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Biomedical subjects

G Curio

Publications and source records attributed to G Curio.

At least 55 records · Page 3Linked to original sources
PubMed1998

Differential recruitment of high frequency wavelets (600 Hz) and primary cortical response (N20) in human median nerve somatosensory evoked potentials.

Human median nerve somatosensory evoked potentials contain a burst of high-frequency (600 Hz) wavelets superimposed on the primary cortical response (N20). These presumably reflect highly-synchronized repetitive thalamic and/or intracortical population spike bursts and are diminished in non-REM sleep with N20 persisting. Here the burst/N20 relation in awake subjects was examined by using eight different intensities of electric median nerve stimuli. In all subjects the amplitude recruitment of both N20 and burst could be modeled adequately as a sigmoidal function of stimulus intensity. While 8/10 subjects showed a parallel recruitment, 2/10 subjects required significantly higher stimulation intensities for burst than for N20 recruitment. This dampened burst recruitment possibly reflects slight vigilance fluctuations in open-eyed awake subjects; a further increase of burst thresholds could explain the burst attenuation when entering shallow sleep.

Adult↗
PubMed1998

fMRI assessment of somatotopy in human Brodmann area 3b by electrical finger stimulation.

Functional magnetic resonance imaging (fMRI) is capable of detecting focal brain activation induced by electrical stimulation of single fingers in human subjects. In eight subjects somatotopic arrangement of the second and fifth finger was found in Brodmann area 3b of the primary somatosensory cortex. In four subjects the representation area of the second finger was located lateral and inferior to the fifth finger; in one subject the somatotopy was reversed. In three subjects representation areas of the two fingers in Brodmann area 3b were found overlapping. Additional activated areas were found on the crown of ipsilateral and contralateral postcentral gyrus (Brodmann areas 1 and 2) and posterior parietal cortex.

Adult↗
PubMed1998

High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system.

Digitally high-pass filtered median nerve SEP show an oscillatory burst of low-amplitude high-frequency (600 Hz) wavelets superimposed on the N20 component which itself is generated by excitatory postsynaptic potentials of area 3b pyramidal cells. Prior studies using magnetoencephalography (MEG) localized one wavelet generator close to the primary somatosensory hand cortex. Since MEG recordings are biased towards tangentially oriented and superficial generators, a dipole source analysis of 32-channel electric SEP recordings was employed here to test for the possibility of deep and/or radially oriented burst generators: in 10 normal subjects low noise (16,000 averages) median nerve SEP were evaluated using dipole source analysis before and after applying a digital 475 Hz high-pass filter. Two main oscillatory 600 Hz burst sources were modeled; (i) a deep burst source close to the thalamus, most active in a time window between the brain-stem P14 and the cortical N20 sources, detectable in 7 of 10 subjects; most probably, this activity originates from deep axon segments of thalamocortical fibers; and (ii) a subsequent burst source timed around the N20 and located in the vicinity of the primary somatosensory hand cortex in all subjects, which was already known from MEG data. This superficial oscillatory source may be dominated by repetitive activity conducted in the terminal segments of the thalamocortical projection fibers initiated by the thalamic burst generator.

Brain Stem↗
PubMed1998

Magnetoneurographic 3D localization of conduction blocks in patients with unilateral S1 root compression.

OBJECTIVES: Tibial nerve somatosensory evoked magnetic fields (tSEFs) over the lower back reflect the propagation of compound action currents along fibers of plexus, nerve roots and cauda equina. One clinical perspective for this 'magnetoneurography' is the non-invasive 3D localization of focal slowing or blocks of conduction. Here, first tSEF mappings in 3 consecutive patients with acute unilateral S1 nerve root compression are reported. METHODS: Right and left tibial nerves were electrostimulated in alternation; tSEF responses were recorded using a multichannel SQUID-detector; additionally, spinal and cortical SEP, F-wave and H-reflex studies were performed. RESULTS: In all patients an intraindividual side-to-side comparison of spinal tSEF mappings was obtained: using a dipolar source model compound action currents could be visualized propagating along plexus, nerve roots and cauda equina on the non-affected side whereas on the affected side normally-propagating dipolar field patterns could be recorded only distal to the spinal transforaminal root entrance; this reflects focal slowing or block of conduction in nerve root fibers as indicated by the SEP, F-wave and H-reflex study results. CONCLUSIONS: With a registration time of 15 min a 3D localization of proximal slowing or block of conduction was successfully performed in patients suffering from acute nerve root lesions.

Cauda Equina↗
PubMed1998

Pictorial pseudohallucinations with an "aperture effect" in a patient with quadrantanopia.

Visual pseudohallucinations are reported on in a patient with a left lower quadrantanopia due to a right parietotemporal surgical defect after tumour removal. Besides metamorphopsia, he hallucinated the lower half of human figures which were limited to within the borders of the anopic defect and appeared "amputated" at the hip with one forearm and hand appearing from above in correct anatomical position. The lower half of these human figures was perceived as correctly scaled in relation to anatomical and environmental coordinates and was recognised as unreal. These pseudohallucinations led to the concept of an "aperture effect" which alludes to the visibility through the anopic field defect of a segment of the contents of the visual association cortex. This supports the idea of a pictorial mode for representation of endogenously generated images in the visual association cortex.

Brain Neoplasms↗
PubMed1997

Somatotopic source arrangement of 600 Hz oscillatory magnetic fields at the human primary somatosensory hand cortex.

Based on low-noise superconducting quantum interference devices (SQUIDs) magnetoencephalography allows the non-invasive detection of low-amplitude high-frequency brain responses evoked about 20 ms after electric hand nerve stimulation. The main spectral energy of these brief oscillatory bursts (near 600 Hz) is in the range typical for rapidly repeated action potentials. Here, the magnetic fields of median and ulnar nerve evoked 600 Hz bursts are shown to exhibit a somatotopic arrangement at the primary somatosensory hand cortex closely resembling that of the concomitant postsynaptic primary cortical response (¿N20m'). Two possible burst generators are discussed: (1) repetitive spike volleys conducted along the terminal segments of somatotopically arranged thalamocortical axons, and (2) early intracortical spike activity in nerve-specific subterritories of the 3b hand area.

Brain Mapping↗
PubMed1997

On the calculation of magnetic fields based on multipole modeling of focal biological current sources.

Spatially restricted biological current distributions, like the primary neuronal response in the human somatosensory cortex evoked by electric nerve stimulation, can be described adequately by a current multipole expansion. Here analytic formulas are derived for computing magnetic fields induced by current multipoles in terms of an nth-order derivative of the dipole field. The required differential operators are given in closed form for arbitrary order. The concept is realized in different forms for an expansion of the scalar as well as the dyadic Green's function, the latter allowing for separation of those multipolar source components that are electrically silent but magnetically detectable. The resulting formulas are generally applicable for current sources embedded in arbitrarily shaped volume conductors. By using neurophysiologically relevant source parameters, examples are provided for a spherical volume conductor with an analytically given dipole field. An analysis of the signal-to-noise ratio for multipole coefficients up to the octapolar term indicates that the lateral extent of cortical current sources can be detected by magnetoencephalographic recordings.

Biophysical Phenomena↗
PubMed1997

Mapping of tibial nerve evoked magnetic fields over the lower spine.

Using a low-noise 49-channel dc-SQUID system spinal somatosensory evoked fields (SEF) were recorded which were generated by compound action currents evoked upon posterior tibial nerve stimulation. The SEF mapping showed the action current propagation along the sciatic nerve, lumbosacral plexus and cauda equina in parallel to simultaneously recorded electrical potentials (SEP). For a reliable intraindividual side-to-side comparison of spinal SEFs the right and left tibial nerves were stimulated in alternating order; this procedure minimizes artifactual inter-nerve SEF map differences due to eventual patient-to-sensor displacements which might occur in serial measurements. These large-area lumbar SEF mappings open up several clinical perspectives for magnetoneurography, in particular with respect to the 3D-localization of proximal conduction blocks.

Adult↗
PubMed1996

Magnetometry of evoked fields from human peripheral nerve, brachial plexus and primary somatosensory cortex using a liquid nitrogen cooled superconducting quantum interference device.

Superconducting Quantum Interference Devices (SQUIDs) can be used to detect neuromagnetic fields evoked in the peripheral and central nervous system. Up to now, such measurements had to be based on SQUIDs with a low critical temperature (Tc) requiring liquid helium cooling. Recent improvements in high-Tc SQUID technology relying on liquid nitrogen cooling led to a significant reduction in the system's noise level. Hare, first high-Tc recordings of weak neuromagnetic fields are demonstrated. In particular, along the entire somatosensory afferent pathway including peripheral nerves, brachial plexus and primary somatosensory neocortex evoked neuromagnetic activities were detected using conventional recording parameters for bandwidth and number of averages. This opens up a wide perspective for cost-effective high-Tc magnetometry in clinical neuroscience.

Brachial Plexus↗
PubMed1994

Localization of evoked neuromagnetic 600 Hz activity in the cerebral somatosensory system.

Upon electrical median nerve stimulation wide-band scalp SEP recordings show a burst of high-frequency low-amplitude wavelets of uncertain origin. Digital high-pass filtering (above 400 Hz) of the primary cortical response ("N20") can separate the burst from the underlying "N20 proper" which itself is known to be generated by excitatory postsynaptic potentials (EPSPs) in area 3b. Here, neuromagnetic multichannel recordings show a close correlation between the spatial field distributions of the magnetic burst and of the magnetic "N20m" proper. It is concluded that somatosensory evoked magnetic high-frequency (600 Hz) wavelets have generators at or near the primary somatosensory cortex. Possible modes of generation comprise repetitive discharges conducted in the terminal segments of thalamocortical axons and postsynaptic contributions from neocortical neurons.

Adult↗
PubMed1993

Non-invasive neuromagnetic monitoring of nerve and muscle injury currents.

Structural damage inflicted on membranes of excitable cells may evoke quasi-DC injury currents driven by the transmembrane resting potential gradient. In contrast to the usually invasive electrophysiological approaches, superconducting quantum interference devices (SQUIDs) measure the concomitant weak biomagnetic fields non-invasively as is shown here for acutely excised rat nerves or muscles. Analysis of the field distributions showed slowly decaying equivalent current dipole moments in the nanoampmeter range as generated by microamp nerve injury currents extending intra-axonally over millimeter distances. The geometric and kinetic parameters of this experimental design may allow in vivo recordings in human patients.

Animals↗
PubMed1991

Exploratory mapping of evoked neuromagnetic activity from human peripheral nerve, brachial plexus and spinal cord.

Upon conventional median nerve stimulation at the wrist early magnetic fields were recorded using a SQUID magnetometer. At the upper arm, mono- and biphasic compound nerve action fields were detected, depending on the subject's distribution of single fiber conduction velocities. At the upper thorax, brachial plexus fields reversed polarity at the level of Erb's point; their distribution was asymmetric, probably due to volume currents. At the upper lateral neck, fields from proximal plexus, spinal cord (P13m) and the primary somatosensory cortex contralateral to the sensor position were detected. The observed P13m field distribution agrees with the electrophysiological concept of a sagittal segmental dorsal horn generator.

Brachial Plexus↗