Michelle L. Hicks

Psychophysical measures of auditory nonlinearities as a function of frequency in individuals with normal hearing.

In order to gain a better understanding of how auditory nonlinear phenomena vary as a function of location along the cochlea, several psychophysical measures of nonlinearity were examined as a function of signal frequency. Six normal-hearing individuals completed three experiments, each designed to measure one aspect of nonlinear behavior: (1) the effects of level on frequency selectivity in simultaneous masking, measured using notched-noise maskers at spectrum levels of 30 and 50 dB, (2) two-tone suppression, measured using forward maskers at the signal frequency (fs) and suppressor tones above fs, and (3) growth of masking, measured using forward maskers below fs at a signal/masker frequency ratio of 1.44. Four signal frequencies (375, 750, 1500, and 3000 Hz) were tested to sample the nonlinear behavior at different locations along the basilar membrane, in order to test the hypothesis that the apical (low-frequency) region of the cochlea behaves more linearly than the basal (high-frequency) region. In general, all three measures revealed a progressive increase in nonlinear behavior as signal frequency increased, with little or no nonlinearity at the lowest frequency, consistent with the hypothesis.

Effects of aspirin on psychophysical measures of frequency selectivity, two-tone suppression, and growth of masking

Three psychophysical measures of nonlinearity were evaluated before and during a course of aspirin ingestion to investigate the role of outer hair cells (OHCs) in these measures, as aspirin is thought to alter the functioning of OHCs. Six normal-hearing individuals received a moderate dose (3.9 g/day) of aspirin for four days, producing essentially flat, temporary hearing losses that ranged from 5-20 dB. The losses were about 2 dB greater for a 300-ms signal than for a 15-ms signal, indicating reduced temporal integration with aspirin. On the final three days of aspirin use, three experiments were completed; each was designed to measure one aspect of nonlinear behavior: (1) the effects of level on frequency selectivity in simultaneous masking using notched-noise maskers, (2) two-tone suppression using forward maskers at the signal frequency (fs) and suppressor tones above fs, and (3) growth-of-masking functions in forward masking using a masking tone below fs. Signal frequencies of 750 and 3000 Hz were used to evaluate the effects of aspirin at relatively low- and high-frequency regions of the cochlea. In experiment 1, aspirin broadened the auditory filters and reduced the effect of level on frequency selectivity. In experiment 2, aspirin reduced or eliminated two-tone suppression. And, in experiment 3, aspirin reduced the slopes of the growth-of-masking functions. Thus, the aspirin was effective in reducing nonlinearity in all three experiments, suggesting that these measures reflect the same (or a similar) active, nonlinear mechanism, namely the compressive nonlinearity provided by the OHCs. In all experiments, aspirin tended to have larger detrimental effects on the nonlinear measures at 3000 Hz than at 750 Hz, which can be explained in terms of greater involvement of nonlinear processing at higher frequencies. Finally, these effects of aspirin were found to be similar to those observed in preliminary measurements in two subjects with mild, permanent hearing loss.

Some factors influencing comodulation masking release and across‐channel masking

The purpose of this study was to determine whether comodulation masking release (CMR) and across-channel masking (ACM) are by-products of a similar across-channel mechanism. This was addressed by examining how the two are affected by stimulus manipulations expected to influence their magnitude. Subjects were required to detect a 1000-Hz signal in the presence of a masker that consisted of a 1000-Hz (on-frequency) component alone or that component and up to six flanking components (500, 600, 700, 1300, 1400, and 1500 Hz). The on-frequency and flanking components typically were sinusoidally amplitude modulated at 10 Hz, although not necessarily in phase with one another. In experiment 1, the amount of CMR and ACM was highly influenced by whether the signal consisted of one or three 50-ms tone bursts; in fact, ACM was only observed when the signal was a train of three 50-ms tone bursts. In experiments 2 and 3, CMR tended to increase as the modulation depth or the number of flanking components increased, whereas ACM was relatively unaffected by these manipulations. In addition, ACM was observed under dichotic situations, whereas CMR was not. Taken together, the results suggest that ACM and CMR may be mediated by different mechanisms.

Factors influencing temporal effects with notched-noise maskers

Temporal effects in simulataneous masking were studied by measuring the reduction in the amount of masking produced by a gated masker when that masker was preceded by a 400-ms noise (the precursor) that was usually spectrally identical to the masker. The signal frequency (fs) was 1.0 or 4.0 kHz. Experiment 1 revealed a temporal effect only when there was a spectral notch (centered at fs) in the masker and precursor. For a relative notchwidth of 0.4 fs, the temporal effect was larger at 4.0 than at 1.0 kHz. In experiment 2. where the masker and precursor both consisted of two bands of noise separated by a spectral notch of 0.4 fs, the size of the temporal effect remained essentially constant as the bandwidth of these noise bands increased from 0.2-0.8 kHz. The results from experiment 3 indicated that the temporal effect was largest when the level fo the precursor was equal to the level of the masker. Finally, the results from experiment 4 suggested that the temporal effect may depend upon the frequency region below as well as above fs, but that the frequency region above fs is probably more important.

The effects of aspirin on a psychophysical estimate of basilar membrane compression

It has been suggested that the amount of basilar membrane (BM) compression to tones at characteristic frequency can be estimated psychophysically by examining the slope of a growth‐of‐masking function for a masker that is lower in frequency than the signal [A. J. Oxenham and C. J. Plack, J. Acoust. Soc. Am. 101, 3666–3675]. In this study, growth‐of‐forward masking functions were measured before and during administration of a moderate dose of aspirin that caused 5–15 dB of temporary hearing loss. The level of a 200‐ms sinusoidal masker required to mask a 5‐ms, 4‐kHz sinusoidal signal presented 2 ms after the offset of the masker was determined for signal levels from 30 to 90 dB SPL. The masker was either the same frequency, or lower in frequency than the signal (signal‐masker ratios of 1.2 to 1.8). High‐pass noise was presented continuously to prevent off‐frequency listening. In general, the aspirin, by presumably altering outer hair cell function, resulted in growth‐of‐masking functions (masker level vers...

The influence of aspirin on temporal effects in simultaneous masking with noise and tonal maskers

Temporal effects in simultaneous masking were measured before and during the administration of a moderate dose of aspirin that caused about 5–15 dB of temporary hearing loss. The 10‐ms, 4.0‐kHz signal was presented at the beginning or in the temporal center of a 400‐ms masker. The masker was either a broadband noise presented at a spectrum level of 10 or 20 dB SPL or a tone with a frequency of 5.2, 5.4, or 5.6 kHz presented at a level of 80 dB SPL. Consistent with previous results [D. McFadden and C. A. Champlin, J. Acoust. Soc. Am. 87, 2634–2642 (1990)], aspirin tended to reduce the temporal effect with the broadband masker (i.e., the overshoot) by decreasing the threshold for the signal at masker onset. Aspirin also tended to reduce the temporal effect in the presence of the tonal masker, but in this case by increasing the threshold for the signal in the temporal center and, to a lesser extent, at masker onset. This differential effect of aspirin suggests that the mechanisms underlying the two temporal effects may differ. [Work supported by NIDCD.]

Temporal integration in the presence of off-frequency maskers.

Temporal integration was measured at a relatively low and a relatively high signal frequency under conditions of off-frequency masking. The masker was typically gated for 300 ms, and the signal was presented 70 ms after masker onset. In experiment 1, the signal frequency was 500 or 2000 Hz. Temporal integration was measured in quiet and in the presence of a masker whose frequency was lower or higher than the signal frequency. In all listening situations, there was less integration at 2000 Hz than at 500 Hz. This effect of frequency was particularly dramatic in the presence of a lower frequency masker, where there was almost no integration at 2000 Hz. Experiment 2 showed that this dramatic effect of frequency cannot be understood in terms of the underlying psychometric functions. Experiment 3 measured temporal integration at 750 and 2000 Hz for a large number of masker-signal frequency separations for both a tonal and a noise masker, and in conditions where the masker was gated or continuous. The results with the gated tonal masker largely confirmed the results of experiment 1. The results with the continuous tonal masker and the gated or continuous noise masker, however, were quite different. In those cases, the amount of temporal integration at both signal frequencies was more or less independent of the masker-signal separation; the masked temporal integration was nearly equal to the integration in quiet. Thus based on the conditions evaluated here, off-frequency masked temporal integration differs substantially from integration in quiet only for gated tonal maskers located considerably lower in frequency than the signal. It is unclear how to account for this finding, although it may be related to attentional factors.

Temporal manipulations in multi‐channel compression effecting sound quality and performance

Multi‐channel compression (in four or more compression channels) is expected in modern hearing aids, but compression temporal characteristics vary without an understanding of what is optimal and why. This study takes a 16 channel system using linearly spaced compressors with minimal overlap and evaluates sound quality ratings and speech recognition performance with the HINT in quiet and noise with various attack and release settings in a 2 x 2 x 2 design. The first variable is the relationship between attack and release times, with asymmetric time constants (attack faster than release) or symmetric time constants (attack and release set the same). The second variable is the relationship of time constants across frequency channels, with uniform settings across frequencies, or frequency dependent time constants (faster settings for higher frequencies with shorter wavelengths). The third variable is absolute speed, with fast or slow time constants. No characteristics other than the temporal settings of the c...

Speech intelligibility in noise with single‐microphone noise reduction implemented in 9‐, 16‐, and 24‐channel compressors

Reception Threshold for Sentences as measured by a modified version of the HINT test [M. J. Nilsson et al., J. Acoust. Soc. Am. 95(2), 1085–1099 (1984)] were collected on hearing‐impaired listeners fit binaurally with digital hearing aids incorporating a 9‐channel, 16‐channel, and 24‐channel spectral subtraction‐like technique of single‐microphone noise reduction. Thresholds were measured in noise presented in a two‐dimensionally diffuse soundfield with the subjects listening unaided, aided without noise reduction, and aided with noise reduction. The ability of noise reduction to reduce the level of steady state noise was quantified using 2 cm3 coupler measures with flat linear fittings (to measure the maximum attenuation possible) as well as using each of the listeners’ prescribed fittings (to better relate to the changes in performance measured in the sound field). Previously measured benefit from noise reduction [(M. J. Nilsson et al., J. Acoust. Soc. Am. (2000)] is hypothesized to occur from frequency...

Some effects of modulator phase on the detection of a signal in a sinusoidally amplitude‐modulated tonal complex

Subjects were required to detect a 1000‐Hz signal in the presence of a masker that consisted of a 1000‐Hz (on‐frequency) component alone or that component and six flanking components (500, 600, 700, 1300, 1400, and 1500 Hz). The on‐frequency component typically was sinusoidally amplitude modulated at 10 Hz and the signal was presented in a dip in the envelope of the on‐frequency component or in three successive dips. In experiment 1, the flanking components could lower (comodulation masking release or CMR) or elevate (across‐channel masking or ACM) threshold, depending upon whether they were modulated in phase with the on‐frequency component, or staggered in phase. CMR was observed for both signal types, although it was smaller for the three‐burst signal. ACM was only observed for the three‐burst signal. These differences were primarily due to lower thresholds in the presence of the on‐frequency component alone for the three‐burst signal. In experiment 2, thresholds were measured as a function of the modu...