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The collision-broadened linewidths and band intensities of three overlapping CO_2 bands near 2600 cm^−1 have been determined from spectra obtained with a Fourier transform spectrometer. These spectra have SNRs in excess of 500 and a spectral resolution of ~0.06 cm^−1. Calculated spectra were matched to the observed spectra by nonlinear least squares regression over 100-cm^−1 intervals. More than forty parameters were estimated simultaneously, and the mean differences between the spectra were about one and a half times the mean noise in the experimental spectra. more...

The input-output characteristics of amplitude and latency of simultaneously recorded auditory-evoked magnetic fields (AEMF) and auditory-evoked potentials (AEP) are significantly different, although they are closely related to the same excitation process of the auditory system. As the source of both AEMF and AEP an equivalent-current dipole lying in the auditory cortex can be assumed. Differences in the input-output characteristics of AEMF and AEP can be explained by changes of one or more parameters of this dipole (depth, location in the tangential x-y plane and direction). Maps of the field distribution obtained at 60 and 80 dB HL indeed reveal a change of the location of the dipole in the x-y plane and the direction of the dipole momentum, whereas the depth of the dipole was found to be more or less constant.Les caractéristiques d'entrée-sortie de Pamplitude et de la latence des champs magnétiques évoqués acoustiquement (CMEA) et des potentiels évoqués auditifs (PEA), enregistrés simultanément, sont significativement différentes, bien qu'elles soient en rapport avec le meme processus d'excitation du système auditif. On peut supposer que la source commune à ces deux types de réponses évoquées (CMEA et PEA) est un courant dipolaire situé dans le cortex auditif. Les différences de caractéristiques entre CMEA et PEA peuvent s'expliquer par un changement d'un ou de plusieurs paramètres de ce dipo˘le (profondeur, localisation dans le plan tangentiel x-y et direction). En effet, la cartographie de la distribution de champs à 60 et 80 dB HL révèle un changement de la localisation du dipo˘le dans le plan x-y et de sa direction alors que sa profondeur reste plus ou moins constante. more...

Both auditory-evoked magnetic fields (AEMF) and auditory-evoked potentials (AEP) mainly consist of three peaks with latencies of about 50, 100 and 160 ms. Comparison of responses to ipsilateral, contralateral and binaural stimulation yields no significant amplitude or latency differences of the AEP peaks whereas the simultaneously recorded AEMF peaks exhibit a 10 ms shorter latency and an approximately 38% greater amplitude for contralateral versus ipsilateral stimulation. This fact can be due to differences in the strength, location (especially the depth) and the direction of the dipole source, and a decision cannot be made considering the data recorded from just one position. Another finding is that binaural stimulation reduces the peak amplitudes by approximately 25% compared with contralateral stimulation. This result indicates some kind of interference between the ipsilateral and contralateral pathways ('binaural interaction').Les champs magnétiques évoqués auditifs (CMEA) et les potentiels évoqués auditifs (PEA) consistent tous les deux principalement en trois pics de latences d'environ 50,100 et 160 ms. La comparaison des réponses à des stimulations ipsilatérales, controlatérales et binaurales ne montre pas de différences significatives dans les amplitudes et les latences des pics des PEA, tandis que les CMEA enregistrés simultanément présentent une latence plus courte d'environ 10 ms et une amplitude approximativement 38% plus grande pour des stimulations controlatérales par rapport aux stimulations ipsilatérales. Ce fait peut etre dü à des différences dans l'intensité, la localisation (spécialement la profondeur) et la direction du dipöle source, et on ne peut pas conclure en considérant les enregistrements à partir d'un seul point. Une autre observation est que la stimulation binaurale diminue les amplitudes maximales d'environ 25% par rapport à la stimulation controlatérale. Ce résultat suggère une sorte d'interférence entre les voies ipsilatérales et controlatérales («interaction binaurale»). more...

A new method of filtering auditory brain stem responses (ABR), which is time-and intensity-dependent, is proposed. The epoch containing the averaged response is divided up into 128 overlapping segments and each segment is differently filtered. The characteristics of a filter that is applied to a segment of the response are derived from the time-dependent spectral composition of normal ABRs at the appropriate stimulus intensity. After filtering, the segments are added together to yield a response with no significant amplitude and/or phase distortion, and with distinctly pronounced peaks that are excellently suitable for evaluation with an automated system.Une nouvelle methode de filtrage des potentiels evoques auditifs du tronc cerebral, fonction a la fois du temps et de l'intensite est proposee. L'epoque contenant la reponse moyenne est divisee en 128 segments qui se chevauchent et chaque segment est filtre differemment. Les caracteristiques du filtre utilise pour chaque segment de la reponse sont determinees a partir de la composition spectrale fonction du temps des PEATC normaux, a l'intensite de stimulation appropriee. Apres filtrage, les segments sont rajoutes pour fournir une reponse sans distorsion significative d'amplitude ou de phase, et avec des pics distincts prononces qui sont tout a fait adaptes a une evaluation automatique. more...

The deconvolution technique of the compound action potential of the auditory nerve consists of a first deconvolution with the unit response, a logarithmic transformation, a second deconvolution with the normalized discharge pattern, and a linear transformation. The simplifying assumptions contained in the process are valid for infinitely short stimuli: The inherent error is growing with increasing duration of the stimulus signal. Similar assumptions can be made for stimuli of sufficiently narrow bandwidth if one interchanges the nonlinear transformation and the second deconvolution. Under these conditions, the error will be within tolerable limits. Application of this method would mean a considerable improvement in the evaluation of compound action potentials elicited by low-frequency tone pulses (carrier frequency < 1000 Hz). The principle and limits of the method are outlined, its efficiency with human compound action potentials elicited by Gaussian tone pulses are shown. more...

Spectra with a resolution of about 0.06 cm−1obtained by a Fourier transform spectrometer have been calibrated by the internal standard method. An accuracy of 3 × 10−3cm−1over intervals of several thousand wavenumbers has been obtained by measuring the lines with a precision of 2 × 10−3cm−1and by using a large number of standard lines. more...

The ability of the polysulfonated antitumor drug suramin and six related polysulfonated azo dyes to inhibit the cell growth, platelet-derived growth factor (PDGF)-receptor binding, and intracellular Ca2+ signaling of Swiss 3T3 fibroblasts was studied. Some of the azo dyes were more potent inhibitors of PDGF binding than was suramin. The concentration giving 50% inhibition (IC50) of PDGF binding was 0.5 µm for the most potent azo dye as compared with 10 µm for suramin. The azo dyes were generally more potent inhibitors of nonmitochondrial Ca2+ uptake and of inositol(1,4,5)trisphosphate-mediated Ca2+ release in permeabilized Swiss 3T3 cells than was suramin, and they were more potent inhibitors of PDGF-induced Ca2+ signaling in intact Swiss 3T3 cells. The azo dyes were only as effective as or less effective than suramin in inhibiting the growth of Swiss 3T3 cells, with IC50 values of between 74 and 361 µm being noted for the dyes as compared with 70 µm for suramin. The difference between the growth-inhibitory activity of the azo dyes and that of suramin could not be explained by metabolism of the compounds, which was not detectable in either Swiss 3T3 cells or human liver slice preparations. The results suggest that suramin and some of the azo dyes have actions on cell growth in addition to inhibition of growth factor binding and of Ca2+ signaling. more...

Systematic studies of the magnetoencephalogram (MEG) in normal and pathological subjects (mainly with focal epilepsies) showed that the MEG may evidence significant brain activities even if they are not present in the electroencephalogram (EEG). They also showed that the MEG has a considerably higher spatial resolution than the EEG. A novel mapping technique was introduced to get such a representation of the data that would enable the investigator to draw his conclusions mainly from inspecting the plots. This technique is characterized by an isospectral amplitude (iso-SA) mapping of the scalp distribution of specified spectral components or frequency bands of the MEG power spectrum. With the above method we were able to localize an epileptiform focus using a noninvasive technique without applying an eliciting stimulus. Furthermore using SQUID measurements we were able to describe the behavior of the MEG when the brains of different subjects were subjected to low frequency sinusoidal binaural stimuli. Under these conditions it has been shown that the MEG tends to organize around discrete frequencies that depend on the interference pattern (beat) between the two inputs. more...

The intensities, widths, and positions of lines of three CO_2 bands near 2750 cm^−1 have been determined. The results are in general agreement with other measured and estimated values.

The waveforms ot the auditory evoked magnetic field (AEF) in normal-hearing individuals and patients suffering from tinnitus are distinctly different. In tinnitus patients, the magnetic wave M200 (corresponding to the electric wave P200, or P2) is delayed and only poorly developed or even completely missing, while the amplitude of the magnetic wave M100 (corresponding to the electric wave N100, or NI) is significantly augmented. A very characteristic feature turned out to be the amplitude ratio of the two waves M200 and M100. Below the age of 50, the amplitude ratio M200/M100 represents a clear-cut criterion to distinguish between tinnitus patients and individuals without tinnitus. In tinnitus patients, the ratio is less than 0.5 independent of age, whereas, in young and middle-aged normal-hearing individuals, it is greater than 0.5. Since in normal-hearing individuals the average amplitude ratio decreases linearly with age, the clusters of amplitude ratios of the two groups begin to overlap beyond the age of 50. The hypothesis is put forward that the decrease of the average amplitude ratio in normal-hearing individuals reflects a degenerative process probably initiated by multiple exogenous and endogenous factors, which leads to both an increased excitability of the generators of a particular component of wave M10 and a sustained neural activity in the generators of one particular component of wave M200 and eventually gives rise to the sensation of tinnitus. The absence or poor development of wave M200 is a concomitant phenomenon, resulting from the involved generators being less responsive to external stimuli. Our hypothesis has been supported by one exemplary case in which we were able to trace the process of tinnitus remission during a period of 256 days after acute onset of tinnitus (due to an acute noise trauma), showing a recovery of the amplitude ratio from an intitial value of 0 to a normal value of approximately 1. more...

The influence of two physical stimulus parameters (frequency and intensity) and of one sensation parameter (pitch) on the auditory evoked magnetic field (AEF) was quantified by approximating the measured magnetic field distribution by that of an equivalent current dipole (ECD) embedded in a homogeneous semi-infinite volume conductor. The main results are as follows: The depth of the ECD increases with increasing frequency, but decreases with increasing intensity. In the case of a complex tone with missing fundamental it is the virtual pitch that determines the ECD location and not the spectral contents of the stimulus. more...

Series of Monte Carlo simulations have been carried out which were based on the assumption that two dipoles with a distance of 0.5-2 cm are located in a homogeneous semi-infinite volume conductor (depth 3 cm), and that the magnetic field component perpendicular to the surface of the volume conductor is recorded by means of a magnetometer with infinitesimal coil diameter. Moving-dipole models (all parameters time-dependent), rotating-dipole models (dipole locations fixed, dipole orientation and amplitudes time-dependent) as well as fixed-dipole models (dipole locations and orientations fixed, amplitudes time-dependent) were considered. The algorithm used to retrieve the model parameters from the simulated field distributions (biomagnetic inverse procedure) was based on a transformation of the standard least-squares fit procedure into a minimization procedure with respect to the nonlinear parameters (dipole locations and orientations), which was solved iteratively by means of the Fletcher-Powell algorithm. It was found that the resolving power of the biomagnetic inverse procedure is highly dependent on the relative orientation of the two dipoles, the temporal overlap of the dipole moments, and the correlation of successive samples of the superimposed noise. The results obtained in this study suggest that the resolving power of the biomagnetic inverse procedure for conditions typically found in the case of auditory evoked magnetic fields is not better than 2 cm for the moving-dipole approach, and not better than I cm for the fixed-dipole approach, provided that no additional a priori information is available. In practice, the situation is probably even worse since the depth of the generators is usually larger than assumed in this study. more...

A statistical analysis of a weighted averaging procedure for the estimation of small signals buried in noise (Hoke et al. 1984a) is given. The weighting factor used by this method is in inverse proportion to the variance estimated for the noise. It is shown that, compred to conventional averaging, weighted averaging can improve the signal-to-noise ratio to a high extent if the variance of the noise changes as a function of time. On the other hand, uncritical application of the method involves the danger that the signal amplitude is underestimated. How serious this effect is depends on the number of degrees of freedom available for the estimation of the weighting factor. The effect can be neglected, if this number is sufficiently increased by means of an appropriate preprocessing. more...

The rotational constants of bands of ^16O^12C^17O and ^16O^12C^18O near 2600 cm^−1 have been estimated by spectral curve fitting. These constants predict the positions of all but the weakest lines with an estimated accuracy of 0.003 cm^−1. They are in reasonable agreement with other published values, although some of these do not predict the observed line positions satisfactorily. The causes of these disagreements are discussed. more...

A new stimulation paradigm is described for eliciting frequency-specific auditory brainstem responses (ABR) by stimulation with a series of seven Gaussian-shaped tone pulses with carrier frequencies descending, in half-octave steps, from 4000 to 500 Hz, and an interstimulus interval between consecutive pulses of 18 ms. The pause between two consecutive series is 54 ms so that the interval between two tone pulses of the same frequency is 162 ms (stimulus repetition rate approximately 6/s). Simultaneously a high-pass noise masker is presented whose lower cut-off frequency is continuously diminished in such a way that, when a tone pulse is presented, the cut-off frequency of the masker is exactly one octave above the carrier frequency of the pulse. Forward masking effects of preceding tone pulses as well as forward and simultaneous masking effects of the high-pass noise suppress activity originating from those regions of the cochlea which are located basalwards to the region to be stimulated by the respective pulse, thus enhancing the frequency specificity, especially for low-frequency stimuli of higher intensity. The new stimulation paradigm was tested in 12 normal hearing subjects and turned out to be suitable to elicit frequency-specific ABR with frequencies as low as 500 Hz and intensities as low as 10 dB nHL. The main advantage of the described technique is that the time required for a complete assessment of the auditory threshold at seven test frequencies (covering the relevant speech frequency range) is substantially shorter as compared to conventional techniques so that it can routinely be employed in pedaudiology, where infants usually have to be investigated in sedation. more...

Hoke M. Objective evidence for tinnitus in auditory - evoked magnetic fields. Acta Otolaryngol (Stockh) 1991; Suppl. 476: 189 - 194.The waveforms of the auditory - evoked magnetic field in normal hearing individuals and patients suffering from tinnitus are distinctly different. In tinnitus patients, the magnetic wave M200 (corresponding to the electric wave P200 or P2) is delayed and only poorly developed or even completely missing, while the amplitude of the magnetic wave M100 (corresponding to the electric wave N100 or N1) is significally augmented. A characteristic feature was the amplitude ratio of the two waves, M200 and M100. Below the age of 50, the amplitude ratio M200:M100 represents a criterion to distinguish tinnitus patients from individuals without tinnitus. In tinnitus patients, the ratio is less than 0.5, independent of age, whereas, in young and middle - aged normal - hearing individuals, it is greater than 0.5. Since in normal - hearing individuals the average amplitude ratio decreases linearly with age, the clusters of amplitude ratios of the two groups begin to overlap after the age of 50. The hypothesis is put forward that the decrease of the average amplitude ratio in normal - hearing individuals reflects a degenerative process, probably initiated by multiple exogenous and endogenous factors, which leads to both an increased susceptibility of the generators of a particular component of wave M100 and a sustained neural activity in the generators of one particular component of wave M200 and eventually gives rise to the sensation of tinnitus. The absence or poor development of wave M200 is a concomitant phenomenon, resulting from the involved generators being less responsive to external stimuli. Our hypothesis has been supported by one exemplary case in which we were able to trace the process of tinnitus remission over 256 days after acute onset of tinnitus (due to an acute noise trauma), showing a recovery of the amplitude ratio from an initial value of 0 to a normal value of approximately 1. more...

The physiological formation of the action potentials is presumed to be well-known and a method of decoding these potentials is therefore based on this knowledge. The method uses the reverse process, i.e. deconvolution of the recorded AP's with a unit response, and a subsequent nonlinear transformation of the obtained waveforms (PST-compound PST) from the time- into the location domain (distance along the cochlear partition). The transformed waveforms are called excitation patterns. Stimulus polarity seems to have only a minor influence on the excitation patterns and it is possible to produce frequency-specific information of the cochlear function. A clinical case demonstrates the concordance between the excitation patterns and the puretone audiogram. more...

In a postlingually deaf individual, the magnetic field evoked by stimulation through a cochlear prosthesis (extracochlear electrodes) as well as of the spontaneous magnetoencephalogram was measured over the hemisphere contralateral to the prosthesis (CP), and the results were compared with those obtained from normal-hearing subjects. The latency of the 2 best developed waves M100 and M200 turned out to be prolonged in the CP patient by approximately 40 ms. The amplitude of wave M100 was significantly diminished, while wave M200 was only poorly developed. Location and direction of the equivalent current dipole (ECD) calculated for wave M100 was in good agreement with normal data, whereas the dipole moment was only about one third of the average dipole moment found in normals. Furthermore, evidence was obtained for another magnetic field wave, preceding the delayed auditory wave M100, which exhibits the same latency, ECD location and direction as reported in the literature for the somatosensory evoked magnetic field. This wave probably results from stimulation, through the intratympanic electrodes, of somatosensory nerves innervating the tympanic cavity. A potential clinical application of neuromagnetic measurements is discussed: The calculation of the ECD moment from the auditory cortical magnetic field evoked by electrical stimulation at the promontory would allow to estimate, prior to CP implantation, the number of persisting, excitable nerve fibres. more...

We have discovered a ca. 40-Hz transient magnetic oscillatory response, evoked in the human brain by the onset of auditory stimuli, consisting of four or more cycles locked in phase to stimulus onset in approximately the 20- to 130-ms poststimulus interval. The response originates in the supratemporal auditory cortex, some millimeters deeper and anterior to the source of the larger-amplitude slow-wave M100 component of the evoked magnetic field and moves in a posterior arcing trajectory 1 cm or more in length. The oscillatory cortical activation elicited by auditory stimuli may be similar to the gamma-band cortical oscillations elicited by olfactory and visual stimuli and may represent an essential component of auditory perceptual processing. more...

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The non-linearity of the ear has been studied to a sound pressure level of approximately -16 dB (relative to 2 · 10-4 µb). It was found that difference oscillations do not appear only above a certain value of the sound pressure level, but, at least in the examined range, do not require any minimal level. Up to a certain sound pressure level the amplitudes of the intermodulation are following a simple linear law which is allowing the introduction of an intermodulational constant. An electronic model which shows the same characteristic of intermodulation is described, and its acting is compaired with that of the ear. more...

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Summary The compound action potential of the auditory nerve can be conceived as a convolution integral of the firing probability densities of the auditory nerve fibres and a unit response. The probability densities can be derived directly — that means bypassing a time consuming model of the mechanical to neural transduction — from the displacement of the basilar membrane, regarding the response of the basilar membrane as the input of a cascade of nonlinearities. In particular we investigated the influence of an absolute refractory period on the probability of firing. more...

Basing on model investigations as well as on theoretical considerations, a method for the processing of frequency-specific information contained in compound action potentials has been developed. This approach consists in doint the reserve process of the physiological formation of compound action potentials. Result of this method is the neural excitation pattern (in the location domain, i.e., as a function of frequency) at the level of primary auditory neurons. Processing originally recorded human compound APs as well, as derived APs could prove the outlined approach. Significance and limits of the method and its impact for clinical application are discussed. more...

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