‐ms, 10‐dB SL sinusoidal signal with a 500‐, 1000‐, or 1500‐Hz, 10‐ms sinusoidal masker were measured using a 2IFC adaptive procedure. Frequency tuning curves for the 10 dB SL, 1000‐Hz signal were also measured for 10‐ and 500‐ms simultaneous maskers. Five normal hearing listeners were employed. The frequency tuning curves for the brief masker were broader than those for the long masker, especially on the high‐frequency side of the signal. Marked differences were not found between the temporal tuning curves for the three masker conditions. The implications of these results for relations between auditory frequency and temporal analysis will be discussed.

Effect of BM compression on masking period patterns (MPPs)

MPPs were measured in listeners with normal and impaired hearing using tonal 4‐Hz SAM maskers and short tonal probes with frequencies that were either identical to or higher than the carrier frequency of the masker. The probe frequencies were 500, 1200, and 3000 Hz for on‐frequency masking, and 1200, 2400, and 6000 Hz for off‐frequency masking. In normal‐hearing listeners MPPs measured with off‐frequency probes had valleys that were much longer and deeper than valleys observed with on‐frequency probes. A similar result was observed in hearing‐impaired listeners in the frequency region of mild hearing losses, where significant residual compression was presumably operating. However, in the frequency region with substantial hearing loss where compression is substantially reduced or absent, MPPs measured with the on‐ and off‐frequency probes were very similar. A model consisting of peripheral filtering, compressive nonlinearity, and a sliding temporal window was used in an attempt to predict the data and to e...