Flash-related synchronization and desynchronization revealed by a multipleband frequency analysis

The fast Fourier transform (FFT) is a good method to estimate power spectra l density (PSD), but the frequency resolution is limited to the sampling wi ndow, and thus the precise characteristics of PSD for short signals are not clear. To relax the limitation, a multiple band-pass filter was introduced to estimate the precise course of PSDs for flash visual evoked potentials (VEPs). Signals were recorded during -200 and 600 ms using balanced nonceph alic electrodes, and sampled at 1,000 Hz in 12 bits. With 1 Hz and 10 ms re solutions, PSDs were estimated between 10 and 100 Hz. Background powers at the alpha- and beta-bands were high over the posterior scalp, and powers ar ound 200 ms were evoked at the same bands over the same region, correspondi ng to P110 and N165 of VEPs. Normalized PSDs showed evoked powers around 20 0 ms and suppressed powers following the evoked powers over the posterior s calp. The evoked powers above the 20 Hz band were not statistically signifi cant, however, the gamma band was significantly evoked intra-individually; details in the gamma bands were varied among the subjects. Details of PSDs were complicated even for a simple task such as watching flashes; both sync hronization and desynchronization occurred with different distributions and different time courses.