This document tries to examine the ways that help us to find the connections among the brain's functional units -- based on the relationships of the EEG recording channels. Searching these connections raises a number of problems, the most important ones are: common reference and the effect of the volume conduction.
The document examines two traditional ways that do not calculate these effects: the classical and the phase coherence. It compares these above with three other methods which were created specially for avoiding these effects: imaginary phase coherence, phase lag index (PLI) and weighted phase lag index (WPLI).
The methods use the time-frequency calculations of the signals. I used three ways of methods to this: short time Fourier transform, wavelet analysing and Hilbert transform. The document examines the differences, too.
Some article says that scalp current density (SCD) transformation of the EEG channels decreases the effect of the volume conduction. But we have to search the reliabilty of the connectivity methods counted on these data.