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Biomedical subjects

M Hoke

Publications and source records attributed to M Hoke.

At least 37 records · Page 2Linked to original sources
PubMed1991

A timesaving BERA technique for frequency-specific assessment of the auditory threshold through tone-pulse series stimulation (TOPSTIM) with simultaneous gliding high-pass noise masking (GHINOMA).

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 4,000 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.

Acoustic Stimulation↗
PubMed1991

On the biomagnetic inverse problem in the case of multiple dipoles.

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 1 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.

Algorithms↗
PubMed1991

Neuromagnetic evidence of functional organization of the auditory cortex in humans.

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.

Auditory Cortex↗
PubMed1991

Auditory cortical basis of tinnitus.

The waveforms of 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 N1) 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 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 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 initial value of 0 to a normal value of approximately 1.

Acoustic Stimulation↗
PubMed1990

Identification of sources of brain neuronal activity with high spatiotemporal resolution through combination of neuromagnetic source localization (NMSL) and magnetic resonance imaging (MRI).

The locations of the origin of wave M100 of the auditory evoked magnetic field in response to tone bursts of different carrier frequencies, obtained through dipole localization methods (DLM), were related to cerebral structures, displayed by coronal MRI (magnetic resonance imaging) tomograms of the respective subjects. This was done by displaying the landmarks which served as reference for the neuromagnetic measurements in MRI tomogram (reference plane). All calculated source locations project exactly onto the transverse temporal gyri (Heschl) in which the primary auditory cortex, the supposed origin of wave M100, is located. The results highlight the exceptional capabilities of a combination of these 2 non-invasive, high-resolution techniques for functional diagnosis.

Auditory Cortex↗
PubMed1990

Objective evidence for tinnitus in auditory-evoked magnetic fields.

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 significantly 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1989

Tonotopic organization of the auditory cortex: pitch versus frequency representation.

According to the place principles of the classical hearing theory, the physical entity frequency is encoded in the auditory periphery as place information (tonotopic representation), which is decoded in more central parts of the auditory system to form the subjective entity pitch. However, this relation is true only for pure-tone signals (spectral pitch); it can be quite different in the case of complex auditory stimuli (virtual pitch), thus requiring a multistage process for pitch formation. Neuromagnetic measurements showed that the tonotopic organization of the primary auditory cortex reflects the pitch rather than the frequency of the stimulus; that is, the pitch formation process must take place in subcortical regions.

Acoustic Stimulation↗
PubMed1989

Neuromagnetic evidence of an amplitopic organization of the human auditory cortex.

It is well known that the location of the source of cortical auditory evoked responses, which can be determined neuromagnetically in humans using the concept of an equivalent current dipole (ECD), shifts with changing stimulus frequency ('tonotopic organization'). Not investigated so far, however, is the question of whether there exists also an 'amplitopic organization' of the human auditory cortex, i.e., a spatial distribution of neurons maximally responsive to respective 'best stimulus intensities.' We measured, in the study presented here, in 3 normally hearing subjects the auditory evoked magnetic field (AEF) in response to tone-burst stimulation with a carrier frequency of 1000 Hz at 6 different intensities (30-80 dB HL in 10 dB steps). The influence of stimulus intensity was quantified in terms of changes in the ECD parameters (amplitude, direction and spatial coordinates) which were determined such that a maximum correspondence between observed and calculated field distributions was obtained. The results of the neuromagnetic measurements presented here prove that the ECD location also shifts with changing stimulus intensity. The depth of wave M100 (latency of about 100 msec) decreases monotonically with increasing stimulus intensity while the horizontal ECD position is slightly shifted in the anterior direction. The results imply that, while topical mechanisms of frequency coding are similar at cortex and at the cochlear level, topical mechanisms of intensity coding are different at these levels.

Acoustic Stimulation↗
PubMed1989

Objective evidence of tinnitus in auditory evoked magnetic fields.

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 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 sustained neural activity in the generators 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.

Acoustic Stimulation↗
PubMed1989

Tinnitus remission objectified by neuromagnetic measurements.

In a previous paper of ours (Hoke et al., 1989a) the hypothesis was put forward that the amplitude ratio of the two major waves of the auditory evoked magnetic field (AEF), M200/M100, is an objective measure which allows to discriminate between individuals suffering from tinnitus (ratio less than 0.5) and individuals without tinnitus (ratio greater than 0.5). We have now been able to trace the process of tinnitus remission in one exemplary case during a period of 256 days after acute onset of tinnitus (due to a noise trauma), in which the amplitude ratio recovered from 0 to a normal value of approximately 1. This very first objectification of tinnitus remission strongly supports our hypothesis and indicates that AEF may become an indispensable, invaluable tool in both tinnitus research and management.

Acoustic Stimulation↗
PubMed1989

Magnetic fields from the auditory cortex of a deaf human individual occurring spontaneously or evoked by stimulation through a cochlear prosthesis.

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.

Acoustic Stimulation↗
PubMed1988

Binaural interaction in brainstem auditory evoked potentials elicited by frequency-specific stimuli.

The frequency specificity of the binaural interaction in brainstem auditory evoked potentials (BAEP) was investigated in ten normal-hearing young adults. A novel stimulus paradigm was devised to reduce the influence of the acoustic reflex (middle ear muscle contraction) on the BAEP, and to minimize the effect of variations in noise level. Sequences of six stimuli (rarefaction clicks or Gaussian-shaped tone pulses with carrier frequencies of 1, 2, 4 and 6 kHz) were periodically presented in the following order: right monaural, left monaural, binaural, left monaural, right monaural, binaural, with an interstimulus interval of 22 ms. Since the sequence of monaural stimuli with binaural stimuli interposed produces a uniform loudness and since the acoustic reflex is a consensual reflex, the relative high stimulus repetition rate (approx. 45/s) causes a muscle contraction which is equal on both sides and rather constant in time. This paradigm turned out to be usable for stimulus intensities as high as 80 dB nHL. The binaural difference potential (BDP) was computed by subtracting the sum of the monaurally (ipsilateral and contralateral) evoked potentials from the binaurally evoked potential. The major binaural interaction occurred in the latency range of BAEP waves V and VI, and there was no evidence of interaction in the earlier portion of the BAEP. Both latency and amplitude of the BDP components were evaluated statistically. The latency of the BDP components - except of the lasted one - showed an almost linear dependence both on stimulus intensity and stimulus frequency. The amplitude grew larger with decreasing frequency, and the visual detection threshold elevated as the stimulus frequency increased. Click stimuli, however, produced the largest amplitudes with lowest visual detection threshold. This novel stimulus paradigm appears to be most suitable for routine clinical investigations since high stimulus intensities can be used.

Brain Stem↗
PubMed1988

Tonotopic organization of the human auditory cortex revealed by transient auditory evoked magnetic fields.

The tonotopic organization of the human auditory cortex has been investigated by systematic measurements of magnetic fields evoked by tone-bursts with carrier frequencies of 250, 500, 1000, 2000 and 4000 Hz. The measured field distribution changes with both time elapsed since stimulus onset and frequency of the stimulus. Nevertheless, the field distribution has always the same overall features and can be approximated by that of an equivalent current dipole located in a semi-infinite volume. This model can be described in terms of 5 parameter values: 3 orthogonal coordinates specifying the dipole location, and amplitude and angle of the dipole moment. The amplitude of the dipole moment is maximal at about 100 msec ('component 100m') and 160 msec ('component 160m') after stimulus onset. The depth estimated for the generator site of the 100m component shows a logarithmic dependence on test frequency whereas no similar behaviour could be observed for the 160m component. Anatomical studies performed in cadaver heads suggest that the equivalent current dipoles of both the 100m and the 160m component are located in the transverse temporal gyri.

Adult↗
PubMed1988

Myocardial Infarction Register in MONICA-Czechoslovakia Centre.

In 1984, 802 coronary events (age 25-64) were registered in six MONICA areas in Czechoslovakia. According to the MONICA study criteria, 63% of these coronary events in males and 51% in females were confirmed as definite myocardial infarction (MI), 26% of events in both males and females as possible MI, 5% of events in males and females were fetal cases with insufficient data, and 6% of events in males and 18% in females were not confirmed as MI (MONICA diagnostic category = 4). Age-standardized annual attack rates of MI per 100,000 population (age 35-64) were 510.4 for males and 99.3 for females. Age-standardized annual incidence rates (first ever MI) per 100,000 population (age 35-64) were 387.4 for males and 89.7 for females. 28-day case fatality was 33.3% in males and 31.5% in females. Of all deaths occurring within 28 days, 75.8% in males and 69.2% in females occurred during the first 24 hours. Of all coronary deaths, 38.5% of males and 64.1% of females died in hospital or other institution.

Adult↗
PubMed1987

Brainstem auditory evoked magnetic fields in response to stimulation with brief tone pulses.

Brainstem auditory evoked magnetic fields evoked by monaural stimulation with Gaussian-modulated tone pulses were recorded in the parieto-occipital region of two normally hearing human subjects. Very small signals with an amplitude of 5-10 fT were detectable only under optimal (with respect to selection of subjects, technique of stimulation, minimization of ambient and instrumental noise level, signal processing, etc.) experimental conditions. The most important findings are that: (a) different components of the magnetic signal show maxima at different recording positions over the scalp, and (b) the magnetic equivalent of the electric wave V exhibits a polarity reversal at symmetrical positions in both hemispheres.

Acoustic Stimulation↗