An apparatus comprises a data processing unit, which causes the apparatus to: receive at least two electroencephalographic signals of a brain of a person; form a first information about at least one of the following: phase-to-phase coupling between at least two electroencephalographic signals, phase-to-amplitude coupling between at least two electroencephalographic signals and amplitude-to-phase coupling between at least two electroencephalographic signals, the first information being related to slow waves having frequencies at or lower than 1 Hz; normalize the first information; and output information about the normalized first information.