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The distribution of EEG oscillation periods in healthy volunteers is symmetrical in the right and left hemispheres. Patients with functional brain disorders showed asymmetrical structure of EEG. The course of acoustic EEG-controlled biological feedback decreases the asymmetry to a level characteristic of healthy individuals. Key Words: electroencephalogram; biological feedback; hemispherical asymmetryNormal brain is characterized by symmetry in electrical activity of the right and left hemispheres [3,5,7,8]. Various organic damages to the brain, bipolar disorders, and schizophrenia disturb the symmetry of bioelectric activity (BEA) in the homologous subdivisions of cerebral hemispheres [1,5,7,8]. By contrast, no deviation in cerebral BEA symmetry was detected in neuroses and some other functional brain disorders [2,3]. Since quantitative assessment of the relationships between cerebral hemispheres is usually performed with the help of cross-correlation and coherent analyses or by hemispherical asymmetry of various EEG indices, it is a common practice to use short epochs (seconds), when EEG is non-stationary [5,7]. At the same time, to reveal the disturbances in BEA symmetry during functional brain disorders, it is expedient to use longer periods of analysis. To this end, we compared the symmetry of cerebral BEA in healthy subjects and in patients with some neurological diseases using a periodometric analysis, which allows to ignore non-stationary character of EEG and to assess cerebral BEA during long-term periods. MATERIALS AND METHODSThe study was carried out on 11 healthy volunteers and 28 patients (23 men and 16 women at the age of 19-61 years) with functional disturbances (asthenoneurotic, neurotic, and depressive states). Psychophysiological treatment included a course of acoustic EEGcontrolled biological feedback (EEG-BFB) [4]. The procedures consisted of sessions of audition of the current sound BEA presentation of patient's own brain. The functional state of patients and volunteers was assessed subjectively and on the basis of feeling-activity-mood (FAM) test, Spilberger--Hanin and Lusher tests. The complete course of procedures consisted of 5-7 sessions. EEG from the right and left hemispheres was recorded during entire session of acoustic EEG-BFB (mean duration 30 min) with digitization frequency of 256 Hz via differential electrodes placed on the forehead and occiput (the reference electrode was placed on the vertex). EEG was analyzed after preliminary band filtration in the frequency range of 4-30 Hz. The whole EEG fragment record during single session was subjected to periodometric analysis. The distribution of EEG oscillation periods in the right and left hemispheres were assessed. The differences were characterized by the sum of absolute values of the differ-0007-4888/00/0002-118525.00 9Kluwer Academic/Plenum Publishers