A statistical technique for measuring synchronism between cortical regionsin the EEG during rhythmic stimulation

The coherence function has been widely applied in quantifying the degree of synchronism between electroencephalogram (EEG) signals obtained from diffe rent brain regions. However, when applied to investigating synchronization resulting from rhythmic stimulation, misleading results can arise from the high correlation of background EEG activity. We, thus propose a modified me asure, which emphasizes the synchronized stimulus responses and reduces the influence of the spontaneous EEG activity. Critical values for this estima tor are derived and tested in Monte Carlo simulations. The effectiveness of the method is illustrated on data recorded from 12 young normal subjects d uring rhythmic photic stimulation.