[Magnetic activity of the brain. Hemispheric differences in acoustic stimulation].
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Acoustic Stimulation↗
Biomedical subjects
C Elberling
Publications and source records attributed to C Elberling.
Evoked acoustic emission: clinical application.
Stimulated acoustic emissions were recorded in response to tonal stimuli at 60 dB p.e. SPL in a small group of normal-hearing adults. Power spectral analysis reveals that the evoked activity from each ear contains energy in preferential frequency bands and the change of stimulus frequency has only a minor effect on the power spectra, i.e. the maximum jumps from one spectral peak to another. Experiments with deconvolution demonstrate that the emission generating system at least at a fixed intensity can be regarded as being linear and characterized by its impulse response which is similar to the emission evoked by click stimuli. It is concluded that significant information is obtained by the click rather than by the tonal stimuli. The click-evoked emissions were also recorded from both ears in a consecutive series of 100 full-term and otherwise normal babies 2-4 days after birth. The emission amplitudes were of the same order of magnitude as those previously found in normal-hearing adults. Cross-correlation analysis was performed in order to evaluate reproducibility and the combination of amplitudes and correlation coefficients from supra-threshold recordings and the no-stimulus recordings reveals presence of a true emission from all ears tested. It is concluded that the cochlear echo can be recorded in normal-hearing newborns with an extremely low rate of type I errors.
Adult↗
Estimation of auditory brainstem response, ABR, by means of Bayesian inference.
The present paper describes a new method to estimate the auditory brainstem response when the electrical activity from the recording electrodes displays non-stationarity, i.e. varies between low and high levels. The method is based on a statistical approach called Bayesian inference and weights the individual components (here blocks of 250 sweeps) inversely proportional to the level of the noise activity during the recording. Fifty sets of data from 10 consecutive patients obtained during stimulation at high intensity are used to evaluate the difference between the classic averaging and the present method which is called Bayes estimation. In approximately 30% of the cases, a significant all-over improvement is obtained by the new method. The classic averaging technique would here require 50% more sweeps to be taken to obtain the same precision of the ABR estimate, on average. Also the latency and amplitude parameters of the Jv wave complex are evaluated and it is shown that the parameter variance decreases by a factor of approximately 2 by using the Bayes estimation. The new technique is compared with a similar technique recently presented by Hoke et al. (1984) and the differences and similarities are discussed.
Bayes Theorem↗
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.
Adult↗
Quality estimation of averaged auditory brainstem responses.
In its clinical use, the auditory brainstem responses, ABR, are recovered from the background noise by averaging a number of post-stimulus time epochs, sweeps. Normally, the test protocol prescribes a fixed number of sweeps to be employed and recording of replications is frequently recommended, too. Since both the ABR and the background noise differ in magnitude among patients and test sessions, such a test protocol can never ensure a given minimum "quality" or signal-to-noise ratio, SNR, of the averaged ABR. In an attempt to solve this problem, a method is proposed to evaluate, in statistical terms, the "quality" or SNR of the averaged recording. This is done by calculating the ratio between the estimated magnitude of the ABR and that of the averaged background noise. The method can be employed "on-line" in an adaptive strategy to estimate the number of sweeps necessary to obtain a given minimum "quality" of the averaged ABR. It can be used in suprathreshold recordings as well as in automatic "threshold" detection and it is a practical tool to analyze and improve the cost-benefit of the ABR test.
Audiometry↗
Evoked acoustic emissions from the human ear. III. Findings in neonates.
Stimulated acoustic emissions were recorded in a consecutive series of 20 full-term and otherwise normal neonates with the equipment and method previously used in adults. One ear randomly chosen was tested in each baby, and otoscopy and tympanometry were normal in all ears. A 2 kHz click stimulus was presented with a repetition rate of 10/sec and the recordings were performed at three intensities, i.e. 70, 50 and -20 dBatt (dBatt approximately dB p.e. SPL). The 50 dB recording was repeated for check of reproducibility. A clear and reproducible response could be identified from all ears at 50 dBatt. However, as in the adults, the response pattern differed significantly from one ear to another, both regarding the number of 'echoes', their latencies, response amplitudes, and frequency content. The 'echo' group latencies and amplitudes were within the same range as in normal adults and the amplitude input-output curves exhibited a clear non-linearity. The relationship between latency and frequency was just as ambiguous as in the adults. Also, in the neonates, the cross correlation analysis proved to be an efficient method to indicate whether or not a true response was present. The results from this investigation are compared with those described in the literature from other audiological tests and it is concluded that the recording of the stimulated acoustic emissions could be applicable as a screening procedure in newborns.
Acoustic Stimulation↗
Cortical magnetic fields evoked by frequency glides of a continuous tone.
Frequency glides from a continuous tone have been shown to produce activity from the human cortex that can be recorded as time-varying magnetic fields outside the scalp in the same way as simpler auditory stimuli such as clicks and tone bursts. Data analysis has been based on a model assuming an equivalent current dipole localized close to the skull surface. Recorded data have shown good agreement with such a model. Interhemispheric differences have been shown in the location of this dipole, as well as with regard to dipole moment and latencies of responses to contralateral stimulation. The location of the equivalent dipole for frequency glide stimulation is close to that previously reported for tone pulse stimulation. However, the results indicate that differences in location of the order of 10 mm may exist. Comparing previously reported electric responses to frequency glides indicates essentially qualitative agreement although some significant differences have also been found. This is interpreted as evidence that at least the major contributions to the two types of response are produced by the same generator in the temporal lobe of the human cortex.
Acoustic Stimulation↗
Auditory magnetic fields from the human cerebral cortex: location and strength of an equivalent current dipole.
Auditory evoked cortical magnetic fields are recorded from human subjects by means of a SQUID gradiometer. The spatial and temporal distributions of the averaged evoked fields normal to the surface of the skull are measured from both hemispheres in response to contra- and ipsilateral 1 kHz stimulation. The evoked magnetic response can be separated into a dominant and a 'residual' signal and the former is analysed with a particular source model consisting of a single equivalent current dipole in each hemisphere. We find that the equivalent current dipoles are located near the superior surface of the temporal lobes approximately 20 mm below the surface of the skull. The dipoles are oriented in the superior-inferior direction. In the left hemisphere the dipole is located approximately 14 mm posterior to that in the right hemisphere, but otherwise no hemisphere/ear difference in dipole location or orientation is found. The strength of the dipole in the left hemisphere is found to be twice as great as that in the right hemisphere when stimulating the right ear, whereas no difference is found when stimulating the left ear. The strength of the dipole is greater in response to contralateral than ipsilateral stimulation. By means of a statistical experiment and using estimates of the variance of the recorded evoked fields we show that the model suggested is adequate to describe the experimental data and that the overall confidence of the extracted dipole parameters can be estimated.
Acoustic Stimulation↗
Location-specific components of the gross cochlear action potential: an assessment of the validity of the high-pass masking technique by cochlear nerve fibre recording in the cat.
The auditory high-pass masking technique has been used in attempts to define the origin, along the cochlear partition, of the gross cochlear action potential (CAP) and the gross brain stem potential. Theoretically, the high-pass masking paradigm should be frequency and location specific at the cochlear level, and some indirect evidence does point to this specificity. However, this hypothesis has not yet been directly substantiated. In the present experiment, click-evoked cochlear nerve activity was recorded simultaneously from the round window and from single fibres of the cochlear nerve, with and without high-pass maskers spaced in octaves from 0.5 to 16 kHz, at three intensities, in the anaesthetized cat. The "derived' CAPs were computed and compared with the mapping of single cochlear fibre responses under the same conditions. With one main exception, the conclusions drawn on the origin of the frequency components of the "derived' potentials were found to be valid in the normal cat. The exception concerned fibres with characteristic frequencies below 1-2 kHz, where the substantial spread towards the high frequencies of their frequency threshold curves, and the effects of lateral suppression or of other "remote masking' phenomena rendered the high-pass masking less location specific. From these results and certain assumptions, we would predict the high-pass masking technique to be valid in electrophysiological investigations in normal humans for frequencies down to 0.5-1 kHz.
Animals↗
Evoked acoustic emissions from the human ear. I. Equipment and response parameters.
Using signal averaging technique, stimulated acoustic emissions can be recorded from the human ear with a probe in the external ear canal. An acoustic click stimulus was used, produced by half a sinusoid of 2 kHz with the polarity corresponding to the rarefaction mode. A number of different techniques were developed in order to evaluate the latency and configuration of the emissions objectively. Recordings from a normal-hearing subject served as an example and a clear response could be traced down and below the psychoacoustic threshold. The threshold was elevated and the response pattern altered when a sensorineural hearing loss was induced by ingestion of acetylsalicylate. No response could be recorded from a deaf ear with an intact eardrum and a mobile ossicular chain.
Acoustic Stimulation↗
Auditory magnetic fields: source location and 'tonotopical organization' in the right hemisphere of the human brain.
The late, acoustically evoked, averaged magnetic field from the right hemisphere of the human brain is composed of two signals. One is dominant, appears generated by an equivalent current dipole within or near the primary auditory cortex and shows a frequency dependent location and/or orientation (tonotopical organization). The other, denoted the 'residual' signal, resembles the electric T-complex and is possibly generated more diffusely in the auditory and adjacent cortical areas.
Auditory Cortex↗
Evoked acoustic emissions from the human ear. II. Normative data in young adults and influence of posture.
Click-evoked acoustic emissions were recorded in 10 normally hearing young adults and evaluated by methods previously described. Five of the subjects were tested on both ears. A clear response could be traced down to or below the psychoacoustic threshold in all ears. However, the response pattern differed significantly from one ear to another, yielding an intra- and intersubject variability of the same order of magnitude. The key parameter of the evoked emission seems to be the group latency, since both peak-to-peak amplitude and frequency content are defined only at a specific latency. Two methods were applied to establish group latencies objectively and the two latency measures were almost identical. The individual input-output functions, based on peak-to-peak amplitude values as well as RMS values, exhibited nonlinearity. The latency versus frequency relationship was ambiguous. Retests of five ears at 4-5 week intervals demonstrated a high stability of the response pattern from the individual ear. Five subjects were tested in the sitting as well as in the recumbent position and the response pattern were unaffected by posture.
Acoustic Impedance Tests↗
High-pass masking in the classification of low-frequency hearing loss.
Invalid BC determinations may be responsible for many diagnostic misinterpretations in the classification of low-frequency hearing losses. To reduce the influence from harmonic distortion, BC thresholds are determined during simultaneous, ipsilateral HP masking and composed with the conventionally obtained BC thresholds. Based on normative data a change of greater than or equal to 10 dB HL caused by HP masking is used as the criterion for judging the validity of the conventional BC testing. In 55% (11/20) of ears with low-frequency hearing loss, a decrease in air-bone gap was obtained with HP masking, and it is concluded that HP masking is of clinical significance for the diagnosis in these patients.
Adolescent↗
Auditory electrophysiology: findings in multiple sclerosis.
15 patients suffering from definite multiple sclerosis were studied by electrocochleography (ECochG) and brain stem electric responses (BSER), including objective analysis of the electrophysiological data. In 7 patients, both ECochG and BSER were performed, in 2 only ECochG and in 6 only BSER. Systematic deviations from normative values were found in all 9 patients tested by ECochG, but the deviations occurred only at 75 dB pe SPL or below. Despite normal or near-normal audiometric thresholds, the cochleae seemed to be involved. At click intensities of 95 dB pe SPL or higher, the BSER revealed a normal AP-V (Jewett) interval in all 13 patients tested. However, a deviant configuration of the BSER following the wave V complex was found in 6 of the 13 patients. The deviation is described as a dominant, relatively low-frequency component of the response. The discrepancies between these results and those reported in the literature indicate a strong need for applying methods that compensate for peripheral hearing dysfunctions and objective methods of analysis to identify and evaluate the various components of the evoked potentials.
Adult↗
ECochG and psychoacoustic tests compared in identification of hearing loss in young children.
Among 149 children (median age 23 months, range 2-123 months) examined by ECochG, a comparison was made between pure-tone and ECochG thresholds in 53 patients. By using linear regression analysis a high correlation was found (correlation coefficient 0.92, p less than 0.0005) proving that ECochG gives valid estimates of the hearing thresholds, also in small children. A comparison between behavioral observation audiometry (BOA) and ECochG thresholds was performed in 62 children. It was concluded that BOA exhibits poor sensitivity in the assessment of hearing losses and underestimates their degree of severity. Furthermore, when BOA diagnoses normal hearing, there is a 9% risk of depriving a "deaf' child of suitable treatment.
Audiometry↗
Slow cortical responses and the diagnosis of central hearing loss in infants and young children.
To evaluate the usefulness of slow cortical responses (ERA) for threshold estimation in infants and young children, 83 children were investigated with combinations of pure-tone audiometry, electrocochleography (ECochG) and ERA. The deviations between ECochG/ and ERA thresholds were correlated to brain function in order to diagnose central hearing losses. By comparing corresponding values of 2 kHz pure-tone and ERA thresholds, 32% (10/31) errors were found, mainly below 35 dB HL. In a group of patients with no sign of brain disorder, an overall error rate of 37% (20/53) was found. Below 35 dB HL, 72% (11/15) errors were found. In a group of patients with brain dysfunction, the overall rate was 70% (21/30), below 35 dB HL it was 84% (21/25). In the range below 35 dB HL, no significant difference (p greater than 0.05) in errors was found between the groups with and without brain disorders. It is concluded that ERA is unreliable for the estimation of moderate hearing losses and cannot per se detect a central hearing dysfunction. Elevated ERA thresholds may indicate a central hearing loss, but to establish this topical diagnosis, ECochG and neuropsychological examinations are necessary.
Audiometry↗
Auditory magnetic fields from the human cortex. Influence of stimulus intensity.
The late averaged magnetic field evoked by contra- and ipsilateral auditory stimulation is recorded by means of a SQUID magnetometer from both hemispheres in four normally hearing, right-handed male adults. The stimuli consist of 1 kHz, 500 ms tone pulses with intensities from 5 to 85 dB HL and averaging is based on 60 sweeps. Stimulating the right ear the averaged magnetic field from the left hemisphere is approx. twice as great as that from the right hemisphere, whereas stimulating the left ear no difference in magnitude is found. The amplitude input-output functions are steeply rising near threshold and more shallow at high intensities. The responses from contralateral stimulation are approx. 9 ms earlier than those from ipsilateral stimulation with no interhemispheric difference.
Acoustic Stimulation↗
Loudness summation across frequency under masking and in sensorineural hearing loss.
The loudness summation across frequency was measured in unmasked and masked normally hearing subects, using noise bands centered at 1 kHz. Homolateral masking reduced the loudness summation of supracritical noise bands to a degree roughly proportional to the degree of threshold shift produced by the masking. Between thresholds and moderate sensation levels, the loudness summation increased at an invariable rate in the unmasked and the masked condition, reflected in an invariable supracritical growth of loudness with increasing stimulus bandwidth at identical sensation levels. At high intensities, the loudness summation decreased to roughly similar values in both conditions. Similar results were obtained by pooling the data from patients with sensorineural hearing losses and comparable threshold elevation. The critical bands appeared to be identical in unmasked and masked normally hearing subjects and in patients with sensorineural hearing losses. Contralateral, 35 dB effective masking produced a slight but consistent central masking effect; compared to the unmasked condition, the loudness summation of supracritical noise bands was reduced between threshold and medium stimulus levels, while it was increased at high levels.
Adolescent↗