Comparative study of the gamma rhythm in normal conditions, during examination stress, and in patients with first depressive episode.

We report here a comparative analysis of measures of spectral power and synchronization of the gamma rhythm (30-40 Hz) in healthy subjects in normal conditions and before examinations (a stress situation) and in patients with major depression (first episode), both without cognitive loading and during performance of tests (arithmetic counting and spatial imagination). The results showed that the power of the gamma rhythm in the frontal and temporal areas of the cortex was significantly greater in patients with depression than in normal subjects. In the stress situation, healthy subjects showed a reduction in the number of differences in this measure as compared with depression patients, both at rest and during performance of the arithmetic counting test. the spatial imagination test resulted in a smaller number of significant differences between patients with depression and healthy subjects, regardless of whether the latter were in normal conditions or in the stress situation. The levels of gamma rhythm synchronization between cortical areas at rest and during cognitive loading were not different in healthy subjects, though synchronization increased in healthy subjects during stress and in patients with depression. Thus, the stress situation in healthy subjects leads to decreases in the differences in EEG measures from those in depression both at rest (power) and during cognitive loading (both measures). This may be evidence that stress is a "trigger mechanism" for depression.

Relationship between EEG power and rhythm synchronization in health and cognitive pathology.

We report here studies of comparative measures of spectral density and cortical interactions in EEG rhythms in health and schizophrenia. In healthy subjects, all rhythms were symmetrical and synchronous. In "acute" schizophrenia, unlike the situation in health, there was asymmetry (predominantly right-sided) in the distribution of the spectral power of EEG rhythms. In chronic patients, asymmetry was less marked, though the power of most EEG rhythms was significantly lower than in the other two study groups. "Acute" patients showed a lack of interhemisphere interactions for all rhythms apart from the alpha rhythm, while the number of cortical interactions in chronic patients was rather lower than that in the "acute" patients, though there were significantly fewer than in healthy subjects. In addition, the gamma range showed only one interhemisphere association in the posterior areas. These neurophysiological characteristics may underlie a number of the impairments of mental activity in patients with schizophrenia. These data may also indicate that the linkage between power characteristics and synchronization of EEG rhythms is a necessary condition for normal perceptive and cognitive activity and the organization of behavior.