MASTER: a Windows program for recording multiple auditory steady-state responses

Statistical Evaluation of ERG Responses: A New Method to Validate Cycle-by-Cycle Recordings in Advanced Retinal Degenerations

Purpose

To describe and evaluate a novel method to determine the validity of measurements made using cycle-by-cycle (CxC) recording techniques in patients with advanced retinal degenerations (RD) having low-amplitude flicker electroretinogram (ERG) responses.

Methods

The method extends the original CxC recording algorithm introduced by Sieving et al., retaining the original recording setup and the preliminary analysis of raw data. Novel features include extended use of spectrum analysis, reduction of errors due to known sources, and a comprehensive statistical assessment using three different tests. The method was applied to ERG recordings from seven patients with RD and two patients with CNGB3 achromatopsia.

Results

The method was implemented as a Windows application to processes raw data obtained from a commercial ERG system, and it features a computational toolkit for statistical assessment of ERG recordings with amplitudes as low as 1 µV, commonly found in advanced RD patients. When recorded using conditions specific for eliciting cone responses, none of the CNGB3 patients had a CxC validated response, indicating that no signal artifacts were present with our recording conditions. A comparison of the presented method with conventional 30 Hz ERG was performed. Bland-Altman plots indicated good agreement (mean difference, -0.045 µV; limits of agreement, 0.193 to -0.282 µV) between the resulting amplitudes. Within-session test-retest variability was 15%, comparing favorably to the variability of standard ERG amplitudes.

Conclusions

This novel method extracts highly reliable clinical recordings of low-amplitude flicker ERGs and effectively detects artifactual responses. It has potential value both as a cone outcome variable and planning tool in clinical trials on natural history and treatment of advanced RDs.

Evaluation of phase-locking to parameterized speech envelopes

Humans rely on the temporal processing ability of the auditory system to perceive speech during everyday communication. The temporal envelope of speech is essential for speech perception, particularly envelope modulations below 20 Hz. In the literature, the neural representation of this speech envelope is usually investigated by recording neural phase-locked responses to speech stimuli. However, these phase-locked responses are not only associated with envelope modulation processing, but also with processing of linguistic information at a higher-order level when speech is comprehended. It is thus difficult to disentangle the responses into components from the acoustic envelope itself and the linguistic structures in speech (such as words, phrases and sentences). Another way to investigate neural modulation processing is to use sinusoidal amplitude-modulated stimuli at different modulation frequencies to obtain the temporal modulation transfer function. However, these transfer functions are considerably variable across modulation frequencies and individual listeners. To tackle the issues of both speech and sinusoidal amplitude-modulated stimuli, the recently introduced Temporal Speech Envelope Tracking (TEMPEST) framework proposed the use of stimuli with a distribution of envelope modulations. The framework aims to assess the brain's capability to process temporal envelopes in different frequency bands using stimuli with speech-like envelope modulations. In this study, we provide a proof-of-concept of the framework using stimuli with modulation frequency bands around the syllable and phoneme rate in natural speech. We evaluated whether the evoked phase-locked neural activity correlates with the speech-weighted modulation transfer function measured using sinusoidal amplitude-modulated stimuli in normal-hearing listeners. Since many studies on modulation processing employ different metrics and comparing their results is difficult, we included different power- and phase-based metrics and investigate how these metrics relate to each other. Results reveal a strong correspondence across listeners between the neural activity evoked by the speech-like stimuli and the activity evoked by the sinusoidal amplitude-modulated stimuli. Furthermore, strong correspondence was also apparent between each metric, facilitating comparisons between studies using different metrics. These findings indicate the potential of the TEMPEST framework to efficiently assess the neural capability to process temporal envelope modulations within a frequency band that is important for speech perception.

Test-Retest Variability in the Characteristics of Envelope Following Responses Evoked by Speech Stimuli

OBJECTIVES

The objective of the present study was to evaluate the between-session test-retest variability in the characteristics of envelope following responses (EFRs) evoked by modified natural speech stimuli in young normal hearing adults.

DESIGN

EFRs from 22 adults were recorded in two sessions, 1 to 12 days apart. EFRs were evoked by the token /susa∫ i/ (2.05 sec) presented at 65 dB SPL and recorded from the vertex referenced to the neck. The token /susa∫ i/, spoken by a male with an average fundamental frequency [f0] of 98.53 Hz, was of interest because of its potential utility as an objective hearing aid outcome measure. Each vowel was modified to elicit two EFRs simultaneously by lowering the f0 in the first formant while maintaining the original f0 in the higher formants. Fricatives were amplitude-modulated at 93.02 Hz and elicited one EFR each. EFRs evoked by vowels and fricatives were estimated using Fourier analyzer and discrete Fourier transform, respectively. Detection of EFRs was determined by an F-test. Test-retest variability in EFR amplitude and phase coherence were quantified using correlation, repeated-measures analysis of variance, and the repeatability coefficient. The repeatability coefficient, computed as twice the standard deviation (SD) of test-retest differences, represents the ±95% limits of test-retest variation around the mean difference. Test-retest variability of EFR amplitude and phase coherence were compared using the coefficient of variation, a normalized metric, which represents the ratio of the SD of repeat measurements to its mean. Consistency in EFR detection outcomes was assessed using the test of proportions.

RESULTS

EFR amplitude and phase coherence did not vary significantly between sessions, and were significantly correlated across repeat measurements. The repeatability coefficient for EFR amplitude ranged from 38.5 nV to 45.6 nV for all stimuli, except for /∫/ (71.6 nV). For any given stimulus, the test-retest differences in EFR amplitude of individual participants were not correlated with their test-retest differences in noise amplitude. However, across stimuli, higher repeatability coefficients of EFR amplitude tended to occur when the group mean noise amplitude and the repeatability coefficient of noise amplitude were higher. The test-retest variability of phase coherence was comparable to that of EFR amplitude in terms of the coefficient of variation, and the repeatability coefficient varied from 0.1 to 0.2, with the highest value of 0.2 for /∫/. Mismatches in EFR detection outcomes occurred in 11 of 176 measurements. For each stimulus, the tests of proportions revealed a significantly higher proportion of matched detection outcomes compared to mismatches.

CONCLUSIONS

Speech-evoked EFRs demonstrated reasonable repeatability across sessions. Of the eight stimuli, the shortest stimulus /∫/ demonstrated the largest variability in EFR amplitude and phase coherence. The test-retest variability in EFR amplitude could not be explained by test-retest differences in noise amplitude for any of the stimuli. This lack of explanation argues for other sources of variability, one possibility being the modulation of cortical contributions imposed on brainstem-generated EFRs.

On the use of envelope following responses to estimate peripheral level compression in the auditory system

Individual estimates of cochlear compression may provide complementary information to traditional audiometric hearing thresholds in disentangling different types of peripheral cochlear damage. Here we investigated the use of the slope of envelope following response (EFR) magnitude-level functions obtained from four simultaneously presented amplitude modulated tones with modulation frequencies of 80-100 Hz as a proxy of peripheral level compression. Compression estimates in individual normal hearing (NH) listeners were consistent with previously reported group-averaged compression estimates based on psychoacoustical and distortion-product oto-acoustic emission (DPOAE) measures in human listeners. They were also similar to basilar membrane (BM) compression values measured invasively in non-human mammals. EFR-based compression estimates in hearing-impaired listeners were less compressive than those for the NH listeners, consistent with a reduction of BM compression. Cochlear compression was also estimated using DPOAEs in the same NH listeners. DPOAE estimates were larger (less compressive) than EFRs estimates, showing no correlation. Despite the numerical concordance between EFR-based compression estimates and group-averaged estimates from other methods, simulations using an auditory nerve (AN) model revealed that compression estimates based on EFRs might be highly influenced by contributions from off-characteristic frequency (CF) neural populations. This compromises the possibility to estimate on-CF (i.e., frequency-specific or "local") peripheral level compression with EFRs.

Intraoperative determination of coupling efficiency of Carina® middle ear implant by means of auditory evoked potentials

OBJECTIVE

The Carina^® implant system is a fully implantable active middle ear implant, which can be coupled to various structures in the middle ear, depending on the nature of the hearing loss and the middle ear anatomy. Currently, there is only one method for determining the coupling efficiency of the actuator of this implant system, and this is limited to incus coupling.

DESIGN

The proposed method is based on the intraoperative recording and evaluation of auditory brainstem responses (ABRs) while directly stimulating the hearing system via the actuator. The acoustic stimulation was achieved using an optimised broadband chirp stimulus (CE-Chirp^®).

STUDY SAMPLES

This study included 10 subjects having moderate to severe sensorineural or mixed hearing loss.

RESULTS

The intraoperative ABR measurements show, that it is possible to derive reliable AEP thresholds via the actuator of the implant. The ABR thresholds correlate well with preoperative BC PTA4 thresholds (r = 0.87) while the postoperative OC-direct thresholds (in situ audiogram via the implant) correlate with r = 0.77.

CONCLUSION

The results demonstrated that the direct actuator stimulation allow for reliable intraoperative ABR measurements and that the proposed method can be used to estimate the coupling efficiency of the actuator and facilitate its positioning.

Subcortical auditory neural synchronization is deficient in pre-reading children who develop dyslexia

Auditory processing of temporal information in speech is sustained by synchronized firing of neurons along the entire auditory pathway. In school-aged children and adults with dyslexia, neural synchronization deficits have been found at cortical levels of the auditory system, however, these deficits do not appear to be present in pre-reading children. An alternative role for subcortical synchronization in reading development and dyslexia has been suggested, but remains debated. By means of a longitudinal study, we assessed cognitive reading-related skills and subcortical auditory steady-state responses (80 Hz ASSRs) in a group of children before formal reading instruction (pre-reading), after 1 year of formal reading instruction (beginning reading), and after 3 years of formal reading instruction (more advanced reading). Children were retrospectively classified into three groups based on family risk and literacy achievement: typically developing children without a family risk for dyslexia, typically developing children with a family risk for dyslexia, and children who developed dyslexia. Our results reveal that children who developed dyslexia demonstrate decreased 80 Hz ASSRs at the pre-reading stage. This effect is no longer present after the onset of reading instruction, due to an atypical developmental increase in 80 Hz ASSRs between the pre-reading and the beginning reading stage. A forward stepwise logistic regression analysis showed that literacy achievement was predictable with an accuracy of 90.4% based on a model including three significant predictors, that is, family risk for dyslexia (R = .31), phonological awareness (R = .23), and 80 Hz ASSRs (R = .26). Given that (1) abnormalities in subcortical ASSRs preceded reading acquisition in children who developed dyslexia and (2) subcortical ASSRs contributed to the prediction of literacy achievement, subcortical auditory synchronization deficits may constitute a pre-reading risk factor in the emergence of dyslexia.

A bayesian approach to the spectral F-Test: Application to auditory steady-state responses

BACKGROUND AND OBJECTIVE

Auditory steady-state responses (ASSRs) represent an objective method used in clinical practice to assess hearing thresholds. The steady-state nature of these signals allows response detection by means of statistical techniques in the frequency domain as spectral F-test. This objective response detection (ORD) compares the power of the response bin against the power of the neighboring frequency noise bins. Most ORD algorithms are based on the Neyman-Pearson approach to the hypothesis test provided that the likelihood ratio test is the most powerful test for a given significance level alpha (also called Type I error). On the other hand, the Bayesian approach allows the inclusion of prior information in the model and enables the updating of this information with posterior knowledge. This approach, however, has not been explored with respect to ORD techniques, thus enabling the exploration of new paradigms, which may contribute to this field of study, especially in terms of the time required for response detection. The aim of this study is to use the Bayesian approach in the implementation of the spectral F-test for application to ASSRs.

METHODS

Monte Carlo simulations were performed to evaluate Neyman-Pearson and Bayesian detectors' performances with the spectral F-test as a function of the signal-to-noise ratio. Then, the two detectors were applied to ASSR recordings of nine normal-hearing individuals subjected to amplitude-modulated tones of various intensities.

RESULTS

Both simulation and ASSR data analyses showed that among the scenarios analyzed, the most promising case was that in which the lowest possible values for the a priori probability were selected for the null hypothesis (H0), allowing detection at low signal-to-noise ratios. The worst performance occurred when the a priori probabilities for both hypotheses were equal. The ASSR data also showed that higher stimulus intensity led to better performance and faster detection due to improvements in the signal-to-noise ratio.

CONCLUSIONS

The a priori probabilities can affect the Bayesian detector's performance, directly impacting the time needed to identify responses. The parallel behaviors observed between the performances of both approaches showed that the Bayesian detector can achieve its ideal performance at lower signal-to-noise ratios compared to the optimal performance of the Neyman-Pearson detector, reflecting the promising applicability of the Bayesian approach to evoked potentials.

Improving the power of objective response detection of evoked responses in noise by using average and product of magnitude-squared coherence of two different signals

Objective response detection (ORD) techniques such as the magnitude-squared coherence (MSC) are mathematical methods tailored to detect potentials evoked by an external periodic stimulation. The performance of the MSC is directly proportional to the signal-to-noise ratio (SNR) of the recorded signal and the time spent for collecting data. An alternative to increasing the performance of detection techniques without increasing data recording time is to use the information from more than one signal simultaneously. In this context, this work proposes two new detection techniques based on the average and on the product of MSCs of two different signals. The critical values and detection probabilities were obtained theoretically and using a Monte Carlo simulation. The performances of the new detectors were evaluated using synthetic data and electroencephalogram (EEG) signals during photo and auditory stimulation. For the synthetic signals, the two proposed detectors exhibited a higher detection rate when compared to the rate of the traditional MSC technique. When applied to EEG signals, these detectors resulted in an increase of the mean detection rate in relation to MSC for visual and auditory stimulation of at least 25% and 13.21%, respectively. The proposed detectors may be considered as promising tools for clinical applications. Graphical Abstract.

Stability of Auditory Steady State Responses Over Time

OBJECTIVES

Auditory steady state responses (ASSRs) are used in clinical practice for objective hearing assessments. The response is called steady state because it is assumed to be stable over time, and because it is evoked by a stimulus with a certain periodicity, which will lead to discrete frequency components that are stable in amplitude and phase over time. However, the stimuli commonly used to evoke ASSRs are also known to be able to induce loudness adaptation behaviorally. Researchers and clinicians using ASSRs assume that the response remains stable over time. This study investigates (1) the stability of ASSR amplitudes over time, within one recording, and (2) whether loudness adaptation can be reflected in ASSRs.

DESIGN

ASSRs were measured from 14 normal-hearing participants. The ASSRs were evoked by the stimuli that caused the most loudness adaptation in a previous behavioral study, that is, mixed-modulated sinusoids with carrier frequencies of either 500 or 2000 Hz, a modulation frequency of 40 Hz, and a low sensation level of 30 dB SL. For each carrier frequency and participant, 40 repetitions of 92 sec recordings were made. Two types of analyses were used to investigate the ASSR amplitudes over time: with the more traditionally used Fast Fourier Transform and with a novel Kalman filtering approach. Robust correlations between the ASSR amplitudes and behavioral loudness adaptation ratings were also calculated.

RESULTS

Overall, ASSR amplitudes were stable. Over all individual recordings, the median change of the amplitudes over time was -0.0001 μV/s. Based on group analysis, a significant but very weak decrease in amplitude over time was found, with the decrease in amplitude over time around -0.0002 μV/s. Correlation coefficients between ASSR amplitudes and behavioral loudness adaptation ratings were significant but low to moderate, with r = 0.27 and r = 0.39 for the 500 and 2000 Hz carrier frequency, respectively.

CONCLUSIONS

The decrease in amplitude of ASSRs over time (92 sec) is small. Consequently, it is safe to use ASSRs in clinical practice, and additional correction factors for objective hearing assessments are not needed. Because only small decreases in amplitudes were found, loudness adaptation is probably not reflected by the ASSRs.

Validation and Benchmarking of a Wearable EEG Acquisition Platform for Real-World Applications

This paper presents the experimental validation of a readout circuit for the acquisition, amplification, and transmission of extremely weak biopotentials with a focus on electroencephalography (EEG) signals. The device, dubbed CochlEEG, benefits from a low-power design for long-term power autonomy and provides configurable gain and sampling rates to suit the needs of various EEG applications. CochlEEG features high sampling rates, up to 4 kHz, low-noise signal acquisitions, support for active electrodes, and a potential for Wi-Fi data transmission. Moreover, it is lightweight, pocket size, and affordable, which makes CochlEEG suitable for wearable and real-world applications. The efficiency of CochlEEG in EEG data acquisition is also investigated in this paper. Auditory steady-state responses acquisition results validate CochlEEG's capability in recording EEG with a signal quality comparable to commercial mobile or research EEG acquisition devices. Moreover, the results of an oddball paradigm experiment prove the capability of CochlEEG in recording event-related potentials and demonstrate its potential for brain-computer interface applications and electrophysiological research applications requiring higher temporal resolution.

Estimation of auditory steady-state responses based on the averaging of independent EEG epochs

The amplitude of auditory steady-state responses (ASSRs) generated in the brainstem of rats exponentially decreases over the sequential averaging of EEG epochs. This behavior is partially due to the adaptation of the ASSR induced by the continuous and monotonous stimulation. In this study, we analyzed the potential clinical relevance of the ASSR adaptation. ASSR were elicited in eight anesthetized adult rats by 8-kHz tones, modulated in amplitude at 115 Hz. We called independent epochs to those EEG epochs acquired with sufficiently long inter-stimulus interval, so the ASSR contained in any given epoch is not affected by the previous stimulation. We tested whether the detection of ASSRs is improved when the response is computed by averaging independent EEG epochs, containing only unadapted auditory responses. The improvements in the ASSR detection obtained with standard, weighted and sorted averaging were compared. In the absence of artifacts, when the ASSR was elicited by continuous acoustic stimulation, the computation of the ASSR amplitude relied upon the averaging method. While the adaptive behavior of the ASSR was still evident after the weighting of epochs, the sorted averaging resulted in under-estimations of the ASSR amplitude. In the absence of artifacts, the ASSR amplitudes computed by averaging independent epochs did not depend on the averaging procedure. Averaging independent epochs resulted in higher ASSR amplitudes and halved the number of EEG epochs needed to be acquired to achieve the maximum detection rate of the ASSR. Acquisition protocols based on averaging independent EEG epochs, in combination with appropriate averaging methods for artifact reduction might contribute to develop more accurate hearing assessments based on ASSRs.

Evaluation of an artifact reduction strategy for electrically evoked auditory steady-state responses: Simulations and measurements

BACKGROUND

Electrically evoked steady-state response (EASSR) recording is a measure of neuronal response strength after continuous electrical stimulation of the auditory system. In order to suppress the large electrical artifact generated by intracochlear electrical stimulation, a sophisticated artifact reduction processing strategy ("Hofmann procedure") has been proposed (Hofmann and Wouters, 2010). So far, EASSR recordings with artifact reduction procedures were reported only in cochlear implant (CI) users implanted with Cochlear devices (Macquarie, Australia).

NEW METHOD

Here, we demonstrate the application of the Hofmann procedure in CI users implanted with MED-EL (Innsbruck, Austria) devices. To demonstrate potential limitations of the procedure, we calculated discrete time Fourier transformations (DTFT) of various pulse patterns which may be used for EASSR.

RESULTS

EASSR recordings were obtained in three subjects and processed with the Hofmann procedure. Neural response amplitude growth functions and phase for modulated and unmodulated pulse trains at various stimulation rates could be assessed. Simulations of three different interpolation methods aimed to suppress the electrical artifact show that the interpolation of a sinusoidal signal in a temporal window between 0 and 1 ms has demonstrated negligible impact on the spectral amplitude of the signal with a superior performance of a spline interpolation.

COMPARISON WITH EXISTING METHOD

The Hofmann procedure, initially developed for recording EASSRs with CIs from the manufacturer Cochlear, was validated for MED-EL devices.

CONCLUSION

It is feasible to record EASSRs with the described measurement setup and CIs from the manufacturer MED-EL.

Using auditory steady-state responses for measuring hearing protector occlusion effect

INTRODUCTION

The currently available methods for measuring the occlusion effect (OE) of hearing protection devices (HPDs) have limitations. Objective microphonic measurements do not assess bone-conducted sounds directly transmitted to the cochlea. Psychophysical measurements at threshold are biased due to the low-frequency masking effects from test participants' physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is used as a technique that might overcome these limitations.

PARTICIPANTS AND METHODS

Pure-tone stimuli (250 and 500 Hz), with amplitude modulated at 40 Hz, were presented to twelve adults with normal hearing through a bone vibrator at three levels in 10-dB steps. The following two conditions were assessed: the unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level were linearized using least-square regressions. The ASSR-based "physiological" OE was then calculated as the average difference between the two measurements.

RESULTS

A significant statistical difference was found between the average threshold-based psychophysical OE and the average ASSR-based OE.

CONCLUSION

This study successfully ascertained that it is possible to objectively measure the OE of HPD using ASSRs collected on the same participant both with and without protectors.

Ear-EEG-Based Objective Hearing Threshold Estimation Evaluated on Normal Hearing Subjects

OBJECTIVE

Hearing threshold levels have been estimated successfully in the clinic using the objective electroencephalogram (EEG) based technique of auditory steady-state response (ASSR). The recent method of ear-EEG could enable ASSR hearing tests to be performed in everyday life, rather than in a specialized clinic, enabling cheaper and easier monitoring of audiometric thresholds over time. The objective of the current study was to evaluate the feasibility of ear-EEG in audiometric characterization of auditory sensitivity thresholds.

METHODS

An ear-EEG setup was used to estimate ASSR hearing threshold levels to CE-chirp stimuli (with center frequencies 0.5, 1, 2, and 4 kHz) from four different electrode configurations including conventional scalp configuration, ear electrode with scalp reference, ear electrode with reference in the opposite ear and ear electrode with reference in the same ear. To evaluate the ear-EEG setup, ASSR thresholds estimated using ear-EEG were compared to ASSR thresholds estimated using standardized audiological equipment.

RESULTS

The SNRs of in-ear ear-EEG recordings were found to be on average 2.7 to 6.5 dB lower than SNRs of conventional scalp EEG. Thresholds estimated from in-ear referenced ear-EEG were on average 15.0 ± 3.4, 9.1 ± 4.4, 12.5 ± 3.7, and 12.1 ± 2.6 dB above scalp EEG thresholds for 0.5, 1, 2, and 4 kHz, respectively.

CONCLUSION

We demonstrate that hearing threshold levels can be estimated from ear-EEG recordings made from electrodes placed in one ear.

SIGNIFICANCE

Objective hearing threshold estimation based on ear-EEG can be integrated into hearing aids, thereby allowing hearing assessment to be performed by the hearing instrument on a regular basis.

Device optimised chirp stimulus for ABR measurements with an active middle ear implant

OBJECTIVE

Active middle ear implants are widely used to treat adults and children with sensorineural, conductive, or mixed hearing loss. Currently, there is no adequate method to determine the performance of active middle ear implant systems.

DESIGN

The proposed method is based on measuring the auditory brainstem response while stimulating the hearing system via the active middle ear implant (Vibrant Soundbridge^TM, VSB; MEDEL, Austria). The acoustic stimulation was achieved via an optimised chirp stimulus (CE-Chirp), implemented in the Eclipse system (Interacoustics, Denmark). To compensate for the frequency-specific delays in the VSB system, the underlying model function of the CE-Chirp was adjusted accordingly (VSB-CE-Chirp). Study samples: The study includes 12 subjects having mild to profound sensorineural, conductive or mixed hearing loss.

RESULTS

The use of an optimised VSB-CE-Chirp instead of the CE-Chirp increases significantly the ABR wave V amplitudes (1.63 times) and so also increases their identifiability (by 15.2%). On average, wave V could be identified at a 7.5 dB lower stimulation level.

CONCLUSION

The constructed VSB-CE-Chirp stimulus, after it had been transmitted through the VSB system, follows well the shape of the original CE-Chirp. Preliminary measurements in VSB patients demonstrated a significantly improved ABR amplitude with the VSB-CE-Chirp.

Neural Correlates of the Binaural Masking Level Difference in Human Frequency-Following Responses

The binaural masking level difference (BMLD) is an auditory phenomenon where binaural tone-in-noise detection is improved when the phase of either signal or noise is inverted in one of the ears (SπNo or SoNπ, respectively), relative to detection when signal and noise are in identical phase at each ear (SoNo). Processing related to BMLDs and interaural time differences has been confirmed in the auditory brainstem of non-human mammals; in the human auditory brainstem, phase-locked neural responses elicited by BMLD stimuli have not been systematically examined across signal-to-noise ratio. Behavioral and physiological testing was performed in three binaural stimulus conditions: SoNo, SπNo, and SoNπ. BMLDs at 500 Hz were obtained from 14 young, normal-hearing adults (ages 21-26). Physiological BMLDs used the frequency-following response (FFR), a scalp-recorded auditory evoked potential dependent on sustained phase-locked neural activity; FFR tone-in-noise detection thresholds were used to calculate physiological BMLDs. FFR BMLDs were significantly smaller (poorer) than behavioral BMLDs, and FFR BMLDs did not reflect a physiological release from masking, on average. Raw FFR amplitude showed substantial reductions in the SπNo condition relative to SoNo and SoNπ conditions, consistent with negative effects of phase summation from left and right ear FFRs. FFR amplitude differences between stimulus conditions (e.g., SoNo amplitude-SπNo amplitude) were significantly predictive of behavioral SπNo BMLDs; individuals with larger amplitude differences had larger (better) behavioral B MLDs and individuals with smaller amplitude differences had smaller (poorer) behavioral B MLDs. These data indicate a role for sustained phase-locked neural activity in BMLDs of humans and are the first to show predictive relationships between behavioral BMLDs and human brainstem responses.

Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation

Introduction:

Present methods of measuring the attenuation of hearing protection devices (HPDs) have limitations. Objective measurements such as field microphone in real-ear do not assess bone-conducted sound. Psychophysical measurements such as real-ear attenuation at threshold (REAT) are biased due to the low frequency masking effects from test subjects’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is explored as a technique which might overcome these limitations.

Subjects and Methods:

Pure tone stimuli (500 and 1000 Hz), amplitude modulated at 40 Hz, are presented to 10 normal-hearing adults through headphones at three levels in 10 dB steps. Two conditions were assessed: unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level are linearized using least-square regressions. The “physiological attenuation” is then calculated as the average difference between the two measurements. The technical feasibility of measuring earplug attenuation is demonstrated for the group average attenuation across subjects.

Results:

No significant statistical difference is found between the average REAT attenuation and the average ASSR-based attenuation.

Conclusion:

Feasibility is not yet demonstrated for individual subjects since differences between the estimates occurred for some subjects.

Atypical neural synchronization to speech envelope modulations in dyslexia

A fundamental deficit in the synchronization of neural oscillations to temporal information in speech could underlie phonological processing problems in dyslexia. In this study, the hypothesis of a neural synchronization impairment is investigated more specifically as a function of different neural oscillatory bands and temporal information rates in speech. Auditory steady-state responses to 4, 10, 20 and 40Hz modulations were recorded in normal reading and dyslexic adolescents to measure neural synchronization of theta, alpha, beta and low-gamma oscillations to syllabic and phonemic rate information. In comparison to normal readers, dyslexic readers showed reduced non-synchronized theta activity, reduced synchronized alpha activity and enhanced synchronized beta activity. Positive correlations between alpha synchronization and phonological skills were found in normal readers, but were absent in dyslexic readers. In contrast, dyslexic readers exhibited positive correlations between beta synchronization and phonological skills. Together, these results suggest that auditory neural synchronization of alpha and beta oscillations is atypical in dyslexia, indicating deviant neural processing of both syllabic and phonemic rate information. Impaired synchronization of alpha oscillations in particular demonstrated to be the most prominent neural anomaly possibly hampering speech and phonological processing in dyslexic readers.

Time-domain analysis of neural tracking of hierarchical linguistic structures

When listening to continuous speech, cortical activity measured by MEG concurrently follows the rhythms of multiple linguistic structures, e.g., syllables, phrases, and sentences. This phenomenon was previously characterized in the frequency domain. Here, we investigate the waveform of neural activity tracking linguistic structures in the time domain and quantify the coherence of neural response phases over subjects listening to the same stimulus. These analyses are achieved by decomposing the multi-channel MEG recordings into components that maximize the correlation between neural response waveforms across listeners. Each MEG component can be viewed as the recording from a virtual sensor that is spatially tuned to a cortical network showing coherent neural activity over subjects. This analysis reveals information not available from previous frequency-domain analysis of MEG global field power: First, concurrent neural tracking of hierarchical linguistic structures emerges at the beginning of the stimulus, rather than slowly building up after repetitions of the same sentential structure. Second, neural tracking of the sentential structure is reflected by slow neural fluctuations, rather than, e.g., a series of short-lasting transient responses at sentential boundaries. Lastly and most importantly, it shows that the MEG responses tracking the syllabic rhythm are spatially separable from the MEG responses tracking the sentential and phrasal rhythms.

Intraoperative auditory steady-state monitoring during surgery in the cerebellopontine angle for estimation of postoperative hearing classes

OBJECTIVE Brainstem auditory evoked potentials (BAEPs) have been used for intraoperative monitoring of the auditory nerve for many years. However, BAEPs yield limited information about the expected postoperative hearing quality and speech perception. The auditory steady-state response (ASSR) enables objective audiograms to be obtained in patients under anesthesia. These ASSRs could be used for intraoperative estimation of hearing classes to improve the postoperative outcome and quality of life. Studies investigating the clinical use of ASSRs during total intravenous anesthesia are currently lacking. The work presented in this article therefore investigates the application of ASSRs for intraoperative estimation of hearing classes. METHODS In 43 patients undergoing surgery for vestibular schwannoma, ASSR measurements were performed at the beginning and end of the surgical procedure. ASSR stimuli consisted of 80-dB hearing level amplitude-modulated tones with 5-minute duration, 90-Hz modulation, and 3 different carrier frequencies: 500, 1000, and 2000 Hz. Stimulation was performed unilaterally with and without contralateral masking, using single and combined carriers. Evoked responses were recorded and analyzed in the frequency domain. ASSRs were compared with extraoperative hearing classes and BAEPs using ANOVA, correlation, and receiver operating characteristic statistics. RESULTS ASSRs yielded high and consistent area under the curve (AUC) values (mean 0.83) and correlation values (mean -0.63), indicating reliable prediction of hearing classes. Analysis of BAEP amplitude changes showed lower AUC (mean 0.79) and correlation values (0.63, 0.37, and 0.50 for Waves I, III, and V, respectively). Latencies showed low AUC values (mean 0.6) and no significant correlation. Combination of several carriers for simultaneous evaluation reduced ASSR amplitudes and respective AUC values. Contralateral masking did not show a significant effect. CONCLUSIONS ASSRs robustly estimate hearing class in patients under total intravenous anesthesia, even when using short measurement durations. The method provides a diagnostic performance that exceeds conventional BAEP monitoring and enables objective and automated evaluation. On the basis of these findings, continuous intraoperative auditory monitoring could become a promising alternative or adjunct to BAEPs.

Multiple Auditory Steady-State Responses in Children and Adults with Normal Hearing, Sensorineural Hearing Loss, or Auditory Neuropathy

Objectives:

We tested the clinical effectiveness of multiple auditory steady-state responses (ASSRs) for the objective assessment of hearing thresholds in patients with and without hearing loss, candidates for cochlear implants, and children with auditory neuropathy.

Methods:

The study sample included 29 subjects with sensorineural hearing loss (SNHL), 18 candidates for cochlear implants, 11 subjects with auditory neuropathy, and 18 subjects with normal hearing thresholds. Behavioral hearing thresholds and ASSRs to carrier frequencies of 0.5, 1, 2, and 4 kHz were obtained. Special care was taken to minimize possible aliasing and high-intensity multiple stimulation effects. Differences and correlations between the ASSRs and the behavioral thresholds were determined.

Results:

The ASSR estimation of behavioral thresholds in the normal-hearing group was elevated, whereas very close predictions were found for the SNHL group. The correlations between the Two measures ranged from 0.86 at 0.5 kHz carrier frequency to 0.94 at 2 kHz. In the cochlear implant candidates and the auditory neuropathy group, the ASSR thresholds generally overestimated the behavioral audiogram. In these groups the number of detected ASSRs was higher than the number of behavioral responses, especially for the high-frequency carrier stimuli.

Conclusions:

Multiple ASSRs may reliably predict the behavioral threshold in subjects with SNHL and may serve as a valuable objective measure for assessing the hearing threshold across different frequencies in candidates for cochlear implants and children with auditory neuropathy.

Aging Affects Neural Synchronization to Speech-Related Acoustic Modulations

As people age, speech perception problems become highly prevalent, especially in noisy situations. In addition to peripheral hearing and cognition, temporal processing plays a key role in speech perception. Temporal processing of speech features is mediated by synchronized activity of neural oscillations in the central auditory system. Previous studies indicate that both the degree and hemispheric lateralization of synchronized neural activity relate to speech perception performance. Based on these results, we hypothesize that impaired speech perception in older persons may, in part, originate from deviances in neural synchronization. In this study, auditory steady-state responses that reflect synchronized activity of theta, beta, low and high gamma oscillations (i.e., 4, 20, 40, and 80 Hz ASSR, respectively) were recorded in young, middle-aged, and older persons. As all participants had normal audiometric thresholds and were screened for (mild) cognitive impairment, differences in synchronized neural activity across the three age groups were likely to be attributed to age. Our data yield novel findings regarding theta and high gamma oscillations in the aging auditory system. At an older age, synchronized activity of theta oscillations is increased, whereas high gamma synchronization is decreased. In contrast to young persons who exhibit a right hemispheric dominance for processing of high gamma range modulations, older adults show a symmetrical processing pattern. These age-related changes in neural synchronization may very well underlie the speech perception problems in aging persons.

Objective detection of auditory steady-state evoked potentials based on mutual information

OBJECTIVE

Recently, we developed a metric to objectively detect human auditory evoked potentials based on the mutual information (MI) between neural responses and stimulus spectrograms. Here, the MI algorithm is evaluated further for validity in testing the auditory steady-state response (ASSR), a sustained potential used in objective audiometry.

DESIGN

MI was computed between spectrograms of ASSRs and their evoking stimuli to quantify the shared time-frequency information between neuroelectric activity and stimulus acoustics. MI was compared against two traditional ASSR detection metrics: F-test and magnitude-squared coherence (MSC).

STUDY SAMPLE

Using an empirically derived threshold (⊖MI=1.45), MI was applied as a binary classifier to distinguish actual biological responses recorded in human participants (n=11) from sham recordings, containing only EEG noise (i.e., non-stimulus-control condition).

RESULTS

MI achieved high overall accuracy (>90%) in identifying true ASSRs from sham recordings, with true positive/true negative rates of 82/100%. During online averaging, comparison with two other indices (F-test, MSC) indicated that MI could detect ASSRs in roughly half the number of trials (i.e., ∼400 sweeps) as the MSC and performed comparably to the F-test, but showed slightly better signal detection performance.

CONCLUSIONS

MI provides an alternative, more flexible metric for efficient and automated ASSR detection.

Towards an optimal paradigm for intraoperative auditory nerve monitoring with auditory steady state responses

Auditory steady state responses (ASSR) may offer an alternative to brainstem auditory evoked potentials for monitoring of the auditory nerve during surgical procedures. In the current study, we evaluated the influence of noise on ASSR characteristics in total intravenous anesthesia (TIVA). Simulated ASSR in real noise recorded during surgery under TIVA were constructed with known parameters. Influence of amplitude, modulation frequency, averaging sweeps and detection threshold on ASSR were evaluated. High amplitude, more sweeps and a liberal threshold facilitated detection. High amplitude ASSR (80 nV) were detected in up to 45 % with 16 s of data, in 80-90 % with 112 s. Near-threshold ASSR were detected in 0.8-25 %. False positives ranged between 0.3 and 10.3 %. Number of sweeps did not influence false positives. Amplitude errors varied between -61 and +39 % and improved with more averages but not with different thresholds. Modulation rate demonstrated the strongest influence on all parameters. 110 Hz yielded best, 90 Hz the worst results. Choice of parameters strongly influences detection and characteristics of ASSR. Optimal parameters enabled detection after 16 s in 45 %. Due to specific noise characteristics, modulation has a critical impact, which is currently not sufficiently recognized in ASSR studies.

A setup for simultaneous measurement of electrophysiological and psychometric temporal encoding in the auditory system

BACKGROUND

Simultaneous assessment of psychometric tasks and electrophysiological recordings is challenging because each requires specific technical and physiological preconditions. Electrophysiological recordings require a comparatively long test time duration to gain sufficient signal-to-noise ratios, whereas test duration of psychometric measurements should be limited to prevent challenges to the attention of the subject. In order to investigate immediate correlation between both measurements a method is described, which combines electrophysiological and psychometrical measurements in a single test procedure. The test may be applied to subjects with deficits in temporal resolution (e.g. auditory neuropathy spectrum disorder, ANSD).

NEW METHOD

Auditory steady state responses (ASSR) and a pitch discrimination task were combined in a single procedure. The setup employed two short-time ASSR sub-stimuli with different fixed modulation frequencies but same carrier frequencies (signal 1 and 2). Simultaneously to the recording of ASSR, the test subject had to determine the signal interval which generated the perception of higher pitch.

RESULTS

The developed setup was successfully tested by means of an artificial EEG signal and in one human subject. ASSR signal as well as pitch discrimination performance.

COMPARISON WITH EXISTING METHODS

To our knowledge the presented method has not yet been described elsewhere.

CONCLUSIONS

The feasibility of a setup to simultaneously perform a pitch discrimination task and electrophysiological measurements was demonstrated for the first time. The method provides the facility to apply sinusoidal amplitude modulated stimuli (SAM) with jittered modulation period lengths.

Auditory steady-state responses in school-aged children: a pilot study

Background

The use of Auditory Steady-State Responses (ASSRs) for auditory screening in school-aged children, particularly in children who are difficult to test and children with disabilities, has not been explored yet. This pilot study investigated the use of ASSR for auditory screening in school-aged children.

Materials and methods

A cross-sectional pilot study of 23 children aged 9 to 11 with normal-hearing thresholds and seven age-matched children with permanent moderate-to-profound bilateral hearing loss were examined. The tested carrier frequencies were 500, 1,000, 2,000, and 4,000 Hz, and the stimulus was modulated between 77 and 107 Hz. The ASSRs decreased according to the tested intensity levels of 50, 40, and 30 dB sound pressure level (SPL). Sensitivity and specificity were estimated from the responses of the children with normal hearing and those with hearing loss.

Results

For the children with normal hearing, the 2,000-Hz frequency was detected more often in both ears and at all intensity levels compared to the other frequencies. The 500- and 2,000-Hz frequencies resulted in different response patterns in both ears. The time until response detection increased in parallel with amplitude reduction, as expected. The overall time required for the test was 15 minutes, including the time spent in volunteer preparation. The sensitivity was 97% for the three intensities, and the best specificity value was 100%, which was observed at 50 dB.

Discussion

The response analysis indicated that a screening protocol for school-aged children could include 1,000, 2,000, and 4,000 Hz and that the recording of ASSRs was highly sensitive to internal and external factors. Fifty dB SPL should be considered a cut-off criterion for screening purposes because this was the intensity level with a sensitivity of 97% and a specificity of 100%.

Conclusion

The use of ASSRs might be particularly useful in school-aged children who have difficulty performing subjective hearing tests. The sensitivity and specificity data suggested that the use of ASSRs was feasible as an auditory screening tool. In order to determine a protocol for screening, future studies should include a larger sample and children with mild hearing loss.

Evolutionary adaptations for the temporal processing of natural sounds by the anuran peripheral auditory system

Sensory systems function most efficiently when processing natural stimuli, such as vocalizations, and it is thought that this reflects evolutionary adaptation. Among the best-described examples of evolutionary adaptation in the auditory system are the frequent matches between spectral tuning in both the peripheral and central auditory systems of anurans (frogs and toads) and the frequency spectra of conspecific calls. Tuning to the temporal properties of conspecific calls is less well established, and in anurans has so far been documented only in the central auditory system. Using auditory-evoked potentials, we asked whether there are species-specific or sex-specific adaptations of the auditory systems of gray treefrogs (Hyla chrysoscelis) and green treefrogs (H. cinerea) to the temporal modulations present in conspecific calls. Modulation rate transfer functions (MRTFs) constructed from auditory steady-state responses revealed that each species was more sensitive than the other to the modulation rates typical of conspecific advertisement calls. In addition, auditory brainstem responses (ABRs) to paired clicks indicated relatively better temporal resolution in green treefrogs, which could represent an adaptation to the faster modulation rates present in the calls of this species. MRTFs and recovery of ABRs to paired clicks were generally similar between the sexes, and we found no evidence that males were more sensitive than females to the temporal modulation patterns characteristic of the aggressive calls used in male-male competition. Together, our results suggest that efficient processing of the temporal properties of behaviorally relevant sounds begins at potentially very early stages of the anuran auditory system that include the periphery.

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.

Viability of intraoperative auditory steady state responses during intracranial surgery

BACKGROUND

For intraoperative monitoring of auditory nerve function, the auditory steady-state response (ASSR) analysis may be an alternative to brain stem auditory evoked potentials, offering frequency specificity and short detection times. Clinical studies investigating the viability of ASSR under total intravenous anesthesia have not been performed.

METHODS

During craniotomy under total intravenous anesthesia with propofol and remifentanil in 20 patients, ASSR were recorded. An additional control patient undergoing cerebellopontine angle surgery was included, in whom the auditory nerve could not be preserved. One-minute sinus tones (500, 1,000, 2,000 Hz) were applied with 60-, 70-, and 80-decibel hearing level. Stimuli were amplitude modulated with 40, 90, or 110 Hz and applied monaurally to the left and right ears. Time to detect a significant response and response amplitudes at 40, 90, or 110 Hz in the evoked EEG spectra was evaluated.

RESULTS

Overall, 90-Hz ASSR were successfully detected in all 20 patients, 110 Hz in 18 patients, and 40 Hz in 14 patients after a median of 10 seconds. No ASSR could be detected in the control patient at the end of the surgical procedure. Time-to-significance and ASSR amplitudes were influenced by stimulus intensity, carrier, and modulation frequency (Scheirer-Ray-Hare test, P < 0.005). Ipsilateral responses were higher than contralateral (P < 0.0001).

CONCLUSIONS

In conclusion, 90- and 110-Hz ASSR can be reliably detected under total intravenous anesthesia. Our results are in line with those from previous studies in awake patients. Auditory steady-state response during anesthesia may enable intraoperative frequency-specific audiometry and monitoring of the auditory nerve.

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.

Comparative study between pure tone audiometry and auditory steady-state responses in normal hearing subjects✩✩Please cite this article as: Beck RM, Ramos BF, Grasel SS, Ramos HF, Moraes MF, Almeida ER, et al. Comparative study between pure tone audiometry and auditory steady-state responses in normal hearing subjects. Braz J Otorhinolaryngol. 2014;80:35-40

Introdução

As respostas auditivas de estado estável permitem avaliar de forma objetiva limiares auditivos frequência-específica. A audiometria tonal é o exame padrão-ouro; no entanto, nem sempre pode ser conclusiva, principalmente em crianças e adultos não colaborativos.

Objetivo

Comparar os limiares auditivos da RAEE aos da audiometria tonal em indivíduos com audição normal.

Materiais e métodos

Foram incluídos neste estudo prospectivo de corte transversal 26 adultos (52 orelhas), de ambos os gêneros, com audiometria normal e sem queixas otológicas. Os pacientes foram submetidos a anamnese, otomicroscopia, audiometria e imitanciometria. A seguir, realizou-se avaliação de respostas auditivas de estado estável. Os resultados obtidos foram analisados estatisticamente e comparados entre si.

Resultados

A diferença entre os limiares (em dB NA) obtidos em ambos os exames para cada frequência testada foi de 7,12 dB para 500 Hz; 7,6 dB para 1000 Hz; 8,27 dB para 2000 Hz e 9,71 dB para 4000 Hz, com limiares mais elevados na RAEE, em todas as frequências. Não houve diferença estatística entre as médias para cada frequência testada.

Conclusão

Os limiares obtidos na RAEE foram comparáveis aos da audiometria tonal em indivíduos normouvintes; entretanto, não deve ser usado como único método de avaliação auditiva.

© 2014 Associação Brasileira de Otorrinolaringologia e Cirurgia Cérvico-Facial. Publicado por Elsevier Editora Ltda. Todos os direitos reservados.

Multiple-ASSR Interactions in Adults with Sensorineural Hearing Loss

The multiple auditory steady-state response (multiple-ASSR) technique, where thresholds for up to 8 frequencies (4 in each ear) are obtained simultaneously, is currently of great interest for audiometric assessment of infants. Although threshold estimates using the multiple-ASSR appear to be reasonably accurate, it is not currently known whether it is more efficient to use multiple stimuli or single stimuli when testing individuals with sensorineural hearing loss (SNHL). The current study investigated the effect of single versus multiple simultaneous stimuli on the 80- and 40-Hz ASSRs in adults with normal hearing or SNHL. Results showed significant interactions (i.e., decreased amplitudes) for both ASSRs going from single to multiple stimuli in one ear. Going from multiple one ear to multiple two ears did not further reduce the amplitude of the 80-Hz ASSR. At the 40-Hz rate, however, there was a further amplitude decrease going from one-ear multiple to two-ear multiple stimuli. Importantly, these interactions did not differ between the normal-hearing and SNHL groups. Although supportive of the multiple-ASSR technique, there are likely situations where it is more efficient to use single stimuli. Future studies are required to assess these interactions in infants with varying degrees and configurations of hearing loss.

Hemispheric asymmetry of auditory steady-state responses to monaural and diotic stimulation

Amplitude modulations in the speech envelope are crucial elements for speech perception. These modulations comprise the processing rate at which syllabic (~3-7 Hz), and phonemic transitions occur in speech. Theories about speech perception hypothesize that each hemisphere in the auditory cortex is specialized in analyzing modulations at different timescales, and that phonemic-rate modulations of the speech envelope lateralize to the left hemisphere, whereas right lateralization occurs for slow, syllabic-rate modulations. In the present study, neural processing of phonemic- and syllabic-rate modulations was investigated with auditory steady-state responses (ASSRs). ASSRs to speech-weighted noise stimuli, amplitude modulated at 4, 20, and 80 Hz, were recorded in 30 normal-hearing adults. The 80 Hz ASSR is primarily generated by the brainstem, whereas 20 and 4 Hz ASSRs are mainly cortically evoked and relate to speech perception. Stimuli were presented diotically (same signal to both ears) and monaurally (one signal to the left or right ear). For 80 Hz, diotic ASSRs were larger than monaural responses. This binaural advantage decreased with decreasing modulation frequency. For 20 Hz, diotic ASSRs were equal to monaural responses, while for 4 Hz, diotic responses were smaller than monaural responses. Comparison of left and right ear stimulation demonstrated that, with decreasing modulation rate, a gradual change from ipsilateral to right lateralization occurred. Together, these results (1) suggest that ASSR enhancement to binaural stimulation decreases in the ascending auditory system and (2) indicate that right lateralization is more prominent for low-frequency ASSRs. These findings may have important consequences for electrode placement in clinical settings, as well as for the understanding of low-frequency ASSR generation.

Effective masking levels for bone-conducted amplitude- and frequency-modulated tones in adults with normal hearing: A behavioural study

OBJECTIVES

(1) to estimate the amount of masking needed to eliminate perceptual responses to 1000- and 4000-Hz bone-conducted mixed amplitude- (AM) and frequency-modulated (FM) tonal stimuli in adults with normal hearing, and (2) to compare these findings to recently reported effective masking levels (EMLs) for bone-conducted 80-Hz ASSRs.

DESIGN

Stimuli were bone-conducted single sinusoidal tones with carrier frequencies of 1000 and 4000 Hz (Mixed modulation (MM): 100% AM & 25% FM at 85-101 Hz) presented to the temporal bone at 15-45 dB HL for 1000 Hz and 25-35 dB HL for 4000 Hz. Air-conducted 1- and 4-Hz narrow-band noise maskers were presented to both ears simultaneously using ER-3A insert earphones. EMLs for each of the stimuli were determined behaviourally.

STUDY SAMPLE

Seventeen adults (mean age: 27.6 years) with normal hearing participated.

RESULTS

Overall, EMLs were 10-17 dB higher for perceptual responses compared to ASSRs for 1000 and 4000 Hz. Linear regression analyses revealed that behavioural and ASSR EMLs were not significantly correlated for most of the stimuli presented except for 1000-Hz presented at 45 dB HL (r =.64, p = .013).

CONCLUSIONS

EMLs are frequency- and testing method-dependent for bone-conducted MM tonal stimuli for normal-hearing adults.