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M Scherg

Publications and source records attributed to M Scherg.

At least 19 recordsLinked to original sources
PubMed1999

Sustained attention modulates the immediate effect of de-afferentation on the cortical representation of the digits: source localization of somatosensory evoked potentials in humans.

Long-term cortical reorganization of the somatotopic arrangement of the digits after alterations of the peripheral input is well established. Studies on the immediate effects of manipulating peripheral input have shown conflicting results indicating that additional factors might modulate cortical reorganization. We present a source localization study using somatosensory evoked potentials (SEP) following electric stimulation of digits one and five before and during anaesthesia of digits two, three and four in 10 normal volunteers. When attention was directed to a stimulus at the dorsal hand, the 3D-distance between digits one and five decreased during as compared to before anaesthesia. In contrast, this distance enlarged when subjects were not attending a particular stimulus. In this condition most subjects focused their attention on the clear sensation of the de-afferented hand region. These results indicate that attention modulates the effect of immediate cortical reorganization of the hand area during partial deafferentation. As an hypothesis: it may be speculated that the sensation of the de-afferentation results in increased synchronized activity of the de-afferented somatosensory cortex and, thus, to its enlarged representation. Conversely, if attention is directed to a different hand region, the representations of the neighboring digits may expand into the de-afferented cortex.

Adult↗
PubMed1999

Common spatial subspace decomposition applied to analysis of brain responses under multiple task conditions: a simulation study.

A method, called common spatial subspace decomposition, is presented which can extract signal components specific to one condition from multiple magnetoencephalography/electroencephalography data sets of multiple task conditions. Signal matrices or covariance matrices are decomposed using spatial factors common to multiple conditions. The spatial factors and corresponding spatial filters are then dissociated into specific and common parts, according to the common spatial subspace which exists among the data sets. Finally, the specific signal components are extracted using the corresponding spatial filters and spatial factors. The relationship between this decomposition and spatio-temporal source models is described in this paper. Computer simulations suggest that this method can facilitate the analysis of brain responses under multiple task conditions and merits further application.

Brain↗
PubMed1999

Deconvolution of 40 Hz steady-state fields reveals two overlapping source activities of the human auditory cortex.

Steady-state auditory evoked fields were recorded from 15 subjects using a whole head MEG system. Stimuli were 800 ms trains of binaural clicks with constant stimulus onset asynchrony (SOA). Seven different SOA settings (19, 21, 23, 25, 27, 29 and 31 ms) were used to give click rates near 40 Hz. Transient responses to each click were reconstructed using a new algorithm that deconvoluted the averaged responses to the different trains. Spatio-temporal multiple dipole modelling in relation to 3D MRI scans revealed two overlapping source components in both the left and right auditory cortex. The primary sources in the medial part of Heschl's gyrus exhibited a N19-P30-N40 m pattern. The secondary, weaker sources at more lateral sites on Heschl's gyrus showed a N24-P36-N46 m pattern. When applied to transient middle latency auditory evoked fields (MAEFs) recorded at SOAs of 95-135 ms, the primary sources imaged activities similar to the deconvoluted steady-state responses, but the secondary source activities were inconsistent. Linear summation of the deconvoluted source waveforms accounted for more than 96% of the steady-state variance. This indicates that the primary activity of the auditory cortex remains constant up to high stimulation rates and is not specifically enhanced around 40 Hz.

Acoustic Stimulation↗
PubMed1999

Stability of high-frequency (600 Hz) components in human somatosensory evoked potentials under variation of stimulus rate--evidence for a thalamic origin.

The generators of spike-like high-frequency (600 Hz) wavelets superimposed on the primary cortical response (N20) in human median nerve somatosensory evoked potentials (SEP) have been localized anatomically both close to the primary somatosensory hand cortex and in deep axon segments of thalamo-cortical projection neurons. Here, N20 and 600 Hz burst components were functionally dissociated by varying the stimulus rate (1.5, 3, 6, 9 Hz). The N20 source amplitudes were significantly reduced at the higher stimulus rates. In contrast, the source amplitudes of the 600 Hz oscillations remained stable across all stimulus rates. This reflects different source origins, confirming a postsynaptic intracortical generation of the N20 component and provides further evidence for a presynaptic origin of the 600 Hz activity like repetitive neuronal population spikes conducted in deep and superficial segments of thalamo-cortical projection fibers.

Brain↗
PubMed1999

Multiple source analysis of interictal spikes: goals, requirements, and clinical value.

When evaluating interictal spikes using dipole source analysis it is important to account for multiple sources and the overlapping background EEG. Analyses of spike peaks may be modeling only propagated sources. Careful filtering of averaged spike data and multiple source analysis can provide useful information about the onset of epileptiform activity. A forward high-pass filter can help to enhance the initial spike activity during onset over the propagated activity. These points are illustrated with examples of a temporal, a parietal, and a frontal averaged spike. Multiple source analysis was applied using a genetic algorithm and a sequential strategy, in one case including a model of background alpha activity. Multiple source analysis could model sources describing the onset activity that were distinct in location and orientation from the propagated activity. In all cases, the prominent peak on the scalp was dominated by the contribution of propagated sources. Clinical interpretation benefits from an approach that combines the temporal evolution of EEG scalp topography and multiple source activities with the information from localization and orientation of equivalent dipole sources to identify the cortical generators underlying the earliest phase of interictal spikes.

Action Potentials↗
PubMed1998

The time course of brain activations during response inhibition: evidence from event-related potentials in a go/no go task.

The cortical organization of executive control was investigated using event-related potentials (ERPs). ERPs were collected while subjects performed a go/no go task that required response inhibition. First, around 260 ms after stimulus onset, an effect of response inhibition on ERPs was observed over inferior prefrontal areas. Generators in these regions were confirmed by source analysis. Later, between 300-600 ms after stimulus onset, a left lateralized fronto-central ERP effect was found which differed in topography from a non-specific effect of task difficulty. Source analysis indicated that generators in anterior cingulate and left premotor areas also contributed to this effect. Orchestrated activation of prefrontal areas and the anterior cingulate subserves executive function whereas relatively late activity of the left premotor cortex is involved in motor control.

Adult↗
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↗
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↗
PubMed1995

Event-related potentials and the categorical perception of speech sounds.

OBJECTIVE: To determine whether there are physiological correlates of categorical perception. DESIGN: Human evoked potentials were recorded in response to computer-modified speech sounds from a nine-stimulus continuum between /ba/ and /da/. In the first experiment, subjects listened to trains composed of 52% /ba/ or /da/ and 6% of each of the other eight stimuli and classified the stimuli as "ba" or "da." In the second experiment, subjects read a book and ignored trains containing a standard stimulus (p = 80%) and two deviant speech sounds (p = 10% each), one within the same category as the standard and the other across the category-boundary. The third experiment was similar to the first except that the subject was reading. The fourth experiment compared the responses to stimuli that deviated from standards in terms of their phonemic category or intensity. RESULTS: An N2-P3 complex was evoked by those stimuli in the more improbable category when the stimuli were attended to in the first experiment. In the second and third experiments, there was a clear mismatch negativity (MMN) for the across-category deviant stimuli when the standard stimulus came from the /ba/ end of the continuum. However, when the standard stimulus came from the /da/ end of the continuum, there was no definite MMN. The overall frequency-content of our /da/ stimulus was broader than that of the /ba/ stimulus. A deviant stimulus from the /da/ end of the continuum thus contained frequencies which were not present in the /ba/-standard stimuli and these frequencies could elicit a MMN. In the fourth experiment the MMN evoked by a small change in intensity was much larger than that evoked by a change in phonemic category. CONCLUSIONS: The N2-P3 complex accurately reflects the phonemic categorization of speech stimuli. The MMN evoked by changes in speech sounds may indicate the detection of acoustic rather than phonetic changes.

Adult↗
PubMed1994

Combined spatial and temporal imaging of brain activity during visual selective attention in humans.

Visual-spatial attention is an essential brain function that enables us to select and preferentially process high priority information in the visual fields. Several brain areas have been shown to participate in the control of spatial attention in humans, but little is known about the underlying selection mechanisms. Non-invasive scalp recordings of event-related potentials (e.r.ps) in humans have shown that attended visual stimuli are preferentially selected as early as 80-90 ms after stimulus onset, but current e.r.p. methods do not permit a precise localization of the participating cortical areas. In this study we combined neuroimaging (positron emission tomography) with e.r.p. recording in order to describe both the cortical anatomy and time course of attentional selection processes. Together these methods showed that visual inputs from attended locations receive enhanced processing in the extrastriate cortex (fusiform gyrus) at 80-130 ms after stimulus onset. These findings reinforce early selection models of attention.

Attention↗
PubMed1994

A multiple source approach to the correction of eye artifacts.

Previously published methods correct eye artifacts by subtracting proportions of the EOG from EEG electrodes. The implicit assumption made by these methods is that the EOG signals are a good measure of eye activity and contain no EEG. In this paper a new multiple source eye correction (MSEC) method of eye artifact treatment based on multiple source analysis is presented, which incorporates a model of brain activity. An accurate, head model-independent estimate of the spatial distribution of eye activity can be obtained empirically from calibration data containing systematic eye movements and blinks. Using the resulting spatial vectors together with the brain model, eye activity in EEG and event-related response data can be estimated in the presence of overlapping brain activity and corrected. A consequence of the MSEC approach is that data at EOG electrodes can be included in analyses of brain activity. In addition, by suitable selection of the spatial vectors, the eye activity can be split into signals which identify vertical and horizontal movements and eyeblinks. Using auditory ERP data sets with and without large eye artifacts, the MSEC method is compared with a "traditional" method in which brain activity is not modelled, particularly with respect to the spatial distribution of the corrected EEG. Traditional eye correction methods are shown to alter the spatial distribution of the EEG, resulting, for example, in changes in location and orientation of modelled equivalent sources. Such distortion is much reduced in the MSEC method, thus enhancing the precision of topographical EEG analyses.

Acoustic Stimulation↗
PubMed1994

A fast method for forward computation of multiple-shell spherical head models.

Using a combination of 3 suitably located dipoles in a homogeneous sphere, the scalp potential due to a dipole source in a 4-shell spherical head model can be approximated with a high degree of precision and a more than 30-fold increase in computing speed. Magnitudes and locations of the 3 equivalent dipoles can be fitted in a homogeneous sphere to data generated from a source at one location in a 4-shell head model. The resulting parameters are used to compute scalp potentials for sources at other locations and orientations. Residual variance measures showed close agreement between the new approximation and a standard 4-shell computation method. Further tests of the method used scalp data from 500 randomly selected pairs of sources generated by the standard 4-shell computation and fitted using, for forward computations, the new approximation and the single-shell Ary-corrected head model. Errors with the new approximation were marginally larger than with the standard computation, but sources were located within 0.5 mm and 0.6 degrees of the original position in 99% of the fits. 99% error limits for the Ary model were up to 18 mm and 25 degrees and depended on the head model parameters.

Brain↗
PubMed1994

Brain source imaging of focal and multifocal epileptiform EEG activity.

Brain electric source analysis (BESA) of the scalp EEG has been used to identify multiple equivalent current sources in the brain during during interictal spikes and seizure onset. To obviate the need for fitting dipole sources to every EEG segment, a new method has been developed on the basis of multiple fixed dipoles, each designed to emphasize functional imaging of particular cortical areas. "FOCUS" can quickly display EEG in various montages including new "sources montages" which provide a high sensitivity for source currents near each dipole while largely suppressing contributions from other brain areas. By comparing this "source EEG" to routine digital EEG in patients with complex partial epilepsy, we have observed that "FOCUS" can more readily determine whether an epileptiform discharge is consistent with a discrete or multifocal generator, characterize likely cerebral source(s), differentiate between spikes and seizures of mesio-basal versus lateral temporal or frontal origin, and estimate the presence and direction of propagation from source potential timing differences. Improved non-invasive EEG evaluations of partial epilepsy will undoubtedly result from this advance.

Brain↗
PubMed1994

From EEG source localization to source imaging.

A new functional imaging technique, "FOCUS", has been developed to transform the traditional scalp EEG into an image of source activities. The image is based on multiple spatio-temporal dipole models and consists of gross spatial patterns and source waveforms reflecting the estimated activities of the different brain regions. The application of the 'FOCUS' technique to the EEG in temporal lobe epilepsy revealed the presence of different activities at the basal and lateral aspects of the temporal lobe. The source waveforms showed propagation patterns consistent with subdural recordings which were not recognizable in the scalp EEG.

Brain↗
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↗