MEASUREMENT SCIENCE REVIEW, Volume 3, Section 2, 2003 17 EEG in the Context of Audiovisual Stimulation M. Teplan, A. Krakovská, S. Štolc Institute of Measurement Science, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovakia michal.teplan@savba.sk Abstract Audiovisual stimulation of the brain and EEG biofeedback has been studied for their possible applications to variety of conditions. One of the most fundamental principles of biofeedback is the necessity of accurate monitoring and feedback of the physiological processes of interest in order to control it. Here we present application of nonlinear measures and their comparison with spectral measures in detection of different brain states. A group of volunteers was involved in audiovisual stimulation training. EEG was recorded during their attempt for general resting. Following measures were tested: linear correlation, averaged mutual information, spectral edge frequency, relative power of alpha, beta, delta and theta band, spectral entropy, correlation dimension, and results of subjective assessment. Results from this study showed their changes in the course of the training. Nonlinear measures employed deserve further attention. Materials and Methods A group of 6 healthy adult volunteers (2 females and 4 males) aged from 23 to 39 years was involved in the audiovisual stimulation training. Commercially available mind machine was used to entrain subjects’ brain waves by a sound and light stimuli from 18 Hz down to 2 Hz (Fig.1). Overall training consisted of 25 single 20 minutes program sessions during 2 months. Subjects were lying in a darkened electrically shielded room. Before and after each training session subjects were instructed to keep their eyes closed and relax. EEG signal was recorded from eight channels: F3, F4, C3, C4, P3, P4, O3 and O4 according to International 10-20 system. The reference electrode was placed at Cz and ground electrode at Fpz. The length of each series was 3 minutes (i.e. 90000 samples). Fig. 1. Entrainment of the EEG dominant frequency during audiovisual stimulation. 20 minute program entrained subject's brain waves from 18 Hz down to 2 Hz and back to 15 Hz.