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Fifty-eight male adolescents aged 15-17 years have been divided into 3 groups: 31 patients with mental infantilism syndrome were included in the study group, 14 with organic brain disorders--in the comparison group and 13 psychiatrically and neurologically normal subjects--in the control group. EEG was recorded from 12 leads monopolarly at rest, during hemisphere-specific cognitive tasks performance and exposure to aversive sound stimulation. Two-Hz wide EEG spectral ranges--A, theta1, theta2, alpha1 and alpha2--were analyzed. Only in the study group, there were decreased values of alpha2 spectral power (SP) and reduced reactivity to functional tests and insufficient lateral differentiation of the reactions during the cognitive tasks performance. Maximal deviation of the SP reactivity indices as compared to the comparison group was found in alpha2 range. The disturbances obtained indicate the delay of brain maturation and retardation in formation of specific neural networks. The results of the study demonstrate that indices of alpha2 SP may be considered as objective criteria of delayed mental maturation.