Steady state responses to multiple amplitude-modulated tones: an optimized method to test frequency-specific thresholds in hearing-impaired children and normal-hearing subjects

Comparison of MASTER and AUDERA for measurement of auditory steady-state responses Comparación de MASTER y AUDERA para la medición de las respuestas auditivas de estado estable

Two approaches to assess auditory steady-state responses (ASSR) are compared under similar test conditions: a monaural single-frequency technique with a detection method based on phase coherence (AUDERA), and a binaural multiple-frequency technique using the F-test (MASTER). ASSR thresholds at four frequencies were assessed with both methods in both ears of ten normal-hearing and ten hearing-impaired adult subjects, within a test duration of one hour. The test-retest reliability and the influence of prolonging the test duration are assessed. For the total subject group the multiple-frequency technique outperforms the single-frequency technique. In hearing-impaired subjects, however, both techniques perform equally well. Hearing thresholds can be estimated with a standard error of the estimate between 7 and 12 dB dependent on frequency. About 55% of the estimates are within 5 dB of the behavioral hearing threshold, and 94% within 15 dB. Prolonging the test duration improves the performance of both techniques.

Auditory steady-state responses (ASSR): effects of modulation and carrier frequencies

Presented are results relevant to extending the utility of the auditory steady-state response (ASSR) in threshold estimation at high-frequency carriers and to the accuracy of thresholds estimated using modulation frequencies near 40 versus 80 Hz. Initially, efforts were directed at confirming various findings reported in the literature apropos effects of several basic ASSR parameters. Results supplement others' observations suggesting that ASSR detection limits overestimate behavioral thresholds for conventional audiometric (carrier) frequencies from 500 to 4000 Hz. Further investigation revealed even greater errors of threshold estimates for 8000 and 12000 Hz, by about 14 and 22 dB on average, respectively. Although suggesting high-frequency ASSR testing to be efficacious, technical advances and additional work is needed to establish clinical utility. Comparison of effects of modulation frequency suggested ASSR thresholds with 40 Hz modulation to fall closer to behavioral threshold than those estimated at 80 Hz. Consequently, when circumstances permit, 40 Hz ASSR testing may be the method of choice (e.g. in the assessment of malingers, who may be tested awake/alert).

80 Hz auditory steady-state responses (ASSR) at 250 Hz and 12,000 Hz

Although measurement of the detection limits of the 80 Hz auditory steady-state response (ASSR) has proven to be a reasonably reliable tool in estimating hearing sensitivity in the mid-conventional audiometric frequencies (e.g. 1000 and 2000 Hz), results in the literature suggest potentially diminishing performance at 500 Hz and above 4000 Hz. The primary goal of this work was to examine the accuracy of threshold estimation via ASSR measurement for carriers more toward the audiometric extremes. At the same time, different stimulus and recording protocols than those commonly employed in ASSR testing were examined. Using a two-channel recording system, ASSRs were obtained from 15 normal-hearing young adults for an amplitude-modulated carrier of 250 Hz and repeated tone bursts of 10,000-16,000 Hz, gated using a Blackman window. Results attest to the ability to record ASSRs to a wide range of carrier frequencies but also suggest that accuracy of threshold estimation suffers toward the audiometric extremes. Feasibility of ASSR-based high-frequency audiometry is demonstrated in principle, but it is not clear that this method will permit adequate sensitivity and accuracy to support such applications as ototoxicity monitoring.

Auditory steady-state responses to MM and exponential envelope AM2/FM stimuli in normal-hearing adults

The present study utilized a commercially available multiple auditory steady-state response (ASSR) system to test normal hearing adults (n=55). The primary objective was to evaluate the impact of the mixed modulation (MM) and the novel proposed exponential AM(2)/FM stimuli on the signal-to-noise ratio (SNR) and threshold estimation accuracy, through a within-subject comparison. The second aim was to establish a normative database for both stimulus types. The results demonstrated that the AM(2)/FM and MM stimulus had a similar effect on the SNR, whereas the ASSR threshold results revealed that the AM(2)/FM produced better thresholds than the MM stimulus for the 500, 1000, and 4000 Hz carrier frequency. The mean difference scores to tones of 500, 1000, 2000, and 4000 Hz were for the MM stimulus: 20+/-12, 14+/-9, 10+/-8, and 12+/-8 dB; and for the AM(2)/FM stimulus: 18+/-13, 12+/-8, 11+/-8, and 10+/-8 dB, respectively. The current research confirms that the AM(2)/FM stimulus can be used efficiently to test normal hearing adults.

Test accuracy and prognostic validity of multiple auditory steady state responses for targeted hearing screening

The test accuracy and prognostic validity of Multiple Auditory Steady State Responses (MSSR) and click Auditory Brainstem Responses (cABR) was compared within the context of a targeted screening protocol. A sample of 508 high-risk babies was first screened using cABR and MSSR (500 and 2000 Hz). All children (failed/ pass) were called back at three to four years of age to determine their hearing status (pure-tone audiometry). Although both methods showed an equally good test performance in the first screen (sensitivity: 100% and specificity: 92-95%), the MSSR may have some potential advantage to identify low-frequency hearing loss. Furthermore, the confirmatory audiometry with MSSR predicted the child hearing status more accurately than the cABR. In conclusion, the MSSR can provide valuable information for the diagnosis and management of infants earlier detected by a screening protocol and further developed might be also useful as a screening test.

Hearing assessment by recording multiple auditory steady-state responses: the influence of test duration

The influence of test duration on the precision of hearing thresholds estimated by recording multiple auditory steady-state responses (ASSRs) was investigated. ASSR thresholds at four frequencies in both ears were assessed in 10 normal-hearing and 10 hearing-impaired subjects. The precision of the estimated hearing thresholds was compared for ASSR recordings of 5, 10 and 15 min per intensity level, corresponding to total test durations of approximately 30, 55 and 70 min for hearing-impaired ears. Furthermore, an intensity step size of 10 dB was compared to a step size of 5 dB. The mean difference scores averaged over the four frequencies were 15 +/- 10, 12 +/- 9 and 11 +/- 8 dB after recordings of 5, 10 and 15 min respectively. The corresponding Pearson correlation coefficients were 0.93, 0.95 and 0.96. Increasing the length of the separate recordings increases the precision of the estimates, independent of tested frequency. A compromise between both will have to be made. With a total test duration of approximately 1 h, four hearing thresholds in both ears can be estimated with a standard error of the estimate of 8 dB.

Longitudinal assessment of air conduction audiograms in a phase III clinical trial of difluoromethylornithine and sulindac for prevention of sporadic colorectal adenomas

A phase III clinical trial assessed the recurrence of adenomatous polyps after treatment for 36 months with difluoromethylornithine (DFMO) plus sulindac or matched placebos. Temporary hearing loss is a known toxicity of treatment with DFMO, thus a comprehensive approach was developed to analyze serial air conduction audiograms. The generalized estimating equation method estimated the mean difference between treatment arms with regard to change in air conduction pure tone thresholds while accounting for within-subject correlation due to repeated measurements at frequencies. Based on 290 subjects, there was an average difference of 0.50 dB between subjects treated with DFMO plus sulindac compared with those treated with placebo (95% confidence interval, -0.64 to 1.63 dB; P = 0.39), adjusted for baseline values, age, and frequencies. In the normal speech range of 500 to 3,000 Hz, an estimated difference of 0.99 dB (-0.17 to 2.14 dB; P = 0.09) was detected. Dose intensity did not add information to models. There were 14 of 151 (9.3%) in the DFMO plus sulindac group and 4 of 139 (2.9%) in the placebo group who experienced at least 15 dB hearing reduction from baseline in 2 or more consecutive frequencies across the entire range tested (P = 0.02). Follow-up air conduction done at least 6 months after end of treatment showed an adjusted mean difference in hearing thresholds of 1.08 dB (-0.81 to 2.96 dB; P = 0.26) between treatment arms. There was no significant difference in the proportion of subjects in the DFMO plus sulindac group who experienced clinically significant hearing loss compared with the placebo group. The estimated attributable risk of ototoxicity from exposure to the drug is 8.4% (95% confidence interval, -2.0% to 18.8%; P = 0.12). There is a <2 dB difference in mean threshold for patients treated with DFMO plus sulindac compared with those treated with placebo.

[Steady-state responses of the auditory system: a comparison of different methods]

BACKGROUND

Determining the hearing threshold in children is one of the most important topics in audiology. Because the existing methods-brainstem evoked response audiometry (BERA) and cortical evoked response audiometry (CERA)-show some severe insufficiencies, it is necessary to look for improved methods. A promising approach may be amplitude modulation following responses (AMFR). In contrast to the conventional transient auditory evoked potentials, these responses show a high-frequency specificity, and they possibly allow statements about the hearing threshold in the low-frequency range.

SUBJECTS AND METHODS

The purpose of our study was to objectively detect the hearing threshold in normal-hearing persons of various ages. Pure-tone audiometry served as a subjective control test. For objective tests, we used the measurement of AMFR (two different systems with distinct parameters) and CERA. We compared the different methods with regard to accuracy of the determination of the hearing threshold and investigated the practicability.

RESULTS

The results showed some large deviations between the subjective hearing threshold and the objectively determined responses. The lowest deviations appeared at low frequencies. With respect to the variability of results, CERA was clearly superior to AMFR.

CONCLUSION

Despite large deviations in the responses objectively determined by AMFR, we think AMFR is suitable to close some gaps in determining objective hearing thresholds, at least at low frequencies.

Comparison of auditory steady-state responses and auditory brainstem responses in audiometric assessment of adults with sensorineural hearing loss

OBJECTIVE

Many of the medico-legal patients who claimed compensation may exaggerate hearing loss that varies in degree, nature, and laterality. The purpose of this study was to investigate whether Auditory Steady-State Response (ASSR) could be used to predict the hearing level of adults, and whether ASSR could become a better testing method than Auditory brainstem response (ABR) in audiometric assessment of adults with sensorineural hearing loss.

METHODS

This was a prospective study, which was conducted in a tertiary referral hospital. From January to June 2007, 142 subjects (284 ears) with varying degrees of sensori-neural hearing impairment were included in this study. Four commonly used frequencies (500, 1000, 2000, 4000Hz) were evaluated. All subjects received pure-tone audiometry, multi-channel ASSR, and ABR tests for threshold measurement. The correlation of pure tone thresholds with ASSR and ABR thresholds were assessed.

RESULTS

Between multi-channel ASSR and pure tone thresholds, a difference of less than 15dB was found in 71% while a difference of less than 25dB was found in 89% of patients. The correlation coefficient (r) of multi-channel ASSR and pure tone thresholds were 0.89, 0.95, 0.96, and 0.97 at 500, 1000, 2000, and 4000Hz, respectively. The strength of the relationship increased with increasing frequency. On the other hand, between ABR and pure-tone thresholds, a difference of less than 15dB was found in 31%; a difference of less than 25dB was found in 62% of patients. The r correlation value for ABR and pure tone thresholds was 0.83.

CONCLUSION

ASSR is a more reliable test for the accurate prediction of auditory thresholds than ABR. It can be a powerful and convenient electro-physiologic examination tool for clinically assessing of adults with sensorineural hearing loss.

Validation of multi-channel auditory steady-state response in adults with sensorineural hearing loss

OBJECTIVE

For various medico-legal and financial reasons, some patients may clinically demonstrate an exaggerated hearing loss that varies in degree, nature and laterality. The purpose of this study was to evaluate whether multi-channel auditory steady-state response measurement can be used as an objective test of auditory thresholds in adults with sensorineural hearing loss.

STUDY DESIGN AND SETTING

This was a prospective, comparative, experimental research design study conducted in an academic medical centre. From January to June 2007, 142 subjects (284 ears) with varying degrees of sensorineural hearing loss were included. Four commonly used frequencies (500, 1000, 2000 and 4000 Hz) were evaluated. Both pure tone thresholds and multi-channel auditory steady-state response thresholds were obtained for each ear in all subjects. The correlation of auditory steady-state response thresholds and pure tone thresholds was assessed. The time taken for multi-channel auditory steady-state response testing was also recorded.

RESULTS

Results for multi-channel auditory steady-state response thresholds and pure tone thresholds were compared for each test frequency. A difference of less than 15 dB was found in 71 per cent of patients, while a difference of less than 20 dB was found in 83 per cent. Correlation between auditory steady-state response thresholds and pure tone thresholds, expressed as the correlation coefficient (r), was 0.89, 0.95, 0.96 and 0.97 at 500, 1000, 2000 and 4000 Hz, respectively. The strength of the relationship between auditory steady-state response thresholds and pure tone thresholds increased with increasing frequency and increasing degree of hearing loss. The recorded auditory steady-state response thresholds were used to calculate regression lines predicting pure tone threshold results. The mean estimated pure tone thresholds calculated from these regression lines were all within 10 dB of the actual recorded pure tone thresholds. The average multi-channel auditory steady-state response test duration was 42 minutes per patient.

CONCLUSION

Measurement of multi-channel auditory steady-state response could be a powerful, convenient electro-physiological examination with which to objectively certify clinical hearing impairment in adults.

A flexible research platform for multi-channel auditory steady-state response measurements

The possibilities of currently commercially available auditory steady-state response (ASSR) devices are mostly limited to avoid unintentional misuse and to guarantuee patient safety as such. Some setups, e.g. do not allow the application of high intensities or the use of own stimuli. Moreover, most devices generally only allow data collection using maximal two EEG channels. The freedom to modify and extend the accompagnying software and hardware is very restricted or inexistent. As a result, these devices are not suited for research and several clinically diagnostic purposes. In this paper, a research platform for multi-channel ASSR measurements is presented, referred to as SOMA (setup ORL for multi-channel ASSR). The setup allows multi-channel measurements and the use of own stimuli. It can be easily extended to facilitate new measurement protocols and real-time signal processing. The mobile setup is based on an inexpensive multi-channel RME soundcard and software is written in C++. Both hardware and software of the setup are described. An evaluation study with nine normal-hearing subjects shows no significant performance differences between a reference and the proposed platform. SOMA presents a flexible and modularly extensible mobile high-end multi-channel ASSR test platform.

Effects of Ambient Acoustic Noise on the Auditory Steady-State Response Thresholds in Normally Hearing Adults

The effect of noise on auditory steady-state response (ASSR) has not been systematically studied, despite the fact that ASSR thresholds are sometimes measured in noisy environments. This study examined the effects of noise (speech babble) on the ASSR thresholds obtained from 31 normal hearing adults aged from 17 to 36 years (mean = 25 years). The ASSR thresholds at 0.5, 1, 2 and 4 kHz were measured in the right ear only using the Biologic MASTER system twice in quiet and in the presence of 55 dB A and 75 dB A of speech babble. The results showed no change in mean ASSR thresholds across the test-retest conditions in quiet. The mean ASSR thresholds obtained in the quiet conditions were 23.8, 22.5, 18.2 and 20.4 dB HL at 0.5, 1, 2 and 4 kHz, respectively. No significant shift in ASSR thresholds across all test frequencies was found when 55 dB A of speech babble was presented. However, when 75 dB A of noise was applied, the mean ASSR thresholds were significantly shifted by 9.5, 3.8, 4.2 and 5.8 dB at 0.5, 1, 2 and 4 kHz, respectively.

Auditory steady-state responses for estimating moderate hearing loss

The auditory steady-state response (ASSR) has gained popularity as an alternative technique for objective audiometry but its use in less severe degrees of hearing loss has been questioned. The aim of this study was to investigate the usefulness of the ASSR in estimating moderate degrees of hearing loss. Seven subjects (12 ears) with moderate sensorineural hearing loss between 15 and 18 years of age were enrolled in the study. Forty-eight behavioural and ASSR thresholds were obtained across the frequencies of 0.5, 1, 2, and 4 kHz. ASSR thresholds were determined using a dichotic multiple frequency recording technique. Mean threshold differences varied between 2 and 8 dB (+/-7-10 dB SD) across frequencies. The highest difference and variability was recorded at 0.5 kHz. The frequencies 1-4 kHz also revealed significantly better correlations (0.74-0.88) compared to 0.5 kHz (0.31). Comparing correlation coefficients for behavioural thresholds less than 60 and 60 dB and higher revealed a significant difference. Eighty-six percent of ASSR thresholds corresponded within 5 dB of moderate to severe behavioural thresholds compared to only 29% for mild to moderate thresholds in this study. The results confirm that the ASSR can reliably estimate behavioural thresholds of 60 dB and higher, but due to increased variability, caution is recommended when estimating behavioural thresholds of less than 60 dB, especially at 0.5 kHz.

Reliability of electric response audiometry using 80 Hz auditory steady-state responses

The reliability of the Auditory Steady State Response (ASSR) has not been thoroughly evaluated despite its recent application as a clinical tool for threshold estimation. The purpose of this study was to examine test-retest (TR) reliability of ASSR threshold estimates in an empirical research design. The ASSR, tested using modulation frequencies approximately 80 Hz and above, was evaluated against pure tone audiometry (PTA), and the slow vertex potential (SVP, N1-P2). Sixteen normal-hearing young female adults were tested twice, one week apart. Varying degrees of sensorineural hearing loss of a notched configuration were simulated with filtered masking noise. Test-retest reliability was assessed using Pearson-product moment correlation analysis, supplemented by other post-hoc analyses. Results demonstrated moderately strong TR reliability for ASSR at 1000, 2000 and 4000 Hz (r = 0.83-0.93); however, the reliability of ASSR at 500 Hz was weaker (r = 0.75). Results suggest that ASSR-ERA is a reliable test at mid-high frequencies, at least with the configuration and degrees of simulated sensorineural hearing loss examined in this study.

Comparison of Auditory Steady-State Responses and Tone-Burst Auditory Brainstem Responses in Normal Babies

OBJECTIVE

To follow the development of tone-burst auditory brainstem response (TB-ABR) and auditory steady-state response (ASSR) thresholds in a group of normal babies through the first 6 wk of life.

DESIGN

This longitudinal study involved assessment at four data-collection points. TB-ABR and ASSR thresholds to 500-Hz and 4-kHz stimuli were established in 17 full-term subjects at 0, 2, 4, and 6 wk of age. Stimulus-modulation rates for ASSR assessment were 74 Hz (for 500-Hz tones) and 95 Hz (for 4-kHz tones). TB-ABR responses were recorded to stimuli presented at 39.1 Hz.

RESULTS

Mean ASSR thresholds (calibrated in dBHL) at 500 Hz ranged from 44.4 to 39.7 dB HL across the recording period, and at 4 kHz they ranged from 37.9 to 32.1 dB HL. TB-ABR thresholds (calibrated in dBnHL) were significantly lower, ranging from 36.8 to 36.2 dB nHL at 500 Hz and from 16.5 to 15.9 dB nHL at 4 kHz. However, when the stimuli used for each test were calibrated in the same units (peak equivalent dB SPL), the results were similar. That is, the differences between the two techniques were only an artifact of the calibration. ASSR thresholds were more variable than TB-ABR, particularly at the neonatal measurement point. Within-subject changes across the test period were observed for ASSR thresholds but not for TB-ABR.

CONCLUSIONS

The longitudinal findings presented in this study suggest that for normal neonates, the TB-ABR technique may offer a more reliable basis for prediction of hearing levels than ASSR assessment. This is not because TB-ABR thresholds (calibrated in dBnHL) are lower, but because the response is less affected by maturational development in the first weeks of life and is less variable across subjects.

Auditory steady-state evoked potentials (ASSEPs): a study of optimal stimulation parameters for frequency-specific threshold measurement in dogs

OBJECTIVE

To define the optimal stimulation parameters (AM/FM vs AM alone and modulation rate) for frequency-specific threshold measurements using ASSEPs in dogs. Dependent variables were thresholds and recording times needed to obtain a response at threshold. To compare the ASSEP threshold results obtained with the optimal stimulation parameters to those obtained with the Tone-Burst/Auditory Brainstem Response (TB/ABR) combination.

METHODS

Thirty-two sedated Beagle puppies were tested at 5 audiometric frequencies (0.5-8 kHz) and 6 ASSEP modulation rates (21-199 Hz).

RESULTS

The ASSEP threshold-modulation rate functions had a high-pass profile with corner frequencies of 101 Hz for 0.5, 1 and 2 kHz carriers and of 151 Hz for 4 and 8 kHz carriers. AM stimuli did not yield higher thresholds than the AM/FM ones except at 1 kHz. Modulation type had no effect on testing duration. Audiometric profiles were obtained much more rapidly with ASSEPs than with TB/ABRs (mean: 50 vs 135 min). Both ASSEP and TB/ABR provided thresholds estimates characterized by low intersubject variability.

CONCLUSIONS

ASSEPs are a valid and rapid method for audiometric assessment in sedated dogs.

SIGNIFICANCE

ASSEPs offer a new, competitive tool for frequency-specific assessment of hearing in the canine species.

Auditory steady-state responses and clinical applications

The auditory steady-state responses to single continuous tones modulated in amplitude have been proposed as an alternative to objective frequency-specific audiometry. The aim of this study was to compare thresholds obtained by pure-tone audiometry (PTA) and by auditory steady-state responses in normal hearing or affected by hearing loss in adults and in order to evaluate the applicability of this objective test in no collaborative hearing-impaired subjects. Eleven people, 6 normal hearing and 5 with hearing loss, underwent PTA and multiple frequency auditory steady-state responses; simultaneous carrier tones (0.5, 1, 2 and 4 KHz) modulated in amplitude at different rates (77-105 Hz) were presented monaurally (TDH 49 earphones) at variable intensities (110-20 dB SPL). The mean threshold difference between PTA and multiple frequency auditory steady-state responses was 28 dB (standard deviation=14.2) and R correlation value at 0.5-1-2-4 kHz was 0.71 (P=0.0012) at the Pearson's test. These differences were significantly smaller considering the hearing-impaired separately (11.7 dB, standard deviation=2.9). The results of this study confirm previous reports showing that the multiple auditory steady-state response method is an accurate predictor of the behavioural audiogram in patients with sensory-neural hearing impairments and can be used as a valid support for behavioural evaluations.

Threshold Estimation in Children Using Auditory Steady-State Responses to Multiple Simultaneous Stimuli

OBJECTIVE

It was the aim of this study to explore the use of auditory steady-state response (ASSR) to multiple simultaneous stimuli for threshold estimation in young children.

METHOD

The subjects consisted of 40 children, aged from 6 months to 5 years, with variant degrees of sensorineural hearing loss. Simultaneous tonepips (0.5, 1, 2 and 4 kHz) with an amplitude modulated at different rates from 77 to 103 Hz were presented to both ears by insert phones. All children were tested with ASSR and age-appropriate behavioral tests.

RESULTS

We found that (1) ASSR thresholds were usually higher than behavioral thresholds with a difference of 8-15 dB, (2) the behavioral thresholds were significantly correlated with ASSR thresholds (p = 0.000), and (3) there was a great difference between ASSR thresholds and behavioral thresholds found in a child with auditory neuropathy.

CONCLUSION

Being objective, frequency specific and well correlated with behavioral thresholds, ASSR to multiple simultaneous stimuli was proven to be a good tool to predict behavioral hearing thresholds.

Comparison of multiple auditory steady-state responses (80 versus 40 Hz) and slow cortical potentials for threshold estimation in hearing-impaired adults

This study evaluated the use of multiple auditory steady-state responses (ASSRs) and slow cortical potentials (SCPs) to estimate behavioural audiograms in adults for compensation cases. Two groups of 23 subjects were assessed using either 80 Hz or 40 Hz multiple simultaneous tones with carrier frequencies of 0.5, 1.0, 2.0, and 4.0 kHz. SCP thresholds for 0.5, 1.0, and 2.0 kHz were obtained for both groups. Mean evoked potential thresholds (dB HL) minus behavioural pure-tone thresholds (dB HL) difference scores were 5-17 dB for the 80 Hz group, 1-14 dB for the 40 Hz group, and 20-22 dB for the SCPs. Thresholds for 40 Hz ASSR were significantly closer to behavioural thresholds than were 80 Hz or SCP thresholds. SCP and 40 Hz ASSR audiogram estimates were obtained more quickly than the 80 Hz ASSR. Multiple 40 Hz ASSR is the method of choice for evoked potential threshold estimation in adults.

Threshold Prediction Using the Auditory Steady-State Response and the Tone Burst Auditory Brain Stem Response: A Within-Subject Comparison

OBJECTIVE

The purpose of this study was to evaluate the accuracy with which auditory steady-state response (ASSR) and tone burst auditory brain stem response (ABR) thresholds predict behavioral thresholds, using a within-subjects design. Because the spectra of the stimuli used to evoke the ABR and the ASSR differ, it was hypothesized that the predictive accuracy also would differ, particularly in subjects with steeply sloping hearing losses.

DESIGN

ASSR and ABR thresholds were recorded in a group of 14 adults with normal hearing, 10 adults with flat, sensorineural hearing losses, and 10 adults with steeply sloping, high-frequency, sensorineural hearing losses. Evoked-potential thresholds were recorded at 1, 1.5, and 2 kHz and were compared with behavioral, pure-tone thresholds. The predictive accuracy of two ABR protocols was evaluated: Blackman-gated tone bursts and linear-gated tone bursts presented in a background of notched noise. Two ASSR stimulation protocols also were evaluated: 100% amplitude-modulated (AM) sinusoids and 100% AM plus 25% frequency-modulated (FM) sinusoids.

RESULTS

The results suggested there was no difference in the accuracy with which either ABR protocol predicted behavioral threshold, nor was there any difference in the predictive accuracy of the two ASSR protocols. On average, ABR thresholds were recorded 3 dB closer to behavioral threshold than ASSR thresholds. However, in the subjects with the most steeply sloping hearing losses, ABR thresholds were recorded as much as 25 dB below behavioral threshold, whereas ASSR thresholds were never recorded more than 5 dB below behavioral threshold, which may reflect more spread of excitation for the ABR than for the ASSR. In contrast, the ASSR overestimated behavioral threshold in two subjects with normal hearing, where the ABR provided a more accurate prediction of behavioral threshold.

CONCLUSIONS

Both the ABR and the ASSR provided reasonably accurate predictions of behavioral threshold across the three subject groups. There was no evidence that the predictive accuracy of the ABR evoked using Blackman-gated tone bursts differed from the predictive accuracy observed when linear-gated tone bursts were presented in conjunction with notched noise. Similarly, there was no evidence that the predictive accuracy of the AM ASSR differed from the AM/FM ASSR. In general, ABR thresholds were recorded at levels closer to behavioral threshold than the ASSR. For certain individuals with steeply sloping hearing losses, the ASSR may be a more accurate predictor of behavioral thresholds; however, the ABR may be a more appropriate choice when predicting behavioral thresholds in a population where the incidence of normal hearing is expected to be high.

Clinical application of dichotic multiple-stimulus auditory steady-state responses in high-risk newborns and young children

Experience with dichotic multiple-stimulus auditory steady-state responses (ASSRs) in clinical practice is described. ASSR thresholds were assessed in a sample of 60 high-risk newborns and young children between birth and 4 years of age. Amplitudes and signal-to-noise ratios (SNRs) of the ASSR were compared between normal-hearing infants and adults. Age-related changes within a group of infants younger than 3 months of age were investigated. A comparison was made between ASSR, the click-evoked auditory brainstem response and behavioral hearing thresholds in infants with a wide range of hearing threshold levels. Mean ASSR thresholds for normal-hearing infants at an average corrected age of 12 days were 42 +/- 10, 35 +/- 10, 32 +/- 10 and 36 +/- 9 dB SPL for 0.5, 1, 2 and 4 kHz, respectively. Compared to adults, these thresholds were elevated by on average 11 dB and SNRs were 1.7 times smaller. However, based on ASSRs, reasonably accurate estimations could be made of behavioral hearing thresholds obtained at a later age (median delay of 7 months). The predicted thresholds were in 61% of the cases within 10 dB of the corresponding behavioral thresholds, and in 83% of the cases within 15 dB. In less than 1 h, thresholds at four frequencies per ear could be obtained. The optimal age of testing is between 1 week and 3 months corrected age. The dichotic multiple-stimulus ASSR technique is a valuable extension of the clinical test battery for hearing-impaired children, as a follow-up diagnostic after the neonatal hearing screening.

[Stable auditory evoked potentials in the study of two patients with auditory neuropathy]

INTRODUCTION

We described the results of the auditory multiple steady state response (MSSR) technique in the assessment of two patients with auditory neuropathy (AN). The aim of this study was to corroborate the correspondence between the MSSR generators elicited by amplitude modulated tones ranging between 80-100 Hz, with the generators of auditory brain stem response (ABR). Moreover, we would also try to demonstrate the validity of the MSSR in the diagnosis of AN in children.

MATERIAL AND METHODS

Two children diagnosed of hyperbilirrubinemia, aged, 18 months and 10 years have been studied with MSSR (500, 1000, 2000 and 4000Hz); ABR with clicks; OAE; behavioural audiometry; MRI and acoustic reflexes.

RESULTS

A difference between electrophysiological and behavioural audiogram in both cases diagnosed with auditory neuropathy have been found. The auditory thresholds were similar using the two types of evoked potentials (MSSR and ABR). Both techniques showed an increment of auditory threshold congruous with a severe auditory impairment, while behavioural audiometry showed only a mild elevation of auditory threshold. We can also see how the threshold differed between frequencies using behavioral audiometry and MSSR.

CONCLUSION

It is concluded that our findings are in agreement with previous studies and they sustain the theory about the coincidence of MSSR at fast rate (80-110 Hz) and ABR generators. Also, we demonstrate the usefulness of the MSSR as an objective [corrected] electroaudiometric tool in patients with auditory neuropathy as ABR. This technique is thus a recommendable test to complete the audiological study in infants with AN, to establish a more precise treatment.

EEG Derivations Providing Auditory Steady-State Responses With High Signal-to-Noise Ratios in Infants

OBJECTIVE

To identify EEG derivations that yield high signal-to-noise ratios (SNRs) of the auditory steady-state response (ASSR) in infants aged 0 to 5 months.

DESIGN

The ASSR was recorded simultaneously from 10 EEG derivations in a monopolar montage in 20 sleeping infants. Stimuli were tones of 0.5 or 2 kHz that were 100% amplitude modulated and 20% frequency modulated, presented at 65 dB SPL for 4.4 minutes in either the right or the left ear. An amplitude modulation frequency of 90 Hz (left ear) or 94 Hz (right ear) was used. From the 10 measured monopolar derivations, all 45 bipolar derivations were calculated mounting up to 55 EEG derivations. EEG derivations were selected in the preferred set if they had the largest SNRs within subjects and if they were obtained significantly more frequently across subjects than was expected by chance (Monte Carlo simulation and Wilcoxon signed ranks test).

RESULTS

The preferred derivations are both mastoids ipsilateral to the stimulated ear with Cz as common reference. These derivations improved SNRs compared with each of several conventional EEG derivations (excluding the preferred derivations) between 16 and 69% (500 Hz, left ear), 9 and 132% (500 Hz, right ear), 31 and 193% (2 kHz, left ear), and 3 and 105% (2 kHz, right ear). In contrast to results reported earlier for adults, high SNRs were not found at the inion-Cz derivation in these infants.

CONCLUSIONS

High SNRs were obtained in infants aged younger than 6 months if the ASSR was recorded from the mastoids ipsilateral to the ear of stimulation referenced to Cz.

Effect of Audiometric Configuration on Threshold and Suprathreshold Auditory Steady-State Responses

OBJECTIVE

The purpose of this study was to examine the correlation between auditory steady-state response (ASSR) thresholds and behavioral thresholds in hearing-impaired adults with two common audiometric configurations. A second goal was to compare suprathreshold ASSR growth functions in these two subject groups and to determine whether these growth functions could be used clinically to improve threshold estimation.

DESIGN

Thirty adults participated, including 10 subjects with normal hearing, 10 subjects with flat moderately severe sensorineural hearing loss, and 10 subjects with sloping high-frequency sensorineural hearing loss. The accuracy of ASSR threshold estimations for the frequencies of 500, 1000, 2000, and 4000 Hz was compared across groups. In addition, ASSRs were recorded at multiple suprathreshold intensity levels, and the growth of the response was compared across subject groups.

RESULTS

ASSR and behavioral thresholds were strongly correlated across all subjects, with no significant difference by audiometric configuration. Slightly poorer correlations were obtained in the normal-hearing subjects than in hearing-impaired subjects and for the 500-Hz test frequency compared with the higher stimulus frequencies. Subtraction and regression methods for predicting behavioral thresholds from ASSR thresholds were equally accurate. Suprathreshold amplitude growth was variable among individuals, but mean growth functions showed steeper slopes in hearing-impaired subjects than in normal-hearing subjects, particularly for the higher frequencies (2000 to 4000 Hz). However, there was not a significant difference in the slopes of amplitude growth functions between subjects with sloping versus flat audiometric configurations for these frequencies, and estimation of threshold from suprathreshold ASSR amplitude growth functions was not advantageous.

CONCLUSIONS

ASSR thresholds accurately predicted behavioral threshold equally well for flat or sloping audiometric configurations. The accuracy of threshold estimation for either audiometric configuration and the similar growth function slopes may suggest that there is little spread of activation to neighboring regions of the cochlea when using multiple 100% AM stimuli in subjects with moderately sloping losses. The small amplitude of the ASSR response and individual variability may make threshold estimation based on extrapolation from suprathreshold ASSR responses impractical.

Auditory steady-state responses in infants with perinatal brain injury

Infants with perinatal brain injury present impairments in motor, visual, auditory, and cognitive functions. The most useful methods for detecting auditory alterations are auditory brainstem responses and otoacoustic emissions. Auditory steady-state responses have been reported as a reliable and objective technique for evaluating the hearing threshold. Auditory brainstem responses and auditory steady-state responses were carried out in 53 infants with perinatal brain injury and abnormal neurologic findings. With auditory brainstem responses, 33 (62.26%) infants presented normal and 20 abnormal results; 8 (15.09%) exhibited mild alterations, 8 (15.09%) moderate, and 4 (7.54%) severe alterations. With auditory steady-state responses, 17 (32.0%) infants were normal and 36 (67.9%) had abnormal results. When auditory steady-state responses were compared with auditory brainstem responses gold standard, the assessment gave 100% sensitivity, 51.51% specificity, 55.55% positive predictive value, and 100% negative predictive value. Abnormalities were mild in 21 (39.6%) infants, moderate in 10 (18.9%), and 5 (9.4%) exhibited severe hearing loss. We conclude that hearing loss is a frequent abnormality in infants with perinatal brain injury, and auditory steady-state responses have a high sensitivity for detecting hearing impairment, which is more evident in mild hearing loss for specific frequencies.

Artifactual Responses When Recording Auditory Steady-State Responses

OBJECTIVE

The goal of this study was to investigate, in hearing-impaired participants who could not hear the stimuli, the possibility of artifactual auditory steady-state responses (ASSRs) when stimuli are presented at high intensities.

DESIGN

ASSRs to single (60 dB HL) and multiple (20 to 50 dB HL; 500 to 4000 Hz) bone-conduction stimuli as well as single 114 to 120 dB HL air-conduction stimuli, were obtained using the Rotman MASTER system, using analog-to-digital (A/D) conversion rates of 500, 1000, and 1250 Hz. Responses (p < 0.05) were considered artifactual when their numbers exceeded that expected by chance. In some conditions, we also obtained ASSRs to "alternated" stimuli (stimuli inverted and ASSRs to the two polarities averaged). A total of 17 subjects were tested.

RESULTS

Bone conduction results: 500 Hz A/D rate: Large-amplitude (43 to 1558 nV) artifactual ASSRs were seen at 40 and 50 dB HL for the 500 Hz carrier frequency. Smaller responses (28 to 53 nV) were also recorded at 20 dB HL for the 500 Hz carrier frequency. Artifactual ASSRs (17 to 62 nV) were seen at 40 dB HL and above for the 1000 Hz carrier frequency and at 50 dB HL for the 2000 Hz carrier frequency. Alternating the stimulus polarity decreased the amplitude and occurrence of these artifactual responses but did not eliminate responses for the 500 Hz carrier frequency at 40 dB HL and above. No artifactual responses were recorded for 4000 Hz stimuli for any condition. 1000 Hz A/D rate: Artifactual ASSRs (15 to 523 nV) were seen at 50 dB HL and above for the 500 Hz carrier frequency and 40 dB HL and above for the 1000 Hz carrier frequency. Artifactual responses were also obtained at 50 dB HL for a 2000 Hz carrier frequency but not at lower levels. Artifactual responses were not seen for the 4000 Hz carrier frequency. Alternating the stimulus polarity removed the responses for the 1000 and 2000 Hz carrier frequencies but did not change the results for the 500 Hz carrier frequency. 1250 Hz A/D rate: Artifactual ASSRs (16 to 220 nV) were seen at 50 dB HL and above for the 500 Hz carrier frequency and 60 dB HL and above for the 1000 Hz carrier frequency. Alternating the stimulus polarity removed the responses for the 1000 Hz carrier frequency but did not change the results for the 500 Hz carrier frequency. There were no artifactual responses at 2000 and 4000 Hz. Air conduction results: 500 Hz A/D rate: Artifactual ASSRs (49 to 153 nV) were seen for 114 to 120 dB HL stimuli for 500 and 1000 Hz carrier frequencies. Alternating the stimulus polarity removed these responses. There were no artifactual responses at 2000 and 4000 Hz. 1000 and 1250 Hz A/D rates: Artifactual ASSRs (19 to 55 nV) were seen for a 120 dB HL stimulus for a 1000 Hz carrier. Alternating the stimulus polarity removed these responses.

CONCLUSIONS

High-intensity air- or bone-conduction stimuli can produce spurious ASSRs, especially for 500 and 1000 Hz carrier frequencies. High-amplitude stimulus artifact can result in energy that is aliased to exactly the modulation frequency. Choice of signal conditioning (electroencephalogram filter slope and low-pass cutoff) and processing (A/D rate) can avoid spurious responses due to aliasing. However, artifactual responses due to other causes may still occur for bone-conduction stimuli 50 dB HL and higher (and possibly for high-level air conduction). Because the phases of these spurious responses do not invert with inversion of stimulus, the possibility of nonauditory physiologic responses cannot be ruled out. The clinical implications of these results are that artifactual responses may occur for any patient for bone-conduction stimuli at levels greater than 40 dB HL and for high-intensity air-conduction stimuli used to assess patients with profound hearing loss.

Near-threshold recordings of amplitude modulation following responses (AMFR) in children of different ages

Amplitude modulation following responses (AMFR) to single near-threshold 40-Hz or 80-Hz amplitude-modulated tones of 1 kHz were recorded in 48 normal children between the ages of 2 months and 14 years. Children under the age of 2 years were tested during natural sleep, and older children were tested awake. The objectives of this study were to find out how the AMFR changes with age and to determine the most effective modulation frequency for objective threshold assessment at different ages. The optimal modulation frequency changed from higher to lower modulation frequencies at about 13 years. In younger children, the 80-Hz AMFR was larger than the 40-Hz AMFR. The 40-Hz response became similar to the adult response by the age of 14 years, while the 80-Hz response had an amplitude greater than half of an adult response by the age of 1 year, changed very little during the investigated period, and reached the amplitude of adult responses at the end of the investigated period.

Frequency specificity of 40-Hz auditory steady-state responses

Auditory steady-state responses (ASSR) to amplitude modulated (AM) tones with carrier frequencies between 250 and 4000 Hz and modulation frequencies near 40 Hz were recorded using a 37-channel neuro-magnetometer placed above the auditory cortex contralateral to the stimulated right ear. The ASSR sources were likely in the primary auditory cortex, located more anteriorly and more medially than the N1m sources. The ASSR amplitude decreased with increasing carrier frequency, the amplitude at 250 Hz being three times larger than at 4000 Hz. The amplitude of the ASSR to a test sound decreased in the presence of an interfering second AM sound. This suppression of the ASSR to the test stimulus was greater when the carrier frequency of the interfering stimulus was higher than that of the test tone and was greater when the test stimulus had a lower carrier frequency. Similar frequency specificity was observed when the interfering sound was a non-modulated pure tone. These results differ from those found for the ASSR elicited by modulation frequencies above 80 Hz or for the transient brainstem and middle-latency responses and suggest substantial interactions between phase-locked activities at the level of the primary auditory cortex.

Objective assessment of frequency-specific hearing thresholds in babies

OBJECTIVE

To report on clinical experience using dichotic multiple-stimulus auditory steady-state responses (ASSRs) as an objective technique to estimate frequency-specific hearing thresholds in hearing-impaired infants.

METHODS

A comparison was made between the click-evoked auditory brainstem response (ABR), auditory steady-state responses and behavioral hearing thresholds (BHTs). Both ears of 10 infants between 3 and 14 months of age were tested. ABR and ASSRs were recorded during the same test session. ABR was evoked by 100 micros clicks. ASSRs were evoked by amplitude- and frequency-modulated tones with carrier frequencies of 0.5, 1, 2 and 4 kHz and modulation frequencies ranging from 82 to 110 Hz. Eight signals (four to each ear) were presented simultaneously. ASSR thresholds were derived after separate recordings of approximately 5, 7.5 and 10 min to compare the influence of test duration. BHTs were defined in later test sessions as soon as possible after the ASSR test, dependent on medical and developmental factors.

RESULTS

For the subjects tested in this study 60% of ABR thresholds and 95% of ASSR thresholds for 1, 2 and 4 kHz were found at an average age of 7 months. Only 51% of frequency-specific BHTs could be obtained but on average 5 months later. The correlation of ABR thresholds and ASSR thresholds at 2 kHz was 0.77. The correlation of ASSRs and BHTs was 0.92. The mean differences and associated standard deviations were 4 +/- 14, 4 +/- 11, -2 +/- 14 and -1 +/- 13 dB for 0.5, 1, 2 and 4 kHz, respectively. The average test duration was 45 min for ABR (one threshold in both ears) and 58 min for ASSR (four thresholds in both ears). By reducing the duration of the separate recordings of ASSR, the precision of the hearing threshold estimate decreased and the number of outlying and missing values increased. Correlation coefficients were 0.92, 0.89 and 0.83 for recordings of maximum 10, 7.5 and 5 min, respectively. A compromise between test duration and precision has to be sought.

CONCLUSIONS

Multiple-frequency ASSRs offer the possibility to estimate frequency-specific hearing thresholds in babies in a time-efficient way.

Auditory Steady-State Responses and Word Recognition Scores in Normal-Hearing and Hearing-Impaired Adults

OBJECTIVE

The number of steady-state responses evoked by the independent amplitude and frequency modulation (IAFM) of tones has been related to the ability to discriminate speech sounds as measured by word recognition scores (WRS). In the present study IAFM stimulus parameters were adjusted to resemble the acoustic properties of everyday speech to see how well responses to these speech-modeled stimuli were related to WRS.

DESIGN

We separately measured WRS and IAFM responses at a stimulus intensity of 70 dB SPL in three groups of subjects: young normal-hearing, elderly normal-hearing, and elderly hearing-impaired. We used two series of IAFM stimuli, one with modulation frequencies near 40 Hz and the other with modulation frequencies near 80 Hz. The IAFM stimuli, consisting of four carrier frequencies each independently modulated in frequency and amplitude, could evoke up to eight separate responses in one ear. We recorded IAFM responses and WRS measurements in quiet and in the presence of speech-masking noise at 67 dB SPL or 70 dB SPL. We then evaluated the hearing-impaired subjects with and without their hearing aids to see whether an improvement in WRS would be reflected in an increased number of responses to the IAFM stimulus.

RESULTS

The correlations between WRS and the number of IAFM responses recognized as significantly different from the background were between 0.70 and 0.81 for the 40 Hz stimuli, between 0.73 and 0.82 for the 80 Hz stimuli, and between 0.76 and 0.85 for the combined assessment of 40 and 80 Hz responses. Response amplitudes at 80 Hz were smaller in the hearing-impaired than in the normal-hearing subjects. Response amplitudes for the 40 Hz stimuli varied with the state of arousal and this effect made it impossible to compare amplitudes across the different groups. Hearing aids increased both the WRS and the number of significant IAFM responses at 40 Hz and 80 Hz. Masking decreased the WRS and the number of significant responses.

CONCLUSIONS

IAFM responses are significantly correlated with WRS and may provide an objective tool for examining the brain's ability to process the auditory information needed to perceive speech.

Audiologic assessment in infants

PURPOSE

The purpose of this review is to provide the reader with current information regarding the standards for audiologic assessment of infants and very young children. The nature of the appropriate test battery and the need for adjusting test procedures to meet the specific needs of infants and toddlers are emphasized.

RECENT FINDINGS

The basic measures in the audiologic test battery include frequency-specific threshold tests by air and bone conduction, predicted by electrophysiologic measures when necessary; immittance measures including tympanometry and acoustic reflex using a high-frequency probe tone for infants under 4 months of age; and otoacoustic emissions. The ABR can be used with frequency-specific stimuli to predict the audiogram in newborns with a great deal of accuracy. Newer techniques, such as Auditory Steady State Response, are promising but need further study before they can be used reliably to predict hearing levels in infants. Finally, infants with hearing loss can be fit with amplification using prescriptive formulae, such as the Desired Sensation Level, which give appropriate hearing aid characteristics for infants based on their hearing thresholds. These fittings must be verified using objective electro-acoustic measures tailored to infants.

SUMMARY

Infants failing newborn hearing screenings can be evaluated by audiologists to predict all necessary audiologic data and those found to have hearing loss can be fitted with appropriate amplification in the newborn period. Procedures must be carefully tailored to this age group.

Auditory steady-state response thresholds of adults with sensorineural hearing impairments

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the degree and configuration of behavioral audiograms of subjects with sensorineural hearing impairments. Place specificity of the multiple-ASSR method was also assessed. Multiple amplitude-modulated (77-105 Hz) tones (500, 1000, 2000 and 4000 Hz) were simultaneously presented to one ear. The results showed that, on average, multiple-ASSR thresholds were 14 +/- 13, 8 +/- 9, 10 +/- 10 and 3 +/- 10 dB above behavioral thresholds for 500, 1000, 2000 and 4000 Hz, respectively. Behavioral and multiple-ASSR thresholds were significantly correlated (r = 0.75-0.89). There were no significant differences between behavioral and multiple-ASSR measures of the audiogram configuration. In subjects with steep-sloping > or = 30 dB/ octave) hearing losses, multiple-ASSR thresholds did not underestimate behavioral thresholds revealing good place specificity. These results indicate that the multiple-ASSR method provides good estimates of the degree and configuration of hearing in individuals with sensorineural hearing impairments.

Human auditory steady-state responses

Steady-state evoked potentials can be recorded from the human scalp in response to auditory stimuli presented at rates between 1 and 200 Hz or by periodic modulations of the amplitude and/or frequency of a continuous tone. Responses can be objectively detected using frequency-based analyses. In waking subjects, the responses are particularly prominent at rates near 40 Hz. Responses evoked by more rapidly presented stimuli are less affected by changes in arousal and can be evoked by multiple simultaneous stimuli without significant loss of amplitude. Response amplitude increases as the depth of modulation or the intensity increases. The phase delay of the response increases as the intensity or the carrier frequency decreases. Auditory steady-state responses are generated throughout the auditory nervous system, with cortical regions contributing more than brainstem generators to responses at lower modulation frequencies. These responses are useful for objectively evaluating auditory thresholds, assessing suprathreshold hearing, and monitoring the state of arousal during anesthesia.

[Steady state multi-frequency auditory evoked potentials as a technique to determine hearing threshold]

OBJECTIVE

Since the set up of Steady-State Auditory Evoked Potentials as a routine technique, it has became necessary the establishment of clinic capabilities and limitations. In this study, we have compared SSEP to behavioural thresholds, considering the last one as "gold standard" technique.

MATERIALS AND METHODS

We have recorded SSEP to multiple frequencies and behavioural thresholds in 84 ears. The obtained data were statistically processed to obtain correlations and others indicators.

RESULTS

SSEP thresholds are slightly higher than behavioural, whose average difference was calculated as 23 dB. It is worth to note that this difference diminishes at high frequencies and hearing loss cases.

CONCLUSIONS

SSEP to multiple frequencies technique is capable to determine auditory thresholds accurately, being closer to the behavioural value at high frequencies and high threshold values. It is necessary the establishment of exploration protocols to assure the highest accuracy of the system. In this work, a diagnostic algorithm has been proposed to reach better results by using SSEP.

Fitting hearing aids on infants and children: a primer for otolaryngologists

Fitting hearing aids on young infants presents a unique set of problems, and to some extent, requires a unique set of skills. First, a complete, frequency-specific evoked potential assessment is needed to define the loss and establish hearing aid fitting targets. The selected hearing aids must be flexible and must be adjusted to account for the acoustic characteristics of a small ear canal. Automatic volume control, wide dynamic range compression, directional microphones, and direct audio input are among the optional features to consider. In general, advanced technology, such as digital, programmable hearing aids offer the greatest flexibility in meeting the instrument-related challenges of the infant hearing-aid fitting. Finally, hearing aids are only one component of the management sequence. An early interventionist must be involved from the start to coordinate rehabilitation and ensure that the needs of the entire family are being met in the process.

Multiple auditory steady-state responses

Steady-state responses are evoked potentials that maintain a stable frequency content over time. In the frequency domain, responses to rapidly presented stimuli show a spectrum with peaks at the rate of stimulation and its harmonics. Auditory steady-state responses can be reliably evoked by tones that have been amplitude-modulated at rates between 75 and 110 Hz. These responses show great promise for objective audiometry, because they can be readily recorded in infants and are unaffected by sleep. Responses to multiple tones presented simultaneously can be independently assessed if each tone is modulated at a different modulation frequency. This ability makes it possible to estimate thresholds at several audiometric frequencies in both ears at the same time. Because amplitude-modulated tones are not significantly distorted by free-field speakers or microphones, they can also be used to evaluate the performance of hearing aids. Responses to amplitude and frequency modulation may also become helpful in assessing suprathreshold auditory processes, such as those necessary for speech perception.

The use of phase in the detection of auditory steady-state responses

OBJECTIVE

To investigate how phase measurements might facilitate the detection of auditory steady-state responses.

METHODS

Multiple steady-state responses were evoked by auditory stimuli modulated at rates between 78 and 95 Hz and with intensities between 50 and 0 dB SPL. The responses were evaluated in 20 subjects after 1, 2, 4, and 6 min. The responses were analyzed in the frequency domain using 4 different detection protocols: (1) phase-coherence, (2) phase-weighted coherence, (3) F test for hidden periodicity, and (4) phase-weighted t test. The phase-weighted measurements were either based on the mean phase of a group of normal subjects or derived for each subject from the phase of the response at higher intensities.

RESULTS

Detection protocols based on both phase and amplitude (F test and phase-weighted t test) were more effective than those based on phase alone (phase coherence and phase-weighted coherence) although the difference was small. Protocols using phase-weighting were more effective than those without phase-weighting. The lowest thresholds for the steady-state responses were obtained using the phase-weighted t test.

CONCLUSION

Threshold detection can be improved by weighting the detection protocols toward an expected phase, provided that the expected phase can be reliably predicted.