Recording of auditory brainstem response at high stimulation rates using randomized stimulation and averaging

Fundamental frequency predominantly drives talker differences in auditory brainstem responses to continuous speech

Deriving human neural responses to natural speech is now possible, but the responses to male- and female-uttered speech have been shown to differ. These talker differences may complicate interpretations or restrict experimental designs geared toward more realistic communication scenarios. This study found that when a male and female talker had the same fundamental frequency, auditory brainstem responses (ABRs) were very similar. Those responses became smaller and later with increasing fundamental frequency, as did click ABRs with increasing stimulus rates. Modeled responses suggested that the speech and click ABR differences were reasonably predicted by peripheral and brainstem processing of stimulus acoustics.

Quick Estimation of Minimum Hearing Levels Using a Binaural Multifrequency Stimulus Paradigm: Proof of Concept

OBJECTIVES

Objective estimation of minimum hearing levels using auditory brainstem responses (ABRs) elicited by single frequency tone-bursts presented monaurally is currently considered the gold standard. However, the data acquisition time to estimate thresholds (for both ears across four audiometric frequencies) using this method usually exceeds the sleep time (ranging between 35 and 49 minutes) in infants below 4 months, thus providing incomplete information of hearing status which in turn delays timely clinical intervention. Alternate approaches using faster rate, or tone-burst trains have not been readily accepted due to additional hardware and software requirements. We propose here a novel binaural multifrequency stimulation paradigm wherein several stimuli of different frequencies are presented binaurally in an interleaved manner. The rationale here is that the proposed paradigm will increase acquisition efficiency, significantly reduce test time, and improve accuracy by incorporating an automatic wave V detection algorithm. It is important to note that this paradigm can be easily implemented in most commercial ABR systems currently used by most clinicians.

DESIGN

Using this binaural multifrequency paradigm, ear specific ABRs were recorded in 30 normal-hearing young adults to both tone-bursts, and narrow-band (NB) iChirps at 500, 1000, 2000, and 4000 Hz. Comparison of ABRs elicited by tone-bursts and narrow-band chirps allowed us to determine if NB iChirps elicited a more robust wave V component compared with the tone-bursts. ABR data were characterized by measures of minimum hearing levels; wave V amplitude; and response detectability for two electrode configurations (high forehead-C7; and high forehead-linked mastoids).

RESULTS

Consistent with the research literature, wave V response amplitudes were relatively more robust for NB iChirp stimuli compared with tone-burst stimuli. The easier identification and better detectability of wave V for the NB iChirps at lower stimulus levels contributed to their better thresholds compared with tone-burst elicited responses. It is important to note that binaural multifrequency hearing levels close to minimum hearing levels were determined in approximately 22 minutes using this paradigm-appreciably quicker than the 45 to 60 minutes or longer time required for threshold determination using the conventional single frequency method.

CONCLUSIONS

Our novel and simple paradigm using either NB iChirps or tone-bursts provides a reliable method to rapidly estimate the minimum hearing levels across audiometric frequencies for both ears. Incorporation of an automatic wave V detection algorithm increases objectivity and further reduce test time and facilitate early hearing identification and intervention.

Multi-response deconvolution of auditory evoked potentials in a reduced representation space

The estimation of auditory evoked potentials requires deconvolution when the duration of the responses to be recovered exceeds the inter-stimulus interval. Based on least squares deconvolution, in this article we extend the procedure to the case of a multi-response convolutional model, that is, a model in which different categories of stimulus are expected to evoke different responses. The computational cost of the multi-response deconvolution significantly increases with the number of responses to be deconvolved, which restricts its applicability in practical situations. In order to alleviate this restriction, we propose to perform the multi-response deconvolution in a reduced representation space associated with a latency-dependent filtering of auditory responses, which provides a significant dimensionality reduction. We demonstrate the practical viability of the multi-response deconvolution with auditory responses evoked by clicks presented at different levels and categorized according to their stimulation level. The multi-response deconvolution applied in a reduced representation space provides the least squares estimation of the responses with a reasonable computational load. matlab/Octave code implementing the proposed procedure is included as supplementary material.

Dolphin short-term auditory fatigue and self-mitigation

Auditory brainstem responses (ABRs) were measured at 57 kHz in two dolphins warned of an impending intense tone at 40 kHz. Over the course of testing, the duration of the intense tone was increased from 0.5 to 16 s to determine if changes in ABRs observed after cessation of the intense sound were the result of post-stimulatory auditory fatigue or conditioned hearing attenuation. One dolphin exhibited conditioned hearing attenuation after the warning sound preceding the intense sound, but little evidence of post-stimulatory fatigue after the intense sound. The second dolphin showed no conditioned attenuation before the intense sound, but auditory fatigue afterwards. The fatigue was observed within a few seconds after cessation of the intense tone: i.e., at time scales much shorter than those in previous studies of marine mammal noise-induced threshold shifts, which feature measurements on the order of a few minutes after exposure. The differences observed between the two individuals (less auditory fatigue in the dolphin that exhibited the conditioned attenuation) support the hypothesis that conditioned attenuation is a form of "self-mitigation."

Temporary threshold shift in bottlenose dolphins exposed to steady-state, 1/6-octave noise centered at 0.5 to 80 kHza)

Temporary threshold shift (TTS) was measured in bottlenose dolphins after 1-h exposures to 1/6-octave noise centered at 0.5, 2, 8, 20, 40, and 80 kHz. Tests were conducted in netted ocean enclosures, with the dolphins free-swimming during noise exposures. Exposure levels were estimated using a combination of video-based measurement of dolphin position, calibrated exposure sound fields, and animal-borne archival recording tags. Hearing thresholds were measured before and after exposures using behavioral methods (0.5, 2, 8 kHz) or behavioral and electrophysiological [auditory brainstem response (ABR)] methods (20, 40, 80 kHz). No substantial effects of the noise were seen at 40 and 80 kHz at the highest exposure levels. At 2, 8, and 20 kHz, exposure levels required for 6 dB of TTS (onset TTS exposures) were similar to previous studies; however, at 0.5 kHz, onset TTS was much lower than predicted values. No clear relationships could be identified between ABR- and behaviorally measured TTS. The results raise questions about the validity of current noise exposure guidelines for dolphins at frequencies below ∼1 kHz and how to accurately estimate received noise levels from free-swimming animals.

Dolphin conditioned hearing attenuation in response to repetitive tones with increasing level

All species of toothed whales studied to date can learn to reduce their hearing sensitivity when warned of an impending intense sound; however, the specific conditions under which animals will employ this technique are not well understood. The present study was focused on determining whether dolphins would reduce their hearing sensitivity in response to an intense tone presented at a fixed rate but increasing level, without an otherwise explicit warning. Auditory brainstem responses (ABRs) to intermittent, 57-kHz tone bursts were continuously measured in two bottlenose dolphins as they were exposed to a series of 2-s, 40-kHz tones at fixed time intervals of 20, 25, or 29 s and at sound pressure levels (SPLs) increasing from 120 to 160 dB re 1 μPa. Results from one dolphin showed consistent ABR attenuation preceding intense tones when the SPL exceeded ∼140-150 dB re 1 μPa and the tone interval was 20 s. ABR attenuation with 25- or 29-s intense tone intervals was inconsistent. The second dolphin showed similar, but more subtle, effects. The results show dolphins can learn the timing of repetitive noise and may reduce their hearing sensitivity if the SPL is high enough, presumably to "self-mitigate" the noise effects.

The hunt for hidden hearing loss in humans: From preclinical studies to effective interventions

Many individuals experience hearing problems that are hidden under a normal audiogram. This not only impacts on individual sufferers, but also on clinicians who can offer little in the way of support. Animal studies using invasive methodologies have developed solid evidence for a range of pathologies underlying this hidden hearing loss (HHL), including cochlear synaptopathy, auditory nerve demyelination, elevated central gain, and neural mal-adaptation. Despite progress in pre-clinical models, evidence supporting the existence of HHL in humans remains inconclusive, and clinicians lack any non-invasive biomarkers sensitive to HHL, as well as a standardized protocol to manage hearing problems in the absence of elevated hearing thresholds. Here, we review animal models of HHL as well as the ongoing research for tools with which to diagnose and manage hearing difficulties associated with HHL. We also discuss new research opportunities facilitated by recent methodological tools that may overcome a series of barriers that have hampered meaningful progress in diagnosing and treating of HHL.

Subspace-constrained deconvolution of auditory evoked potentials

Auditory evoked potentials can be estimated by synchronous averaging when the responses to the individual stimuli are not overlapped. However, when the response duration exceeds the inter-stimulus interval, a deconvolution procedure is necessary to obtain the transient response. The iterative randomized stimulation and averaging and the equivalent randomized stimulation with least squares deconvolution have been proven to be flexible and efficient methods for deconvolving the evoked potentials, with minimum restrictions in the design of stimulation sequences. Recently, a latency-dependent filtering and down-sampling (LDFDS) methodology was proposed for optimal filtering and dimensionality reduction, which is particularly useful when the evoked potentials involve the complete auditory pathway response (i.e., from the cochlea to the auditory cortex). In this case, the number of samples required to accurately represent the evoked potentials can be reduced from several thousand (with conventional sampling) to around 120. In this article, we propose to perform the deconvolution in the reduced representation space defined by LDFDS and present the mathematical foundation of the subspace-constrained deconvolution. Under the assumption that the evoked response is appropriately represented in the reduced representation space, the proposed deconvolution provides an optimal least squares estimation of the evoked response. Additionally, the dimensionality reduction provides a substantial reduction of the computational cost associated with the deconvolution. matlab/Octave code implementing the proposed procedures is included as supplementary material.

Auditory brainstem responses obtained with randomised stimulation level

OBJECTIVE

To present randomised stimulation level (RSL) - a stimulation paradigm in which the level of the stimuli is randomised, rather than presented sequentially as in the conventional paradigm.

DESIGN

The value of RSL was evaluated by (i) comparing the morphology of auditory brainstem responses (ABRs) elicited by the conventional and RSL paradigms, and by (ii) an online survey investigating the hearing comfort of the stimulus sequence.

STUDY SAMPLE

ABRs were obtained from 11 normal-hearing adults (8 females, 25-29 years). The online survey was administered to 238 adults from the general community.

RESULTS

Results showed that (i) both stimulation paradigms elicit ABR signals of similar morphology, (ii) RSL provides a faster comprehensive representation of the ABR session, and that (iii) the general population found RSL stimuli to be more comfortable.

CONCLUSIONS

The simultaneous evaluation of all ABR traces of the session provided by RSL has potential to improve the identification of ABR components by enabling clinicians to make use of the response tracking strategy from the start of the test, which is critical in situations where ABRs present an abnormal morphology. New research opportunities and the clinical potential of RSL are discussed.

The Variability in Potential Biomarkers for Cochlear Synaptopathy After Recreational Noise Exposure

PURPOSE

Speech-in-noise tests and suprathreshold auditory evoked potentials are promising biomarkers to diagnose cochlear synaptopathy (CS) in humans. This study investigated whether these biomarkers changed after recreational noise exposure.

METHOD

The baseline auditory status of 19 normal-hearing young adults was analyzed using questionnaires, pure-tone audiometry, speech audiometry, and auditory evoked potentials. Nineteen subjects attended a music festival and completed the same tests again at Day 1, Day 3, and Day 5 after the music festival.

RESULTS

No significant relations were found between lifetime noise-exposure history and the hearing tests. Changes in biomarkers from the first session to the follow-up sessions were nonsignificant, except for speech audiometry, which showed a significant learning effect (performance improvement).

CONCLUSIONS

Despite the individual variability in prefestival biomarkers, we did not observe changes related to the noise-exposure dose caused by the attended event. This can indicate the absence of noise exposure-driven CS in the study cohort, or reflect that biomarkers were not sensitive enough to detect mild CS. Future research should include a more diverse study cohort, dosimetry, and results from test-retest reliability studies to provide more insight into the relationship between recreational noise exposure and CS. Supplemental Material https://doi.org/10.23641/asha.16821283.

Relevance of Artifact Removal and Number of Stimuli for Video Head Impulse Test Examination

OBJECTIVE

To evaluate the effect of artifacts on the impulse and response recordings with the video head impulse test (VHIT) and determine how many stimuli are necessary for obtaining acceptably efficient measurements.

METHODS

One hundred fifty patients were examined using VHIT and their registries searched for artifacts. We compared several variations of the dataset. The first variation used only samples without artifacts, the second used all samples (with and without artifacts), and the rest used only samples with each type of artifact. We calculated the relative efficiency (RE) of evaluating an increasingly large number of samples (3 to 19 per side) when compared with the complete sample (20 impulses per side).

RESULTS

Overshoot was associated with significantly higher speed (p = 0.005), higher duration (p < 0.001) and lower amplitude of the impulses (p = 0.002), and consequent higher saccades' latency (p = 0.035) and lower amplitude (p = 0.025). Loss of track was associated with lower gain (p = 0.035). Blink was associated with a higher number of saccades (p < 0.001), and wrong way was associated with lower saccade latency (p = 0.012). The coefficient of quartile deviation escalated as the number of artifacts of any type rose, indicating an increment of variability. Overshoot increased the probability of the impulse to lay on the outlier range for gain and peak speed. Blink did so for the number of saccades, and wrong way for the saccade amplitude and speed. RE reached a tolerable level of 1.1 at 7 to 10 impulses for all measurements except the PR score.

CONCLUSIONS

Our results suggest the necessity of removing artifacts after collecting VHIT samples to improve the accuracy and precision of results. Ten impulses are sufficient for achieving acceptable RE for all measurements except the PR score.

Latency-dependent filtering and compact representation of the complete auditory pathway response

Auditory evoked potentials (AEPs) include the auditory brainstem response (ABR), middle latency response (MLR), and cortical auditory evoked potentials (CAEPs), each one covering a specific latency range and frequency band. For this reason, ABR, MLR, and CAEP are usually recorded separately using different protocols. This article proposes a procedure providing a latency-dependent filtering and down-sampling of the AEP responses. This way, each AEP component is appropriately filtered, according to its latency, and the complete auditory pathway response is conveniently represented (with the minimum number of samples, i.e., without unnecessary redundancies). The compact representation of the complete response facilitates a comprehensive analysis of the evoked potentials (keeping the natural continuity related to the neural activity transmission along the auditory pathway), which provides a new perspective in the design and analysis of AEP experiments. Additionally, the proposed compact representation reduces the storage or transmission requirements when large databases are manipulated for clinical or research purposes. The analysis of the AEP responses shows that a compact representation with 40 samples/decade (around 120 samples) is enough for accurately representing the response of the complete auditory pathway and provides appropriate latency-dependent filtering. MatLab/Octave code implementing the proposed procedure is included in the supplementary materials.

Optimizing Auditory Brainstem Response Acquisition Using Interleaved Frequencies

Auditory brainstem responses (ABRs) require averaging responses to hundreds or thousands of repetitions of a stimulus (e.g., tone pip) to obtain a measurable evoked response at the scalp. Fast repetition rates lead to changes in ABR amplitude and latency due to adaptation. To minimize the effect of adaptation, stimulus rates are sometimes as low as 10 to 13.3 stimuli per second, requiring long acquisition times. The trade-off between reducing acquisition time and minimizing the effect of adaptation on ABRs is an especially important consideration for studies of cochlear synaptopathy, which use the amplitude of short latency responses (wave 1) to assess auditory nerve survival. It has been proposed that adaptation during ABR acquisition can be reduced by interleaving tones at different frequencies, rather than testing each frequency serially. With careful ordering of frequencies and levels in the stimulus train, adaptation in the auditory nerve can be minimized, thereby permitting an increase in the rate at which tone bursts are presented. However, widespread adoption of this stimulus design has been hindered by lack of available software. Here, we develop and validate an interleaved stimulus design to optimize the rate of ABR measurement while minimizing adaptation. We implement this method in an open-source data acquisition software tool that permits either serial or interleaved ABR measurements. The open-source software library, psiexperiment, is compatible with widely used ABR hardware. Consistent with previous studies, careful design of an interleaved stimulus train can reduce ABR acquisition time by more than half, with minimal effect on ABR thresholds and wave 1 latency, while improving measures of wave 1 amplitude.

Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear

Previous studies have demonstrated that some species of odontocetes can be conditioned to reduce hearing sensitivity when warned of an impending intense sound; however, the underlying mechanisms remain poorly understood. In the present study, conditioned hearing attenuation was elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 28 kHz. Testing was performed in air, with sounds presented via contact transducers. Hearing was assessed via noninvasive measurement of monaural auditory nerve responses (ANR) and binaural auditory brainstem responses (ABR). ABRs/ANRs were measured in response to 40-kHz tone bursts, over 2 to 3-s time intervals before and after the conditioned and unconditioned stimuli. Results showed reductions in ABR/ANR amplitude and increases in latency after pairing the warning and more intense tones. Monaural ANRs from the left and right ears were attenuated by similar amounts when the warning and more intense sounds were preferentially applied to the right ear. The data support a neural mechanism operating at the level of the cochlea and/or auditory nerve and suggest the involvement of neural projections that can affect the contralateral ear.

The Effects of Random Stimulation Rate on Measurements of Auditory Brainstem Response

Electroencephalography (EEG) signal is an electrophysiological recording from electrodes placed on the scalp to reflect the electrical activities of the brain. Auditory brainstem response (ABR) is one type of EEG signals in response to an auditory stimulus, and it has been widely used to evaluate the potential disorders of the auditory function within the brain. Currently, the ABR measurements in the clinic usually adopt a fixed stimulation rate (FSR) technique in which the late evoked response could contaminate the ABR signals and deteriorate the waveform differentiation after averaging, thus compromising the overall auditory function assessment task. To resolve this issue, this study proposed a random stimulation rate (RSR) method by integrating a random interval between two adjacent stimuli. The results showed that the proposed RSR method was consistently repeatable and reliable in multiple trials of repeated measurements, and there was a large amplitude of successive late evoked response that would contaminate the ABR signals for conventional FSR methods. The ABR waveforms of the RSR method showed better wave I–V morphology across different stimulation rates and stimulus levels, and the improved ABR morphology played an important role in early diagnoses of auditory pathway abnormities. The correlation coefficients as functions of averaging time showed that the ABR waveform of the RSR method stabilizes significantly faster, and therefore, it could be used to speed up current ABR measurements with more reliable testing results. The study suggests that the proposed method would potentially aid the adequate reconstruction of ABR signals towards a more effective means of hearing loss screening, brain function diagnoses, and potential brain–computer interface.

Matrix-based formulation of the iterative randomized stimulation and averaging method for recording evoked potentials

The iterative randomized stimulation and averaging (IRSA) method was proposed for recording evoked potentials when the individual responses are overlapped. The main inconvenience of IRSA is its computational cost, associated with a large number of iterations required for recovering the evoked potentials and the computation required for each iteration [involving the whole electroencephalogram (EEG)]. This article proposes a matrix-based formulation of IRSA, which is mathematically equivalent and saves computational load (because each iteration involves just a segment with the length of the response, instead of the whole EEG). Additionally, it presents an analysis of convergence that demonstrates that IRSA converges to the least-squares (LS) deconvolution. Based on the convergence analysis, some optimizations for the IRSA algorithm are proposed. Experimental results (configured for obtaining the full-range auditory evoked potentials) show the mathematical equivalence of the different IRSA implementations and the LS-deconvolution and compare the respective computational costs of these implementations under different conditions. The proposed optimizations allow the practical use of IRSA for many clinical and research applications and provide a reduction of the computational cost, very important with respect to the conventional IRSA, and moderate with respect to the LS-deconvolution. matlab/Octave implementations of the different methods are provided as supplementary material.

Signal-to-noise ratio of auditory brainstem responses (ABRs) across click rate in the bottlenose dolphin (Tursiops truncatus)

Although the maximum length sequence (MLS) and iterative randomized stimulation and averaging (I-RSA) methods allow auditory brainstem response (ABR) measurements at high rates, it is not clear if high rates allow ABRs of a given quality to be measured in less time than conventional (CONV) averaging (i.e., fixed interstimulus intervals) at lower rates. In the present study, ABR signal-to-noise ratio (SNR) was examined in six bottlenose dolphins as a function of measurement time and click rate using CONV averaging at rates of 25 and 100 Hz and the MLS/I-RSA approaches at rates from 100 to 1250 Hz. Residual noise in the averaged ABR was estimated using (1) waveform amplitude following the ABR, (2) waveform amplitude after subtracting two subaverage ABRs (i.e., the "±average"), and (3) amplitude variance at a single time point. Results showed that high stimulus rates can be used to obtain dolphin ABRs with a desired SNR in less time than CONV averaging. Optimal SNRs occurred at rates of 500-750 Hz, but were only a few dB higher than that for CONV averaging at 100 Hz. Nonetheless, a 1-dB improvement in SNR could result in a 25% time savings in reaching criterion SNR.

The Parallel Auditory Brainstem Response

The frequency-specific tone-evoked auditory brainstem response (ABR) is an indispensable tool in both the audiology clinic and research laboratory. Most frequently, the toneburst ABR is used to estimate hearing thresholds in infants, toddlers, and other patients for whom behavioral testing is not feasible. Therefore, results of the ABR exam form the basis for decisions regarding interventions and hearing habilitation with implications extending far into the child's future. Currently, responses are elicited by periodic sequences of toneburst stimuli presented serially to one ear at a time, which take a long time to measure multiple frequencies and intensities, and provide incomplete information if the infant wakes up early. Here, we describe a new method, the parallel ABR (pABR), which uses randomly timed toneburst stimuli to simultaneously acquire ABR waveforms to five frequencies in both ears. Here, we describe the pABR and quantify its effectiveness in addressing the greatest drawback of current methods: test duration. We show that in adults with normal hearing the pABR yields high-quality waveforms over a range of intensities, with similar morphology to the standard ABR in a fraction of the recording time. Furthermore, longer latencies and smaller amplitudes for low frequencies at a high intensity evoked by the pABR versus serial ABR suggest that responses may have better place specificity due to the masking provided by the other simultaneous toneburst sequences. Thus, the pABR has substantial potential for facilitating faster accumulation of more diagnostic information that is important for timely identification and treatment of hearing loss.

Comparison of maximum length sequence and randomized stimulation and averaging methods on the bottlenose dolphin auditory brainstem response

The purpose of the present study was to compare auditory brainstem responses (ABRs) using two approaches that allow the use of high stimulation rates, but with different temporal variability in the interstimulus interval: maximum length sequences (MLS) and iterative randomized stimulation and averaging (I-RSA). ABRs were obtained to click stimuli in six bottlenose dolphins (Tursiops truncatus). In experiment 1, click level was held constant and click rate varied from 25 to 1250 Hz. For MLS, interstimulus intervals varied by a factor of 6 at each rate, while for I-RSA the interstimulus intervals varied by ± 0.5 ms regardless of rate. In experiment 2, stimulus rates ranged from 100 to 1000 Hz and click level varied from 105 to 135 dB re: 1 μPa. For experiment 1, MLS and I-RSA showed similar decreases in ABR peak amplitudes and increases in ABR peak latencies and interwave intervals with increasing rate. For experiment 2, there was an increase in peak latency and a decrease in peak amplitude with decreasing click level; however, the effects of click level were reduced at higher rates. The results indicate that the greater jitter for MLS compared to I-RSA does not substantially affect the dolphin ABR.

Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns

Conditioned reductions in hearing sensitivity were elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 20, 40, or 80 kHz. Hearing was assessed via noninvasive measurement of auditory brainstem responses (ABRs) to 20 - to 133-kHz tone bursts presented at randomized intervals from 1 to 3 ms. ABRs within each trial were obtained by averaging the instantaneous electroencephalogram, time-locked to tone burst onsets, over 2- to 3-s time intervals. In initial testing, ABR amplitudes were reduced (relative to baseline values) in one dolphin after the conditioned stimulus, but before the unconditioned stimulus, demonstrating conditioned hearing attenuation. In subsequent testing with both dolphins, ABRs were attenuated throughout the entire 31-s trial. Maximum ABR threshold shifts occurred at and above the unconditioned stimulus frequency and were above 40 dB for some conditions. The results (1) confirm that dolphins can be conditioned to reduce hearing sensitivity when warned of an impending noise exposure, (2) show that hearing attenuation occurs within the cochlea or auditory nerve, and (3) support the hypothesis that toothed whales can "self-mitigate" some effects of noise if warned of an impending exposure.

Bottlenose dolphin (Tursiops truncatus) auditory brainstem responses recorded using conventional and randomized stimulation and averaging

Auditory brainstem response (ABR) measurements using conventional averaging (i.e., constant interstimulus interval, ISI) are limited to stimulus rates low enough to prevent overlapping of the ABRs to successive stimuli. To overcome this limitation, stimuli may be presented at high rates using pseudorandom sequences (e.g., maximum length sequences) or quasi-periodic sequences; however, these methods restrict the available stimulus sequences and require deconvolution to recover the ABR from the overlapping responses. Randomized stimulation and averaging (RSA) is an alternate method where evoked responses at high rates are obtained by averaging responses to stimuli with ISIs drawn from a random distribution. The RSA method enables precise control over stimulus jitter, is flexible with respect to stimulus sequence parameters, and does not require deconvolution to extract the ABR waveform. In this study, ABRs were measured in three normal-hearing dolphins using conventional averaging and RSA. Results show the RSA method to be effective in dolphins if the ISI jitter ≥ ∼1.5 ms and that the influence of stimulus artifacts in the averaged ABR can be substantially reduced by alternating stimulus polarity on successive presentations rather than employing digital blanking or iterative processes.

The effects of click rate on the auditory brainstem response of bottlenose dolphins

Rate manipulations can be used to study adaptation processes in the auditory nerve and brainstem. For this reason, rate effects on the click-evoked auditory brainstem response (ABR) have been investigated in many mammals, including humans. In this study, click-evoked ABRs were obtained in eight bottlenose dolphins (Tursiops truncatus) while varying stimulus rate using both conventional averaging and maximum length sequences (MLSs), which allow disentangling ABRs that overlap in time and thus permit the study of adaptation at high rates. Dolphins varied in age and upper cutoff frequency of hearing. Conventional stimulation rates were 25, 50, and 100 Hz and average MLS rates were approximately 50, 100, 250, 500, 1000, 2500, and 5000 Hz. Click peak-equivalent sound pressure levels for all conditions were 135 dB re 1 μPa. ABRs were observed in all dolphins, at all stimulus rates. With increasing rate, peak latencies increased and peak amplitudes decreased. There was a trend for an increase in the interwave intervals with increasing rate, which appeared more robust for the dolphins with a full range of hearing. For those rates where ABRs were obtained for both conventional and MLS approaches, the latencies of the mean data were in good agreement.

New metric for optimizing Continuous Loop Averaging Deconvolution (CLAD) sequences under the 1/f noise model

Continuous loop averaging deconvolution (CLAD) is one of the proven methods for recovering transient auditory evoked potentials (AEPs) in rapid stimulation paradigms, which requires an elaborated stimulus sequence design to attenuate impacts from noise in data. The present study aimed to develop a new metric in gauging a CLAD sequence in terms of noise gain factor (NGF), which has been proposed previously but with less effectiveness in the presence of pink (1/f) noise. We derived the new metric by explicitly introducing the 1/f model into the proposed time-continuous sequence. We selected several representative CLAD sequences to test their noise property on typical EEG recordings, as well as on five real CLAD electroencephalogram (EEG) recordings to retrieve the middle latency responses. We also demonstrated the merit of the new metric in generating and quantifying optimized sequences using a classic genetic algorithm. The new metric shows evident improvements in measuring actual noise gains at different frequencies, and better performance than the original NGF in various aspects. The new metric is a generalized NGF measurement that can better quantify the performance of a CLAD sequence, and provide a more efficient mean of generating CLAD sequences via the incorporation with optimization algorithms. The present study can facilitate the specific application of CLAD paradigm with desired sequences in the clinic.

Noise Attenuation Estimation for Maximum Length Sequences in Deconvolution Process of Auditory Evoked Potentials

The use of maximum length sequence (m-sequence) has been found beneficial for recovering both linear and nonlinear components at rapid stimulation. Since m-sequence is fully characterized by a primitive polynomial of different orders, the selection of polynomial order can be problematic in practice. Usually, the m-sequence is repetitively delivered in a looped fashion. Ensemble averaging is carried out as the first step and followed by the cross-correlation analysis to deconvolve linear/nonlinear responses. According to the classical noise reduction property based on additive noise model, theoretical equations have been derived in measuring noise attenuation ratios (NARs) after the averaging and correlation processes in the present study. A computer simulation experiment was conducted to test the derived equations, and a nonlinear deconvolution experiment was also conducted using order 7 and 9 m-sequences to address this issue with real data. Both theoretical and experimental results show that the NAR is essentially independent of the m-sequence order and is decided by the total length of valid data, as well as stimulation rate. The present study offers a guideline for m-sequence selections, which can be used to estimate required recording time and signal-to-noise ratio in designing m-sequence experiments.

Simulation on the Comparison of Steady-State Responses Synthesized by Transient Templates Based on Superposition Hypothesis

The generation of auditory-evoked steady-state responses (SSRs) is associated with the linear superposition of transient auditory-evoked potentials (AEPs) that cannot be directly observed. A straightforward way to justify the superposition hypothesis is the use of synthesized SSRs by a transient AEP under a predefined condition based on the forward process of this hypothesis. However, little is known about the inverse relation between the transient AEP and its synthetic SSR, which makes the interpretation of the latter less convincible because it may not necessarily underlie the true solution. In this study, we chose two pairs of AEPs from the conventional and deconvolution paradigms, which represent the homo-AEPs from a homogenous group and the hetero-AEPs from two heterogeneous groups. Both pairs of AEPs were used as templates to synthesize SSRs at rates of 20–120 Hz. The peak-peak amplitudes and the differences between the paired waves were measured. Although amplitude enhancement occurred at ~40 Hz, comparisons between the available waves demonstrated that the relative differences of the synthetic SSRs could be dramatically larger at other rates. Moreover, two virtually identical SSRs may come from clearly different AEPs. These results suggested inconsistent relationships between the AEPs and their corresponding SSRs over the tested rates.

Comparison of Auditory Middle-Latency Responses From Two Deconvolution Methods at 40 Hz

GOAL

Auditory middle-latency responses (MLRs) are reported to be particularly susceptible to stimulation rate. Deconvolution methods are necessary to unwrap the overlapping responses at a high rate under the linear superposition assumption. This study aims to investigate and compare the MLR characteristics at high and conventional stimulation rates.

METHODS

The characteristics were examined in healthy adults by using two closely related deconvolution paradigms, namely continuous-loop averaging deconvolution and multirate steady-state averaging deconvolution at a mean rate of 40 Hz, and a conventional low rate of 5 Hz.

RESULTS

The morphology and stability of the MLRs can benefit from a high-rate stimulation. It appears that stimulation sequencing strategies of deconvolution methods exerted divergent rate effects on MLR characteristics, which might be associated with different adaptation mechanisms.

CONCLUSION

MLRs obtained by two deconvolution methods and the conventional reference feature differently from one another.

SIGNIFICANCE

These findings have critical implications in our current understanding of the rate effects on MLR characteristics which may inspire further studies to explore the characteristics of evoked responses at high rates and deconvolution paradigms.

Habituation of Auditory Steady State Responses Evoked by Amplitude-Modulated Acoustic Signals in Rats

Generation of the auditory steady state responses (ASSR) is commonly explained by the linear combination of random background noise activity and the stationary response. Based on this model, the decrease of amplitude that occurs over the sequential averaging of epochs of the raw data has been exclusively linked to the cancelation of noise. Nevertheless, this behavior might also reflect the non-stationary response of the ASSR generators. We tested this hypothesis by characterizing the ASSR time course in rats with different auditory maturational stages. ASSR were evoked by 8-kHz tones of different supra-threshold intensities, modulated in amplitude at 115 Hz. Results show that the ASSR amplitude habituated to the sustained stimulation and that dishabituation occurred when deviant stimuli were presented. ASSR habituation increased as animals became adults, suggesting that the ability to filter acoustic stimuli with no-relevant temporal information increased with age. Results are discussed in terms of the current model of the ASSR generation and analysis procedures. They might have implications for audiometric tests designed to assess hearing in subjects who cannot provide reliable results in the psychophysical trials.

Auditory brainstem and middle latency responses recorded at fast rates with randomized stimulation

Randomized stimulation and averaging (RSA) allows auditory evoked potentials (AEPs) to be recorded at high stimulation rates. This method does not perform deconvolution and must therefore deal with interference derived from overlapping transient evoked responses. This paper analyzes the effects of this interference on auditory brainstem responses (ABRs) and middle latency responses (MLRs) recorded at rates of up to 300 and 125 Hz, respectively, with randomized stimulation sequences of a jitter both greater and shorter than the dominant period of the ABR/MLR components. Additionally, this paper presents an advanced approach for RSA [iterative-randomized stimulation and averaging (I-RSA)], which includes the removal of the interference associated with overlapping responses through an iterative process in the time domain. Experimental results show that (a) RSA can be efficiently used in the recording of AEPs when the jitter of the stimulation sequence is greater than the dominant period of the AEP components, and (b) I-RSA maintains all the advantages of RSA and is not constrained by the restriction of a minimum jitter. The significance of the results of this study is discussed.

Automatic quality assessment and peak identification of auditory brainstem responses with fitted parametric peaks

The recording of the auditory brainstem response (ABR) is used worldwide for hearing screening purposes. In this process, a precise estimation of the most relevant components is essential for an accurate interpretation of these signals. This evaluation is usually carried out subjectively by an audiologist. However, the use of automatic methods for this purpose is being encouraged nowadays in order to reduce human evaluation biases and ensure uniformity among test conditions, patients, and screening personnel. This article describes a new method that performs automatic quality assessment and identification of the peaks, the fitted parametric peaks (FPP). This method is based on the use of synthesized peaks that are adjusted to the ABR response. The FPP is validated, on one hand, by an analysis of amplitudes and latencies measured manually by an audiologist and automatically by the FPP method in ABR signals recorded at different stimulation rates; and on the other hand, contrasting the performance of the FPP method with the automatic evaluation techniques based on the correlation coefficient, FSP, and cross correlation with a predefined template waveform by comparing the automatic evaluations of the quality of these methods with subjective evaluations provided by five experienced evaluators on a set of ABR signals of different quality. The results of this study suggest (a) that the FPP method can be used to provide an accurate parameterization of the peaks in terms of amplitude, latency, and width, and (b) that the FPP remains as the method that best approaches the averaged subjective quality evaluation, as well as provides the best results in terms of sensitivity and specificity in ABR signals validation. The significance of these findings and the clinical value of the FPP method are highlighted on this paper.