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.

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.

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.

Release from upward spread of masking in regions of high-frequency hearing loss

The upward spread of masking was compared for 500-Hz quasifrequency-modulated (QFM) and sinusoidally amplitude-modulated (SAM) maskers. The modulation rate was 20 Hz. These maskers had identical magnitude spectra but different envelopes, which were relatively flat for the QFM masker and strongly fluctuating for the SAM masker. At signal frequencies more than an octave above the masker, masked thresholds for the SAM masker were lower than for the QFM masker, revealing "masking release" (QFM-SAM masked threshold differences) exceeding 30 dB in normal-hearing ears. In ears with high-frequency sensorineural hearing loss, but normal hearing in the region of the masker, masking release was markedly reduced or completely absent in regions of hearing loss. The data were evaluated with a model of masking based on the linearized response growth (LRG) of basilar membrane transfer functions associated with cochlear damage in animals. The LRG model predicted more gradual slopes of the growth of masking and reduced amount of masking in regions of hearing loss. The reduced masking release seen in regions of hearing loss could be largely accounted for by a more rapid growth of response to the probe tone in regions of hearing loss.

Electrode ranking of "place pitch" and speech recognition in electrical hearing

The ability to distinguish electrical stimulation of different electrodes on the basis of "pitch or sharpness" was evaluated with an electrode ranking procedure in 14 individual users of the Nucleus cochlear implant. Prior to the electrode ranking test, absolute thresholds and maximum comfortable loudness levels were measured, and loudness balancing was accomplished across all usable electrodes. Performance on the electrode ranking task was defined in terms of d' per mm of distance between comparison electrodes. Large individual differences were found among cochlear-implant users. In subjects with good to excellent place-pitch sensitivity, the electrode ranking task was limited by a ceiling effect; however, in those with poor to moderate sensitivity d'/mm was relatively constant with spatial separation between electrodes. Place pitch was typically ordered from apical to basal electrodes, i.e., basal electrodes were judged to be higher in pitch than more apical electrodes. However, instances of reversals in place-pitch ordering were seen on some electrodes in some subjects. Instances were also seen of better electrode ranking in the apical half of the electrode array than in the basal half, and vice-versa. Analyses of the electrode ranking functions in terms of d' per stimulus indicated that, in some subjects, perfect performance was reached with as little as 0.75 mm between comparison electrodes, the minimum possible. In other subjects, perfect performance was not reached until the spatial separation between comparison electrodes was over 13 mm, more than three quarters of the entire length of the electrode array. Ten of the subjects also participated in a closed-set recognition task of intervocalic consonants. Although the maximum transmitted information for place of consonant articulation (which is based primarily on spectral speech cues) was only 34%, correlations between place-pitch sensitivity and transmitted speech information were as high as 0.71. This was surprising considering the excellent place-pitch sensitivity exhibited by some of the subjects, and may reflect limitations of the Nucleus speech coding strategy for representing spectrally coded speech information. The two prelingual subjects performed notably poorer on the speech task than the postlingual subjects, even though one of the prelingual subjects demonstrated very good place-pitch sensitivity.

Critical bandwidth for phase discrimination in hearing-impaired listeners

Monaural phase discrimination was evaluated in normal-hearing and hearing-impaired listeners as a function of the frequency separation among components in three-tone complexes. The phases of the center components of 100% sinusoidal amplitude-modulated (SAM) waveforms were shifted by 90 degrees to yield quasi-frequency-modulated (QFM) waveforms that had identical long-term spectra but different envelopes and temporal fine structure. Normal-hearing listeners can distinguish QFM waveforms from SAM waveforms as long as the modulation frequency (frequency separation between components) is less than about 40% of the carrier frequency (center component). Phase discrimination performance (d') was measured as a function of modulation frequency, and critical bandwidths for phase discrimination (CBphs) were specified as the modulation frequency corresponding to a performance index (d') of 1.0. In normal-hearing ears, CBphs increased with stimulus SPL. In hearing-impaired ears, CBphs estimates were equal to or narrower than normal when comparisons were made at the same SPLs; CBphs estimates from hearing-impaired ears were broader (better) than normal only when comparisons were made at equivalent SLs. Differences between CBphs estimates in normal-hearing and hearing-impaired ears are explained by level-dependent auditory filtering and the sensation levels at which comparisons are made, without the necessity to postulate abnormal tuning in hearing-impaired ears.

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.