Reproducibility and validity of electric source localisation with high-resolution electroencephalography

The present study investigates the reproducibility and validity of the EEG source localisation of somatosensory evoked potentials (SEPs) using high-resolution EEG (61 scalp electrodes) and a source reconstruction on the basis of the individual brain morphology as obtained from magnetic resonance images (MRIs). The somatosensory evoked potentials (SEPs) to electrical stimulation of the right median nerve were repeatedly collected from the scalp of one healthy subject in 9 replications run on 9 different days. The source reconstruction for the 19 ms SEP component was performed by using a single moving dipole model as a source model. Two different head models were used: a spherical 3 shell model and a more realistically shaped 3 compartment model computed using the boundary element method (BEM). The source locations of the 19 ms SEP component were found to be highly reproducible using both head models: the mean standard deviation of the dipole locations was found to be 2.6 mm for the 3 shell model and 4 mm for the more realistically shaped head model. By projection into the individual MRI, the dipoles resulting from either head models were found to be located within the postcentral gyrus. The electric source locations were consistent with the maximum of the task-specific changes seen in a functional magnetic resonance imaging (fMRI) experiment when using the same somatosensory stimulation protocol.

The bereitschaftspotential paradigm in investigating voluntary movement organization in humans using magnetoencephalography (MEG)

In 1965, Kornhuber and Deecke first described the bereitschaftspotential (BP), a paradigm for investigating the organization of voluntary movement in humans, using electroencephalography (EEG). This paradigm has since been used in many studies for investigating motor control in healthy humans and patients. Over the last years, the advantages of magnetoencephalography (MEG) have been applied to the BP paradigm by a number of researchers. The main advantage of magnetoencephalography over electroencephalography is that MEG has a higher localization accuracy. This is due to the fact that the different structures of the head (brain, liquor cerebrospinalis, skull and scalp) influence the magnetic fields less than the volume current flow that causes the EEG. Additionally, the MEG is reference free, so that the localization of sources with a given precision is easier for MEG than it is for EEG. The present protocol shows in detail how the bereitschaftspotential paradigm can be applied using MEG. Some additional paradigms for investigating motor plasticity, somatosensory gating, Parkinson disease, and the efference copy theory are suggested as well.

Slow potentials, event-related potentials, "gamma-band" activity, and motor responses during aversive conditioning in humans

We examined slow potentials, transient event-related potentials, and oscillatory-like responses in the electroencephalogram during aversive conditioning in humans, in order to determine what is happening in the neocortex when behavioral adaptations are learned. Pictures of an angry and a happy human face served as reinforced (CS+) and unreinforced (CS-) conditioned stimuli, respectively, in one group, and either the reversed condition or two discriminably different neutral faces in two other groups (total n = 48 subjects). The unconditioned stimulus (US) was intracutaneous shock delivered to the left hand 5 s after CS+ onset. The electroencephalographic (EEG) activity was recorded from Fz, Cz, Pz, C3, and C4, electromyographic (EMG) activity from bilateral forearm and corrugator muscles, and skin conductance from the right hand. During acquisition a negative slow potential developed after CS+ (not CS-), which was more pronounced when a neutral face served as CS+. Early (iCNV, initial contingent negative variation) and late (tCNV, terminal contingent negative variation) components of the slow-potential response were positively related to the magnitude of conditioned EMG responses. Differentiation of tCNV was larger when neutral faces signaled the US; iCNV persisted during extinction when a happy face served as CS+. Late-occurring event-related potentials (ERPs) elicited by the US diminished over conditioning, whereas short-latency US components and ERPs elicited by CS events did not. Fourier analysis revealed oscillatory ("gamma-band") activity between 30 and 40 Hz, which persisted up to 3 s after US delivery and diminished as conditioning progressed. Our findings indicate that learning is expressed in neocortical structures at the earliest stages of conditioning. The functional roles of the three types of EEG response in learning are discussed.

Neuromagnetic study of movement-related changes in rhythmic brain activity

Neuromagnetic fields from the left cerebral hemisphere of five healthy, right-handed subjects were investigated in a typical Bereitschaftspotential paradigm consisting of self-paced voluntary movement of the right index finger. To assess movement-related spectral changes of the spontaneous magnetoencephalogram. latency-dependent short-time spectra were obtained by Fourier analysis for each single trial. The number of trials in which the spectral estimate for a certain frequency and latency deviated from reference values was then transformed into a probabilistic relative power measure. A spectral power depression around 20 Hz was observed starting about 2.5 s before movement onset, followed by elevated power in the 20-35 Hz range starting about 500 ms after movement onset. Generally, the power increase differed from the prior depression in both spectrum and topography, suggesting different generating processes rather than just a 'rebound' effect of the idling rhythm generator. The time course and topography of spectral power changes are discussed in relation to the corresponding properties of the movement-related neuromagnetic fields (readiness field, motor field, and movement-evoked field I).

Oscillatory neuromagnetic activity induced by language and non-language stimuli

Event-related oscillatory brain activity during language perception differs from activity occurring during the processing of comparable non-language stimuli. This fact became apparent in the observation of changes in the normalized spectral power of magnetoencephalographic (MEG) signals during the subject's processing of these stimuli. MEG was recorded over the left and right hemispheres of 12 right-handed subjects. During the experimental session, bisyllablic content words and physically similar non-language stimuli were presented with equal probability in a randomized order in either the visual or auditory modality. Approximately 15% of these stimuli were marked and the subject's task was to detect these marked stimuli. As a major characteristic of language vs. non-language processing, we obtained an enhancement of the normalized spectral power around 240 ins in the 60-65-Hz band over the left hemisphere for the language condition and over the right hemisphere for the non-language condition, independent of the modality of stimulus presentation. Starting at approximately the same latency but in lower-frequency bands (15-45-Hz), an extended (250-600 ms) reduction of normalized spectral power was observed. This reduction, although it generally confirmed previous results, differed in the no hemisphere-specific reduction was found for the processing of words. A domain-specific enhancement of normalized spectral power was also evident around 800-1200 ms in the 15-30-Hz band. In the auditory condition, this enhancement of the normalized spectral power was larger after the presentation of language stimuli whereas in the visual condition a larger enhancement of the normalized spectral power was obtained after presentation of non-language stimuli. As this latter effect appears relatively late after the stimulus onset and differs in expression for both modalities of stimulus presentation, a simple relationship between language perception and oscillatory brain dynamics can be excluded for this enhancement. In contrast, the left hemispheric enhancement of the normalized spectral power present around 240 ms in the 60-65-Hz band seems to reflect oscillatory pattern specific to the processing of words.

Changes in movement-related brain activity during transient deafferentation: a neuromagnetic study

Neuromagnetic fields from the left cerebral hemisphere of three healthy, right-handed subjects were investigated preceding and during voluntary index finger movements performed every 8-15 s under two different experimental conditions: before (stage A) and during (stage B) anesthetic block of median and radial nerves at the wrist. The anesthesia caused blocking of cutaneous receptors and some of the proprioreceptors from a wide hand area, including the entire index finger. However, the index finger movements were not impaired because the muscles participating in the task were not anesthetized. The magnetic signals of the brain sources corresponding to the main components of the movement-related neuromagnetic fields (motor field, MF and movement-evoked field I, MEFI) were mapped and localized using a moving dipole model. In the three investigated subjects the MF and MEFI dipole sources were stronger (30% on average) during stage B than during stage A. No significant changes in spatial coordinates of the estimated dipole locations between stages A and B were observed. This was true for both MF and MEFI. The results show that the MEFI reflects not only proprioceptive input from the periphery but cutaneous inputs as well. In this way the results support the view that cutaneous inputs play a specific role in the cortical control of movement.

High-frequency cortical responses reflect lexical processing: an MEG study

Meaningful words and matched pseudowords, such as moon vs. noom, are of equal perceptual complexity, but invoke different cognitive processes. To investigate high-frequency cortical responses to these stimuli, biomagnetic signals were recorded simultaneously over both hemispheres of right-handed individuals listening to words and pseudowords. Consistent with earlier EEG studies, evoked spectral responses recorded from the left hemisphere revealed depression of spectral power in the low gamma band (around 30 Hz) after pseudowords but not after words. Similar differences between stimulus categories were present in the beta range. These results indicate that distinct patterns of high-frequency cortical responses correspond to the different cognitive processes invoked by words and pseudowords. It is hypothesized that differential high-frequency cortical responses signal the activation or activation failure of distributed Hebbian cell assemblies representing words and other elements of cognitive processing.

Oscillatory brain activity during a motor task

Changes in spectral power in neuromagnetic fields associated with a manual task requiring a high level of sensorimotor integration (SMI) were investigated by analysing spontaneous, non-invasively recorded activity during motor preparation (WAIT), task performance (SMI), and control (REST) conditions in four healthy, right-handed human subjects. Neuromagnetic fields were recorded over the left sensorimotor cortex using a 37-channel instrument. In all subjects, a prominent narrow-band motor preparation rhythm centered near 19 Hz was consistently observed during the WAIT state. During SMI, mean relative increases in 26-30 Hz activity appeared in two of the subjects, paralleling gamma band enhancement recently observed during SMI in monkeys.

[Choline sulphate in the lichen cladonia convoluta]

After thin layer chromatographic separation of (35)S-labelled metabolic compounds from the lichen Cladonia convoluta choline sulphate was found as an accumulation product.