Search PubMed⌕ Search

Biomedical subjects

B C Moore

Publications and source records attributed to B C Moore.

At least 163 records · Page 9Linked to original sources
PubMed1989

Synthetic two-formant vowel perception by some of the better cochlear-implant patients.

Synthetic two-formant vowel recognition was tested in some of the better cochlear-implant users to determine their ability to resolve sounds that differ essentially in their frequency content. The vowels /i, epsilon, alpha, u/ were synthesized with equal duration and similar sound pressure level. The formant values were chosen to approximate the values of these vowels in French, German and American English. Performance ranged from 29 to 71% for the 6 patients with the Chorimac implant, from 25 to 67% for the 9 patients with the 3M/Vienna implant, from 63 to 92% for the 10 patients with the Nucleus implant from Hannover, from 17 to 79% for the 10 patients with the Duren/Cologne implant, from 54 to 100% for the 9 patients with the Symbion implant, and from 79 to 100% for the 10 Nucleus patients from the USA. Patients with each of these devices can utilize some spectral information when recognizing steady-state vowels.

Cochlear Implants↗
PubMed1989

Psychoacoustic abilities of subjects with unilateral and bilateral cochlear hearing impairments and their relationship to the ability to understand speech.

This paper is concerned with deficits in the ability to distinguish sounds, which accompany hearing loss of cochlear origin, and with the relationship of those deficits to the ability to understand speech in quiet and in background noise. Nine subjects with moderate unilateral cochlear hearing loss and 6 with moderate bilateral cochlear hearing loss took part in a series of psychoacoustic and speech perception tests. The impaired ears showed deficits in several of the psychoacoustic tests, including: detection of temporal gaps in bands of noise; frequency discrimination of pure tones; frequency discrimination of complex tones; and frequency selectivity as measured by the masking of tones by notched noise. The impaired ears showed near-normal performance in the detection of changes in intensity and for detecting temporal gaps in sinusoidal signals. Speech reception thresholds (SRTs--defined as the level of speech required for 50% intelligibility) were measured both in quiet and in speech-shaped noise, and were invariably higher for the impaired than for the normal ears. Correlational analysis, principal-components analysis and multiple-regression analysis were used to explore the relationships between the psychoacoustic measures and the SRTs. The results suggest that SRTs in quiet are determined primarily by absolute thresholds as measured by the pure-tone audiogram. SRTs in noise are related more to supra-threshold discrimination abilities, such as the detection of temporal gaps in noise and the frequency discrimination of pure and complex tones, and to age. Possible clinical applications of the results are briefly discussed.

Adult↗
PubMed1988

Gap detection with sinusoids and noise in normal, impaired, and electrically stimulated ears.

Thresholds for the detection of temporal gaps were measured using two types of signals to mark the gaps: bandpass-filtered noises and sinusoids. The first experiment used seven subjects with relatively flat unilateral moderate cochlear hearing loss. The normal ear of each subject was tested both at the same sound-pressure level (SPL) as the impaired ear, and at the same sensation level (SL). Background noise was used to mask spectral "splatter" associated with the gap. For the noise markers, gap thresholds tended to be larger for the impaired ears than for the normal ears when the comparison was made at equal SPL; the difference was reduced, but not eliminated, when the comparison was made at equal SL. Gap thresholds for both the normal and impaired ears decreased as the center frequency increased from 0.5 to 2.0 kHz. For the sinusoidal markers, gap thresholds were often similar for the normal and impaired ears when tested at equal SPL, and were larger for the normal ears when tested at equal SL. Gap thresholds did not change systematically with frequency. Gap thresholds using sinusoidal markers were smaller than those using noise markers. In the second experiment, three subjects with single-channel cochlear implants were tested. Gap thresholds for noise bands tended to increase with increasing center frequency when the noise bandwidth was fixed, and to decrease with increasing bandwidth when the center frequency was fixed. Gap thresholds for sinusoids did not change with center frequency, but decreased markedly with increasing level. Gap thresholds for sinusoids were considerably smaller than those for noise bands.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1988

The shape of the ear's temporal window.

This article examines the idea that the temporal resolution of the auditory system can be modeled using a temporal window (an intensity weighting function) analogous to the auditory filter measured in the frequency domain. To estimate the shape of the hypothetical temporal window, threshold was measured for a brief sinusoidal signal presented in a temporal gap between two bursts of noise. The duration of the gap was systematically varied and the signal was placed both symmetrically and asymmetrically within the gap. The data were analyzed by assuming that the temporal window had the form of a simple mathematical expression with a small number of free parameters. The values of the parameters were adjusted to give the best fit to the data. The analysis assumed that, for each condition, the temporal window was centered at the time giving the highest signal-to-masker ratio, and that threshold corresponded to a fixed ratio of signal energy to masker energy at the output of the window. The data were fitted well by modeling each side of the window as the sum of two rounded-exponential functions. The window was highly asymmetric, having a shallower slope for times before the center than for times after. The equivalent rectangular duration (ERD) of the window was typically about 8 ms. The ERD increased slightly when the masker level was decreased, but did not differ significantly for signal frequencies of 500 and 2000 Hz. The temporal-window model successfully accounts for the data from a variety of experiments measuring temporal resolution. However, it fails to predict certain aspects of forward masking and of the detection of amplitude modulation at high rates.

Adult↗
PubMed1988

Failure to obtain comodulation masking release with frequency-modulated maskers.

These experiments were intended to determine whether comodulation masking release (CMR) occurs for maskers that are modulated in frequency rather than in amplitude. In experiment I, thresholds for a sinusoidal signal were measured in the presence of two continuous sinusoidal maskers: one was centered at the signal frequency (1.0 kHz), and the other was positioned at flanking frequencies ranging from 0.5 to 2.0 kHz. The two maskers were frequency modulated (FM) by the same low-pass-noise modulator (correlated condition) or by independent noise modulators (uncorrelated condition). Thresholds were the same for the correlated and uncorrelated maskers, i.e., no CMR occurred. This was also true when the flanking band was presented in the ear opposite to that containing the signal and the on-frequency masking band. In experiment II, 25-Hz-wide noise maskers were used. The on-frequency band was sinusoidally frequency modulated, while the off-frequency band either had the same FM or no FM. Thresholds were similar for the two conditions, again indicating that no CMR occurred. The results suggest that, unlike amplitude modulation, correlated FM of the masker in different frequency bands does not give rise to a release from masking.

Acoustic Stimulation↗
PubMed1988

Duration discrimination of steady and gliding tones: a new method for estimating sensitivity to rate of change.

Thresholds for the detection of differences in duration were measured in a two-alternative, forced-choice task for four types of signals, all centered at 2000 Hz: (1) sinusoids fixed in frequency and level; (2) sinusoids of fixed frequency whose level was swept up or down by 5 or 10 dB; (3) sinusoids of fixed level whose frequency was swept up or down by 100 Hz; and (4) sinusoids whose level was swept up or down by 10 dB and whose frequency was swept up or down by 100 Hz. For types (2)-(4), the direction of the sweeps was fixed within a run. The duration of the standard was either fixed at 750 ms or was varied randomly from trial to trial by up to +/- 7% about 750 ms. The duration of the comparison signal was initially 100 ms greater than that of the standard and was varied adaptively to determine threshold. The pattern of results was similar for all four subjects tested. Duration-discrimination thresholds for the signals that were swept in level and/or frequency were lower than those for the fixed signal, typically by 15-20 ms. This indicates that subjects were sensitive to the rate of change of frequency and/or level and could use this as a cue for duration discrimination. The Weber fraction for rate of change was estimated to be about 0.05-0.06 and was similar for changes in level and in frequency.

Adult↗
PubMed1988

A comparison of four methods of implementing automatic gain control (AGC) in hearing aids.

Hearing impairment of cochlear origin is usually associated with loudness recruitment. As a consequence, the dynamic range between threshold and the highest comfortable level is smaller than normal. To ensure that low-level sounds can be heard, while avoiding discomfort at high levels, a hearing aid with automatic gain control (AGC) is required. This paper compares four different systems for implementing AGC, and compares each of them with unaided listening and with linear amplification. The systems were evaluated by measuring thresholds for understanding speech in quiet and in five types of background sound: speech-shaped noise, 12-talker babble, cafeteria noise, traffic noise and a single competing speaker. The first system used a new dual-action AGC (called dual front-end AGC) operating on the whole speech signal. A slow-acting control voltage (recovery time 5 s) held the average level of speech at the output constant, regardless of the input level. In response to sudden intense transients, a fast-acting control voltage (recovery time 150 ms) reduced the gain rapidly and then returned the gain to the value set by the slow-acting component. In the second system, referred to as the mark II aid, the output of the dual front-end AGC was split into two frequency bands, and fast-acting (syllabic) compression was applied in the high-frequency band only. The bands were then recombined. The third system resembled the mark II aid except that fast-acting compression was applied in both bands. The fourth system resembled the 2-channel aid evaluated in previous trials (Moore, 1987). It was similar to the third system, but had only single-action front-end AGC with a recovery time of 400 ms. Six subjects with moderate sensorineural hearing loss accompanied by recruitment were used. Best results overall were obtained using the mark II aid. Speech reception thresholds (SRTs) in noise were, on average, 4 dB better than for linear amplification and 2.4 dB lower than for the previous 2-channel aid. There was a significant advantage of having fast-acting AGC in the high-frequency band, but no advantage of having AGC in the low-frequency band.

Acoustics↗
PubMed1987

Formulae describing frequency selectivity as a function of frequency and level, and their use in calculating excitation patterns.

The auditory filter may be considered as a weighting function representing frequency selectivity at a particular centre frequency. Its shape can be derived using the power-spectrum model of masking which assumes: (1) in detecting a signal in a masker the observer uses the single auditory filter giving the highest signal-to-masker ratio; (2) threshold corresponds to a fixed signal-to-masker ratio at the output of that filter. Factors influencing the choice of a masker to measure the auditory filter shape are discussed. Narrow-band maskers are unsuitable for this purpose, since they violate the assumptions of the power-spectrum model. A method using a notched-noise masker is recommended, and typical results using that method are presented. The variation of the auditory filter shape with centre frequency and with level, and the relationship of the auditory filter shape and the excitation pattern are described. A method of calculating the excitation pattern of any sound as a function of level is presented, and examples and applications are given. The appendix gives a Fortran program for calculating excitation patterns.

Auditory Perception↗
PubMed1987

Gap detection and masking in hearing-impaired and normal-hearing subjects.

Subjects with cochlear impairments often show reduced temporal resolution as measured in gap-detection tasks. The primary goals of these experiments were: to assess the extent to which the enlarged gap thresholds can be explained by elevations in absolute threshold; and to determine whether the large gap thresholds can be explained by the same processes that lead to a slower-than-normal recovery from forward masking. In experiment I gap thresholds were measured for nine unilaterally and eight bilaterally impaired subjects, using bandlimited noise stimuli centered at 0.5, 1.0, and 2.0 kHz. Gap thresholds were usually larger for the impaired ears, even when the comparisons were made at equal sensation levels (SLs). Gap thresholds tended to increase with increasing absolute threshold, but the scatter of gap thresholds was large for a given degree of hearing loss. In experiment II threshold was measured as a function of the delay between the onset of a 210-ms masker and the onset of a 10-ms signal in both simultaneous- and forward-masking conditions. The signal frequency was equal to the center frequency of the bandlimited noise masker, which was 0.5, 1.0, or 2.0 kHz. Five subjects with unilateral cochlear impairments, two subjects with bilateral impairments, and two normal subjects were tested. The rate of recovery from forward masking, particularly the initial rate, was usually slower for the impaired ears, even when the maskers were presented at equal SLs. Large gap thresholds tended to be associated with slow rates of recovery from forward masking.

Acoustic Stimulation↗
PubMed1987

Distribution of auditory-filter bandwidths at 2 kHz in young normal listeners.

Auditory-filter shapes at 2 kHz were estimated for 95 young normally hearing subjects using a notched-noise masker with spectrum level of 45 dB. Excluding two subjects with a recent history of noise exposure, the equivalent rectangular bandwidths (ERBs) of the filters were approximately normally distributed but the distribution had a slight positive skew. The mean ERB was 308 Hz and the standard deviation was 32 Hz. The two noise-exposed subjects had ERBs of 404 and 497 Hz.

Acoustic Stimulation↗
PubMed1987

Gap detection and the auditory filter: phase effects using sinusoidal stimuli.

Psychometric functions were determined for the detection of temporal gaps in sinusoidal signals at center frequencies between 0.2 and 2.0 kHz. A continuous notched-noise masker was used to restrict listening to the signal frequency region. The gap always started when the signal was at a positive-going zero crossing. There were three different conditions for the starting phase of the signal at the termination of the gap. In the standard-phase condition the signal restarted at a positive-going zero crossing, in the reversed-phase condition at a negative-going zero crossing, and in the preserved-phase condition at the phase the signal would have had if the gap had not been present. In the standard-phase and reversed-phase conditions the psychometric functions were nonmonotonic, showing oscillations with a period equal to that of the signal; maxima in the functions for the standard-phase condition coincided with minima in the functions for the reversed-phase condition, and vice versa. In the preserved-phase condition the psychometric functions were monotonic and the 75% points were roughly independent of center frequency, having a value of about 5 ms. The general form of the results can be modeled by a filter bank followed by a square-law device and a temporal integrator, but good agreement between the data and the model could not be attained across the whole range of gap durations. The deviations between data and model suggest that subjects are sensitive to the brief transitions in phase (or, equivalently, in frequency) in some conditions.

Attention↗
PubMed1987

The temporal course of masking and the auditory filter shape.

Recent experiments have shown that frequency selectivity measured in tone-on-tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched-noise masker, threshold was measured for a 20-ms signal presented at the beginning, the temporal center, or the end of the 400-ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1-kHz signal frequency. Growth-of-masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched-noise experiment in terms of the level of a flat-spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high-frequency side, which was attributed to suppression.

Adult↗
PubMed1987

Transient masking and the temporal course of simultaneous tone-on-tone masking.

Previous studies have shown that threshold for a signal in tone-on-tone simultaneous masking is sometimes lower when the masker is continuous than when it is gated. Threshold may also decline as signal onset is delayed relative to the onset of a longer duration masker, though it may increase again near masker offset. In the present study, the level of a 1250-Hz sinusoidal masker was found which would just mask a 20-ms, 1000-Hz sinusoid presented at 10-dB sensation level (SL). Masker duration was 20 or 400 ms; in the latter case, the signal was presented in one of three temporal positions within the masker. The level of the 1250-Hz masker necessary to mask the signal was reduced, sometimes by as much as 20-25 dB, by a 20-ms, 500-Hz sinusoid (transient masker) presented at the times when the signal might occur, but at a level 30 dB below that at which it would mask the 10-dB SL signal. This suggests that, in the earlier studies, at least some of the elevation in threshold in the presence of a short-duration masker or at the beginning (or end) of a longer duration masker may have been due to the transient responses to the masker affecting detection of the signal, but not necessarily masking the signal in terms of excitation in the signal "channel."

Acoustic Stimulation↗
PubMed1987

Factors affecting thresholds for sinusoidal signals in narrow-band maskers with fluctuating envelopes.

When a signal is higher in frequency than a narrow-band masker, thresholds are lower when the masker envelope fluctuates than when it is constant. This article investigates the cues used to achieve the lower thresholds, and the factors that influence the amount of threshold reduction. In experiment I the masker was either a sinusoid (constant envelope) or a pair of equal-amplitude sinusoids (fluctuating envelope) centered at the same frequency as the single sinusoid (250, 1000, 3000, or 5275 Hz). The signal frequency was 1.8 times the masker frequency. At all center frequencies, thresholds were lower for the two-tone masker than for the sinusoidal masker, but the effect was smaller at the highest and lowest frequencies. The reduced effect at high frequencies is attributed to the loss of a cue related to phase locking in the auditory nerve. The reduced effect at low frequencies can be partly explained by reduced slopes of the growth-of-masking functions. In experiment II the masker was a sinusoid amplitude modulated at an 8-Hz rate. Masker and signal frequencies were the same as for the first experiment. Randomizing the modulation depth between the two halves of a forced-choice trial had no effect on thresholds, indicating that changes in modulation depth are not used as a cue for signal detection. Thresholds in the modulated masker were higher than would be predicted if they were determined only by the masker level at minima in the envelope, and the threshold reduction produced by modulating the master envelope was less at 250 Hz than at higher frequencies. Experiments III and IV reveal two factors that contribute to the reduced release from masking at low frequencies: The rate of increase of masked threshold with decreasing duration is greater at 250 Hz than at 1000 Hz; the amount of forward masking, relative to simultaneous masking, is greater at 250 Hz than at 1000 Hz. The results are discussed in terms of the relative importance of across-channel cues and within-channel cues.

Acoustic Stimulation↗
PubMed1987

Comodulation masking release (CMR): effects of signal frequency, flanking-band frequency, masker bandwidth, flanking-band level, and monotic versus dichotic presentation of the flanking band.

In experiment I, thresholds for 400-ms sinusoidal signals were measured in the presence of a continuous 25-Hz-wide noise centered at signal frequencies (fs) ranging from 250 to 8000 Hz in 1-oct steps. The masker was presented either alone or together with a second continuous 25-Hz-wide band of noise (the flanking band) whose envelope was either correlated with that of the on-frequency band or was uncorrelated; its center frequency ranged from 0.5 fs to 1.5 fs. The flanking band was presented either in the same ear (monotic condition) as the signal plus masker or in the opposite ear (dichotic condition). The on-frequency band and the flanking band each had an overall level of 67 dB SPL. The comodulation masking release, CMR (U-C), is defined as the difference between the thresholds for the uncorrelated and correlated conditions. The CMR (U-C) showed two components: a broadly tuned component, occurring at all signal frequencies and all flanking-band frequencies, and occurring for both monotic and dichotic conditions; and a component restricted to the monotic condition and to flanking-band frequencies close to fs. This sharply tuned component was small for low signal frequencies, increased markedly at 2000 and 4000 Hz, and decreased at 8000 Hz. Experiment II showed that the sharply tuned component of the CMR (U-C) was slightly reduced in magnitude when the level of the flanking band was 10 dB above that of the on-frequency band and was markedly reduced when the level was 10 dB below, whereas the broadly tuned component and the dichotic CMR (U-C) were only slightly affected. Experiment III showed that the sharply tuned component of the CMR (U-C) was markedly reduced when the bandwidths of the on-frequency and flanking bands were increased to 100 Hz, while the broadly tuned component and the dichotic CMR (U-C) decreased only slightly. The argument here is that the sharply tuned component of the monotic CMR (U-C) results from beating between the "carrier" frequencies of the two masker bands. This introduces periodic zeros in the masker envelope, which facilitate signal detection. The broadly tuned component, which is probably a "true" CMR, was only about 3 dB.

Attention↗
PubMed1987

Design and evaluation of a two-channel compression hearing aid.

The design of a two-channel compression hearing aid for persons with moderate sensorineural hearing losses with recruitment is described. The aid applies slow-acting automatic gain control (AGC) to the whole signal, and then splits the signal into two bands, with separate fast-acting (syllabic) AGC in each band. Trials evaluating the aid have shown that it allows speech in quiet to be understood over a wide range of sound levels without any need to adjust the controls on the aid. It also gives speech intelligibility in noise superior to that allowed by a comparable linear (non-compression) aid, a comparable single-channel compression aid, and by unaided listening. Pilot experiments comparing two different methods for fitting the aid suggest that fitting using speech as the test signal is superior to fitting using narrow band tonal signals.

Acoustics↗
PubMed1986

Temporal effects in simultaneous pure-tone masking: effects of signal frequency, masker/signal frequency ratio, and masker level.

The effect of the temporal relationship between a pure-tone masker and a pure-tone signal in simultaneous masking was investigated in three experiments. The experiments extend previous work by: studying the temporal effect over a wide range of signal frequencies, studying the change in masking over time for several masker/signal frequency ratios, and studying the growth of masking for a brief signal at different temporal positions within a longer duration masker. In the first experiment, threshold was measured for a 20-ms signal temporally centered in a masker whose duration ranged from 20 ms to continuous. Signal frequency (fs) was 0.5, 1.0, 2.0, 4.0, or 8.0 kHz; masker frequency (fm) was 1.2 fs. For all signal frequencies, the amount of masking decreased as masker duration increased. In the second experiment, threshold was measured for a 20-ms, 1.0-kHz signal as a function of the signal's temporal position within a 400-ms masker whose frequency ranged from 1.0 to 1.25 kHz. For all but the 1.0-kHz masker, for which threshold was almost independent of the signal's temporal position, threshold decreased as signal onset was delayed relative to masker onset, but then increased slightly as the signal approached masker offset. In the final experiment, growth-of-masking functions were measured for a 20-ms, 1.0-kHz signal positioned at the beginning, at the temporal center, or at the end of a 400-ms masker whose frequency was 1.20 or 1.25 kHz. The masking functions generally were steepest for a signal at the onset of the masker and, for a given temporal position, steepest for the 1.20-kHz masker.

Adaptation, Physiological↗