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Biomedical subjects

M Hoke

Publications and source records attributed to M Hoke.

At least 55 records · Page 3Linked to original sources
PubMed1987

Biomagnetic measurements using squids.

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.

Acoustic Stimulation↗
PubMed1986

MEG measurements with SQUID as a diagnostic tool for epileptic patients.

In experimental studies with a SQUID (Super-conducting QUantum Interference Device) second order gradiometer, we recently registered the magnetoencephalogram (MEG) from different subjects under different physiological and psychological conditions from which we will determine normal and abnormal function of the human brain. Thus with our first measurements using the MEG spectra, we have succeeded in identifying the exact location of the abnormality in the human brain as shown in several individuals.

Brain↗
PubMed1986

Randomized data acquisition paradigm for the measurement of auditory evoked magnetic fields.

The high variability of both amplitude and latency measures of the components of the auditory evoked magnetic field (AEMF), which we have attributed primarily to changes in the state of vigilance, makes it often impossible to compute significant isofield contour maps. Using a randomized data acquisition paradigm we have been able to considerably reduce the time-dependent fluctuations of the state of vigilance resulting in more stable latencies and in more stable and higher amplitudes of the AEMF components.

Evoked Potentials, Auditory↗
PubMed1986

Comparison between simultaneously recorded auditory-evoked magnetic fields and potentials elicited by ipsilateral, contralateral and binaural tone burst stimulation.

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').

Acoustic Stimulation↗
PubMed1986

Causes of differences in the input-output characteristics of simultaneously recorded auditory evoked magnetic fields and potentials.

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.

Acoustic Stimulation↗
PubMed1985

Possibilities and limitations of weighted averaging.

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, compared 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.

Acoustic Stimulation↗
PubMed1984

Weighted averaging--theory and application to electric response audiometry.

A weighted averaging technique has been developed to overcome the shortcomings of conventional, unweighted averaging in the case of nonstationary noise. The technique is based on the linear least-mean-square estimate of a periodic signal in a simple model of non-stationary noise. This estimate weights each recorded epoch according to the magnitude of the noise within the epoch. In the estimation of a known signal, the weighted averaging procedure yielded smaller root-mean-square errors in comparison with the normal unweighted average. The weighted averaging procedure offers many advantages over conventional averaging or averaging with automatic gain control preamplifiers.

Audiometry↗
PubMed1984

Time- and intensity-dependent low-pass filtering of auditory brain stem responses.

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.

Acoustics↗
PubMed1980

Influence of refractory properties on the response of single auditory nerve fibres to sinusoidal stimuli.

The influence of refractoriness upon the response of single auditory nerve fibres to sinusoidal stimuli has been investigated by means of a simple 'recovery' model. It turned out that refractoriness causes not only a saturation of the mean discharge rate for high stimulus intensities, but also affects the shape of period histograms. If the stimulus intensity is increased, the mode of the histogram is shifted to the left, and finally, at high intensities, the histogram assumes a bimodal shape. The saturation of the discharge rate and the skewness of period histograms can also be explained by a transmitter release mechanism (Schroeder-Hall model). The investigations presented in this paper suggest that the influence of refractoriness on the response of auditory nerve fibres is greater than hitherto assumed.

Acoustic Stimulation↗
PubMed1980

Deconvolution of compound action potentials and nonlinear features of the PST histogram.

Deconvolution of the compound action potential (CAP) with a uniform unit response (UR) results in the compound PST histogram (CPST). After a logarithmic transformation of the CPST, a second deconvolution of the obtained transformed PST histogram (TPST) with an intensity-dependent norm PST histogram (NPST) may be feasible to determine the excitation pattern (E), reflecting the contributions from the individual nerve fibres. The presented model investigations show that, in spite of nonlinearities, a normalized PST histogram can be assumed. Application to recorded CAPs yields excitation patterns almost as predicted by the model.

Action Potentials↗