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Biomedical subjects

V Diekmann

Publications and source records attributed to V Diekmann.

At least 19 recordsLinked to original sources
PubMed2001

Sensor fusion by neural networks using spatially represented information.

A neural network model based on a lateral-inhibition-type feedback layer is analyzed with regard to its capabilities to fuse signals from two different sensors reporting the same event ("multisensory convergence"). The model consists of two processing stages. The input stage holds spatial representations of the sensor signals and transmits them to the second stage where they are fused. If the input signals differ, the model exhibits two different processing modes: with small differences it produces a weighted average of the input signals, whereas with large differences it enters a decision mode where one of the two signals is suppressed. The dynamics of the network can be described by a series of two first-order low-pass filters, whose bandwidth depends nonlinearly on the level of concordance of the input signals. The network reduces sensor noise by means of both its averaging and filtering properties. Hence noise suppression, too, depends on the level of concordance of the inputs. When the network's neurons have internal noise, sensor noise suppression is reduced but still effective as long as the input signals do not differ strongly. The possibility of extending the scheme to three and more inputs is discussed.

Artifacts↗
PubMed1998

Localisation of epileptic foci with electric, magnetic and combined electromagnetic models.

We compare the localisation of epileptic foci by means of (1) EEG, (2) magnetoencephalography (MEG) and (3) combined EEG/MEG data in a group of patients suffering from pharmaco-resistant focal epilepsy. Individual epileptic events were localised by means of a moving dipole model in a 4-shell spherical head approximation. A patient's epileptic activity was summarised by calculating the spatial density distribution (DD) of all localised events, and the centre of gravity of DD was considered the most likely locus of seizure generation. To verify these loci a subgroup of 6 patients was selected, in which seizures could be related to a clearly identifiable lesion in MRI. On average, the combined EEG/MEG approach resulted in the smallest error (1.8 cm distance between calculated locus and the nearest lesion border); using only MEG yielded the largest error (2.4 cm), while EEG resulted in an intermediate value (2.2 cm). In the individual patients, EEG/MEG would also rank intermediate, but never worst. In summary, combining EEG/MEG appears to be a more robust approach to localisation than using only EEG or only MEG. Finally, we also report on the use of the barbiturate methohexital as a safe method of increasing the number of spike events during an EEG/MEG recording session.

Algorithms↗
PubMed1998

Bereitschaftspotential in amyotrophic lateral sclerosis (ALS): lower amplitudes in patients with hyperreflexia (spasticity).

In a pilot study the Bereitschaftspotential (BP) was investigated in 16 patients suffering from amyotrophic lateral sclerosis (mean age 58.6, mean severity of the illness according to Norris ALS score 76.4 points). Comparing the total ALS group (n=16) with matched controls no significant differences in the BP amplitude parameters were found. However, a subgroup of 7 ALS patients with signs of pronounced spasticity (hyperreflexia) differed significantly at the central midline from matched controls and significantly in addition from patients with a lower degree of spasticity. Controls as well as patients with a lower degree of spasticity had significantly higher BP amplitudes at the midline (electrode positions Cz and Pz, P<0.05, H-test). The correlation coefficient between the hyperreflexia Norris score and the various BP parameters for the total ALS sample (n=16) revealed a significant correlation especially over the midline. Stronger signs of spasticity (hyperreflexia) are associated with lower amplitudes of the BP.

Adolescent↗
PubMed1996

Changes in EEG order in neuroleptically treated normal volunteers during a voluntary movement task.

15 normal volunteers were treated over three weeks with haloperidol (HAL) and in the third week additionally with biperidene (BIP). The order of the EEG spectra at different topographical locations and in different frequency bands during a movement task was analyzed using uncertainty analysis (UA), a multivariate analysis technique based on information-theoretical methods. Different patterns of drug-induced changes were found. HAL decreases the theta and alpha band order at the fronto-central lateral areas but increases it at the fronto-central midline in the theta band and at the parietal areas in the alpha band. With the exception of the fronto-central midline locations, BIP more or less counterbalances the effect of HAL. Volunteers felt unwell and had motor disturbances during HAL and felt well again during HAL + BIP. Reaction time values were increased during HAL and normalized during HAL + BIP.

Adult↗
PubMed1994

Changes in functionally determined order processes of the EEG in a motor task due to various dosages of biperidene.

Animal experiments have demonstrated that the cholinergic system plays an important role in the activation of the cerebral cortex in conjunction with motor activities. In order to study the significance of the cholinergic system in the generation of voluntary movements in man, the effects of the anticholinergic drug biperidene on EEG states were analyzed. The effects depend strongly on the dosage and are shown in various frequency bands, topographic loci and time periods. Different functional significances of various frequency bands were found. In the alpha band motoric control processes are seen as an expression of mechanisms which substitute the cholinergic system. A shift of functions from the theta to the delta band with the increase of the medication seems to parallel an enhancement of motivational or volitional effort. The emergence of highly ordered EEG states is seen to be meaningful in view of the underlying processes of voluntary movements.

Adult↗
PubMed1993

First experiences with a multichannel software gradiometer recording normal and tangential components of MEG.

We describe a 28-channel magnetometer that operates as a 22-channel 'software gradiometer'. Gradiometer function is achieved by subtraction of weighted signals representing the noise field along three orthogonal axes as detected by six compensation channels. The instrument measures both normal and tangential field components; the user can select a total of 22 normal and/or tangential measuring sites from two arrays of 16 sites each. First experiences indicate that the combination of a shielded room with a software suppression of the residual field entering the room is amply sufficient to allow biomagnetic measurements in a hospital environment.

Brain Mapping↗
PubMed1993

EEG-blocking before and during voluntary movements: differences between the eyes-closed and the eyes-open condition.

Movement-related EEG periods of 1 s duration were analyzed by fast Fourier transformation in the Bereitschaftspotential (BP) paradigm in 21 healthy, right-handed normals. Spectra of the rest period and the BP period before the onset of the voluntary movement, and of the rest period and the movement period, were compared in order to describe blocking effects before and during movement execution (fast fist closure of the right hand). When comparing the rest period and the movement period with the eyes closed as well as with the eyes open, blocking was pronounced in both conditions within the total frequency range of the alpha-band, especially at positions contralateral to the movement over the motor cortex (C3') and the parietal cortex (P3) as well as at the parietal midline electrode (Pz). Moreover, when comparing the spectra of the rest and the BP periods in the eyes-closed condition, significant differences were again seen, with lower values during the BP period at C3' and P3. We were, however, able to observe over the parietal cortex an increase of alpha power in the 10 Hz range during the BP period with the eyes opened and fixated. This increase during the BP period in the eyes-open condition was not found when the eyes were closed. This paradoxical phenomenon of an increase of power with the eyes opened instead of blocking during the BP period, as observed in the eyes closed, may be due to an additional activation effect on the already activated (blocked) EEG in the eyes-open condition during fixation.

Adult↗
PubMed1992

Spectral analysis of EEG during self-paced movements: differences between untreated schizophrenics and normal controls.

Thirteen untreated schizophrenic patients, among them nine who had never been treated, were compared with a corresponding number of matched normal controls with regard to changes of the spectral composition of the electroencephalogram (EEG) accompanying voluntary movements. Triggered by self-paced movements of the right fingers (fast fist closure), the spectral composition of three epochs was analyzed: (1) rest (2,5-1,5 sec before movement), (2) movement preparation (last sec before movement onset), and (3) movement execution (1st sec following movement onset). For frequencies above 6 Hz, marked differences between schizophrenics and controls were evident, in particular over the parietal electrodes. Whereas patients exhibited a clear decrease of power density during movement as compared to rest, controls showed only a small decrease (left and mid parietal) or virtually none (only right parietal). Consequently there were significant differences over the right parietal area (P4) between patients and controls in the theta, alpha- and beta-bands with regard to the mean power density and center frequencies of these bands. Also at parietal positions, schizophrenics lacked the enhancement of theta-power during the preparatory epoch that was characteristic for normal controls at all parietal positions. The results are discussed with regard to the well-known disturbances of voluntary motor behavior in schizophrenia.

Adolescent↗
PubMed1992

Anticholinergic influences of biperidene on the EEG effects of haloperidol.

15 normal volunteers demonstrated after 14 days of haloperidol (HA) medication (3 mg/day) a higher mean power density (mpd) in all frequency bands of the EEG power spectra combined with a higher theta and a lower alpha center frequency (cf). After further 7 days of HA medication together with the additionally applied anticholinergic biperidene (BI) (6 mg/day) the HA effects were antagonized especially in the theta cf and the alpha mpd. The higher delta and theta mpd were not antagonized. The EEG spectra of the pre- and posttreatment condition did not differ. There was a significant effect of HA on the "Adjective Mood Scale" and on the "Complaint List", which disappeared during the additional BI medication. The reaction times were prolonged with HA and decreased with BI. Blocked dopaminergic impulses on the cholinergic EEG mechanisms possibly produce higher amplitudes during HA medication. Schizophrenic patients treated with neuroleptic drugs showed similar EEG spectra in previous studies during rest but not during motor activation.

Adult↗
PubMed1991

A comparison of normal and tangential magnetic field component measurements in biomagnetic investigations.

Because of the way most available hardware gradiometers are designed and in view of the prediction, by theory, that the normal magnetic field component provides all available information on the intrinsic current source, MEG and MCG measurements generally consider only the field vector normal to the head or truck surface. However, when looking for single events, the information contained in the normal component often cannot be fully sampled, because the sensor array has limited dimensions and therefore covers only a fraction of the field's spatial extension. Simulation of a current dipole in a sphere using realistic parameters shows that there is a considerable area where the amplitude of the tangential field components is larger than that of the normal one. Measurements using a 28-channel magnetometer system with normal and tangential pick-up coils and a current dipole in a phantom model confirm this prediction; depending on dipole orientation, the signal-to-noise ratio (SNR) could improve by a factor of up to 20 if the total field was considered instead of only the normal component. MCG recordings with the same instrument demonstrated a broad area above the heart where the tangential SNR was clearly better than the normal one. Preliminary measurements indicate that tangential components can also be recorded in the MEG; it is suggested that they may help source localisation.

Brain↗
PubMed1990

Slower theta activity over the midfrontal cortex in schizophrenic patients.

Disturbances of the topographical distribution of theta activity in the EEG spectra before and during voluntary movements were investigated in 31 neuroleptic-treated and in 13 untreated schizophrenics and matched controls as well as in 15 normals medicated with haloperidol. All 4 groups demonstrated similar topographical distribution of theta mean power density, with highest values over the midfrontal region. In the center frequency of the theta band, however, treated and untreated schizophrenics had lower values over the midfrontal region than at parietal electrodes. In controls and normals medicated with haloperidol, this frontoparietal "gradient" demonstrated the inverse picture, with highest values at the frontal midline electrode. Patients and controls differed significantly in this gradient. The slower theta activity over the midfrontal cortex in the schizophrenic patients is related to the hypofrontality hypothesis.

Adult↗
PubMed1990

EEG-spectra parameters distinguish pathophysiological and pharmacological influences on the EEG in treated schizophrenics.

Movement related EEG segments of one second duration were analyzed by Fast Fourier Transformation in the Bereitschaftspotential paradigm in 31 neuroleptic treated schizophrenics compared to 21 matched controls. Schizophrenics demonstrated a higher theta mean power density and center frequency together with a lower radius of gyration, skewness and kurtosis in the theta band than controls. The higher theta power in the faster frequency range of the theta band is probably due to neuroleptic medication, but the higher theta power in the lower frequency range may be related to the disease. In the alpha band the schizophrenic group differed from controls by a lower center frequency, higher radius of gyration and skewness. Both groups did not differ in the alpha mean power density. The lower center frequency reflects the fact that alpha activity is distributed towards the lower frequency range which seems to be a consequence of the neuroleptic treatment. The more widespread alpha activity (higher radius of gyration) in schizophrenics indicates less frequency stability, i.e., more desynchronized alpha rhythm which corresponds to a highly activated functional state. By means of the applied spectral parameters more detailed quantitative descriptions of the power distributions in each frequency band are available, allowing to test hypothesis in relation to underlying physiological processes.

Adolescent↗
PubMed1988

Event-related EEG-spectra in a concept formation task.

Event-related spectra of short EEG epochs were investigated in a concept formation paradigm. In this task, subjects had to learn to transform letters into Morse codes. Related to a resting state, cognitive performance in this learning task was characterized (i) by an increased mean power density (MPD) within the Theta frequency band (theta) in recordings over the frontal lobes and (ii) by a reduced Alpha (alpha) MPD in all recording sites. The performance-related increase of the theta-MPD, as obtained in left frontolateral and frontomedial recordings, separated (i) the learning task from an appropriate control and (ii) a group of successful learners from a group which performed less efficiently. MPDs of the alpha- and the delta-(Delta) frequency band did not differ between tasks and groups. A consistent finding in the learning tasks was a temporal dissociation of performance-related alpha-attenuation between parietal and frontal recordings: in the period preceding the presentation of the informative stimuli, the alpha-rhythm is attenuated especially at parietal recordings, whereas in frontal recordings, alpha-attenuation accompanied information processing and response selection.

Adult↗
PubMed1987

Uncertainty analysis of human EEG spectra: a multivariate information theoretical method for the analysis of brain activity.

Short time Fourier power spectra yield activity related data of the EEG. The mean power densities of frequency bands have proved a reliable descriptive parameter of that activity. Due to their distributional properties of these data their analysis by conventional statistics is restricted to nonparametric univariate or at most bivariate tests. In order to account for the many-relational characteristics of EEG data, uncertainty analysis was employed and specific data transformations were developed. This technique was applied to some of our experimental records. The results confirm prior knowledge of the functional diversity of brain areas and demonstrate the general applicability of uncertainty analysis to this problem. In addition, the spatio-temporal coordination of these activities suggests a systems interpretation: These neuronal processes follow the self-organizing principle.

Analysis of Variance↗