To know not only where but how information is processed in the brain one needs to have the picture of cortical areas interaction, not available with the most brain imaging methods. Approaching the problem the method was elaborated, based on the idea that cortical connections were promoted with the neuronal oscillators synchronization, revealing particularly in the precise coincidence of the EEG and ERP spectral components in the interacting cortical areas. The method was applied for the study of the brain mechanisms in thinking operations. The main result was that the cortical connections at mental task solving converged to the particular cortical zones typical for e.g. abstract vs. imaginative thinking. These cortical areas were named the interaction foci. We hypothesize that these connections centers realize the function of the comparison and the synthesis of information coming to the focus resulting in decision making. The method was applied also to the mental pathology study.