Scalp potentials of normal and hearing-impaired subjects in response to sinusoidally amplitude-modulated tones

None of the current electrical audiometric procedures, alone or in combination, has yet achieved the precision of conventional audiometric testing that is used to assess hearing in verbally capable children and adults. The reason for this, in part, lies in the use of stimuli which have a wide frequency content. We have measured scalp potentials which follow the envelopes of sinusoidally amplitude-modulated tones: a frequency-specific stimulus. In normal subjects such amplitude-modulation following responses (AMFRs) appear to be generated by two sources. One source has a latency of about 30 ms, generates large responses and is only observed at modulations below 55 Hz, while the other source has a latency of 7-9 ms, generates smaller responses, and is only observed at modulations from 100-350 Hz. The latencies of these two sources are consistent with origins in the cortex and midbrain, respectively. We examined AMFRs to low frequency (50 Hz) modulations as a possible audiometric tool. In normal subjects, the amplitude of the AMFR increased as a function of intensity, decreased as a function of carrier frequency, and could be evoked across the whole audiometric range (250-8000 Hz). In hearing-impaired subjects, the AMFR amplitudes as a function of carrier frequency accurately reflected the pattern of hearing loss on a frequency-by-frequency basis. In most subjects, the threshold for evoking the AMFR was within 0-25 dB of hearing threshold. It therefore appears that the AMFR may be a potentially useful tool to assess hearing in those unable to undergo conventional audiometric testing.     

Human auditory evoked potentials to frequency-modulated tones

Three experiments evaluated steady state and transient auditory evoked potentials (EPs) to tones that were sinusoidally modulated in frequency and to tones that alternated between two frequencies with a linear ramp. The steady state responses to sinusoidal FM were small and difficult to record at both the first and second harmonics. Ramp FM evoked larger and more consistent second harmonic steady state responses than the sinusoidal FM. Only the ramp FM stimuli elicited transient EPs and these only at low modulation rates. These responses were larger to upward ramps than to downward ramps. The response to two simultaneously presented ramp FM tones differed from the sum of responses to the individual tones, indicating some interaction in the processing of the two stimuli.     

Steady-state evoked potentials to amplitude modulated tones in the monkey.

A frequency-specific, objective assessment of hearing thresholds is required for use in subjects unable to perform behavioural audiometry. One such method using steady-state evoked potentials (SSEPs) in response to amplitude-modulated tones was evaluated in an experimental animal, the macaque monkey. An amplitude-modulation frequency of 165 Hz was found to produce optimum response detection in the anaesthetised animal. Auditory thresholds determined by a computerised automatic response detection system accurately reflected behavioural thresholds previously described in this species.     

Thresholds for short-latency auditory-evoked potentials to tones in notched noise in normal-hearing and hearing-impaired subjects

The thresholds for the short-latency auditory evoked potentials (SLAEPs) to short-duration tones presented in notched-noise masking were evaluated in 20 normal-hearing and 20 hearing-impaired subjects. The differences (dB) between these thresholds (dB nHL) and the pure-tone behavioral thresholds (dB HL) across all 40 subjects were 11.6, 6.1, 6.3 and 0.8 dB for 500, 1,000, 2,000 and 4,000 Hz, respectively. These differences were significantly smaller for the hearing-impaired subjects than for the normal-hearing subjects. Ninety-eight percent of the SLAEP threshold estimations were within 30 dB of the subjects' pure-tone behavioral thresholds and 91% were within 20 dB.     

Local auditory evoked potentials and effects of pure tones on click evoked potentials in the pigeon

Local auditory evoked potentials (AEPs) in the pigeon were recorded from the nucleus magnocellularis (NM), nucleus angularis (NA) and Field L with tungsten microelectrodes. In the NM and NA, AEPs in response to clicks were always suppressed by application of continuous pure tones at specific frequencies as is usual for simultaneous masking. In the NA, frequencies of continuous pure tones which produced maximum suppression and frequencies of tone bursts which elicited maximum response both centered around 0.8 kHz. The NM tended to respond similarly. In Field L, however, amplitudes of the AEPs to clicks were suppressed, enhanced, both suppressed and enhanced, or unaffected by presentation of continuous pure tones at specific frequencies. The frequencies of tone bursts which caused maximum AEP were vaguely related to the frequencies of continuous pure tones which elicited maximum suppression of the AEPs to clicks. On the other hand, enhancement was produced by 1-2 kHz continuous pure tones independent of the frequency of tone bursts that produced maximum AEP. It was concluded that enhancement, suppression and lack of effect in Field L were due to some central neural mechanism.     

Steady-state evoked responses to sinusoidally amplitude-modulated sounds recorded in man

Steady-state potentials evoked in response to binaural, sinusoidally amplitude-modulated (AM) pure tones and broadband noise signals were recorded differentially from position F4 and the ipsilateral mastoid on the human scalp. The responses elicited by the AM stimuli were approximately periodic waveforms whose energy was predominantly at the modulation frequency of the stimulus. The magnitude of responses was between 0.1 and 4 microV for modulation frequencies between 2 and 400 Hz imposed on a 1-kHz carrier signal. The magnitude of the responses increased linearly with log modulation depth for low (4 Hz) and high (80 Hz) modulation rates. The response magnitude also increased linearly with the mean intensity of the sound for intensities up to 60 dB above the subject's pure tone threshold; at higher levels the response saturated. The relationship between response magnitude and modulation frequency (the modulation transfer function) was a lowpass function for both pure tone and broadband noise carrier signals. The modulation transfer functions were similar to those obtained from human psychophysical measurements where spectral cues are either unavailable or not used by the subject. The responses also contained a significant component at the second harmonic of the modulation frequency. The magnitude of this component was greatest at modulation rates between 5 and 20 Hz. The responses elicited by ipsilateral and contralateral monaural stimulation were approximately equal in magnitude, and binaural stimulation produced a potential 30% greater than the individual monaural responses. It is suggested that the evoked response represents the entrained neural activity to temporal amplitude fluctuations, and reflects the psychophysically measured performance of the auditory system for the detection and analysis of amplitude modulation.     

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

OBJECTIVE: To evaluate, using statistical methods, the usefulness of the binaural multiple frequency auditory steady state responses (MF SSRs) for objective, frequency-specific audiometry in a large sample of hearing-impaired children and normal-hearing subjects. DESIGN: The MF SSRs were recorded in a sample of 43 hearing-impaired children (86 ears) and 40 normal-hearing young adults (80 ears). Simultaneous carrier tones (0.5, 1, 2, and 4 kHz) modulated in amplitude at different rates (77 to 105 Hz) were presented binaurally (TDH 49 earphones) at variable intensities (110 to 20 dB SPL). For each subject the response thresholds (RTHs) at 0.5, 1, 2, and 4 kHz, were determined automatically (F test) and compared with the corresponding behavioral thresholds (BTHs). RESULTS: In the normal-hearing subjects, RTHs were detectable, on average, between 11 and 15 dB above the BTH. These differences were significantly smaller in the hearing-impaired (5 to 13 dB). Also a close correspondence was found between the subjective and objective audiogram curves in both groups. The within subject Spearman correlation coefficients calculated between the two curves, were in most cases above the significance cut off point (p < 0.05). Also in 1-way repeated measures analysis of variance, the overall error in the estimation of the audiogram (vector across frequency of absolute distances between the curves) did not differ significantly from zero. CONCLUSIONS: The binaural MF SSR was proven to be a valid technique for the estimation of an objective audiogram, in a large sample of hearing-impaired children and normal-hearing subjects. With this method, frequency-specific thresholds at 0.5, 1, 2, and 4 kHz could be determined in all subjects (and both ears) with no appreciable loss in accuracy and a considerable reduction in testing time (average recording time = 21 minutes) when compared with other frequency-specific techniques.     

Human auditory steady-state responses to amplitude-modulated tones: phase and latency measurements

Human auditory steady-state responses were recorded to four stimuli, with carrier frequencies (f(c)) of 750, 1500, 3000 and 6000 Hz, presented simultaneously at 60 dB SPL. Each carrier frequency was modulated by a specific modulation frequency (f(m)) of 80.6, 85.5, 90.3 or 95.2 Hz. By using four different recording conditions we obtained responses for all permutations of f(m) and f(c). The phase delays (P) of the responses were unwrapped and converted to latency (L) using the equation: L=P/(360xf(m)). The number of cycles of the stimulus that occurred prior to the recorded response was estimated by analyzing the effect of modulation frequency on the responses. These calculations provided latencies of 20.7, 17.7, 16.1 and 16.1 ms for carrier frequencies 750, 1500, 3000 and 6000 Hz. This latency difference of about 4.5 ms between low and high carrier frequencies remained constant over many different manipulations of the stimuli: faster modulation rates (150-190 Hz), binaural rather than monaural presentation, different intensities, stimuli presented alone or in conjunction with other stimuli, and modulation frequencies that were separated by as little as 0.24 Hz. This frequency-related delay is greater than that measured using transient evoked potentials, most likely because of differences in how transient and steady-state responses are generated and how their latencies are determined.     

Coding of amplitude-modulated tones in the central auditory system of catfish

In the catfish central acoustic system information is coded by two subsets of units. Type I units show little or no adaptation, type II units adapt rapidly, and some units are transitional, showing moderate adaptation. The two groups of units also respond differently when exposed to sinusoidal amplitude modulation of the signal's carrier frequency. The tonic, less readily adapting type I units code over an intensity range of about 30 dB, are fairly insensitive to intensity changes, and follow stimulus envelopes of 60 Hz and less. They apparently discharge in response to the actual intensity of the signal rather than in response to something in its temporal pattern. The onset-sensitive, fast-adapting type II units on the other hand are restricted to an intensity range of only 10 dB, show greater sensitivity to intensity changes, and are capable of following the temporal pattern of amplitude-modulated stimuli exceeding 100 Hz. These units appear to code the temporal changes in the stimulus intensity irrespective of the absolute intensity of the signal.     

A comparison of auditory brain stem evoked potentials in hyperlipidemics and normolipemic subjects

Twenty-five hyperlipidemic, neurologically and audiologically asymptomatic patients were compared to 20 normolipemic control subjects regarding different interpeak latency differences (IPLD) of auditory brain stem evoked potentials (ABEP). ABEP were recorded in response to click stimuli presented at a rate of 10/sec and 55/sec. The net effect of increasing stimulus rate (ISR) on IPLD of ABEP was significantly greater in the hyperlipidemic patients for IPLD (V-I) and IPLD (III-I). ISR of ABEP indicates a trend of subclinical impairments of brain stem function in hyperlipidemic patients, probably due to ischemia accelerated by their condition.     

Coherence analysis of scalp responses to amplitude-modulated tones

Magnitude squared coherence is a method of auditory evoked potential (AEP) frequency-domain analysis, measuring the degree to which the AEP is determined by the stimulus as a function of frequency. In 10 normal-hearing human subjects, scalp responses to an amplitude-modulated (AM) tone (500 Hz carrier modulated with a 40 Hz envelope) were recorded. Critical value criteria were utilized in the statistical analysis of coherence-intensity functions for determination of threshold responses. When compared with subjective determination of AEP threshold from time-domain waveforms, coherence analysis provided a significantly (p less than 0.001) more sensitive threshold measure. Coherence analysis provides information on the spectral content of the response, and allows for the objective determination of threshold which may potentially be utilized in a more expeditious threshold measurement scheme.     

Development of auditory evoked potentials.

The development and maturation of the human auditory system appears to occur in parallel at all levels from middle ear to cortex. The maturation of evoked potentials from auditory brainstem to auditory cortex can be described by equal percentage changes in equal time periods. This is essentially the exponential growth model with equal maturation rate for each station. The auditory nerve maturation occurs at a rate considerably faster than that for more central parts of the nervous system. Premature birth does not seem to affect the maturation rate and time to maturity of the auditory brainstem potentials, so that experience does not seem to affect physiological maturation. However, there is a clear difference in maturation rates for different frequency regions, suggesting that the time course of structural maturation has an effect. Behavioral changes in hearing threshold show maturation rates similar to the physiological ones for the central nervous system.     

Amplitude--intensity functions for auditory middle latency responses in hearing-impaired subjects

Results from animal studies show that hearing loss can result in increased neural responsiveness within the central auditory system. This study employed auditory middle latency evoked responses to compare central auditory responsiveness in human males with and without hearing loss. Measurements of auditory middle latency responses (MLRs) recorded from 14 normal hearing males were compared with MLR measures from 14 males with high-frequency, sensorineural hearing loss. Sixteen toneburst stimuli (four frequencies x four intensities) were used. Slopes of the amplitude-intensity functions for the several components of the MLR were obtained for each subject at frequencies below, near, and above the audiometric edge. Analysis of variance (ANOVA) revealed a significant group effect for MLR component Pa-Na, with less steep slopes at all frequencies for the hearing-impaired group. The ANOVA also showed a trend towards a significant group effect for Pb-Nb. Two-sample t-tests performed for Pb-Nb for each of the four tonebursts showed a frequency-specific effect. For Pb-Nb there was a statistically greater mean slope for the hearing-impaired group at the toneburst frequency associated with the audiometric edge.     

Digital filtering of auditory evoked potentials

Digital filters have been proposed by many authors for analysis and enhancement of the auditory brain stem response and related potentials. Because these techniques are unfamiliar to many, and require attention to underlying assumptions, an overview of the field is presented, with emphasis on limitations and risks of each method. Topics include Fourier transforms, finite impulse response (convolutional) filters, and infinite impulse response (recursive) filters.     

Steady-state analysis of auditory evoked potentials over a wide range of stimulus repetition rates: Profile in adults

Abstract

Objective: Quasi-steady-state responses were assessed over a wide range of stimulus repetition rates embracing well the traditionally measured transient AEPs (obligatory auditory evoked potentials of all latencies). Repetition rates of ≤10 Hz have received little attention in the context of the ASSR stimulus-response analysis approach which is speculated to provide technical advantages, if not additional information, over more traditional transient stimulus-response paradigms. Design: A measure introduced and defined as the sum of the response at the stimulus frequency and its harmonics. The magnitude of steady-state responses were measured at repetition rates from 0.75 to 80 Hz, using trains of repeated tone-burst stimuli. Study sample: Twenty-five normal-hearing adults. Results: Results show that the magnitudes of the response across repetition rates are largest at the two lowest rates, following trends expected from the transient AEP literature. Good reliability overall was observed for the harmonic sum. Conclusions: The analysis methods used in this paper may give information that will have application for clinical testing. Of pragmatic importance is that the rate profile could be determined without subjective wave identification and/or interpretation, and thus by a method that is inherently more objective than conventional AEP tests.

Sumario

Objetivo: Se evaluaron respuestas de cuasi-estado estable en un amplio rango de tasas de repetición del estímulo, que correspondían a los AEP tradicionalmente medidos (potenciales evocados auditivos obligatorios de todas las latencias). Las tasas de repetición de ≤10 Hz han recibido poca atención en el contexto del enfoque de análisis de estímulo-respuesta para ASSR, que se especula proporciona ventajas técnicas, además de información adicional, sobre paradigmas más tradicionales de estímulo-respuesta transitorios. Diseño: Una medida introducida y definida como la suma (armónica) de la respuesta, a la frecuencia de estímulo y de sus armónicos. La magnitud de las respuestas de estado estable fueron medidas a tasas de repetición de 0.75 a 80 Hz, usando series de estímulos de ráfagas tonales repetidas. Muestra del Estudio: Veinticinco adultos con audición normal. Resultados: Los resultados muestran que las magnitudes de las respuestas en las tasas de repetición son mayores en las dos tasas más bajas, siguiendo la tendencia esperada de la literatura de AEP transitorios. Se observó una buena confiabilidad para la suma armónica. Conclusiones: Los métodos de análisis usados en este trabajo pueden dar información que tendrá aplicaciones para evaluación clínica. De importancia pragmática es que el perfil de la tasa podría ser determinado sin identificación y/o interpretación subjetiva de ondas, y por tanto, por un método que es inherentemente más objetivo que las pruebas convencionales de AEP.     

Electrically evoked potentials in cochlear implant subjects

A series of experiments was performed to study electrically evoked potentials as indicators of subject response to cochlear implantation. 1. Brain stem evoked responses to electrical stimulation were compared to those obtained by acoustic stimulation in guinea pigs. The response pattern was similar and was independent of the site of placement of the stimulus electrode (cochlear base or apex) or of the extracochlear ground electrode (eustachian tube or temporalis muscle) when evoked electrically. 2. Electrically evoked middle latency responses were recorded and compared to subjective behavioral thresholds in patients who had received a single channel cochlear implant (House-Urban). The behavioral responses to the same stimuli were similar. 3. Electrically evoked auditory brain stem responses were studied in single channel cochlear implant subjects (3M/House). When evoked electrically, potential latencies were shorter and interpeak intervals narrower than acoustically evoked potentials.