A study on phase relationship in human alpha activity. Correlation of different regions

Reflection of the pattern of cortical activation in the phase structure of the human EEG

A total of 22 healthy subjects in the EEG laboratory and 62 patients in the clinical functional diagnostic unit were studied. Spontaneous EEG recordings were made using the 10-20 scheme relative to combined ear electrodes in the state of rest with the eyes closed and open and during various functional loads. Traces were analyzed by computer animation of the EEG phase structure. The main concept of the method for extracting phase structure was based on not using a single reference lead. Time shifts were measured only between neighboring electrodes, with the result that the oscillations being compared were highly coherent. Time discordance was assessed in terms of the shift in the peak of the cross-correlation function. The results showed that from the point of view of phase structure, the differences between the high-and low-frequency EEG rhythms were purely quantitative. Qualitatively, the properties of the rhythms were identical and were reduced to slow (in the seconds range) oscillations of phase shifts. Low-frequency activity was characterized by large (in absolute terms, msec) phase shifts from electrode to electrode as compared with high-frequency activity. The phase shifts of potentials formed a structure which was overall very similar in different subjects and was reproduced in different leads. The initial appearance of EEG waves was statistically linked with the main sensory projections--the visual (occipital areas), auditory (temporal areas), and somatic (parietal areas), with addition of the frontal areas. Rearrangement of phase leadership in favor of the occipital pole at the expense of both temporal areas was observed on opening the eyes. This appears to depend on the level of sensory influx to this cortical area from the thalamus. It is suggested that the direction of the phase gradient reflects a gradient of cortical current density parallel to the surface. This can be used to locate compact sources lying close to the surface.

Transverse relationships of the alpha rhythm on the scalp

Using auto- and cross-spectral analyses on data of 3 normal adult subjects, two components of rhythmic alpha activity were distinguished with respect to their spatial distribution and spatial relationships on the scalp. Derivations from 21 scalp points vs. linked ears were analyzed. The alpha activity with power maxima in the bilateral occipital areas (component A) showed high coherence with activity in the anterior areas and somewhat lower coherence with activity in the central/parietal areas. The other component (B) appeared dominantly in the central areas, showing extremely low values of coherence between the anterior and posterior regions, and an unstable phase relation among recording points. Components A and B appear to correspond to the 'generalized' and 'localized' alpha components described earlier. It is suggested that these two components account for the major properties of the rhythmic alpha activity.

Averaging of spectral power and phase via vector diagram best fits without reference electrode or reference channel

A method is proposed to average results of Fourier analysis over epochs without assigning zero amplitude values to any electrode or zero phase values to any channel. The Fourier sine and cosine values of each electrode define its entry location into a vector diagram for the frequency component. The entry constellations thus formed are translated so that the mean of all entries (the virtual average reference which is the location of best fit) is at the centre of each constellation, and then they are rotated around the means into angles of best fit so that the sum of the squared distances between corresponding entries is minimal. The method leads to non-ambiguous mean results, with variance information for all recording points. Approaches for the assessment of the constellations, their transformation into field maps, and the problems with conventional phase measurements vs. real electrodes are discussed. The virtual average reference (spatial high pass filter) is proposed for assessing the time sequence (phase differences) between entry points, thus describing the internal structure of the constellations without bias and without reference to a zero phase point. Comparison of voltage and current density result constellations and consideration of reference-independent descriptors of the scalp field potential maps support the validity of the average reference for the approach.