Context-dependent EEG spectral characteristics during performance of tasks

Spatial and frequency EEG characteristics of two groups of healthy adult subjects were examined in two series of experiments, which differed in conditions of the second cognitive task in a trial. The first task was the same in the two series: subjects had to evaluate size relationship between two closely spaced circles. The second task successively presented in trials of the first series consisted in the recognition of words/pseudowords, and in the second series, subjects had to localize a target letter in a matrix. It was assumed that the cognitive performance in the first series predominantly involved the ventral visual system, whereas during task performance in the second series, predominant involvement of the ventral and dorsal visual systems alternated. Multichannel EEG fragments recorded prior to the presentation of the task pairs were analyzed. Analysis of variance of the EEG spectral power revealed the generalized significant effect of the factor of the second task in the pair for delta band and lower beta subband, the power being higher in the first series. Factor brain hemisphere had a significant effect for the alpha band in the occipital area, the spectral power being lower in the left hemisphere for both experimental series. The task x hemisphere interaction was significant in the temporal cortical areas for the EEG power in alpha2 band, i.e., the predominant involvement of the ventral visual system was associated with stronger asymmetry of alpha2 rhythm and lower spectral power in this band in the left temporal area. Thus, the character of the forthcoming cognitive activity was shown to be reflected in spatio-frequency characteristics of the preceding EEG.     

Relations of the EEG local and spatialtemporal spectral characteristics changes under hypoxia in humans

Spatial temporal and local EEG characteristics were studied in healthy subjects during inhalation of hypoxic oxygen-nitrogen gas mixture with 8 % content of oxygen. Analysis of spectra power density, coherence, phase shift, similarity of dominant frequencies in the EEGs of different derivations was performed separately for the EEG epochs with and without visually detected patterns of spatial synchrony of the EEG. Apart from this, a fact of dominance of the frequency in the EEG spectra of corresponding derivation was taken into account when estimating spectral parameters. Results of the study showed that, in general, under hypoxia, the EEG coherence in alpha- and delta-frequency range decreases as compared to the background level, in beta-range growth of this parameter is observed, in theta-range ambiguous changes occur: in the epochs with patterns of spatial synchrony--growth, in other epochs--lowering. Under hypoxia, also occurs growth of frontal and temporal EEGs' phase shift (corresponding to EEGs other derivations) in delta- and theta-range. In beta-range, on the contrary, average level of the phase shift decreases. It was revealed that taking into account the fact of dominance of frequency in the local EEG spectra is necessary for correct interpretation of the EEG spatial and temporal parameter analysis' results. A mathematical model of interaction between processes with different frequency characteristics is suggested, which explains some facts obtained in the study.     

Spatial synchronization of the segmental EEG in humans

Topographic features of spatial synchronization of sharp changes, or rapid transition processes (RTP), were studied in human EEG recorded from longitudinal and transversal electrode arrays. A new algorithm, the EEG Threshold Scanning, was proposed for the detection of the RTP. Synchronization of the RTP was estimated by Operational Synchrony Index (OSI) based on the difference between the actual and stochastic frequency of RTP coincidence in a pair of EEG channels. The relationship between the OSI and interelectrode distance was not monotonous. The OSI depended also on the extent of morpho-functional similarity between two cortical areas. Similar results were obtained for crosscorrelation calculated for the same pairs of the EEG derivations. The existence of dynamic spatial modules which incorporate different brain areas by complementary stabilization of their functional states is discussed.     

Dynamic properties of segmental characteristics of EEG alpha activity in rest conditions and during cognitive tasks

Segmental structure of the alpha activity of human EEG was studied in the state of rest and during arithmetical task performance with mono- and binaural listening of task instruction and binaural listening of music. The amplitude, duration, amplitude variability and steepness of between-segment transitions were calculated as segmental characteristics. It was shown that all of these characteristics are sensitive to the cognitive tests but the degree of this sensitivity depends on the cortex area and type of the cognitive task. Most pronounced changes in the segmental characteristics were observed during arithmetic task with left-side instruction presentation and minimal changes were seen during binaural listening of music.     

Dynamics of spectral characteristics of theta- and alpha-range EEG during negative emotional reactions]

Power characteristics of the EEG theta and alpha rhythms were studied in a human in neutral state and during a conditioned negative emotional reaction (Fp1, Fp2, F3, F4, C3, C4, P3, P4, O1, O2, F7, F8, T3, T4, T5, and T6 derivations). A significant increase in the relative spectral power in the narrow theta band of 7.4-8.1 Hz in the frontocentral and temporal brain regions was observed during the development of the negative emotional reaction. The alpha-rhythm dynamics during the negative reaction was substantially individual and could be expressed in either an increase, or decrease in relative spectral power of different alpha-frequencies. No pronounced changes in their dynamics could also be observed. In some subjects the spectral power of the medium-frequency alpha-rhythm significantly decreased, that of the high-frequency rhythm increased, and changes in the spectral power of the low-frequency alpha range varied.     

Classification methods for ongoing EEG and MEG signals

Classification algorithms help predict the qualitative properties of a subject's mental state by extracting useful information from the highly multivariate non-invasive recordings of his brain activity. In particular, applying them to Magneto-encephalography (MEG) and electro-encephalography (EEG) is a challenging and promising task with prominent practical applications to e.g. Brain Computer Interface (BCI). In this paper, we first review the principles of the major classification techniques and discuss their application to MEG and EEG data classification. Next, we investigate the behavior of classification methods using real data recorded during a MEG visuomotor experiment. In particular, we study the influence of the classification algorithm, of the quantitative functional variables used in this classifier, and of the validation method. In addition, our findings suggest that by investigating the distribution of classifier coefficients, it is possible to infer knowledge and construct functional interpretations of the underlying neural mechanisms of the performed tasks. Finally, the promising results reported here (up to 97% classification accuracy on 1-second time windows) reflect the considerable potential of MEG for the continuous classification of mental states.     

Normative EEG spectral characteristics in healthy subjects aged 7 to 89 years

The EEGs of 885 healthy subjects of both sexes aged 7 to 89 years were recorded in two modes: with the subjects’ eyes closed and with the eyes open. The subjects were divided into 20 age groups, for each of which the normative values of the EEG spectral characteristics were determined: the total EEG power spectra and the EEG independent component power spectra in the Δ, ϑ, α, and β frequency bands. Tables of confidence intervals with a level of confidence of 0.95 were constructed for each electrode channel in the case of the EEG power spectra and for each component in the case of the EEG independent component power spectra. The normative values obtained may provide EEG specialists with objective criteria for assessing cerebral dysfunction.     

Classification of lymphoproliferative disorders by spectral imaging of the nucleus

Spectral nuclear morphometry was used for the classification of lymphocytes in lymphoproliferative disorders. May-Grunwald-Giemsa-stained blood specimens were taken from thirty patients with infectious mononucleosis, non-Hodgkin lymphoma or chronic lymphocytic leukemia, and from ten healthy individuals. Blood specimens were analyzed by spectral imaging. Seventeen distinct spectra were collected into a spectral library and a distinct pseudo color was assigned to each one of them. The library was used to scan all the cells in the database and to create a spectrally classified image of each cell. The spectral map, per cell, reveals distinct spectral-response regions in each cellular compartment, via the distinct region colors. Computational analysis of the spectral maps allows for the objective quantification of a set of parameters, or features, representing the cell. The features used in this work include the area and perimeter of the nucleus, circularity, edginess and the spectral pattern. The analysis pursued showed that each class of cells is associated with a set of unique parameters. We conclude that spectral analysis combined with feature analysis provides significant information in the analysis of lymphoproliferative disorders and may serve as an additional tool for the histopathological evaluation of disease.     

EEG spectral characteristics at different stages of the unconscious visual set in two motivation conditions

Multichannel EEG were recorded in young healthy subjects in two series of experiments during formation, actualization, and extinction of the visual unconscious set to the perception of unequal circles under conditions of increased motivation of subjects to the result of their performance. In the first series of experiments, subjects were promised to be rewarded (a small money price) for each correct response (the "general" rise of motivation). In the second series, subjects were promised to be rewarded for correct responses only in cases when one of the circles was larger than the other one (the "directed" rise of motivation). The dynamics of the EEG spectral power derived under these two conditions was compared with similar indices obtained earlier during formation of the same set without any special motivation of subjects (control). In all experimental conditions, before the presentation of the stimuli the EEG power in the alpha range was higher in subjects with the stable set. The dynamics of changes in the alpha power at set stages was principally similar in all conditions. In all the experimental conditions, in subjects with unstable set the EEG power in the delta range was highest at the stage of set actualization. The most pronounced generalized changes in the EEG power in the theta-range during the "general" rise of motivation in subjects with stable and unstable forms of set and greater variability of the reaction time to the probe stimulus suggest that the task performance under these conditions required greater tension than under conditions of the "directed" rise of motivation.     

Analysis of the segmental structure of EEG alpha-activity in humans

Phasic organisation of human EEG alpha activity was studied in a pilot investigation using a previously suggested EEG segmental analysis methodology. The EEG was recorded in three normal subjects under resting conditions. The segmentation procedure enabled effective identifying of the periods with different amplitude in alpha band and the short-term transitions between them. Mean intersegmental variability of amplitude envelope were computed for the eyes closed and eyes open EEG in each of 16 standard derivations. Analysis of segment amplitude distributions showed that the difference between the average alpha activity amplitude in these conditions were determined mainly by variations in the number of segments of different amplitude classes and not by a shift of the distribution or by change of its width. Distribution and quartile analysis of mean segment amplitudes provide evidence for possible functional heterogeneity of upper and middle subranges of the amplitude range.     

Effect of cold stimulation of the arm fingers on the spectral/coherent EEG characteristics in humans

Examination of modifications of EEG in humans induced by cold stimulation of the arm fingers showed that the EEG frequency composition noticeably depended on this thermal influence (in the relaxed state with no movements or during realization of voluntary cyclic movements by the fingers of another arm). In the resting state, cold stimulation mostly induced intensification of the delta activity, while, when coinciding with the performance of voluntary movements, it also resulted in increases in the powers of oscillations of the alpha1 and beta1 ranges. The structure of changes in the coefficients of coherence under the influence of cooling also depended on the conditions of testing (in the resting state or during motor activity). Therefore, the effect of tonic cold stimulation on the interaction between synchronizing and desynchronizing cerebral systems and interrelations between different cortical zones was modified under conditions of realization of a motor function. Neirofiziologiya/Neurophysiology, Vol. 40, No. 3, pp. 268–270, May–June, 2008.     

Pre-stimulus spectral EEG patterns and the visual evoked response

The relationship between the latencies and amplitudes of the N1 and P2 components of the visual evoked potential (VEP) and the psychophysiological state of the brain immediately preceding the time of the stimulus has been investigated in 7 male subjects. Power spectral measures in the delta, theta, alpha and beta bands of the 1 sec pre-stimulus EEG were used to assess the brain state, and low intensity flashes, delivered randomly between 2 and 6 whole seconds, were used as the stimuli. Trials were ranked separately according to the relative amounts of pre-stimulus power in each EEG band and were partitioned into groups by an equal pre-stimulus spectral power criterion. Averaged EPs were computed from these groups and multiple regression analysis was used to relate pre-stimulus spectral power values to EP features. Five of the 7 subjects displayed consistent increases in N1-P2 amplitude as a function of increasing pre-stimulus relative alpha power. The between-subjects effect of pre-stimulus EEG on N1 latency was small, but was moderate for P2 latency (both significant). Both N1 and P2 latency were found to decrease with increasing amounts of pre-stimulus relative delta and theta power.     

The spectral dynamics and its applications in EEG

The method of spectral dynamics is introduced to process slow changes in EEG. This method is based on evaluation of distance between EEG spectra. The typical application field is pharmaco-EEG after single dose drug administration, where distances between pre and post drug EEG spectra are computed. The distance between two spectra may be, however, defined in several different ways. The paper deals with the well known L _p metrics and with the metrics that plays an important role in discriminating stationary processes with different spectra. The L _p and metrics are non-equivalent and their properties, consistency and robustness, are studied in a simple statistical model.     

Changes in spectral coherent characteristics of EEG in the early postpartum period in mothers with anxious and depressed mood

Postpartum EEG spectral and coherence characteristics were estimated in mothers with or without postpartum depressions. In mothers without affective disorders the power of oscillations in the delta, theta, and alpha 1 frequency bands was increased as compared to controls. Intrahemispheric EEG coherence between the left frontal and adjacent derivations in the delta and theta bands and interhemispheric coherence in the central areas was increased and decreased over the remaining cortical surface. These changes led to a significant decrease in EEG asymmetry. It is suggested that during normal postpartum the influence of the limbicodiencephalic and lower brainstem structures on the cortex is augmented and a certain kind of dominanta is formed. In mothers with postpartum depressions the EEG alpha-band power was lower than in the control and normal groups, coherence changes in the delta and theta bands diminished the EEG asymmetry. The insufficiency of limbicodiencephalic influence and impairment of adaptive brainstem reactions are suggested to be responsible for problems in the formation of maternal dominanta, which results in the development of postpartum depressions.     

A comparative analysis of the change in the EEG spectral characteristics of rats during the activation and suppression of the catecholaminergic systems]

To find EEG-markers of catecholaminergic activation shifts EEG power spectra of white rats were computed before and after intraperitoneal injections of propranolol, metaproterenol, haloperidol, amantadine, or isotonic sodium chloride solution. Differential spectral characteristics were undergone to factor analysis and discriminant analysis. Factors with similar structure for both catecholaminergic systems were revealed in EEG-reactions to mutually antagonistic injections and relatively specific factors as well. The leading factor of adrenotropic injections described the augmentation of the spectral power in the range of 9-16 Hz induced by propranolol and its reduction by metaproternol. Similar factor was also revealed in reactions to dopaminotropic injections with the smaller value of discriminant function coefficient. One more common feature of EEG-reactions to catecholaminergic disturbances was found to consist of the reciprocal narrow-band shifts in the theta- and delta-diapasons. The leading factor for the recognition of dopaminotropic disturbances described the increase of EEG power in the band of 19-30 Hz at activation and its reduction at suppression of the transmitter system.