3D-EEG rhythm mapping during flash stimulation using zero-crossings analysis

3D-EEG rhythm mapping was performed to study alpha desynchronization using the zero-crossing analysis (ZCA) method. 32-electrode EEGs were recorded from five healthy volunteer subjects ranging from 21 to 36 years old during relaxed eye-closed state with flash-light stimulation (at 1 Hz, 10 Hz and 40 Hz inter-stimulus intervals). These EEG signals were filtered for the alpha band (8.0-14.0 Hz) using a fast Fourier transform-based digital filter. From thresholded zero-crossing points, periodic intervals of the alpha waves were determined and converted to instantaneous frequencies (IFs). The power spectrum of the IF fluctuation plot during a 81.92-second length of recorded EEG was calculated and plotted. From the power spectrum of an IF plot, information regarding the rhythm of the waveforms and of the envelopes, which is not represented in the filtered alpha waveforms alone, can be extracted. The spectrum slope values (SSVs) of the best-fit inclination lines for the frequencies of interest on the power spectrum were determined. Finally, 3D-EEG rhythm mapping displayed the scalp distribution of SSVs to show regions of synchronized or desynchronized alpha activity. We can observe the spatial dynamics of the alpha rhythm during flash stimulation using over-minute recordings. 3D-EEG rhythm mapping is useful in the investigation of cortical structures and the interactions of the underlying rhythmic activity