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In 43 healthy subjects passing from the state of calm alertness to various conditions of perception of short sounds of different height, interchanging at random, the most expressed is a generalized reduction of individual values of the EEG factor I positively related to the index, mean amplitude and regularity of the alpha-activity and also to mean beta- and theta-amplitudes and negatively related to the theta-index. Maximum reaction by this factor is observed at the beginning of the stimulation; at the second minute of the stimulation, the factor values return towards the initial background level, and again, to a lesser degree, are reduced when carrying out tasks on the sound analysis of stimuli, and the more difficult is the task the stronger is the reduction. In all cases the parietal regions are more reactive than the central ones. By the EEG factor II positively related to the index, frequency and regularity of the beta-activity, the reaction is less expressed and is more local: the factor values in the parietal areas augment only at the beginning of stimulation and diminish at the second minute; and in the central parts, on the contrary, there is a tendency to increase only at the second minute of stimulation. At the sound analysis, reaction is poorly expressed by this factor. Changes of values of the EEG-factor III, positively related to the mean period of the alpha-waves and theta-index, are not great and have only a general tendency to increase at the beginning of the stimulation, and to lower to the background level at the second minute.

The paper describes the results of factor analysis of interconnections between 20 EEG parameters in each of 4 leads from the central and parietal areas of both hemispheres at inter-individual variation of parameter values in 54 healthy subjects. Three of the chosen factors reveal a stable structure in all leads, and also significant differences in factor values between various areas. Marked fluctuations of factor values are found in some subjects in different moments, which presumably reflect spontaneous fluctuations of the brain functional state and open a perspective for evaluation of these states by means of factor values. An attempt has been made to interpret the physiological meaning of some factors. Factor I is presumably connected with activity of non-specific activating systems controlling the level of wakefulness and stipulating the processes of synchronization of the bioelectrical activity. Factor II is interpreted as a reflection of the processes of cortical excitation, factor III--as a reflection of active selective inhibitory processes in the cerebral cortex.