The effect of broadband noise on the human brainstem auditory evoked response. II. Frequency specificity

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high-pass masking noise. Experiment 1 used 1000- and 4000-Hz, 60-dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000-Hz tone bursts, while amplitude decrements were similar for both tone-burst frequencies. Experiment 2 varied high-pass masker cutoff frequency and the level of subtotal masking in the presence of 50-dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived-band frequency. Experiment 3 covaried high-pass masker cutoff frequency and subtotal masking level for 1000- and 4000-Hz tone-burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone-burst frequencies when the response-generating region of the cochlear partition was limited by high-pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low-frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000-Hz tone-burst stimuli is made more complex by multiple technical factors discussed herein.     

Human brain-stem auditory evoked responses obtained by cross correlation to trains of clicks, noise bursts, and tone bursts

Brain-stem auditory evoked responses (BAERs) were obtained in eight normal-hearing young adults. Stimuli included clicks, noise bursts, and tone bursts. Tone bursts included carrier frequencies of 1, 2, 4, and 8 kHz. All stimuli were presented at 60 dB nHL. BAERs were obtained by presenting stimuli in pseudorandom trains, called maximum length sequences (MLSs). BAERs were recovered by cross correlating the responses with a recovery sequence. MLS-BAERs were obtained with minimum pulse intervals (MPIs) of 6, 4, and 2 ms. Conventional BAERs were also obtained for stimuli presented at a rate of 30 Hz. BAERs were obtained for all stimuli, for both the conventional averaging technique and for the cross-correlation technique. BAERs were observed for MPIs as short as 2 ms for all stimuli. Wave V was the only peak consistently identifiable for these stimuli. For all stimuli, wave V latency increased and wave V amplitude decreased with decreasing MPI. This is the first demonstration of the use of maximum length sequences combined with cross correlation to obtain BAERs to noise burst and tone burst stimuli.     

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies.

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.     

Frequency specificity of human auditory brainstem responses as revealed by pure-tone masking profiles

The frequency specificity of the auditory brainstem response (ABR) was examined by means of pure-tone masking profiles using click, 4000-Hz, and 1000-Hz filtered-click stimuli. Simultaneous pure-tone maskers were presented at one-half octave intervals around stimulus center frequency. Masking profiles at two intensities (60 and 40 dB SL) were obtained by measuring both latency and amplitude shifts in wave V as a result of the discrete-frequency maskers. Both latency and amplitude analyses showed masking profiles at 40 dB SL that were narrow and centered around stimulus frequency, whereas profiles at 60 dB SL showed high-frequency spread of the cochlear excitation area.     

Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). I: Effects of click level, rate, and polarity

Three experiments evaluating the effects of various stimulus manipulations on the click-evoked gerbil brain-stem auditory-evoked response (BAER) are reported. In experiment 1, click polarity and level were covaried. With increasing click level, there is a parallel decrease in the latency of the first five BAER peaks (i-v) and an increase in BAER peak amplitudes. Mean wave i amplitude was greater for rarefaction than condensation clicks at high click levels; mean wave v amplitude was greater for condensation clicks at higher click levels. Experiment 2 covaried click rate and polarity. The latency of the BAER peaks increased with increasing click repetition rate. This rate-dependent latency increase was greater for the later BAER peaks, resulting in an increase in the i-v interval with increasing click rate. As rate increased, the amplitudes of waves i and v decreased monotonically, whereas the amplitudes of waves ii-iv were largely uninfluenced by click rate. As in experiment 1, mean wave i amplitude was greater for rarefaction clicks, whereas mean wave v amplitude was greater for condensation clicks. The magnitude of these polarity dependencies on waves i and v amplitude decreased with increasing click rate. Experiment 3 evaluated the effects of click polarity on BAERs to high-intensity (100 dB pSPL) clicks presented at a rate of 10 Hz. In eight of ten gerbils evaluated, wave i amplitude was greater to rarefaction clicks, and, in all ten animals, wave v amplitude was greater to condensation clicks. The effects of click level and rate on BAER peak amplitudes, latencies, and interwave intervals are reminiscent of stimulus dependencies reported for the human BAER. The effects of click polarity on the amplitudes of waves i and v of the gerbil BAER have also been reported for the human BAER.     

An acoustic model of a multiple-channel cochlear implant

A set of bandpass filtered, pulsed noise stimuli presented to three normally hearing subjects was found to have psychophysical properties similar to those of a set of pulsed electrical stimuli presented to two cochlear implant patients. Identical procedures were used to compare the performances of the two groups of subjects in the following tasks: (a) pulse rate difference limen measurements, (b) pitch scaling for stimuli differing in pulse rate, (c) pitch scaling and categorization of stimuli differing in filter frequency or electrode position, and (d) similarity judgments of stimuli differing in pulse rate and filter frequency or electrode position. By choosing the parameters of the acoustic stimuli appropriately, a high level of agreement between the two sets of results was achieved. Electrical stimuli on electrodes at different sites in the cochlea were matched with pulsed noise passed through bandpass filters with different center frequencies. Matching was achieved for equal electrical and acoustic pulse rates.     

Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). III: Additivity of click level and rate with noise level

Two experiments were performed that evaluated the effects of ipsilateral-direct broadband noise maskers on the gerbil brain-stem auditory-evoked response (BAER) to click stimuli. In experiment 1, clicks were presented at 27 Hz at levels including 70, 80, 90, and 100 dB pSPL. Noise conditions included a no-noise control, and included noise levels varying in 10-dB increments from 20 dB SPL to a maximum noise level of 50, 60, 70, and 80 dB SPL for click levels of 70, 80, 90, and 100 dB pSPL, respectively. Gerbil BAER peaks were labeled with small roman numerals to distinguish them from human BAER peaks. The dependent variables included waves i and v latencies and amplitudes. Peak latencies increased and peak amplitudes decreased with decreasing click level and increasing noise level. To a first approximation, peak latencies and amplitudes showed changes with increasing noise level that were similar across click level. With increasing click level, there was little or no effect on the i-v interval. There was an increase in the i-v interval with increasing noise level. In experiment 2, click level was held constant at 90 dB pSPL, and click rates included 15, 40, 65, and 90 Hz. For each click rate, noise conditions included a no-noise control, and noise levels included 20, 30, 40, 50, 60, and 70 dB SPL. With increasing click rate and noise level, there was an increase in peak latencies, an increase in the i-v interval, and a decrease in peak amplitudes. The magnitude of peak latency and amplitude shifts with increasing click rate was dependent on noise level. Specifically, the magnitude of rate-dependent changes decreased with increasing level of broadband noise. These data are compared to human BAER experiments, and are found to be in fundamental agreement.     

Frequency specificity of the human auditory brainstem and middle latency responses using notched noise masking

This study investigated the frequency specificity of the auditory brainstem and middle latency responses to 80 and 90 dB ppe SPL 500-Hz and 90 dB ppe SPL 2000-Hz tonebursts. The stimuli were brief (2-1-2 cycle) linear-gated tonebursts. ABR/MLRs were recorded using two electrode montages: (1) Cz-nape of neck and (2) Cz-ipsilateral earlobe. Cochlear contributions to ABR wave V-Na and MLR waves Na-Pa and Pa-Nb were assessed by plotting notched noise tuning curves which showed amplitudes and latencies as a function of center frequency of the noise masker [Abdala and Folsom, J. Acoust. Soc. Am. 97, 2394 (1995); ibid. 98, 921 (1995)]. Maxima in the response amplitude profiles for the ABR and MLR to 80 dB ppe SPL tonebursts occurred within one-half octave of the nominal stimulus frequency, with minimal contributions to the responses from frequencies greater than one octave away. At 90 dB ppe SPL, contributions came from a slightly broader frequency region for both stimulus frequencies. Thus, the ABR/MLR to 80 dB ppe SPL tonebursts shows good frequency specificity which decreases at 90 dB ppe SPL. No significant differences exist in frequency specificity of: (1) ABR wave V-Na versus MLR waves Na-Pa and Pa-Nb at either stimulus frequency or intensity; and (2) ABR/MLRs recorded using the two electrode montages.     

Auditory brain stem responses from human infants: pure-tone masking profiles for clicks and filtered clicks

The effects of simultaneous pure-tone maskers on ABR wave V latency and amplitude were examined in three-month-old infants as a means of delineating the frequency specificity of these responses in the immature auditory system. Masking profiles at two intensities (60 and 40 dBn HL) were obtained for click, as well as 4000- and 1000-Hz filtered-click stimuli. Infant profiles, obtained by measuring both latency and amplitude shifts as a result of the discrete-frequency maskers, were compared to adult data obtained under an identical masking paradigm. Both latency and amplitude analyses showed masking profiles for infants which reveal greater low-frequency contribution to responses than found in adult profiles. Additionally, the infant profiles reveal clear differences in the degree of high-frequency spread of masking when comparisons are made to the adult data.     

Comparison of echolocation clicks from geographically sympatric killer whales and long-finned pilot whales (L)

The source characteristics of biosonar signals from sympatric killer whales and long-finned pilot whales in a Norwegian fjord were compared. A total of 137 pilot whale and more than 2000 killer whale echolocation clicks were recorded using a linear four-hydrophone array. Of these, 20 pilot whale clicks and 28 killer whale clicks were categorized as being recorded on-axis. The clicks of pilot whales had a mean apparent source level of 196 dB re 1 μPa pp and those of killer whales 203 dB re 1 μPa pp. The duration of pilot whale clicks was significantly shorter (23 μs, S.E.=1.3) and the centroid frequency significantly higher (55 kHz, S.E.=2.1) than killer whale clicks (duration: 41 μs, S.E.=2.6; centroid frequency: 32 kHz, S.E.=1.5). The rate of increase in the accumulated energy as a function of time also differed between clicks from the two species. The differences in duration, frequency, and energy distribution may have a potential to allow for the distinction between pilot and killer whale clicks when using automated detection routines for acoustic monitoring.     

Normal short-latency electrophysiological filtered click responses recorded from vertex and external auditory meatus.

We recorded normal electrophysiological responses to third-octave filtered clicks from external auditory meatus (EAM) and vertex electrodes referred to coupled earlobe electrodes (forehead ground). From both vertex and EAM, polarity-sensitive responses predominated at low frequencies and exhibited characteristics of both phase-locked neural responses (frequency-following response or FFR) and cochlear microphonics (CM). The FFR-like response predominated at the vertex site and the CM-like response predominated at EAM. At high frequencies, polarity-insensitive responses closely resembled rectangular-pulse click action potentials and brainstem evoked potentials, with clearly defined N1 and V peaks recorded from EAM and vertex, respectively. As frequency was lowered, the N1 and V peak latencies increased, the peaks broadened, and the latency-intensity curves steepened with greater prolongation occurring at lower click intensities. Lowering click frequency also shortened the N1-V interval and caused the plot of N1-V interval versus click intensity to become steeper. Plots of polarity-insensitive response amplitudes and thresholds against frequency revealed a high frequency bias for both N1 and V, but the V "frequency response" was flatter. A possible explanation of the shortened N1-V interval at low click frequencies based on this flatter V "Frequency response" is presented.     

A glimpsing account for the benefit of simulated combined acoustic and electric hearing

The benefits of combined electric and acoustic stimulation (EAS) in terms of speech recognition in noise are well established; however the underlying factors responsible for this benefit are not clear. The present study tests the hypothesis that having access to acoustic information in the low frequencies makes it easier for listeners to glimpse the target. Normal-hearing listeners were presented with vocoded speech alone (V), low-pass (LP) filtered speech alone, combined vocoded and LP speech (LP+V) and with vocoded stimuli constructed so that the low-frequency envelopes were easier to glimpse. Target speech was mixed with two types of maskers (steady-state noise and competing talker) at -5 to 5 dB signal-to-noise ratios. Results indicated no advantage of LP+V in steady noise, but a significant advantage over V in the competing talker background, an outcome consistent with the notion that it is easier for listeners to glimpse the target in fluctuating maskers. A significant improvement in performance was noted with the modified glimpsed stimuli over the original vocoded stimuli. These findings taken together suggest that a significant factor contributing to the EAS advantage is the enhanced ability to glimpse the target.     

Effects of relative starting phase and frequency separation of two-tone stimuli on the brain-stem auditory-evoked response

Brain-stem auditory-evoked responses (BAERs) were obtained in six normal-hearing adults using single-tone and two-tone stimuli arithmetically centered around 4000 Hz. Two-tone stimuli varied in frequency separation from 200 to 3200 Hz, and started in-phase (homophasic) or 180 deg out-of-phase (antiphasic) with each other. Responses to each of the single-tone components of the two-tone stimuli were elicited and then summed for comparison with responses to the two-tone stimuli. Results indicated no significant difference in wave V latency between homophasic or antiphasic two-tone conditions, and summed single-tone conditions. Under the homophasic condition, the mean latency for the widest frequency separation of the tones was significantly longer than those for narrower separations. A significant difference in wave V amplitude between two-tone phase conditions was found for frequency separations of 200, 400, and 3200 Hz only. Summed single-tone BAERs demonstrated a significantly larger wave V amplitude than responses from either two-tone phase condition at all frequency separations.     

The feasibility of maximum length sequences to reduce acquisition time of the middle latency response

Maximum length sequences (MLS) have been used to improve the signal-to-noise ratio (SNR) of otoacoustic emissions [Thornton, J. Acoust. Soc. Am. 94, 132-136 (1993)] and the auditory brainstem response [Thornton and Slaven, Br. J. Audiol. 27, 205-210 (1993)]. By implication, a shorter recording time would be required to give equal signal-to-noise ratio (SNR). This study aimed to establish whether it is also possible to improve the SNR of the auditory-evoked potential termed the middle latency response (MLR) using maximum length sequences (MLS). Recordings of 180 s each were made using a conventional recording rate and MLS rates of 42, 89, and 185 clicks/s. Three different stimulus intensities were used in the range 30 to 70 dB nHL. The rate of 89 clicks/s was found to produce most improvement in SNR for both the Na-Pa region of the MLR and the Na-Pb region. This improvement in SNR using MLS implies that an MLS rate of 89 clicks/s would produce a fourfold reduction in recording time for equal SNR over conventional recording for the Pa-Nb region of the MLR at a stimulus intensity of 70 dB nHL. The latency of the Nb wave was found to reduce significantly using MLS. An MLR could not be recorded from every subject in this study, but more subjects had an identifiable response for MLS than for conventional recordings. Use of MLS to record the MLR appears to offer the potential for reduction in test time and better wave identification.     

Object-related brain potentials associated with the perceptual segregation of a dichotically embedded pitch

The cortical mechanisms of perceptual segregation of concurrent sound sources were examined, based on binaural detection of interaural timing differences. Auditory event-related potentials were measured from 11 healthy subjects. Binaural stimuli were created by introducing a dichotic delay of 500-ms duration to a narrow frequency region within a broadband noise, and resulted in a perception of a centrally located noise and a right-lateralized pitch (dichotic pitch). In separate listening conditions, subjects actively discriminated and responded to randomly interleaved binaural and control stimuli, or ignored random stimuli while watching silent cartoons. In a third listening condition subjects ignored stimuli presented in homogenous blocks. For all listening conditions, the dichotic pitch stimulus elicited an object-related negativity (ORN) at a latency of about 150-250 ms after stimulus onset. When subjects were required to actively respond to stimuli, the ORN was followed by a P400 wave with a latency of about 320-420 ms. These results support and extend a two-stage model of auditory scene analysis in which acoustic streams are automatically parsed into component sound sources based on source-relevant cues, followed by a controlled process involving identification and generation of a behavioral response.     

Response timing constraints on the cortical representation of sound time structure

The precision of spike response timing of 94 primary auditory cortex neurons was studied using conventional extracellular recording techniques in barbiturate-anesthetized cats to which tone- and/or noise-burst stimuli were presented using sealed sound delivery systems. Precision of spike timing was indexed using the standard deviation of the first-spike latent period in responses evoked by repeated presentation of tonal stimuli systematically varied in frequency, amplitude, and/or repetition rate. Within a neuron, variability of first-spike timing was usually proportional to the mean first-spike latency, in agreement with previous reports. In cases where there was a systematic relation between the precision of response timing and the mean latency, a linear correlation accounted for up to 90% of the data variance. Across the 94 neurons, standard deviations seen in responses of minimum latency were related to minimal mean latencies, and were typically in the range from 0.15-1.5 ms. The data suggest that responses to transients in the cortex show a precision of spike timing which is only slightly worse than that seen in cochlear-nerve fibers. This, however, is in dramatic contrast to previous evidence on the steady-state temporal response of cortical cells, which is at least an order of magnitude poorer than that seen in auditory-nerve fibers and many cochlear nucleus cells. These observations may be directly relevant to the known consequences of auditory cortex pathology in man.     

Masking-level differences for trains of clicks

Masking-level differences (MLDs) were measured for trains of 2000-Hz bandpass clicks as a function of the interclick interval (ICI) and the number of clicks in the train. The magnitude of the MLD grew as the number of clicks in the train was increased from 1 to 32. While the MLDs tended to be larger at longer ICIs, the effect was mediated by changes in detectability in the homophasic conditions. For click trains consisting of 4-32 clicks, the improvement in detectability in the antiphasic conditions with increases in the number of clicks appears to be the result of integration of acoustic power, as is the case for the homophasic conditions. The absence of MLDs for short trains of high-frequency transients remains quite puzzling, since large MLDs are found with single, low-frequency transients.     

Auditory brain stem responses from human adults and infants: wave V tuning curves

Decrement in ABR wave V amplitude was measured in the presence of simultaneous tonal maskers. Probe stimuli were 1.0, 4.0, and 8.0-kHz third-octave-filtered clicks. Adults and 3-month-old infants served as subjects. The resultant amplitude-decrement functions for each tonal masker were fit with regression lines. The sound pressure level (SPL) required to reduce wave V to 50% of the unmasked probe amplitude was plotted for each masker to develop tuning curves. The tuning curves were quantified by calculations of tip-to-tail difference, Q 10, and SPL at maximum masker frequency (MMF). Tuning curves for adult and infant subjects were similar for the 1.0-kHz probe. For the high-frequency probes (4.0 and 8.0 kHz), smaller tip-to-tail differences and lower Q 10 values were observed for the infant subjects. Ranges of MMF level were similar across adult and infant subjects. For the 8.0-kHz probe, tuning curves from infant subjects consistently showed maximum masker frequencies which were lower than the probe.     

中华白海豚和东亚窄脊江豚回声定位信号分析与比较

为了增进珍稀齿鲸物种的了解和保护,对中华白海豚(Sousa chinensis)和东亚窄脊江豚(Neophocaena asiaeorientalis sunmeri)的回声定位信号特性进行了分析和比较.通过船只观测与声学监听的方式对厦门海域中华白海豚和东亚窄脊江豚的回声定位信号进行了调查,并对其声学参数进行了统计和对...