Detection of phase synchronization applied to audio-visual stimulation EEG M. Teplan, K. Šušmáková, M. Paluš, M. Vejmělka Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovak republic Institute of Computer Science, Academy of Sciences of the Czech republic, Praha, Czech republic

Outline synchronization phenomena phase synchronization detection brief illustration of the method audio-visual stimulation conclusions

Introduction - synchronization phenomena play a key role in organization of biological structures - for application of existing theories and models dfficulties may arise with experimental data contaminated by noise: problem to distinguish phase synchronized regime from asynchronous one. - we illustrate a method that is able to quantify a degree of phase synchrony - demonstration on audio-visual stimulation data

Synchronization phenomena - biomeasurements may yield complex and broadband data - emergence of dynamical order: spontanous development of structural organizations in complex systems - in some cases phase synchronization is sufficient for order maintaining, or it may be a sign for aproaching general synchronization - simplest system: two coupled 1D harmonic oscilators - 3D nonlinear system: Lorentz, Rosler - two or N coupled 3D nonlinear system: - coupling in one or both directions - common inherent frequencies or (slightly) different ones

Phase synchronization detection - increasing coupling constant: asynchrony, phase synchrony, general synchrony: M. Rosenblum et al.: Phase synchronization of chaotic oscillators. Physical Review Letters, 1996, 76/11. - phase synchronization can be expected to occur in real biosystems at different organizational levels due to nonlinear nature and feedback ties - synchronization may not be obvious but masked - phase locking: nφ 1 - mφ 2 = const - applied in study of epilepsy - onset and predictions - cardio-respiratory data - atmoshere CO 2 concentration and sun spot cycles - estimation of degree of phase dependence - method is sensitive to number of parameters - approaches are still under development

EEG and wavelet transform Complex wavelet transform

Phase extraction

Phase difference in unwrapped mode flat regions represent synchronized periods, slopy intervals mark phase slips 2π2π

Phase differences distribution evaluation - evaluation of irregularity of the distribution by mean phase coherence (MPC): values between 0 (uniform) and 1 (fully concentrated) and reflects how the relative phase is distributed over the unit circle - high values of MPC do not always identify synchrony clearly during stimulation before stimulation

Surrogate data - Testing hypothesis that value of dependence index is due to selected data properties - FT1 surrogates preserves the power spectrum, however randomly changes the phases of the Fourier coefficients for each time serie separately - If the null hypothesis stating that surrogates possess the same synchronization as EEG is refused, occurrence of linear phase synchronization is assumed

Mind machines?

Audio-visual stimulation of the brain (AVS): - brain influence through audio and visual sensory channel - adaptation of dominant EEG frequency – wave brain entrainment - potential use: strokes, head injury, headaches, dental anxiety, premenstrual syndrome, cognitive and behavioral functioning, performance improvement - AVS training: direct, short and long term effects M. Teplan et al.: EEG responses to long-term audio-visual stimulation. International Journal of Psychophysiology, 2006, 59/2 M. Teplan, A. Krakovská, S. Štolc: Direct and transient effects of audio-visual stimulation on EEG, submitted

Brain wave entrainment course of the stimulation & brain response - EEG spectrogram:

Direct effects of stimulation evaluation with ratios of relative powers in narrow frequency band: during stimulation referenced to interval prior to stimulation high statistical significancy of AVS effect, quite large variations the strongest effect at visual cortex and for higher frequencies

Phase synchronization during AVS Difference in phase synchronization during stimulation in respect to nonstimulation conditions: Difference of medians of z-scores. Z-scores evaluate distance of synchronization measure MPC from testing surrogate MPCs. Higher positive values mean higher increase of synchronization during the stimulation. Mann-Whitney U test (paired Wilcoxon) reflects a strong level of statistical significance (p- values from to 0.03) in all selected pairs for both stimulating frequencies. Stronger effect for 17 Hz than for 4 Hz stimulation. Interhemispheric relations: without monotonous decrease towards frontal locations - top values for centro-parietal regions. Possible enhanced communication also between cross areas (F3C3-P3O1, F4C4-P4O2).

Comparison to coherence methods during stimulation: prior to stimulation: linear correlation coefficient < 0.5

- approaches determining phase synchrony level are usefull for EEG studies - in the case of audio-visual stimulation we were able to determine regions with different synchronization relations - decrease in synchronization roughly follows power attenuation - possible impact on higher brain functions across the cortex - future work: application of directionality index – related technique for finding direction of the information flow Conclusions