Dimensional complexity of EEG brain mechanisms in untreated schizophrenia.

The dimensional complexity of left temporal-parietal and parietal-occipital electroencephalographic (EEG) recordings was assessed by computing the correlation dimension during 20 sec in six recording conditions from 15 first-episode acute schizophrenics before medication, 12 other medication-free individuals clinically and socially remitted after a first schizophrenic episode, 17 medication-free neurotics and 17 controls. The correlation dimension of the temporal-parietal EEG differed between groups [analysis of variance (ANOVA)] (p < 0.004), whereas neurotics (different from schizophrenics at p < 0.002) and remitted schizophrenics showed intermediate values. There was no overall significant difference between groups in the parietal-occipital EEG. Differences of the correlation dimension of the temporal-parietal versus the parietal-occipital EEG were significant between groups (ANOVA p < 0.05); first-episode schizophrenics differed from controls (p < 0.002) and remitted patients (p < 0.08). Increased dimensional complexity of schizophrenic EEG was found in one of two examined brain regions. The higher dimensional complexity of functional brain mechanisms in schizophrenics versus normals is reminiscent of the loosened organization of thought, and of suggestions of certain superior abilities in the patients.

Source localization of brain electric field frequency bands during conscious, spontaneous, visual imagery and abstract thought.

This paper addresses the issue of mind-brain correspondence, using a novel way to reduce brain electric field data in the frequency domain to estimates of intracerebral model source locations, and applying this method to brain electric data collected during the 2-s epochs immediately before the randomly solicited reports of spontaneous, conscious, covert experiences from 12 normal volunteers. The mentation reports were classified into visual imagery and abstract thought. The mean locations of the EEG model sources associated with abstract thoughts were generally more anterior and deeper than those of visual imagery, particularly significant for the delta/theta band; the finding was common across subjects. Thus, different brain functional states involving different geometries of activated neural populations exist during conscious, spontaneous, task-free mentations of the visual imagery type and of the abstract thought type.

42-channel potential map series to visual contrast and stereo stimuli: perceptual and cognitive event-related segments.

Event-related potential maps to perceptual (stimulus type) and cognitive (stimulus relevance) manipulations were studied in 12 healthy volunteers using 42-channel mapping. Perceptual manipulation used three types of visual stimuli: rectangles constituted by: (1) contrast; (2) different densities of monocular Dynamic Random Dots (Flat DRD); and (3) different binocular disparities of Dynamic Random Dots (Stereo DRD). Cognitive manipulation within each stimulus type consisted of presenting the rectangles horizontally and vertically, one of the two with a probability of 33%, and requesting the subjects to count and thus attend to the 'rare' rectangles. Spatial characteristics of the maps were analyzed; this allowed conclusions about the generating sources. The map series were adaptively segmented using the minima points of the grand mean Global Field Power curve. Segment strength (Global Field Power) and segment landscape (locations of extreme potentials) were assessed. Stimulus type had effects from 78 to 310 ms, stimulus relevance was effective from 210 to 1000 ms. In the 78-174 ms segment, Stereo DRD and Flat DRD stimuli produced similar map landscapes, while contrast stimuli produced different map landscapes. Attended and ignored stimuli produced contrary effects on landscapes at 210-310 ms as compared to those at 310-546 ms, indicative of different neural populations activated by attention processes during these late event-related potential segments. Interaction between perceptual and cognitive manipulation occurred at 210-310 ms when perceiving stereo stimuli and attending to relevant monocular visible stimuli produced similar map landscapes, suggesting a common brain resource during this segment for automatic figure perception and voluntary attention. The observed functional differences of the segments contribute to the identification of global functional microstates of brain electric activity.

Brain electric microstates and momentary conscious mind states as building blocks of spontaneous thinking: I. Visual imagery and abstract thoughts.

Prompted reports of recall of spontaneous, conscious experiences were collected in a no-input, no-task, no-response paradigm (30 random prompts to each of 13 healthy volunteers). The mentation reports were classified into visual imagery and abstract thought. Spontaneous 19-channel brain electric activity (EEG) was continuously recorded, viewed as series of momentary spatial distributions (maps) of the brain electric field and segmented into microstates, i.e. into time segments characterized by quasi-stable landscapes of potential distribution maps which showed varying durations in the sub-second range. Microstate segmentation used a data-driven strategy. Different microstates, i.e. different brain electric landscapes must have been generated by activity of different neural assemblies and therefore are hypothesized to constitute different functions. The two types of reported experiences were associated with significantly different microstates (mean duration 121 ms) immediately preceding the prompts; these microstates showed, across subjects, for abstract thought (compared to visual imagery) a shift of the electric gravity center to the left and a clockwise rotation of the field axis. Contrariwise, the microstates 2 s before the prompt did not differ between the two types of experiences. The results support the hypothesis that different microstates of the brain as recognized in its electric field implement different conscious, reportable mind states, i.e. different classes (types) of thoughts (mentations); thus, the microstates might be candidates for the 'atoms of thought'.

Localization of sources of brain alpha/theta/delta activity and the influence of the mode of spontaneous mentation.

A method is described that accounts for multichannel brain field data (EEG) epochs, after transformation into the frequency domain, by a single oscillating dipole source in terms of phase angles. The method produces a potential distribution for each frequency point ('FFT dipole approximation'). These maps can be subjected to conventional equivalent dipole source fittings in terms of amplitudes. We studied the equivalent source locations for the different temporal EEG frequency bands (delta/theta/alpha) in 12 normal subjects during the collection of reports of spontaneous thoughts. Some of the thought reports were classed into two modes, 'visual imagery' and 'abstract', and the associated equivalent source locations during the 2 s immediately prior to these reports were computed. Different equivalent source locations were found for the different spectral components of the EEG, implying that different neural generator populations generate the different frequencies. Further, the different types of spontaneous thought, i.e. different modes of cortical functioning, were found to be associated with the activity of different neuronal generator sources that operated at the same frequency at different source locations.

Localization of the sources of EEG delta, theta, alpha and beta frequency bands using the FFT dipole approximation.

FFT dipole approximation and 3-dimensional dipole modelling were used to determine the locations of the equivalent dipole model sources of the delta, theta, alpha, beta-1 and beta-2 frequency bands in 13 normal subjects during resting. From each subject, 2 successive data sets were analysed, each consisting of 10 epochs of 2 sec randomly collected during 30 min. ANOVAs showed that over subjects, the source locations of EEG frequency bands differed significantly in the vertical and antero-posterior dimensions. Results of data set 2 confirmed those of data set 1. The source of delta was deepest and most anterior, theta more posterior and less deep, alpha most posterior and highest on the vertical dimension, beta-1 deeper and slightly more anterior than alpha, and beta-2 again more anterior and deeper than beta-1. Thus, the depth of source location was not linearly related to temporal frequency. The sources of all 5 bands were oriented in the sagittal direction; delta mean fields had steeper gradients anteriorly, alpha and beta-1 posteriorly. The power map for any frequency was well described by a single phase angle. The results indicate that the different EEG frequency bands during a given EEG epoch are generated by neural populations in different brain locations.