Early and Late Averaged Electroencephalic Responses at Low Sensation Levels

Early and late averaged electroencephalic responses (AER) were elicited from eight listeners under comparable conditions. The AERs were compared with respect to their (1) absolute amplitudes (N_a-P_a and N₁-P₂) and the variability of amplitudes and (2) absolute latencies (N_a and N₁) and the variability of latencies. Although the early AER amplitude is small, it's relative stability makes it a promising candidate for electric response audiometry     

Reexamination of Effects of Stimulus Rate and Number on the Middle Components of the Averaged Electroencephalic Response

The middlecomponents of the evoked cortical response (8–50 msec) were examined under improved signal processing conditions. 512 click stimuli were presented at 5 rates ranging from 1–16/sec to 10 normal-hearing subjects. The influence of stimulus numbers of 32, 64, 128 and 512 and stimulus rates of 1, 2, 4, 8 and 16/sec was examined.

Identifiable and repeatable responses were found with as few as 128 stimuli. Stimulus rate had little effect on middle component waveform or its identifiability. An enhancement of signal-to-noise ratio through improved filter conditions is suggested as one reason for the ability to identify middle components to fewer stimuli.     

Threshold of the Early Components of the Averaged Electroencephalic Response Determined with Tone Pips and Clicks During Drug-Induced Sleep

The early components of the averaged electroencephalic response (AER) were elicited from eight adults by narrow-spectrum tone pips centered at 250, 1 000, and 4 000 Hz, and by wide-band clicks. Brief naps were induced with secobarbital. Stimuli at 0, 10, 20, and 30 dB above behavioral threshold were presented in random order for each frequency. Control conditions in which no acoustic stimuli were presented were also employed. Responses were stored on magnetic tape and later scored independently on a yes-no basis by two judges who employed predetermined response criteria. Threshold for hearing (50-percent response) as measured with the early components was 20 dB sensation level (SL) for 250 Hz tone pips, and at 10 dB SL for 1 000 and 4 000 Hz tone pips and clicks. Thus, threshold estimated with the early components of the AER from sleeping adults closely approximated their behavioral threshold     

A comparison of the mismatch negativity and a differential waveform response

A mismatch negativity response (MMN) and a new differential waveform were derived in an effort to evaluate a neural refractory or recovery effect in adult listeners. The MMN was elicited using oddball test runs in which the standard and deviant stimuli differed in frequency To derive the differential waveform, the same standard and deviant stimuli were presented alone. MMN responses were obtained by subtracting the averaged responses to standards from the deviants. The differential waveforms were obtained by subtracting the averaged responses to standards presented alone from deviants presented alone. Scalp topography for the MMN and differential waveforms were similar. A significant (p <.05) positive and negative correlation was found between the earlier and later components of the bimodal MMN and the NI and P2 component of the differential waveform, respectively. Further, N1 and P2 of the differential waveform were significant (p <.05) predictor variables of early and late peak amplitudes of the MMN. These results suggest that refractory effects may overlay/modify the morphology of the MMN waveform.     

Objective detection of averaged auditory brainstem responses

Detection of an auditory brainstem response, ABR, usually relies on visual evaluation of two or more data acquisition runs of a fixed number of sweeps to determine if there is sufficient replication of the averaged waveforms to indicate a response. Visual interpretation can be difficult when the signal-to-noise ratio is poor because of either a small response or high levels of physiological background noise. Moreover, variations in the background noise from run to run can result in poor or spurious replications of component peaks and troughs in the waveform. A previous study (Elberling & Don, 1984) described a statistical approach for objective evaluation of the quality of an ABR recording. The method uses variance analysis in calculating the ratio of the magnitude of the ABR to the estimated averaged background noise. This study further applies this method to obtain a quantitative definition of the ABR threshold, to demonstrate its application in automatic threshold detection, and to estimate the number of sweeps required to reach detection criterion. Application of this method is valuable in reducing the variability of test interpretation and in maximizing the efficiency of recording ABRs by avoiding the averaging of excessive or insufficient numbers of sweeps. These improvements enhance the cost-benefit of ABR testing to the patient.     

Reexamination of effects of stimulus rate and number on the middle components of the averaged electroencephalic response.

The middle components of the evoked cortical response (8-50 msec) were examined under improved signal processing conditions. 512 click stimuli were presented at 5 rates ranging from 1-16/sec to 10 normal-hearing subjects. The influence of stimulus numbers of 32, 64, 128 and 512 and stimulus rates of 1, 2, 4, 8 and 16/sec was examined. Identifiable and repeatable responses were found with as few as 128 stimuli. Stimulus rate had little effect on middle component waveform or its identifiability. An enhancement of signal-to-noise ratio through improved filter conditions is suggested as one reason for the ability to identify middle components to fewer stimuli.     

Asymptotic Noise-Induced Temporary Threshold Shift in Chinchilla Measured by the Auditory Evoked Response

Monaural chinchillas were exposed to octave band (4 kHz center frequency) noise at 80 dB SPL for 96 h. Auditory threshold at frequencies of 1, 2, 4, and 8 kHz was estimated by the auditory evoked response (AER) technique prior to noise exposure, and on a fixed schedule, both during exposure and following removal from the noise. Results were analyzed in terms of both threshold and threshold shift. Results indicate that the AER is an adequate measure of asymptotic noise-induced temporary threshold shift. Advantages of the use of AER as opposed to behavioral means of threshold determination are discussed. Threshold during this long-term noise exposure was found to be determined by the spectral and intensity components of the noise independent of pre-exposure thresh-old. Implications for future research in long-term noise exposure are discussed.     

Effect of consonant-vowel ratio modification on amplitude envelope cues for consonant recognition

This investigation examined the degree to which modification of the consonant-vowel (C-V) intensity ratio affected consonant recognition under conditions in which listeners were forced to rely more heavily on waveform envelope cues than on spectral cues. The stimuli were 22 vowel-consonant-vowel utterances, which had been mixed at six different signal-to-noise ratios with white noise that had been modulated by the speech waveform envelope. The resulting waveforms preserved the gross speech envelope shape, but spectral cues were limited by the white-noise masking. In a second stimulus set, the consonant portion of each utterance was amplified by 10 dB. Sixteen subjects with normal hearing listened to the unmodified stimuli, and 16 listened to the amplified-consonant stimuli. Recognition performance was reduced in the amplified-consonant condition for some consonants, presumably because waveform envelope cues had been distorted. However, for other consonants, especially the voiced stops, consonant amplification improved recognition. Patterns of errors were altered for several consonant groups, including some that showed only small changes in recognition scores. The results indicate that when spectral cues are compromised, nonlinear amplification can alter waveform envelope cues for consonant recognition.     

Low-Level frequency-Following responses

In man, the scalp-recorded 'Frequency-following response' (FFR) can be well observed only at fairly high levels of stimulation. As a consequence, it is difficult to ascertain whether the low or the high-frequency channels of the peripheral auditory system mediate this response. It is shown that the sensitivity of the recording method is considerably increased when the averaged recorded waveform is subjected to frequency analysis. Results from experiments with high-pass masking noise become unequivocal and they suggest that the low-level FFR is caused by nervous activity arising in the apical part of the cochlea and mediated by the low-frequency channels in the brain stem.

Résumé

Les réponses évoquées par le tronc cérébral à la stimulation acoustique par des tons de basse fréquence (500 Hz) ont une courbe d'onde qui ressemble beaucoup à celle du stimulus. Pour un ton assez long, la réponse moyennée (FFR) est presque sinusoïdale. On peut améliorer la sensibilité de la méthode d'enregistrement en effectuant une analyse de Fourier de la réponse moyennée. Avec cette méthode Sensibilisée il est possible d'obtenir des enregistrements satisfaisants pour des stimuli faibles (45-60 dB SPL). Le problème central de l'origine du FFR est le suivant: pour des stimuli acoustiques assez forts, il est absolument incertain que la réponse résulte des canaux des voies auditives pour les basses ou les hautes fréquences. Avec la méthode sensibilisée, il est facile de résoudre ce problème: en appliquant un bruit masquant la région des hautes fréquences (1 000 Hz et plus) on pourra observer que la réponse FFR ne disparaiˇt pas. On peut en conclure que la réponse FFR à un stimulus faible est vraiment une réponse évoquée par les canaux pour les basses fréquences des voies auditives. Pour l'usage du FFR en clinique, il est recommandé d'utiliser la měme méthode afin d'éviter une contamination par des réponses dues aux canaux pour hautes fréquences.     

Age-Related Morphological Changes in Auditory Middle-Latency Response

Age-related changes in the waveforms of the middle latency response (MLR) were investigated in 9 adults and 28 children aged between 4 and 14 years. The children were classified into three groups according to their age. For obtaining characteristic configurations in the responses for each group, composite group averaging was performed by sum-mating the individual recordings in each group. With high-pass digital filtering at 20 Hz, composite MLR for adults showed a well-defined Na-Pa-Nb-Pb complex with peak latencies at about 17, 30, 45 and 63 ms, respectively. The composite response for children aged 4-7 years was characterized by a broad positive deflection (Pa) followed by a negative peak (Nb) at about 40 and 60 ms after stimulus onset, respectively. The peak latency of Pa was close to the adult value in the composite MLR for subjects aged 8-11 years, while the complete adult pattern in the later part of the response was not reached even in the composite response for subjects aged 12-14 years.     

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.     

Hemispheric asymmetry of the right ear advantage in dichotic listening

ERP waveforms evoked by target-right and target-left stimuli in a directed-attention, dichotic-listening paradigm were examined using cross-correlation analysis. We analyzed data from two experiments involving linguistic processing. They involved listening for (1) a phonemic feature, and (2) a series of morpho-syntactic anomalies. The maximum correlation between target-right and target-left waveforms was achieved when the target-right waveform was delayed relative to the target-left waveform (the tau shift), reflecting the shorter latency of the target-right waveform. We interpret the direction of displacement as equivalent to a "right-ear advantage" in the dichotic listening paradigm. In both tasks, tau shifts were not uniformly distributed across the parietal electrode array. They were greatest on the extreme left side of the head and systematically declined as the electrode site moved rightward, indicating a temporal gradient in the relative latencies of the two waveforms. Results are interpreted in relation to both structural and attentional aspects of dichotic listening.     

Auditory evoked response test strategies to reduce cost and increase efficiency: the postauricular muscle response revisited

We describe a number of techniques in auditory evoked response (AER) testing for hearing loss which should decrease its cost and increase its efficiency, making its use in infant hearing screening more viable. We demonstrate the use of bit-stream averaging of the electrical signals from the head as a cheap alternative to analogue averaging and show that the average waveforms obtained are similar with both techniques. We demonstrate how the postauricular muscle response (PAMR) can be potentiated by lateral rotation of the eyes and argue that uncontrolled eye movements in previous studies have led to an unfounded belief that the PAMR is not stable. When eye rotation is used to potentiate the PAMR, the response becomes very stable and so large in most subjects that it is clearly visible in the raw traces. We also demonstrate that when the PAMR is potentiated by eye rotation, stable PAMR waveforms can be reliably obtained with tone bursts with frequencies up to and above 8 kHz and with sound levels within 30 dB of the subjective detection threshold. As a result the PAMR can be used to rapidly determine an objective audiogram in most subjects within minutes. Finally, we demonstrate a correlation technique for detecting the PAMR without waveform averaging and the need for an expensive computer. We are sure that a combination of these techniques can be used to increase the efficiency of AER screening for infant deafness and lower its cost dramatically.     

Active noise reduction audiometry: a prospective analysis of a new approach to noise management in audiometric testing

OBJECTIVES: To develop a new method of screening audiometry that reduces the adverse effects of low frequency background noise by using active noise reduction (ANR) headphone technology. DESIGN: Prospective testing within an anechoic chamber evaluated the physical properties of ANR headphones. A prospective clinical crossover study compared standard audiometry with ANR headphone audiometry. METHODS: Bose Aviation X circum-aural ANR headphones were tested for both active and passive attenuation properties in a hemi-anechoic chamber using a head and torso simulator. Thirty-seven otology clinic patients then underwent standard audiometry and ANR audiometry, which was performed in a 30- and/or 40-dB sound field. RESULTS: Objective ANR headphone attenuation levels of up to 12 dB were achieved at frequencies below 2,000 Hz. In standard audiometric testing, 40 dB of narrow-band background noise decreased patient pure tone thresholds by 24 dB at 250 Hz. The use of ANR technology provided 12 dB of additional attenuation. This resulted in a significant improvement in test results despite the 40 dB of background noise (P = <0.001). In a 30-dB sound field, standard audiometric thresholds were shifted down by an average of 12 dB. The use of ANR technology completely attenuated this effect and resulted in a significant improvement in results (P = <0.01). These results were identical to those obtained in a quiet sound booth. CONCLUSIONS: Despite a 30-dB sound field, ANR audiometry can produce an audiogram identical to that obtained in a double-walled sound booth. ANR headphone audiometry improves the sensitivity of audiometric screening for mild low-frequency hearing loss. This technology may have important applications for screening in schools, industry, and community practices.     

Improving evoked response audiometry

Normative studies were carried out with 8 adult subjects whose averaged evoked responses to auditory stimuli were scored visually and by a number of differently defined quantitative methods (machine scoring). In order to compare different scoring methods a common signal-to-noise ratio measure was introduced based on a model where noise is distributed normally and signal is additive. Visual and machine scoring proved to be approximately equally sensitive, but the latter lends itself to a superior testing procedure which takes only one-third as long for equivalent results and is not contaminated by subjective error. The authors believe that the greatest promise for improving evoked response audiometry lies in the exploration of the new techniques proposed earlier. Progress in proving out one of these techniques, fast periodic stimulation, is reported.     

Strain Comparison in Rats Differentiates Strain-Specific from More General Correlates of Noise-Induced Hearing Loss and Tinnitus

Experiments in rodent animal models help to reveal the characteristics and underlying mechanisms of pathologies related to hearing loss such as tinnitus or hyperacusis. However, a reliable understanding is still lacking. Here, four different rat strains (Sprague Dawley, Wistar, Long Evans, and Lister Hooded) underwent comparative analysis of electrophysiological (auditory brainstem responses, ABRs) and behavioral measures after noise trauma induction to differentiate between strain-dependent trauma effects and more consistent changes across strains, such as frequency dependence or systematic temporal changes. Several hearing- and trauma-related characteristics were clearly strain-dependent. Lister Hooded rats had especially high hearing thresholds and were unable to detect a silent gap in continuous background noise but displayed the highest startle amplitudes. After noise exposure, ABR thresholds revealed a strain-dependent pattern of recovery. ABR waveforms varied in detail among rat strains, and the difference was most prominent at later peaks arising approximately 3.7 ms after stimulus onset. However, changes in ABR waveforms after trauma were small compared to consistent strain-dependent differences between individual waveform components. At the behavioral level, startle-based gap-prepulse inhibition (gap-PPI) was used to evaluate the occurrence and characteristics of tinnitus after noise exposure. A loss of gap-PPI was found in 33% of Wistar, 50% of Sprague Dawley, and 75% of Long Evans rats. Across strains, the most consistent characteristic was a frequency-specific pattern of the loss of gap-PPI, with the highest rates at approximately one octave above trauma. An additional range exhibiting loss of gap-PPI directly below trauma frequency was revealed in Sprague Dawley and Long Evans rats. Further research should focus on these frequency ranges when investigating the underlying mechanisms of tinnitus induction.     

A comparison of high precision F0 extraction algorithms for sustained vowels.

Perturbation analysis of sustained vowel waveforms is used routinely in the clinical evaluation of pathological voices and in monitoring patient progress during treatment. Accurate estimation of voice fundamental frequency (F0) is essential for accurate perturbation analysis. Several algorithms have been proposed for fundamental frequency extraction. To be appropriate for clinical use, a key consideration is that an F0 extraction algorithm be robust to such extraneous factors as the presence of noise and modulations in voice frequency and amplitude that are commonly associated with the voice pathologies under study. This work examines the performance of seven F0 algorithms, based on the average magnitude difference function (AMDF), the input autocorrelation function (AC), the autocorrelation function of the center-clipped signal (ACC), the autocorrelation function of the inverse filtered signal (IFAC), the signal cepstrum (CEP), the Harmonic Product Spectrum (HPS) of the signal, and the waveform matching function (WM) respectively. These algorithms were evaluated using sustained vowel samples collected from normal and pathological subjects. The effect of background noise and of frequency and amplitude modulations on these algorithms was also investigated, using synthetic vowel waveforms.     

Use of the median method to enhance detection of the mismatch negativity in the responses of individual listeners

The median method was evaluated as an alternative way of expressing the mismatch negativity (MMN). Traditionally, signal averaging has been used to extract these event-related potentials from unwanted background noise. However, mean values are biased by unrejected artifact that skews the relatively small distribution of values on which the MMN is based. Because the median is a more valid measure of central tendency in asymmetric distributions, it may describe MMN data more accurately. Better representation of the signal in the median waveform might enhance detection of the MMN in the responses of individual listeners. Mean and median waveforms were computed from previously recorded MMN data. Visually identified MMNs were validated using area and onset latency criteria. Detectability of the MMN was not improved using median waveforms. Despite this result, a theoretical argument for use of the median is presented.     

Predicting the temporal responses of non-phase-locking bullfrog auditory units to complex acoustic waveforms

Axons from the basilar papilla of the American bullfrog (Rana catesbeiana) do not phase lock to stimuli within an octave of their best frequencies. Nevertheless, they show consistent temporal patterns of instantaneous spike rate (as reflected in peristimulus time histograms) in response to repeated stimuli in that frequency range. We show that the second-order Wiener kernels for these axons, derived from the cross-correlation of continuous (non-repeating), broad-band noise stimulus with the spike train produced in response to that stimulus, can predict with considerable precision the temporal pattern of instantaneous spike rate in response to a novel, complex acoustic waveform (a repeated, 100-ms segment of noise, band-limited to cover the single octaves above and below best frequency). Furthermore, we show that most of this predictive power is retained when the second-order Wiener kernel is reduced to the highest-ranking pair of singular vectors derived from singular-value decomposition, that the retained pair of vectors corresponds to a single auditory filter followed by an envelope-detection process, and that the auditory filter itself predicts the characteristic frequency (CF) of the axon and the shape of the frequency-threshold tuning curve in the vicinity of CF.     

A Comparison of the mismatch negativity and a differential waveform response

A mismatch negativity response (MMN) and a new differential waveform were derived in an effort to evaluate a neural refractory or recovery effect in adult listeners. The MMN was elicited using oddball test runs in which the standard and deviant stimuli differed in frequency. To derive the differential waveform, the same standard and deviant stimuli were presented alone. MMN responses were obtained by subtracting the averaged responses to standards from the deviants. The differential waveforms were obtained by subtracting the averaged responses to standards presented alone from deviants presented alone. Scalp topography for the MMN and differential waveforms were similar. A significant (p<.05) positive and negative correlation was found between the earlier and later components of the bimodal MMN and the N1 and P2 component of the differential waveform, respectively. Further, N1 and P2 of the differential waveform were significant (p<.05) predictor variables of early and late peak amplitudes of the MMN. These results suggest that refractory effects may overlay/modify the morphology of the MMN waveform.

Sumario

Se obtuvo una respuesta de negatividad desigual (MMN) y una nueva onda “diferencial” en un esfuerzo por evaluar un efecto neural refractario o de recuperación en sujetos adultos. La MMN fue generada utilizando cursos peculiares de prueba en los que el estimulo estándar y el alterado tenían frecuencias diferentes. Para derivar la onda diferencial, se presentaron el mismo estímulo estándar y el alterado en forma aislada. Las respuestas MMN se obtuvieron restando las respuestas promediadas estándar de las alteradas. Las formas de onda diferenciales se obtuvieron restando las respuestas promediadas a estímulos estándar presentados aisladamente, de los estímulos alterados presentados también en forma aislada. La topografía craneana de los MMN y las onda diferenciales fueron similares. Se encontraron correlaciones positivas y negativas significativas (p<.05) entre los componentes tempranos y tardíos de la MMN bi-modal y de los componentes N1 y P2 de la onda diferencial, respectivamente. Más aún, los componentes N1 y P2 de la onda diferencial fueron variables de predicción significativas (p<.05) de los picos tempranos y tardíos de amplitud de las MMN. Estos resultados sugieren que los efectos refractarios pueden cubrir/modificar la morfología de las onda de la MMN.