Yuko Matsumoto

Localization of activated areas and directional EEG patterns during mental arithmetic

In a search for the physiological correlates of mental activity, localized EEG desynchronization induced by mental arithmetic was assessed by the entropy of the EEG power spectrum (irregularity index II). The topographic mapping of II can show a localized cortical activation. Directional EEG patterns during mental arithmetic were further examined with a directed mutual information measure (information flow). These two information measures were compared between resting and mental arithmetic conditions in 10 right-handed subjects as well as one left-handed subject. II significantly increased during mental arithmetic in the left temporo-centro-parietal region in right-handed subjects, thus exhibiting its activation. Significant changes in information flow during mental arithmetic were found in the directions mostly from the temporo-centro-parietal region to other areas and within the frontal region in right-handed subjects: information flow from the left temporal and the mid-frontal areas increased and information flow mostly from the left hemisphere decreased. One left-handed subject showed greater II in right post-temporal area. These findings suggest that the left temporo-centro-parietal activation is specific to calculation processing, and the frontal information flow is related to the active performance of mental arithmetic.

Potential flow of frontal midline theta activity during a mental task in the human electroencephalogram

The movement of potential field (potential flow) of frontal midline theta activity (Fm theta) as well as its potential distribution was examined in 7 subjects by using optical flow detection techniques in image processing. Electroencephalograms (EEGs) over the fronto-central region were recorded from 13 electrodes near the frontal midline (Fz) while the subjects were performing a mental task. The potential flow of Fm theta was estimated on a frame consisting of a square grid with Fz at its center. In regions anterior to Fz, the direction of potential flow was from lateral to medial, whereas it was from medial to lateral in regions posterior to Fz. The peak-to-trough amplitude distribution was round or oval with a maximum just anterior to Fz. The source density distribution showed the greatest potential along the midline in the frontal region and bilaterally symmetric smaller maximum areas mostly in the prefrontal regions. Our findings suggested the presence of 2 different source areas of opposite direction in each hemisphere in spite of a round or oval amplitude distribution.

A new segmentation method of electroencephalograms by use of Akaike's information criterion

Non-stationary EEGs, whose statistical properties change with time, were segmented into stationary segments to closely track the behavior of EEG characteristics. We have developed a new segmentation method of optimizing segmentation parameters by using AIC (Akaikes information criterion) as an objective criterion. We applied the segmentation method to EEGs. The instantaneous power spectra of EEGs estimated with wavelet transform were compared with the segmented EEGs. EEGs were recorded from F3, F4, C3, C4, P3, P4, 01 and 02 in 13 normal subjects. Artifact-free 15-s epochs were taken at each electrode location. Each epoch was divided into stationary segments, consisting of several fixed intervals, by optimizing 2 segmentation parameters (interval length and starting point) so that the sum of AICs for several sequences of segments could be the smallest. The EEG segmentation could represent differences in the power spectra between segments. The average length of segments during relaxed wakefulness was 6.0 +/- 3.8 s. The EEG segmentation during mental arithmetic could detect the start of mental arithmetic.

INTER-SITE EEG RELATIONSHIPS BEFORE WIDESPREAD EPILEPTIFORM DISCHARGES

EEG interactions of the background among locations can start to change just before epileptiform discharges. Such interactions were investigated with relative power contribution analysis based on a multivariate autoregressive model, which permits determination of multiple causal relations of EEGs among locations. EEGs from F3, F4, P3, P4, T3 and T4 were examined in 10 epileptic patients with asymmetric spike and wave complexes (SWCs). A 12.5-s epoch just before SWCs was divided into stationary segments throughout 6 locations with a segmentation method. In segments long before SWCs, most power at each location was generated from its own location. In segments immediately preceding SWCs, contributions from other locations, particularly from the hemisphere with smaller SWCs, increased. Overall EEG relationships among 6 locations were examined by an entropy which measures the uniformness of the spatial distribution of power contribution. The entropy significantly increased gradually toward SWCs. Our findings demonstrated stronger interactions among locations just before epileptiform discharges, suggesting a transitional state from background EEG to epileptiform discharges.

Increases in the power spectral slope of background electroencephalogram just prior to asymmetric spike and wave complexes in epileptic patients

Moment-to-moment electroencephalogram (EEG) changes just before spike and wave complexes (SWCs) were investigated by using a non-stationary power spectral analysis in 10 epileptic patients with asymmetric SWCs. The instantaneous power spectra of background EEG were estimated for a 10-s period just before SWCs in 10 locations. The spectral shape, showing a l/f amplitude characteristic, was assessed by the negative slope (spectral slope) of a straight line on a plot of log power versus frequency. Spectral slope significantly increased toward SWCs at locations with greater SWCs. This finding suggested structural changes in EEG frequency composition just before SWCs because of an epileptogenic process.

Changes in the fractal dimension of alpha envelope from wakefulness to drowsiness in the human electroencephalogram

Changes in the fractal dimension of the waxing and waning of alpha activity from wakefulness to drowsiness were investigated in 10 normal subjects. The envelope of filtered alpha activity was estimated with the Hilbert transform. A subset time series was constructed from the alpha envelope data taken at an interval (k). The points of the logarithm of interval k vs. the logarithm of the curve length of the subset time series (log L(k)) were fitted by a two-segment curve, consisting of small and long line segments (segments 1 and 2). The fractal dimension was estimated from the slope of straight line for each segment by using Higuchis method. The mean fractal dimension was 1.16 for segment 1 and 1.95 for segment 2 during wakefulness and 1.15 for segment 1 and 1.89 for segment 2 during drowsiness. The fractal dimension significantly decreased in both segments during drowsiness, thus, indicating decreases in the complexity of alpha envelope.

Moving potential field of frontal midline theta activity during a mental task

The movement of potential field (potential flow) of frontal midline theta activity (Fm theta) was examined by using optical flow detection techniques in image processing in 6 normal subjects. Electroencephalograms (EEGs) over the fronto-central region were recorded from 13 electrodes around Fz. The potential flow of Fm theta was estimated on a frame which consisted of a square grid of 19 x 19 points in which each of the 13 electrodes was positioned with Fz at the center of grid. The peak-to-trough amplitude distribution was rounded or oval with a maximum just anterior to Fz. In each hemisphere, potential field moved medially in a region anterior to Fz, posteriorly near or at the midline and laterally in a region posterior to Fz. In the temporal sequence of potential field, at each point velocity directions tended to change progressively clockwise or counterclockwise. In the whole frame, a rotary movement of potential flow was found with a center at the middle on each side. Our findings suggested that a potential maximum at the midline may be due to overlapping of fields of opposite direction over the frontal scalp, arising from sources of similar orientation in both hemispheres. It can be assumed that Fm theta propagates in a clockwise direction in the left hemisphere and in a counterclockwise direction in the right hemisphere.

Potential flow of a activity in the human electroencephalogram

Abstract The movement of potential field of a activity was investigated with a potential flow technique. a activity was divided into slow, medium and fast subbands. The peak-to-peak amplitude and source density distribution of each subband was also constructed. The potential flow of medium a was from medial to lateral in the anterior region, and from lateral to medial in the posterior region. The flow patterns of slow and fast a were more complicated. Similar flow directions formed a group. Different groups met or diverged at discontinuous grid points around the local maxima of potential distribution. The discontinuous points of medium a were centered in the parieto-occipital region, and those of slow and fast a were more equally distributed over the scalp. Against potential gradients, several groups of similar flow directions participated in the formation of potential field. The generation of potential field was assumed to be controlled widely for medium a and locally for slow and fast a.

The 3-dimensional representation of EEG distance by use of Shannon-Gelfand-Yaglom information measure during mental arithmetic

SummaryEEG relationships in δ, θ, α and β frequency bands as well as the whole frequency range among 12 electrode locations during resting and mental arithmetic were examined by use of Shannon-Gelfand-Yaglom information measure (SGY) in 10 normal subjects. The EEG relationships were represented in a 3-dimensional (3-D) feature space with multidimensional scaling (MDS). MDS resulted in an anterior-posterior, a left-right and an upper-lower axis. In all 5 frequency bands, the 12 electrodes on the anterior-posterior and left-right axes resembled their actual positions on the scalp during resting and mental arithmetic. During mental arithmetic, prominent changes in the 3-D representation of electrode sites were visualized for activity. The isolation of left and right mid-temporal locations, together with greater EEG distances between left and right mid-temporal locations, were notable during mental arithmetic, suggesting the simultaneous activation possibly in dominant and non-dominant hemispheres and contralateral influences from left to right mid-temporal locations.