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M Scherg

Publications and source records attributed to M Scherg.

At least 19 recordsLinked to original sources
PubMed2001

Is frontal lobe involved in the generation of auditory evoked P50?

This study examined the functional substrate of P50 suppression. Auditory evoked potentials (AEPs) and magnetic fields (AEFs) were recorded from healthy subjects simultaneously and analyzed using spatio-temporal source analysis. The resulting equivalent dipole model for the AEP consisted of one source in the auditory cortex (AC) of each hemisphere and an radially oriented medial frontal source, both with maximum AEP activity around 50 ms. The frontal source was functionally separated from the AC sources since it peaked significantly later and showed significantly larger P50 amplitude suppression. P30m showed neither suppression nor substantial frontal activity. In sum, this study relates P50 suppression to reduction of AC source activity and is the first to yield direct evidence for frontal involvement in P50 suppression.

Acoustic Stimulation↗
PubMed2001

Interaction of tactile input in the human primary and secondary somatosensory cortex--a magnetoencephalographic study.

Interaction of simultaneous tactile input at two finger sites in primary (SI) and secondary somatosensory cortex (SII) was studied by whole-head magnetoencephalography. Short pressure pulses were delivered to fingers of the right and left hand at an interstimulus interval of 1.6 s. The first phalanx of the left digit 1 and four other sites were stimulated either separately or simultaneously. We compared four sites with increasing distance: the second phalanx of left digit 1, left digit 5, and digits 1 and 5 of the right hand. The temporal evolution of source activity in the contralateral SI and bilateral SII was calculated using spatiotemporal source analysis. Interaction was assessed by comparing the source activity during simultaneous stimulation with the sum of the source activities elicited by separate stimulation. Significant suppressive interaction was observed in contralateral SI only for stimuli at the same hand, decreasing with distance. In SII, all digits of the same and the opposite hand interacted significantly with left digit 1. When stimulating bilaterally, SII source waveforms closely resembled the time course of the response to separate stimulation of the opposite hand. Thus, in bilateral simultaneous stimulation, the contralateral input arriving first in SII appeared to inhibit the later ipsilateral input. Similarly, the separate response to input at two unilateral finger sites which arrived slightly earlier in SII dominated the simultaneous response. Our results confirm previous findings of considerable overlap in the cortical hand representation in SII and illustrate hemispheric specialization to contralateral input when simultaneous stimuli occur bilaterally.

Adult↗
PubMed2000

Fast temporal interactions in human auditory cortex.

The temporal resolution of the human primary auditory cortex (AC) was studied using middle-latency evoked fields. Paired sounds with either the same or different spectral characteristics were presented with gaps between the sounds of 1, 4, 8 and 14 ms. Spatio-temporal modelling showed (1) that the response to the second sound was recognizable with gaps of 1 ms and rapidly increased in amplitude with increasing gap durations, (2) an enhanced N40m amplitude at gaps > 4 ms, (3) delayed N19m-P30m latencies when the stimuli were different. The median psychoacoustical thresholds were 1.6 ms for the same stimuli and 2.5 ms for different stimuli, confirming the electrophysiological evidence for rapid pattern-specific temporal processing in human primary auditory cortex.

Acoustic Stimulation↗
PubMed2000

Magnetic source imaging of tactile input shows task-independent attention effects in SII.

We investigated whether attention to different stimulus attributes (location, intensity) has different effects on the activity of the secondary (SII) somatosensory cortex. Tactile stimuli were applied to the left index finger and somatosensory evoked fields (SEFs) were recorded using a whole-head magnetoencephalography (MEG) system. Two oddball paradigms with stimuli varying in location or intensity were performed in an ignore and an attend condition. Brain sources were estimated by magnetic source imaging. No attention effect was observed for the primary SI area. However, attention enhanced SII activity bilaterally from 55 to 130 ms by 52% in the spatial and 64% in the intensity discrimination task. SII attentional enhancement was very similar in both paradigms and occurred both for deviants and standards.

Adult↗
PubMed2000

The correction of ocular artifacts: a topographic perspective.

OBJECTIVE: To evaluate the scalp topography of the potentials related to saccades and blinks. METHODS: The scalp topographies of the potentials associated with saccades and blinks were recorded in 60 subjects. The topographies were analyzed using both source components and attenuation factors, with each factor representing the fraction of the potential recorded in peri-ocular electrodes that contributes to the EEG recorded from a particular scalp location. RESULTS: Blinks and upward saccades generated potentials with very different topographies. Left and right saccades and up and down saccades generated equal but inverted fields except at peri-ocular locations where subtle inequalities occurred. The potentials associated with lateral saccades were consistently larger in female subjects than in male subjects. CONCLUSIONS: The differences in the scalp topographies between blinks and vertical saccades can be explained by the different ways in which they are generated. Blink potentials are caused by the eyelids sliding down over the positively charged cornea, whereas saccade potentials are caused by changes in the orientation of the corneoretinal dipole. Any compensation procedure for ocular artifacts must take into account the topographic differences between blinks and upward saccades.

Adolescent↗
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↗