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PubMed1994

Conventional and cross-correlation brain-stem auditory evoked responses in the white leghorn chick: rate manipulations.

Rate-dependent changes in the chick brain-stem auditory evoked response (BAER) using conventional averaging and a cross-correlation technique were investigated. Five 15- to 19-day-old white leghorn chicks were anesthetized with Chloropent. In each chick, the left ear was acoustically stimulated. Electrical pulses of 0.1-ms duration were shaped, attenuated, and passed through a current driver to an Etymotic ER-2 which was sealed in the ear canal. Electrical activity from stainless-steel electrodes was amplified, filtered (300-3000 Hz) and digitized at 20 kHz. Click levels included 70 and 90 dB peSPL. In each animal, conventional BAERs were obtained at rates ranging from 5 to 90 Hz. BAERs were also obtained using a cross-correlation technique involving pseudorandom pulse sequences called maximum length sequences (MLSs). The minimum time between pulses, called the minimum pulse interval (MPI), ranged from 0.5 to 6 ms. Two BAERs were obtained for each condition. Dependent variables included the latency and amplitude of the cochlear microphonic (CM), wave 2 and wave 3. BAERs were observed in all chicks, for all level by rate combinations for both conventional and MLS BAERs. There was no effect of click level or rate on the latency of the CM. The latency of waves 2 and 3 increased with decreasing click level and increasing rate. CM amplitude decreased with decreasing click level, but was not influenced by click rate for the 70 dB peSPL condition. For the 90 dB peSPL click, CM amplitude was uninfluenced by click rate for conventional averaging. For MLS BAERs, CM amplitude was similar to conventional averaging for longer MPIs.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1994

Single neurons in the frog inferior colliculus exhibit direction-dependent frequency selectivity to isointensity tone bursts.

The effects of sound direction on frequency selectivity of inferior colliculus (IC) neurons were investigated by measuring the neuron's isointensity frequency responses (FRs) to tone bursts emanating from a free-field loudspeaker at several sound levels. The loudspeaker was rotated across the frontal field at 0 degrees elevation through 180 degrees of azimuth (from contralateral 90 degrees or C90 degrees to ipsilateral 90 degrees or I90 degrees). At each frequency, to assess the magnitude of response change with sound direction, the mean spike count obtained at an azimuth was compared to that at C90 degrees. The FR of most IC neurons (75/83 or 90%) was direction dependent. For most of these neurons, bandwidths of FRs were narrower when sounds originated from ipsilateral azimuths. Remarkably, with a change in sound azimuth, some segments of these FRs showed very distinct changes in shape, while other portions of the same FRs remained essentially unchanged. These narrow-band changes associated with restricted portions of the FR, were also exhibited by neurons with direction-dependent frequency-threshold characteristics (Gooler et al., 1993). Additionally, the most frequent direction-dependent change in the FRs occurred in a narrow frequency band around the units' best excitatory frequency.

Acoustic Stimulation↗
PubMed1994

Profile analysis and comodulation detection differences using narrow bands of noise and their relation to comodulation masking release.

Experiment 1 examined the ability to compare relative level across frequency (profile analysis) for stimuli that were dynamically varying over time. The task was to detect an increment in level of a narrow band of noise (the target) in the presence or absence (reference condition) of four flanking bands (FBs). The envelopes of the FBs were either the same as that of the target (correlated condition), independent of that of the target but the same as each other (co-uncorrelated condition), or all independent (all-uncorrelated condition). The overall level of the stimuli was either fixed or randomly varied from one stimulus to the next. The results showed that subjects can make effective use of spectral-shape cues even for stimuli whose amplitudes vary markedly over time. In the correlated condition, the threshold for detecting an increment in the level of the target band was decreased (relative to the reference condition) both when the overall level was fixed and when it was varied randomly from stimulus to stimulus. In the uncorrelated conditions, the FBs did not lead to better performance when the overall level was fixed; rather they produced a small interference effect. When the overall level was randomized, the presence of uncorrelated FBs produced thresholds that decreased with increasing bandwidth and, for a bandwidth of 64 Hz, produced an improvement in performance (relative to the reference condition) that was almost as large as that produced by the correlated FBs. It seems that the more rapid fluctuations in the wider bands of noise were smoothed by the auditory system, enabling information about the long-term spectral shape to be extracted effectively. Experiment 2 used similar stimuli, but the task was to detect the target, rather than to discriminate its level. Detection thresholds in the presence of FBs were lowest in the co-uncorrelated condition, higher in the correlated condition, and highest in the all-uncorrelated condition. Thus the presence of correlated FBs improved discrimination thresholds in the profile analysis task, but impaired performance in the detection task. Reasons for the discrepancy between the effects of correlated FBs in the two tasks are discussed in the context of the cues available to the listener.

Adult↗
PubMed1994

Effect of masker level on overshoot in running- and frozen-noise maskers.

Masked thresholds were measured with running- and frozen-noise maskers. The 5-kHz signal was 2 ms in duration. The masker was low-pass noise (20 Hz-10 kHz); its total duration was 300 ms. The overall level of the masker was 30, 50, or 70 dB SPL. The onset of the signal was delayed by 0, 3, 8, 18, 198, or 278 ms relative to the onset of the masker. In all frozen-noise measurements, the signal was added to the same fine structure of the noise. Overshoot in frozen noise was measured for two starting phases of the signal that led to a 10-dB difference for large signal-onset delays. In all three configurations (running noise and frozen noise with two different signal phases) masker level had a similar influence on overshoot. At the intermediate masker level (50 dB SPL), a significant amount of overshoot (up to 15 dB) was observed in all three conditions. At the low and the high masker levels, overshoot was very much reduced, and even became negative in most conditions for the 30-dB-SPL masker. For the 50-dB frozen-noise masker, the total variation of thresholds with signal phase was 8 to 11 dB for long signal-onset delays, but only 3 to 6 dB for short delays. For the low- and high-level maskers, where only a small overshoot was observed, the threshold variation with phase for a signal at masker onset was the same as that for the long-delay condition. An explanation for the variation of signal detectability with masker level is proposed that refers explicitly to the compressive input-output characteristic of the basilar membrane at intermediate levels.

Adult↗
PubMed1994

Modulation detection and discrimination with three-component signals.

In an attempt to study the processing of amplitude and frequency modulation (AM and FM), detection and discrimination tasks using mixed modulation (MM) signals were performed. Modulation detection thresholds were obtained for three-component signals that span the parameter space between AM and quasi-FM. A single-cue modulation detection model predicts the thresholds with reasonable accuracy. If one assumes that the AM and FM components are extracted separately, thresholds are also well predicted if the d' of the MM signals is equal to the sum of the separate d's of the AM and FM components (two-cue summation model). This could arise from common internal noise that puts the AM and FM information along a single decision axis. A modulation discrimination task was then examined in which the subjects discriminate between signals with both different modulation depths and different modulation types. The single-cue model predicts performance well. In order for the two-cue model to predict the results, the AM and FM cues must be combined into a single statistic before a decision can be made; the listener cannot process the cues separately.

Adult↗
PubMed1994

New data on the short-term effects of tone exposure.

Three experiments studied the short-term effects of moderate- and high-level tone exposures on threshold and loudness. Experiment 1 measured temporary threshold and loudness shifts (TTS and TLS) due to a 1000-Hz exposure at a moderate level (65 dB SPL) as a function of exposure duration. Whereas there was no significant TTS, TLS was as high as 11 dB and recovered rapidly. TLS was significantly larger for the highest test levels. TTS and TLS were practically independent of exposure duration. Experiment 2 measured TTS and TLS as a function of test frequency following exposures to 500-, 1000-, or 3000-Hz tones at 65 dB SPL. The frequency patterns of TLS were established at test levels of 20-60 phons. TLS and TTS had similar frequency patterns with a maximum at the exposure frequency, but TLS patterns were higher and more extended than those of TTS. In experiment 3, subjects were exposed to a 1000-Hz tone at a high level (90 dB SPL). Frequency patterns for TTS or TLS at a 60-dB test level were measured repeatedly during 5 1/2 min after the end of exposure. Two short-term effects were observed immediately after the exposure: (1) a slight TTS (4.5 dB) at the exposure frequency which disappeared 2 to 3 min later; and (2) a large TLS (9 dB) which extended over the whole range of test frequencies, recovered rapidly, and had disappeared by 330 s after exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1994

Intensity resolution and loudness in high-pass noise.

Intensity resolution and loudness growth for a 1000-Hz tone were studied in the presence of high-pass noise (cutoff: 1800 Hz). Intensity resolution was measured for gated and continuous standards using a three-interval forced-choice (3-IFC) adaptive procedure. Loudness matches were obtained using an adaptive, alternate binaural loudness balance procedure. Three subjects listened in three conditions (1) quiet; (2) high-pass noise with a spectrum level of 32 dB SPL; and (3) high-pass noise with a spectrum level of 42 dB SPL. Noise levels were selected so that detection thresholds were minimally affected at the test frequency; however, for frequencies in the noise passband, thresholds were shifted to either 50 or 60 dB SPL, depending on the spectrum level of the noise. On average, loudness growth and intensity resolution were unaltered by the presence of the noise for tonal levels below 40 dB SPL; above 40 dB SPL the following was generally true: (1) intensity resolution for gated standards was well described by Weber's law except at the highest levels where the Weber fraction decreased; (2) intensity resolution for continuous standards showed a "near-miss" to Weber's law, but just-noticeable differences (jnd's) were slightly larger than those in quiet for the same SPL; (3) loudness was reduced. A comparison of jnd's for equally loud tones showed that loudness is less dependent on excitation spread than the jnd. That is, jnd's in the threshold-shifted ear were larger than the ones in quiet when the comparison was made for tones judged to be equally loud.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Lateralization of bands of noise as a function of combinations of interaural intensive differences, interaural temporal differences, and bandwidth.

Listeners indicated the intracranial position of bands of noise (from 50 to 400 Hz in width) for several combinations of interaural intensive differences (IID), and interaural temporal differences (ITD), and/or interaural phase differences (IPD). All ITD and IPD combinations produced an interaural delay of 1500 microseconds at the center frequency of the noise. The interaural phase spectra were constructed to produce several patterns of putative cross-correlation functions. Potency of IIDs depended greatly on particular combinations of bandwidth, ITD and IPD. For some combinations, changing the IID by only 3 dB resulted in large shifts in laterality (sometimes moving the image from near one ear to near the other). The complex interactions observed make the results incompatible with the traditional notion that IIDs simply act as weights or scalars. Rather, IIDs act in two distinct manners: (1) as independent scalar quantities and (2) by interacting with specific combinations of bandwidth and ITD/IPD, which is believed to reflect an action within the cross correlation surface.

Acoustic Stimulation↗
PubMed1994

Low-pass filtering in amplitude modulation detection associated with vowel and consonant identification in subjects with cochlear implants.

Temporal auditory analysis of acoustic events in various frequency channels is influenced by the ability to detect amplitude modulations which for normal hearing involves low-pass filtering with a cutoff frequency around 100 Hz and a rejection slope of about 10 dB per decade. These characteristics were established in previous studies measuring modulation transfer functions. For cochlear implant subjects, the delivery of detailed amplitude modulation information has been recently shown to result in very significant improvements in speech understanding. Several previous studies on cochlear implant subjects have reported capacities for temporal resolution rather equivalent to those of normally hearing subjects but with some notable individual differences. Recently two studies on some cochlear implant subjects indicated modulation transfer functions often quite similar to those of normal hearing but exhibiting marked individual differences in shape and absolute sensitivity. The present study compared amplitude modulation detection and phonetic recognition in a group of cochlear implant subjects to determine the extent to which the two tasks are correlated. Nine individuals who had been implanted with an Ineraid device and who demonstrated open speech understanding ranging from excellent to poor were chosen and tested in the present study. For each subject modulation transfer functions were measured at the most apical electrode and phonetic recognition of isolated vowels and intervocalic consonants was assessed. Results showed a strong correlation between the depth of high-frequency rejection in modulation transfer functions and success in vowel and consonant intelligibility. These results emphasize the importance of temporal speech features and offer perspectives for customizing signal processing in cochlear implants.

Adult↗
PubMed1994

Effect of compression ratio in a slow-acting compression hearing aid: paired-comparison judgments of quality.

Paired-comparison judgments of quality were obtained from 20 hearing-impaired listeners (half with a small dynamic range and half with a large dynamic range) for speech-in-noise (vent, apartment, and cafeteria) processed through a slow-acting compression hearing aid. Compression ratio was varied (1, 1.5, 2, 3, 5, and 10:1). Compression threshold, attack time, and release time were fixed. Sound quality judgments were significantly affected by compression ratio, noise, and dynamic range. Preference decreased with increasing compression ratio. The selection of compression ratio. The selection of compression ratios < or = 2:1 was significantly higher than of compression ratios > 3:1. Less compression (no compression or 1.5:1) was preferred with the highest level noise (cafeteria noise) than with the lower level noises (vent or apartment). In particular, the small dynamic range group preferred compression with the vent and apartment noises (noise below the compression threshold), but preferred a linear hearing aid with the cafeteria noise (above the compression threshold). The large dynamic range group showed a slightly greater preference for the linear hearing aid for all three noises.

Adolescent↗
PubMed1994

Relationships between the variability of magnitude matching and the slope of magnitude level functions.

Binaural loudness matching and intermodal magnitude matching experiments were performed to test systematically the origins of the phenomenon observed earlier that the variability of binaural loudness matches was larger when sound intensity was varied in the normal ear than when it was varied in the contralateral ear with raised threshold and loudness recruitment. In the experiments, the raised threshold and loudness recruitment were produced by masking a 1-kHz tone with narrow-band random noise. In intermodal experiments, magnitude matches were performed between the masked tone and the length of lines projected on a translucent window pan. The results are consistent with the earlier observations. They show in addition that the variability depends on the slope of the matching functions in a complicated, not previously anticipated way, irrespective of whether the functions are intra- or intermodal. More specifically, for moderate slopes, the variability in the ear with loudness recruitment decreased as the slope increased. The reverse was true for the unmasked ear or the line length--when the slope was large, the variability increased with the slope. Since the variability decreased in one ear and increased in the other, the ratio of the variabilities increased as the slope increased. When the slope was equal to one, both variabilities tended to be the same.

Female↗
PubMed1994

Relation between the growth of loudness and high-frequency excitation.

To test the conjecture that cochlear impairment and a masking noise in normal ears produce analogous effects on the growth of loudness, loudness was measured for 21 listeners, all with bilateral symmetrical high-frequency cochlear losses. Beyond the normal-hearing region, thresholds increased by 30-49 dB/octave for 10 listeners in group I and by 51-71 + dB/octave for 11 listeners in group II. Measured by absolute magnitude estimation and production at two frequencies where thresholds were normal, the dynamic range over which loudness grows was not altered by the adjacent hearing loss. This was not true however, for the slope. Whereas the overall shape and slope of the loudness functions were essentially the same for group I, the mid-to-high level slope for group II was noticeably flatter for a tone at the cutoff frequency than for a lower-frequency tone. In contrast, markedly steeper loudness functions, characteristic of loudness recruitment, were obtained for both groups at a frequency in the region of impaired hearing. The reduced rate of loudness growth observed in the region of normal hearing for listeners with steep low-pass losses, like the reduction usually found for a tone masked by an adjacent high-frequency noise, is ascribed to the tone's restricted excitation pattern. These results indicate that high-frequency auditory cues are effectively eliminated by a steeply sloping high-frequency hearing loss just as they are eliminated by the addition of an external noise.

Adult↗
PubMed1995

Timbral aspects of reproduced sound in small rooms. I.

This paper reports some of the influences of individual reflections on the timbre of reproduced sound. A single loudspeaker with frequency-independent directivity characteristics, positioned in a listening room of normal size with frequency-independent absorption coefficients of the room surfaces, has been simulated using an electroacoustic setup. The model included the direct sound, 17 individual reflections, and the reverberant field. The threshold of detection and just-noticeable differences for an increase in level were measured for individual reflections using eight subjects for noise and speech. The results have shown that the first-order floor and ceiling reflections are likely to individually contribute to the timbre of reproduced speech. For a noise signal, additional reflections from the left sidewall will contribute individually. The level of the reverberant field has been found to have an effect on the contribution of the individual reflections. An increase in the level of individual reflections are most likely to be audible for the first-order floor and ceiling reflections, and certain reflections from the sidewalls.

Acoustics↗
PubMed1995

Intensity resolution and loudness in broadband noise.

Intensity resolution and loudness matching behavior for pure tones was studied in quiet and in two levels of noise (15 and 40 dB spectrum level) to test the predictions of several models for relating these phenomena. Five normally hearing listeners participated. The Weber functions in noise were elevated compared to the ones in quiet when comparisons were made at equal sound-pressure levels (SPLs). Loudness matching functions (dB SPL in quiet versus dB SPL in noise) showed a slope greater than unity. Modified power functions with threshold corrections were fitted to the matching data to estimate the loudness function exponent. The best-fitting loudness exponent for individual and group data (range = 0.24-0.35) was in the range of values typically found using magnitude estimation procedures for the loudness equation yielding the best fit. Three models for predicting Weber functions from loudness were evaluated with this loudness representation. One of these models, the subjective analog to Weber's law, yielded results inconsistent with the observed data. The other models, McGill and Goldberg's [J. Acoust. Soc. Am. 44, 576-581 (1968)] neural counting model and the proportional-jnd theory, predicted Weber functions that are consistent with the observed data if the variance of the decision variable is assumed to change in quiet and noise backgrounds. The proportional-jnd theory has such a change built into the model, but the underlying physiological mechanisms responsible for its success are unknown.

Adult↗
PubMed1996

Sources and effects of low-frequency noise.

The sources of human exposure to low-frequency noise and its effects are reviewed. Low-frequency noise is common as background noise in urban environments, and as an emission from many artificial sources: road vehicles, aircraft, industrial machinery, artillery and mining explosions, and air movement machinery including wind turbines, compressors, and ventilation or air-conditioning units. The effects of low-frequency noise are of particular concern because of its pervasiveness due to numerous sources, efficient propagation, and reduced efficacy of many structures (dwellings, walls, and hearing protection) in attenuating low-frequency noise compared with other noise. Intense low-frequency noise appears to produce clear symptoms including respiratory impairment and aural pain. Although the effects of lower intensities of low-frequency noise are difficult to establish for methodological reasons, evidence suggests that a number of adverse effects of noise in general arise from exposure to low-frequency noise: Loudness judgments and annoyance reactions are sometimes reported to be greater for low-frequency noise than other noises for equal sound-pressure level; annoyance is exacerbated by rattle or vibration induced by low-frequency noise; speech intelligibility may be reduced more by low-frequency noise than other noises except those in the frequency range of speech itself, because of the upward spread of masking. On the other hand, it is also possible that low-frequency noise provides some protection against the effects of simultaneous higher frequency noise on hearing. Research needs and policy decisions, based on what is currently known, are considered.

Auditory Threshold↗
PubMed1996

Temporal integration of loudness as a function of level.

Temporal integration of loudness for 1-kHz tones and broadband noises was compared over a wide range of levels. Absolute thresholds and levels required to produce equal loudness were measured for 5-, 30-, and 200-ms stimuli using an adaptive, two-interval, two-alternative forced-choice procedure. Levels ranged from 5 to 80 dB SL for noises and from 5 to 90 dB SL for tones. Results for six listeners with normal hearing show that the amount of temporal integration, defined as the level difference between equally loud 5- and 200-ms stimuli, varies nonmonotonically with level. The average amount of temporal integration varies from about 10-12 dB near threshold, to a peak of 18-19 dB when the 5-ms tone is about 56 dB and the 5-ms noise is about 76 dB SPL; the amount of temporal integration decreases to 10 dB for tones and 13 dB for noises with levels around 100 dB SPL. The data for tones are in good agreement with the majority of existing data. The data for noises are within the range of the few previous data, but the effect of level differs from that obtained in previous studies. The present results indicate that the growth of loudness may, at least in part, be consistent with the nonlinear input/output function of the basilar membrane.

Adult↗
PubMed1996

Loudness enhancement and intensity discrimination under forward and backward masking.

There is a large deterioration in intensity discrimination performance at medium levels for a 30-ms sinusoidal pedestal presented 100 ms before or 100 ms after an intense masker [Plack and Viemeister, J. Acoust. Soc. Am. 92, 3097-3101 (1992)]. It has also been demonstrated that the loudness of a 30-ms sinusoidal tone burst, presented 100 ms after a masking tone burst, is enhanced at mid-levels [Zeng, J. Acoust. Soc. Am. 96, 2127-2131 (1994)]. The present experiment measured intensity discrimination and loudness enhancement in both forward and backward masking. A double-staircase adaptive procedure was used to match the loudness of a 30-ms, 1-kHz standard sinusoid presented in quiet to the loudness of a 30-ms, 1-kHz sinusoid presented 100 ms after (forward masking) or 100 ms before (backward masking) a 110-ms, 90-dB, 1-kHz masking sinusoid. The mean of the thresholds from the two staircases was used to determine the amount of enhancement, and the difference between the thresholds from the two staircases was used to determine the intensity just noticeable difference (jnd). Four listeners were tested at a range of standard levels between 30 and 90 dB. For all listeners, in both forward and backward masking, the jnd and loudness were greatest at mid-levels (40-70 dB). For a given listener, there was no substantial difference between the form of the results under forward and compared to backward masking, although there was considerable variability in the size of the effects between the individual listeners. Combining all the data, for both forward and backward masking there was a positive correlation between the size of the jnd and the magnitude of the loudness enhancement, although the correlation was only significant in backward masking (p < 0.005). Taken with the results of Zeng, these data suggest a link between loudness enhancement and the jnd increase, and a link between the mechanisms underlying the effects of forward and backward masking on intensity discrimination. It is suggested that all these effects may be caused by long-term loudness integration in the auditory system.

Humans↗
PubMed1996

Relationships of intensity discrimination to sensation and loudness levels: dependence on sound frequency.

The main purpose of the experiments described in this article was to establish the frequency dependence of auditory intensity jnd's (just noticeable differences) for pure tones as functions of loudness level (LL). For this purpose, two sets of experiments were performed. In the first, the jnd's were measured as functions of sensation level (SL) at sound frequencies of 0.125, 0.5, 1, 2, 4, 6, and 8 kHz. The SLs were set at 10, 20, 40, 60, and 80 dB in a random order. In the second, LLs corresponding to the set of the SL at 1 kHz were obtained, and the relationship of the jnd's to LL was determined. We investigated to what extent the constant loudness-constant jnd relationship found previously intrafrequency can be applied interfrequency. The detection experiments were performed with a continuous-pedestal paradigm and an adaptive two-alternative, forced-choice (2AFC) procedure that converges on 75% of correct responses. We found that the jnd's in dB plotted versus SL decreased roughly according to power functions, the rate of decrease depending on sound frequency. The jnd's increased with sound frequency at low SLs but became practically constant at high SLs. According to the second experiment, the jnd's followed approximately the same function of LL at all sound frequencies, except for a multiplicative constant, irrespective of the slope of the loudness level function. Thus, the constant loudness-constant jnd relationship appears to apply interfrequency except for a multiplicative constant that, with the continuous-pedestal paradigm, grew monotonically with the frequency.

Adult↗