Loudness perception for short-duration tones in masking noise

The effect of masking noise on the temporal summation of loudness is investigated here by performing loudness balances between a standard 500-msec tone burst (1000 Hz at either 20-, 50-, or 80-dB SPL) and either masked or unmasked comparison tone bursts (1000 Hz with durations between 10 and 640 msec). In all but two instances, the obtained functions relating SPL for equal loudness to stimulus duration could be plotted as two line segments that met to form a knee. The slopes of the line segments at durations less than the kneepoints are altered by the masking noise, becoming less steep with increased masking. The rate of the slope decrease is related to the standard sound pressure level (SPL) and is greatest using the 80-dB SPL standard and least with the 20-dB SPL standard. Temporal summation of loudness continues at durations above the kneepoints. However, the obtained slopes are less than those found below the knee, and are independent of the test conditions. The slope changes are found to be related to the noise producing a power transformation on the operating characteristics of the auditory system. These latter findings are discussed in relation to Zwislocki's quantitative theory of the temporal summation of loudness.     

Diotic loudness summation in normal and impaired hearing

Diotic loudness summation at 500 and 2000 Hz was measured in 10 normal-hearing and 10 cochlear-impaired listeners. Diotic stimuli were matched in loudness to monaural "standards" of 70, 80, and 90 dB SPL. Diotic loudness summation averaged about 9 dB at 500 Hz for both groups. At 2000 Hz, the hearing-impaired listeners showed reduced diotic loudness summation at the 70-and 80-dB levels, but showed normal diotic loudness summation (about 9 dB) at the 90-dB level. The results indicate that diotic loudness summation is normal in cochlear-impaired ears, provided that the stimuli are presented sufficiently above threshold.     

Auditory evoked potentials in the detection of interaural intensity differences in children and adults

OBJECTIVE: The goal of the current study was to identify neurophysiological indices of binaural processing in children with no history of hearing loss or listening problems. The results will guide our efforts to diagnose children for whom impaired binaural processing may contribute to difficulty understanding speech in adverse listening environments. Our main aim was to document the cortical auditory evoked potentials (AEPs) in response to interaural intensity differences (IIDs) in children. It is well known, however, that the morphology of AEPs is substantially different in children and adults. Comparison of AEPs in response to IIDs between children and adults allowed us to evaluate age-related differences in the AEP waveform of binaural processing. DESIGN: Nine children (ages 7 yr 0 mo to 9 yr 4 mo) and 11 adults (23 to 34 yr) with normal hearing and no known or suspected neurological or academic problems listened to click stimuli under earphones. Click trains consisted of broadband noise of 1-msec duration with a click rate of 100 Hz. In the experimental condition (IID-present) 50-msec intervals containing an interaural intensity difference of 20 dB were introduced periodically in the continuous stream of otherwise diotic click trains. The diotic trains alternated in intensity between 50 and 70 dB peSPL. In the baseline condition (IID-absent), the same continuous diotic click stream utilized in the IID-present condition was presented with no IID. Finally, for comparison with existing literature on AEPs in children and adults, we presented monaural click trains of 50-msec duration, like those used in the IID stimulus (but with no continuous stream) to the left ear at 70 dB peSPL, with an interstimulus interval of 750 msec. Stimuli were presented in separate blocks for each stimulus type and AEPs were recorded in a passive listening condition. RESULTS: A prominent AEP activation was present in both age groups for the IID-present condition; the IID-absent condition did not evoke substantial AEPs. Adult waveform characteristics of the AEPs to monaural clicks and IID-present around 100 and 200 msec were comparable to previous reports. The children demonstrated the expected AEP activation patterns in response to monaural clicks (i.e., positivity around 100 msec, followed by prominent negativity around 250 msec); however their AEP waveforms to IIDs were mainly comprised of a prolonged positivity around 200 to 250 msec after stimulus onset. A two-step temporal-spatial Principal Component Analysis (PCA) was used to evaluate the temporal (time) and spatial (electrode location) composition of the AEP waveforms in children and adults in response to IID-present and IID-absent conditions. Separate repeated-measures ANOVAs with factor scores as the dependent variable were conducted for each temporal factor (TF) representing the waveform deflections around 100, 200 and 250 msec (i.e., TF110, TF200, and TF255) at the frontocentral spatial factor (SF1). Significantly greater negative activation was observed in adults than in children in response to IID-present for TF110. The IID-present condition evoked a significantly greater waveform inflection for TF200 in both age groups than IID-absent. A positive going activation for TF255 was observed in the IID-present condition in children but not in adults. CONCLUSIONS: This study compared obligatory AEPs in response to binaural processing of IIDs in children and adults with normal hearing. The morphology of the AEP waveform in children was different for monaural clicks and IID-present stimuli. The difference between AEPs for monaural clicks and IID-present did not occur in adults. It is likely that polarity reversal of the AEPs in response to the IID accounts for the observed AEP morphology in children.     

Auditory stream segregation in goldfish (Carassius auratus)

Goldfish were classically conditioned to a mixture of two pulse trains differing in both repetition rate and the spectral profile of the pulses. Animals were then tested for generalization to single pulse trains having one or the other spectral profile presented at a variety of repetition rates. Generalization functions of repetition rate were qualitatively similar to those obtained following conditioning to either of the pulse trains alone. Thus, the spectral profile of each pulse type was appropriately associated with the repetition rate at which that pulse type was presented during conditioning. These results indicate that the two concurrent pulse trains making up the conditioning stimuli were analyzed independently, forming two auditory streams. When either of the two pulse trains were presented with a 500 ms onset asynchrony, stream segregation was enhanced. These and other results suggest that many fundamental features of the human sense of hearing are widely shared among vertebrate animals, and may have developed first among fishes.     

Effect of Pulse Rate on Loudness Discrimination in Cochlear Implant Users

Stimulation pulse rate affects current amplitude discrimination by cochlear implant (CI) users, indicated by the evidence that the JND (just noticeable difference) in current amplitude delivered by a CI electrode becomes larger at higher pulse rates. However, it is not clearly understood whether pulse rate would affect discrimination of speech intensities presented acoustically to CI processors, or what the size of this effect might be. Intensity discrimination depends on two factors: the growth of loudness with increasing sound intensity and the loudness JND (or the just noticeable loudness increment). This study evaluated the hypothesis that stimulation pulse rate affects loudness JND. This was done by measuring current amplitude JNDs in an experiment design based on signal detection theory according to which loudness discrimination is related to internal noise (which is manifested by variability in loudness percept in response to repetitions of the same physical stimulus). Current amplitude JNDs were measured for equally loud pulse trains of 500 and 3000 pps (pulses per second) by increasing the current amplitude of the target pulse train until it was perceived just louder than a same-rate or different-rate reference pulse train. The JND measures were obtained at two presentation levels. At the louder level, the current amplitude JNDs were affected by the rate of the reference pulse train in a way that was consistent with greater noise or variability in loudness perception for the higher pulse rate. The results suggest that increasing pulse rate from 500 to 3000 pps can increase loudness JND by 60 % at the upper portion of the dynamic range. This is equivalent to a 38 % reduction in the number of discriminable steps for acoustic and speech intensities.     

Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. I. Stimulation with acoustic clicks

The coding of fine-temporal structure of sound, especially pulse repetition rate, was investigated on the single-unit level in the auditory midbrain of the grassfrog. As stimuli periodic click trains and Poisson distributed click ensembles have been used. The response to periodic click trains was studied in two aspects, focussing on two types of possible codes: a rate code and a synchrony code. From the iso-intensity rate histogram five basic average response rate characteristics as function of pulse repetition rate have been established: low-pass, band-pass, high-pass, bimodal and non-selective unit types. The synchronization capability, expressed in a synchronization index, was for a small majority of units non-significant and a low-pass function of pulse repetition rate for most of the other units. The rate code showed the largest diversity of response types and an enhanced selectivity to pulse repetition rate. The stimulus-response relation to Poisson distributed click ensembles was investigated by a non-linear system theoretical approach. On the basis of first- and second-order Poisson kernels possible neural mechanisms accounting for temporal selectivity were determined. A considerable fraction of units exhibited response characteristics that were invariant to changes in sound pressure level and average click rate. These units may function as feature detectors of fine-temporal structure of sound. The spectro-temporal sensitivity range of the auditory midbrain of the grassfrog appeared to be broad and not particularly tuned to the ensemble of conspecific cells.     

Coding of FM sweep trains and twitter calls in area CM of marmoset auditory cortex

The primate auditory cortex contains three interconnected regions (core, belt, parabelt), which are further subdivided into discrete areas. The caudomedial area (CM) is one of about seven areas in the belt region that has been the subject of recent anatomical and physiological studies conducted to define the functional organization of auditory cortex. The main goal of the present study was to examine temporal coding in area CM of marmoset monkeys using two related classes of acoustic stimuli: (1) marmoset twitter calls; and (2) frequency-modulated (FM) sweep trains modeled after the twitter call. The FM sweep trains were presented at repetition rates between 1 and 24 Hz, overlapping the natural phrase frequency of the twitter call (6–8 Hz). Multiunit recordings in CM revealed robust phase-locked responses to twitter calls and FM sweep trains. For the latter, phase-locking quantified by vector strength (VS) was best at repetition rates between 2 and 8 Hz, with a mean of about 5 Hz. Temporal response patterns were not strictly phase-locked, but exhibited dynamic features that varied with the repetition rate. To examine these properties, classification of the repetition rate from the temporal response pattern evoked by twitter calls and FM sweep trains was examined by Fisher’s linear discrimination analysis (LDA). Response classification by LDA revealed that information was encoded not only by phase-locking, but also other components of the temporal response pattern. For FM sweep trains, classification was best for repetition rates from 2 to 8 Hz. Thus, the majority of neurons in CM can accurately encode the envelopes of temporally complex stimuli over the behaviorally-relevant range of the twitter call. This suggests that CM could be engaged in processing that requires relatively precise temporal envelope discrimination, and supports the hypothesis that CM is positioned at an early stage of processing in the auditory cortex of primates.     

Binaural loudness summation in the hearing impaired

Binaural loudness summation was measured using three different paradigms with 10 normally hearing and 20 bilaterally symmetrical high-frequency sensorineural hearing loss subjects. An adaptive paradigm and a loudness matching procedure measured summation at the lower and upper level of comfortable loudness and the loudness discomfort level (LDL). Monaural and binaural LDLs also were obtained with a clinical procedure designed to select maximum output of hearing aids. Stimuli for all three tasks consisted of 500- and 4000-Hz pure tones and a speech spectrum noise. Binaural summation increased with presentation level using the loudness matching procedure, with values in the 6-10 dB range. Summation decreased with level using the adaptive paradigm, and no summation was present with the clinical LDL task. The hearing-impaired subjects demonstrated binaural summation that was not significantly different from the normally hearing subjects. The results suggest that a bilaterally symmetrical sensorineural hearing loss does not affect binaural loudness summation. The monaural and binaural dynamic range widths were similar, and the LDL results suggest that binaural loudness summation may not be an important factor in selecting maximum output of hearing aids.     

Maturational aspects of periodicity coding in cat primary auditory cortex.

The click-following responses for single units in the primary auditory cortex of the cat were explored as a function of age. Recordings were obtained in kittens from 9-53 days of age and assembled in four age groups; 10-15 days, 16-21 days, 22-27 days and 30-60 days. Age group means were compared to results obtained in adult cats. The stimulus consisted of one second long click trains presented every three seconds with click rates ranging from 1-32 clicks per second. The response was characterized by entrainment, rate Modulation Transfer Function (rMTF), vector strength (VS) and temporal Modulation Transfer Function (tMTF). Maturational effects on periodicity coding comprised changes in overall responsiveness as well as click-rate dependent changes. The number of spikes elicited by single stimuli increased on average 3-fold between the second post-natal week and adulthood, probably as a result of more efficient synapses in the central auditory pathway and some improvement in thresholds. Adaptation became less pronounced with age; neurons started to respond to the later clicks in the 8/s and 16/s click trains from the third post natal week on. By the end of the first post-natal month the click following responses resembled the adult ones qualitatively, however, increased firing rates and spontaneous rates together with rebound responses continued to produce quantitative differences between the 30-60 days olds and the adults. Limiting rates for the tMTF (50% of the response at 1/s) increased from 6 Hz in the 10-15 day old to 12 Hz in adults. The decrease in the duration of the post-activation suppression coupled with the increased response with age to trains with higher click rates suggested that the maturation of inhibitory processes in the cortex play a major role in this rate dependence.     

Temporal nonlinearity revealed by transient evoked otoacoustic emissions recorded to trains of multiple clicks

A series of detailed experiments is described that investigates how a transient evoked otoacoustic emission (TEOAE) recorded to one-click stimulus is affected by the presence of a variable number of preceding clicks presented over a range of interclick intervals (ICIs). Part of the rationale was to determine if the resulting nonlinear temporal interactions could help explain the amplitude reduction seen when TEOAEs are recorded at very high click rates, as when using maximum length sequence stimulation. Amongst the findings was that the presence of a preceding train of clicks could either suppress or enhance emission amplitude, depending on the number of clicks in the train and the ICI. Results also indicated that the duration of the click trains, rather than the ICI, was the important factor in yielding the most suppressed response and that this seemed to depend on stimulus level. The results recorded at two levels also suggested that the cochlear temporal nonlinearity being monitored was in part related to the nonlinear process that determines the compressive input/output function for stimulus level. It is hypothesised that nonlinear temporal overlap of vibration patterns on the basilar membrane may underlie much of the pattern of results.     

Spectral loudness summation as a function of duration for hearing-impaired listeners

Spectral loudness summation was measured for 10- and 1000-ms long bandpass-noise signals with bandwidths in the range of 0.2 to 6.4 kHz centered at 2 kHz for nine listeners with hearing impairment of primarily cochlear origin. Loudness matches between signals with the same duration and different bandwidth were obtained using an adaptive two-interval, two-alternative forced-choice procedure with interleaved tracks. The 3200-Hz wide reference signal had a level of 45 dB or 65 dB SPL. Every signal was individually frequency equalized prior to presentation in order to ensure audibility of all spectral components for the two reference input levels. Generally the same amount of spectral loudness summation for 10- and 1000-ms long signals was obtained for both reference levels. However, some hearing-impaired listeners show, similarly to normal-hearing listeners, a larger spectral loudness summation for short than for long duration signals, indicating that duration effects in spectral loudness summation are caused by retrocochlear processes.     

Spectral loudness summation as a function of duration for hearing-impaired listeners

Spectral loudness summation was measured for 10- and 1000-ms long bandpass-noise signals with bandwidths in the range of 0.2 to 6.4 kHz centered at 2 kHz for nine listeners with hearing impairment of primarily cochlear origin. Loudness matches between signals with the same duration and different bandwidth were obtained using an adaptive two-interval, two-alternative forced-choice procedure with interleaved tracks. The 3200-Hz wide reference signal had a level of 45 dB or 65 dB SPL. Every signal was individually frequency equalized prior to presentation in order to ensure audibility of all spectral components for the two reference input levels. Generally the same amount of spectral loudness summation for 10- and 1000-ms long signals was obtained for both reference levels. However, some hearing-impaired listeners show, similarly to normal-hearing listeners, a larger spectral loudness summation for short than for long duration signals, indicating that duration effects in spectral loudness summation are caused by retrocochlear processes.     

Relationship between gap detection thresholds and loudness in cochlear-implant users

Gap detection threshold (GDT) is a commonly used measure of temporal acuity in cochlear-implant (CI) recipients. This measure, like other measures of temporal acuity, shows considerable variation across subjects and also varies across stimulation sites within subjects. The aims of this study were (1) to determine whether across-site variation in GDTs would be reduced or maintained with increased stimulation levels; (2) to determine whether across-site variation in GDTs at low stimulation levels was related to differences in loudness percepts at those same levels; and (3) to determine whether matching loudness levels could reduce across-site differences in GDTs. Thresholds and maximum comfortable loudness levels were measured in postlingually deaf adults using all available sites in their electrode arrays. All sites were then surveyed at 30% of the dynamic range (DR) to examine across-site variation. Two sites with the largest difference in GDTs were then selected and for those two sites GDTs were measured at multiple levels of the DR (10%, 30%, 50%, 70%, and 90%). Stimuli consisted of 500 ms trains of symmetric-biphasic pulses, 40 μs/phase, presented at a rate of 1000 pps using a monopolar (MP1+2) electrode configuration. To examine perceptual differences in loudness, the selected sites were loudness-matched at the same levels of the DR. Variations in GDTs and loudness patterns were observed across stimulation sites and across subjects. Variations in GDTs across sites tended to decrease with increasing stimulation levels. For the majority of the subjects, stimuli at a given level in %DR were perceived louder at sites with better GDTs than those presented at the same level in %DR at sites with poorer GDTs. These results suggest that loudness is a contributing factor to across-site variation in GDTs and that CI fittings based on more detailed loudness matching could reduce across-site variation and improve perceptual acuity.     

Temporal loudness integration and spectral loudness summation in normal-hearing and hearing-impaired listeners.

The aim of this study was to test for differences between normal-hearing and hearing-impaired listeners regarding two fundamental aspects of intensity perception: loudness integration and loudness summation. Loudness functions for three different stimuli were measured using categorical loudness scaling in 8 normal-hearing and 12 hearing-impaired subjects. The results indicated that temporal loudness integration, defined as the difference in SPL between 16.25-ms and 300-ms noise bursts of equal loudness, was larger in the hearing-impaired than in the normal-hearing listeners. Loudness summation, defined as the difference in SPL between a 300-ms, 1,600-Hz tone pip and a white noise burst of the same duration and loudness, did not differ between the two groups. Implications of these results for hearing aid fitting strategies based on loudness normalization are discussed.     

Loudness and auditory steady-state responses in normal-hearing subjects

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the growth of loudness in listeners with normal hearing. Individual intensity functions were obtained from measures of loudness growth using the contour test and from the electrophysiological amplitude measures of multiple amplitude-modulated (77–105Hz) tones (500, 1000, 2000, and 4000Hz) simultaneously presented to both ears and recorded over the scalp. Slope analyses for the behavioural and electrophysiological intensity functions were separately performed. Response amplitudes of the ASSRs and loudness sensation judgements increase as the stimulus intensity increases for the four frequencies studied. A significant relationship was obtained between loudness and the ASSRs. The results of this study suggest that the amplitude of the ASSRs may be used to estimate loudness growth at least for individuals with normal hearing.     

Loudness and auditory steady-state responses in normal-hearing subjects

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the growth of loudness in listeners with normal hearing. Individual intensity functions were obtained from measures of loudness growth using the contour test and from the electrophysiological amplitude measures of multiple amplitude-modulated (77-105 Hz) tones (500, 1000, 2000, and 4000 Hz) simultaneously presented to both ears and recorded over the scalp. Slope analyses for the behavioural and electrophysiological intensity functions were separately performed. Response amplitudes of the ASSRs and loudness sensation judgements increase as the stimulus intensity increases for the four frequencies studied. A significant relationship was obtained between loudness and the ASSRs. The results of this study suggest that the amplitude of the ASSRs may be used to estimate loudness growth at least for individuals with normal hearing.     

Auditory-evoked responses to a monaural or a binaural click, recorded from the vertex, as in two temporal derivations; effect of interaural time differences (author's transl)]

The auditory-evoked responses have been recorded on 5 subject by vertex, right temporal and left temporal electrodes simultaneously. 30 dB sensation level clicks were used as stimuli; one click was presented only to the right ear, or one click only to the left ear, or one click to the right ear and another click to the left ear with a variable interaural time difference in this latter case (0-150 ms). The N-P amplitude variations and the N and P latency variations have been studied and compared to those observed in the perceived lateralizations of the sound source.     

Audiological evaluation of the middle ear implant--temporal auditory acuity

Temporal auditory acuities provided by the middle ear implant (MEI) and by the hearing aid (HA) were compared in ten patients implanted with the MEI. Test sounds used in the experiment were tone bursts [rise and fall time: 25 msec, duration (t): 982-1000 msec, interval (T): 1000 msec] of 500 Hz or 2000 Hz at 70 dBSPL. A speaker was placed in front of a subject one meter apart. At first, the subject adjusted the gain control of outer unit at the most comfortable loudness level. He heard totally fifty times of the test sounds, which varied in duration each 2 msec difference between 982 and 1000 msec and were provided in random manner. Therefore, sound of each duration was given 5 times. The notch [= (T-t) x 2] of the test sounds recognized by the patients was regarded as time gap. Fifty percent discrimination thresholds, which were indicated by the gap times of the test sounds half identified, were obtained as the index of the temporal auditory acuity. Following the above test, performance of the HA was investigated by use of the same procedure. The results indicated that the temporal auditory acuities of the MEI were superior to those of the HA at 500 Hz (P less than 0.01) and 2000 Hz (P less than 0.001). When the intensity of the test sound at 2000 Hz decreased from 70 dBSPL to 60 or 50 dBSPL, the temporal auditory acuities of the MEI and the HA were almost depreciated together with consistent difference in the test of 6 patients (P less than 0.01, 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Twenty-stimulus train for rapid acquisition of auditory brainstem responses in humans

This study addressed the clinical need to obtain frequency-specific auditory brainstem responses (ABRs) more rapidly than is currently possible. ABRs were obtained from 20 subjects using two different methods: a conventional method with tone bursts presented singly and a multiple-stimulus method using a train of 20 tone bursts. For both methods, tone bursts were presented at frequencies 1, 2, 4, and 8 kHz, shaped with a Blackman-Harris window and having intensity levels up to 105 dB peak equivalent sound pressure level (peSPL). The single tone bursts were presented at a 17.2/sec repetition rate. The 20 tone-burst train used the four frequencies at five intensities each and a repetition rate of 3.7/sec (separations between tone bursts of 9-12 msec, with 77 msec off-time between trains). Mean latencies and mean amplitudes for wave V were compared using t-tests for each of 12 conditions (four frequencies, each at the three highest output levels). For latency, only one comparison was significantly different (2 kHz, 77 dB peSPL). Similarly, only one comparison was significant for amplitude (2 kHz, 97 dB peSPL). There was, however, a trend for the tone bursts presented in trains to have longer latencies and reduced amplitudes compared to the respective responses for the single tone-burst condition. These results indicate the presence of some response adaptation when tone bursts are presented in a train. The use of a properly designed stimulus train can result in a significant time savings for obtaining frequency-specific ABRs as compared with single tone-burst presentations.     

Multichannel electrical stimulation of the auditory nerve in man. II. Channel interaction

A multichannel cochlear implant can be an effective prosthesis only if its channels are independent of each other. Presumably independence is achieved by stimulating different populations of surviving neurons. Two types of interaction might occur between channels: electrical current field summation peripheral to stimulation of the nerves and neural-perceptual interaction following stimulation. Two psychophysical techniques to assess channel independence are discussed. In one technique a masker is presented on one channel in order to adapt the nerves responding to that channel. The forward masked threshold of a signal is then measured on all other channels and elevation of threshold is assumed to indicate overlapping neural populations. In the second procedure channel interaction is evaluated by measuring the loudness summation of stimuli presented simultaneously to two channels. The magnitude, distribution, and phasic components of the loudness summation are measures of interaction between channels. Data from two subjects suggests that monopolar stimulation produces broader interaction patterns than bipolar stimulation as a function of electrode separation. Considerable differences in the extent of channel interaction were observed between the two subjects, possibly because of the difference in the absolute current levels needed for equivalent sensation levels.