P. Rappelsberger

Probability mapping: Power and coherence analyses of cognitive processes

SummaryIn recent years different mapping techniques have proved to be an efficient means in studying cognitive processes. Our approach is based on recordings with 19 EEG channels and the computation of spectral parameters. Power and coherence values are obtained from the EEG of control recordings and recordings during the performance of cognitive tasks and are compared for significant differences. The statistical procedures used yield descriptive error probabilities for the rejection of the null hypothesis which are mapped either colour coded or by black and white squares. The usefulness of this method is demonstrated on a mental cube rotation study involving 31 students, 13 males and 18 females. One essential result during the performance of this test is an increase of theta power only in females. As far as local coherence and the theta band are concerned, males show increased values right parietally and right temporo-occipitally; opposed to this, in females the left hemisphere is more affected. In the beta bands, local coherence increases in males right parietally and left temporo-occipitally. Contrary to this, in females the parietal increase of local coherence is more accentuated in the left hemisphere but temporo- occipital coherence is increased in the right hemisphere. In both sexes interhemispheric parietal coherence increases during cube rotation suggesting increased functional couplings between the hemispheres. This study demonstrates that the inclusion of the coherence parameter in brain mapping furthers the detection of sex and hemispheric differences during the performance of cognitive tasks.

The contribution of the cortical layers to the generation of the EEG: Field potential and current source density analyses in the rabbit's visual cortex ☆

Intracortical spontaneous field potentials (EEG) were simultaneously recorded from the different layers of the rabbits visual cortex using multielectrodes produced by thin-film technology. The signals were subjected to spectral analyses. Additionally, current source density analysis was applied to spontaneous delta waves. The results are based on 10 experiments. The spontaneous EEG of the rabbits visual cortex consists mainly of slow delta waves. The spectral analytical studies of the delta band showed maximum power in layers I and II and in layers V and VI. Significant power minima were found in the middle layers IV and/or III. Essentially, the zones of power minima agree with the zones of minimum coherence and those zones where a phase reversal is observed. These properties of the intracortical spontaneous delta activity are best described as dipole-like. The spontaneous delta waves can be surface negative or surface positive; accordingly they show different source-sink distributions from which different generation mechanisms can be inferred. The possible mechanisms for the generation of these current source-sink distributions (dipoles) are discussed, taking into consideration the cortical anatomy. In many cases these discussions must be speculative because, for an unambiguous explanation of the observed phenomena, a much better knowledge of the cortical anatomy, the fibre connections with other cortical anatomy, the fibre connections with other cortical areas and with the deep cerebral structures is necessary.

Penicillin-induced epileptic phenomena in the rabbit's neocortex II. Laminar specific generation of interictal spikes after the application of penicillin to different cortical depths.

To elucidate the generation mechanisms of interictal spikes we analyzed the electrical events in the different layers of the rabbits neocortex after application of penicillin to distinct cortical layers. Field potentials (FP) recorded simultaneously from all layers with a 16-fold micro-electrode-assembly were subjected to current-source-density analysis (CSD), which allows a more accurate description of local electrical events. To demonstrate the influence of neocortical architectonics on the generation of interictal spikes, 3 different areas-area occipitalis 1, precentralis 1 and 2, according to Fleischhauer et al., were studied. Interictal spikes develop regardless of the depth of PNC application. Moreover the appearance of spikes does not seem to depend exclusively on the diffusion of PNC into a distinct cortical layer (i.e. layers IV and V). The CSD analysis indicated that different generation mechanisms within the different cortical layers underly the spikes. According to CSD configurations maximum sinks occur at depths where the dendrites join closely to form bundles. This finding supports the importance of dendrites for the generation of interictal spikes. Finally, the distribution of CSD patterns is similar in all 3 neocortical areas studied, if PNC is applied to the same cortical depth.

Is There any Message Hidden in the Human EEG

The term “electroencephalogram,” or EEG, is ambiguous: it was coined by Hans Berger (1929) who understood by it both the human electrical brain activity as it manifests itself in scalp recordings, and its representation as a potential-time diagram. For activities recorded from the cortex, the proper generator of the EEG, the word “electrocorticogram” was created, a term that did not, however, win much favor; today the term “EEG” is generally used for the designation of electrical brain activity regardless of where and how it is recorded. This also holds true for this chapter.

On-going EEG in depression: A topographic spectral analytical pilot study☆

Spontaneous EEG activities recorded with 19 electrodes (10-20 system) on the scalp were quantified by spectral analytical methods (power and coherence spectra) to reveal changes of power and coherence during different conditions (eyes closed/relaxed, eyes opened/relaxed, flicker stimulation, reading, listening to a story). Control subjects and depressive patients were compared. Power as well as coherence changes were found in all frequency bands. Changes in the organization of the EEG were found in different areas of both hemispheres. The EEG organization was different in control subjects and depressive patients, indicating different levels of vigilance and/or different cognitive strategies in verbal tasks in the two groups.

Simultaneous laminar intracortical recordings in seizures

Abstract Some results obtained by intracortical recording in different cortical regions by a multielectrode probe, produced by thin-film technology, are presented.

An approach to a synopsis of EEG parameters, morphology of brain convolutions and mental activities.

SummaryA method for the projection of EEG data on the brain surface is proposed. The EEG data are obtained during the performance of mentaltasks and represented as probability maps of power and coherencechanges with respect to the averaged EEG at rest. The morphological data are obtained from 3D reconstructions of the brain by means of serial slices provided by an MRI scanner. Before scanning the positions of the EEG electrodes were marked by dummy electrodes of plexiglass filled with a contrast medium. Changes of power are color-coded and entered at the respective electrode positions, changes of coherence between the respective positions. The applicability of this procedure in the fields of psychology, psychiatry and neurology is discussed.

Alpha Activity of NREM Sleep

A term like “alpha activity in sleep” seems to be contradictory. Since H. Berger’s first publication (1929), a rhythmically organized activity in the alpha frequency range is usually associated with a relaxed waking state, and this explicitly serves as a criterion for the definition of the posterior alpha background rhythm (IFSECN, 1974). The designation of a frequency band, however, is purely descriptive and should not imply a specific functional meaning, state of vigilance, or clinical correlation. In this chapter, therefore, the term “alpha” refers only to “in the alpha frequency range,” and the particular activity is specified by indicating the state during which it is recorded. Non-rapid eye movement (NREM) alpha activity is equivalent to rhythmical activity in the alpha frequency range recorded during physiological NREM sleep.

Evaluation of Relationships Between Seizure Potentials

For the evaluation of relationships between seizure potentials different methods can be applied. The choice of method largely depends on the kind of signals to be analyzed and on the question which is to be answered. An essential problem in epileptology concerns the development of a seizure initiated by a focus and the mechanisms leading to spreading and generalization. However, since in most cases the site of the focus is unknown the reverse problem arises, namely, the localization of the focus. Therefore, examinations and analyses have to be aimed at this localization problem.