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Biomedical subjects

H Buchner

Publications and source records attributed to H Buchner.

At least 55 records · Page 3Linked to original sources
PubMed1995

Somatotopy of human hand somatosensory cortex revealed by dipole source analysis of early somatosensory evoked potentials and 3D-NMR tomography.

Somatosensory evoked potentials (SEPs) to median nerve and finger stimulation were analyzed by means of spatio-temporal dipole modelling combined with 3D-NMR tomography in 8 normal subjects. The early SEPs were modelled by 3 equivalent dipoles located in the region of the brain-stem (B) and in the region of the contralateral somatosensory cortex (T and R). Dipole B explained peaks P14 and N18 at the scalp. Dipole T was tangentially oriented and explained the N20-P20, dipole R was radially oriented and modelled the P22. The tangential dipole sources T were located within a distance of 6 mm on the average and all were less than 9 mm from the posterior bank of the central sulcus. In 6 subjects the tangential sources related to finger stimulation arranged along the central sulcus according to the known somatotopy. The radial sources did not show a consistent somatotopic alignment across subjects. We conclude that the combination of dipole source analysis and 3D-NMR tomography is a useful tool for functional localization within the human hand somatosensory cortex.

Adult↗
PubMed1994

The timing of visual evoked potential activity in human area V4.

Studies of human occipital visual cortex have demonstrated functional specializations for colour and for motion, with a pivotal area for colour processing (area V4) being located in the fusiform gyrus. To study the timing of arrival of signals in area V4 we have recorded multi-channel visual evoked potentials (VEPS) to colour and grey 'Mondrian' stimuli, spatio-temporal dipole source analysis being computed on two independent group averages of five and six subjects respectively. Three active brain regions were identified, which we interpret to correspond to areas V1, V2 and V4; they showed sequential but overlapping activity in time with no difference in magnitude between colour and grey stimulated VEPs. Source analysis of the difference potentials, colour minus grey, isolated source activity resulting from colour stimulation and located it in the region of area V4. Activity in area V4 started at 100 ms and peaked at 135 ms after the onset of the visual stimulus.

Adult↗
PubMed1994

Source analysis of median nerve and finger stimulated somatosensory evoked potentials: multichannel simultaneous recording of electric and magnetic fields combined with 3D-MR tomography.

At the current state of technology, multichannel simultaneous recording of combined electric potentials and magnetic fields should constitute the most powerful tool for separation and localization of focal brain activity. We performed an explorative study of multichannel simultaneous electric SEPs and magnetically recorded SEFs. MEG only sees tangentially oriented sources, while EEG signals include the entire activity of the brain. These characteristics were found to be very useful in separating multiple sources with overlap of activity in time. The electrically recorded SEPs were adequately modelled by three equivalent dipoles located: (1) in the region of the brainstem, modelling the P14 peak at the scalp, (2) a tangentially oriented dipole, modelling the N20-P20 and N30-P30 peaks, and part of the P45, and (3) a radially oriented dipole, modelling the P22 peak and part of the P45, both located in the region of the somatosensory cortex. Magnetically recorded SEFs were adequately modelled by a single equivalent dipole, modelling the N20-P20 and N30-P30 peaks, located close to the posterior bank of the central sulcus, in area 3b (mean deviation: 3 mm). The tangential sources in the electrical data were located 6 mm on average from the area 3b. MEG and EEG was able to locate the sources of finger stimulated SEFs in accordance with the somatotopic arrangement along the central fissure. A combined analysis demonstrated that MEG can provide constraints to the orientation and location of sources and helps to stabilize the inverse solution in a multiple-source model of the EEG.

Adult↗
PubMed1994

Preoperative localization of the central sulcus by dipole source analysis of early somatosensory evoked potentials and three-dimensional magnetic resonance imaging.

Surgery of lesions within or close to the central area of the brain always carries the risk of iatrogenic motor or sensory deficits. Functional localization by means of intraoperative direct stimulation of the motor area or by recording somatosensory evoked potentials (SSEP's) from the surface of the somatosensory cortex is believed to reduce the operative risk. The authors introduce the combination of dipole source analysis of scalp-recorded SSEP's with three-dimensional (3-D) magnetic resonance (MR) imaging as a tool for preoperative localization of the central sulcus. This provides information on both functional and structural localization for preoperative planning. Four repeated measurements of right and left median nerve SSEP's were obtained from 20 subjects. Dipole source analysis showed a retest reliability of the 3-D localization error of 2.9 +/- 2.0 mm. Compared to the MR evaluation, dipole source analysis was found to mark the central sulcus within 3 mm for 15 conditions (subjects x side of stimulation), while the 3-D MR measurement was accurate to within 6 mm for 10 conditions and 9 mm for 14 conditions. Dipole locations were confirmed in six patients who underwent surgery of the central region. With respect to this application, dipole source analysis combined with 3-D MR imaging appears to be a valuable tool for preoperative functional localization. The accuracy in localization will be further improved when realistic head models become available that can take into account individual head geometry. Further development of the proposed new method holds promise that evoked potentials and electroencephalography will gain greater use in presurgical functional localization.

Brain Diseases↗
PubMed1993

Facilitation of somatosensory evoked potentials by exploratory finger movements.

Modification of somatosensory processing depending on the behavioral setting was studied. Active alternating movements of the fingers, passive tactile stimuli to the hand, and active exploration of objects were performed during recording of somatosensory evoked potentials (SEPs). SEPs were elicited by compound electrical median nerve stimulation and electrical stimulation at detection threshold of cutaneous median nerve fascicles identified by microneurography. Electrical stimulation was not time-locked to the studied condition. In comparison with SEPs at rest there was attenuation of early cortical potentials up to 25 ms post-trigger in all nonresting conditions. In stimulation of the compound median nerve as well as of isolated cutaneous fascicles of a hand actively exploring an object there was an additional increased negativity, peaking at 28 ms. This facilitory effect was independent of attentional focusing and was absent during exploration using the ipsilateral, non-electrically stimulated hand. In patients with parietal lesions the facilitatory effect was diminished on the affected side. Spline interpolated brain maps at this latency based on 32-channel recordings in healthy volunteers showed a shift of local contralateral positive maximum from frontal to parietal during exploration, indicating enhancement of a tangential dipole. It is suggested that in conditions involving close sensorimotor interaction such as exploratory hand movements there is preactivation of a cortical area which is located in the central sulcus and receives cutaneous somatosensory inputs.

Adult↗
PubMed1993

Somatosensory evoked potentials and magnetic fields: separation of multiple source activities.

Median nerve somatosensory evoked potentials (SEP) and magnetic fields (SEF) were recorded in two subjects with multichannel (32 SEP, 24 SEF channels) devices in Aachen and Helsinki. Single-moving- and multiple-stationary-dipole models were compared with the brain electric source analysis (BESA) program of Scherg. Subcortical sources, reflecting the afferent neural volley when entering the brainstem and leaving the thalamus, were found only in the SEP. The analysis of SEF and SEP revealed a minimum of four overlapping source activities in the region of the contralateral post-and precentral cortical projection areas. Two sources in the depth of the central sulcus could not be resolved unambiguously. The third, more superficial source, which probably reflects activation of area 1, was better defined in the source analysis of the SEP, because dipole orientation was close to radial. The fourth source was more posterior. Its initial activity around 30 ms was seen consistently in SEP and SEF. Several problems observed in the analysis of the present MEG and EEG data suggest that the simultaneous measurement and analysis of multichannel EEG and MEG data will substantially increase spatio-temporal resolution.

Adult↗
PubMed1992

[Pre- and postoperative NMR tomographic studies in avascular femur head necrosis].

46 patients with avascular necrosis of the femoral head were examined by T2- and T1-weighted MR before and after infusion of Gd-DTPA. Both sides were involved in 12 cases. The classification was done according to Ficat. In early stages and in postoperative studies a correlation of signal intensity after infusion of Gd-DTPA and clinical symptoms was found. Hyperintensity of the avascular area or of the implanted material was assumed to be vascularised or vital components in 4 cases of Ficat I and in 15 postoperative studies. Contrariwise, we found in 5 patients with severe postoperative symptoms, low signal intensity areas due to avascular regions.

Contrast Media↗
PubMed1992

Transcranial magnetic stimulation in pontine infarction: correlation to degree of paresis.

Transcranial magnetic stimulation was performed in 20 patients with pontine infarction who had initially some degree of hemiparesis. Only patients with a well defined lesion on magnetic resonance imaging that was appropriate for the neurological signs were included. Recordings were made from the abductor pollicis brevis muscle (APB) bilaterally. The degree of hand paresis was estimated clinically and related to the following parameters: central motor conduction time (CMCT), interside latency difference of total latency, and amplitude ratio of affected to unaffected side. Increasing degree of paresis was associated with increasing latency parameters and decreasing amplitude ratio. In the four patients with severe paresis a low amplitude response could be evoked and CMCT was delayed by up to 10 ms. When the paresis had resolved at the time of transcranial magnetic stimulation CMCT was normal. However, amplitude ratio was less than 100% in all but one patient, with most of the values ranging between 40% and 60%, which indicates a subclinical pyramidal tract lesion. Median nerve sensory evoked potentials (SEP) and related interside latency difference to amplitude ratio N20/P25 were also recorded. In contrast to TCMS, decreased amplitude ratio of SEP was not associated with delayed latency. Clinically, the mild degree of and good recovery from paresis in ventral pontine infarction was remarkable.

Adult↗
PubMed1992

[Topography of early cortical median nerve somatosensory evoked potentials: results for routine use of the method].

The results from a topographic analysis of the early median nerve SEPs allowed to design a method for recording and analysing the SEPs in the routine laboratory. The detailed analysis of the topography of 50 normal subjects revealed: 1. An inter- and intraindividual variability of the location of the maximal amplitudes, 2. A significantly longer latency of the cortical potentials after left side stimulation, 3. A significantly higher amplitude of N20 after left side stimulation and 4. A significantly higher amplitude of the later potentials P25 and N30 after right side stimulation. It was shown that a 4-channel recording from the neck at C7 with a Fz-reference as well as from stimulus contralateral F3, CP3 and P3 or F4, CP4 and P4 with an stimulus contralateral earlobe reference provides all necessary parameters in comparison to an 20-channel recording. The definition of normal values has to take into account these results. Absolute maximum values were taken instead of the standard deviation because all amplitude values were proven to be not distributed normally. In 7 out of 30 MCA-stroke patients pathological SEP amplitudes were obtained using the 4-channel montage, whereas the 1-channel recording from CP3 and CP4 with a Fz-reference revealed normal amplitudes.

Adult↗
PubMed1992

[Spatial distribution of the action potentials of the sural nerve].

We investigated the spatial distribution of the sural nerve sensory nerve action potential (SNAP) in 25 healthy subjects between 21 and 50 years. Stimulation was achieved through surface electrodes at the lateral malleolus. Recordings were made 15-18 cm proximal to the site of the stimulation from different positions on a line perpendicular to the sural nerve using needle electrodes insulated except for the tip. The amplitude of the SNAP decreased on both sides of the potential of highest amplitude. 10 mm lateral to the potential of highest amplitude the mean amplitude was reduced to 55%. The latency of the first positive phase decreased within increasing distance from the largest SNAP. We explained this with different influence of more distal, earlier depolarized nerve segments on the locally generated SNAP. In 20 subjects we performed an additional recording using an uninsulated needle electrode that was placed 3-4 cm subcutaneously perpendicular to the sural nerve. The latency of the SNAP recorded in this way was similar to the latency of the largest SNAP using the insulated needle electrode; however, the amplitude was smaller by 19%. We recommend for clinical practice to use the uninsulated needle in case the SNAP is smaller than 4 microV.

Action Potentials↗
PubMed1992

[Excitability of the blink reflex during self-elicitation or elicitation by others].

Electrically evoked blink reflexes were studied in 24 healthy subjects in different conditions of elicitation. In one condition the electrical shock was delivered by the experimenter; in a second condition the subject triggered the stimulus himself by manually operating a switch. The different conditions were investigated relaxed or clenching the fist with left hand. Moreover, in 3 subjects the stimulus was given in various delays after the occurrence of the EMG activity of the forearm flexor muscles due to a ballistic wrist flexion. The latencies and peak to peak amplitudes of the ipsi- and contralateral early and late responses were analysed. Voluntary sustained contraction of the left hand (fist) caused no significant modification of the blink reflex. Self-triggering of the stimulus had a facilitating influence on the early components (R1, R1') and an inhibitory effect on the late components (R2, R2'). In EMG triggered self-stimulation the R2-inhibition could already be seen at a delay of 0 ms. From a delay of 10 ms on the suppression was still more pronounced and recovered over the following 500-1000 ms. We conclude, that the reflex alterations are not due to the outflow of the motor cortex, but to the conditioning effect of self-elicitation. The influence on the excitability of the blink reflex resemble that described in the literature for the conditioning effect of an acoustic or visual stimulus. We suggest involvement of the same interneuron network on the condition of self-eliciting.

Adult↗
PubMed1992

High frequency vibration induced gating of subcortical and cortical median nerve somatosensory evoked potentials: different effects on the cervical N13 and on the P13 and P14 far-field SEP components.

Subcortical and cortical somatosensory evoked potentials (SEP) to median nerve stimulation were recorded before, during and after high frequency (270 Hz) vibration of the fingers 1-3 in 8 healthy subjects. A marked decrease of the amplitude of all potentials was observed. The attenuation of the sensory nerve action potential (SNAP) of the median nerve and the attenuation of SEP components N9, N11 and N13 showed no differences, while the attenuation of the subcortical P14 component was significantly higher. This is in accordance with a generator of the cervical N13 in the interneurons beside the lemniscal pathway. The cortical N20 (post-rolandic) was significantly more decreased in amplitude than P14 while P22 (pre-rolandic) remained reduced in amplitude like P14. An increased latency of the far-field subcortical P14 was observed, while P13 recorded in the same montage remained unchanged in latency. These findings suggest different generators of these peaks. A generator of P14 above the nucleus cuneatus is confirmed. A presynaptic generator of P13 is suspected.

Action Potentials↗
PubMed1991

Short-term memory performance with magnetic stimulation of the motor cortex.

Whether transcranial magnetic stimulation of the motor cortex has an influence on memory was investigated. In a first experiment with 21 healthy volunteers six pronounceable nonsense words were visually presented, immediately followed by a magnetic stimulus. There were three blocks of stimulation with field intensities of 60, 80 and 100% (referring to a maximal intensity of 2 Tesla), each block comprising six magnetic stimuli and six nonsense words. After each block there was a free recall test and at the end another free recall trial as well as a multiple-choice recognition test for all 18 words. Eighteen subjects served as controls, undergoing the same procedure, except that the field intensity was zero. A significant but small reduction of short-term memory performance was observed only for 100% field intensity. In a second experiment with 16 subjects who had not participated in experiment I, the effect of 100% intensity cortical magnetic stimulation was compared with a control stimulation over the cervical spine. There was no difference in free recall or in the multiple-choice test between the sites of stimulation, suggesting that the difference in the 100% intensity block in experiment I was not due to a specific cortical effect of the magnetic field on memory function. With respect to the effect on memory functions, transcranial magnetic stimulation of the motor cortex is thought to be a safe method.

Adult↗
PubMed1991

[Clinical aspects of acute lesions of the brain stem of inflammatory origin].

The diagnostic considerations of an inflammatory brainstem disease and its symptoms are shown in 22 cases. The diagnosis based on a topodiagnostic decision and the demonstration of an inflammatory genesis. The topodiagnosis has to take into consideration that a symptom can be caused by both a central lesion as well as a peripheral nerve lesion. Electrophysiological methods (EEG, Nerve conduction velocity, reflex studies and evoked potentials) were of only minor use in these decisions. The somatosensory evoked potentials demonstrated 5 central lesions and were far more useful than the other methods. The CCT demonstrated a brainstem lesion in one case. In fifty percent of the cases, symptoms caused by both peripheral and central nerve lesions were demonstrated. Thus, there is no clear border between a brainstem encephalitis and a peripheral neuropathy (Fisher-Syndrome or Guillain-Barré-Syndrome). The inflammatory genesis was proven by CSF in 12 cases. An inflammatory disease was supported in the other cases through the exclusion of another genesis by means of CCT, NMR, Doppler sonography, angiography and an observation of the course of the illness.

Adolescent↗
PubMed1991

[Spinal and subcortical somatosensory evoked potentials: a comparison with the localization of spinal, medullary and pontine lesions and in brain death].

The spinal and subcortical median nerve SEPs were recorded in 65 patients with lesions of the cervical cord, medulla oblongata, pons and in brain death. A recording technique including cephalic, non-cephalic and anterior neck referenced leads was used. The location of the lesions corresponded to different types of SEP alterations: Cervical extramedullary lesions compressing the spinal cord corresponded to a prolonged P9-P14 interpeak latency. Cervical intramedullary lesions corresponded to the loss of N13, normally generated in the spinal interneurons. Vascular lesions of the medulla oblongata (Wallenberg's syndrome) showed normal SEPs. Space occupying medulla oblongata lesions corresponded to reduction in amplitude or loss of P14. Pontine lesions showed normal spinal and subcortical SEPs. In brain death P14 showed a graduate decrease in amplitude or alternatively vanished abruptly. The spinal and subcortical SEPs provide a good tool for testing the function of the lemniscal pathways and the spinal interneurons.

Brain Death↗
PubMed1991

[Analysis of the generators of early cortical somatosensory evoked potentials (N. medianus) using dipole source analysis: initial results].

There is still much controversy about the contribution of the brainstem, the thalamus and the somatosensory and motor areas of the various scalp recorded peaks of the somatosensory evoked potentials (SEP) after median nerve stimulation. This study addressed the generator problem of the scalp recorded potentials using brain electric source analysis. In 11 normal subjects median nerve SEPs were recorded from 32 locations. The brain-electric-source-analysis revealed a minimum of 5 sources with overlapping activities in the interval of 12-35 ms post stimulus. The initial deflections were in the time range of the scalp peaks (P14, P18, N20, P22, N30), but there was no single source to fully explain a scalp peak except for the brainstem source of P14. The other sources appeared to reflect activities of the thalamo-cortical-pathway (P18), of the somatosensory areas 3b (N20) and 1 (P22) and of a fifth source (contribution maximal around 30 ms) with no consistent location. The close location of multiple sources makes the precise separation and localisation of the various sources quite difficult in individual data sets.

Electroencephalography↗
PubMed1991

[Evoked potentials in diagnosis of ischemic brain stem lesions].

We present an overview on the impact of evoked potentials in diagnosis of ischemic brainstem lesions. Brainstem auditory evoked potentials and somatosensory evoked potentials depict abnormalities, whereas visual evoked potentials are normal in most cases. In patients with basilar artery thrombosis and primary pontine hemorrhage these evoked potentials are mostly abnormal and can indicate the location of the lesion. Furthermore, they are of prognostic value. Whether evoked potentials are abnormal in strokes with branch occlusion of the basilar artery, depends on the location of the infarction. In infarctions of the basis pontis BAEP and SEP may be normal. Hemiparesis in brainstem strokes are associated with abnormalities in transcranial magnetic stimulation. However, no further evaluation of the level of the pyramidal tract lesion is possible by this method. During fibrinolytic therapy of basilar artery thrombosis a continuous monitoring is possible by means of BAEP. Thus, information can be obtained that is not available from neurological examination of the patient under sedative drugs.

Brain Ischemia↗